Latex composition distribution

The latexes were cleaned by ion exchange and serum replacement, and the number and type of surface groups were determined by conductometric titration. The molecular weight distributions of the polymers were determined by gel permeation chromatography. The stability of the latexes to added electrolyte was determined by spectrophotometry. The compositional distribution was determined by dynamic mechanical spectroscopy (Rheovibron) and differential scanning calorimetry, and the sequence distribution by C13 nuclear magnetic resonance. 

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

In these density profiles the latex particles, added before starting the experiment, migrate to that position in the cell where their density coincides with the density of the surrounding medium. The position of the particles can be recorded by schlieren optics or, if there is a particle density distribution, more precisely by scanning extinction measurements normally used for the characterization of proteins. Thus the density and extinction profile in the ultracentrifugation cell yield a criterion for the density distribution and hence, because of the correlation between chemical composition and particle density, a criterion for the composition distribution or heterogeneity of the latex particles. 

All latex samples prepared by emulsion polymerization are characterized by a broad distribution of molar masses, and in the case of copolymer latexes, a distribution of copolymer composition. Since the diffusion coefficient for a polymer depends upon both the chain length and the chemical structure, the polymers in any one film sample will be characterized by a rather broad distribution of Dcm values. Experiments to detormine in such systems actually yield a value averaged over the distribution, Dts. As will be seen below, since different components of the system contribute to the measured signal at different times, and the fastest diffusing species dominate the diffusion at early times, experi-mental values of Detr decrease with the ext t of interdiffiision. For such sanqiles, one is normally less int sted in the absoluie values of than in how extonal... 

This difference in copolymer composition distribution was confirmed by transmission electron microscopy of microtomed sections stained with hydrazine and osmium tetroxide to show much larger butyl acrylate-rich particle cores for the semi-continuous latexes (39). 

Figure 4.9 Copolymer composition distribution of the BA/MMA latex produced in seeded batch emulsion copolymerisation (a) initial molar ratio of BA and MMA is one (b) initial molar ratio of BA and MMA is nine.

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

The polymerization of a mixture of more than one monomer leads to copolymers if two monomers are involved and to terpolymers in the case of three monomers. At low conversions, the composition of the polymer that forms from just two monomers depends on the reactivity of the free radical formed from one monomer toward the other monomer or the free radical chain of the second monomer as well as toward its own monomer and its free radical chain. As the process continues, the monomer composition changes continually and the nature of the monomer distribution in the polymer chains changes. It is beyond the scope of this laboratory manual to discuss the complexity of reactivity ratios in copolymerization. It should be pointed out that the formation of terpolymers is even more complex from the theoretical standpoint. This does not mean that such terpolymers cannot be prepared and applied to practical situations. In fact, Experiment 5 is an example of the preparation of a terpolymer latex that has been suggested for use as an exterior protective coating. 

Composites can also be prepared by electropolymerization from solutions containing dissolved polymer 307). Since films of polypyrrole or polythiophene are normally porous, it seems most likely that the dissolved polymer is simply entrained in the pores. Similarly, composites have been prepared by polymerization of pyrrole in the presence of acrylic latex, giving blends with 10-30 % polypyrrole that are conducting yet processable 808). Presumably the polypyrrole is distributed throughout the latex particles. 

For the development, production, and application of polymer latices the determination of the size distribution and the analysis of the chemical composition and heterogeneity of the latex particles are important. The size distribution can be determined rapidly by ultracentrifugation, electron microscopy or light scattering > but for the analysis of the... 

In Figure 5 the results obtained by rapid density gradient centrifugation of a polymer latex of unknown composition are shown. The gradient system consists of pure H2O and D2O. The density distribution of the particles has two broad peaks at the density values... 

Thus the density and the density distribution and consequently the chemical composition and heterogeneity and also the cross-linking of polymer latex particles can be determined in a few minutes by rapid density gradient centrifugation. 

This microscopic difference in the copolymer composition could influence the particle morphology, especially the distribution of the carboxyl groups within the latex particle, which in turn could be expected to influence the alkali-swelling behavior. 

Although MAA monomer possesses a larger reactivity ratio than MMA monomer, more MAA was found to exist in the outer side of the particle in the batch latex, as shown in Figures 5 and 6. This behavior could be explained if one can accept the fact that the MAA-rich polymers, which are formed early on during the polymerization, can migrate to the surface of the particle due to their higher hydrophilicity and plasticization of the polymer with the monomer. In the semi-continuous process, it could be expected that copolymer with the same composition as the comonomer feed is formed, and the particle contains a uniform distribution of carboxyl groups. 

Complex colloids can be characterized advantageously by a combination of Fl-FFF with different analytical or other FFF techniques, yielding supplemental information. Examples reported in the literature are combinations of Fl-FFF and S-FFF for size (Fl-FFF) and density (S-FFF) as well as the thickness and density of the shell of core shell latexes [402],El-FFF for the charge and composition of emulsions [403],Th-FFF for the characterization of the size and composition of core shell latexes [404] and, finally, with SEC for the particle size distribution and stoichiometry of gelatin complexes with poly(styrene sulfonate) and poly(2-acrylamido-2-methylpropane sulfonate) [405]. 

In the semibatch experiments, the particle size distributions of the final latexes were affected by the residual surfactant in the seed latex, which tended to facilitate homogeneous nucleation during the entire feed period. The monomer feedrate determined the polymerization rate and had little effect on copolymer composition. The polymer compositions for the runs with different monomer feeding modes tended to be identical at very low feedrate. 

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