Microscopy of polymers

Microscopy

• Experimental methods to obtain magnification of morphological structures

• Optical microscopy (OM)• Scanning electron microscopy (SEM)• Transmission electron microscopy (TEM)• Scanning probe microscopy (SPM)

– Atomic force microscopy (AFM) is the most commonly used

Main features

Gedde, U., Polymer physics 1995

Optical microscopy

• The magnification is obtained via a two-lense system, referred to as the objective and the eyepiece, respectively

• The maximum magnification obtained is about 2000x• Surface topography is studied in reflected light mode• Bulk structure is studied with the light transmitted through the

specimen– More often used for polymers– Sample thickness important, usually 5-40 mm

OM

• Phase contrast microscopy

• Differential interference-contrast microscopy

Electron microscopy

• Acceleration voltage in SEM is 1–30 kV, typically 15 kV• A typical voltage in TEM is 100 kV

• The samples are inserted into a vacuum chamber; the vacuum conditions mean that samples must not be liquid

• Samples must be conducting– Coated with gold or platinum

• NOTE: Artificial structures– Artefacts are not true features of the structure of a material, but

are created by the preparation method or during examination (radiation damage), particularly TEM

Scanning electron microscopy

• In scanning electron microscopy (SEM), an electron beam is focused into a small probe and scanned in a raster pattern across the surface of a sample

• The electron beam interacts with the sample, generating different signals. By detecting these signals and correlating signal intensity with probe position, images of the sample surface are generated

• The nature of the image depends on the type of signal collected:• secondary electrons for imaging surface morphology• backscattered electrons for compositional imaging• X-rays for compositional analysis

http://cime.epfl.ch/introduction-to-em

SEM

• The electron probe channels a current onto the sample, which must be conducted away to prevent charge accumulation on the sample surface. Samples must therefore be conductive

• Non-conductive samples can be made conductive by coating with carbon or metallic films

• The coating is achieved with by vacuum evaporation or sputtering of a heavy metal (Au or Pd) or carbon

Backscattering image of surface

250 x magnification

SEM, composite materialsImportant to check the magnification when comparing data!

SEM, biological materials

Require chemical fixation and dehydration

TEM

• The transmission electron microscope uses a high energy electron beam transmitted through a very thin sample to image and analyze the microstructure of materials with atomic scale resolution

• The electrons are focused with electromagnetic lenses and the image is observed on a fluorescent screen, or recorded on film or digital camera

• The electrons are accelerated at several hundred kV, giving wavelengths much smaller than that of light: 200kV electrons have a wavelength of 0.025 Å– The electron microscope is limited to about 1-2 Å

TEM

• Typical specimen examined with TEM consists of a series of thin (50-100 nm) sections of stained polymer on a microscopy grid

• Thin sections are produced by ultra microtome• Natural variation in density is seldom sufficient to achieve adequate

contrast• Contrast is obtained by staining or by etching followed by

replication• Sample can be embedded in epoxy, polyesters or methacrylates

Electron microscopy

Gedde, U., Polymer physics 1995

AFM• High resolution method• Development from scanning tunnelling microscope• Designed to measure the topography of a non-

conductive sample• A very sharp tip is dragged across a sample

surface and the change in the vertical position (denoted the "z" axis) reflects the topography of the surface

• By collecting the height data for a succession of lines it is possible to form a three dimensional map of the surface features

• Contact mode – non-contact mode– Tapping Mode (intermittent contact Mode)

AFM

• An Atomic Force Microscope can reach a lateral resolution of 0.1 to 10 nm

• Spherulites• Poly(ferrocenyl-di-

butylsilane) • Contact Mode AFM images

of a reduced sample, • Left height image, z range

1,5 mm; right deflection• Isothermal crystallization

temperature 120 oC

AFM

Degradation and stability

General overview

• During polymerization• During processing• Product

– Shelf life– Stability or controlled degradation

• Stability of polymers can be affected by; – Chemical

• Water, oxygen, ozone, acids– Physical

• Heat, mechanical action, radiation– Biological environmental effects

Effect of environmental agents on polymers

Effects of degradation on polymers

• Changes in chemical structure• Changes on the surface• Loss in mechanical properties• Embrittlement• Reduction in molecular weight due to chain scission or increase

due to crosslinking• Generation of free radicals• Toxicity of products formed due to thermal degradation, pyrolysis or

combustion• Loss of additives and plasticizers (leaching)• Impairment of transparency (hazing)

Hamid et al. Handbook of polymer degradation (1992)

Thermal degradation

• Chain scission– Random degradation (chain is broken at random sites)– Depolymerization (monomer units are released at an active

chain end)• Tc ceiling temperature; rates of propagation and depolymerization

are equal– Weak-link degradation (the chain breaks at the lowest energy

bonds)• Non-chain scission reactions

– One example is dehydrohalogenization which results from the breakage of carbon-halogen bond and subsequent liberation of hydrogen halide (PVC)

PVC

• Partial dehydrochlorination of a repeating unit of PVC resulting in double bond formations and the liberation of hydrogen chloride

Picture: Fried

Polymers with high temperature stability

• For use at high T, the best polymers are those with highly aromatic structures, especially heterocyclic rings

• Polymers having high temperature stability as well as high performance properties are specialty polymers for limited use in aerospace, electronics, etc.

• Factors contributing to high temperature stability also contribute to high Tg, high melt viscosity and insolubility in common organic solvents– Polymers are difficult or impossible to process by usual

methods such as extrusion or injection molding

Examples of thermally stable polymers

Radical reactions and stabilization

Oxidation of hydrocarbons in liquid phase:

These primary processes are followed by secondary branching reactions initiated by hydroperoxide (ROOH) thermolysis and/or photolysisHandbook of degradation

Effect of processing on PP and PE

Oxidative and UV stability

Oxidation

• Most polymers are susceptible to oxidation particularly at elevated temperature or during exposure to UV light

• Oxidation leads to increasing brittleness and deterioration of strength

• Mechanism of oxidative degradation is free radical and is initiated by thermal or photolytic cleavage of bonds

• Free radicals react with oxygen to yield peroxides and hydroperoxides

• Photolysis: combined effect of light and oxygen• Ozonolysis: effect of ozone

Oxidation and UV

• Unsaturated polyolefins are susceptible to attack by oxygen and by ozone

• Absorbed energy can break bonds and initiate free radical chain reactions that can lead to discoloration, embrittlement and eventual degradation

Oxidation

• Rate of oxidation is different for polymers

• Oxidation in saturated polymers is slow even at 100 °C, and is enhanced by UV or metal ions. Activation energy of oxidation for saturated polymers are 149 - 230 kJ/mol

• For unsaturated polymers, such as isoprene or polybutadiene, physical properties are quickly affected even at room temperature by oxidation. Activation energies are 100 - 110 kJ/mol

Time (h)

Oxi

datio

n

Oxidation of saturated polyolefins

• Heat and UV affect PE. The influence of heat on the oxidation of LDPE has an induction period depending on the T; effect of UV has no induction period:

Effect of ozone on polymers

• Formation of ozonide

C CO3

+ O C CO O

CO

CO3

C

CO

O-

+

Amfoteerinenioni

Varsinainen otsonidiAmphoteric ionAmphoteric ion Actual ozonide

Radiation effects

Radiation• High energy ionization radiation (radiolysis)

– Gamma radiation– Electron beams– X-rays

• Causes degradation and crosslinking in polymers– Whether it causes chain scission or crosslinking depends on the

chemical structure• Can be used on purpose

– Sterilization of medical devices and equipment can be done using g or electron radiation

– Can be used to prepare graft polymers• Polystyrene and polysulfone very resistant to radiation, PP susceptible to

degradation• Antioxidants (radical scavenges) are effective stabilizers for radiation-

oxidative degradation

Mechanically induced degradation of polymers

Mechanodegradation

• Degradation can result from stress, such as high shear deformation of polymer solutions and melts

• Solids can be affected by machining, stretching, fatigue, tearing, abrasion or wear

• Particularly severe for high molecular weight polymers in a highly entangled state

• Stress induced degradation causes the generation of macro-radicals originating from random chain rupture

Mastication

• Mastication is the process where natural rubber is softened by passing between spiked rollers

• In this process, fillers and other additives (accelerators, vulcanizers, and antioxidants) are dispersed

C

CH3

CH CH2 CH2 C

CH3

CH CH2 C

CH3

CHC

CH3

CH CH2 +

Hydrolytic degradation

Hydrolytic degradation

• Some polymers are susceptible to degradation due to water• Acidic conditions can enhance degradation• Requires labile chemical bonds such as ester-, ether, amide • Naturally occurring polymers such polysaccharides and proteins

• Chemical structure and physical properties influence hydrolysis rate greatly– Glass transition temperature – Amorphous polymers are more easily hydrolyzed than partly-

crystalline; water penetrates to the amorphous regions first

Effect of microorganisms

Effect of micro-organisms

• Biodegradation is a process by which bacteria, fungi, yeasts and enzymes consume a substance

• Most synthetic polymers are not attacked by microorganisms but some stabilizers or plasticizers may act as hosts

0 days effect of enzymes after 5 days

Microbial degradation

Examples

Stabilizers

Stabilizers

• Applications:– To prevent premature polymerization of the monomer– To prevent degradation caused by heating during processing– To reduce the environmental (weathering) effects such as

radiation (UV, visible), moisture, temperature cycling and wind

• Short term– To protect against T and oxygen during processing– Typically low molecular weight such as hindered phenols and

aromatic amines; serve as free-radical scavengers

Stabilizers

Handbook of degradation (1992)

Stabilizers

When adding stabilizers, the following properties and effects must be considered:

– Miscibility with the polymer– Toxicity– Volatility– Effect on the colour and odour of the product– Applicability to processing– Compatibility with possible fillers– Economical effects, price

Elimination of active sites

• Active sites are where primary reactions occur. The site is either inherently part of the structure or it is developed during processing or aging of the product. Examples of active sites:

– Carbon double bonds– Labile Cl atoms (PVC)– Catalyst residues– OH-end groups (polyacetal)– Hydrogen atoms at branching sites– Groups formed by oxidation

Prevention of oxidations

• Too high a dose of inhibitor will have a negative effect

Oxidation

Inhibitor contentwt.-%

Metal complexes

• Metal catalyst residues have been found to enhance the effects of oxidation, for example on PVC

• Metal traces are bound with substances forming complexes with them. Most common are chelates due to their great stability

CH

OH

N

Cu

OH

NCH

O O

sivuvalenssi

heteropolaarinen sidos

Against the effects of UV

• Pigments can be added to protect the polymers from the effects of UV light

• Carbon black is added (2-3 wt.%) to the formulation

• Titanium oxide, Zn white and other lighter pigments prevent the penetration of radiation but will not protect the effects on the surface i.e. yellowing and brittleness

• UV-light absorbents are colorless, organic compounds which change the UV energy, making it less harmful to the polymer by:– Fluorescence– Heat production– Disproportionation

Free radical graft copolymerization of microfibrillated celluloseKuisma LittunenMaster’s thesis

Microfibrillated cellulose

• Can be produced by combining mechanical, chemical and enzymatic treatments of cellulose

• Microfibrils (MFC) have a very large specific surface area and impressive mechanical properties

• Raw material is cheap and abundant in nature

• MFC can only be stored as a dilute water suspension to avoid aggregation of fibrils

• Incompatible with many common polymers

MFC graft copolymerization

• Redox initiated free radical polymerization with Ce(IV) initiator – Reaction can be done in an aqueous medium and low

temperature– Radicals are formed selectively on cellulose chains– Method is well known and tested

MFC graft copolymerization

• Experiments were done with several acrylic monomers• Goals:

– To achieve hydrophobization and/or functionalization of MFC– To find out differences in grafting tendency between different

acrylates and methacrylates• Monomer/MFC ratio, initiator concentration and polymerization time

were variedO

O

O O

O

O

O

O

O

O

O

OH

GMA EA MMA

BuA HEMA

Characterization

• Starting material and some products were studied by AFM imaging to compare fine structures

• Monomer conversion, polymer weight fraction and the amount of homopolymer were determined gravimetrically

• Graft copolymerization was verified by FTIR, XPS and solid state NMR

• Grafted polymer was cleaved by acid hydrolysis and analyzed with GPC

• Thermal behavior was studied by DSC and TGA

• High graft yield and low homopolymer formation (depending on monomer)

• Yield could be adjusted with monomer/MFC ratio. Initiator concentration and reaction time had only a minor effect

C = conversionwG = graft polymer weight fractionGE = graft efficiency

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35 40 45

n(M)/m(MFC) (mmol/g)

G (

%) GMA

EA

MMA

BuA

HEMA

Results

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70

t (min)

G (

%)

Visual observation of dried products

AFM images

MFC MFC-g-PGMA

FTIR spectrometry

FTIR spectrometry

Solid state 13C NMR

• Polymeric carbon signals detected and assigned in all samples

• Signals were integrated to calculate molar and weight fraction of polymer

XPS (X-ray photoelectron spectroscopy)

• MFC that was used as raw material contains some nonpolar impurities

• Possible sources are lignin residues or surface contamination

XPS spectrometry

• MFC-g-PGMA spectrum matched pure PGMA, indicating a very dense polymer coating

• MFC-g-PMMA sample showed also cellulosic O-C-O bonds

GPC analysis

• Hydrolysis was successful with products grafted with PEA, PMMA and PBuA

0

500000

1000000

1500000

2000000

2500000

3000000

15 20 25 30 35 40 45

n(M)/m(MFC) (mmol/g)

Mn

(g

/mo

l)

EA

MMA

BuA

DSC results

DSC results

TGA results

• Initial decomposition temperature was increased by 10 oC and 20 oC in products grafted with PEA and PBuA

• Other grafted polymers had only minor effects

Temp Cel500.0400.0300.0200.0100.00.0

TG

mg

3.000

2.500

2.000

1.500

1.000

0.500

DTG

ug/m

in

400.0

350.0

300.0

250.0

200.0

150.0

100.0

50.0

0.0

265.0Cel3.022mg

318.3Cel438.2ug/min

395.7Cel408.9ug/min

351.1Cel1.956mg

Conclusions

• MFC was successfully grafted in aqueous solution• Selected polymerization method was efficient and quite selective• Results varied greatly between different monomers• Reaction seemed suitable for scale-up• MFC was hydrophobized to some degree by all tested

modifications• Nanostructure was at least partially preserved• PBuA grafting improved the heat resistance of MFC