Temperature and Relative Humidity Dependency of Film Formation of

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Temperature and Relative Humidity Dependency of Film Formation of Polymeric Latex Dispersions Xuelian Chen, Stefan Fischer, and Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P. R. China

bS Supporting Information ABSTRACT: Thermogravimetric analysis and a synchrotron small-angle X-ray scattering technique were employed to characterize the structural evolution of a polymeric latex dispersion during the first three stages of film formation at different temperatures and relative humidities. Three intermediate stages were identified: (1) stage I*, (2) stage I**, and (3) stage II*. Stage I* is intermediate to the conventionally defined stages I and II, where latex particles began to crystallization. The change of drying temperature affects the location of the onset of ordering, whereas relative humidity does not. Stage I** is where the latex particles with their diffuse shell of counterions in the fcc structure are in contact with each other. The overlapping of these layers results in an acceleration of the lattice shrinkage due to a decrease of effective charges. Stage II* is where the latex particles, dried well above their Tg, are deformed and packed only partially during film formation due to incomplete evaporation of water in the latex film. This is because of a rapid deformation of the soft latex particles at the liquid/air interface so that a certain amount of water is unable to evaporate from the latex film effectively. For a latex dispersion dried at a temperature close to its minimum film formation temperature, the transition between stages II and III can be continuous because the latex particles deform at a much slower rate, providing sufficient surface area for water evaporation.

’ INTRODUCTION Latices are dispersions of polymeric particles in a suspending medium, usually water. They are important industrial products used as raw materials for paper coatings, adhesives, paints, and so on. Understanding the film formation process is thus essential and received extensive attention from both academic and industrial researchers.1
8 It is generally accepted that film formation during the drying of latex dispersions includes three main steps (Scheme 1): evaporation of water, which brings the particles into close contact; deformation of particles; and finally coalescence of particles by diffusion of polymer chains across particle boundaries and formation of a continuous film. Correspondingly, four stages (I, II, III, and IV) are defined marking the starting and ending of the three steps during film formation. According to the most common model of film formation in latex dispersions, deformation of particles can occur above the minimum film formation temperature (MFFT), which corresponds to the lowest possible temperature for particles to reduce the size of interstitial void to well below the wavelength of light. The mechanism of deformation of particles during film formation has received the most attention in the literature and is also most controversial. Brown proposed that the capillary force is the primary driving force for the deformation of particles and the polymer/air and the polymer/water interfacial tensions contribute to a certain extent.9 Vanderhoff claimed that particle deformation is due to the surface tension between the particles and the solvent, typically water.10 Dobler et al. similarly observed r 2011 American Chemical Society

Scheme 1. Three Main Stages of the Film Formation Process

particle compaction due to the polymer/water surface tension.11 Particle deformation driven by the polymer/air surface tension has initially been proposed by Dillon et al.12 and is supported by Sperry et al., who provided important experimental evidence that the presence of water is not essential for particle deformation at temperatures far below the glass transition temperature.13 This result was supported by Keddie et al.14 However, Sheetz argued that a skin may form at the air
film interface during film formation at high evaporation rates of water, creating a large osmotic pressure in the fluid below and thus causing compaction.15 Recently, Routh and Russel presented a comprehensive model for film formation by which many results of the earlier film formation studies can be explained.16,17 They stated that the evaporation rate of water has a large impact on the film formation process. This variable mainly depends on temperature and relative humidity. Received: June 19, 2011 Revised: September 23, 2011 Published: September 26, 2011 12807

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Figure 1. Small-angle X-ray scattering data of a latex dispersion. The dashed line shows the best fit using the Percus
Yevick model of particles having a radius of 69 nm and Gaussian-type particle size distribution of 3%.

Although the final properties of the polymeric latex film require a thorough understanding of the interplay between film formation and water evaporation, studies containing a comprehensive analysis spanning all stages are scarce. Keddie and coworkers studied the microstructure of acrylic latices during all four stages of film formation by using a combination of multipleangle-of-incidence ellipsometry (MAIE) and environmental SEM (ESEM).14 Their observation and analysis identified an additional stage (II*), intermediate to the conventionally defined stages II and III. Film formation from latex dispersions with varying concentration of sodium persulfate was studied via sorption balance by Erkselius et al.18 Their samples showed different types of film formation mechanisms, revealing large variations in critical volume fraction of polymer where the evaporation rate of water decreases strongly and total drying time, which was mainly due to differences in electrostatic stabilization. More advanced studies about mechanism of film formation have been performed, including studies on the variation of temperature and colloidal stability19
21 as well as the influence of surfactants22,23 and of the particle size24,25 on the film formation process. In this paper, we utilized a combination of thermogravimetric analysis (TGA) and synchrotron small-angle X-ray scattering (SAXS) techniques to follow the dynamic drying process of latex dispersion (ordering and deformation of particles) at various temperatures and relative humidities. The effects of temperature and relative humidity on the onset of latex particle ordering (crystallization), shrinkage rate of crystalline lattice, evaporation rate of water as well as amount of residual water after the deformation of latex particles have also been investigated.

’ EXPERIMENTAL SECTION Latex Properties. A styrene/n-butyl acrylate copolymer (PS-coBA, Tg = 20 °C) with about 50% fraction by weight of each monomer is used. It is a commercially available raw material for coating and adhesion applications produced by BASF China. The system was stabilized against coagulation by negative surface charges on the latex particles originating from polyelectrolyte. It additionally contains emulsifiers and some amount of electrolyte. The ζ-potential of the latex spheres was about
43 mV by measuring the latex dispersion at a concentration of 0.05 wt % using a ζ-potential analyzer (EF9
010, Brook Haven). Figure 1 shows a 1D SAXS intensity distribution of the latex dispersion with a solid content of ca. 28%. This curve can be satisfactorily fitted by

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Figure 2. Photograph of experimental setup with controlled temperature and relative humidity and the droplet attached to the cell window. considering a hard sphere system with a Percus
Yevick structural factor, including a polydispersity of latex particle size and instrumental smearing.26,27A radius of 69 nm with a 3% particle size distribution of Gaussian type is obtained from the fit. Sample Chamber and Setup. The sample chamber used in this work was a small aluminum box with two Kapton windows on the two parallel sides of the chamber for passing through of the X-ray beam. A small droplet of latex dispersion (about 10 μL) was placed onto one Kapton window by injection from inside of the chamber. A photograph showing the experimental cell including a latex drop for SAXS experiments was presented in Figure 2. To achieve temperature control, the chamber was equipped with four semiconductor chilling plates and a heating plate. The relative humidity in the chamber was regulated by using different saturated salt solutions and was measured by a humidity sensor. Characterization. The weight loss during film formation at various temperatures (relative humidity fixed at 60%) and relative humidities (temperature fixed at 25 °C) was followed by TGA (TGA/DSC 1 STARe of Mettler-Toledo). In order to minimize instrumental uncertainty during TGA experiments, approximately 85 mg of the latex dispersion with a solid content of ca. 30 wt % was used, which is much more than that used in SAXS measurements. The instrumental uncertainty comes from the fact that the reestablishment of the preset experimental temperature and relative humidity other than atmospheric condition after loading a sample in the TGA setup requires a certain period. In addition, the weight loss of a latex dispersion during drying at 14 °C was recorded by an analytic balance (BT 25S, Sartorius) with a precision of 0.01 mg, using a thermo- and humidistatic chamber (LHS100CB, Shanghai, China). In-situ synchrotron SAXS measurements were carried out at beamline BW4 at HASYLAB am DESY, Hamburg, Germany. The energy of the X-ray radiation was 8.979 keV, resulting in a wavelength of 0.13808 nm. The size of the primary X-ray beam at the sample position was 0.4  0.4 mm2. The sample chamber was mounted onto a translational stage at the beamline. To prevent the flow of the latex drop, approximately 10 μL of the latex dispersion with a solid content of ca. 30 wt % was placed onto the Kapton window in the sample chamber. The initial diameter of the drop is around 3.5 mm. The area probed by SAXS is in the middle of the drop and has a final thickness of approximately 0.5 mm. After drying, a ring-shaped deposit forms along the perimeter of the drop, indicating that water flow transports particles toward the periphery of the drop due to the pinning of the contact line of the drop. In-situ SAXS experiments took 1
2 h, depending on the drying condition. The sample-to-detector distance was 13 621 mm. The effective scattering vector q [=(4π/λ)sin θ, where 2θ is the scattering angle and λ the wavelength] at this distance ranges from 0.025 to 0.26 nm
1. The drying behavior of the dispersion has been studied by changing temperature and relative humidity. SAXS patterns were collected with an exposure time of 100 s every 120 s with a 2D MarCCD 12808

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the original weight of latex as a function of time elapsed since the deposition of the latex at different temperatures and relative humidities. Clearly, there is a pronounced increase in initial slope in the water loss curves with increasing drying temperature or decreasing humidity due to increasing evaporation rates. The initial slopes of the water loss curves for the latex dispersions are linear and show similar trends in drying behavior. This is because the particles are able to move freely, following Brownian motions. As a consequence, this initial stage is called the constant rate period. This stage is the longest of the three and lasts until the latex dispersion has reached approximately 60
70% volume fraction.28 After this constant rate period, the overall rate of water evaporation is much smaller than in the first stage due to irreversible contact and deformation of particles, which contributes to the decrease of surface area available to water evaporation. A sharp change in the evaporation rate marks the end of the intermediate stage and the beginning of the final stage. During this period, differences in the water content in the latex films submitted to different drying condition were found. This finding supports the notion that clear films are not necessarily completely dry but can contain water-filled domains smaller than the wavelength of light.29 Interestingly, the data presented in Figure 3b reveal that the remaining water content in the latex film obtained at 14 °C (below the polymer’s Tg) is lower compared to those dried above room temperature. The remaining water leaves the film mainly via any remaining interparticle channels formed due to membrane material (nonpolymeric materials such as salts, emulsifiers, and surfactants) at the interstices between the particles, which prevents the complete coalescence of adjacent particles at room temperature.30 Osmotic Pressure
Distance Dependence. The osmotic pressure of colloidal dispersions is measured as a resistance to a decrease in the volume available to the particles; this resistance originates from an increase of energy (interparticle force) or from a loss of entropy (configuration). In a closed sample cell, the osmotic pressure of the latex droplet on the substrate during water evaporation can be expressed as following:31   kT Pw ln Π¼
ð1Þ v P0

Figure 3. Time dependency of weight loss analyzed at various temperatures [(a) above room temperature and below room temperature (b)] and relative humidities (c). Inset: all the curves were enlarged for clarification of the difference of volume fraction at different conditions at the last stage. detector (2048  2048 pixels, pixel size 79.1 μm). The SAXS data were calibrated for background scattering and normalized with respect to the primary beam intensity.

’ RESULTS AND DISCUSSION Water Loss. The impact of temperature and relative humidity on evaporation rate of water during drying was studied by means of TGA. Figure 3 presents the variation of weight with respect to

where kT = 4.12  10
21 J at room temperature and v = 30  10
24 cm3 is the molecular volume of water.32 P0 is the saturation vapor pressure for the solvent at the same temperature. Pw is the vapor pressure of water surface, which has a relation with the evaporation rate of water. In a drying experiment the mass flow rate is proportional to the difference in vapor pressure between the water surface and the surrounding air and this can be written as18 J ¼ Kp AðPw
Pa Þ

ð2Þ

where J is the mass flow rate, Kp is the mass transfer coefficient depending not only on the fluid properties but also on the hydrodynamics of the gas phase. It can be determined by measuring the drying behavior of pure water cast on the substrate under the same conditions as those used for drying the latex dispersions. The evaporation rate was followed over time and the mass transfer coefficient (Kp) could be calculated. A is the exposed surface area, Pa is the vapor pressure of the air depending on both the temperature and the 12809

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Table 1. Peclet Number during Latex Film Formation under Various Conditions drying condition T (°C)/RH (%)

Ea (nm/s)

Peclet numberb

14/60

1

0.2

25/60 35/60

4 6

0.7 0.9

45/60

12

1.2

55/60

43

3.4

25/25

29

5.0

25/43

11

2.0

25/60

4

0.7

25/75

1

0.2

a

Evaporation rate of water is determined from TGA measurement under various conditions. b The values presented in the table were based on calculations according to eq 5 using R0 = 69 nm, H = 550 μm.

Figure 4. Osmotic pressure as a function of water layer film thickness between particles during drying at different temperatures under the condition of fixed relative humidity of 60% (a) and relative humidities under the condition of fixed temperature of 25 °C (b). Temperatures and relative humidities are indicated in the plot. Water layer film thickness is obtained from eq 2.

relative humidity:33 Pa ¼ RHP0

ð3Þ

where RH denotes the relative humidity. Since the latex dispersions initially contain roughly 30% volume fraction of particles, it is possible to calculate the thickness of interparticle water film. In the concentrated system, water film between particles is considered as a layer of thickness 2h, and a foamlike structure of connected locally flat water domains with the topology of a dodecahedron is considered. Hence, we have31 h¼R

ð1
ϕÞ ð2 þ ϕÞ

ð4Þ

where R is the radius of the average foam cell and ϕ is the volume fraction of solid. Osmotic pressures as a function of water layer film thickness at various temperatures and relative humdities are presented in Figure 4. It can be seen that the variation of osmotic pressure are not primarily different when the latex dispersion was dried at various temperatures and relative humidities. Initially, osmotic pressure rises gradually with a decrease of water layer film thickness, indicating that the interaction between particles can

be described by electrostatic repulsion of charged spheres due to the overlapping of their ionic clouds. This kind of interaction has been found in a PS latex system by a gradual increase of osmotic pressure with increasing volume fraction of latex particles, as was reported by Bonnet-Gonnet et al.34 However, there was a steep increase in osmotic pressure in a P(S-BA) latex when the volume fraction increases to approximately 40%, which was attributed to the strong resistance to dehydration between polyelectrolytes located on the particles. Furthermore, osmotic pressure of the system reaches a maximum value at a certain water layer film thickness. The location of maximum osmotic pressure at various conditions is determined by temperatures and relative humidities. Such an obvious difference in location of maximum osmotic pressure is explained by assuming that the deformation of latex particles during film formation is so fast that an amount of water remains in the film when drying at high temperature, as can be seen in Figure 4a. To visualize the variation of osmotic pressure during water evaporation, osmotic pressure as a function of water layer film thickness at various conditions in log
log scale is presented in the Supporting Information (Figure S1). During the formation of latex film, the distribution of particles is determined by the Peclet number, which is the ratio of the rates of evaporation and diffusion and reads35,36 Pe ¼ HE=Do ¼ 6πμHR0 E=kT

ð5Þ

where H is the initial film thickness, E is evaporation rate of water determined by TGA experiment depending on relative humidities and temperatures, Do is the Stokes
Einstein diffusion coefficient, and μ is the viscosity of water. The influence of evaporation rates on Pe during water evaporation is presented in Table 1. As mentioned earlier, the vertical distribution of latex particles during drying is described by the Pe, and accumulation of latex particles near the top surface is likely to occur with Pe > 1.37 In the present case, the data show that the Pe numbers were decreasing with decreasing temperature and increasing relative humidity. For the sufficiently low Pe numbers (i. e., Pe = 0.2 at 14 °C and 75%), the diffusion of latex particles in the dispersion is rapid enough that they are able to distribute uniformly during water evaporation. However, pinning of the contact line at the periphery of the drop during drying at low temperature and high relative humidity was observed. Crystalline Structure. The drying process of the latex dispersion under various conditions to solid films was followed by means of synchrotron SAXS. Integrated 1D SAXS intensity 12810

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Figure 5. Integrated 1D SAXS data plotted for various evaporation temperatures at a fixed relative humidity of 60% (a) and humidities at fixed temperature of 25 °C (b). All curves were converted from the last patterns of in situ SAXS experiments at various conditions. Solid lines represent the latex film with an fcc structure, dried at room temperature for 2 weeks, as a reference for the other curves. The curves were shifted vertically for the sake of clarity.

distributions of latex directly after being dried at different temperatures and relative humidities are presented in Figure 5. Distinct Bragg peaks can be clearly identified, and the position of the X-ray diffraction peaks indicates an fcc crystalline structure as expected.38 The Miller indices of the corresponding peaks are included in the plot. The solid lines represent a completely dried latex film, in which the latex particles were deformed to full extent after water evaporation for two weeks. The SAXS data show that the crystalline structure is preserved in the system even after film formation. The strong SAXS signal has its origin in the electron density difference between latex particles and membrane material surrounding the particles, which contains emulsifiers and salt. The particles in the film are separated by the membrane material, which prevents or retards interdiffusion of the polymeric chains between particles.39 When water has been evaporated at a higher rate (i.e., at 35 °C or at 25% relative humidity), one observes a broadening of the diffraction peaks, which indicates an increase in lattice faults during film formation compared to room temperature. This effect can be easily understood, since the high evaporation rate leads to a faster crystal growth, which in turn results in a less well-defined crystalline structure.40 Furthermore, a shift of the corresponding q positions to larger values with increasing temperature or decreasing relative humidity is clearly identifiable. This might be attributed to a difference in the degree of shrinkage of the crystalline lattice, meaning that the deformation of the latex particles proceeded to a lesser degree after film formation at various conditions compared to a completely dried latex film. However, the film dried at 14 °C does not follow this

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Figure 6. Time-dependent interparticle distance dried at various temperatures at fixed relative humidity of 60% (a) and at different relative humidities at fixed temperature of 25 °C (b). The insets are schematics of the structural changes within the colloidal crystallites. Crosses denote the location of transitions in the shrinkage rate of crystalline lattice.

trend. The positions of diffraction peaks of the resultant film are identical to those originated from fully dried latex film, implying a complete deformation of the particles in the latex film formed at 14 °C. To elucidate the relationship between the microstructure development and variation in water loss, the interparticle distance as a function of drying time at various drying conditions was investigated. Crystalline Structural Evolution during Drying Process. The interparticle distance in the colloidal crystals can be derived from the 1D scattering intensity distribution by using the equation d110 = 4π/q220. Figure 6 shows the interparticle distance plotted as a function of drying time. Three distinct regions in the curves can be identified according to the changes in their slope. The variation of the volume fraction of latex particles in the crystalline domains during water evaporation can be calculated after the starting of crystallization of latex particles according to38  V ¼ 74:1% 

d d110

3 ð6Þ

where 74.1% is the maximum volume fraction in a system of close packed spheres and d is the diameter of those spheres, which is equivalent to d110 at 74.1% volume fraction. One finds a pronounced change in volume fraction at the onset of crystallization from 0.407 to 0.304, when temperature increases from 14 to 55 °C at a constant relative humidity of 60%. However, at the condition of constant temperature, relative humidity has minor influence on the onset of crystallization of colloids during drying. 12811

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Langmuir Subsequently, the evolution of the interparticle distance shows that the shrinkage rates of lattice at various drying condition were almost constant until a transition point denoted by crosses in Figure 6 is reached. The rate of crystalline lattice shrinkage is influenced by the evaporation rate of water, which is determined by drying temperature and humidity. Over this transition point, there is a pronounced increase in the shrinkage rate of lattices with respect to the initial shrinkage rate observed before this transition point. Clearly, for the sample dried at a temperature higher than MFFT, the shrinkage rate of crystalline lattice keeps constant until interparticle distances are close to approximately 135
125 nm, indicating that the corresponding volume fraction are larger than the volume fraction of close-packed structure (74%). However, when sample dried at a temperature of 14 °C, which is close to the MFFT, the lattice shrinks to the volume fraction of 74% at a constant rate, followed by the particle deformation during water evaporation. In the last region, the shrinkage rate of the lattice drops to nearly zero. Interestingly, only in the case of the latex dispersion dried at 14 °C did the particles form a completely deformed close-packed structure, which is identified by comparing the interparticle distance between this condition and sample dried for 2 weeks. However, one finds obvious differences in the interparticle distance at different drying conditions, demonstrating that the deformation of the latex particles proceeds to different extents. Further gradual compaction and deformation of particles in the films have been accomplished via water evaporation over a long enough drying time. In that case identical positions of the Bragg peaks for all of latex films can be detected, suggesting that the films are fully dried up (data not shown). Mechanism of Film Formation. We shall discuss the data in Figure 6 in terms of the four stages of film formation as mentioned in the Introduction. Obviously, stage IV cannot be observed in the current case, due to the remaining electron density difference between deformed spheres consisting of polymers and membrane material (nonpolymeric additives in the system such as salt, emulsifiers, and surfactants) locating in the interstices of deformed particles. Thus, only the first three stages of film formation will be discussed. The system always starts from stage I, where latex particles are homogeneously dispersed in water. As a result, the SAXS experiment showed no distinct Bragg peak in this stage. As the water evaporation proceeds, the SAXS data clearly show that colloidal crystallization occurs far before a close packing of the latex particles is reached. The system thus transforms from stage I to an ordered state before reaching the conventionally defined stage II. Consequently, this intermediate state marking the onset of ordering will be referred to as stage I* from now on. In the present case, the minimum volume fraction to induce crystallization of particles at the water
air interface during water evaporation is influenced by temperature. With increasing the temperature, there is a pronounced shift in minimum volume fraction from 0.407 to 0.304. In order to elucidate the pronounced change in volume fraction at the onset of crystallization at different temperatures, temperature-dependent SAXS data of latex dispersion of about 30 wt % are presented in the Supporting Information (Figure S2). The data show that a shoulder in the spectra at 14 °C around q = 0.06 nm
1 could be detected, indicating a much stronger interaction at 14 °C than at higher temperatures. Furthermore, a stronger depression at low q at 14 °C than at 55 °C also demonstrates that the particle interaction at 14 °C is stronger than at other temperatures. As a consequence, the lower volume

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fraction to induce a crystallization driven by equilibrium forces should be observed when the latex dispersion was dried at 14 °C, which is contrary to the minimum volume fraction referring to the onset of crystallization obtained from Figure 6a. As was discussed in the last section, even the Peclet number is very low in present case, and pinning of the contact line at the periphery of the drop during drying causes the kinetics of drying to not simply be driven by equilibrium forces. The constant shrinkage rate of crystalline lattices after stage I* may relate to the water evaporation rate. It was established that ordered particles at water
air interface can approach each other by an increase of attractive capillary force between them. Such an increase in capillary force owing to the decrease of local curvature is caused by evaporation of water from this region. Consequently, the shrinkage rate of lattice is determined by water evaporation rate, which is supposed to be a constant at a given temperature and relative humidity. Hence, it should be understood that the shrinkage rate of lattice decreases gradually with increasing relative humidity or decreasing temperature. A close inspection of the data presented in Figure 6 reveals another intermediate stage (I**) where the shrinkage of the crystalline lattice began to accelerate before reaching stage II. Similar phenomenon was reported by Wong et al., who observed a gradual shrinkage in the lattice over time followed by a sudden lattice shrinkage to reach the close-packed structure during the formation of colloidal crystals in water.41 The sudden decrease of the interparticle distance corresponds well to the onset of overlapping of electric double layers for the colloidal suspension, which have a thickness on the order of the Debye screening length (1/k). In the case of a symmetrical z:z electrolyte, the Debye screening length is given by34,42 k
2 ¼

εkT 2e2 z2 n0

ð7Þ

where ε is the electrical permittivity, e is the elementary charge, z is the valency of the electrolyte, k is the Boltzmann constant, T is the temperature, and n0 is the bulk concentration of the electrolyte. At room temperature with 1:1 electrolyte at molar concentration Co, the Debye length is given by 0.304/Co (nm). The estimated Debye length of 10 nm agrees well with our result of 11.2 nm at 25 °C obtained from the transition point at stage I** at this temperature. The thickness of electric double layer depends on the ionic strength of electrolyte in the colloidal dispersion. Since overlapping of the double layers of latex particles occurs due to further water evaporation, the contribution from electrostatic repulsive interaction between particles can be gradually weakened, resulting from the decrease of effective charges (including changes from the double layer and surface of particles) accompanied by increasing osmotic pressure. The influence of the interparticle distance on the surface charge regulation and interaction between particles has been thoroughly studied.43,44 Clearly, the pronounced increase in the rate of lattice shrinkage upon water evaporation in the present case can be attributed to the decrease of effective charges caused by the condensation of electric double layer around the particle surface. After this intermediate stage, stage II with a close-packed array with water-filled interstices is reached, as can be seen from the dashed line in Figure 6a. It has been reported that the second step of film formation begins at the point of irreversible contact of the particles (stage II) and ends with particle deformation to form a densely 12812

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Scheme 2. Schematic Presentation of the Film Formation Process during Water Evaporation

packed structure (stage III). However, our data show that the deformation of latex particles proceeds to different extents at different drying temperatures and relative humidities. This finding is explained as follows: the lattice shrinkage proceeds from stage I** to an intermediate stage (after stage II) within a much shorter time for the latex dried well above its Tg in comparison to one observed at 14 °C, implying that particle deformation and the condensation of electric double layer may proceed simultaneously. The proposed notion can be explained as follows: first, stage II is unable to be identified due to fast lattice shrinkage from stage I** (the volume fraction of solid is about 50%) to an intermediate stage when the latex dried above its Tg, including the lattice dried at various relative humidities compared to one observed at temperature of 14 °C. The differences in the shrinkage rate of lattice from stage I** to an intermediate stage at various temperatures and relative humidities obey the kinetics of deformation of particles, which is known to be a function of polymer’s Tg. These all support our notion that lattice shrinkage from stage I** to an intermediate stage was controlled by particle deformation during drying. As a result, an incomplete deformation of particles can be attributed to the presence of residual water in the film resulting from very fast deformation of particles. This can also be evidenced by the TGA data on the difference in the water layer thickness at the location of maximum osmotic pressure under different conditions. It should be mentioned that there is evidence that water is evaporated primarily through the optically clear regions.45 As a consequence, the small amount of water must be retained in the interstice regions of films created at high evaporation rates. In the present case, the deformation of latex particles proceeded to different extents, resulting in a latex film containing a certain amount of water, depending on the evaporation rate, which is controlled by drying temperature and relative humidity. Unlike in stage III, the system in this stage is not completely dry and the particles are not completely deformed to a polyhedral shape. We shall refer to this intermediate stage as II*, after the finding of Keddie et al., where a similar result has been gained by using MAIE and ESEM to investigate the kinetics of film formation of latices of different Tg.14 The rate of the transition from stage II* (partial particle deformation) to stage III depends on the glass transition of the latex. Interestingly, as can be seen in Figure 6a, the latex film obtained at 14 °C close to its MFFT has identical Bragg peaks compared to completely dried latex film. This indicates that there is no water left in the latex film obtained at 14 °C due to a very slow deformation of particles, allowing water to be evaporated from the optically clear

region effectively. For a latex dried at temperatures well above its Tg, the transition from stage II* to III proceeds on a much longer time scale. To clarify the process of film formation of latex, we present in Scheme 2 the stages discussed above during film formation.

’ CONCLUSIONS The main results from this experimental study can be summarized as follows. A simple experimental cell was designed that allows one to investigate dynamic drying process of latex dispersion at different temperatures and relative humidities by means of in situ synchrotron SAXS. In addition to the conventionally agreed four stages of film formation from latex dispersions, three intermediate stages (I*, I**, and II*) have been identified. Stage I* marks the onset of colloidal crystallization of latex particles. The volume fraction of latex particles at stage I* increases with increasing temperature, depending strongly on temperatures but not on relative humidities. As water evaporation proceeds, stage I**, where the colloid spheres with their diffuse layer of counterions are in contact with each other, was observed. The further overlapping of counterion clouds results in a decrease in effective charge. Densification and microstructural development continue in all latices after stage I**. When the drying temperature is higher enough than MFFT, the particles are able to deform at a much faster rate after coming into contact, so stage II was not observed in these latices. Furthermore, it was found that the particles deform to different extents under different conditions. This stage occurs during the transition between stages II and III and was marked as stage II*. Further evaporation of the remaining water in the latex film can drive the particles to deform further. Stage II* is not experienced when the latex is dried at 14 °C, i.e., a temperature lower than the polymers Tg, due to a much slower deformation rate of the particles under this condition, ensuring a complete evaporation of water. ’ ASSOCIATED CONTENT

bS

Supporting Information. A figure containing small angle X-ray scattering curves of latex dispersion of about 30 wt % at different temperature and a log
log plot giving variations of osmotic pressure as a function of water layer film thickness between particles during water evaporation at different conditions. This material is available free of charge via the Internet at http://pubs.acs.org/.

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’ AUTHOR INFORMATION Corresponding Author

*Tel: +86 431 85262907. Fax: +86 431 85262954. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (20874101, 51050110442 and 50921062), and HASYLAB project II-20080190. S.F. is supported by Chinese Academy of Sciences fellowship for young international scientists (Grant No. 2010Y1GB9). We thank Dr. Jens Rieger and Dr. Volodymyr Boyko (BASF SE) for helpful discussions and Dr. Jan Perlich and Dr. Rainer Gehrke for their assistance during SAXS measurements. The authors thank reviewer 1 for his/her insightful and helpful suggestions. ’ REFERENCES

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dx.doi.org/10.1021/la202300p |Langmuir 2011, 27, 12807–12814