Handbook of Modern Coating Technologies

Film formation using pure and mixed latexes using energy transfer method

In a two-stage process, the PMMA particles are used to prepare the mixtures for P- and N- labeled and/or pure N-labeled latex films on an individual basis [154]. On the first stage, MMA polymerization to low conversion in cyclohexane is performed with PIB having 2% iso- prene to support grafting. The production of graft copolymer as a dispersant helps to make 
dispersions at the second phase in polymerization, where MMA is polymerized in a cyclo-hexane dissolution of copolymer. The detailed information have been released in another paper [155]. The stability in the dispersions of spherical polymer latexes is achieved in the radii ranging from 0.5 to 0.7 and from 1 to 3 pm for both P- and N-labeled particles, respec-tively. A combination of 1H-NMR and UV analyses demonstrates that these latexes contain PIB of 6 mol.% and N of 0.37 mmol and P of 0.037 mmol/g of polymer, respectively. These particles are referred to N and P.
A specific technique is used to prepare the latex films. A dispersion with the equal amounts of N and P particles in heptane is put inside a test tube including solid content of 0.24% for the mixed films. The sample films are prepared using this dispersion with a specific number of drops fell on glass plates, providing with heptane evaporation. For pure films the preparation method is applied simply to disperse N particles in heptane, and this is repeated as for the mixed ones. At this point, the liquid dispersion from droplets is carefully got it to fill the whole surface area of the plate to evaporate the heptane. The samples are weighed pre- and postcast¬ing to estimate the film thickness and the mean value is computed as c. 20 pm. The pure latex films and their mixture are presented in Fig. 6—31A and B, respectively.
Following the exposure of both types of latex films to vapor of different mixtures of chlo-roform and heptane in seven trials the fluorescence lifetimes for N are followed throughout
the film formation induced by vaporization. In the current study, the pure (N) and mixed (N + P) samples were put into a quartz cell containing the blend of chloroform and heptane as placed at the bottom and illuminated using the excitation light (286 nm) and N fluores¬cence emission was detectible at 335 nm.
Energy transfer method: in the mixed (N + P) film, the naphthalene N is excited by inci¬dent light with intensity of I0, and then the following reactions are observed
N * 1P -!%N 1P* (6.34)
N* ! N 1 hvN (6.35)
P* ! P 1 hvp (6.36)

The energy transfers without radiation from the excited naphthalene N* to the pyrene P with the rate constant kET (the first mechanism). The excited naphthalene and pyrene are inverted to their original states via light emission at the frequencies of vN and vP (the second and third mechanisms). These are described for N* and P* using the time-dependent rate equations as follows:
drN *1
dt =10AN 1 (kfN 1 knjv) [N*] — kET[P][N*] (6.37)
drp*l
~dtT =10AP — {kfP 1 KP) [P*] 1 kET[P][N*] (6.38)
where kfN and kfP are the radiative rate constants for N and P fluorescence, and knN and knP the rate constants representing the corresponding nonradiative decay constants, respectively. The deactivation of the excited N and P molecules can be demonstrated with kN = kfN 1 knN and kP = kfP 1 knP, respectively. By the way, AN and AP are the Beer's law absorbances for N and P, respectively. These equations reveal that pyrene fluorescence has two source terms: direct excitation from the incident light and energy transfer from naphthalene. Following the excitation of naphthalene in the presence of pyrene by a 6-pulse of light, the naphthalene IN(t), and pyrene IP(t), the following equations work out for emission intensity decays
IN (t) = Aexp( — t/rN) (6.39)
Tv1 = ToN 1 kET[P] (6-40)
where kN = T0N and A = kfN[N*]0
Ip (t) = A1exp( — t/тр) 1 A2[exp( — t/Tv) — exp {t/тр)] (6.41)
where kP = тр1) and A1, A2 are the related preexponential factors. Eq. (6.39) defines the exponential decay of naphthalene, as observed in the experiments depending on the system under consideration, regardless of use of pyrene. 
To investigate the film formation induced by vaporization, the fluorescence decay curves are fitted to Eq. (6.39), and the values of A and TN are produced at each step of the process using the linear analysis of least squares.
Lifetimes: The TN values measured for the pure (N) and mixed (N + P) films made in the content of 60% chloroform are shown in Fig. 6—32A and B. In both case TN values exponen-tially decrease with rising te. For objectivity of these results, a Stern—Volmer quenching mechanism is suggested for the fluorescence decay of N in the two latex film types during film formation caused by vaporization, where Eq. (6.40) is applicable. For low efficiency of quenching, kET [M ] < < 1, Eq. (6.40) can now be rewritten as TN ^ T0N (1 2 kET[M]) (6.42) where [M] is the quencher concentration at te. At this point it is noted that [M] may be taken as vapor concentration of the pure (N) film without pyrene molecules. However, the quench-ing mechanisms in mixed (N + P) films may account for the behavior of pyrene molecules. In other words, chloroform vapor have great part in quenching process for the pure film. Furthermore the chloroform vapor has trivial effect in this mechanism for the mixed (N + P) film, where pyrene quenching is the dominant process when solvent molecules are featured as plasticizers. In the final situation [M] can be taken as pyrene concentration. The volume integration of Eq. (6.42) may offer the relationship between the lifetime of N and [M] in both films and the following equality is provided — = 1 - C — т 0N MN where C = T0kqMN/v while v is the swollen volume of the film. Eq. (6.30) is used for calcula-tion of the quantity of vapor and/or pyrene based on the differential volume. Both Eqs. (6.30) and (6.31) and an assumption that vapor proportionally penetrates through  plasticization depending on the chain interdiffusion; that is, the better vapor sorption leads to higher crossing density, and the following equation is written as [154]: 1 - — = Btl/2 (6.44) T0N where B = CR. Based on Eq. (6.44) the data in Fig. 6—32A and B are plotted in Fig. 6—33A and B, respectively. B values can be found with the slopes of the linear functions (R2 from 0.92 to 0.98) as shown in Fig. 6—33. The B values obtained for both pure and mixed films are plot¬ted corresponding to the chloroform fraction in Fig. 6—34. It is observed that B values are higher as the fraction is increasing. To interpret this behavior it is suggested that higher  FIGURE 6-35 SEM micrographs of (A) P, (B) N, and (C) (N + P) films before vapor exposure. N, Naphthalene; P, pyrene; SEM, scanning electron micrograph [154]. content of chloroform vapor helps polymer chains reptate at higher frequencies within the film formation processes, resulting in greater B values. It is very likely that the fast repta- tion of polymer chains makes B values higher due to higher crossing densities, that is, plas-ticization accelerates the film formation at the high content of chloroform vapor. Additionally it is found that the polymer chains reptate at a slower rate in the mixed films compared with the pure films. Such a behavior may account for poor mixtures of N and P particles in (N + P) film. Fig. 6—35A—C displays the SEM micrographs of P, N, and (N + P) films prior to exposure to vapor, respectively. Polymer chains can slowly reptate in the interparticle interface due to the dimensional disparities of N and P particles, which results in lower values of B in the mixed (N + P) film. 6.3.2 Film formation from blends of hard and soft latexes 6.3.2.1 Swelling of interpenetrating network like particles in a soft polymer matrix In this study small (P-labeled) and/or large (N-labeled) PMMA latexes were embedded into the low-molecular-weight PIB matrix at different compositions to prepare several films. The hard latexes were placed in the soft, low-molecular-weight PIB matrix, using PMMA parti-cles. In this study, these films were annealed at higher temperatures within equal periods to enable low-molecular-weight PIB homopolymers to penetrate into the interpenetrating PIB channels existing in the PMMA particles. The preparations of N- and P-labeled latex particles (P and N particles) accorded with the procedures discussed in Section 6.3.1.4.2. The graft PMMA molecules weighed Mw = 1.10 X 105 g/mol when its polydispersity level was 2.3. A quantity of N and/or P particles were embedded into a low-molecular-weight (Mw = 1.35 X 103 g/mol) PIB matrix to prepare nine different films on an individual basis. For this purpose, redispersion of each N and/or P particle in heptane solutions was carried out with higher quantities of PIB in nine test tubes. Nine different films were prepared using N and/or P particles in these different stock solutions with a same number of droplets onto the glass plates. Following heptane evaporation, it is necessary to help particles swell by annealing the films at higher temperatures within the range of 60°C—210°C at a 30-min interval to help particles swell. Formed using N and/or P particles the films individually include different quantities of latexes in the soft PIB matrix. The swelling process of hard latexes in a soft poly-mer matrix was controlled using fast transient fluorescence (FTRF) technique [155]. Fluorescence lifetimes, T, were quantified for each sample of N and P films, followed by illu-mination with excitation light (286 and 345 nm) to observe the fluorescence emissions at 335 and 395 nm, respectively. As presented in Fig. 6—36A (for N) and Fig. 6—36B (for P) the fluorescence decay profiles were constructed from the annealed latex films (80%) at different temperatures. The fluorescence decay curves were plotted using Eq. (6.24) to analyze the swelling processes of the latexes. Fig. 6—37A (for N) and Fig. 6—37B (for P) display decay curves, fitting to Eq. (6.24) the annealed films at 60° C, and T values were measured at each stage using least-squares method. These values for N at 60° C in Fig. 6—38A and for P at 200° C in Fig. 6—38B are plot-ted corresponding to PIB content. On the other hand the lifetimes of N and P (TN and TP) films altered only a bit with annealing temperature of 60° C. However when it reached to 200° C, T values decrease with greater PIB content for both types, which suggests that the excited states of N and P molecules could be more strongly quenched. Now the lower values of TN and TP may account for low-molecular-weight PIB chains having penetrated from the PIB matrix into the interpenetrating PIB channels in the latex particle. This process let latex particles swell through film annealing by sufficiently increasing the temperatures. Fig. 6—39A (before) and Fig. 6—39B (after) include the cartoon representa¬tions of the latexes pre- and postswelling [155]. (A) (B) FIGURE 6-36 Fluorescence decay profiles from (A) N and (B) P films containing 80% latex annealed at various temperatures. I is the intensity of N and/or P. The sharp peaked curve is the lamp profile. The numbers on each curve present the annealing temperature in °C. N, Naphthalene; P, pyrene [155].   (A) (B) FIGURE 6-37 The fits of decay curves of the (A) N and (B) P films annealed at 60°C according to the logarithmic form of Eq. (6.24). The sharp peaked curve is the lamp profile. N, Naphthalene; P, pyrene [155]. FIGURE 6-38 Plots of the (A) N lifetimes, TN and (B) P lifetimes, Tp, versus PIB content in the film for annealing temperatures of (I) 60°C and (II) 200°C. N, Naphthalene; P, pyrene; PIB, polyisobutylene [155].  Our observations were evaluated using a Stern—Volmer quenching mechanism in terms of the fluorescence decay of N and P following the completion of each step in annealing, where Eq. (6.27a) can be utilized, being rewritten as T 1 5 T21 1 kq[PIB] (6.45) The assumption is made that the single quencher is low-molecular-weight PIB for the excited N and P molecules, and T0 values are identified as 40 for N and 200 ns for P, respec-tively. For this, given that the [PIB]0 value is 7.41 X 10_3 M, kq is measured in pure PIB and computed as 3.1 X 106 for the N and 7.6 X 105/M s for the P. Based on Eq. (6.45) and at the given values of T, T0, and kq, the local concentration of PIB in the interpenetrating PIB channels offers an opportunity to make calculation following each step in annealing of a cer¬tain film. The trial results are plotted in Fig. 6—40A for the N and Fig. 6—40B for the P corre¬sponding to different PIB contents. As shown in Fig. 6—40 the local PIB content escalates with rising temperature of annealing; That is, PIB whose molecular weight is low can penetrate into the integrated PIB channels in the latex particle with rising annealing temperatures. Then it is noted that the local con¬centration of PIB in the large (N) particles was observed to be a bit greater than that in the small (P) particles while PIB film content ratio was 80%. The N particles with greater local PIB content included more low-molecular-weight PIB in the matrix, whereas for the P parti¬cles it was determined that local concentrations were almost similar in all the samples. It is very likely that as PIB chains increase the penetration into the PIB channels deeper in large (N) than in small (P) latexes is getting easier. For examination of the influence of the dimensions on the latexes' swelling, T values are graphed corresponding to the annealing temperatures in Fig. 6—41A for the N films and Fig. 6—41B for the P. It is found that the low-molecular-weight PIB content in the films dra-matically decreases T values higher than a specified temperature, which differentiates for the  FIGURE 6-40 The calculated local PIB concentrations in the interpenetrated PIB channels after each annealing step for the (A) N and (B) P films containing 60% low-molecular-weight PIB. N, Naphthalene; P, pyrene; PIB, polyisobutylene [155]. FIGURE 6-41 The plots of (A) TN and (B) TP versus annealing temperatures for the N and P samples with 50% low- molecular-weight PIB. N, Naphthalene; P, pyrene; PIB, polyisobutylene [155]. N and P samples. The 160°C is a thermal point where T values begin to decrease in the N sample while the corresponding point to the P samples display reduction in T values is around 120°C. As discussed in Section 6.3.1.4.2. The N-labeled particles (1—3 pm) are greater than the P-labeled particles (0.5—0.7 pm), thus throughout the annealing process the small (P) particles swell much easier than the large (N) particles. Namely, low-molecular-weight PIB not only penetrates but also quenches the excited P molecules almost at the shortest time compared with the excited N molecules [155]. The large N particles require higher annealing temperatures to swell and absorb the free PIB homopolymers deeper into their interpenetrating channels. Nevertheless, the pene-tration process in small P particles involves annealing at lower temperatures.  FIGURE 6-42 AFM images of (A) P-PS and (B) PBA latexes produced for this study. AFM, Atomic force microscopy; PBA, poly(n-butyl acrylate) [140].