Film Formation of High Tg Latex Using Hydroplasticization: Explanations from NMR Relaxometry.

The film formation of acrylic latex dispersions, containing different amounts of carboxylic acid functional groups by the incorporation of methacrylic acid (MAA), was studied with GARField 1H NMR at various relative humidities (RH). Polymer particles with glass-transition temperatures in the range from 26 to 50 °C formed films at room temperature because of hydroplasticization. It was found that with an increased drying rate due to lower RH, the evaporation flux of water was limited by the latex polymer. Only in the second stage of drying this phenomenon was more obvious with increasing MAA content. 1H NMR relaxometry was used to study the change of hydrogen mobilities during film formation and hardening of the films. This showed that the drying rate itself had no impact on the hydrogen mobility in the latex films as measured via the T2 relaxation time. Hydrogen mobilities of water and the mobile polymer phase only significantly decrease after most water has evaporated. This implies that the rigidity of the polymers increases with the evaporation of water that otherwise plasticizes the polymer through hydrogen bonding with the carboxylic acid groups. This hardening of the polymer phase is essential for applications in a coating. The hydrogen mobilities were affected by the MAA concentration. Densities of mobile hydrogens increase with increasing MAA content. This is expected if the mobile protons are contained in the MAA groups. The result thus confirms the role of carboxylic acid groups in hydrogen bonding and plasticization of the copolymers. Hydrogen mobilities, however, decrease with increasing MAA content, which is hypothesized to be caused by the formation of dimers of carboxylic acid groups that still hold water. They still enable short-range polymer hydrogen mobility due to hydroplasticization but limit long-range polymer mobility due to interaction between the carboxylic acid groups.

Introduction

For several decades, latex film formation has been a broadly studied topic. This process is typically divided into three stages.[1,2] One, water evaporates from the aqueous dispersion, resulting in the increased concentration of the polymer particles and overcoming their colloidal stability.[3] Two, particles deform due to capillary effects, forming a close-packed polyhedral structure.[4,5] Three, polymer chains cross the interparticle boundaries through a process called interdiffusion, leading to a cohesive coating.[6,7] It would be more precise to describe film formation as a continuous process rather than a sequential one since these stages overlap in time and different processes may take place simultaneously. For example, it is known that the drying rate of the latex dispersion can lead to heterogeneous distributions of water and polymer in the drying film, with polymer particles accumulating on the latex–air interface.[8,9] This phenomenon can inhibit the evaporation of water from the bulk of the film. When the particles on the surface coalesce, often referred to as skin formation, water evaporation is impeded, resulting in prolonged drying times.[10,11] Pohl and co-workers showed that particle coalescence can overlap with drying and particle deformation.[12]

Only mobile polymers can undergo particle deformation and polymer interdiffusion. Hence, in an application, the user has to deal with the dilemma of choosing between a hard polymer with a volatile plasticizer, which contributes to the emission of volatile organic components (VOCs), or a soft polymer that does not provide good surface protection. This choice between two extremes is called the “film formation dilemma”.[2,13,14] However, hydroplasticization, that is, plasticization of polymers by water, offers a way to avoid the problem of VOCs. The degree of hydroplasticization can be influenced by the polymer chemistry. More specifically, increasing the polymer polarity promotes interaction with water.[15]

The monomer building blocks that are incorporated into the polymer chains interact with water, meaning that they can either bind water, for example, through hydrogen bonding, or repel water due to the hydrophobic nature of the monomers.[15] The effect of polymer composition on the film formation process has been investigated by probing the minimum film formation temperature (MFT) using an MFT bar.[16−18] This technique, however, provides macroscopic information on the drying process without any fundamental basis. Feng and Winnik provided more fundamental proof on the effect of polymer hydrophilicity on polymer interdiffusion.[19] They proved that the water–polymer interaction plays a crucial role in the coalescence of the polymer particles. Incorporation of hydrophilic monomers into the polymer chain appeared to promote polymer interdiffusion.

For acrylic copolymers, examples of highly hydrophilic monomers are acrylic acid and methacrylic acid (MAA).[20,21] These monomers are often used to incorporate charged functionalities via deprotonation of the carboxylic acid groups, enabling colloidal stability of the polymer particles.[22] Moreover, these monomers can bind high amounts of water by hydrogen bonding with the carboxylic acid constituents ensuring hydroplasticization of the copolymers, which lowers their effective glass-transition temperature Tg.[15,23,24] The hydroplasticization effect can be used as a way to obtain film formation of polymers with a dry Tg above the ambient film formation temperature. Hard polymer particles do not deform and coalesce to make a film.[25,26] Hydroplasticized particles are able to form a film, and a hard coating is expected upon the loss of water. However, rigorous investigations of the phenomenon are lacking in the literature.

GARField 1H NMR profiling has already been used to study the film formation process of different polymer systems, including alkyd and polyurethane resins and acrylic latex dispersions.[8,11,27−29] Due to its high gradient in the magnetic field, hydrogen distribution profiles can be produced during drying of films, from which drying rates can be derived. Moreover, hydrogen mobilities and densities can be obtained, which can be particularly useful to study how water mobility is affected by the interaction with the polymer, for example, due to hydrogen bonding.

Here, we study the film formation process of acrylic latex dispersions with variable MAA contents using GARField 1H NMR to gain valuable insights into the role of the carboxylic acid constituents on water and polymer mobility. The experiments measure the molecular mobility of the polymer phase in the presence of water and as the film hardens after evaporation of water to make a hard coating. Hence, the aim of this study is to understand the influence of the hydroplasticization effect on the film formation behavior of the latex dispersions.

Experimental and Theoretical Details

Latex Synthesis

Latex dispersions were synthesized by seeded emulsion polymerization. The monomers methyl methacrylate (MMA; Lucite, >99.8%), MAA (BASF, >99.8%), and butyl acrylate (BA; Arkema, >99.8%) were used as received. The following synthetic procedure is for the latex dispersion containing 2 wt % MAA on total solids. In Table , the monomer amounts and the total solid content for each dispersion are listed. The lower limit of MAA was chosen at 2% MAA to provide the polymer particles with the necessary colloidal stability. The upper limit was chosen at 20% due to the possibility of gelation of the latex dispersions at a higher MAA content. The ratio of MMA and BA is adjusted for each dispersion to maintain the dry polymer glass-transition temperature at 25 °C according to the Flory–Fox equation.[30] The solid contents are determined gravimetrically. Although the ester moieties of MMA and BA are known to bind water, this will have a negligible influence on the hydroplasticization and concomitant Tg decrease of the copolymers.[15,24]

Table 1

Monomer Amounts of the Latex Dispersions Used for this Study and the Solid Weight Fractions sw of the Total Dispersions

monomer fraction (wt % on total solids)
methacrylic acid	methyl methacrylate	butyl acrylate	sw (wt %)
2.0	51.0	47.0	33.8
5.0	47.0	48.0	34.1
10.0	40.4	49.6	33.6
15.0	33.7	51.3	30.5
20.0	27.0	53.0	21.7

A 2000 cm3 flask equipped with a thermometer, N2 inlet, and overhead stirrer was charged with water (799.1 g) and ammonium persulphate (0.35 g). In a funnel, an emulsified monomer feed was prepared by mixing demineralized water (305.97 g), sodium lauryl sulfate (4.62 g of 30 wt % solution in water), methyl methacrylate (MMA, 353.59 g), n-butyl acrylate (n-BA, 325.85 g), and methacrylic acid (MAA, 13.87 g). In another funnel, an initiator solution was charged by dissolving ammonium persulfate (3.12 g) in demineralized water (111.38 g). The reactor was heated to 83 °C, and 5 wt % of the emulsified monomer feed was added to the reactor, and the reaction temperature was allowed to increase to 85 °C. At 83–87 °C, the remainder of the monomer mixture was fed to the reactor in 100 min. At the end of the monomer feed, demineralized water (28.3 g) was used to rinse the funnel holding the monomer mixture. The reaction was kept at 85 °C for 30 min. Next, the batch was cooled to room temperature, and 1,2-benzisothiazolin-3-one (Proxel Ultra 10, 6.9 g of a 10 wt % solution) was added to prevent bacterial and fungal growth in the dispersions, followed by demineralized water (45.3 g). Finally, the batch was filtered through a filter cloth to remove any coagulum formed during the reaction.

Wang and co-workers found that the water loss rate of latex containing a poly(acrylic acid) copolymer was slower in the later stages of drying, when the pH was less than 7.[31] In the present experiments, the pH of the latex dispersions was adjusted by addition of a dilute solution of ammonia (5 wt % in water) under mild stirring. See Table for the pH values obtained. pKa values for PMAA are reported to be as high as 9.5,[15] which indicates that the latex dispersions studied here might not be fully deprotonated. The pH of the various latex dispersions are set at similar values. Therefore, pH effects are the same in all experiments and not of interest here.

Table 2

Physical Characteristics of the Laticesa

composition (wt % MAA)	Zav (nm)	Đ	ζ (mV)	pH	η (mPa s)
2	345	0.04	–72	8.1	6
5	323	0.06	–66	7.9	9
10	308	0.05	–51	7.3	17
15	301	0.07	–57	7.1	193
20	293	0.03	–59	7.0	368

Zav is the average particle diameter, Đ is the polydispersity index of the particle size distribution, ζ is the ζ-potential determined at similar pH as the latex dispersion, and η is the latex dispersion viscosity.

The average particle diameter (Zav), polydispersity (Đ), and ζ-potential at similar pH were determined with a Malvern Zetasizer Nano ZS at 25 °C. Values of ζ indicated good stability of the latex dispersions. The dispersion viscosities were determined with a viscometer (Brookfield DV-E) with spindle 1 at a speed of 60 rpm. Results of the analyses are listed in Table .

Modulated differential scanning calorimetry (TA Instruments Q2000 DSC) analyses were performed on dried material from the latex dispersions to determine the dry polymer Tg values. Results of these measurements are listed Table . The DSC thermograms are presented in Figure S1 in the Supporting Information.

Table 3

Results of the Dry Polymer Tg Analyses of the Various Latex Dispersions

composition (wt % MAA)	Tg (°C)	Tg,onset (°C)	Tg,end (°C)	Tg,width (°C)
2	25.9	19.2	32.6	13.4
5	30.2	23.0	37.4	14.4
10	37.3	28.4	46.2	17.8
15	46.9	36.0	57.8	21.8
20	50.2	36.8	63.7	26.9

The Tg values in Table appear to be higher than predicted. An explanation for this is the dimerization of carboxylic acid groups in the dry material, which effectively increases the polymer Tg.[24][24] This behavior is not predicted by the Tg determination based on the Flory–Fox equation.

Another observation is the increasing width of the Tg transition region Tg,width with increasing MAA content. A plausible explanation for this phenomenon is the nonuniformity of the latex particles. Moreover, with increasing MAA content, increasing concentrations of MAA oligomers are expected in the water phase. Due to the low concentration of these oligomers, no distinguishable signal is observed in the GARField 1H NMR experiments.

Wet Tg values were determined using modulated DSC (TA Instruments Q2000 DSC). Samples were sedimented overnight in a centrifuge. Afterward, the serum was decanted, and the residue was used for measurement. Details and results of the measurements are listed in Table . The DSC thermograms are presented in Figure S2 in the Supporting Information.

Table 4

Results of the Wet Polymer Tg Analyses of the Various Latex Dispersionsa

composition (wt % MAA)	Tg (°C)	Tg,onset (°C)	Tg,end (°C)	Tg,width (°C)
2	18.4	≤14.6	22.3	−
5	18.6	≤14.5	22.6	−
10	19.1	≤15.0	23.2	−
15	19.3	≤16.6	22.1	−
20	nm	nm	nm	nm

nm: not measured.

Clearly, Tg values of the wet polymer phase are lower than those of the dry polymer, which is a result of the hydroplasticization effect. Values of Tg,width could not be calculated since Tg,onset could not be accurately determined. Values of Tg,onset are expected to be close to the lower temperature limit (5 °C) of the DSC analyses.

Garfield 1H NMR

The use of GARField 1H NMR imaging and its design and principles have first been described by Glover et al.[32] This technique is a useful tool to study the drying process of thin films. The current equipment has a magnetic field strength of 1.5 T with a static gradient of 42.2 ± 0.2 T/m. An Ostroff–Waugh[33] pulse sequence (90°-τ-[90°-τ-echo-τ]) is used to obtain signal decays from which T2 relaxation times of both water and the polymer are obtained. Unless stated otherwise, the echo time te = 2τ used for this study is 40 μs with an acquisition time tac of 35 μs, resulting in a spatial resolution of 14 μm. The long delay ld was set at 1.7 s and the number of echoes n at 128. To reduce the signal-to-noise ratio, signal decays were obtained by averaging multiple measurements as follows: for t < 0.6 h, 32 averages, for 0.6 < t < 3 h, 256 averages, and for t > 3 h, 1024 averages. The results were normalized with a signal decay of an aqueous 0.025 M CuSO4 solution, with te = 40 μs, tac = 35 μs, ld = 0.3 s, and n = 2048 and 4096 averages.

The NMR setup is equipped with a temperature- and humidity-controlled chamber in which the sample is placed directly onto the RF coil. This sample holder is a 140 μm-thick cover glass, covered with a microscope glass slide having a circular hole with a 10 mm diameter. Herein, 50 μL of latex was placed using a 100 μL volumetric pipette, resulting in a wet coating of about 400 μm thickness. Initially, the sample holder is closed with a silicone stopper to perform a measurement without any initial drying. Subsequently, the cover is removed and sequential drying measurements are performed.

The RH in the climate chamber was set at 46 ± 1, 62 ± 1, or 79 ± 1%. The temperature was set at 23 °C, which is below the Tg of the dry copolymers listed in Table .

Ostroff–Waugh signal decays of a latex dispersion at a certain position in the film can be fitted with an exponential decay functionwhere S(x) is the total signal at time t = nte (s), T2,(x) (s) is the transversal relaxation time of the kth hydrogen pool of the sample with amplitude A(x), and S0 is the signal noise level. P(x) is a weighing factor necessary to correct for heterogeneities in the coil profile and echo modulations introduced by the Ostroff–Waugh sequence. P(x) is obtained using an aqueous 0.025 M CuSO4 solution. For details, we refer to.[28] With this, eq can be rewritten aswhere I(x) is the relative signal intensity and ρ(x) is the relative hydrogen density of the kth hydrogen pool with respect to water. Aref(x) is the signal amplitude obtained with a reference measurement of the 0.025 M CuSO4 solution.

T2 relaxations can be split into dipole effects T2,dip and a diffusion part T2,diffIn a previous work, we have shown that with the current experimental conditions with te = 40 μs, mainly hydrogen relaxations due to dipolar interactions T2,dip are probed.[29] Dephasing due to diffusion in the NMR gradient at this echo time does not play a significant role.

Influence of Relative Humidity on the Drying Rates

The drying rate E (m/s) can be externally limited by the environment or internally limited by transport through the film. In the case of only external limitation, water can evaporate freely from the drying filmwhere m is the mass of the evaporated water, t is the evaporation time, A is the area of evaporation, and ρw is the density of water. Since E is determined by the water activity difference of the film surface af and the environment a, it can be expressed aswith E′ (m/s) being the weighted drying rate taking account of the water activity difference. With this, eq can be written for E′with the weighted drying time t′Thus, t′ can be calculated if both a and af are known. For the latex dispersions, af ≈ 1 and a can be directly calculated from the climate chamber RHeq can be used to study driving forces for water evaporation from the latex dispersions.

One phenomenon that can limit E is the accumulation of polymer particles at the surface of the latex dispersion layer, forming a barrier for water evaporation. During drying, the distribution of polymer particles in a latex dispersion layer can be affected depending on the evaporation time of water tevap (s)with H (m) being the initial thickness of the latex dispersion layer. The characteristic diffusion time tdiff (s) of the polymer particles is inversely proportional to the particle diffusion coefficient D0 (m2/s)with D0 given by the Stokes–Einstein equation in the dilute regime aswhere T (K) is the temperature of the latex dispersion, η (Pa s) is the solvent viscosity, and R (m) is the polymer particle radius. If tdiff ≫ tevap, particles would accumulate on the film–air interface. Basically, the Peclet number described the ratio between advection of particles to the latex film surface due to water evaporation and diffusion of particles from the surface due to Brownian motion. The Peclet number Pe is used to predict this heterogeneous distribution of polymer particlesIf Pe ≫ 1, heterogeneous polymer particle distributions are predicted, whereas for Pe ≪ 1 homogeneous distributions are expected.

Results and Discussion

Drying Rates of Latex Dispersions

Film formation occurs at 23 °C in the magnetic field, resulting in crack-free and transparent films. This can be understood by the plasticization of the copolymers in the wet latex, lowering the Tg values below the film formation temperature as can be seen in Table .[23]

During the drying process of a latex film, bulk water in the film evaporates. Water evaporation rates can be derived from temporal hydrogen distribution profiles obtained with GARField 1H NMR. In Figure , examples of hydrogen distribution profiles of the 2 and 20% MAA dispersions during drying at 79 ± 1% RH and 23 °C are shown.

Figure 1

Hydrogen distribution profiles of a latex containing (a) 2% MAA and (b) 20% MAA drying at 79 ± 1% RH. The color change of the profiles from red to green indicates profiles from early to most recent measurement.

The hydrogen distribution profiles in Figure can be used to determine the film thickness decrease ΔHwhere H(0) is the film thickness before drying and H(t) is the film thickness at t.

The hydrogen distribution profiles in Figure show spatial signal intensities I(x), proportional to mobile hydrogen densities according to eq , during drying of the latex film. This clearly shows latex-air fronts, which are receding faster for the 2% MAA latex dispersion. This front shows a gradient, which is caused by the low spatial resolution of approximately 14 μm and averaging over multiple measurements as described in the Experimental Section. The drying surface is at the receding right side of the profiles, whereas position 0 reflects the bottom of the latex film. ΔH of the latex dispersion layer can be estimated from the front and bottom positions, which are taken at I = 0.5Imax, with Imax being the maximum signal intensity of each profile. In Figure a,b ΔH as a function of drying time is shown for the 2 and 20% MAA latex dispersions.

Figure 2

(a) and (b): ΔH of the 2% and 20% MAA latex dispersions, respectively, dried at 79 ± 1, 62 ± 1, and 46 ± 1% RH. The red lines represent linear fits with slope E. (c) and (d): ΔH of the 2% and 20% MAA latex dispersions, respectively, dried at 79 ± 1, 62 ± 1, and 46 ± 1% RH, with the drying time t adjusted with the water activity difference (af – a) to t′. The dashed line indicates the transition point from shrinking films to films with a stable film thickness.

Figure shows two phases. First, a linear increase of ΔH is observed, which is a result of the water evaporation and the concomitant shrinkage of the film. Second, ΔH stabilizes to a constant thickness. In this phase, further evaporation of residual water in the coating does not significantly change ΔH. From linear fits of the plots in the first phase, E can be determined from the slopes.

The linear fits for the 2% MAA latex dispersion in Figure a clearly show an increased E with decreasing RH, which is in line with the expectation that a higher ambient RH leads to slower evaporation of water. The results for the 20% MAA latex dispersion shown in Figure b, however, do not show this relation, since at 46 ± 1% RH E appears to be lower than at the other, higher RH’s. This already indicates that additional phenomena are influencing E.

To obtain a better insight into the cause of the deviations in the drying behavior, eq is used to calculate Pe for all of the dispersions at the different RH’s. For this, E values obtained from the linear fits as shown in Figure a,b were used. Results are shown in Figure a. Moreover, eq was used to calculate the weighted drying time t′. Figure c and d show ΔH as a function of t′ for the 2 and 20% MAA latex dispersions, respectively. E′ is obtained from linear fits of the slopes, which can be used to evaluate how drying rates are affected by the climate chamber RH. The results are shown in Figure b.

Figure 3

(a) Pe dependence on the latex dispersion MAA content ([MAA]). (b) Dependence of E′ on [MAA].

The values found for E, and therefore E′, are significantly higher than those found in the literature for pure water or latex films drying in static air.[11,34,35] It should be noted, however, that the values reported here are obtained with an airflow in the climate chamber.

As discussed, fast drying of latex dispersions can result in heterogeneous water–polymer distributions due to accumulation of polymer particles on the latex–air interface. Figure clearly shows that Pe ≫ 1 for all of the latex dispersions at any of the applied RH, indicating that this particle accumulation is indeed taking place. This could inhibit E. Although the hydrogen distribution profiles shown in Figure are expected to show a gradient in the NMR signal, similar to those found by Carter who showed the appearance of a lower signal intensity at the latex film surface,[11] this is not visible due to the low spatial resolution of the current profiles.

E is proportional to the water activity difference af – a, as is schematically shown in Figure , and is expected to be inversely proportional to the polymer barrier thickness δAccording to eq , a decrease of af or an increase of δ would result in a decrease of E during drying. The data in Figure , however, show linear slopes, and Figure b shows that E is proportional to af – a. This implies that af remains constant during drying and the increase of δ does not affect the evaporation of water. This implies that evaporation of water is not hindered by particle accumulation due to high Pe values. Hence, no significant particle deformation and coalescence of the accumulated particle take place.

Figure 4

Schematic drawing of a drying latex film resulting in heterogeneous water and polymer distributions. The water activity on the latex surface af is approximately 1 and remains constant during drying of the latex films, as reflected by a constant E. The polymer barrier thickness δ, which is formed rapidly between the first two 1H NMR measurements (0–3 min.), is expected to increase during drying since Pe ≫ 1. The water evaporation flux J is not affected by this, meaning that the surface of the latex layer remains open during the bulk drying of the latex films.

The values of E′ in Figure b show a trend of decreasing E′ with an increasing MAA concentration [MAA]. This suggests that the increased MAA content of the particles plays a role in the drying behavior of the latex dispersions. A plausible explanation would be that hydrogen bonding of carboxylic acid groups with water can inhibit E.[36] Therefore, the effect of MAA will be considered next.

Hydrogen Mobilities during Film Formation

Analysis of the Ostroff–Waugh Signal Decay

As was shown in the previous section, the values of E and E′ can be affected by the presence of MAA and thus carboxylic acid constituents of the polymers. Figure b shows that increasing the MAA concentration in the polymer results in a decrease of E′. To determine the influence of MAA on the drying process and film formation of latex polymers with Tg below the ambient temperature, 1H NMR relaxometry has been used to study the water and polymer hydrogen mobilities during drying. As an example, in Figure , an Ostroff–Waugh decay of the 2% MAA latex dispersion before drying is shown.

Figure 5

Ostroff–Waugh decay of the 2% MAA latex dispersion before drying.

From the Ostroff–Waugh decays, T2 relaxation times can be obtained. These relaxation times are a measure of hydrogen mobility and hence water and polymer mobility. Increasing values indicate higher mobilities. For each observed T2, a hydrogen density ρ is also obtained, which is a measure of the density of hydrogens at a given mobility.

The Ostroff–Waugh decay in Figure and subsequent decays during drying of the various latex dispersions show the presence of two hydrogen pools with different mobilities. This means that they can be fit with eq using k = 2 resulting in a short T2,short and long T2,long relaxation with their respective hydrogen densities ρshort and ρlong. T2,short was previously attributed to hydrogens of rigid polymer domains in the latex, whereas T2,long is attributed to hydrogens of water and soft polymer domains.[29] These soft polymer domains may arise from an MAA-enriched surface layer of the particles, which is also indicated by the DSC analyses as discussed in the Experimental Section.

Effect of RH on Hydrogen Mobilities

The transversal relaxation times T2 and the hydrogen densities ρ during the film formation process of each latex dispersion at the various RH values can provide valuable insights into the influence of latex MAA content on the process. Figure shows T2 and ρ as a function of drying time t for the 2% MAA latex dispersion.

Figure 6

(a) and (b): Short T2,short transversal relaxation times and proton densities ρshort of the 2% MAA latex dispersion as a function of drying time. (c) and (d): Long T2,long transversal relaxation times and proton densities ρlong of the 2% MAA latex dispersion as a function of drying time.

The short relaxation time T2,short in Figure a does not show any significant variation during drying. This could indicate the presence of a rigid polymer fraction with low hydrogen mobility, having no apparent interaction with water. The hydrogen density ρshort shown in Figure b, however, increases during the drying process.

Since ρshort represents the hydrogen density of a rigid polymer phase, its value would scale with the polymer concentration during drying. Relative polymer concentrations can be obtained from the minimum film thickness Hmin, where the solid weight sw approaches 100%, and the thickness of the latex film at any point during the drying process H(t). In Figure a, ρshort is shown versus Hmin/H(t) of films of the various latex dispersions dried at 79 ± 1% RH. This indeed shows linear behavior of ρshort with the latex solid content for all dispersions and that ρshort increases due to concentrating of the polymer phase.

Figure 7

(a) Short hydrogen densities ρshort and (b) long hydrogen densities ρlong of all latex dispersions versus the relative film thickness Hmin/H(t) of the coatings during drying at 79 ± 1% RH. Dashed lines are drawn to guide the eye.

The long relaxation time T2,long in Figure c shows a gradual decrease down to a constant value. This behavior was explained before by the evaporation of water and the decrease in polymer hydrogen mobility.[29] The relative hydrogen density ρlong for T2,long decreases, as can be seen in Figure d. Figure b shows the correlation of ρlong with Hmin/H(t), proving the relation between the decrease of ρlong and the loss of mobile hydrogen atoms due to evaporation of water. Although Figure clearly shows the concentration effect, a more in-depth analysis of T2,long is needed to elucidate the role of water–polymer interaction on hydrogen mobility.

To investigate the influence of the RH in the climate chamber on the bulk drying process of the latex dispersion films, the relation between T2,long and the corresponding ρlong can be investigated for each dispersion. In Figure a, T2,long is shown as a function of ρlong for the 2% MAA dispersion at the various RH values. Evidently, E controlled via the RH has no impact on the relaxation behavior of the hydrogens, which supports the results in Figure b that the RH does not significantly influence the drying process of the latex dispersions.

Figure 8

(a): T2,long as a function of ρlong at the various RH values for the 2% MAA latex dispersion. (b): T2,long as a function of ρlong for the various latex dispersions at 79 ± 1% RH.

The MAA content strongly influences the hydrogen relaxation behavior. Figure b shows T2,long as a function of ρlong for all dispersions dried at 79 ± 1% RH. When comparing the hydrogen mobilities during drying of the different latex dispersions, clear differences are found. Hydrogen mobilities decrease with increasing MAA content of the latex at a given ρlong. The decrease of water hydrogen mobility due to hydrogen bonding with carboxylic acid groups can explain this behavior.

Influence of MAA Content on Film Formation

As was shown in Figure , both ρlong and T2,long decrease and ρshort increases during drying of the latex dispersions. A better understanding of the effect of water evaporation on the hydrogen density and mobility changes can be obtained by plotting T2,long and the hydrogen densities ρshort and ρlong as a function of H, shown in Figure for the 10% MAA latex dispersion dried at 79 ± 1% RH.

Figure 9

ρshort, ρlong, and T2,long as a function of the latex coating thickness during drying of the 10% MAA latex dispersion at 79 ± 1% RH. The solid line represents H at which T2,long gradually decreases, coinciding with the close packing of the latex particles. The dashed line indicates the minimum film thickness Hmin at which T2,long shows a sharp decrease.

Clearly, during an initial drying stage, T2,long does not change significantly. This indicates that the overall hydrogen mobility present in this pool during this stage of the drying process is hardly affected by the evaporation of water. The solid content of the latex dispersion at this point is approximately 68 wt %. Previously, it was found that at this point the drying film reaches its closest particle packing.[29] When H approaches Hmin, T2,long shows a sharp decrease, indicating a decrease of hydrogen mobility, when most water has evaporated.

To provide a better insight into the hydrogen mobilities near the end of the drying process, Figure shows the inverse of T2,long as a function of ρlong. Even after the coatings reached their minimum thickness and the minimum value of ρlong, the hydrogen mobilities still decrease drastically. This shows that the rigidity of the polymers still increases, most likely due to continued evaporation of small amounts of water that provide a high degree of hydroplasticization to the polymers. At this point, differences between the various latex dispersions are clearly visible.

Figure 10

Inverse of T2,long as a function of ρlong. The arrow is drawn to stress the effect of increasing MAA content on the hydrogen mobility of the various latex dispersions during the drying process.

Apparently, with an increase in MAA content, the rigidity of the polymers at similar ρlong increases. This can be explained by the dimerization of carboxylic acid constituents on the polymer chains (Figure ), resulting in a lower polymer mobility with increasing MAA content. The density of mobile hydrogens, however, decreases to lower values with decreasing MAA content as indicated by the lower values of ρlong in the dried coatings. This indicates a lower water content of the coating at the end of the drying process due to the lower polarity of the latex coating.

Figure 11

Schematic drawing of a dimer of carboxylic acid constituents on copolymer chains.

Fourier transform infrared-Attenuated total reflection (FTIR-ATR) measurements support this hypothesis, as dimers of carboxylic acid were shown to be present. For details on measurements and results, we refer to our previous work.[24] For the 15 and 20% MAA films, however, charged carboxylate groups are still present, apparent from a band at 1542 cm–1 representing COO– stretch vibrations as shown in Figure .[31] This would imply that evaporation or partial evaporation of ammonia during the drying process leads to protonation of the carboxylate groups. Since carboxylic acid groups bind less water (1.3 mol/mol COOH) than carboxylate groups (11.3 mol/mol COO–),[15] the degree of hydroplasticization of the copolymers decreases significantly.

Figure 12

FTIR-ATR absorbance spectra of the dried latex films in the characteristic region for ionic interaction of carboxylate constituents. The spectra were shifted vertically for the sake of clarity.

Discussion

The study of the drying process of the various latex dispersions at different RH’s has shown that variations of the MAA content of the polymer, and therefore the carboxylic acid content, affect the drying behavior of films of the latex dispersions. Figure shows a schematic picture of the observed phenomena, divided into four phases. Accumulation of polymer particles occurs in phase I, followed by bulk water evaporation until close packing of particles in phase II, evaporation of water between packed particles and particle deformation in phase III, and evaporation of plasticizing water resulting in rigidification of the polymer in phase IV.

Figure 13

Schematic illustration of the four phases during the drying process of the various latex dispersions. Different phenomena can be distinguished. I. Particle accumulation on the latex surface occurs early during the drying process. II. Water evaporation continues, with increasing ρshort and decreasing ρlong. T2,long remains constant. III. After close packing of the particles, ρshort increases faster and ρlong decreases faster. T2,long also starts decreasing gradually. IV. When the film thickness reaches the minimum film thickness, hydroplasticization of the polymer chains disappears due to the low amount of water present in the coatings, resulting in a decreased polymer hydrogen mobility. Due to this, T2,long shows a sharp decrease.

During phase I of the drying process, particles accumulate on the latex–air interface. It was shown that heterogeneous distributions are expected to occur for all latex dispersions based on the estimation of the Pe values. Particle accumulation occurs early during the drying process, which is followed by a constant evaporation rate E. This implies that an increase of the particle boundary thickness δ at the interface does not slow down water evaporation further. Hence, no particle deformation or interdiffusion occurs, which would result in a continuous decrease of E. Therefore, the accumulated particle layer allows water to evaporate via a tortuous pathway.

The bulk drying of water in phase II of the drying process studied by 1H NMR relaxometry provides a deeper understanding of the drying process. The two relaxation times found can be ascribed to hydrogens with different mobilities, with T2,short reflecting the mobility of hydrogens of more rigid polymeric parts and T2,long reflecting more mobile hydrogens. In a previous work,[29] it was shown that hydrogens from both water and polymer contribute to T2,long and ρlong and that T2,short and ρshort arise from more rigid polymer domains. From the T2 relaxation times and the corresponding hydrogen densities of the latex dispersions during drying, three important observations are made. One, hydrogens of the rigid polymer fraction reflected by T2,short and ρshort increase linearly with the solid fractions of the latex dispersions during drying and do not significantly interact with water, as is obvious from the constant values for T2,short. Two, for the individual latex dispersions, variations of E set by the climate chamber RH do not affect the hydrogen relaxations in the more mobile hydrogen phase represented by T2,long and ρlong. Thus, only E is affected. This indicates that E does not affect water–polymer interaction or polymer interdiffusion. Three, between the individual latex dispersions, differences in hydrogen mobilities are observed from the T2,long data. At similar hydrogen densities ρlong, mobilities appear to decrease with increasing carboxylic acid content. An explanation for this behavior is the interaction of water with carboxylic acid groups through hydrogen bonding, which decreases overall hydrogen mobilities.

Upon evaporation of water, two different processes take place that affect T2,long and ρlong: a decrease of hydrogen atoms due to the evaporation of water and a decrease of hydrogen mobility in this phase due to increased rigidity of polymer segments, which were previously plasticized by water. Initially, as depicted by phase II in Figure , the evaporation of bulk water does not appear to impact T2,long significantly, indicating that the hydrogen density and mobility of the mobile phase are hardly affected by the loss of water. Only when close packing of the particles is reached, T2,long starts decreasing due to confinement of water. Only when the bulk water has evaporated and the thickness of the film reaches the minimum film thickness by closest packing of the particles, represented by phase IV in Figure , T2,long sharply decreases. Apparently, the rigidity of the polymer increases due to a decreased hydroplasticization of polymer segments. At this point, clear differences appear between the various latex dispersions. ρlong decreases to lower values with decreasing MAA content. This is logical since carboxylic acid groups can bind high amounts of water (1.3 mol/mol −COOH and 11.3 mol/mol −COO–) and as such account for a high degree of hydroplasticization.[15]

The relaxometry data in Figure show that the mobility of the polymer hydrogens in the mobile phase decreases with increasing MAA content. For these latex dispersions, it was found that carboxylic acid groups form dimers, which can hold water.[24] Hence, this may be due to the formation of these “open” dimers of carboxylic acid groups. Polymer hydrogen mobility is still limited, however, due to this dimerization.[37,38]

Conclusions

Acrylic copolymers with glass-transition temperature Tg values above 23 °C were film-formed at that temperature because of hydroplasticization resulting from the presence of MAA monomers. This hydroplasticization during film formation and film hardening of acrylic latex dispersions with different concentrations of methacrylic acid (MAA), ranging from 2 to 20 wt % on total solid content of the dispersions, was studied at various RH values by NMR techniques.

It is found that the polymer MAA content affects the film formation process of the latex dispersions. The drying rate E decreases with increasing MAA content. In a later stage of the drying process, when the latex film thickness approaches its minimum thickness, rigidity of the polymers increases. Due to evaporation of residual water that enabled hydroplasticization of the polymers, polymer segments become less mobile.

It is shown that varying the RH of the climate chamber, and hence the drying rate, does not affect the water–polymer interaction of the various latex dispersions during drying. Hydrogen mobilities at similar hydrogen densities, however, appear to decrease with increasing carboxylic acid content. This phenomenon is explained by the hydrogen-bonding interaction between water and carboxylic acid groups, resulting in an overall decrease of hydrogen mobility in the dispersion.

This work shows that polymers with hydrophilic functional groups can be used to form films at room temperature from polymers that are glassy when dry. Because the hydroplasticized polymer phase hardens as the water evaporates and water is not a volatile organic component (VOC), this material offers a way to solve the film formation dilemma. Although the typical MAA content of a latex for paints is between 2 and 5 wt % on total solids,[20] the results are a viable starting point for further development of fully hydroplasticizable binders.