Handbook of Modern Coating Technologies

Film formation of nanosized hard latex in soft polymer matrix: an excimer study

Our study aims to investigate how the latex film formation is and can be performed using the mixture of two types of latex, first of the noncompatible nanosized hard P-labeled nano-sized PS and second of soft poly(n-butyl acrylate) (PBA) through fluorescence and UVV tech-niques in consideration of hard/soft latex weight fraction [140]. The emulsion polymerization process creates the P-labeled nanosized (100 nm) hard PS particles (see Section 6.3.1.3). The samples of soft PBA latexes were prepared by semicontinuous process [156]. BA (99wt.%) and a small percentage of acrylate acid (1 wt.%) composed the synthesized core—shell lattice. These really monodisperse particles have in common the mean diameter (97 nm) and Tg (— 41 °C) lower than room temperature. Eight different blend films were prepared in the order of wt.% of PS (i.e., the values at room temperature of 100, 80, 60, 50, 30, 20 and 10) together with 10-min annealing at rising temperatures higher than Tg of PS within the range of 100°C—300°C. Monomer (IP) and excimer (IE) intensities for the P samples were estimated following the annealing process step by step to provide data for monitoring activity of film formation steps. Preannealing AFM images of each PS and PBA latex elements were dis¬played in Fig. 6—42A and B, respectively. In Fig. 6—42A, the failure to form a film with rough surface is illustrated so that PS particles were not reshaped and contrarily their original (spherical) shapes remained so.
Nevertheless, AFM image of pure PBA film (Fig. 6—42B) displays the film surface being completely flat and smooth. The continuous, voidless films can be achieved using these par-ticles form at room temperature [140].
Fig. 6—43A—C presents the monomer and excimer emission spectra of blend films with 100, 80, and 30 wt.% PS content, respectively, annealed for 10 min at higher temperatures. 
(A) (B) (C)

Wavelenght, X (nm) Wavelenght, X (nm) Wavelenght, X (nm)
FIGURE 6-43 Monomer (/P) and excimer (/E) spectra of (A) 100 wt.%, (B) 50 wt.%, and (C) 30 wt.% P-PS content blend films after being annealed at various temperatures for 10 min [140].

Annealing temperature, Annealing temperature,
T (°C) T (°C)
FIGURE 6-44 Plot of /E//P versus annealing temperature for various PS content blend films. Numbers on each curve represent PS content in the film. Here, Th is the healing temperature. PS, Polystyrene [140].

It is observed that IE increases for 100 and 50 wt.% PS content film with rising annealing tem-perature, T. By the way, the decreased monomer intensity, IP for the blend films depends on the computational performance with the excimer formation at the first stage. Further anneal-ing of the blends increased IE factor and decreased IP factor.
However, 30 wt.% PS content film never shows the excimer emission (IE), except for monomer emission (IP). Plotting IE/IP ratios corresponding to T are shown in Fig. 6—44, for the films with 50 and 10 wt.% PS content. It is found that when wt.% PS content is 50, IE/IP ratio first goes up to a maximum point at Th, then goes down as annealing is continued. This means that PS particles in the film undergo complete film formation.
As for 10 wt.% PS, the ratio of IE/IP first increases and then remains constant despite higher annealing temperature, which means that film formation is unavailable in such mixtures. That is why, the behavior of IE/IP for wt.% PS in the range of 50—100 which is appropriate to the complete film formation can be illustrated with schema in Fig. 6—18 (see Section 6.3.1.3).

Annealing temperature, Annealing temperature,
T (°C) T (°C)
FIGURE 6-45 Plot of Itr versus annealing temperatures for various PS content blend films. Numbers on each curve represent PS content in the film. Tv is the void closure temperature. PS, Polystyrene [140].

Fig. 6—45 includes the curve of transmitted light intensity, Itr corresponding to the annealing temperatures. When annealing, the transmitted light intensity, Itr for pure PS film (100 wt.%) commenced with decreasing, showed a minima at Tv and then increased again as previously discussed in Section 6.3.1.3. However, Itr values for PBA content blend films remain almost unchanged up to a particular temperature, then dramatically decreased. Furthermore Itr values of the mixture films are higher than those of pure PS film. In fact, the PS particles (100 nm) are smaller than the visible light photons, leading to nonscattering of it, light is scattered mostly by the voids and the interparticle interfaces in the film. In PS/PBA blend films, low Tg of PBA polymer forces to coalesce at room temperature without thermal aging (Fig. 6—42B) and cover hard PS particles by filling the interinterface voids. Thus, most of the light passing the sample without scattering results in higher Itr values unlike pure PS film. A specific temperature provides deformation and coalescence of the PS particles. The immisci- bility of PS and PBA polymers helps them undergo a phase separation process because of the break up and coarsening of the domains separated phase by phase on this step [137,138]. Despite sort of difference between the refractive indices of two polymers [157] (the spreads of 0.12), it is realized that lower Itr (turbidity) values primarily associate with such domains [158]
which are able to scatter the light. Fig. 6—46A—C has AFM images PS/PBA blend films annealed at 100°C, 170°C, and 250°C, respectively, which are supportive for those results. At 100° C (Fig. 6—46A), PS particles are in no way reshaped as much as observed.
A cluster of PS particles may potentially form close-packed domains for 100 and 80 wt.% PS content films. In 50 and 20 wt.% PS content films, the nano-PS particles are observed to be well-dispersed in the soft PBA matrix. It is evident that the addition of PBA may improve particle packing resulting in less void concentration in the film and higher Itr values are yielded compared with the pure nano-PS film. For the annealed films at 170°C (Fig. 6—46B), the surface of 100 wt.% PS content film appears completely flattened, which indicates that the polymer chain interdiffusion occurs. However, AFM topography of 80, 50, and 20 wt.% PS content films has a pitted structure of (around 100 nm sized) surface. Our observation demonstrates that nano-PS particles in these films were a bit deformed according to the orig-inal spherical shape despite the prevention from any contact with one other by the soft PBA phase. After the annealing at 250°C (Fig. 6—46C), in spite of a little air bubble the pure nano-PS film structurally exhibits regular and continuous surface. In the 80 wt.% PS content film, small pits tend to disappear. With this, it is suggested that further annealing allows for PS particles to keep in touch with each other and coalesce forming a continuous percolated network. Nevertheless, small pits combine and form larger separated domains for 50 wt.% PS
І
500.00nm 1.00*1.00^m
(C) 50
^ Л
500.00nm ' ГМхГОДцш
FIGURE 6-46 (A) AFM micrograph of 100, 80, 50, and 20 wt.% P-PS content blend films annealed at 100°C for 10 min. (B) AFM micrograph of 100, 80, 50, and 20 wt.% P-PS content blend films annealed at 170°C for 10 min. (C) AFM micrograph of 100, 80, 50, and 20 wt.% P-PS content blend films annealed at 250°C for 10 min. AFM, Atomic force microscopy [140].

FIGURE 6-46 (Continued)

FIGURE 6-46 (Continued)

FIGURE 6-47 Logarithmic plots of IE/IP data in Fig. 6-45 versus inverse of annealing temperatures (721) for the films annealed at 10 min time intervals. The slope of the linear relations produces AH and ДЕ values, listed in Table 6-7 [140].

content film. When the surrounding PBA phase keeps the domains isolated, they can make no contribution to film formation. All the hard particles were not percolated, and the avail-ability of such separated domains thus accounts for the significant differences in both IP and Itr curves.
As mentioned before, the increased IE/IP values are derived from the void closure pro¬cess, then Eq. (6.21) is used for the values of IE/IP at lower than Th for the mixture films. Fig. 6-47 illustrates the graph of the Ln(IE/IP) corresponding to T~1 and the information of AH activation energies were obtained as listed in Table 6-7.
It is observed that activation energies do not significantly change with increasing PBA content, that is, the heating requirement for one mole polymer to jump in the viscous flow remains the same despite varying compositions of mixture. It is determined that the average AH value (2.16 kcal/mol) estimated for blend systems accords with AH value (2.46 kcal/mol) already obtained for nanosized pure PS latex system (see Section 6.3.1.3). Accordingly it is suggested that the energy amount required for the viscous flow related to the nanosized PS particles is under no influence of blend composition.
At higher than Th, the decreased values of IE/IP is previously related to the disappearance of interparticle interface as presented in Fig. 6-44. The activation energy of backbone 
1(A)
OlrU X7.500
FIGURE 6-48 SEM micrograph of (A) NaMMT and (B) PS latex particles. NaMMT, Na-montmorillonite; PS, polystyrene; SEM, scanning electron microscopy [159].
motion, AE is produced by least squares method fitting the data in Fig. 6—48 to Eq. (6.23) as listed in Table 6—7. The average value is estimated as 8.15 kcal/mol. In this study, the AE value is produced at 8.15 kcal/mol for PS/PBA system, being smaller than the AE ( = 12.00 kcal/mol) of the nanosized pure PS system (see Section 6.3.1.3). Moreover, this is close to the healing activation energy, AEh ( = 9.00 kcal/mol) for pure PS system (the energy requirement for carrying out the healing process using minor chains). This result shows that the PS chains in PS/PBA mixing system can apply no interdiffusion across the junction sur¬face at all. The contacting of PS particles is avoided by soft PBA phase, and so interpenetra¬tion of PS chains in mixture films is constrained by soft matrix. Thus, AE value of the mixture system is lower, compared with pure PS film where PS chains mix and its interdiffu¬sion process ends in a complete manner.