high tg polymeric materials for heated microfluidic systems1029357/fulltext02.pdfenes, enabling the...

MASTER'S THESIS

High Tg Polymeric Materials for HeatedMicrofluidic Systems

Yitong Liu

Master of ScienceMaterials Engineering

Luleå University of TechnologyDepartment of Engineering Science and Mathematics

High Tg polymeric materials for heated microfluidicsystems

Yitong Liu

MICROSYSTEM TECHNOLOGYSCHOOL OF ELECTRICAL ENGINEERING

ROYAL INSITUTE OF TECHNOLOGYAMASE PROGRAMME

LULEA UNIVERSITY OF TECHNOLOGY

2011-12 - 09

Abstract

In this thesis work a novel material platform for microfluidics applications has been developedusing dual-cure ternary monomer thiol-ene-epoxy system. The introduction of epoxy com-ponent to the OSTE (off-stoichiometric thiol-ene) system has significantly increased the glasstransition temperature and mechanical properties of the material compared with regular thiol-enes, enabling the applications with a temperature resistant requirement (such as PolymeraseChain Reaction, PCR) and offering opportunities of ease of fabrication of multilayer devices,facile surface modification and biocompatible bonding to biosensors (such as biofunctionalizedQuartz crystal microbalance, QCM).

The majority of the work in this thesis has been devoted to the formulation of the novelternary monomer system and the different curing procedures. Different options of epoxy monomerswere appraised and UV/thermal and thermal/UV two-stage curing procedures with different per-spectives were evaluated with respect to the dynamic mechanical property, biocompatibility andbondability. Formulations of ternary thiol-ene-epoxy system with different off-stoichiometricratios were designed and characterized by DMTA in order to investigate the relation between theoff-stoichiometric ratios and Tg and mechanical properties. Raman spectra were used to assessthe conversion after each curing step. The novel thiol-ene-epoxy (hereafter named OSTE(+))system was compared with the OSTE system, and the advantages and disadvantages of thenewly developed system were summarized.

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Contents

Abstract iii

1 Introduction 11.1 Lab-on-a-chip and Microfluidic devices . . . . . . . . . . . . . . . . . . . . . 11.2 Demands on microfluidic device materials . . . . . . . . . . . . . . . . . . . . 3

1.2.1 General requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.2 Specific demands on PCR microfluidic devices . . . . . . . . . . . . . 51.2.3 Wish-list of microfluidic device materials . . . . . . . . . . . . . . . . 5

1.3 Generality of microfluidic device materials . . . . . . . . . . . . . . . . . . . . 61.3.1 Glass and silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3.2 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Thiol based polymers for microfluidic devices . . . . . . . . . . . . . . . . . . 71.4.1 Thiol-ene click chemistry . . . . . . . . . . . . . . . . . . . . . . . . 71.4.2 Off-stoichiometry thiol-enes (OSTE) in microfluidic device fabrication 91.4.3 Thiol-epoxy systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 High Tg polymeric materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.5.1 monomer modification . . . . . . . . . . . . . . . . . . . . . . . . . . 121.5.2 Hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Purpose Of Study 152.1 Research Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2 Proposed Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Experimental 193.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1 Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.2 Initiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2 Techniques and instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.1 Dynamic mechanical analysis (DMA) . . . . . . . . . . . . . . . . . . 213.2.2 Fourier Transfrom-RAMAN spectroscopy (FT-RAMAN) . . . . . . . . 213.2.3 Thermogravimetric analysis (TGA) . . . . . . . . . . . . . . . . . . . 213.2.4 UV-light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.5 Ultraviolet-Visible spectroscopy (UV-Vis Spectroscopy) . . . . . . . . 22

3.3 Experimental Procedure and methods . . . . . . . . . . . . . . . . . . . . . . 223.3.1 General mixing procedure . . . . . . . . . . . . . . . . . . . . . . . . 223.3.2 Curing schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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3.3.3 Test series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.4 Characterization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.4.1 DMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.4.2 RAMAN Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.4.3 TGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.4.4 Extraction test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.5 Fabrication of device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4 Results and Discussion 294.1 Processing development-Development of an universal mixing procedure for

OSTE (+) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2 Feasibility study of the ternary monomers system . . . . . . . . . . . . . . . . 30

4.2.1 Thiol-epoxy polymerization . . . . . . . . . . . . . . . . . . . . . . . 304.2.2 UV cure as the first polymerization stage in dual cure scheme . . . . . 314.2.3 Control over mechanical properties by partial cure . . . . . . . . . . . 334.2.4 Material A - PETMA/triallyl/BADGE ternary monomer system . . . . 344.2.5 Material B - PETMA/triallyl/TGIC ternary monomer system . . . . . . 354.2.6 Functional group conversion via RAMAN Spectra characterization . . 37

4.3 Evaluation of effect of processing . . . . . . . . . . . . . . . . . . . . . . . . 384.3.1 Influence of homogeneity on polymer properties . . . . . . . . . . . . 384.3.2 Influence of dual cure mechanism on polymer properties . . . . . . . . 394.3.3 Evaluation of the effect of monomer structure . . . . . . . . . . . . . . 414.3.4 Evaluation of the effects of off-stoichiometric ratios . . . . . . . . . . 43

4.4 Thermal stability study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494.5 Extraction test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504.6 Bonding and leaching test of device . . . . . . . . . . . . . . . . . . . . . . . 50

5 Conclusion 53

6 Outlook 55

Acknowledgement 61

Chapter 1

Introduction

1.1 Lab-on-a-chip and Microfluidic devices

Lab-on-a-chip (LOC) is a term for devices that integrate laboratory functions on a single chipwhich measures from only millimeters to a few centimeters in size. The lab-on-chip technol-ogy was developed from the early Miniaturized Total Analysis System (µ-TAS)[1], and is alsoknown as microfluidic chip or microfluidic lab-on-chip. In a similar manner to the IntegratedCircuit revolution, where the invention of the chip led to the miniaturization of the computer,LOC promises to realize the miniaturization of the clinical laboratory. Therefore, compared toconventional macro scale chemical or biological analysis systems, LOC systems have distinctadvantages: reduced consumption of samples and reagents; reduced size and power require-ment; lowered system cost while simultaneously allowing for faster analysis time and massivelyparallel (high-throughput) analysis capabilities. Indeed, due to the small amount of disposedwastes, it is also a ’green’ technology.

Microfluidic products are already on the market and in many applications microfluidic de-vices have found a widespread use, e.g. pregnancy tests, drugs of abuse tests, devices forcardiac marker quantification and also tests designed for bio-warfare protection. In the re-search field, microfluidic technologies have widely influenced various areas, especially in lifesciences, including in-vitro diagnostics, drug discovery, biotechnology, and ecology[2]. Ar-guably, microfluidic devices have their greatest potential in in-vitro diagnostics. An exampleis the application in point of care (POC), which is defined as a close to the patient and easyto perform medical test. By further increasing the integration of functions and portability ofdiagnostic devices, the technology would realize quick, close, and accurate diagnostic testingwhere the need is the greatest, i.e. at the point of care. For instance, recently Samuel K. Siaet al. developed a chip system (figure1) for blood diagnostics to test allergies and diseases likeHIV, which is designed to benefit people in the poorest region of the world [3]. As shown inpicture 1.1, the device consists of a disposable chip and a detector. The test need only a dropletof blood and can process the HIV analysis in less than 15 minutes.

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2 CHAPTER 1. INTRODUCTION

Figure 1.1: Samuel K. Sia, at Columbia Engineering and team have invented the mChip (mobilemicrofluidic chip) that can interpret quantitative blood test results[3]. (Photo: popsci.com /

Sangeeta G D).

Figure 1.2: Microfluidic chemostat made from polydimethylsiloxane (PDMS) used to study thegrowth of microbial populations – now routinely incorporate intricate plumbing. This deviceincludes a high density of pneumatic valves. The colours are dyes introduced to trace the

channels.[4]

To realize microfluidic devices, operations have to be done on a miniaturized scale. Thus,microfluidics system is fundamentally a field dedicated to miniaturized plumbing and fluidicmanipulation with channel dimensions from tens to hundreds of micrometers. To obtain func-tional devices, the following capabilities are needed: a means of introducing reagents and sam-ples is required, so called chip-to-world connections; methods for moving samples and reagentsaround the chip, and for merging and mixing fluid streams; various other components and unitoperations such as sensors for microanalytical tasks, and components for purification and sam-ple preparation of e.g. patient samples used in diagnostics, such as PCR (Polymerase Chain

1.2. DEMANDS ON MICROFLUIDIC DEVICE MATERIALS 3

Reaction).Since many challenges are posed for microfluidic devices, the development of microfluidic

systems is a multidisciplinary challenge, requiring knowledge not only in fluidics, microma-chining, electro-magnetics, but also in materials, biology and chemistry. Indeed, the ultimategoal of microfluidic platforms is to change the traditional chemical and biological analysis intosimple and inexpensive operations conducted on desktop readers to benefit many people in un-derserved regions of the world. Given the fact that the amount of publications in microfluidicshas constantly grown in recent years, it is credible that this technology will have an impact onmany research areas and consequently change our lives, maybe even in a revolutionarily fashionin the future.

1.2 Demands on microfluidic device materialsThere are some general demands on microfluidic device materials, such as: adequate mechan-ical and physical properties; good and strong bonding between device components and sub-strates; and precisely controlled surface properties. In addition, low cost simple fabricationprocesses are needed to ensure commercial viability. Beyond that, the demands on microfluidicdevices vary widely depending on the application, and each device needs to be addressed on anindividual basis.

1.2.1 General requirementsMechanical and physical properties

Elastomeric materials such as polydimethylsiloxane (PDMS), are needed in microfluidic de-vices for various functions, such as in valves, pumps, mixers, or switches, to control the flowof fluids in complex devices [5]. A soft material can also be preferred in certain cases as a sub-strate material, like in bonding process to adapt the micro-irregularities on the surface in orderto attain a perfect sealing, or in demoulding to facilitate the process without applying a greatforce which risks breaking devices made from stiffer materials. Apart from the requirementof soft and elastomeric materials for certain applications, stiffer materials are also needed inmicrofluidic chips for structural support, packaging and interfacing.

Mechanical strength is needed for microfluidic devices in the sense that the material needs tobe sufficiently stiff to ensure device integrity during transport and use, also ideally, the deviceshould not be permanently deformed when subjected to heating, as in PCR where 96◦C isrequired, or during the demoulding process. In addition, the microfluidic device material shouldnot be fragile or brittle, since dicing or drilling is also required in certain fabrication schemes.

As a package for microfluids, the material of the device often needs to be impermeable, i.e.have good barrier properties, to small molecules, while simultaneously being able to preventadsorption of biomolecules such as proteins. In summary, the substrate material should notinteract with the contained liquids and influence the analytical results by addition or removal ofany chemical compounds into or out of the sample.

Solvent resistance is also a crucial property for microfluidics devices, as it relates to ab-sorption and adsorption of the small molecules of solvents. For example, absorption of organic


solvents in PDMS can have a ruinous effect by swelling of the chip, which can make the flowof the fluids in the channels of the device very difficult[6].

Bonding properties

Sealing of different microfluidic components plays a significant role in the fabrication process ofmicrofluidic devices due to the requirement of enclosed fluidic paths and leakage-free packagingof other microfluidic elements. Since various materials are utilized in different components ofmicrofluidic devices, bonding between different materials is usually required, where substratescan include silicon, glass, gold, copper, aluminum, and various polymers. In addition, simplebonding of the microfluidic component to amine and alcohol-functionalized surfaces withoutthe need for secondary surface modification reactions is highly desired since these chemicalcompounds are prevalent in substrates designed for DNA and protein array assays.

In general, the bonding force between two substrates resulte from four basic mechanisms:covalent bonds, Van der Waals bonds, metallic bonds, ionic bonds [7]. Among these bondingtechniques, covalent bonding and Van der Waals bonding are the most commonly used. Also,comparing with Van der Waals bonding (with energy content of 0.08-42 kJ/mol [8], the bondstrength of covalent bonding (with energy content of 563-710 kJ/mol[8]) is much higher. De-pending on the different nature of substrates, several common bonding techniques are utilizedto achieve Van der Waals bonds and covalent bonds, for example, thermal bonding, plasma ac-tivation, solvents or adhesives[9]. However, by considering the important factors such as thebonding quality, the efficiency and the biocompatibility, all these available techniques have no-ticeable limitations, e.g., excessive temperature requirements, strict operation conditions, therisk of deforming channels or leachable adhesives.

Because of the disadvantages of the current bonding techniques, such as the high tempera-ture and difficult operation conditions, a new bonding concept of direct bonding at low temper-ature of the microfluidic device is preferred to facilitate the process. For instance, as describedin the paper of Carlborg et al.[10], two pieces of OSTE (Off-Stoichiometric Thiol-Ene) withrespectively thiol and allyl functional groups on the surface can bond to each other through asimple photo-initiated ’click’ chemistry and form strong covalent bonds.

Surface properties in microfluidic devices

The utility of microfluidic devices is often limited by interactions between channel surfacesand dissolved molecules contained in the device. Thus, surface properties should be takeninto consideration during the design and manufacture of microfluidic devices. In general, sur-face modification is a vital process in fabrication of microfluidic chips, since it facilitates theincorporation of multiple functionalities and provides the ability to tune and enhance surfaceproperties such as adhesiveness, hydrophobicity, biocompatibility, antifouling, surface hard-ness, and surface roughness. For example, spatially controlled hydrophobic or hydrophilicsurfaces are usually required in droplet microfluidics devices for water-in-oil emulsion and oil-in-water emulsion applications. Also, hydrophilic portions are required for capillary flow, andhydrophobic patches are required for stopping fluid flow[11].

The surface energy can be altered by grafting hydrophilic, e.g. amino or hydroxyl groups orhydrophobic, e.g. fluorinated compounds, functional groups. Another common surface modi-

1.2. DEMANDS ON MICROFLUIDIC DEVICE MATERIALS 5

fication technique is plasma treatment. This functions by breaking covalent bonds and exposereactive groups that are inherently hydrophilic, which can be used as is or used for furtherreactions (e.g. for PDMS, treatment generates silanol groups (Si-OH) on the surface by the ox-idation of methyl groups[12]). In contrast, an easier approach to modify the surface property isfavored, for example, the thiol functional groups displayed on the surface of OSTE can graft enefunctional groups and change the surface properties by using the UV-initiated ’click’ reactionto form a thio-ether bond. Furthermore, the density of unreacted groups present on the surfacesof the OSTE polymers can be controlled by the degree of off-stoichiometry and is stable overtime and evenly distributed over the entire surface. Also, by using a simple stencil mask, thegrafting on the surface can be precisely patterned[10].

1.2.2 Specific demands on PCR microfluidic devicesThe polymerase chain reaction (PCR) is a technique to amplify a single or a few copies ofDNA several orders of magnitude, generating thousands to millions of copies of a particularDNA sequence. Nowadays, it has become an indispensable technique that is extensively usedin biological and medical analysis for numerous applications. Currently, thanks to the advent ofmicro-electromechanical systems (MEMS) technology, the development of miniaturized PCRchips has become possible and indeed, is becoming more prevalent[13]. The miniaturizationof PCR has largely reduced the power and sample consumption, providing the possibilities ofa short assay time and integration of multiple processing modules compared with their macro-scopic counterparts. For example, to rapidly analyze bacteria or viruses in the blood stream,PCR coupled with a DNA-assay is needed.

The PCR reaction relies on repeated heating and cooling cycles for rapid DNA melting andenzymatic replication, during which the temperature oscillates between 52◦C and 96◦C. Thus,in addition to the importance of surface properties and impermeability, the characteristic ofwithstanding high temperatures and relatively high pressures is necessary in PCR chips.

Due to the temperature resistance requirement, the most common material for fabricationof PCR chips is glass and silicon. Recently, thermoplastics, which have high glass transitiontemperatures, e.g. PC (Polycarbonate), or COC (Cyclicolefin copolymer), have gained muchattention thanks to their low materials cost and ease of fabrication via replication molding.However, PCR LoCs typically comprise dissimilar materials, i.e. a polymeric cap and a bio-functionalized substrate, and the bonding process involving these materials is often cumbersomeand ineffective[14]. Hence, new alternative materials for PCR LoCs are demanded.

1.2.3 Wish-list of microfluidic device materialsTo summarize the demands of microfluidic device materials, a ’wish list’ has been drawn upwhere the materials system:

• allows for easy surface modification

• has a wide range of mechanical properties

• provides chemical inertness, no interaction with sample

• allows for low temperature, biocompatible bonding


• has high temperature resistance

• is easy to use and compatible with soft lithography

1.3 Generality of microfluidic device materialsMaterials used for fabrication of microfluidic devices generally include glass, silicon, polymersand hydrogels for different functions[15]. In this section, glass and silicon are mentioned astraditional materials and polymers are highlighted as popular materials for microfluidic appli-cations.

1.3.1 Glass and silicon

Silicon and glass are the earliest utilized device materials because of their mechanical dura-bility, surface stability and well-developed techniques for fabrication[16]. Also, the low auto-fluorescence, and high transparency of glass makes it suitable for optical detection. Therefore,silicon and glass are still very important materials in microfluidic device fabrication and theyhave been used until now. However, the processes used for fabricating microfluidic devicesfrom glass and silicon are too time-consuming and expensive for mass production[17], whichhave driven researchers to seek new alternatives.

1.3.2 Polymers

Compared with silicon and glass, polymers, which include thermoplastics and thermosets, havegained attention as materials for lab-on-a-chip applications, because of their biocompatibility,low-cost manufacturability, ease of fabrication, rapid prototyping, disposability, and wide vari-ety of surface properties[18]. Recently, many researchers have focused on the development ofpolymeric microfluidic devices with the aim of achieving the optimal device efficiency, through-put, and cost effectiveness.

Thermoplastics

Thermoplastics is a category of polymers that soften when heated and can be melted to liquidabove a characteristic melting temperature, Tm, above which all crystalline structure disappearsand the chains become randomly inter dispersed, while returning to their original glassy stateupon cooling[19]. This results in the unique feature of thermoplastics that they can be remeltedand remoulded. In light of the lower cost of the raw material and fabrication, thermoplasticssuch as PMMA, PC, COC/COP, became popular material for microfluidic devices. However,many solvents can dissolve thermoplastics, and the surface modification is difficult to realize ina lot of cases.

Thermosets

A thermoset is a crosslinked polymer that forms a three-dimensional network after an irre-versible polymerization process. Unlike the thermoplastics, once the three dimensional network

1.4. THIOL BASED POLYMERS FOR MICROFLUIDIC DEVICES 7

is formed, it’s impossible to melt the material again. A good solvent can swell a thermoset ma-terial but cannot dissolve it. Usually, thermoset polymers are formed by polymerization ofmonomers where the average monomer functionality > 2.

Owing to their good dimensional stability, thermal stability, chemical resistance and elec-trical insulation properties, thermosets constitute a good solution to the demands posed on amaterial for fabrication of microfluidics devices.

Figure 1.3: The different structures of the network between thermoplastic and thermoset

Nowadays, the thermoset most commonly used in research microfluidic devices is PDMS(Poly(dimethylsiloxane)). PDMS is an elastomer, in which the molecular chains are long, phys-ically entangled and very loosely crosslinked. PDMS behaves ’elastically’, which means if anexternal force is applied, the chains will disentangle and allow the polymer to stretch, and if theforce is withdrawn it will return its original shape. The popularity of PDMS in the microfluidicsresearch community are due to the rapid prototyping capacity[20], good biocompatibility andalso its attractive mechanical and chemical properties [21]. Nevertheless, PDMS has also manydrawbacks where adsorption of proteins, swelling in non-polar solvents and non-permanencyof surface modifications stand out. Furthermore, PDMS has one of the lowest Tgs known forpolymers because Si-O-Si is a very flexible link. All these shortcomings render PDMS inade-quate for lab-on-chip systems. Thus, new alternatives are emerging in recent years, includingmodification of PDMS, and the use of thiol-ene polymers.

1.4 Thiol based polymers for microfluidic devices

1.4.1 Thiol-ene click chemistryThe so-called ’Click’ chemistry, pioneered by the 2001 Nobel Prize laureate Barry Sharpless,compared an efficient, simple, reliable chemical formation of carbon-hetero atom bonds (C-X-C) to a ’click and lock’ mechanism[22]. This newly developed chemistry is undeniably one ofthe most important trends in contemporary chemistry, and has all the promises of predictability,tunability and toughness required for novel polymeric microfluidic device materials.

The reaction of thiols with enes can be carried out via radical (termed thiol-ene reaction) oranionic chain (termed thiol Michael addition) mechanisms, both of which carry many of the at-tributes of click reactions, including quantitative yields, small concentrations of catalysts, rapid


reaction rates, insensitivity to oxygen inhibition[23]. Cross-linked polymers formed from thesesystems are the most ideal homogeneous network structures ever formed by free-radical poly-merization, which leads to narrow glass transition regions and extremely low polymerizationshrinkage stress [24].

Classic radical-based photopolymerization, e.g. using (meth)acrylate prepolymers, are ofgreat interest in many applications, such as coatings, contact lenses and photolithographicprocesses[6]. Nevertheless, classic radical-based photo-polymerization is plagued by somecritical problems, including inhibition by oxygen, complicated volume relaxation and stressdevelopment, complex polymerization kinetics, etc.

The thiol-ene photopolymerization reactions address each of the critical limitations of con-ventional photopolymerization by forming homogeneous polymer network through a control-lable step addition polymerization, and at the same time inherit the rapid production and con-venience of the photopolymerization process. Therefore, in recent years, thiol-ene photopoly-merizations have gained considerable importance in the technological and scientific work onradiation curing because of the unique advantages and have been used in the fabrication ofmicrofluidic devices[25].

The ideal thiol-ene radical reaction revolves around the alternation between thiol radicalpropagation across the ene functional group and the chain-transfer reaction, which involvesabstraction of a hydrogen radical from the thiol by the carbon-centered radical.

Figure 1.4: General mechanism of thiol-ene ’click’ chemistry


Initiation

There are many different approaches to initiate thiol-ene photopolymerization, includingthe excitation of a diarylketone followed by hydrogen transfer, the excitation of an arylaliphaticketone followed by bond cleavage, and the direct excitation of the thiol followed by a lysis ofthe sulfur-hydrogen bond [24]. Although the reaction can be excited by a low wavelength UVlight spontaneously, usually it is carried out with an unimolecular photoinitiator, i.e. norrishtype I initiator, which dissociates into two radical fragments under UV exposure.

Propagation

After initiation, two stages of propagation take place. The first step involves an additionof the thiyl radical to the carbon of an ene functional group, then the second step is the chaintransfer to thiol monomer, regenerating a thiyl radical by abstraction of atomic hydrogen l fromthe thiol functional group by the carbon-centered radical.

In the ideal step-growth thiol-ene reaction, there is no homopolymerization (i.e., chaingrowth), in which the carbon-centered radical propagates through the ene moiety[26]. How-ever, for some thiol-vinyl polymerizations the vinyl monomer, e.g (meth)acrylate, participatesin both step growth propagation and chain transfer events as well as chain growth homopoly-merization. In this case, the vinyl monomer will achieve higher final consumption rate than thatof the thiol monomer, and consequently a higher Tg.

The propagation 2 is responsible for the gelation of the polymer since crosslinks are formedin this step. Due to the chain transfer in propagation 3, a delayed gelation results, where gelsform at a much higher conversion for thiol-ene systems compared with multifunctional acrylatemonomer polymerization. Thus, the shrinkage stress that is caused by the transformation fromvan der waals to covalent bond distances in the polymerization occurring after the gelation isdramatically decreased.

1.4.2 Off-stoichiometry thiol-enes (OSTE) in microfluidic device fabrica-tion

The use of thiol-enes as a material for microfluidic devices is one of the approaches that havebeen taken in pursuit of PDMS alternatives. Regarding its unique reaction mechanism, thiol-ene photopolymers have been used to fabricate microfluidic devices or to contribute to theperformance, alter the surface chemistry, etc.

At Microsystem technology department at KTH, Carlborg et al.[10] have developed a novelplatform of multi-functional and UV-curable OSTE polymers by tailoring the stoichiometrybetween the content of thiol functional groups and ene functional groups. It has been provenparticularly effective in solving many difficult microfluidic packaging tasks such as, bondingto bio-functionalized substrates, rapid and simple assembly of layers via low temperature mi-crofluidic layer bonding, simple micromachining (dicing and CNC milling), low permeabilityto small molecules and solvents and simple surface modification schemes.


Figure 1.5: The formulation of stoichiometric thiol-ene and off-stoichiometric thiol-ene[10]

From PDMS to OSTE, a big step has been made in the area of the fabrication of polymericmicrofluidics devices since there have been very few materials specially designed for lab-on-chip applications. Both the polymerization characteristics and the material properties of OSTEsystems make it very suitable for fabrication of microfluidics devices.

Despite great progress that OSTE have made in changing the concept of microfluidic ma-terials, there are still some demands not fulfilled in the wish list. For example, the bulk ofthe commercially available non-acrylated vinyl monomers and thiol monomers typically havea flexible core and a flexible S-C bond, making the development of high glass transition tem-perature (highly cross-linked) materials more difficult. It is problematic to use most thiol-eneformulations due to the poor temperature resistance, due to relatively low Tgs, which limitedthe applications of OSTEs to room temperature applications, which excludes, for example, PCRcycling. Furthermore, there is 8±2 % leakage of thiol monomer in chloroform for the OSTE (90% excess of thiol group) because of the uncured thiol excess content[10]. This leakage problemmakes the material potentially risky since leaking monomers might influence the analysis result.Therefore, this leakage problem should also be addressed by further development.

1.4.3 Thiol-epoxy systems

Epoxy, as a thermoset, is widely used in the areas of adhesives, coating, lamination, electronicencapsulation, and composite applications due to its excellent mechanical properties and adhe-sive properties.

The curing reaction of the epoxy can be realized generally through three kinds of poly-merizations: step-growth polymerizations through reaction with curing agent; chain-growthpolymerizations initiated by Lewis acids; or chain-growth polymerizations initiated by Lewisbases.

The crosslinking network of pure epoxy resins is a very complex process, and it is impossi-ble to give a definitive structure for the final product. The properties and performance of epoxyresins are dependent on the type of epoxy resin, the curing agent, and curing conditions. Con-cerning the curing conditions, it was found that the curing reaction was both dependent on thecuring temperature and curing time, where the curing temperature has more impact than thecuring time on the curing reaction in terms of mechanical properties and Tg [27].


Thiol-Epoxy polymerization

Thiols react with epoxies in a base catalyzed addition reaction or nucleophilic displacementreaction[28], [29]. More precisely, as shown in the equations, a thiol functional group creates areactive intermediate with a tertiary amine (1), resulting in a thiol anion. The thiol anion attacksan epoxide group via an anionic addition (2,3). Meanwhile, the tertiary amine can react withthe epoxide group creating an epoxide anion (4). Although both the thiol anion and the epoxideanion can proceed the polymerization in a nucleophilic displacement, it is demonstrated thatthe relative rate of nucleophilic displacement of a thiol anion is over 3 orders of magnitudefaster than a similar alcohol anion[30]. The dominant reactions that discussed in this study arepresented by equation (1) to (5).

Figure 1.6: General mechanism of thiol-epoxy chemistry

Thus, as demonstrated in the mechanism of polymerization, parameters such as the monomerconcentrations and the initiator concentration may alter the polymerization rate and hence pro-mote different reaction mechanisms.


1.5 High Tg polymeric materialsThe glass transition event, which is dependent upon the chemical make-up of the material, isone of the most important characteristic properties of a polymeric material. Theoretically, toincrease Tg, the principle is that less flexible chains result in higher Tgs.

To obtain thermosets that maintain glassy properties even at a higher temperature (highTg), many researchers have made efforts to form high levels of cross-linking in the cured resinor to introduce rigid chemical structures in the polymer backbone. To counter the inherentlylow Tg of most thiol-ene polymers, researchers have made efforts to obtain high Tg thiol-enebased polymers. After a thorough look into the present literature, two appealing approaches aredeemed the most attractive for this thesis work:

1.5.1 monomer modification

Owing to the relative ease of synthesis of new multifunctional thiols and enes, there is a largepotential for developing new monomer systems. Therefore, a potential first approach to achievehigher Tg is to modify the monomers. One of the most interesting reports showed that Tgsas high as 94◦C was achievable in thiol-norbornene are resins[31]. In another work it wasreported[32] that the incorporation of thiourethane linkages into the thiol-ene networks resultedin TUTE films with high Tg and increases of Tg were achieved by aging at room temperatureand annealing the UV cured films at 85◦C.

This approach has many advantages since it is: tailorable, controllable, and we can modifyboth the enes and the thiols to achieve preferred properties. However, the additional requirementof easy surface modifications to reduce non-specific binding of DNA may lower Tg’s too muchsince off stoichiometry formulations, e.g. the OSTE approach, reduce the crosslink density.

1.5.2 Hybrid system

Hybrid polymerizations, systems where one or both of the monomers used in the polymer-ization contain functional groups which are polymerizable by multiple curing methods, offersynergistic material properties in the resultant material[33].

Carioscia and co-workers[34] have developed the thiol-epoxy/thiol-ene hybrid network whichhas a relatively high Tg of 77◦C. Other researchers[35] also reported that an increase in glasstransition temperature and in storage modulus was observed for the hybrid thiol-ene/epoxy coat-ings when compared with the pristine thiol-ene UV-cured system.

Thus, it is possible to have a (radical polymerizing) thiol excess OSTE as the base formula-tion and an epoxy that reacts with excess thiol groups via an anionic mechanism that is initiatedwhen so desired. The potential advantages are simple bonding to substrates using both the thioland epoxy functionality combined with a low Young’s modulus for perfect fit to the substrate’smicrostructure after the first cure, and upon secondary polymerization, a high Tg reached.

In addition, apart from the approaches of monomer modification and the ternary system, thepresence of a large amount of rigid particles, e.g. silica fillers, may also increase the temperatureresistance and yield a relatively high E-modulus after the glass transition to prevent deforma-tion under loads at high temperature. As reported in literature, the incorporation of significant

1.5. HIGH TG POLYMERIC MATERIALS 13

amounts of inorganic fillers, particularly nano-sized particles, greatly increases hardness andabrasion resistance[36]. However, no article describing a very high Tg material through thisapproach was found.

To compare the findings in the literature, a summarized table is made regarding differentapproaches and different Tgs:

Table 1.1: methods to achieve high Tg thiol-ene system in litterature

Approach chemical structrue Achieved highest Tg (◦C) Reference

Monomer modification thiol-norbornene 94 [31]Monomer modification Thiourethane based thiol-ene 85 [35]Hybrid ternary system Thiol-ene-epoxy 77 [34]

Chapter 2

Purpose Of Study

2.1 Research GoalThe overall purpose of this study is to investigate the possibility and feasibility of a novel ap-proach to improve the properties of polymeric microfluidic device material. The OSTE polymerplatform has already opened up many opportunities for material development in various aspects.However, there are some remaining challenges that need to be addressed, and this Master’sthesis focuses on two important limitations experienced in microfluidic application currently;Firstly (Primary goal), resistance to high temperature (>96◦C) and secondly (Secondary goal),simple bonding of the polymeric microfluidic component to many kinds of substrates withoutthe need for secondary surface modification reactions.

2.2 Proposed MethodsAfter a thorough literature study, an approach was proposed to achieve the research goals byintroducing a tertiary epoxy component which can reacts with the remaining thiol functionalgroups through the anionic polymerization in the OSTE network, and this new material platformwas named ’OSTE(+)’, where the ’+’ means the additional epoxy component and also theimproved properties.

In the novel OSTE(+) scheme, there are three monomers: thiol monomer, ene monomerand epoxy monomer. We denote the functionalities of thiol, ene, and epoxy as n, m and l. Thenumber of moles are defined as x, y, z.

In this project, only systems with thiol excess after the first stage curing were studied be-cause of the sequential reaction of thiol-ene and thiol-epoxy. Then the thiol excess content ? isdefined by the uncured amount of thiol functional groups after the UV cure, in other words, thequantity of the thiol functional groups that will react with epoxide groups:

Thiol excess α =

(nx−my

my× 100%

)(2.1)

For example, the sample with thiol excess = 50% contains the stoichiometric ratios of1:1.5:0.5 with respect to allyl, thiol and epoxy functional groups. The thiol excess groupsand unreacted epoxy monomer after the first stage cure provide the possibility to obtain a ratherlow Tg, due to low crosslink density and dangling chain ends, in order to acquire preferable

15

16 CHAPTER 2. PURPOSE OF STUDY

mechanical properties for dry bonding at room temperature or slightly heated conditions. Byvarying off stoichiometry, preferable properties can be found for a certain application. In ad-dition, the unreacted thiol functional groups and epoxy functional groups (or ene functionalgroups) displayed on the surface can facilitate the bonding process by creating covalent bondsto various kinds of surfaces such as silicon, glass, gold, copper, aluminum, polymers, and alsobio-functionalized substrates, either directly or in the ease of ene excess, through a grafting stepwith a functional molecule.

In order to achieve the optimal mechanical strength and avoid leaching phenomena, thefully connected cross-linked network was designed to be stoichiometric which means having noexcess content after the two-stage curing of thiol-ene and thiol-epoxy. Therefore, the amount ofthiol functional groups should equal to the sum of the allyl functional groups and the epoxidefunctional groups, then we have:

nx = my + lz (2.2)

The ideal situation is that after the two stages curing process, there are very few or noreactive groups remaining in the polymer and thus non-leachable and optimized temperatureresistant properties are ensured.

To be detailed, the study can further be divided into separated parts:

Effect of monomer composition on final materials properties

The initial goal is to modify the temperature resistance property of the packaging of microflu-idics devices. The introduction of the tertiary epoxy monomer to the thiol-ene scheme of OSTEcan change the chemical architecture of the network to form a stiffer and highly cross-linkedmaterial in order to achieve a Tg > 96◦C) which is the uppermost temperature in PCR cycling.Therefore, a proper epoxy structure should be chosen to enhance the network rigidity, hencefinally obtain sufficient final good enough mechanical properties at high temperature.

2.2. PROPOSED METHODS 17

Effect of the curing process (dual cure mechanism)

In this thesis project, a clearly separated two-stage reaction is highly preferable in terms of theease of controlling of bonding, surface modification and mechanical properties. According tothe nature of the curing mechanisms, the dual cure mechanism can be pursued by initiatingeither:

• the thiol-ene polymerization at temperature 1 or at light wave length 1; or

• the unreacted epoxy-thiol excess polymerization at temperature 2 or at wave length 2

To evaluate the separation of the two stages curing procedure is of great importance, sincethe first stage properties can be influenced by the conversion rate of each monomer. To ob-tain increased knowledge on how the mechanism and overall thiol-ene reaction rate depends onthe introduction of the tertiary component with competing thiol-epxoy anionic polymerization,evaluation tests needed to be performed. The FT-Raman analysis of partly polymerized systemsis a very powerful tool for thiol-ene systems since both the thiol and allyl functional groups givestrong Raman signals and analysis can be performed on liquid as well as solid samples. Fur-thermore, the order of the sequential dual cure process can be an interesting factor for alteringthe different properties after the first cure.

Possibility to obtain in-situ surface functional groups

To improve the bonding property of OSTE, simple bonding of the polymeric microfluidic com-ponent to amines and alcohols without the need for secondary surface modification reactions isalso an ambition of the thesis work. Since the dual cure process is used, after the first stage cure,there will be two different kinds of functional groups, i.e. thiols and epoxies or thiols and enes,on the surface that can be used to bond directly to a surface modified substrate, e.g. protein andDNA arrays.

Tunability of the ternary system

Different off-stoichiometries of thiol or ene monomers offers the opportunity to tailor the prop-erties both after the first stage cure and the second stage cure. Control of the stoichiometrybetween the monomers can further be used to obtain a sequential structure build up with one re-action mechanism dominating initially while the second contributes at later stages. A schematicstudy of the effect of off-stoichiometry on first stage and second stage material properties willbe helpful to design material suitable for a certain application.

18 CHAPTER 2. PURPOSE OF STUDY

Chapter 3

Experimental

To investigate the feasibility of the ternary monomer dual cure system to achieve high Tg, differ-ent formulations were designed and tested. Subsequently, DMA, FT-Raman, TGA and leachingtest were performed to characterize dynamic mechanical properties, dual cure kinetics, thermalstability and leaching properties. At last, devices for demonstration were fabricated to test thereal microfluidic properties such as bonding and leaching.

3.1 Materials

Three kinds of monomers (thiol, ene and epoxy) and two kinds of initiators (UV initiator andanionic initiator) were used in this investigation.

3.1.1 Monomers

Different kinds of thiols, allyls and epoxies were utilized in this investigation to study the rela-tionship between the structure and the properties of the final cured polymer.

Table 3.1: Thiol monomers used in this study

Name of monomer Chemical structure Mn [g/mol] ρ [g/mL]

PETMA 432.55 1.385(pentaerythritol tetrakis

(2-mercaptoacetate))

PETMP 488.66 1.28(pentaerythritol tetrakis(2-mercaptopropionate))

19

20 CHAPTER 3. EXPERIMENTAL

Table 3.2: "Ene" monomer used in this study


TATATO 249.27 1.159(triallyl-1,3,5-diazine

-2,4,6(1H,3H,5H)-trione)

Table 3.3: Epoxy monomers used in this study


BADGE 340.41 1.16(bisphenol-A diglycidyl ether)

TGIC 297.3 1.420(Triglycidylisocyanurate)

TMPTE 302.36 1.157(Trimethylolpropane

triglycidyl ether)

All monomers were used without additional purification. Each monomer was obtained fromSigma Aldrich GmbH, Germany.

3.1.2 Initiators

Two kinds of initiators for thiol-ene photo cure and thiol-epoxy anionic cure were utilized toinitiate the two polymerizations separately.

3.2. TECHNIQUES AND INSTRUMENTATION 21

Table 3.4: Photo-initiator and anionic initiator used in this study

Name Chemical structure

Anionic initiator:DBN (1,5-diazabicyclo[4.3.0]non-5-ene)

Photo-initiator:TPO-L

(Ethyl-2,4,6-Trimethylbenzoylphenylphosphinate)

DBN was obtained from Sigma Aldrich GmbH Germany. TPO-L was obtained from BASFGmbH Germany.

3.2 Techniques and instrumentation

3.2.1 Dynamic mechanical analysis (DMA)Dynamic mechanical analysis was performed on a DMA Q800 from TA instruments equippedwith TA software for evaluation and data analysis. All measurements were conducted by usingthe tension film mode at a frequency of 1 Hz with a strain oscillation amplitude of 15 microm-eters (125% autostrain) under a heating rate of 5◦C/min.

3.2.2 Fourier Transfrom-RAMAN spectroscopy (FT-RAMAN)FT-Raman measurements were performed with a Perkin-Elmer Spectrum 2000 NIR-Ramanequipment equipped with Spectrum software. The laser power used in the measurements was500 mW, and 64 scans were taken for every sample to achieve results with low noise.

3.2.3 Thermogravimetric analysis (TGA)The TGA Q500 (TA Instruments) was used in this study and the TA software was used duringthe collection of the data and analysis of the results.

3.2.4 UV-light sourcesFore UV-curing, a UV-lamp (EFOS Lite, EFOS, Ontario, Canada) equipped with a 365 nmwavelength band pass filter was used to obtain a constant irradiance of 4 mW/cm2. Alternativelyan UV fusion conveyor MC6R equipped with fusion electrodless bulbs standard type BF9 (UV-fusion lamp) was employed.


3.2.5 Ultraviolet-Visible spectroscopy (UV-Vis Spectroscopy)Ultraviolet-Visible spectroscopy was carried out on a double-beam Cary E1 UV-Vis spectropho-tometer. The scan resolution of the UV-Vis spectrometer was 1.0 nm. Scans were taken in a1cm path length quartz cuvette.

3.3 Experimental Procedure and methods

3.3.1 General mixing procedureThe mixing procedures of different monomers were developed in this study. According to theviscosity of the monomers, there are mainly two altered methods, represented by the mixingprocedure of material A and that of material B.

Material A - PETMP/TATATO/BADGE system

At room temperature, the three monomers of PETMP, TATATO and BADGE are all at liquidstate. The amount of the three components were calculated for a certain off-stoichiometricformulation and they were added together in a glass vial and then stirred using Vortex mixeruntil the mixture was homogeneous. The photoinitiator and anionic initiator were added andthe prepolymer was mixed again with the ternary monomer system.

Material B - PETMP/TATATO/TGIC system

At room temperature, the thiol monomer PETMP and the triallyl momomer TATATO are liquid,while the TGIC is a white, granular solid with a melting point of 90-125◦C. In order to dissolvethe epoxy and obtain a homogeneous mixture of the three components, a mixing method wasdeveloped in this project.

Ultrasonic processing combined with heat treatment in an oven was used in the prepolymermixing as a means to prevent particle agglomeration and achieve a homogeneous mixture.

Specifically, the desired amount of the anionic initiatior DBN was firstly mixed with 1/3 ofthe total amount of the thiol monomer, calculated according to the previously shown equation(Mixture1). The rest of thiol monomer which is needed for the formulation was mixed withthe ene monomer and epoxy monomer (Mixture2). Mixture 2 was put in the ultrasonic bathfor 10 min and then put in the oven set at 90◦C for 20 min in order to let the TGIC dissolve inthe liquid phase. Afterwards, mixture 1 and 2 were mixed together and stirred after adding thephotoinitiator TPO-L.

3.3.2 Curing schemesUV/thermal dual cure process

The prepolymer formulations were firstly mixed in predetermined off-stoichiometric ratios andpoured into a Teflon mold, and then irradiated with a standard tabletop UV-lamp at a curing

3.3. EXPERIMENTAL PROCEDURE AND METHODS 23

distance of 20 cm, which gives the intensity of 4 mW/cm2. After UV exposure for 100 seconds,the pre-cured samples were immediately put in the oven at 90 ◦C for 2 hours to afford full cure.

3.3.3 Test seriesInitial experiments on ternary monomers system

In order to achieve a ternary monomer two-stage curing procedure, firstly it is important toshow that the system can be cured and have reasonable properties after the first stage cure.Therefore, the feasibility of curing thiol-ene is in the presence of the unreacted epoxy monomerwas studied. Subsequently, the dual cure process was carried out to investigate the possibilityto obtain a fully cured network with high Tg.

(1) Thiol-epoxy polymerization with DBN

The first step of the feasibility study is to realize the thiol-epoxy anionic polymerization with theavailable anionic initiator DBN. PETMA (pentaerythritol tetrakis(2-mercaptoacetate)) was usedas thiol monomer and BADGE (bisphenol-A diglycidyl ether) was used as epoxy monomer.The mass of each monomer was calculated through stoichiometric formulation. Approximately1wt% and 0.5wt% DBN of the total weight respectively were measured and added to the stoi-chiometric mixture of PETMA and BADGE, and a formulation without DBN served as a refer-ence:

Table 3.5: Stoichiometric formulation of thiol-epoxy with different concentration of anionicinitiator

PETMA BADGE DBN1 1 1 1%wt2 1 1 0.5%wt3 1 1 0%

(2) UV cure of the ternary system

According to Fredrik’s work, thiol-ene can be cured under an UV fusion lamp at degree 4. Inorder to test if the polymerization is effective without photoinitiator, formulation 4 which iswithout TPO-L was tested initially.


PETMA Triallyl BADGE TPO-L(0.5wt%) DBN(0.5wt%)4 50% excess 1 1 1

Afterwards, a series of different formulation series were evaluated in order to reveal the ef-fect of different formulation parameters, including the chemical structure of the thiol monomer,the content of the thiol excess, and the presence of anionic initiator DBN. The contents of everycomponent were calculated according to the equation. All the formulations were made using the


general mixing procedure described previously. The composition of the different formulationsseries are listed in table


PETMA Triallyl BADGE TPO-L(0.5wt%) DBN(0.5wt%)5 no excess 1 16 50% excess 1 1 17 100% excess 1 1 18 50% excess 1 1 1 19 100% excess 1 1 1 1

Formulations 5-9 were designed with added TPO-L as photoinitiator (0.5wt%). Two kindsof thiols with different degrees of functionality were evaluated. A series of formulations of thiol-ene-epoxy with different content of excess thiol (0%, 50wt% and 100wt%) were calculated totest. The first three formulations were in absence of DBN for comparison. The following twoformulations contained DBN (0.5wt%).

The samples of different formulations were mixed, molded in a Teflon mold (thickness2.1mm) and then put under the UV exposure for a certain time to check the time needed forgelation.

(3) Dual cure scheme of the ternary monomer system

To evaluate the possibility of increasing the Tg by introducing epoxy monomer via a dual cureprocess, the UV/thermal sequential curing process was carried out. Two kinds of materialswith different epoxies were defined as Material A (PETMA/triallyl/BADGE) and Material B(PETMA/triallyl/TGIC). For Material A, the test sample was made through formulation 8. Formaterial B, the formulation was also followed formulation 8, except that the TGIC was used asepoxy monomer instead of BADGE. The prepolymer was UV-exposed for 100s and then put inthe oven at 70◦C overnight.

Dual cure ternary monomers system with different off-stoichiometric ratios

To evaluate how different off-stoichiometric ratios influence the first stage properties as wellas the ultimate properties of the thiol-ene-epoxy ternary monomer system, a series of MaterialB formulations were prepared by varying the thiol-ene stoichiometric ratios (contents of thiolexcess after first stage curing) with interval of 20%: 20%, 40%, 60%, 80%, and 100%. Thecontents of each component were calculated according to the equation 2.1 and 2.2. All theformulations were made using the previously reported general mixing procedure.

3.4 Characterization Methods

3.4.1 DMADynamic Mechanical Thermal Analyzer (DMTA or DMA) has been widely used to monitorthe material mechanical properties, including the changes of these properties that occur at the

3.4. CHARACTERIZATION METHODS 25

glass transition temperature (Tg). Its working principle is the use of a perpendicular sinusoidalvibration of a plunger pressed or clamped onto a sample of known geometry. For a known stress,the sample will then deform a certain amount, which is related to its stiffness. The frequencycan be selected in the range of 10 mHz - 100 Hz. The amplitude can be pre-set and thereforeto reach this amplitude that is automatically tuned by the apparatus in a closed loop[37]. As aresult, the storage modulus E’ and loss modulus E” were determined.

The glass transition is detected as a sudden and considerable (several decades) change inthe loss modulus and an attendant peak in the tan δ curve. This underscores the importance ofthe glass transition as a material property, for it shows clearly the substantial change in rigiditythat the material experiences in a short span of temperatures. Accordingly, the glass transitiontemperature is a key factor in deciding the usefulness of a polymer.

Generally, Tg can be determined by several different approaches, including the onset of thestorage modulus, the peak of loss modulus or the peak of tan δ. For most commercial DMTA,a relatively accurate value of the damping factor tan δ can be measured compared with otherfactors such as forces or strains. Thus the dissipation spectra of tan δ = f (δ, T) are very practicalanalytical tools. In this study, all the Tgs were determined by the temperatures of tan δ peak.

The width of the glass transition of tan δ can be related to the degree of inhomogeneityof the spatial distribution of crosslink density. Presumably, the narrower the width, the morehomogeneous the network is.

Sample preparation

All the samples were prepared by molding in a Teflon mold with a dimension of approxi-mately 15mm×6mm×2mm. Then the samples were cured in the mold by application of UV orheat. Tests were taken after 20 minutes of making the samples.

Experimental method

The temperature scan scale was from -10-80◦C for the first stage cured polymer and 20-150◦C for the second stage cured polymer. The results were plotted to E’, E” and tan δ as afunction of temperature.

3.4.2 RAMAN Spectra

RAMAN Spectra is used to study the vibrational, rotational and other low-frequency modes ofchemical groups in a polymer system.

This instrument uses a holographic notch filter as a beam splitter to couple the laser lightinto the microscope and onto the sample and to reject laser light from the backscattered signalthat goes to the spectrometer while passing the Raman-shifted spectrum.

It has been shown that Raman is an effective characterization method for monitoring theevolution of polymerization. Since the thiol and allyl peaks are clearly visible in the RAMANspectra, in this study, it is used for monitoring the conversion of the polymerization of theternary monomer system.


Sample preparation

Samples for Raman spectroscopy were drawn from the mixed prepolymer with a glass cap-illary and applied to cleaned, bare EG6060 steel panels with a draw down bar. After the scanof prepolymer, the sample was irradiated under UV lamp with a filter of 365nm wavelength for100s. Then after the scan of the UV-cured polymer, the sample was put in oven at 90◦C for 2hours.

Experimental method

Mid-Raman spectra (6000-400cm−1) have been collected before any curing procedure, shortlyafter the UV cure, and shortly after the thermal treatment to compare the evolution of the threecomponents monitoring the peaks at 2575cm−1 (thiol stretching vibration), 1644 cm−1 (allylstretching vibration), and 1260 cm−1 (epoxide).

The percent conversion was calculated using Equation in which A is the area of the peakbefore and after cure.

3.4.3 TGA

Thermogravimetric Analysis (TGA) measures the absolute mass and rate of change in the massof a material as a function of temperature or time in a controlled atmosphere. Measurements areused primarily to determine the decomposition of materials and to predict their thermal stabilityat temperatures up to 500◦C. Its typical applications include the measurements of thermal sta-bility of materials, compositional analysis of multi-component systems, oxidative stability anddecomposition kinetics of materials, and moisture and volatiles content of materials etc.

Sample preparation

The specimens for TGA were cut from the pre-made samples with a knife. According to theinstrument instruction, the specimen mass should be in the range of 5mg to 20mg. The finalspecimens under tests were controlled to about 10mg in mass.

Experimental method

The TGA tests were made using a temperature range from room temperature to 500◦Cand the TGA-curves were plotted as weight % of the initial mass as a function of temperature.When the material reaches the temperature of decomposition, there is normally a steep decreaseof weight can be observed in the obtained curves. Hence, the temperatures of decompositioncan be determined.

3.4.4 Extraction test

The OSTE extraction problem in chloroform motivated leaching tests for OSTE(+). The sam-ples were cut in size about 1*1cm2 and immersed in chloroform for 24 hours, then followedby drying in a vacuum oven at 50 ◦C. The extractable weight was calculated by weighing thesamples before and after extraction and converted to percentage of initial mass.

3.5. FABRICATION OF DEVICE 27

3.5 Fabrication of deviceThe fabrication and commercialization of microfluidics devices must become a very active areafor academic research. The successful project can significantly decrease the development cycleof the devices and offer limitless possibilities for growth of the field of lab-on-a-chip.

The fabrication process of the Thiol-ene-epoxy material is compatible with standard micro-molding. The devices for demonstration with 200 microns wide and 40 microns high microflu-idic channels using OSTE(+) of Material B (40% excess) were fabricated using the procedureshown in Figure 3.1. Firstly, the prepolymer was mixed and then debubbled in a vacuum cham-ber. Secondly, the prepolymer that contains three kinds of monomers was poured into themicropatterned PDMS master. Thirdly, spacers and a transparent cover were used to make aflat chip with a certain thickness. Fourthly, the master with prepolymer was UV exposed for10 seconds to ensure a minimum thiol-epoxy reaction due to the heat generated by excessiveUV irradiation. Fifthly, a soft polymer layer was obtained and it was demolded easily from thePDMS master. To make sealed channels, the soft polymer with thiol and epoxy groups wascarefully bonded to the intended substrates (silicon, glass, aluminum) at room temperature andaccess ports were drilled. Finally, the bonded chips were put in oven and baked at 90◦C for 2hours to obtain the fully cured high Tg and high E-modulus final device.


prepolymer

Epoxy + thiol + alkene monomers

O SH

R R R

Master

1

2 UV-cure (10 sec @ 365 nm, 6 mW/cm2

remaining epoxy and thiol monomers

O SH

R R

Master

Soft polymer layerR

SH

R’

Thiol-ene + network

+ +

Surface with epoxy and thiol

O OSH

Substrate

RS

HR’ Soft polymer layer

3

4

polymer layer Master

2

Second cure step

Second cure

Substrate

90˚C

pressure

No remaining unreacted groups

2hrs substrate

OSTE+

Hard polymer layer

R’C C S

OH

H H

H

R

RS

HR’

5

Figure 3.1: Fabrication procedures of device demonstrator using UV/thermal two-stage cureand bonded to another substrate

Chapter 4

Results and Discussion

In this chapter, first an universal mixing procedure for OSTE (+) was designed and then thefeasibility of the ternary monomers system was investigated. Different formulations were de-veloped and characterized by DMA, FT-RAMAN, etc. Moreover, how various parameters in-fluence the properties was discussed and studied.

4.1 Processing development-Development of an universal mix-ing procedure for OSTE (+)

In real fabrication, processing methods are usually crucial to the ultimate material propertiessince the real processing conditions are often not identical to the ideal states. In this study, themixing procedure and dual cure procedure are two very important processing factors affectingthe final properties of the polymer. Thus, through the study, the processing method is also aparameter that can be used to alter and optimize material properties.

The homogeneity of the mixture of the ternary monomer system prepolymer (before curing)will contribute to the efficiency of the formation of the crosslinked network, thereby highlyinfluence the final properties of the sample. Thus, a good mixing procedure is a prerequisite forachieving optimal polymer properties. In this study, methods are used to accelerate the diffusionof different monomers and initiators, and realize a homogeneous mono-phasic mixture.

The feasibility of obtaining a homogeneous prepolymer is dependent on the nature of themonomers in the formulation. For example, Material A (PETMA/triallyl/BADGE ternary monomersystem) is formed from monomers that can be easily blended. On the contrary, the threemonomers of Material B (PETMA/triallyl/TGIC ternary monomer system) are not miscibleat room temperature due to the high viscosity of TGIC. Hence, an ultrasonic bath was used tohomogenize the mixture and the combination of the usage of the ultrasonic bath and heating inan oven provides a time-efficient option of mixing according to the experiments.

A good dispersion of initiator is also important since the reaction rate is dependent on theinitiator concentration. Thus, in a non-homogeneous mixture different cure rates are experi-enced throughout the prepolymer, resulting in an inhomogeneous polymer. If the neat DBNis added directly after mixing the three monomers, the reaction will start immediately wherethe DBN contacts the prepolymer, while the rest remains unreacted. Thus a phase separation

29

30 CHAPTER 4. RESULTS AND DISCUSSION

tends to appear and lead to the inhomogeneity in the formed cross-linked network. After ther-mal treatment in formulations using non-optimized mixing protocols, reddening or yellowingspots were observed because of the violent reaction at high DBN concentrations. Hence, inthe processing procedure used in this work, the DBN was dissolved in thiol monomer first toreduce the concentration and then added to the ternary monomer mixture in order to achieve ahomogeneous mixture. Thanks to the optimized dispersion of initiator, a homogeneous networkwas formed.

4.2 Feasibility study of the ternary monomers systemThe main purpose of the project is to obtain a polymeric material with optimal mechanicaldynamic properties in terms of a high E-modulus at 96◦C and good bonding properties at bio-compatible temperatures by introducing multifunctional epoxy monomers to the OSTE system.Therefore, the epoxy component was designed to react completely with the thiol excess afterthe second stage cure (being cured is defined as passing from a liquid, wetting state to a solid,load-bearing state).

The evaluation of the feasibility of the ternary monomers scheme includes several aspects:

• the possibility to polymerize the thiol monomer and epoxy monomer with the availablestrong base initiator

• the feasibility of photopolymerization of OSTE in the presence of ternary epoxy monomerwith and without the photo induced initiator

• the feasibility of a dual cure process and separation of the two-stage curing process byusing different curing mechanisms

• the feasibility of increasing the Tg by introducing ternary epoxy to OSTE

4.2.1 Thiol-epoxy polymerizationInitiator for thiol-epoxy anionic polymerization

DBN is an amidine-type tertiary amine with exceptional basicity, which is attributed to theconjugative interaction of the two nitrogen atoms via the carbon-nitrogen double bond. Asan anionic initiator, it is widely used in the area of thiol-epoxy polymerization in form of thethermally accelerated-DBN and photolatent-DBN.

Nevertheless, even though DBN is a very strong amine base, it is not an efficient catalyst forthe epoxide homopolymerization[38]. Thanks to this property, it’s convenient to use in OSTE(+) since homogeneous networks are ensured, and good control the amount of epoxy functionalgroups that react to the thiol functional groups is reached.

Formulation of thiol-epoxy with initiator

The stoichiometric thiol-epoxy formulations with different quantity of added anionic initiatorDBN were evaluated so as to determine the feasibility of thiol-epoxy polymerization and theproper amount of added initiator. At room temperature, formulation 1 (referred to section 3.3.1)

4.2. FEASIBILITY STUDY OF THE TERNARY MONOMERS SYSTEM 31

is cured within 5 min after mixing the three components, formulation 2 is not cured after 5 hoursand formulation 3 is not cured after 1 day. The results indicate a relation between the concen-tration of the anionic initiator and the time of curing: at room temperature the formulation ofthiol-epoxy without initiator is rather stable, and when the concentration of DBN increase, thereaction time decreases considerably. In fact, a quick reaction is not preferred in thiol-epoxypolymerization, since a more controllable reaction kinetic is favored. The reasons for this are:

• the sequential order of thiol-ene and thiol-epoxy reactions is important to ensure a homo-geneous network structure;

• fast reactions prevent proper dispersion of the initiator molecules throughout the prepoly-mer;

• liquid prepolymers are required for the initial molding step.

Furthermore, according to Cariocia et al.[34], increasing of the amount of amine initiatordoes not significantly change the final conversion values for any of the monomers, meaningthat the reduction of the amount of DBN will not decrease the conversion rate of thiol or epoxymonomers. Hence, a relatively low concentration of DBN was chosen for the formulations.

4.2.2 UV cure as the first polymerization stage in dual cure scheme

Thiols have been reported to cleave into thiyl and hydrogen radicals upon irradiation by 254nm light and reacted with enes without any photoinitiator, and it has been proved by Cramerand coworkers[39], demonstrating a wide variety of achievable polymers without the commondisadvantages imparted by photoinitiator molecules, which can lead to the inaccuracy of theanalytical results.

However, unlike the thiol-ene photopolymerization without photoinitiator, formulation 4which contains no photoinitiator was not cured after 15 runs under the UV fusion lamp atdegree 4. The explanation for this difference is attributed to the UV absorption of the DBN, seefollow section.

Photoinitiators for thiol-ene polymerization

The photobleaching TPO-L is one of the most efficient initiators on the market. It absorbs UV atwavelengths below 380 nm. Photo-fragmentation produces benzoyl and phosphinyl radicals thatcan initiate the polymerization of formulations containing acrylates, unsaturated polyesters andstyrene. The initiator fragments have UV absorption bands below that of the parent molecule,wherefore higher wavelength UV penetrates deeper into the sample as the TPO-L is consumed,hence the term photobleaching.

According to BASF technical sheets, TPO-L is added in proportions of 0.3 to 5wt%. In thisproject, the concentration of 0.5%wt was chosen.

Fomulation 5, in which there is no epoxy load, was cured for approximately 30s under UVexposure. The curing time was estimated by checking if the polymer had passed from the liquidstate to solid state (gelation) every 10 seconds. For formulation 6-9, the different curing timewere listed in the following table:


Figure 4.1: Comparison of different curing times of thiol/ene/epoxy formulations with andwithout DBN

As shown in the table 4.1, the ternary thiol-ene-epoxy system can be partially cured bysimple UV exposure for a short time with photoinitiator TPO-L. At the same time, from thetable it’s observed that the formulations with the presence of DBN show a retarded curing time.This phenomenon is probably related to the UV absorption of the basic anionic initiator. To testthis hypothesis, an UV-vis scan was performed. As shown in Figure 4.2 , the UV absorptionof the 0.1wt% DBN solution (diluted in water) is nearly 0 from the wavelength of 500nm to350nm. From 300nm, the absorption starts to increase dramatically. At about 250nm, theabsorption reaches the limit of the measurement capability of the equipment. This explainswhy formulation 0 is not cured in the exposure under the UV fusion lamp, where the highintensity at wavelength of about 250nm, at which the initiator free thiol-ene cures, is blocked.In a similar manner, in formulation 2-5, DBN have absorbed a certain amount of UV-light, sothat the curing time was postponed.

Figure 4.2: UV-vis spectra for

After the UV curing procedure, a homogeneous network, which contains free thiol and


epoxy functional groups, was formed owing to the thiol-ene ’click-reaction’. To ensure thatthe mechanical properties evaluated after the first cure only came from radical reactions, for-mulations without DBN were used. The mechanical properties of formulations with differentstoichiometric ratios, without DBN, show a clear trend where the Tg of the polymer decreaseswhen increasing the content of the excess of thiol. This is due to the uncured content of epoxyand thiol functional groups (figure 4.3), and hence a lowered crosslink density, perhaps com-bined with softening of the network by the unbound epoxy monomers. The reduction of Tgprovides the potential for easy bonding properties at room temperature, while the variation ofTg shows a great freedom of tailoring the material properties.

Figure 4.3: comparison of Tgs for thiol-ene-epoxy systems without DBN with different thiolexcess content

4.2.3 Control over mechanical properties by partial cure

Generally, in the early stages of cure the thermoset can be described by an increase in itsviscosity η, and the gel point coincides with the first appearance of an equilibrium (or time-independent) modulus as shown. Polymerization continues beyond the gel point to completethe network formation, where physical properties such as modulus build to levels characteristicof a fully developed network. The cured polymer should be at a stage in between the gel pointand the Hookean solid. To ensure the fully curing of the network, a rather long curing time wereused in further tests.

On the other hand, the mechanical properties can be potentially tailored by controlling thecuring time and curing condition in order to achieve different conversions of monomers. Thiscan be very useful in real applications and more extensive studies are needed to draw any furtherconclusions on the usefulness of this approach.


Figure 4.4: network formation with increase of conversion rate

4.2.4 Material A - PETMA/triallyl/BADGE ternary monomer system

In this thesis work, we first developed a ternary monomer system, material A, by introduc-ing BADGE as the epoxy monomer to evaluate the second stage ternary monomer dual curescheme. Samples followed formulation 8 with 50% excess thiol is treated by UV/thermal se-quential dual cure procedure. According to DMA measurements, after the UV cure, a Tg of50◦C was achieved with a broad glass transition phenomenon 4.5. After the thermal treatment,Tg increased from 50◦C to 72◦C with a sharper glass transition, indicating that some or all ofthe remaining thiol excess has reacted with the epoxy monomers at the second stage thermalcure, resulting in a more crosslinked network. Therefore, it is proved that it’s feasible to use thethiol-ene-epoxy ternary monomer system to improve the mechanical and temperature resistanceproperties.

Compared with Cariocia’s paper[34], a similar Tg is obtained with the same off-stoichiometricratio, though a different thiol monomer and anionic initiator were used (we used PETMA asthiol monomer and DBN as initiator instead of PETMP and DMP-30). This material has subse-quently been used in many applications due to the unique dual cure process, e.g. for dry transferbonding of porous silicon membrane[40].

However, as shown in figure 4.5, the dual cured polymer starts to soften at around 50◦C,which is far from sufficient when it comes to the PCR thermal cycling where the material needsto withstand a temperature of 96◦C, wherefore the temperature resistance property has to befurther increased. To this end, another ternary thiol-ene-epoxy formulation is proposed, alteringthe chemical structure of epoxy and keeping other experiment parameters unchanged, in pursuitof the formation of a denser and more rigid crosslinked network.


E−m

odul

us(M

Pa)

20 30 40 50 60 70 80 90

101

102

103

20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Temperature (°C)

Loss

tang

ent

Figure 4.5: DMA curves of Loss tangent and E-modulus of material A (50% excess sample)

4.2.5 Material B - PETMA/triallyl/TGIC ternary monomer system

TGIC is an epoxy that has been widely used for weather-resistant powder coating systems.It has a similar chemical build-up as the ene monomer TATATO that contains a triazine ringand three reactive epoxy functional groups in its structure, which form more rigid structuresthan possible with the difunctional BADGE. Like other epoxies, it can react with a variety offunctional groups, i.e. thiols, hydroxyls and amines. To evaluate the possibility to enhance thetemperature resistant property, TGIC is introduced in the thiol-ene system to achieve a stiffernetwork than that formed with BADGE.

The dynamic mechanical properties of the sample PETMA/triallyl/TGIC with 50% excessof thiol have been evaluated by DMA measurements. The results (Figure4.6) demonstrate thatthe introduction of TGIC as epoxy monomer has significantly increased the Tg and mechanicalproperty of the ternary monomer network. As shown in Figure 4.6, the Tg after UV cure is52◦C, which is similar to the Tg of material A after UV exposure, and after thermal cure the Tgobtained is 99◦C with a sharp well-defined relaxation peak, indicating a homogeneous network.


E-m

odul

us (M

Pa)

0 20 40 60 80 100101

102

103

104

0 20 40 60 80 1000

0.5

1

1.5

Temperature (°C)

Loss

tang

ent

Figure 4.6: Comparison of loss tangent and E-modulus before and after second stage cure ofMaterial B (50% excess sample)

Comparing the ideal network formed by PETMA/triallyl/TGIC ternary monomer schemewith the stoichiometric PETMA/triallyl network with the same off-stoichiometric ratio (50% forinstance), it’s noticed that the only difference of the network is the extra hydroxyl groups thatcreated during the thiol-epoxy anionic polymerization. From this observation, we propose thatthe higher Tg is owing to the hydrogen bonding generated among the hydroxyl groups and fromhydroxyl groups to other electronegative atoms, such as N, O (see Figure4.7). Moreover, ideally,the amount of hydroxyl groups created during the thermal cure equals to the amount of thiolexcess groups content. Therefore, theoretically, the more thiol excess content the formulationhas, the more hydroxyl groups will be created, thus the Tg will be higher.

Figure 4.7: hydrogen bonding in polymerized ternary monomer system after dual cure process


4.2.6 Functional group conversion via RAMAN Spectra characterization

In Raman spectra, the thiol, allyl ether, epoxy absorbance peaks were monitored at 2575cm−1,1644cm−1 and 1260cm−1, respectively. Since the thiol and allyl peaks are more pronouncedthan the epoxide peak in Raman spectra, they were chosen to analyze the evolution of thepolymerizations. By comparing the change in area under the thiol peak (2630-2520cm−1) andunder the allyl ether peak (1670-1625cm−1), the quantity of functionality of allyl ether and thiolreaction during first and second polymerization was evaluated.

Figure4.8 and figure4.9 shows the evolution of thiol peak and allyl peak during two stagecuring process of the 50%thiol excess sample. After the UV cure, the thiol absorbance peak islowered compared with that of the prepolymer, which implies the thiol functional groups wereconsumed in the UV initiated polymerization with ene functional groups. At the same point intime, the allyl peak is nearly undetectable, implying the photopolymerization has nearly con-sumed all the allyl functional groups and the reaction is complete. Then after the thermal cure,it is shown that the thiol absorbance peak almosthas disappeared, indicating that the remainingthiol functional groups after the first stage cure have polymerized with epoxy functional groupsand that the conversion rate of thiol functional groups in both thiol-ene and thiol-epoxy poly-merizations is very high. In addition, the absence of the thiol functional groups after the dualcure process also suggests that there is no ene homopolymerization since the overall formula-tions of functional groups are stoichiometric, i.e. nene+nepoxy=nthiols.

Therefore, Raman spectra characterization has indicated the possibility to achieve the fa-vored well-separated thiol-ene and thiol-epoxy polymerizations using the UV/thermal sequen-tial dual cure approach. As expected, after the two-stage cure process, there will be no excessfunctional groups in the bulk and on the surface, which leads to the potential non-leachableproperty.

Figure 4.8: Raman spectra for the evolution of the thiol peak


Figure 4.9: Raman spectra for the evolution of the allyl peak

4.3 Evaluation of effect of processing

4.3.1 Influence of homogeneity on polymer properties

The morphology of thermosets has not been studied thoroughly compared with that of thermo-plastics [41]. If the polymer is formed as an ideally structured infinitive crosslinked network,then there is one giant molecule. However, the fact that thermosets have Tg reveals the inho-mogeneity of the network. The inhomogeneity can be defined by the existence of defects andfluctuations of crosslink density of the composition.

In general, the inhomogeneity is stimulated by the fast bulk reaction that is difficult to con-trol. Logically, the homogeneity of the cured polymer should largely depend on the homogene-ity of the prepolymer. For example, if there are some undissolved solid state monomers in theprepolymer, then the accumulation of one certain monomer will result in unreacted monomerin the bulk of network and the deviation of the precalculated stoichiometry of the formulation,thus softening the crosslinked network. Moreover, the concentration of initiator can also havean effect on the homogeneity of the cured polymer. As observed in the experiments of thiol-epoxy thermal polymerization, the formulations with different concentrations of DBN exhibitdifferent curing rates and homogeneities of the cured polymers. In general, the greater the con-centration is, the faster the reaction proceeds. Thus, high concentration of initiator will possiblylead to the inhomogeneity of the cured polymer. Since thermal treatment is an easier and mildermethod to tune the polymerization, little initiator is preferred to add in the ternary monomersystem in order to achieve better homogeneity.

The DMA measurements can be used to evaluate the homogeneity of the cured materialby studying the glass transition curve. A more narrow Tg transition, for example, indicatesa more homogenous structure, which is often seen for step-wise polymerizing systems, whilea broadening indicates an increased contribution of chain-wise reactions during the networkformation. Take material B (40% excess thiol) as an example, after UV cure, the glass transition

4.3. EVALUATION OF EFFECT OF PROCESSING 39

experiences a blunt slope implying an inhomogeneous crosslinked network, which is attributedto dangling chain ends and the presence of unreacted monomers. After the thermal cure, theglass transition becomes sharp and tan delta curve has a narrow peak.

The ternary monomer system of 100%excess of thiol group is a special formulation sincethe amount of the ene functional groups and epoxy functional groups are equivalent. In otherwords, the expected amounts of thiol-ene and thiol-epoxy networks are the same. Therefore,the network build-ups through the UV cure and the thermal cure as the first stage cure should benearly identical ideally. Through studying on this formulation, the difference of the networksbuild up during the UV cure and thermal cure was further explored. Figure 4.10 shows the losstangent curves of differently cured samples of 100% excess of thiol group. It’s demonstrated thatthe Tgs after UV cure and thermal cure are 22◦C and 43◦C respectively. This result indicatesthat hydrogen bonding created by thiol-epoxy anionic polymerization. However, a narrowerpeak was obtained by UV cure compared to that obtained by thermal cure, reflecting a morehomogeneous network is formed by UV cure than that formed though thermal cure. Again, thismay be attributed to the influence of hydrogen bonding.

-20 -10 0 10 20 30 40 50 60 70 800

0.5

1

1.5

2

2.5

Temperature (°C)

Loss

Tan

gent

UV cureThermal cure

Figure 4.10: the plotted loss tangent curves as a function of temperature after UV cure andthermal cure of Material B (100% excess thiol)

4.3.2 Influence of dual cure mechanism on polymer properties

The curing process of the ternary monomer system consists of two different mechanisms: thethiol/ene radical polymerization and the thiol/epoxy anionic polymerization. Hypothetically,these two systems don’t influence reciprocally so that the order of the curing processes shouldbe interchangeable without adverse effects. In fact, the parameter of the curing order is a veryinteresting factor that can be used to control the first stage property and second stage property,


respectively. In this thesis work, two kinds of sequential dual cure processes were investigatedand compared in different applications.

As mentioned before, after the first stage curing, the OSTE(+) polymer is designed to have arelatively low Tg (measured by DMA) when it softens and adapts to substrate surface to conformto the micro-irregularities on the surface in order to attain a perfect sealing.

After the second curing stage, the excess reactive thiol functional groups will be used tobuild a network that is uniformly crosslinked. Thus, the homogeneity of the network will behighly improved and Tg will be dramatically increased due to the high crosslink density.

On one hand, different orders of sequential dual cure mechanisms can offer similar me-chanical properties by tuning the off-stoichiometric ratios (see also section 4.3.4), both after thefirst stage and the second stage curing process. Precisely, after first stage cure, a relatively softmaterial will be obtained, while after the second cure, a stiff material will be obtained. On theother hand, the dual cure scheme gives the opportunity to alter the surface properties after thefirst cure since different excessive functional groups will be exposed on the surface.

UV/thermal sequential dual cure process

The UV-initiated curing process of the thiol/ene radical polymerization firstly offers a networkthrough ’click chemistry’ with a large number of reactive thiol excess functional groups andepoxy monomers distributed throughout the network.

According to the DMA results, after the first cure, the Tg is reduced compared with the purethiol-ene polymer with the same amount of thiol excess due to the unreacted epoxy monomerdispersed in the network, and the material has a fairly broad glass transition4.10, which reflectsthe heterogeneity of the network. Theoretically, the step-growth nature of the polymerizationresults in uniform polymer networks with narrow glass transition regions and reduced brittle-ness. However, when the total conversion rate of the monomers is insufficient and there is alarge number of unreacted monomer, the glass transition exhibit a blunt slope with a width of30-40 ◦C.

The remaining reactive thiol and epoxy functional groups can be potentially used for variousapplications, such as bonding to kinds of substrates via different mechanisms, and grafting tobiocompatible anchors for surface modification of the channels.

After the UV exposure, the thiol excess react with the epoxy monomer at 90◦C in the ovenand achieve a highly cross-linked network. In this manner, a high Tg was obtained and the glasstransition was sharp, reflecting the network homogeneity.

Through the high temperature thermal treatment, the system of thiol-epoxy is highly ac-tivated with high mobility of monomers. Therefore, the UV/heat curing system provides anoptimal approach of getting the highest Tg.

Thermal/UV sequential dual cure process

In light of the demand of low temperature biocompatible bonding for certain applications,the possibility of thermal/UV sequential dual cure process was studied and compared withUV/thermal dual cure system. The heat accelerated curing process of the thiol-epoxy anionicpolymerization firstly takes place and provides a homogeneous network with the presence of


"ene" monomer, and then the excess thiol functional groups will react with the allyl functionalgroups by photopolymerization and achieve a highly crosslinked network.

After the thermal treatment, a viscous cross-linked system has been formed so that themobility of the monomers for the thiol-ene photopolymerization is limited. Due to the abilityof the network to take up load, stress will be built up during the second stage cure due to strainfrom the polymerization shrinkage. Thus, in theory, the ultimate mechanical properties of thissystem should be worse than that of UV/thermal curing system. While by this approach, thesecond heating treatment was not needed after first stage bonding, thereby fulfilling the featureof biocompatibility of the device.

As mentioned in Carioscia’s paper[34], changing the sequence of polymerization thiol-eneand thiol-epoxy significantly affects the overall conversion and network development in thehybrid resins. In the thermal/UV approach, thiol-epoxy polymerization can reach a high con-version. However, the cure of the thiol-epoxy network first decreases monomer mobility suchthat the final conversion of the thiol and vinyl in the hybrid systems is lower than in the hybridsystems where the thiol-ene network is cured first.

According to loss tangent curves form DMA characterization(figure4.11), the UV cure givesa more homogeneous network than thermal cure due to the ’click’ reaction nature of the thiol-ene photopolymerization. Meanwhile, the UV/thermal sequential cure results in a more uniformnetwork than thermal/UV sequential cure for a certain off-stoichiometric ratio (in the range from20% to 100%). For instance, as shown in figure4.11 (the sample of 40% excess of thiol group),comparing the loss tangent peaks of after the first stage thermal cure and after the first stage UVcure, the UV cured polymer has a higher Tg and a narrower peak. Similarly, the UV/thermalcure provides a narrower peak than the thermal/UV cure, which is probably caused by thedifficulty of radical diffusion of thiol-ene polymerization in the cured thiol-epoxy network atroom temperature and the stress built up in the network during the UV cure.

4.3.3 Evaluation of the effect of monomer structureThe monomer structure and architecture play a significant role on the flexibility and mobilityof the polymerized chain and consequently the final properties of the cross-linked network.The structure/property relationship between the original monomer structure and the final per-formance of the thermoset has been studied by varying the chemical structure of the monomers.

The thiol monomer

In this study, a flexible structured tetrafunctional thiol was used to form the three-dimensionalnetwork and a triazine ene was used to increase the stiffness of the structure.

It has been reported[42] that thiols based on mercaptopropionate esters can result in ele-vated reaction rate than the thiols based on mercaptoacetate esters because of a weakening ofthe sulfur-hydrogen bond by hydrogen bonding of the thiol hydrogen groups with the ester car-bonyl [24], and the reaction rate is 6 times greater. Due to the excessive curing rate of themercaptopropionate based thiol, the mercaptoacetate based thiol was chosen for the bulk ofthe experiments in order to prolong the shelf life to handle the molding of the prepolymer andwell separate the mechanisms of the dual cure process in previous tests. On the other hand, thelower viscosity of mercaptopropionate based thiol monomer provides the possibility to achieve


20 40 60 80 100 120 1400

0.5

1

1.5

Temperature (°C)

Loss

Tan

gent

UV+thermalThermalUVThermal+UV

Figure 4.11: plotted loss tangent curves as a function of temperature after UV cure, thermalcure, UV/thermal sequential cure and thermal/UV sequential cure of Material B (40% excess

thiol)

a better-dissolved prepolymer of thiol, ene and epoxy, and hence potentially better optical andmechanical properties.

Table 4.1: Comparison of the Tgs after first stage UV cure and UV+thermal cure of formula-tions (60% and 80% excess) using different thiol monomers

Tg of 60% excess sample Tg of 80% excess sample

Different thiol monomer after UV after UV+thermal after UV after UV+thermalcure (◦C) cure(◦C) cure (◦C) cure (◦C)

mercaptoacetatebased thiol 45 101 33 96

mercaptopropionatebased thiol 40 95 23 92

With the same stoichiometric ratios and curing conditions, the dynamic properties of mer-captopropionate based thiol and mercaptoacetate based thiol were compared in terms of theirinfluences on Tg values.

However, as seen in table 4.1, the Tg decreased by replacing mercaptoacetate based thiolwith mercaptopropionate based thiol. The phenomenon can be owing to the lower crosslinkdensity in the network formed by mercaptopropionate based thiol, since the molecular weightof mercaptopropionate based thiol is greater than mercaptoacetate based thiol by 56 g/mol.


The epoxy monomer

Three types of epoxies with different chemical structures and functionalities were studied inthis project in order to investigate the relationship between the epoxy chemical build-up and thefinal property of the material.

As a di-functional epoxy, BADGE has a symmetric nature and contains two benzene ringswhich enhance the stiffness of the network. Compared with BADGE, TGIC has a very simi-lar chemical build-up as the ene monomer, containing three epoxide groups which give cross-linking properties to the chemical structure. Importantly, the isocyanurate ring can provide amore rigid network and increase the Tg of the cross-linked polymer. As a comparison, TMPTEis also a trifunctional epoxy as TGIC, but contains no rigid ring structure component in itsformula. In contrast, the center ether group will make the chain more flexible. For the sameoff-stoichiometric ratio (50% excess of thiol groups) and curing conditions, the Tg values offormed polymers with these three different epoxies are shown in table 4.2.

Table 4.2: Comparison of the Tgs after first stage UV cure and UV+thermal cure of 50% excessformulations using different epoxy monomers

Epoxy monomer Tg after UV cure (◦C) Tg after UV+thermal cure (◦C)

TGIC 53 100BADGE 51 72TMPTE 51 71

As expected, after UV cure, there is only a slight difference in the Tg values of the UVcured polymer with different epoxy monomers since the epoxies remain uncured and exist inthe form of monomer scattered in the thiol-ene network. In contrast, the Tg values of the UV andthermally cured samples vary in the range from 71◦C to 100◦C, reflecting the different rigidityand crosslink density of the crosslinked networks.

4.3.4 Evaluation of the effects of off-stoichiometric ratiosThe off-stoichiometric ratios of thiol functional group affect the quantities of each monomer dueto the different molecular weights of the thiol, ene and epoxy, respectively. For the same reason,different crosslink densities result for different off-stoichiometry ratios. Thus, the possibility totailor the properties of the polymer, such as mechanical properties, dynamic properties andsurface properties is provided.

In addition, the order of the UV/thermal dual cure process is also a crucial factor that hasa great impact on the properties of the polymers. By controlling both the order of dual cureprocess and the off-stoichiometric ratios, a variety of crosslinked networks for different appli-cations can be formed because of the different curing mechanisms and different amounts ofremaining thiol excess to react with the ternary monomer after the first stage cure.

For the UV-thermal dual cure procedure, the thiol-ene crosslinked network forms with theepoxy monomer load in it after the first stage cure. The off-stoichiometric ratios define theamount of excess of thiol groups and epoxy functional groups. Theoretically, the more excess


content there is, the looser the formed network is, also the unreacted monomers will act assoftening agents in the network, hence the lower the Tg of the polymer.Tgs as determined by thetan δ peak values were plotted as a function of off-stoichiometric ratios (excess of thiol groupsfrom 20% to 100%), showing the trend of decreasing of the Tg varying from 70◦C to 23◦Cwhile increasing the content of excess of thiol (figure4.12). As demonstrated, the variationof the Tg is nearly linear with respect to the amount of excess of thiol groups while the E-modulus (at 25 ◦C) is varying much more from 40% to 80% off stoichiometry. During thesecond stage thermal cure, remaining thiol groups polymerized with epoxy groups via anionicaddition polymerization and formed a denser and more uniform network to achieve a high Tg.

Due to the reinforcement effect of hydrogen bonding among the network formed by thiol-epoxy polymerization, hypothetically the elevated amount of excess of thiol group should resultin a elevated value of Tg. However, as shown in figure 4.13, the maximum Tg obtained byvarying off-stoichiometric ratios is around 115◦C appearing at 40% excess of thiol group insteadof 100%. This is possibly owing to the inhomogeneity of the prepolymer of the high percentageepoxy content formulations. More specifically, when there is large amount of epoxy content, itis not possible to fully dissolve the epoxy in thiol and ene monomers, which may lead to a lowerconversion of epoxy monomer. As the off-stoichiometric ratios increase, there’s an increasingtendency for the accumulation of epoxy molecules due to the undissolved epoxy component,which will result in greater inhomogeneity of the overall network. At the mean time, becauseof the pre-calculated content of remained thiol groups after the first cure equals to the epoxygroups, the unreacted thiol groups due to the accumulation of epoxy molecules will have a lessconversion rate also. Then the unreacted thiol groups will form dangling chain ends, resultingin an overall lower crosslink density in the network. Figure 4.14 demonstrates the curves of tandelta (loss tangent) plotted as a function of temperature. It’s noticed that in general the peaksare well defined and the peak of the 20% and 40% excess of thiol are the most narrow, whichindicates the best homogeneity among different off-stoichiometric formulations.

0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100

Off−stoichiometric ratios (Thiol excess)

Tg (°

C)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

2000

4000

E−m

odul

us (M

Pa)

E−modulusTg

Figure 4.12: The plotted Tg and E-modulus as a function of off-stoichiometric ratios after firststage UV cure of Material B


0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

90

95

100

105

110

115

Off-stoichiometric ratios (thiol excess)

Tg (°

C)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

2200

2400

2600

2800

3000

3200

E-m

udul

us (M

Pa)

TgE-modulus

Figure 4.13: The plotted Tg and E-modulus as a function of off-stoichiometric ratios after UVcure of Material B

Figure 4.14: comparison of the plotted loss tangent as a function of temperature of Material Bfor samples with different stoichiometric ratios

In contrast, during the thermal-UV two-stage cure, the thiol-epoxy crosslinked network wasformed first with the ene monomer dispersed throughout the polymer. Theoretically, the moreexcess content there is, the stronger the formed network is, and the higher the Tg. However,according to figure4.15, the expected trend of increased Tg when increasing the amount ofexcess from 20% to 100% was not shown. Instead, there is a maximum Tg appearing at theoff-stoichiometric ratio of 60%. From 20% to 60% excess, Tg increases. However, from 60%to 100% content of excess, the Tgs decrease. As mentioned before, the reason of the devia-tion from the theoretical perspective can be the poor homogeneity of the prepolymer where thesolid epoxy content reached a solubility saturation level in the mixture of the thiol-ene-epoxy


resulting in incomplete epoxy conversion. Another possible reason is the conversion of anionicpolymerization at a certain period of time. When the amount of epoxy content increased, it’sless likely to achieve an as high conversion during the same period of time than the formula-tion with a lower off-stoichiometric ratio. In other words, it’s suggested that the thiol-epoxypolymerization may be incomplete at high off-stoichiometric ratios.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

44

46

48

50

52

54

Off-stoichiometric ratios (thiol excess)

Tg (°

C)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

400

600

800

1000

1200

1400

E-M

odul

us (M

Pa)

Figure 4.15: The plotted Tg and E-modulus as a function of off-stoichiometric ratios after firststage thermal cure of Material B

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

90

95

100

105

110

Off-stoichiometric ratios (thiol-excess)

Tg (°

C)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1400

1600

1800

2000

2200

E-m

odul

us (M

Pa)

TgE-modulus

Figure 4.16: The plotted Tg and E-modulus as a function of off-stoichiometric ratios aftersecond stage UV cure of Material B

After the second stage UV cure, as shown in figure 4.16, the Tgs of the polymers are notablyincreased, varying in a range from 90◦C to 107◦C. The two peak values appear at 40% and80% excess of thiol group instead of 60%. The discrepancy of the peak value of Tg afterthe first cure and the Tg after second cure can possibly be explained by the homogeneity of


the formed network. Specifically, the smaller the off-stoichiometric ratio, the more thiol andene formed network will be present in the bulk of the final crosslinked polymer. Given thefact that the network formed by thiol-epoxy is less uniform than that of thiol-ene (accordingto DMA results), the most uniform network should be present at the lowest off-stoichiometricratio. Thus, while increasing the off-stoichiometric ratio, the enhancement of hydrogen bondingcompetes with the inhomogeneity due to the greater amount of thiol-epoxy network.

For PCR application, the 40% excess thiol sample after firstly UV curing and secondlythermal curing is perfectly suited for the temperature cycle since it exhibits high E-modulus(>2GPa) at 100◦C.

Relation between prepolymer composition and crosslink density

Cross-links are bonds that link one polymer chain to another via covalent bonds or ionic bonds.In thermosets, crosslinks form during cure and increase mechanical strength by restricting irre-versibly the degree of freedom of segmental motions.

Crosslink density is one of the most important structural parameters that control the proper-ties of thermoset resins. To understand the relation between the mechanical and thermal prop-erties of the material and the different formulations from the aspect of actual network structure,an approach is to study the relation between different formulations and cross-link densities.

For thermosets, one commonly employed method to obtain a value for the crosslink densityis to calculate Mc, the average molar mass of elastically effective chains between crosslinks,according to the following equation derived from the theory of rubber elasticity [41] by consid-ering the entropy phenomenon:

E =3RTρ

Mc

= 3RTυ (4.1)

where E is the modulus of elasticity (or storage modulus) in the rubbery plateau region, Ris the ideal gas constant, T is the absolute temperature at which the experimental modulus isdetermined, υ is the crosslink density, and ρ is the polymer density. Since the density beforecuring and after curing would not have a great variation thanks to the low shrinkage polymer-ization reactions, the densities of the cured polymers of different formulations were attained bycalculating the average densities of the prepolymers.

On the other hand, theoretically, if we consider the Mc the molecular mass between twocrosslinks in the idea network built up perfectly, then the value of Mc’ can be calculated by:

M′c =

(1

1 + α× Mene

3+

α

1 + α× Mepoxy

3+Mthiol

4

)(4.2)

where α is the thiol excess percentage calculated according to equation 2.1, Mthiol, Mene

and Mepoxy are the molar mass of the thiol, ene and epoxy monomers.

The DMA curves show well-defined transition regions and rubbery plateau regions. In therubbery plateau region, the storage modulus changes only slightly with temperature. To a firstapproximation, the moduli in the rubbery plateau were considered constant up to the chemicaldegradation temperature. In this study, the maximum temperature reached in the test was usedto define the E- modulus in the rubbery plateau region (at 150◦C). However, the measurement of


modulus is very sensitive to experimental conditions. Therefore, factors such as the calibrationof the DMA instrument before testing and the geometry of the samples are crucial in order toobtain accurate modulus data. For the UV/thermal and thermal/UV cured Material B samplesof different off-stoichiometric ratios, the densities and Mcs were calculated respectively andshown in table 4.3 and 4.4.

Table 4.3: (A) UV/thermal cured Material B samples with different off-stoichiometric ratios

Excess of thiol E-modulus @150◦C (MPa) ρ (g/mL) Mc’(g/mol) Mc(g/mol)

20% 19 1.297 194 72440% 30 1.312 196 46660% 18 1.323 197 77480% 19 1.332 198 744

100% 22 1.339 199 654

Table 4.4: (B) thermal/UV cured Material B samples with different off-stoichiometric ratios

Excess of thiol E-modulus @150◦C (MPa) ρ (g/mL) Mc’(g/mol) Mc(g/mol)

20% 20 1.297 194 69940% 25 1.312 196 55760% 16 1.323 197 88080% 13 1.332 198 1119

100% 16 1.339 199 857

As shown in the tables, there is a great inconsistency between the values calculated byequation 4.1 from the theory of rubber elasticity and the theoretical values calculated by themonomer composition (equation 4.2) in an ideal network. Specifically, the calculated valuesfrom equation 4.1 are much larger than the ideal ones. Precisely, the calculated values fromequation are much greater than the ideal ones. This discrepancy may indicate the imperfec-tion of the formed network, considering the presence of cyclization and loose ends. Figure C’demonstrates the formed intercycle structure with one trifunctional monomer (ene or epoxy)and one thiol monomer when two functional groups of the same thiol molecule with one tri-functional molecule, while Figure C’ shows the ideal network. Since during the thermal/UVdual cure procedure, none of the thiol or ene has a great mobility in the photopolymerization,there is a pronounced chance of C’ being created. The likelihood of the formation of structureC’ leads to an increase of Mc by creating loops between two crosslinks.

4.4. THERMAL STABILITY STUDY 49

Figure 4.17: Schematic illustration depicting two functional groups of ene or epoxy monomercan react with the same thiol monomer and create loops between two cross-links in the network,

so that increase the values of Mc

Figure 4.18: Schematic illustration depicting each functional group of ene or epoxy monomerreacts with different thiol monomers, which contributes to the network crosslinking density

From both table (A) and (B), it’s noticed that the sample of 40% excess has the lowest valueof Mc, representing the highest crosslink density, while the sample of 80% excess has highestvalue of Mc, hence the lowest crosslink density. Also, it should be noted that the differencesamong modulus are rather small hence these data indicate that the crosslink densities are allwithin the same range. More extensive studies are needed to draw any further conclusions.

4.4 Thermal stability study

TGA analysis indicated that up to 300◦C all crosslinked polymers have not degraded and nosmall molecule was detected as a volatile. The OSTE(+) system has typically shown to havehigh temperature resistance and the lack of unreacted material gives little to no weight loss atthis temperature. From around 300◦C, the polymer started degrading dramatically which can beconsidered as the decomposition of the polymer. To 400◦C, the decomposition reached a steadylevel, and the remaining mass is about 20% of the initial mass.


Figure 4.19: TGA curve of Material B (100%excess) sample

4.5 Extraction testAfter the extraction in chloroform after 24 hours, no swelling phenomenon has been observedfor the immersed samples. By weighing them before and after the extraction, a weight loss of0.22±0.05% was observed. This result shows the excellent solvent resistance of the ternarymonomer dual cured polymer comparing to other microfluidic substrate materials like PDMS(5±1%) and PMMA (dissolves). In addition, this indicates the significant improvement of theleakage problem of OSTE by polymerizing the excess functional groups.

Table 4.5: Comparison of the leaching test results of different materials

Different materials Weight loss in extraction in chloroform

PDMS 5±1%PMMA dissolvesOSTE 2% ± 1% to 8% ± 2%

OSTE (+) 0.22±0.05%

4.6 Bonding and leaching test of deviceThe bonding of OSTE(+) to different kinds of microfluidic substrates relies on a few differentbonding mechanisms depending on the surface chemistry of the substrate.

Metal surfaces are actually hydrated oxides. For example, the surface of aluminum includesmetal oxide, hydroxyl layer, and hydrated water layer[43]. Therefore, the bonding to metal is

4.6. BONDING AND LEACHING TEST OF DEVICE 51

in fact bonding to the hydrated oxides. Similarly, the bonding mechanism to silicon and glassis also considered as bonding to the outer layer that consists hydrated silicon dioxide. Hence,the epoxide groups displayed on the surface of OSTE(+) can subsequently bond to the hydroxylgroups on the surface of another substrate, as shown in figurebondingmech.

Figure 4.20: the surface of metal, which is actually hydrated oxides [43]

Figure 4.21: the bonding mechanism of first stage OSTE(+) with substrates that have hydroxylgroups on the surface

As shown in picture4.22, the bonding to silicon and glass performed great sealing and flowability with dyed water (A and B). However, when it bonds to aluminum substrate (C), thefluidic channel was not perfectly sealed and a bit leaching was observed in this case, whichmight be due to the irregularities on the surface of the material that we used.


Figure 4.22: bonding to different kinds of substrates and leaching tests (A. to silicon substrateB. to glass substrate C. to Aluminum substrate)

Chapter 5

Conclusion

The interdisciplinary thesis presented a novel polymeric material platform OSTE(+) for mi-crofluidic devices to meet various demands by introducing a tertiary epoxy component to theprevious OSTE polymers. In this investigation, a ternary monomer polymerization system witha unique dual cure mechanism was developed, and several goals have been achieved, including:

• A high temperature resistant thiol-ene-epoxy based formulation that is sufficient to thehigh temperature thermal cycling of PCR (>96◦C)

• A unique dual cure process system of thiol-ene photopolymerization and thiol-epoxythermal anionic polymerization, which allows the two stage different dynamic mechani-cal properties (from a PDMS-like material to a thermoplastic-like material) and surfaceproperties (in-situ surface functional groups)

• Leaching-free device after dual cure procedure

• Tailorable mechanical properties by altering the off-stoichiometry of thiol excess contentand the order of the sequential dual cure procedure

• Facile fabrication process and soft lithography microstructuring

Furthermore, different polymer formulations were characterized by DMA, FT-Raman, and TGAin order to study the polymerization kinetics and structure-property relation. From these char-acterizations, summaries have been conducted:

• Material B (with PETMA, TATATO and TGIC) provides the highest Tg and best mechan-ical properties among various formulations due to the rigid ring structure in the chemicalbuild-up of TGIC.

• Mercaptopropionate based thiol reacts with a faster rate with enes than mercaptoacetatebased thiol, while the Tgs achieved are slightly lower.

• The two-stage polymerization of thiol-ene and thiol-epoxy can be separated and the con-version of each monomer is close to 100% (sample Material B (50% excess of thiol)).

• The order of the UV/thermal sequential cure can be altered and it can influence boththe first stage properties e.g. surface properties and second properties e.g. mechanicalproperties.

53

54 CHAPTER 5. CONCLUSION

• The Tgs of different formulations varying with off-stoichiometry of 20%-100% have beenstudied and summarized in the table below:

Tg of UV/Thermal Cure (◦C) Tg of Thermal/UV cure (◦C)

After first cure 23 - 70 45 - 54After second cure 92 - 115 90 - 107

• According to the extraction test in chloroform for 24h, a weight loss of 0.22±0.05% wasmeasured and thus proved the significantly improved leaching property of OSTE.

Chapter 6

Outlook

This thesis project has conducted the research goals of a high Tg dual cured polymeric materialfor microfluidic applications. Thanks to the opened up possibilities for a novel platform ofOSTE(+), more studies are interesting to be further investigated.

The two stage photo curing

According to the nature of the curing mechanisms, the two-stage approach can also be pursuedby:

• the interface bonding at light wave length 1

• the unreacted epoxy-thiol excess bonding at wave length 2

The two-stage curing procedure can also be realized by separate the two different curingwave length if the appropriate UV-induced initiator is available for the thiol-epoxy polymeriza-tion. This approach can significantly reduce the processing time and the energy consumption.

Shelf life

Shelf life is a critical characteristic of the prepolymer regarding the storage of the prepolymer,also the debubbling and molding issues. In this study, as mentioned, the prepolymer MaterialB has a high viscosity and once the three monomers are mixed with the two initiators, thepolymerization will start, and the shelf life of the mixture will be dependent on the concentrationof the initiators.

In order to increase the shelf life of the prepolymer, several methods are proposed:

• lower the concentration of initiators, and protect the bottle containing prepolymer fromUV-light and heat

• add inhibitor to prepolymer formulation

• modify the structure of monomer in order to achieve a lower viscosity

55

56 CHAPTER 6. OUTLOOK

Surface modification

Firstly, the quantity and density of the functional groups after the first cure is interesting to beinvestigated. The measurement can be carried out by XPS (X-ray photoelectron spectroscopy)analysis. Furthermore, the surface hydrophilicity can be altered by grafting hydrophilic, e.g.amino or hydroxyl groups or hydrophobic, e.g. fluorinated compounds, functional groups, thencontact angles can be tested after grafting.

This dual cure ternary monomers system has its unique qualities, such as the stages-separatedcure and controllable properties of the final product. Thus, it can be potentially used in lots ofinnovative applications out of the area of microfluidics.

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Acknowledgement

This interdisciplinary master thesis work was performed with the help of both Micro SystemTechnology Lab and Polymer Technology Lab at KTH, Stockholm, Sweden. There are manypeople who have provided beneficial ideas and supportive help during the project, here I wouldlike to mention some of them.

First of all, I would like to thank my supervisors: Tommy Haraldsson, for his genial ideasand great support throughout the whole project, I appreciate all the fun conversations that wehave had, and the optimist atmosphere that has been created; Mats Johansson, for his helpof profound knowledge of polymer materials and positive comments on my project; LennartWallstrom, for his attention on the project and help out for administrative details; Wouter Vander Wijngaart, who impressed me by his attitude on work and research, also the great supportthat has given me.

Moreover, I have to thank all the colleagues and friends at MST. All of them have been veryfriendly and helpful and are extremely good people to work with, several of whom have beenproviding important help on my thesis work: Carl Fredrik Carlborg, the ’doctor OSTE’, for thegood research foundation for OSTE(+); Farizah Saharil, for the cooperation and the hard work;Kim Oberg, for the experiments of leaching test; Andreas Fisher, for the use of the vacuummachine.

Specially, I would like to thank my friends Albert Mola, Valentin Dubois at MST and TingYang at Polymer Technology, for their kind acompany and the joyful time passed both in andout of the lab.

Finally, I thank my family and my friends in China, for their understanding and constantencouraging during the time I’m aboard.

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high tg polymeric materials for heated microfluidic systems1029357/fulltext02.pdfenes, enabling the...

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