Influence of Ethylene Glycol and Propylene Glycol on Polymer

Feb 17, 2010 - acrylate-co-methyl methacrylate) Latex Films. Walter F. Schroeder,†,‡ Yuanqin Liu,† J. Pablo Tomba,†,‡ Mohsen Soleimani,§ Wi...
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J. Phys. Chem. B 2010, 114, 3085–3094

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Influence of Ethylene Glycol and Propylene Glycol on Polymer Diffusion in Poly(butyl acrylate-co-methyl methacrylate) Latex Films Walter F. Schroeder,†,‡ Yuanqin Liu,† J. Pablo Tomba,†,‡ Mohsen Soleimani,§ Willie Lau,| and Mitchell A. Winnik*,†,§ Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada, Department of Chemical Engineering and Applied Chemistry, UniVersity of Toronto, 200 College Street, Toronto Ontario, M5S 3E5 Canada, and Rohm and Haas Company, 727 Norristown Road, Spring House, PennsylVania 19477 ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: January 14, 2010

We describe fluorescence resonance energy transfer (FRET) experiments carried out to examine the effect of ethylene glycol and propylene glycol on the early stages of polymer diffusion in poly(butyl acrylate-comethyl methacrylate) latex films. In our approach, we temporarily arrest the drying process of a wet latex film by sealing the film in a previously cooled airtight sample chamber. This arrests propagation of the drying front and suppresses polymer diffusion during the measurements. We then measure donor fluorescence decays from 0.5 mm diameter spots at various locations on the film. From our analysis, we obtain information about the earliest stages of polymer diffusion as the film is still drying. We also investigate the effect of these glycols on polymer diffusion at longer aging times on predried latex films. Ethylene glycol and propylene glycol retard polymer diffusion at early times immediately after the passing of the drying front but enhance the rate of polymer diffusion at later aging times. This behavior is described quantitatively in terms of freevolume theory and the partitioning of the glycols between the aqueous and polymer phases in the film. Introduction Concern for the environment is driving changes in the coating industry, to replace solvent-based coatings with waterborne systems and to improve the environmental compliance of aqueous systems.1 Most waterborne coatings are based upon latex dispersions (i.e., latex paints) and almost all of these formulations contain volatile organic compounds (VOCs) that provide essential performance features.2,3 It is a difficult scientific and technological challenge to remove these solvents and maintain performance. The intense research on latex film formation carried out over the past decades has been driven by the hope that a deeper understanding of the film formation process by latex dispersions will help provide the knowledge base needed to develop environmentally friendly products with improved performance at similar or reduced cost.4 In this report, we are concerned with the role of two water-soluble diols, ethylene glycol and propylene glycol, which are commonly added to latex paint formulations.5 Their intended role is to provide freeze-thaw stability to the colloidal latex dispersion as well as to reduce the rate of water evaporation toward the later stages of the drying process. We show that they have other and subtle effects on the film formation process. The classic description of the process that transforms an aqueous latex dispersion into a fully developed continuous film involves three major steps.6 First, water evaporation brings latex particles into close contact. As more water evaporates, the * Author for correspondence. E-mail: [email protected]. † Department of Chemistry, University of Toronto. ‡ Permanent address: Institute of Materials Science and Technology (INTEMA), National Research Council (CONICET), University of Mar del Plata, Juan B. Justo 4302, (7600) Mar del Plata, Argentina. § Department of Chemical Engineering and Applied Chemistry, University of Toronto. | Rohm and Haas Co.

particles undergo deformation to form a void-free solid structure consisting of space-filling polyhedral cells. The first two steps transform the latex from a turbid dispersion into a transparent film, indicating that any remaining inhomogeneities in the film, such as voids, precipitated salts, and surfactant, or pools of water, must be small enough to not scatter light significantly.7 Finally, the film develops useful mechanical properties as polymer molecules diffuse across the intercellular boundaries and create entanglements that provide mechanical strength.8,9 Most of the attention in the scientific literature about latex film formation has been devoted to stages 2 and 3, trying to develop an understanding of the specific forces (osmotic, surface-energy derived) which drive the particle deformation, and to understand the factors that affect the rate of polymer diffusion across interfaces in latex films.10-12 Much less attention has been paid to the transition between stages 2 and 3.13 For instance, the mechanism that brings the system from the particle compaction stage to the onset of polymer diffusion is expected to depend upon specific characteristics of the latex formulation, such as chemical composition of the latex particles, including surface functionality, type and amount of surfactant or the presence of external additives. We are particularly interested in developing a better understanding of this transition from compaction to coalescence and how various factors associated with latex polymer composition and the details of the drying process affect the onset of polymer diffusion in the process of latex film formation. While it is clear that polymer diffusion leads to the growth in mechanical strength of polymer films, we believe that factors that promote early onset of polymer diffusion will lead to the most rapid development of useful mechanical properties of latex films. A necessary prerequisite to the onset of diffusion of polymer molecules across intercellular boundaries is the intimate contact between the two polymer faces. When a latex dispersion is

10.1021/jp9118875  2010 American Chemical Society Published on Web 02/17/2010

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TABLE 1: Physical Properties of the Dye-Labeled Latex Particles

a

latex samplesa

Mn (g/mol)

Mw/Mn

Tg (°C)

particle diameter (nm)

solids content (wt %)

Phe-P(BA60-MMA39) NBen-P(BA60-MMA39)

45 000 35 000

2.5 2.7

3 3

134 ( 9 147 ( 10

33.0 48.9

Both latex polymers contain 1 wt % methacrylic acid as a comonomer.

allowed to dry, polar solutes present in the formulation can be trapped in the boundary between adjacent latex particles to form membranes that block intimate contact between particles. Chevalier et al.,14 point out that coalescence and polymer interdiffusion can only occur once the hydrophilic membranes break up to allow the core polymer in the adjacent cells to come into intimate contact. A considerable difficulty has been encountered when we attempt to track details of the diffusion process at the earliest stages. One complicating feature is that very early in the diffusion process the latex film is still partially wet. Another feature is that latex dispersions in the disordered state dry as a moving front that separates the dry region from the wet dispersion.15 When a latex dispersion is placed on a flat substrate, drying begins at the edges, where the wet film is thinnest. As drying proceeds, the wet region contracts in area, surrounded by a dry latex film. It has also been reported that when a latex dispersion is placed in a dish with vertical walls, drying takes place first at the center and the drying front propagates outward.16,17 This is due to the formation of a concave meniscus of the fluid with a thicker edge near the wall and a thin central region. In contrast, latex dispersions with low ionic strength which order in the liquid state tend to dry uniformly without a drying front.10,14 Overall, and despite the fact that all these characteristics of drying front propagation have been well described by various authors,16,18-23 the events occurring at the molecular level, within or close to this front, mechanistically linked to the onset of polymer diffusion, have not been examined and remain poorly understood. For many years, we have used fluorescence resonance energy transfer (FRET) measurements as a tool to monitor polymer diffusion across interfaces in latex films.9,24 In our approach, we synthesize two nearly identical latex dispersions, one sample labeled with a donor chromophore, and its complement labeled with an acceptor chromophore. We then mix donor-labeled and acceptor-labeled latex particles, cast the mixed dispersion onto an appropriate substrate, and let the dispersion to dry to form the film. Polymer interdiffusion leads to mixing of donor and acceptor groups, which is measured as an increase in energy transfer between them. We monitor this energy transfer and the extent of polymer diffusion by measuring the fluorescence decay of the donor as a function of diffusion time. We recently reported the development of new instrumentation to measure diffusion near the edge of the drying front, also based on FRET measurements.25 For these experiments we place a partially dried latex film into a previously cooled, hermetically sealed sample chamber. Under these conditions we suppress further water evaporation, arrest the movement of the drying front, and effectively stop polymer diffusion. We then measure donor fluorescence decays at various locations in the film with submillimeter resolution; in each measurement one examines an approximately 0.5 mm diameter spot on the sample, at a known distance from the drying front. In this way, we experimentally access the earliest stages of polymer diffusion, as the film is still drying, a kind of information that was previously unattainable. In a previous report, we described the use of this new apparatus to investigate the effect of humidity on the onset of

polymer diffusion in butyl acrylate/methyl methacrylate/methacrylic acid copolymer latex films.26 We found that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome, high humidity plasticizes the polymer and increases polymer diffusivity. To explain these results, we proposed that after the drying front has passed, there still remains a very thin film of water between the compacted latex particles that blocks polymer diffusion. As a consequence, particle coalescence and polymer interdiffusion can occur only after this thin water barrier is largely removed. In the experiments described here, we fix the humidity level and examine the effect of ethylene glycol and propylene glycol on polymer diffusion at room temperature for films formed by a similar latex. As mentioned above, these high boiling temperature diols are commonly added to provide freeze-thaw stability to latex formulations, although their influence on latex coalescence and polymer diffusion is not well understood. First, we investigate the effect of these diols on the onset of polymer diffusion in the region near the drying front, using our recently developed instrumentation. Then, we explore how they influence polymer diffusion at longer aging times, on dry latex films, using our traditional approach. Remarkably, we found that the effect of ethylene glycol and propylene glycol on polymer diffusion changes over time. The presence of the diol leads to a retardation effect on polymer diffusion at early stages but enhances polymer diffusion at later times. We discuss and explain these results on a quantitative basis, in terms of free-volume theory and the redistribution of the diols in the film. Experimental Section Materials. Organic solvents, ethylene glycol (EG) (ReagentPlus, g99%), propylene glycol (PG) (ACS reagent, 99.5%), and Texanol (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) (mixture of isomers, 99%) were purchased from Aldrich and used as received. Phenanthrylmethyl methacrylate (PheMMA) was used as received from Toronto Research Chemicals Inc., and 4′-(dimethylamino)-2-(methacryloxy)-5-methylbenzophenone (NBenMA) was synthesized and characterized as described elsewhere.27,28 Procedures for the preparation and characterization of the donor-labeled (PheMMA) and acceptor-labeled (NBenMA) poly(butyl acrylate-co-methyl methacrylate) P(BAMMA) (BA:MMA weight ratio 60:39) latex samples were reported previously.29 The latex samples contain 1 wt % MAA to confer colloidal stability to the dispersion. The physical properties of the dye-labeled latex particles employed in this work are summarized in Table 1. Partition Coefficient Measurement. The equilibrium partition coefficients for EG, PG, and Texanol between water and polymer phases were measured by gas chromatography. Nonlabeled latex P(BA60-MMA39-MAA1) samples containing known amounts of additive in the range between 5 and 15 wt % based on polymer were prepared and allowed to equilibrate under gentle stirring at room temperature for 3 days. The suspensions were then centrifuged in an Eppendorf Centrifuge-5415C at 14 000 rpm for 2 h at 23 °C. Once separated, the supernatant

Poly(butyl acrylate-co-methyl methacrylate) Latex Films aqueous phase was decanted from the polymer phase and directly injected into the gas chromatograph (AutoSystem XL, PerkinElmer). A DBWax column (30 m × 0.53 mm i.d., film thickness 1 µm) was used for EG and PG, and a Simplicity I column (30 m × 0.32 mm i.d., film thickness 0.25 µm) was used for Texanol. The split flow was 30.0 mL/min, the initial and final temperatures were 50 and 220 °C, respectively, and the rate was 25 °C/min. A flame ionization detection system was used, maintained at 280 °C. Calibration curves were established for each additive. Finally, the partition coefficients were calculated as the ratios of the concentrations of each additive in the water and polymer phases (Cw/Cp). Latex Film Preparation. Latex dispersions were prepared by adding weighed amounts of EG or PG to a 1:9 weight ratio of mixed donor-labeled to acceptor-labeled latex particles. Dispersions containing 0, 5, 10, and 15 wt % additive based on polymer were prepared and allowed to equilibrate under gentle stirring for 3 days. Latex dispersions were cast onto quartz substrates and allowed to partially or fully dry depending on the nature of the experiment. To measure polymer diffusion at early stages, 0.05 mL of the latex dispersion was spread onto a 25 mm diameter quartz disk and allowed to partially dry under fixed conditions of humidity and temperature, with no air flow. To control the humidity level during drying, the latex dispersion, right after casting, was quickly enclosed in a small chamber containing a saturated aqueous solution of magnesium nitrate which fixes the relative humidity (RH) within 54 ( 1%.30 To track polymer diffusion over longer times, the latex films were prepared by spreading 2 drops of the latex dispersion on a small quartz plate (20 × 8 mm), and the system was allowed to fully dry uncovered in a cold room at 4 °C over 5 h, to minimize polymer diffusion during particle coalescence. To promote polymer diffusion between particles, the latex film was further annealed at room temperature and 54% relative humidity, a condition that was maintained during the FRET measurements. We also performed FRET measurements on solvent-cast films. To prepare these films, a latex dispersion was allowed to dry and the obtained film was dissolved in a small amount of tetrahydrofuran (THF). The solution was then recast onto a small quartz plate and allowed to dry at room temperature overnight. Fluorescence Decay Measurements and Data Analysis. Donor fluorescence decay profiles were recorded at room temperature using nanosecond time-correlated single photon counting (IBH).31 The excitation source was a 295 nm pulsed diode (NanoLED, IBH), and the emission was collected at 350 ( 16 nm. A 335 nm cutoff filter was mounted in front of the emission monochromator (IBH) to minimize the amount of scattered light from the sample entering the detector. Data were collected until 5000 counts in the channel of maximum intensity, which here required less than 2 min. The instrumental response function was obtained by using p-terphenyl (0.96 ns fluorescence lifetime) as a mimic standard.32 To measure early stage polymer diffusion, we used a “wet sample accessory” (WSA)25 interfaced with the time-correlated single photon counting system described above. This device allows us to measure donor decay profiles at different positions across a partially wet latex film. The partially dried sample is sealed in a chamber cooled at 10 °C to suppress further water evaporation, arrest the drying front, and greatly retard polymer diffusion during FRET measurements. The sample chamber is placed on a X-Y stage that allows measurements at different positions across the film. A pulsed diode was used as the excitation source, with focusing optics to limit the spot size to less than 1 mm2 on the latex film. The film was also illuminated

J. Phys. Chem. B, Vol. 114, No. 9, 2010 3087 with 900 nm diodes, and a visible/near-IR sensitive camera (Edmond Industrial Optics) placed on the top of the apparatus was used to image the film and to monitor the position of sample excitation. The donor fluorescence emitted was focused on the entrance of the monochromator and then detected with a backcooled microchannel plate detector (Hamamatsu). Further details of the design and operation of this device are reported elsewhere.25 We measured fluorescence decays at different positions along a single radial direction, moving outward from the center to the edge of the film. In this way, we were able to obtain the decay profiles as a function of the distance from the drying front for each latex sample analyzed. The as-measured time-resolved decays were numerically processed to obtain the decay area. We base our analysis on the fitting of the decay profile to a suitable function. Studies in our laboratory on the film formation from latex particles have found that the donor fluorescence decay profile can be satisfactorily analyzed by a stretched exponential function.24 As the decays are distorted by the width of the excitation pulse, we include the effect by convoluting the fitting function with the instrumental response (lamp profile).33 The processing of the decay data was carried out using the ANALYZER software package version 4.1.1, Digital Intelligence. ANALYZER yields the parameters of the fitting function through which we evaluate analytically the area under each decay profile. The details of this treatment are described in Supporting Information. Analysis of the data from these energy transfer experiments is developed in terms of the quantum efficiency of energy transfer ΦET(t)

ΦET(t) ) 1 -

∫0∞ ID(t',t) dt' area(t) )1∞ 0 τD ∫0 ID(t') dt'

(1)

where ∫0∞ID0(t′) dt′ (equal to the unquenched donor lifetime τD) is the area under the donor decay profile of a film containing only donor. The term ∫0∞ID(t′,t) dt′ describes the area under the donor decay profile of a film containing both donor and acceptor dyes; t′ is the fluorescence decay time, and t refers to the time that a given sample was annealed prior to the fluorescence decay measurement. The fraction of mixing fm is a useful parameter to quantify the extent of growth of ΦET due to polymer interdiffusion and is defined in such a way that it corrects for the energy transfer efficiency in the nascent film. Values of fm are calculated in the following way

fm(t) )

ΦET(t) - ΦET(0) ΦET(∞) - ΦET(0)

(2)

where the numerator represents the change in energy transfer efficiency between the freshly prepared film and that annealed for time t, and the denominator describes the difference in energy transfer efficiency between the initial and the fully mixed films. Results and Discussion The experiments described here examine the effect of ethylene glycol and propylene glycol on coalescence and polymer diffusion in poly(butyl acrylate-co-methyl methacrylate) (P(BAMMA)) latex films containing 1 wt % methacrylic acid. We use the extent of energy transfer based upon fluorescence decay measurements as a tool to monitor polymer diffusion across interparticle boundaries in the latex films. In our approach, we

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Figure 1. Time-sequence of photographs taken during the drying at 23 °C and 54% humidity of a latex film from the sample where no additive was added: (A) freshly cast dispersion; (B) after 67 min; (C) after 114 min of drying. The black line in (C) indicates the direction along which energy transfer measurements were performed.

synthesize two virtually identical dispersions of dye labeled latex particles. One set of particles is labeled with a small amount of phenanthrene (Phe) that acts as the donor chromophore in a FRET experiment. The second set of particles is labeled with an aminobenzophenone chromophore (NBen) that acts as the acceptor. We then mix the donor- and acceptor-labeled latex particles, cast the dispersion on a substrate to form a film, and allow this film to dry under controlled conditions. As polymer interparticle diffusion proceeds, donor and acceptor groups are brought into proximity leading to an increase in the measured extent of ET. In our previous work, we often used a 1:1 ratio of acceptorlabeled to donor-labeled particles to follow polymer diffusion. Here, as we are particularly interested in tracking polymer diffusion at early stages, we have chosen a different ratio, i.e., 9:1. By increasing the acceptor/donor ratio, we make the experiment more sensitive to early stage polymer diffusion. At the same time, decreasing the fraction of donors also decreases the overall intensity that reaches the detector. The 9:1 A:D ratio yields a good balance between sensitivity and signal strength. A comparison of the evolution of the quantum efficiency of energy transfer ΦET with the aging time for systems prepared using 1:1 and 9:1 ratios of donor-labeled to acceptor-labeled polymer without additive added, is presented in Figure S1 (Supporting Information). Equilibrium Distribution of Diols between Aqueous and Polymer Phases. We begin by examining how the diols distribute between the aqueous and polymer phases of the latex dispersion, through the analysis of their equilibrium partition coefficients (K). K values for EG and PG, defined as the ratio of additive concentrations in the aqueous and polymer phases (K ) Cw/Cp), were measured at 50 wt % latex solids in the P(BA60-MMA39-MAA1) latex system, as described in the Experimental Section. A known amount of diol was allowed to distribute for three days at fixed temperature between the aqueous and latex particle phases. These phases were then separated by centrifugation. From analysis of the aqueous phase by gas chromatography, the content of diol in each phase was quantified. For comparison, we also determined K values for 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol, a classic coalescent aid used in the coating industry) in the same latex system.34 We obtained values of K > 200 for both EG and PG, at room temperature, which indicate that the equilibrium concentrations of the diols in the aqueous phase are more than 200 times higher than in the polymer phase. This behavior is a consequence of the strong capacity of EG and PG molecules to interact via hydrogen bonding with water molecules, as shown by their large hydrogen bonding parameters.35 On the other hand, Texanol partitions preferentially into the polymer phase. Texanol is supplied commercially as a mixture of two isomers. The two

isomers have slightly different K values: Kisom1 ) 2.2 × 10-3, Kisom2 ) 2.6 × 10-3, and Kglobal ) 2.5 × 10-3. In this case, the results indicate that the equilibrium concentration of Texanol in the polymer phase is around 400 times higher than its concentration in the aqueous phase. The K values obtained are in accord with values reported by Hoy for these additives in similar latex systems.36 Locus of the Onset of Polymer Diffusion. In this section, we explore the locus of the onset of polymer diffusion when our cast latex dispersions are allowed to dry under controlled conditions of humidity and temperature. Latex dispersions were cast onto 25 mm diameter quartz disks and allowed to dry at 23 °C and 54% humidity. The drying process was monitored visually. Figure 1 shows a time sequence of photographs of the drying of a latex film from the base latex sample, where no additive was added. Figure 1A shows the freshly cast dispersion; panels B and C of Figure 1 show the same sample after 67 and 114 min of drying respectively. The vertical black line in Figure 1C indicates the direction along which energy transfer measurements were carried out; see below. The sequence of images shows that the wet region on the drying latex film contracts inward as the samples dries. Under the experimental conditions selected, different regions can be identified through well-defined boundaries that advance concentrically toward the center of the drying film, as shown in Figure 1B,C. One can see four different regions when moving radially from the center of the film: a turbid wet spot, followed by a translucent halo, a turbid ring, and finally a transparent (dry) latex film. On the basis of cryo-scanning electron-microscopy (cryoSEM) experiments, Scriven and co-workers19 identified several distinct propagating fronts during the drying of latex coatings. They identified a consolidation front where particles concentrate as a boundary between the turbid suspension and a more translucent region, which consists of consolidated wet particles that scatter light to a lesser degree. This front is followed by a compaction front at the wet-dry boundary that separates the translucent region from an opaque region containing air voids. Here opacity is produced by an increase in light-scattering associated with the particle/air interfaces in the pore spaces occupied by air. Finally, they describe a coalescence front where deformed particles come into intimate contact. In this stage the latex film becomes transparent as the air-filled interstitial spaces gradually disappear. It is interesting to note that we can identify for this sample, in the images in Figure 1B,C, zones with the optical properties described by Scriven et al.19 The boundaries between the zones we observe here likely correspond to the consolidation, compaction, and coalescence fronts that they identify for their system by cryo-SEM. In this context and given the complexity of the transition region between the wet spot and the transparent region, we need to carefully identify the locus of the onset of polymer diffusion.

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Figure 2. Efficiency of energy transfer ΦET as a function of position (distance measured from the edge of the substrate) for the latex film shown in Figure 1, immediately after the photograph in Figure 1C was taken. The position in the film is measured along the vertical black line shown in Figure 1C. The location of the turbid wet spot, translucent halo, and turbid ring are indicated on the plot.

Our initial expectation was the onset of polymer diffusion would occur within or close to the turbid ring formed during the drying of our latex films. To test this idea, we carried out a series of donor fluorescence decay measurements along the vertical black line shown in Figure 1C, to determine the extent of polymer interdiffusion as a function of position in the latex film. The partially dried film was quickly removed from the controlled humidity chamber and sealed in the WSA sample chamber (previously cooled to 10 °C) of the fluorescence decay instrument. These conditions suppress further water evaporation and arrest the movement of the propagating boundaries over the time necessary to carry out fluorescence decay measurements at various positions across the latex film (see Figure S2 in Supporting Information). Figure 2 shows the results plotted as efficiency of energy transfer ΦET as a function of position (distance measured from the edge of the substrate) obtained immediately after the photograph shown in Figure 1C was taken. Figure 2 shows that ΦET is low and approximately constant in the turbid wet spot as well as in the translucent halo (ΦET = 0.1), which indicates that little or no polymer interdiffusion has occurred in the region of the film that is still wet. It is noteworthy that the calculated values of ΦET are not equal to zero in these regions. This result may be due to a small but measurable amount of ET, caused by occasional particle fusion or to the diffusion in the aqueous phase of a small amount of watersoluble dye-labeled oligomers resulting from the latex synthesis.26 Moving outward from the boundary between the translucent halo and the turbid ring, one notes that ΦET increases quickly (see Figure 2). For example, ΦET ) 0.19 at a distance of 1 mm outside from this boundary, and ΦET ) 0.30 at 3 mm away from the same boundary. These results suggest that the onset of polymer interdiffusion occurs within the turbid ring and the extent of interdiffusion increases with the distance from this ring, due to the fact that more distant regions have been aged for longer periods of time than regions closer to the turbid ring. On the basis of these observations, for all our experiments concerned with the study of early stages of polymer diffusion, we have established the well-defined sharp line dividing the translucent halo from the turbid ring as the locus of the onset of polymer diffusion. By analogy with the work of Scriven and co-workers,19 we may identify this interface as the compaction front in the drying process. It is interesting that polymer diffusion begins in a region of the film containing sufficient voids to be turbid. Since polymer diffusion can only occur between deformed polymer particles in intimate contact, this result indicates that some deformed particles contact each other at times at which voids in the system have not yet disappeared.

Figure 3. Photograph taken during drying at 23 °C and 54% humidity of a latex film from the sample containing 10 wt % ethylene glycol (based on polymer). The white arrow indicates the position of the drying front.

We are fortunate to be able to distinguish the two edges of the inner turbid ring in the images shown in Figure 1. In our first study of latex film drying using this same wet-sample accessory, we did not see this well-defined ring. Rather, we observed only a single sharp boundary between the turbid center and the clear edges, which we referred to as “drying front”. We have no good explanation for this difference in observations. It may have to do with differences in film thickness or drying rate between these different samples. Latex dispersions containing ethylene glycol or propylene glycol exhibit a different appearance after drying at 23 °C and 54% humidity. Figure 3 shows a photograph taken during drying of the latex sample containing 10 wt % ethylene glycol. We see that the dry region of the film, which looks transparent in the sample where no diol was added (Figure 1B,C), is cloudy in the sample containing ethylene glycol. When photographs of the latex films containing different amounts of EG were compared, we observed that the intensity of this turbid region increases with the content of diol in the formulation. One possible explanation for this observation is that, once the drying front has passed, the diol remains as a separated phase within the latex film, along with tiny amounts of water remaining. Although the films containing diol look cloudy, the drying front can be clearly identified as the line dividing the translucent halo from the external turbid region, as indicated by the white arrow in Figure 3. Films containing propylene glycol presented similar features. One implication of this behavior is that in these glycolcontaining films, we can no longer distinguish separate compaction and coalescence fronts. For convenience in describing all of our polymer diffusion experiments, we will refer to the boundary representing the onset of polymer diffusion as the “drying front”. Effect of Diols on the Early Stages of Polymer Diffusion. We now consider how the presence of EG and PG influence the onset of polymer diffusion in regions close to the drying fronts. Each latex dispersion containing a known amount of diol was cast and allowed to dry inside a controlled humidity chamber at room temperature and 54% relative humidity. Over the course of drying, a series of photographs was taken to determine the position of the drying front as a function of time. Each sample was allowed to dry long enough for the drying

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Figure 4. Efficiency of energy transfer ΦET as a function of position in the film for latex samples containing different amounts of ethylene glycol (based on polymer): (A) 0 wt %; (B) 5 wt %; (C) 10 wt %; (D) 15 wt %. The position in the film is measured from the edge of the substrate, along the “measurement line” (see Figure 1C as an example). The gray shaded zone indicates the wet region of the film, limited by the drying front. The dotted vertical lines indicate the time that each position in the film has been aging after passing the drying front before FRET measurements were carried out.

Figure 5. Efficiency of energy transfer ΦET as a function of position in the film for latex samples containing different amounts of propylene glycol (based on polymer): (A) 0 wt %; (B) 5 wt %; (C) 10 wt %; (D) 15 wt %. The position in the film is measured from the edge of the substrate, along the “measurement line” (see Figure 1C as an example). The gray shaded zone indicates the wet region of the film, limited by the drying front. The dotted vertical lines indicate the time that each position in the film has been aging after passing the drying front before FRET measurements were carried out.

front to retract about 6 mm from the edge of the coating. At that moment, the sample was quickly removed from the controlled humidity chamber and sealed in the WSA sample chamber precooled at 10 °C to arrest the movement of the drying front and stop polymer diffusion. A series of donor fluorescence decay measurements was then carried out along a single measurement line, moving radially outward from the center of the film. An example of the measurement line is given in Figure 1C. From the analysis of images collected during the drying, we were able to determine the position of the drying front as a function of time, along the same line followed for the FRET measurements. In Figure 4, we plot the values of ΦET as a function of position in the film for latex samples containing 0, 5, 10, and 15 wt % of ethylene glycol. The position is measured from the edge of the substrate, which is noted as 0 mm. The location of the wet region of the film, limited by the drying front, is indicated by the gray shaded zone for each plot in Figure 4. The vertical dotted lines in these plots represent the position of the drying front at various times during the drying process. Each of these vertical lines is labeled with a time. This time label corresponds to how long a particular position of the film has been aged after the passing of the drying front before the FRET measurements were performed. For example, the position labeled as 28 min in Figure 4A had been dry and aged for 28 min before the film was removed from the controlled humidity chamber for the FRET measurements. One can see that the aging time increases with increasing distance from the drying front. A remarkable feature of the data presented in Figure 4 is that one has access to the first minutes of the polymer interdiffusion as the latex film is still drying. By comparing the results plotted in the different panels in Figure 4, we see that increasing the ethylene glycol content of the dispersion produces a retardation effect in the rise of ΦET

close to the drying front. For example, at the position that has been annealed for 30 min after passage of the drying front, ΦET is approximately 0.25 for the latex film without EG, 0.22 for 5 wt % EG, 0.19 for 10 wt % EG, and 0.17 for 15 wt % EG. In Figure 5, we present similar plots for latex samples containing 0, 5, 10, and 15 wt % of propylene glycol. Here, we see that propylene glycol also produces a delay in the rise of ΦET close to the drying front, but this effect is less pronounced than that observed with ethylene glycol. To compare the data of Figures 4 and 5, we replot these results in terms of ΦET as a function of aging time. The aging time is counted as the time elapsed after the passing of the drying front. Each ΦET value was assigned to an aging time based on the distance from the drying front at which the FRET measurement was made. As an example, we consider the data in Figure 4A. The ΦET value determined at the position of the drying front is assigned an aging time of 0 min. Moving outward from the drying front into the dry region of the film, the next fluorescence decay measurement was performed at a position that had aged for less than 12 min. We assigned this position an aging time of 9 min, determined by interpolating from the time-position data. In this way, we assigned an aging time for each value of ΦET presented in Figures 4 and 5. In Figure 6A,B, we plot the values of ΦET as a function of aging time for films containing different amounts of EG and PG, respectively. In Figure 6A,B, one sees that close to the drying front (i.e., 0 min on the x-axis) the rise in energy transfer occurs more slowly as the diol content increases. These data demonstrate that EG and PG delay polymer diffusion at early stages, in the region close to the drying edge of the film. As one can see, this effect is more pronounced for EG than for PG. In Figure 6A, we see a crossover between the samples with 0 and 5 wt % EG that occurs approximately 70 min after the passing of the drying front. A crossover is also seen between samples with 0 and 5

Poly(butyl acrylate-co-methyl methacrylate) Latex Films

Figure 6. Efficiency of energy transfer ΦET as a function of aging time after passing the drying front for latex films containing different amounts of (A) ethylene glycol and (B) propylene glycol: (2) 0 wt %; (∆) 5 wt %; (9) 10 wt %; (O) 15 wt % diol. Lines are drawn to guide the eye.

wt % PG at 45 min aging, and between 0 and 10 wt % PG at 65 min aging (Figure 6B). These results indicate that the effect of EG and PG on polymer diffusion changes over time; that is to say, the presence of diol produces a retardation effect on polymer diffusion at early stages but enhances polymer diffusion at later times. In the experiments described in our previous paper,26 we found that high humidity retarded the onset of polymer diffusion but then, at later annealing times, enhanced the polymer diffusion rate. To keep the influence of humidity constant in the work reported here, we carried out experiments at constant humidity while varying the diol content of the dispersions. Our results show that the presence of diol has an effect beyond what is seen with water alone. Our explanation for these results is based on the hydrophilic character of these two diols, reflected in their large partition coefficients between the aqueous and polymer phases (K > 200). One of the consequences of this behavior is that even at the very late stages of the drying process, much of the EG and PG remains in the aqueous phase (see below for detailed calculations). Because in the final stages of drying, polar or ionic groups attached to the latex particle surface end up in the interface between cells,37 we imagine that diol molecules interact with those functional groups and remain trapped at these regions, where they hinder coalescence and the early stages of polymer diffusion. With the progressive evaporation of water, the diol molecules (and remaining water molecules) are forced to dissolve in the polymer phase, where they enhance the rate of polymer diffusion by plasticizing the polymer. The competition between these two effects would explain the crossover observed in the diffusion data. The relative rates of these processes would fix the position of the crossover between the additive delaying and enhancing polymer diffusion in the experiment. As seen in Figure 6, the crossovers for EG and PG occur at different times after passing the drying front. We speculate that the interaction via hydrogen bonding between the diol molecules and carboxylic acid groups located at the latex particle surface (introduced by methacrylic acid) plays an important role in the kinetics of diol transport toward the polymer phase. Because EG has a higher capacity

J. Phys. Chem. B, Vol. 114, No. 9, 2010 3091 to interact via hydrogen bonding compared to PG, as shown by a higher hydrogen bonding parameter,35 one would expect that EG molecules remain more strongly retained in the interface between latex cells, once the drying front has passed. As a consequence, EG will migrate to the polymer phase more slowly than PG, leading to a more pronounced retardation effect on polymer diffusion at the early stages of film formation. Effect of Diols on Polymer Diffusion in Fully Dried Latex Films. To investigate the effect of EG and PG on polymer diffusion in latex films at longer times, i.e., after the disappearance of the wet spot, we dried films at low temperature (4 °C over 5 h). These conditions suppress polymer diffusion but allow small-molecule diffusion, i.e., redistribution of diol molecules throughout the latex film. The films were then stored in the controlled humidity chamber at 54% humidity and room temperature and were periodically removed to carry out FRET measurements at a fixed position on the sample. To quantify the extent of intercellular diffusion over time, we calculated the evolution of quantum efficiency of energy transfer ΦET and fraction of mixing fm through eqs 1 and 2, respectively. To evaluate fm by means of eq 2, one should know the limiting value of quantum efficiency of energy transfer in the state of full mixing (ΦET(∞)). This value corresponds to ΦET in a film comprising a random mixing of donor- and acceptor-labeled polymers. To obtain a sample to serve as a model for the full mixing state, we cast a latex film from the labeled latex dispersion and dissolved it in THF. In solution, one expects full mixing of the donor- and acceptor-labeled polymers. This solution was recast onto a quartz plate and allowed to dry at room temperature for 24 h. The donor florescence decay profile for this film yielded a value of ΦET ) 0.62. As a test for random distribution of dyes in the film, we fitted the decay profile to the Fo¨rster equation38 (see eq S2 in the Supporting Information) that describes the donor fluorescence intensity decay for donors and acceptors randomly distributed in a three-dimensional rigid medium. We obtained a good fit (χ2 ) 1.1), strong evidence for a random distribution of labeled polymer chains throughout the film. As a second test, we calculated analytically a ΦET(∞)cal value using the Fo¨rster model with the P-parameter computed, on the basis of the acceptor (quencher) concentration in the labeled latex dispersion and with the pre-exponential factor A ) 1 (eqs S2 and S3 in the Supporting Information). We obtained ΦET(∞)cal ) 0.61, in very good agreement with the experimental value. These results confirm that the limiting value of quantum efficiency of energy transfer obtained from the solvent-cast film (ΦET(∞) ) 0.62) truly represents the state of full mixing at molecular level of donor- and acceptor-labeled polymers. Parts A and B of Figure 7 show the evolution of ΦET as a function of aging time for films containing various amounts of EG and PG, respectively. We see that, in all the samples, ΦET increases with the aging time, showing a rapid initial growth followed by a much slower increase after several hundred minutes aging. Comparing latex formulations with the same type of diol, we see that ΦET increases at higher rates with an increase in diol content. These results show that both EG or PG enhance the polymer interdiffusion rate under the conditions of the experiment, and that this effect is more pronounced as the amount of diol in the formulation increases. We also calculated the fraction of mixing fm as a function of aging time (eq 2) using the ΦET(∞) value obtained from the THF-cast film. The results are plotted in Figure 7C,D, for EG and PG, respectively. Qualitatively, the fm values present the same trend observed in ΦET.

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Figure 7. ΦET as a function of aging time for fully dried samples containing different amounts of (A) ethylene glycol and (B) propylene glycol. Fraction of mixing fm as a function of aging time for fully dried samples containing different amounts of (C) ethylene glycol and (D) propylene glycol: (2) 0 wt %; (∆) 5 wt %; (9) 10 wt %; (O) 15 wt % diol. Lines are drawn to guide the eye.

Figure 8. Plots of apparent diffusion coefficient Dapp as a function of fraction of mixing fm for newly dried latex films containing different amounts of (A) ethylene glycol and (B) propylene glycol: (2) 0 wt %; (∆) 5 wt %; (9) 10 wt %; (O) 15 wt % diol. The lowermost curve labeled as “MC” is the master curve of the Dapp data shifted to 0 wt % diol, as described in the text. For visual clarity, the master curve was translated by 1 unit along the y-axis. Lines are drawn to guide the eye.

Values of fm can be used to calculate apparent diffusion coefficients Dapp for the process. This calculation is carried out by fitting the fm data to a Fickian diffusion model for spherical geometry,39,40 with Dapp as adjustable parameter (the calculation procedure is described in Supporting Information). We remind the reader that although Dapp are not true center-of-mass diffusion coefficients, Dapp values track very well changes in polymer diffusion rates due to changes in temperature or the presence of additives in latex films.29,39,41,42 Figure 8 shows a plot of Dapp values as a function of fm on a semilog scale for latex films containing different amounts of ethylene glycol (Figure 8A) and propylene glycol (Figure 8B). An important

Schroeder et al. feature of the data is that, for a given latex formulation, Dapp decreases as fm increases. The pattern is observed in all the systems shown in Figure 8A,B. Dapp is a cumulative mean value taken over all the species that contribute to the growth in FRET up to a given value of fm. At lower extents of mixing, the diffusive process is dominated by the lowest molecular weight and most mobile species; while as the extent of mixing increases, slower moving species having much higher molecular weight and/or long-chain branches begin to contribute to the energy transfer signal, leading to a decrease in the computed Dapp value.12,29 We now turn our attention to how the additive influences the rate of polymer diffusion. We see from Figure 8A that, for a given value of fm, Dapp increases with the content of EG. For example, at fm ) 0.57 the calculated Dapp value is 8.8 × 10-17 cm2/s for the film without EG, and 3.9 × 10-16 cm2/s for 15 wt % EG. A similar behavior is seen for propylene glycol in Figure 8B, where the addition of increasing amounts of the additive progressively increases Dapp. The magnitude of the enhancement is less striking than we found previously for Texanol in a different system, in which the addition of 6 wt % additive to poly(butyl methacrylate) latex films increased Dapp by more than 1 order of magnitude.43 To quantify the enhancement effect on polymer diffusion rates, we carried out a master curve analysis on Dapp data. For each amount of additive, we determine the shift factor (aD) that superimposed the data set for that diol onto the data for 0 wt % additive, taken arbitrarily as the reference. The resulting master curves are shown in Figure 8A,B, for EG and PG, respectively. These curves, labeled “MC” in the plots, were translated by 1 unit along the Dapp-axis for visual clarity. We use these shift factors in the next section to analyze the mechanism by which these diols enhance polymer diffusion rates at long aging times. Mechanism of Diffusion Promotion at Long Aging Times. We have seen that the diols retard polymer diffusion at early stages but they act as plasticizers to increase polymer diffusivity at longer times. The plasticizing effect can be explained and analyzed in terms of free volume effects. There are two features to note in this analysis. First, the plasticizer efficiency of both EG and PG is relatively small compared with that of other additives of comparable molecular size. Second, our attempts to describe the data in the framework of classic treatments (i.e., Fujita-Doolitle; see below) failed when we assumed that all of the glycol added to the dispersion was dissolved in the polymer. These two aspects are consistent with the idea that only part of the glycol can effectively act as plasticizer and that the remaining part of the additive is localized in tiny phaseseparated domains that persist in the system even at long drying times. To test these ideas, we use measured values of the partition coefficient (K ) Cw/Cp) to calculate the weight fraction of additive contained in the polymer phase (wp) in terms of the weight percent of latex solids (Sf):

wp )

Sf K(100 - Sf) + Sf

(3)

We used eq 3 to obtain the mass of additive dissolved in the polymer phase under equilibrium conditions for different extent of drying. As water evaporates and film drying evolves, the solids content in the system Sf increases, and the additive should partition from the aqueous to the polymer phase. Figure 9 shows the predictions of eq 3 for EG and PG and compares them to

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Figure 9. Content of additive in the polymer phase as a function of weight percent solids in the P(BA60-MMA39-MAA1) latex system for EG, PG, and Texanol. In the case of the diols, the profiles for three different K values are plotted. The vertical dashed line represents the critical concentration for the onset of film formation.

the behavior calculated for Texanol in the P(BA60-MMA39MAA1) latex system. Since our partition measurements for EG and PG establish only a lower bound (K > 200), curves in Figure 9 were computed for three different values of K: 200, 300, and 400. The vertical dashed line at 74 wt % corresponds to the solids content of close packed spheres (assuming that wt fraction ∼ vol fraction and fcc packing), which one might think of as the critical concentration for the onset of coalescence. One observes that as water evaporates, EG and PG remain almost exclusively in the aqueous phase. Only at the very late stages of the drying process, far above 95% solids content, is the amount of glycol dissolved in the polymer phase above 10 wt % of its total content; i.e., 90 wt % of the original glycol added to the formulation still remains outside the latex polymer, mixed with the remaining water phase. This profile is very different from that observed for Texanol, which resides exclusively in the polymer phase during the whole drying process, as shown in Figure 9, and for which we anticipate a rather different behavior. We carried out independent measurements to estimate the percentage of water remaining in the polymer film over long aging times. In this experiment, latex films without diol additives were equilibrated for 2 days at 23 °C and 54% humidity. The films were then weighed, dried at 80 °C under vacuum for one week and then reweighed to determine the amount of residual water.44 We found that the equilibrium water percent that remains in the system after 2 days aging is 0.8% (i.e., Sf,eq ) 99.2%). To complete our analysis, we examine if the fraction of EG or PG distributed in the polymer phase under the aging conditions (i.e., 23 °C and 54% humidity), acts as a classical plasticizer by increasing the amount of free volume in the polymer matrix. If so, the enhancement in polymer diffusion rates should obey the Fujita-Doolittle equation.45 This equation has been used extensively to characterize the effect of free volume produced by low molecular weight additives on polymer diffusion coefficients,

D(T,φ) ln D(T,0)

[

-1

]

fp2(T,0) ) fp(T,0) + φβ(T)

(4)

Figure 10. Fujita-Doolittle plots for P(BA60-MMA39-MAA1) latex films containing ethylene glycol and propylene glycol, annealed at 23 °C and 54% humidity. For both diols, K ) 200 was considered.

TABLE 2: β Parameter Values Obtained as Fitting Polymer Diffusion Data to the Fujita-Doolittle Model for Ethylene Glycol and Propylene Glycol as Additives in P(BA60-MMA39-MAA1) Latex Films at 23 °C, Calculated Using Different Values of the Partition Coefficient K β diol

K ) 200

K ) 300

K ) 400

ethylene glycol propylene glycol

0.06 0.05

0.08 0.06

0.1 0.08

fractional free volume in absence of plasticizer and is directly related to the C1 parameter in the Williams-Landel-Ferry (WLF) equation though fp(T) ) 1/(2.303C1(T)).46 To test this model, we fitted our diffusion data to eq 4. The term ln[D(T,φ)/D(T,0)] ) ln D(T,φ) - ln D(T,0) was evaluated from the shift factors used to construct the master curves in Figure 8. To estimate the φ values, i.e., the actual amount of diol dissolved in the polymer phase, we used eq 3 along with the estimate of the amount of water remaining upon long time aging (Sf,eq). The value of fp(T) was calculated from the C1 parameter that we reported in a previous paper for the same latex system at 25 °C (C1 ) 10.0).29 β(T) was then the only adjustable parameter in eq 4. In Figure 10, we plot values of {ln[D(T,φ)/D(T,0)]}-1 against 1/φ for EG and PG, using K ) 200 for both diols. We observe that in both cases the linear fit is very good, confirming that the effect on polymer diffusion follows nicely the behavior predicted by the Fujita-Doolittle model. Very good fits were also obtained considering K ) 300 and K ) 400, as shown in the Supporting Information. We list the β fitting parameters for the different K values in Table 2. The β values obtained here are in good agreement with the values reported for traditional plasticizers in similar latex systems. For example, in the past we have employed this kind of analysis to investigate the effect of Texanol on the diffusion of poly(butyl methacrylate) (PBMA) at 36 °C, where we found β ) 0.07.34 We conclude that EG and PG in latex dispersions locate almost exclusively in the aqueous phase. Only at very late stages of drying do these diols migrate into the polymer phase for P(BA-MMA) latex films. Once in the polymer phase, the diols act as plasticizers and promote the rate of polymer diffusion via an increase in the amount of free volume. Summary

where D is the polymer diffusion coefficient, φ is the volume fraction of the plasticizer in the polymer phase, and β(T) describes the fractional free volume difference at temperature T between the additive and the polymer. The term fp(T) is the

We have investigated the effect of ethylene glycol and propylene glycol on polymer diffusion in P(BA60-MMA39MAA1) latex films. These films dry via a drying front that propagates from the edges toward the center of the film. We

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found that EG and PG delay the onset of polymer diffusion at early stages, in the region close to the drying front. This effect was more pronounced for EG than for PG. We attribute this behavior to the strong hydrophilic character of diols, reflected in their large partition coefficients favoring the aqueous over the polymer phase. At the late stages of the drying process, diol molecules dissolved in the remaining aqueous phase retard coalescence and hinder the early stages of polymer diffusion. Because EG has a higher capacity than PG to interact via hydrogen bond with the polar groups at the particle surface, EG will migrate to the polymer phase more slowly than PG and will give place to a more pronounced retardation effect on polymer diffusion at the early stages. In the nearly dry film, the partition equilibrium leads to elevated levels of diol in the polymer phase. Under these circumstances, both EG and PG act as plasticizers to promote the rate of polymer diffusion. The β values calculated from a Fujita-Doolittle analysis of the diffusion data show that the intrinsic plasticizing ability of EG and PG are comparable to that of a traditional coalescing aid such as Texanol. EG and PG are less effective at promoting polymer diffusion because only a fraction of the diol added dissolves molecularly in the polymer matrix. Some of it remains trapped in interstitial spaces in the film until it evaporates. We also observed that the effect of these diols changes over time, leading to an enhancement of polymer diffusion at later aging times. Acknowledgment. We thank Rohm & Haas, Rohm and Haas Canada, and NSERC Canada for their support of this research. Y.L. thanks the Province of Ontario for an Ontario Graduate Scholarship in Science and Technology (OGSST) scholarship. Supporting Information Available: Figure showing ΦET versus aging time for latex films formed from different ratios of donor- and acceptor-labeled latex particles; photographs showing a latex film immediately before being sealed in the WSA and after the decay measurements; text explaining the analysis of donor fluorescence decay profiles, the Fo¨rster model, and the calculation of apparent diffusion coefficients; FujitaDoolittle plots for different partition coefficient values. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bezinski, J. J. In Paint and Coating Testing Manual: Fourteenth Edition of the Gardner-Sward Handbook; Koleske, J. V., Ed.; American Society for Testing and Materials: Philadelphia, 1995. (2) Patton, T. C. Paint Flow and Pigment Dispersion, 2nd ed.; WileyInterscience: New York, 1979; pp 194-197. (3) Walker, K. In AdditiVes for Water-based Coatings; Karsa, D. R., Ed.; Special Publ. No. 76; Royal Chemical Society: London, 1990; p 198. (4) (a) Dobler, F.; Holl, Y. Trends Polym. Sci. 1996, 4, 145–151. (b) Winnik, M. A. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M., Eds.; John Wiley & Sons Ltd: New York, 1997; p 467. (c) Steward, P. A.; Hearn, J.; Wilkinson, M. C. AdV. Colloid Interface Sci. 2000, 86, 195–267. (5) Calbo, L. J. Handbook of Coatings AdditiVes; Marcel Dekker: New York, 1992. (6) Voyutskii, S. S. Autohension and Adhesion of High Polymers; Interscience: New York, 1963; p 74. (7) van Tent, A.; te Nijenhuis, K. J. Colloid Interface Sci. 1992, 150, 97–114. (8) Yoo, J. N.; Sperling, L. H.; Glinka, C. J.; Klein, A. Macromolecules 1990, 23, 3962–3967. (9) Zhao, C. L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990, 23, 4082–4087. (10) Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.; Lindner, P.; Rios, L.; Cabane, B. Prog. Colloid Polym. Sci. 1990, 81, 175–183.

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