explanations for water whitening in secondary...
TRANSCRIPT
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Explanations for Water Whitening in Secondary Dispersion and Emulsion Polymer Films
Yang Liu, 1 Agata M. Gajewicz,1 Victor Rodin,2 Willem-Jan Soer,3 Jürgen Scheerder,3 Guru Satgurunathan,3 Peter J. McDonald1 and Joseph L. Keddie1
1Department of Physics, University of Surrey, GU2 7XH, Guildford, UK
2NMR Center, Institute of Organic Chemistry, Johannes Kepler University Linz, Altenbergerstrae 69, Linz 4040, Austria
3DSM Coating Resins B.V., Sluisweg 12, 5145 PE Waalwijk, The Netherlands
Correspondence to: Joseph L. Keddie (E-mail: [email protected])
(Additional Supporting Information may be found in the online version of this article.)
INTRODUCTION
Barrier coatings are essential for the prevention of the corrosion of metal surfaces and the degradation of porous substrates, such as wood. Ideally, a barrier coating should prevent the transport of water and all ionic species, as they are the culprits in metal corrosion.1,2 Polymers are frequently employed in barrier coatings, despite the fact that water has some solubility even in hydrophobic polymers.
Coatings can be deposited by casting from solutions in organic solvents, but that process emits volatile organic compounds (VOCs) into the atmosphere, which is prohibited by law in many regions of the world. Alternatively, coatings can be manufactured using UV curing or powder processing. Waterborne polymer coatings, deposited from polymer colloids in water (known as latex), reduce VOC emissions and are compliant with environmental legislation. Yet latex films have been found to be
ABSTRACT
The loss of optical transparency when polymer films are immersed in water, which is called “water whitening,” severely limits their use as clear barrier coatings. It is found that this problem is particularly acute in films deposited from polymers synthesized via emulsion polymerization using surfactants. Water whitening is less severe in secondary dispersion polymers, which are made by dispersing solution polymers in water without the use of surfactants. NMR relaxometry in combination with optical transmission analysis and electron microscopy reveal that some of the water sorbed in emulsion polymer films is contained within nano-sized “pockets” or bubbles that scatter light. In contrast, the water in secondary dispersion polymer films is mainly confined at particle interfaces, where it scatters light less strongly and its molecular mobility is reduced. The addition of surfactant to a secondary dispersion creates a periodic structure that displays a stop band in the optical transmission. The total amount of sorbed water is not a good indicator of polymers prone to water whitening. Instead, the particular locations of the water within the film must be considered. Both the amount of water and the size of the local water regions (as are probed by NMR relaxometry) are found to determine water whitening.
KEYWORDS: Latex; barrier coating; impedance analysis; NMR relaxometry; Rayleigh scattering
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a poor barrier to water vapor and liquid, in comparison to their solvent-cast counterparts, because hydrophilic groups and surfactants are used in their synthesis.3-6
Thermoplastic latex particles are transformed into a film via a series of processes known as film formation.2 Figure 1 illustrates the idealized stages of film formation that are related by three processes. Briefly outlined, water evaporation reduces the spacing between particles and brings them into close packing. Particle deformation occurs under the influence of capillary pressure and surface energy reduction. Diffusion of the polymer chains dissolves the boundaries between the particles, leading to a homogeneous coalesced film. If the particle deformation is incomplete, then nanovoids remain in the film and can increase the permeability of the film. If the particle coalescence is inhibited, then the particle/particle interfaces remain in the film and provide pathways for water transport. Hence, it is understandable why the water permeability of solvent-cast films has been found to be lower than that of latex films.7,8 The use of elevated temperatures of film formation to speed the particle coalescence has been found to reduce the amount of water sorption.9
A particular problem with waterborne polymers, which limits their use in clear barrier coatings, is their tendency to develop opacity or “whiten” when exposed to water in liquid or vapor form. This so-called phenomenon of “water whitening” (or “blushing”) is explained by light scattering from regions of water that have a refractive index that is different from that of a polymer film. 4,10 According to optical modelling, if the water “pockets” or “bubbles” are of sufficient size and number, they will scatter light significantly and impart opacity visible to the eye.11 However, it is a non-trivial calculation to relate the distribution of water in a polymer film to its visual appearance and optical properties. At present, studies of whitening are only an indirect way to probe the concentration and distribution of water in polymer films.
Latex is conventionally synthesized via emulsion polymerization. Research efforts are underway to reduce or remove the hydrophilic species in latex dispersions with the aim of reducing water sorption and associated whitening. The choice of surfactant in a latex has been found previously to influence the equilibrium water sorption and subsequent whitening.5 However, the presence of surfactant has been found to have only a small effect on the rate of diffusion of water into latex coatings.12
Researchers have designed waterborne polymer systems to reduce water sorption and associated water whitening. Chenal et al.13 synthesized highly asymmetric diblock copolymers that self-assembled into core-shell particles during polymerization in water. These films contained no conventional (i.e. small molecule) surfactant and were reported to be resistant to water whitening even after 72 h of immersion. Aguirreurreta et al.4 found that the water whitening of pressure-sensitive adhesives was reduced when polymerizable surfactants were used in the place of conventional surfactants. Feng et al.9 found that the addition of hydrophilic acrylic acid groups to a latex copolymer reduced the amount of water whitening and presumed that the water was being confined to smaller domains that did not scatter light strongly.
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Figure 1. Stages of film formation. (a) A stable dispersion of polymer colloids. (b) Evaporation of water leads to close-packed particles. (c) Particles deform to fill the interparticle void space. (d) Polymer chains diffuse across particle boundaries to make a homogenous film.
A relatively recent and promising development has been the use of methods of secondary dispersion, in which a solution polymer in organic solvent is emulsified in water using hydrophilic components contained within the copolymer. The organic solvent is removed to leave a dispersion in water, self-stabilized by hydrophilic groups on the particles.14,15 This method eliminates the use of surfactants, which are an expensive ingredient in the formulation of a waterborne coating.
In our previous work, we studied the equilibrium sorption and diffusion of water vapor in secondary dispersion (SD) coatings in comparison to emulsion (Em) polymer coatings, and solution-cast (SL) coatings.8 The sorption and diffusion were affected by the form of the polymer (colloid versus solution) and by the presence of surfactants. The SD coatings were found to have the lowest equilibrium vapor sorption and permeability coefficient at high relative humidities and also the lowest water
diffusion coefficient at low humidities, compared to the other waterborne polymer systems.
The penetration of water into polymer films has received increasing attention in organic coatings research. Equilibrium sorption7,16,17 and the kinetics of diffusion of water (in the forms of liquid and vapor)8,12,18 in latex films has been extensively investigated by several research groups.
Although gravimetry is commonly used to measure the mass uptake of liquid water5,16,19,20 or water vapor,21-23 it does not provide information about where the water resides inside a polymer coating. It merely determines how much water is present. One might speculate that the local microstructural environment of the water is important for determining whether water can penetrate through a coating to cause substrate corrosion or degradation or whether it remains trapped in isolated pockets within the bulk of the coating or is bound within the polymer phase. When coatings are exposed to water, there are also changes to their microstructures and the growth of water “bubbles” that have been observed with electron microscopy.24,25
Electrochemical impedance spectroscopy (EIS) has previously also been applied to measure the water uptake in polymers.26-28 It provides information on the mobility and connectivity of ionic species (including water) that is relevant to barrier properties.
NMR spectroscopy and relaxometry analysis have similarly been used to characterize water molecules in latex films during the drying process and have identified three different environments for water: (1) isolated in the surfactant layers or aggregated in pockets; (2) interfacial layers between nanoparticles; and (3) dissolved in the polymer.29,30 In an NMR profiling study, the presence of interfaces between the polymer phase and inorganic pigments was determined to provide a pathway for diffusion and to increase the kinetics of transport.31
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Baukh et al.32 used NMR relaxometry to identify the distribution of water in different domains or environments within coatings made from a waterborne acrylic/polyurethane dispersion. After exposing the coatings to liquid water, they found that the majority of the sorbed water was localized in the hydrophilic dispersant phase. They were able to observe the plasticization of the hydrophilic dispersant phase by sorbed water. However, they did not investigate the effect of the type of dispersant on water distribution, nor did they consider the correlation between water sorption and the whitening phenomena.
Despite this progress in the characterization of latex films when exposed to liquid water, there is currently a gap in understanding. In particular, there is a need to understand how the particle type influences the distribution of water in films as a function of time. Furthermore, the relationship between the local environment of water and the resulting water whitening characteristics is poorly understood. Our present research aims to fill this gap in knowledge.
In this work, our aim is to study the sorption of liquid water into secondary dispersion coatings in comparison with emulsion polymer coatings with exactly the same monomer composition. We answer several important questions. What is the local environment into which water is absorbed in a colloidal polymer coating? Does this water environment differ for secondary dispersion polymers in comparison with emulsion polymers? What is the cause of water whitening and how can the phenomenon be reduced?
EXPERIMENTAL
Materials
The same monomers of styrene (S), 2-ethylhexyl acrylate (2-EHA), n-butyl methacrylate (n-BMA), and acrylic acid (AA) were used in all copolymers in this proportion: 35.8 wt.% S; 19.8 wt.% 2-EHA; 36.3 wt.% n-BMA; and 8.1 wt.% AA. Iso-octyl thioglycolate (0.75 wt.% on the monomers) was
included in the monomer feed as a chain transfer agent. The copolymers were synthesized by three different methods.8 These methods and our abbreviated names for the polymers are (1) emulsion (Em) polymerization; (2) solution (SL) polymerization; and a secondary dispersion (SD) process. A full description of the polymerization methods was presented previously,8 so only the essential details will be presented here. Emulsion polymerization was carried out using ammonium persulfate as the initiator and ammonium dodecylbenzene sulphonate (ADBS) as the emulsifier. This choice of reagents ensured that the concentration of ionic species was low in the final film because the ammonium is volatile. Cations have been reported to increase water sorption in latex films.9
A schematic diagram showing the steps in the preparation of the SD polymers is presented in the Supporting Information (Figure S1). The SD process15,33 was initialized by a solution polymerization process in which methyl ethyl ketone (MEK) was used as the solvent. Then the resultant solution polymer was heated to 60 °C in a reactor. When the temperature was stable, N,N-dimethyl ethanolamine (DMAE) was added. The mixture was then stirred before demineralized water was added over 10 min., upon which the temperature dropped to 42 °C. The SD particles are charge-stabilized by the presence of -COO- groups on their surface. The temperature was set to 40 °C, and after 15 minutes the MEK was removed using vacuum distillation, until the residual level of MEK was below 200 ppm, as determined by gas chromatography. The characteristics of the Em, SL and SD polymers (particle sizes, glass transition temperatures, and molecular weights) are presented in Table S1. For comparison to the three main types of polymer preparations, other materials were prepared. The Em polymer was dialyzed over a period of one week by placing it in dialysis tubing (Sigma-Aldrich, D6191) which retains species with a molecular weight greater than 12 kDa.
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The resulting dispersion is referred to hereafter as the dialysed emulsion polymer (DEm). After dialysis, the surface tension of the emulsion increased by 20 mN/m to 40 mN/m, which indicates that some surface-active compounds were removed.8 Comparison of DEm to Em provides information on the effect of surfactants and water-soluble oligomers. A dry emulsion polymer film was dissolved in methyl ethyl ketone (MEK) to make a solution, which was then used to cast films. This material, referred to as Em-MEK, has no residual particle boundaries when in the form of a film and is a useful comparison to the Em films.
To determine the effect of surfactant on the SD films, 3 wt.% ADBS (measured on the polymer) was pre-added to the polymer solution during the SD process. This material is hereafter called SD-pre.
Films were formed by spreading the liquid dispersion onto polypropylene sheet or polytetrafluoroethylene (PTFE) substrate. Standard film formation conditions were chosen such that films were dried at 25 °C for 24 h in the open air then heated at 50 °C for another 24 h in a convection oven. Some films were treated under various conditions for analysis by the techniques described hereafter.
Methods and Characterization
Scanning Electron Microscopy. A JEOL 7000 FE scanning electron microscope (SEM) with an operating voltage of 5 kV or 10 kV was used to obtain images of the polymer films before and after immersion in water. The films were cast on a PTFE block using a 200 μm cube applicator. They were film-formed under our standard conditions. Fully dried films peeled off from the substrate were divided into two groups: one was kept in air at room temperature and directly observed in the SEM; the other group was immersed in H2O for 4 h and then taken out to examine. The water on the sample surface was blotted using fiber-free tissue. All films were coated with 3 nm thick Au layers via thermal evaporation using two angles of incidence to
achieve complete coverage. Silver paint was also applied to improve conductivity between the specimens and the conductive sample holder.
UV Visible Spectroscopy. Films were cast onto polypropylene sheets and formed under the standard drying procedure. The initial thickness of the films in all cases was approximately 90 µm. The films were cut into thin strips with a dimension of 8 cm × 1 cm and were inserted into 4.5 ml cuvettes that were filled with deionized water. The films were immersed in the water within the cuvettes for different times of 1 h, 4 h, 24 h, 120 h, and 144 h. Scans of incident light with wavelengths over the range from 300 to 900 nm were made using a Shimadzu UV/Visible spectrometer. The transmission measurements were made while the films were immersed in water inside of the cuvettes.
Electric impedance spectroscopy. The coatings were applied by roller bar applicator at a 100 µm wet film thickness on aluminum Q-panels with a chromate finish (Alloy 3003H14). After application, the films were dried at 52 °C for 16 h, to reach a typical film thickness of 40 µm. EIS measurements were recorded with a Zahner Zennium Electrochemical Workstation and Thales® Z 1.13 software. The measurements were performed with an AMZ60 electrochemical cell. An acrylic tube containing 0.1M K2SO4 aqueous solution was clamped onto a coated panel by means of a rubber sheet to avoid leaking of the fluid. The exposed area of the coated substrate was 6 cm2. The measurements were made over a frequency range from 105 Hz to 0.03 Hz with an AC voltage of 20 mV, every 10 min. during soaking in water for a period of at least 4 h.
NMR measurement. A Kea2 nuclear magnetic resonance benchtop spectrometer (Magritek, New Zealand) was used to collect the 1H NMR data. The operating frequency is ca. 20 MHz. A solid echo sequence was used to determine the ratio of “solid”, i.e. very short T2, to “mobile” 1H. For these measurements, a solid-echo sequence34 (90x – τ – 90y – Acquisition) was employed, in which 90x and 90y are the
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excitation pulses at different phases, and τ refers to the time gap between two consecutive pulses. The composite signal of the solid echo appearing
at time after the second excitation pulse and the free induction decay of mobile 1H unaffected by the second pulse was recorded. The detailed time diagram has been described in the work of McDonald et al. 35 In order to increase the signal-to-noise ratio, 256 scans of 4096 points with a
dwell time of 1 s per cycle were employed with a repetition time of 1.5 s. The time gaps (τ) were chosen at 12, 15, 19, 24, 30, 37 and 45 μs. In all cases, a small residual NMR signal emanating from the empty NMR probe was subtracted from the measured data, and all acquisitions were normalized by the number of averages.
To acquire the spin-spin relaxation time (T2) of the mobile components, a Carr-Purcell36-Meiboom-Gill37 (CPMG) sequence consisting of a 90° excitation pulse followed by an echo train induced by successive 180° pulses was employed. In this experiment, the echo separation parameter, ‘τ’, was incremented logarithmically from 26 to 2000 µs. A total number of 400 echoes was acquired. 256 averages were applied with a repetition time of 1.5 s. Each echo was recorded by 32 points with a dwell time of 1 µs. The NMR signal of an empty cavity was subtracted from the measured data, and all acquisitions were normalized by the number of averages.
The samples free of substrates were immersed in the liquid water or deuterium oxide (99.9% D2O, Aldrich Chemical) in a sealed bottle for 4 h, 24 h, or one week. The samples were then put into 10 mm NMR tubes, which were sealed with paraffin film to prevent evaporation and to maintain a stable humidity inside. D2O was used to determine to extent whether the polymer T2 was affected by the presence of water. Care was taken to see that the sample was contained within the volume of the probe coil to ensure good radiofrequency field homogeneity. All NMR measurements were made at a temperature of 30 °C.
NMR Data Analysis
Solid Echo (SE) Analysis. The second pulse of the solid echo sequence does not influence the signal of mobile hydrogen, which thus shows an exponential decay with an apparent origin at the end of the first pulse that decays with a time constant, T2
*, that is normally determined by the homogeneity of the magnet. For rigidly-coupled spin-½ pairs in solids, the transversal magnetization de-phases rapidly after the first pulse and refocuses equally rapidly after the application of the second pulse due to static dipole-dipole interactions. In polymers, with more complex couplings, the echo signal from ‘solid-like hydrogen’ has an approximate Gaussian shape centered at a time 2τ following the first pulse. Thus the total signal can be fit to35
𝐼 (𝑡, 𝜏) = 𝐼𝑚(𝜏) exp(−𝑡
𝑇2∗) +
𝐼𝑠 (𝜏) exp (−(𝑡−2𝜏
𝜎)2) (1)
where Im and Is are respectively the mobile and solid intensities proportional to the respective amounts of 1H, τ is the echo time, σ is the Gaussian width (approximately equal to the “solid T2” relaxation time), and t is time measured from the first pulse. Since the solid refocussing is incomplete in polymers due to more complex spin couplings, measurements of Is (τ) were made as a function of τ and back extrapolated to τ = 0 to discover the true “solid intensity” using Gaussian fitting. For completeness, the Im (τ) was also back-extrapolated using a linear fit. The raw data set were processed using bespoke MatLab code.
CPMG analysis. The CPMG sequence was used to separate different T2 component fractions of the mobile hydrogen. The signal of rigid polymer chains and chemically bonded groups are considered to be substantially relaxed by the time of the first echo. The analysis is made using an inverse Laplace transform of the echo intensities using the algorithm developed by Venkataramanan et al.38 The data were processed using bespoke MatLab code. To allow comparison between experiments, the intensity
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in the CPMG T2 distributions was normalized by dividing by the mass of the dry sample.
RESULTS AND DISCUSSION
Solid Echo Analysis
Figure 2 shows the echo attenuation and decay fit amplitudes for the Em film (after being soaked in water for 24 h) as a function of the varying
pulse gap, . It is seen that the solid signal amplitude is strongly dependent on the pulse gap, yet the mobile signal amplitude is only weakly dependent. The solid and mobile amplitudes have been fit to a Gaussian curve and linear curve, respectively, as a function of the pulse gap and extrapolated back to τ = 0. The corresponding signal amplitudes at τ = 0 are 23.4 and 3.6, respectively. These amplitudes reveal the ratio of the solid component to the mobile component to be 6.5:1.
Figure 2. The dependence of the solid echo (black solid circles) and the mobile (red open circles) signal intensities on the pulse gap for the Em film after being soaked in water for 24 h. The solid lines are Gaussian (solid component) and linear (mobile component) fits to the data.
Similarly, the changes of mobile hydrogen in all types of polymer films in the dry state and after being soaked in either H2O or D2O for 4 h and 24 h were analyzed and presented in Table S2 and Table S3. All the signal intensities were normalized by dividing by the mass of the dry sample. The signal intensities of the samples
after soaking in D2O were normalized by their counterpart soaked in H2O according to the mass-intensities correlation. The mobile component in a ‘wet’ sample is expressed as a percentage of the total by a simple calculation:
Mobile % = 𝐼𝑚
(𝐼𝑚 + 𝐼𝑠) × 100% (2)
where Im and Is are the signal intensities already defined. This represents the mobile component of both the polymer and the sorbed water. To calculate the percentage of mobile component arising from the immersion in H2O (H) or D2O (D), another parameter is defined to subtract out the mobile component in the original dry film:
Δ𝐻 𝑜𝑟 𝐷 =I𝑚−I𝑠×
𝐼𝑚𝑑𝑟𝑦
𝐼𝑠𝑑𝑟𝑦
I𝑚+I𝑠 × 100 %, (3)
where 𝐼𝑚𝑑𝑟𝑦 is the mobile 1H amplitude of the
initial dry sample (before immersion in water),
and 𝐼𝑠𝑑𝑟𝑦 is the solid 1H amplitude of the dry
sample. The subscript on Δ designates whether the samples were in H2O or D2O.
A parameter, called , is used to represent quantitatively the amount of water inside the polymer films. It measures the signal intensity increment caused only by the sorption of molecular H2O and ignores the effect of plasticization of the polymer and other species by water (in the forms of either H2O or D2O). When glassy polymers or organic molecules are plasticized, a T2 peak appears in the distribution at a value that increases in proportion to the
polymer mobility.32 is defined simply as the difference between 𝛥𝐻 and 𝛥𝐷:
Δ = 𝛥𝐻 − 𝛥𝐷 (4)
Errors on the were calculated from the standard deviation on replicate measurements.
Effects of boundaries. Summaries of the intensities of mobile 1H components and their increments in percentage for different polymers after soaking in H2O and D2O for 4 h and 24 h are presented in the Supporting Information (Table
0 10 20 30 40 500
10
20
30
Sig
na
l In
ten
sity (
a.u
.)
Pulse Gap (s)
8
S2 and S3). The calculated 𝛥 values of the various polymer films after being immersed in water for 4 h and 24 h are shown in Figure 3.
The SL film has no particle boundaries as it is deposited from a solution, hence its value of Δ = 0.6 (at 24 h) is the lowest of all the samples. In the absence of particle boundaries, water is presumed to be molecularly dissolved in the film. A comparison of the Em and Em-MEK films enables the isolation of the effect of particle boundaries. When the Em polymer is cast from an MEK solution, and hence there are no particle boundaries, Δ = 5.4% at 24 h. This value is higher than that for the SL film, probably because of the effects of the hydrophilic species e.g. surfactants or oligomeric species that are polymerized in the water phase in the Em polymers. Due to the water solubility of some of the monomers and the use of a water-soluble initiator, initiation of the polymerization takes place in the water phase.39 Once a few (< 10) monomers have reacted into a chain, the propagating chain is no longer water-soluble and will enter the particles where it further polymerizes. However, some of the chains in the water phase will terminate while still in the water phase, resulting in water-soluble material in the emulsion product. This “water phase” material is not expected to be present in the solution polymer, since here all monomers are in the same phase with the initiator.
The Δ for Em-MEK films is significantly lower than the Δ = 9.5% found for the Em films. The difference of 4.0% between them can be explained by the presence of particle boundaries in the Em films. Water is able to be transported in the surfactant bilayers at particles boundaries and through the channels created by Plateau borders. Furthermore, disorder in the packing of particles creates voids in which the water can accumulate.
Figure 3. The percentage of water (Δ) in various polymer films after being soaked for 4 h and 24 h.
Comparison of the Δ for SD and SL films also provides insight into the effect of particle boundaries. The SD films are formed from emulsion particles, which could lead to some remnants of particle/particle interfaces in the final film, which would facilitate water transport. That is one explanation why the Δ for SD is higher with a value of 8.6 %. However, the SD polymer is neutralized before it is emulsified in water, and it becomes more hydrophilic than the polymer in solution.
Effects of surfactants and hydrophilic species. Surfactant was purposely added to the SD polymers as a means of identifying the effect of the surfactant on the water sorption. The value of Δ increased from an initial value of 8.6% in SD film to 12.5% when the surfactant was pre-added. This increase is attributed to the hydrophilic nature of the surfactant. The pre-addition method is expected to produce a thin layer of surfactant at the polymer/water interface. Upon film formation, surfactant bilayers could remain at the particle boundaries, which would enable water transport.
The Δ value for Em films is higher than for the SD films. This difference is determined by several factors acting together. The Em polymer contains surfactant, which will increase the film’s hydrophilicity, and additionally the surfactant is adsorbed on the particle surface and could
Em DEm Em-MEK SD SD-pre SL0
4
8
12
16
20
24
24 h
4 h
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persist there after film formation. In the SD films, there are no obvious impediments to interdiffusion and the resulting loss of particle boundaries.
Dialysis of the Em films is expected to remove some - but not all - of the surfactant. Comparison of the DEm and Em films thus provides information on the effect of surfactant on water sorption. The removal of hydrophilic species might be expected to decrease the water sorption. However, the Δ for DEm films is 20.1 %, which is nearly double the value found for Em films. Because of the relatively high molecular weight and corresponding viscosity of the Em polymer, particle deformation and coalescence can take significant periods of time, on the order of several days or more. Thus, even in the absence of surfactant and hydrophilic species, particle boundaries can exist throughout the Em film. Besides, the water-soluble species and surfactant might plasticize the polymer phase, and thereby help film formation. After dialysis, heterogeneities in the distribution of the hydrophilic species could result in the formation of localized water pockets. With reduced hydrophilic compounds, a continuous network cannot be formed.
Comparison with Equilibrium Vapor Sorption
Previously, the equilibrium sorption of water vapor in these same polymers was reported.8 Figure 4 presents a comparison of sorbed liquid water content (wt. %), obtained gravimetrically after 24 h of immersion and the equilibrium vapor sorption (wt. %). The sorbed vapor at an activity of aw = 1 (100% relativity humidity) was obtained by extrapolation of the equilibrium sorption isotherms (from Liu et al..8). Models of sorption predict that the sorption of liquid water should equal vapor sorption at aw = 1.40,41 The data in Figure 4 show that the water vapor sorption follows the same trend for the series of polymers, but it is consistently lower than the liquid water sorption. In particular, the dialyzed emulsion polymer (DEm) film sorbs significantly more liquid water than water vapor. There are several possible explanations. First, liquid water
can plastically deform the polymer microstructure through the growth of “bubbles” of water, whereas vapor does not modify the microstructure in this way.16 Secondly, the water vapor data was extrapolated to aw = 1 using the ENSIC model-dissolution theory.42 This extrapolation may lead to an underestimation. Microstructural evaluation using SEM was undertaken to explore the extent of microstructural change induced by the liquid water sorption.
Figure 4. Comparison of sorbed liquid water content (wt. %, measured gravimetrically) after 24 h of immersion and the equilibrium sorbed water vapor (from Liu et al.8) extrapolated to aw = 1.
Film Morphology after Water Immersion
Figure 5 shows the surface structures of the various polymer films after immersion in water for 4 h. Prior to immersion, the surfaces were featureless when examined with SEM. Figure 5a presents the SL film after immersion. The surface is homogenous, and no voids or defects are found. In Figure 5b, an Em film shows randomly distributed voids appearing on the surface. The average diameter of voids is ca. 110 nm. These voids are assumed to originate from the Plateau borders between polymer particles, which are the final remaining spaces when particles deform under the action of capillary pressure.43 Under the external pressure of the water acting on the film, nano-sized spaces could be expanded to sizes greater than 100 nm. Similar conclusions were drawn by Agarwal et al.16 The
DEm SD-pre Em SD Em-MEK SL0
5
10
15
20
sorb
ed w
ate
r conte
nt
(wt%
)
Vapor
Liquid
10
latex from which the Em films are deposited contains surfactant and other water-soluble species in the water phase. Hence, when the dry films are exposed to liquid water, the hydrophilic species could dissolve to leave voids in the film.
In contrast, the Em-MEK film does not have boundaries between individual particles because it was deposited from solvent, but it contains the same hydrophilic species as the Em film. Figure 5c shows that the surface is featureless after immersion in water. This result provides support to the idea that Plateau borders and particle boundaries are nucleating sites for larger voids.
In Figure 5d, the surface of the DEm film is shown to be covered with voids approximately 50 nm in diameter. Out of all of the films studied, the void density is highest in the DEm films. In some regions, the voids are arranged in a hexagonal pattern. This structure is consistent with the voids being at the Plateau borders of hexagonally-packed particles flattened to make a hexagonal “honeycomb” structure. This void development is consistent with the high water sorption in these materials, as was presented in Figures 3 and 4.
In contrast to the Em film, the SD film (Figure 5e) shows no voids. As both the Em and SD films are deposited from colloidal particles, the results imply that the presence of surfactant and water-soluble species in the Em films plays a role in the development of voids during water immersion. Thus, when surfactant is pre-added to the SD, a few isolated voids appear in the resulting film (shown in Figure 5f), although they appear with a lower density than in the Em and DEm films.
Figure 5. Representative SEM images of various
polymer films after soaking in H2O for 4 h: a) SL;
b) Em; c) Em-MEK; d) DEm; e) SD; f) SD-pre. All
scale bars are 1 µm.
Optical Transmission
After soaking the films, water is adsorbed as a second phase. According to the SEM analysis, the water forms water bubbles that lead to micro-voids being distributed within the dry film. When one phase is distributed in another phase with a different refractive index, the scattering of light from the interfaces between the phases results in a decrease in the optical transmission. van Tent et al.44 have used Mie scattering theory to show that when air void sizes are smaller, the film is more transparent. Also, there is a greater loss of transparency when the concentration of voids is higher. Their results tell us that the loss of transparency depends on both the total amount of water that is sorbed and the size of water bubbles that scatter light.
1 µm 1 µm
1 µm
1 µm 1 µm
1 µm
a) b)
c) d)
f) e)
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The optical transmission as a function of variable
wavelength, , of incident light is presented in
Figure 6 for Em, DEm, SD and SD-pre films. (It was found that the optical properties of the SL and Em-MEK films did not change upon immersion in water, and hence their spectra are not shown here.) A loss of transparency, i.e. the water whitening, is observed to develop with an increasing time of immersion. Up to 4 h of immersion in water, all the films are nearly transparent (> 95 %) across the entire spectrum with only a small amount of transmission loss at lower wavelengths. After 8 h of immersion in water, the Em film loses its transparency significantly and shows a strong dependence on the wavelength of incident light.
Analysis of these results provides a method to characterize the number and size of water bubbles in a polymer film. Previous workers11 have used Rayleigh scattering theory to characterize the optical transmission (T) assuming that the scattering objects are spheres with a radius, R. This model assumes there are spherical scatterers with one refractive index in a continuous phase of another refractive index. If the scattering objects in a film are spherical
water voids present with a volume fraction of w, the optical transmission is given by the equation:
ln 𝑇 = (−32𝜋2𝑅3∅𝑤𝑑
𝜆4 (𝑚2−1
𝑚2+2)
2
) (5)
where d is the film thickness, and m is the ratio of the refractive indices of the water (the scattering object) and the polymer as the continuous phase. The refractive index, n, of a copolymer can be calculated via the Maxwell-Garnett equation from knowledge of the refractive indices of the corresponding homopolymers (nA, nB, and nC), given as:
𝑛2−𝑛𝐴2
𝑛2+2𝑛𝐴2 = 𝜙𝐵
𝑛𝐵2 −𝑛𝐴
2
𝑛𝐵2 +2𝑛𝐴
2 + 𝜙𝐶𝑛𝐶
2−𝑛𝐴2
𝑛𝐶2+2𝑛𝐴
2 (6)
where B and C are the volume fractions of polymers B and C, respectively. In this work, the refractive index of the copolymer was calculated to be n = 1.50, using literature values for the refractive indices of the homopolymers: poly(styrene), nA = 1.589; poly(BMA), nB = 1.483; and poly(EHA) nC = 1.436.45 The refractive index of water was taken as 1.33.46 Hence, the value of the refractive index ratio, m (in Equation 5) was calculated to be 0.88. If we let
𝐶 = 32𝜋2𝑅3𝜙𝑤𝑑 (𝑚2−1
𝑚2+2)
2
(7)
then Equation 5 can be re-written as simply
T = 𝑒−
𝐶
𝜆4 (8)
12
Figure 6. Optical transmission spectra of various film after immersion in water for the times written on the figures. The wavelength of incident light ranged from 300 nm to 900 nm. Data for four types of polymers are presented: a) Em; b) DEm; c) SD; d) SD-pre. The smooth, green solid lines in a, b and c represent the Rayleigh fit using Eq. 8.
We used Eq. 8 to fit the optical transmission spectra of the Em film. The results are presented in
Figure 6. It shows that the optical transmission data can be well fit by Eq. 8 over the range of wavelengths from 300 nm to 900 nm, except for a small deviation in the UV region. Note that if there were not discrete somewhat spherical scatterers in a continuous film, the transmission would not show this characteristic dependence on wavelength. The values of the fitting parameter, C, used to characterize the optical transmission spectra are presented in the Supporting Information (Table S4). Estimates of
the volume fraction of water, w, were obtained gravimetrically from other samples of films with the same thickness; values of d and m were reported here earlier. These parameters were used to find the scatterers’ average radius, R via Equation 7. The resulting values of R are only estimates of the size of the scattering objects in our experiments and are best used to compare the various types of polymer film rather than as
an absolute measure. Nonetheless, the apparent R for the Em film increases with the water immersion time, as is presented in Figure 7a, and reaches values above 10 nm after 120 h. Table S4 lists the data used to calculate the values of R.
Similarly, the optical spectra for the DEm films are adequately fit to the Rayleigh scattering model (Figure 6b), and the calculated radius of scatterer, R, increases slightly with increasing water immersion times (Figure 7b). The values for the DEm film are lower than the Em film, as is consistent with the observed higher optical transmission for the former.
The optical transmission of the SD film decreases only slightly with increasing immersion times (Figure 6c). The optical transmission depends
only weakly on after short immersion times,
and it cannot be adequately described by a -4 dependence. Although the SD film absorbs a significant amount of water when immersed, this water does not create scattering objects that can be described by the Rayleigh model. The
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13
relatively high optical transmission for the SD films suggests that the water is finely dispersed. After 120 h of water immersion, the wavelength dependence of transmission is stronger, but it still cannot be adequately described by the Rayleigh model.
The optical transmission of the SD-pre film decreases with increasing immersion times, but the wavelength dependence is not consistent with the Rayleigh model (Figure 6d). Most notably, a sharp transmittance minimum is
observed at = 485 nm for SD-pre films after soaking in water for 4 h (shown in
Figure 6d). We suspected that this peculiar optical feature was a stop band or Bragg peak. According to the kinematical Bragg law, the wavelength of the stop band is given as47
𝜆𝑠𝑡𝑜𝑝 = 2𝑛𝑒𝑓𝑓𝑧 (9)
where 𝑛𝑒𝑓𝑓 is the effective refractive index of
the polymer/water composite, and z is the lattice–plane distance. Assuming hexagonal close-packing of spheres, z can be calculated as:
𝑧 ≈ 2√2
3r (10)
where r is the radius of the particles (ca. 100 nm in this case). 𝑛𝑒𝑓𝑓 is calculated48,49 from an
interpolation between the refractive index of water (n = 1.33) and the copolymer (n = 1.50), assuming a linear dependence on the weight fraction of water. Using a value 𝑛𝑒𝑓𝑓 = 1.498,
obtained for an immersion time of 4 h, the wavelength of the stop band is calculated to be 𝜆𝑠𝑡𝑜𝑝 = 488 nm. This value is very close to the
experimental value of 485 nm and hence supports our interpretation of this spectral feature.
The result indicates a strong diffraction effect. It occurs as a result of long-range ordered array of colloidal particles. The general shape of the spectrum away from the stop band is quite similar to the SD latex film, which is caused by the scattering properties of the SD film. The stop
band is expected to redshift when the volume fraction of water increases, because of an increased lattice-plane distance and a decreased effective refractive index. This explains the shift of the stop band to 491 nm after 120 h in water.
Next, we consider the extent of correlation between the total water sorption and water whitening. As not all of the films can be adequately described by the Rayleigh model, we use the optical transmission at a wavelength of
= 550 nm as a measure of the whitening. Figure 7c shows the optical transmission plotted as a function of the percentage of water (Δ) for various polymer films after 24 h of water immersion. There is a poor correlation between the two. For instance, the SD-pre film sorbs a higher amount of water, but does not scatter light more, compared to the SD and Em films. The DEm film sorbs the greatest amount of water but shows a greater transparency than the Em film. The SD film has a relatively high transparency compared to the Em film, which sorbs nearly the same amount of water. These results indicate that greater light scattering and a loss of optical transparency cannot solely be attributed to a greater amount of sorbed water. Because the results in Figure 7c imply that factors other than the total water sorbed influence the water whitening, additional experiments were conducted to characterize the local environments of the water in the various films.
14
Figure 7. Radius of scatters, R as a function of the time of water immersion for a) Em films and b) DEm films. R was calculated by fitting the transmission to Eq. 8, and solving Eq. 7 for R. c)
Optical transmission (at = 550 nm) plotted as a function of the percentage of water (Δ) in various polymer films after 24 h of immersion. Film thicknesses are 90 µm.
Electrical Impedance Properties
Electrical impedance analysis was used to gain a deeper understanding of the distribution of water in the three types of polymer films (Em, SD, and SL). This technique provides information on the total amount of water (or electrolyte) in a material and also provides crucial information on the continuity and connectedness of water pathways in polymer coatings.50,51
Spectra obtained from the electrical impedance analysis of materials that contain water are classified as being I-type or D-type. An I-type material exhibits a simple linear decrease in impedance with an increase of frequency, which is caused by the scarcity of continuous aqueous pathways. In contrast, a D-type material shows a plateau region in the impedance spectra at low frequencies and a linear decrease in impedance with increasing frequency in the high frequency region. The plateau arises from the formation of interconnected aqueous pathways. Leidheiser et al.52 found that polymer coatings can exhibit both I-type and D-type impedance behavior.
Figure 8 compares the impedance spectra for Em, SD, and SL films after their deposition and after immersion in water for 4 h and 24 h. The figure shows the impedance modulus decreases as the water content increases in the Em, SD and SL films. The original dry SL film shows a straight linear relation as the frequencies increase, which indicates an I-type impedance behavior. The spectra are unchanged after the SL film has been immersed in water. The results indicate that the SL film sorbs a negligible amount of water, even after 24 h, and there is not enough water to create continuous pathways through it.
For a dried Em film and a film after being immersed in water for 4 h, there is a linear decrease in the impedance modulus at high frequencies (above 103 Hz), which overlays on the SL data. In the dry film, there is a deviation from the I-type behavior at lower frequencies. This result shows that some pathways for conductivity exist, probably arising from traces of water and ions in residual boundaries
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15
between particles. After 4 h in water, the Em film exhibits a plateau in impedance modulus at frequencies below 103 Hz, which is a signature of D-type behavior. Water at particle boundaries and in the Plateau borders between particles in the Em film is likely to be creating the conductive pathways throughout the film.
Figure 8. The electrical impedance modulus as a function of frequency (Bode plot) for dry Em, SD, and SL films and after being immersed in water for 4 h and 24 h.
The SD film displays impedance behavior that is intermediate between the SL and Em films. The impedance spectrum of a dry SD film and the dry films after being immersed in water for 4 and 24 h overlay the SL data at high frequencies (above 103 Hz). Whereas the dry SD film presents an I-type spectrum, there is a slight deviation from the linear I-type behavior after 4 h and 24 h of immersion. There is thus an indication that the sorbed water is creating paths along residual particle boundaries in the SD film. However, the pathways do not have continuous connections that would yield a plateau in the spectra, which is characteristic of D-type behavior.
From the impedance modulus values at low frequency, it is concluded that the volume fraction of water contain in the Em films is higher than that in the SD film, which in turn is higher than in the SL film. This result is consistent with the findings obtained from solid echo analysis. More importantly, the SD film lacks the continuous pathway of water that is found in the
Em film. This result can be explained by the presence of surfactant and water-soluble oligomers between the Em particles, creating a continuous network in the final film. The coalescence of the Em particles is inhibited such that there is a continuous hydrophilic pathway. In the original SD dispersions, the water phase is purer chemically, and the particle boundaries in the dry film can be partially dissolved because of coalescence.
Distinguishing Water Environments using NMR Relaxometry
The impedance analysis revealed differences in the water pathways between the SD and Em films. However, this difference alone cannot explain the differences in the extent of water whitening as was presented in Figure 7c. NMR relaxometry was therefore used to provide information on the local environments of the water in the various polymer films.
The distribution of T2 relaxation times of 1H from water in porous micro-structures provides a sensitive characterization by identifying the different compartments or environments in which water is present. Typically, there are multimodal distributions of T2 times with several peak values. The average T2 value of each peak is a measure of the molecular mobility of the 1H-containing molecules, and hence it can characterize the extent of the confinement of the water. Water in more confining environments, such as hydrogen-bonded on solid surfaces or trapped at particle interfaces, will have a lower T2 value. When water is in a larger space, less confined and more mobile, the T2 takes higher values. The area under a particular T2 peak provides an estimate of the amount of water in a particular environment. Thus, NMR relaxometry determines not only how much water is present, but also how it is distributed within a nano/microstructure.
In the present experiments, complementary experiments used D2O. Then the signal from 1H covalently bonded to C in mobile hydrocarbon molecules can be seen separately from water. If
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16
hydrocarbon molecules become more mobile, such as when plasticized by water, the T2 peak shifts to higher values.
Rottstegge et al.29 employed NMR spectroscopic techniques to study water environments in partially-dried poly (vinyl acetate) and poly (vinyl acetate-ethylene) copolymer latex films. They identified three different environments: 1) water absorbed inside hydrophilic polymer particles; 2) water bound at the interface between the particles and bound with ionic and nonionic groups of the surfactant; and 3) free water that is external to the polymer and is the most mobile. In the experiments presented here, it was therefore expected that at least three water environments would be found in the T2 relaxation data.
T2 relaxation distributions of the various polymer films after being immersed in either H2O or D2O for 4 and 24 h (and 168 h for some samples) are presented in Figure 9. The solid black lines show the T2 relaxation distributions of 1H in the polymer films after being immersed in H2O for different times, whereas the distributions shown with the blue circles correspond to films in D2O. T2 distributions are also shown for the films prior to immersion in H2O or D2O, as a comparison. Note that the 1H with a T2 time much less than 2τ = 60 µs is not visible here.32 The area under each peak is proportional to the total number of 1H in that same environment.
Figure 9a shows the T2 distributions for the Em film. The initial nominally-dry film has a single peak centered around T2 ≈ 0.1 ms. 1H species with this relatively low T2 value have highly restricted mobility. This peak is attributed to the 1H in the mobile groups of the copolymer and any water dissolved in the polymer phase. In the forthcoming discussions, we refer to this environment as being in Region I.
After the Em film is immersed in water for 4 h, two new peaks are observed: one is in the T2 region from 0.3 ms to 1.25 ms, and the other in the region from 1.7 ms to 5.4 ms. The first of these emerging peaks, centered around 0.7 ms,
is tentatively attributed to water that is bound or confined at the interfaces between particles. The area under the peak in the H2O experiment is greater than found for the D2O experiment, which confirms that the signal is being obtained from 1H in water molecules. However, the existence of the peak in the same position using both H2O and D2O suggests that non-aqueous molecules, such as surfactants and oligomers, have a mobility that is similar to that of the water. (Additionally, there could also be some chemical exchange between the OH in the copolymer’s acrylic acid (8 wt.% of the copolymer) and the OD in deuterium oxide.) This environment is hereafter called Region II.
In the original dry Em film (with no sorbed
water), there is a tiny peak centered at T2 0.7 ms, which can be explained as residual water confined between the polymer particles. When the film is immersed in water, additional water is seen to be confined in this same environment, and the hydrocarbon molecules in the confined regions are plasticized.
The second emerging peak for the Em film, centered around 3 ms, has the highest T2 value of the three peaks. It is attributed to more mobile water within the film. This “free” water is likely to be found in bubbles or pockets within the film, in agreement with the SEM images shown in Figure 5b. In comparison, bulk water with no confinement of its molecules has a much higher T2 value on the order of three seconds.53,54 We will refer to this environment as being in Region III of the T2 distribution.
Vertical dashed lines are drawn in Figure 9 to delineate the T2 regions for the three environments. For consistency in the discussion and analysis, the boundary between Regions I and II is set to be at 0.3 ms in all data sets; the boundary between Regions II and III is set to be at 2.5 ms. Table 1 summarizes the regions and their meaning. The three different environments are illustrated in the drawings in Figure 10.
Previous NMR CPMG studies of water in starches55-57 and in gluten57 have likewise
17
identified multiple populations of 1H. Notably, water between granules of rice starch was attributed to a population centered around a T2 of ca. 20 ms, whereas bound intragranular water was attributed to a T2 range between 2 ms and 10 ms.56 Our interpretation of the T2 distributions (Regions II and III) for the two types of water is in broad agreement with these previous studies.
The states of water in different hydrophilic polymers have also been examined elsewhere using differential scanning calorimetry (DSC).58,59 Similar to the findings here, three types of water were found in polymer films containing hydrophilic groups. The first type is “bound” water, which has polar interactions or H-bonding directly between a polymer’s chemical group(s) and water molecules. The second type is sorbed and bound water that is attributed to a second hydration layer that has H-bonding with the directly bound layer. The third type is bulk water that is not directly bound to the polymer and is not in the second hydration layer. The three regions of the T2 distributions for water identified via NMR relaxometry in Figure 9 are consistent with the three types of water found via DSC.
For the Em film after 24 h of immersion (Figure 9a, bottom), the area of the T2 peak for the sample in H2O is significantly greater than the sample in D2O. This implies that the majority of
the 1H is in sorbed water, rather than in plasticized polymers or small molecules. However, a significant difference exists between the H2O and D2O. The T2 distribution obtained with D2O exhibits three separate peaks, whereas in the distributions obtained with H2O immersion, the second and third peaks have merged. This implies that the Region II environments is not distinct from the Region III environment, and the two could be connected physically. It is proposed that the growth of the peak in Region II at 4 and 24 h in the D2O experiments arises from the plasticization and mobilization of the surfactant and hydrophilic oligomers at the particle boundaries. Additionally, chemical exchange of D and H will make a small contribution to the peak.
It can be observed that the T2 peak assigned to the mobile polymer phase (Region I) slightly right-shifts over time towards a higher value. The shift is explained by the plasticization of the polymer by water molecules. Similar effects have been observed previously in poly(vinyl alcohol),60 cellulose acetate,61 acrylic/polyurethane hybrids,32 and polyurethane.62 This interpretation is also supported by our NMR solid echo results. Evidence for plasticization in Region I is similarly found for the other types of polymer film in Figure 9.
18
Figure 9. Normalized T2 distributions of 1H in the mobile components of the various types of polymer film: a) Em; b) DEm; c) SL; d) Em-MEK; e) SD; and f) SD-pre. In each panel, the results are presented in the order of increasing soaking time, from top to bottom: 0 (dry film), 4 h, and 24 h. (168 h of soaking time is also shown for SL and Em-MEK samples.) The solid black line represents the T2 distributions of samples after immersing in H2O, and the blue symbols display the distributions of samples in D2O. The T2 distributions for the corresponding dry samples prior to immersion in either H2O or D2O are also identified by black lines and blue symbols, respectively.
In Figure 9b, the distribution of the dry DEm film contains a single peak at around 0.1 ms. As most of the small species (molecular weight less than 12 kDa) are assumed to be removed by dialysis, we propose that the peak at the 0.1 ms position belongs to the acrylic-styrene copolymer. A
similar value was obtained for a low molecular-weight polymer by Baukh et al.32 Note that for the dry DEm film there is no peak in Region II, whereas there is a small peak in Region II for the dry Em film. This difference can be explained by attributing the dry Em film’s Region II signal to a
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19
combination of surfactant, low molecular-weight species, and associated water. When the surfactant and low molecular-weight species are removed through dialysis, this Region II peak does not appear.
After soaking the DEm film in water for 4 h, the distributions have an additional 1H component at 1 ms for samples immersed in D2O and at approximately 2 ms for samples in H2O. The area under the peak in Region II obtained with H2O is significantly greater than under the same peak when using D2O, which reveals that liquid water is making a strong contribution to the signal. After soaking for 24 h, the H2O peak in Region II increases strongly (the peak area increasing about 3.8 times that of the Region II peak for 4 h) and shifts to higher times, which indicates that the water is less confined. The peak in Region II obtained after immersion in D2O for 24 h is essentially unchanged in comparison to 4 h, which confirms that the increased H2O signal is the result of liquid water uptake and not plasticization of organic molecules.
After 24 h of soaking the DEm film in H2O, a new peak appears in Region III, centered around 7 ms. The Region II peak has shifted to higher T2
values, so it sits near the boundary with Region III. When using D2O, there is no significant peak in these regions. Thus, it is concluded that a new environment for water has developed. However, based on the relative peak areas, it is found that most of the water is in the more confined environment of the Region II peak. This result for DEm differs somewhat from what was found for the original Em film in which the area of the Region II peak is smaller than in DEm and partially merged with the Region III peak. Also, in the original Em film, the Region III peak appeared after soaking for only 4 h.
The SEM image in Figure 5d revealed that there is a narrow distribution of void sizes in the DEm film after water immersion, and the mean void size is smaller than seen in the Em film (Figure 5b). The T2 distributions reflect these differences in void sizes and distribution. The Region II and III peaks of the DEm film are narrower than those
in the Em film, and the maximum T2 value in the distribution is lower, which is consistent with the confinement of water in smaller spaces in the DEm film.
Table 1. Regions in the T2 distributions of mobile 1H and their physical meaning
T2 range (ms) Physical Meaning
I < 0.3 Dissolved water; mobile polymer, surfactants, etc.
II 0.3 to 2.5 Interfacial water bound between particles
III > 2.5 Free or mobile water in Plateau boundaries, voids in film
Figure 10. Schematic representation of water located in different types of environment. I) dissolved inside the polymer; II) bound at the interfaces between particles or surfactant layer attached around particles; III) “free” or mobile water in inter-particle voids or holes distributed in the films.
The SL film, which has no particle boundaries and no surfactant, is considered next. The T2 distributions of the film in the dry state and after immersion in H2O and D2O for varying times are presented in Figure 9c. As expected from our previous discussion, original dry SL film and the films in H2O or D2O show only a single peak in the T2 distributions, centered around 0.1 ms, which is within Region I. The areas under the peaks obtained for 24 and 168 h in H2O is higher than under the corresponding peaks for D2O immersion. This difference means that the water molecules are dissolved in the polymer phase, and the two components have a similar
20
molecular mobility. Since there are no surfactants and particle boundaries in the SL films, water exists only in the more restricted environment within the polymer phase, and its T2 value is low. The position of the T2 peak near 0.1 ms is broadly in the same position observed for the Region I peak for the Em film. Hence, the assignment of the Region I in the T2 distribution for the Em film as being due to the polymer phase along with dissolved water is supported.
In the case of Em-MEK, the boundaries between particles are not present and hence a more homogenous film was observed in SEM analysis (Figure 5c). In Figure 9d, the T2 distributions of the dry Em-MEK film exhibit two components: around T2 ≈ 0.1 ms (Region I) and around T2 ≈ 0.5 ms (Region II). The existence of two peaks in the dry film is consistent with what was found in the original Em film. This result provides evidence that the Region II peak is associated with surfactants and low-molecular weight compounds present in the emulsion polymer. The result shows that a cellular structure of a latex film is not necessary for this T2 component to be present.
After immersion in water (H2O or D2O) for 4 h, there are very little changes in the T2 spectra for the Em-MEK films. The populations for the D2O and H2O experiments are similar. There is no evidence for water sorption. This result is consistent with the NMR solid echo data in which
the water sorption was very low (= 2.4 %), and it can be explained by the absence of particle boundaries that would provide a pathway for water transport. Unlike what was found for the Em film, there are no components in Region III for the Em-MEK film after 4 h in water. This result shows that the Region III signal in the Em film is most likely arising from water pockets developing in the film.
After immersing the Em-MEK film in water for 168 h, the total amount of water (proportional to the area under the T2 peak) increases in Region II, whereas there is only a small amount of water uptake in Region III (around T2 ≈ 12 ms),
despite the long exposure time. Furthermore, the Region II peak right-shifts to T2 ≈ 2.2 ms when the time increases from 24 h to 168 h, which indicates greater mobility in this environment, perhaps because of swelling of the hydrophilic components. The peak heights for the D2O and H2O experiments are very similar in Region III, which can be explained by the plasticization of the polymeric dispersant (and hydrophilic oligomers) by water. There is no evidence for liquid water in the Region III environment, which is consistent with the lack of inter-particle spaces able to develop into water pockets. The total amount of absorbed water is much less than the amount in the Em and DEm films.
Next, we consider the T2 distributions of the SD film (see Figure 9e). As discussed already, there is no surfactant present in this dispersion. The Region II peak is attributed to residual water confined at the particle boundaries. After soaking in H2O for 4 h, the peak in Region II at T2 ≈ 0.7 ms for H2O is significantly higher than found for D2O. This result implies that water exists in this Region II environment, probably at the particle/particle boundaries. After 24 h in H2O, the Region II peak broadens to span the range from 0.3 ms to 2.6 ms. The H2O peak is significantly higher than in the D2O immersion, which shows that the signal is primarily from confined liquid water rather than organic molecules. In contrast, the Region III peak (centered around 15 ms) is much less high, and nearly identical for D2O and H2O immersions. This result indicates that the signal is not from water but from some plasticized molecular species.
There are notable differences between the Em and SD films. In the Em film, there is more water in the environment of Region III than in Region II in comparison to the SD film. The majority of the water is mobile and present in pockets within the Em film. In strong contrast, in the SD film, the water is almost entirely in the more confined environment of Region II rather than in the more mobile environment of Region III (with higher T2
21
values). The majority of the water is confined at the particle interfaces in the SD film.
The T2 distributions in a film from the secondary dispersion polymer with pre-added surfactant (SD-pre, Figure 9f) is broadly similar to what was found for the SD film. However, the increase in the intensity of the Region II peak after H2O immersion is larger than that found for the pure SD film. It is concluded that the added surfactant creates an environment where more water is absorbed but it is still confined (hence the resulting T2 peak lies in Region II). The T2 peak in Region II grows significantly after 24 h of water immersion, and the peak right-shifts. This result means that more water is absorbed in the environment and it is less confined, perhaps because of a growth in the size of the surrounding environment. As with the SD film, there is a Region III peak for the SD-pre film, but it is minor. The T2 distributions there for H2O and D2O immersions are similar, which indicates that they represent plasticized species rather than confined water.
Correlation between T2 Relaxometry and Optical Transmission
We showed in Figure 7c that the total water
uptake () did not correlate with the optical transmission of films exposed to water. Here, we investigate the extent to which the water whitening can be explained by the distribution of mobile water in the film, as determined by NMR relaxometry.
Equation 5 shows that the natural logarithm of the transmission, lnT, is proportional to the volume of the scatterer (R3) and the volume
fraction of the scatterers (w). The CPMG analysis has identified water in different environments within the film, each with a different extent of confinement, as indicated by the T2 value.
For water in porous solids, the value of the inverse relaxation time (1/T2) is proportional to the ratio of the surface area, A, of the domain over its volume, V.63,64 For a spherical bubble of
radius, R, it is easily shown that A/V ~ 1/R. Hence, for a particular population of scattering water bubbles, R3 should scale with T2 raised to a power of 3.
The area under the peak in the T2 distribution for a particular water environment, Aw, is proportional to number of 1H in that environment, and hence to the volume of the water there. In a solid echo measurement, the total signal intensity, Itot = Im + Is, for a wet film is approximately proportional to the film volume (through the total number of 1H). Through this
reasoning, the water volume fraction, w, is proportional to Aw/Itot for each of water environments. Taken together, these proportionalities lead to a new insight into the connection between the optical transmission and the CPMG T2 distributions:
𝑙𝑛𝑇 ~ − 𝑅3𝜙𝑤 ~ − 𝑇23 (
𝐴𝑤
𝐼𝑡𝑜𝑡) . (11)
The area under a peak in the distribution is found from the sum of the intensities that create that peak. Accordingly, we define a volume-weighted T2
3 parameter, which is calculated by summing the product of T2
3 and the distribution’s normalized intensity, I, at each T2 position, i, in the distribution within Regions II and III. It is written as:
(𝑇23)𝑣𝑤 =
1
𝐼𝑡𝑜𝑡∑ [(𝑇2
𝑖)3
𝐼𝑖] 𝑖=𝑛𝑖=1 . (12)
We expect that lnT will be proportional to this newly-defined parameter. Our analysis assumes that the effects of the scatterers are additive.
To test our argument, Figure 11 shows the optical transmission (at a wavelength of 550 nm, on a natural logarithm scale) plotted against the volume-weighted T2 3 values (in Region II and III) for the various types of film after 24 h in H2O. It shows a linear inverse relationship in support of our argument. The correlation (coefficient, R2 = 0.93) seen here is much better than the correlation between the optical transmission (T) and the total water uptake, for which there was no clear trend, as was shown in Figure 7c. The
22
CPMG analysis is better than simple analysis of water uptake to explain the water whitening of the films. The result indicates that when there are more scatterers of larger size (corresponding to a greater number of mobile water molecules with a longer T2) there is a greater loss in the transparency.
The only exception to the linear inverse trend is the DEm film (not shown in Figure 11). The value of (T2
3)vw for the measured optical transmission is lower than expected from the trend from the other films. The SEM image in Figure 5d provides evidence for very small voids at the Plateau borders of the particles; these voids are smaller than what is observed in Em films. The water in Region III of the CPMG distributions (and in Region II near the boundary with Region III) for the DEm film could be confined to these nanovoids. The T2 value in Region III is particularly low in comparison to the T2 for the other films, and accordingly the (T2
3)vw is lower. Our analysis has made the assumption that the surface relaxation mechanism is the same in all films.
In summary, the water whitening problem in coatings is not explained solely by the amount of water sorbed in waterborne film. Instead, it is primarily a function of the total number of larger water pockets that are found in Region III of the CPMG T2 distributions.
Figure 11. Optical transmission (at 550 nm) plotted as a function of the volume- weighted T2
3 values in Regions II and III for the various films after being in H2O for 24 h. Dry film thicknesses are 90 µm.
CONCLUSIONS
This paper has presented information on the water environments inside various polymer films with the same copolymer composition cast from emulsion and secondary dispersions and also from a polymer solution.
Large differences in the optical transmission are seen between the types of films over time when immersed in water. Solution-cast films, without any surfactants or particle boundaries, and emulsion films cast from MEK solvent have the greatest transparency. The optical transmission does not correlate with the total amount of water sorbed. In particular, although the
0 25 50 75 100
-0.20
-0.15
-0.10
-0.05
0.00
Ln (
T)
((T2)3)vw (ms3)
SL
Em-MEK
SD-pre
SD
Em
23
emulsion film does not sorb the greatest amount of liquid water, its optical transparency is the lowest of all the types of film. Secondary dispersion films, which are surfactant-free, have a higher transmission than the emulsion films.
Analysis of the optical transmission of the emulsion and secondary dispersion films using the Rayleigh scattering model shows that the data are consistent with the films having water in spherical bubbles. The bubbles are larger in the emulsion films. A ’stop band’ appears in the optical spectrum of SD-pre film because of the periodic cellular structure defined by water in the surfactant at the particle boundaries.
Impedance analysis identified in emulsion films the existence of continuous water pathways, which were not found in solution-cast films. Secondary dispersion films, which are surfactant free, showed intermediate impedance properties, suggestive of a partial conductive pathway.
CPMG analysis identified three regions in the T2 relaxation time distributions in films that were immersed in water. Three regions (labelled as I, II, and III) were attributed to molecularly dissolved water (and mobile polymer), to bound interfacial water, and to more mobile water in pockets or bubbles. A key difference was found when comparing emulsion films with secondary emulsion films. Most of the water in emulsion films has a higher mobility (higher T2 relaxation times), which is found in less confining environments. In contrast, the water in the secondary emulsion films is primarily more confined (having intermediate T2 values), probably because it is bound at particle interfaces.
It is concluded that the optical transmission is determined by the total amount of water in combination with the type of local water environment. There is a linear inverse correlation between the optical transmission and the amount of mobile, less-confined water, as determined to have a longer T2 by CPMG analysis. The results show that a greater number
of scatterers (mobile water in large pockets) and larger scatterers both contribute to a greater loss of the optical transparency. This concept explains why the optical transmission of the emulsion films with more mobile water is lower than the secondary dispersion films with mainly interfacial water, even though the total amount of water in them is similar. Dissolving the particle boundaries (e.g. using MEK) and removing hydrophilic compounds (e.g. via dialysis) both increase the optical transparency.
The understanding obtained in this research can be applied to the development of future generations of environmentally-friendly water-borne coatings that resist water whitening. When water is confined to small geometries, such as at the particle interfaces in secondary dispersions, light scattering is reduced. This concept could also be applied to other types of porous coatings, such as sol-gel films exposed to moisture.
ACKNOWLEDGEMENTS
V.R. was funded by EPSRC. We are grateful for technical assistance from Mrs. Violeta Doukova and Mr. Robert Derham (University of Surrey). We thank Sophie Godefroy, Brett Ryland and Paul T. Callaghan of Victoria University of Wellington, New Zealand for providing 2D Fast Laplace Inversion software. We also thank DSM Coating Resins B.V. for funding the project.
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26
GRAPHICAL ABSTRACT
AUTHOR NAMES
Yang Liu, Agata M. Gajewicz, Victor Rodin, Willem-Jan Soer, Jürgen Scheerder, Guru Satgurunathan, Peter J. McDonald and Joseph L. Keddie
TITLE
Explanations for Water Whitening in Secondary Dispersion and Emulsion Polymer Films
TEXT
The loss of optical transparency when polymer films are immersed in water, called “water whitening,” limits their use as clear coatings. This problem is especially acute in films deposited from emulsion polymers, compared to surfactant-free secondary emulsion polymers. The greatest water whitening occurs when the water is contained in voids, rather than being confined at interfaces between colloidal particles. The total amount of sorbed water is not a good indicator of polymers subject to water whitening.
GRAPHICAL ABSTRACT FIGURE
Water Whitening
Emulsion Secondary Dispersion
Solution