No abstract

Atomic force microscopy has been used to image deformed latex films. The films were made from core/shell latex particles having a soft shell and a hard core so that when the films were formed, the continuous phase was composed of the shell polymer in which the hard cores formed long range hexagonal orderings. Upon small amounts of film elongation, linear necklaces of core particles, perpendicular to the elongation direction, were observed at the surface of the films. This observation, which is mainly due to matrix deformation and has been analyzed theoretically in the companion paper (preceding paper in this issue), can be easily understood. When the film is elongated, it becomes thinner in the direction perpendicular to the elongation; as a result, the core particles are pushed together in the direction perpendicular to the elongation, whereas, at the same time, the core particles are pulled apart along the direction of elongation. However, as the elongation increases further, AFM images show that, besides the matrix deformation process, another deformation mechanism, which is a geometrical rearrangement of the core particles, appears. The response of the film to the strain is then characterized by the appearance of breaks in the linear necklace of core particles, which now form zigzags or chevrons. Such a geometrical rearrangement of the core particles was anticipated from the failure of the theoretical analysis to account for the experimental strain-stress curves at large film elongations. Therefore, future theoretical analysis of the mechanical behavior at finite strain of coalesced core/shell latex films should take into account both deformation mechanisms.

Seeded emulsion polymerizations of vinyl acetate with and without poly(viny1 alcohol) were carried out in a batch reactor. ' h o types of poly(viny1 alcohol) (different molecular weights but the same degree of hydrolysis) have been investigated. In the absence of poly(viny1 alcohol) no new nucleation was observed, while in its presence new particles were formed, giving a bimodal particle size distribution. The number of new particles was greater with the higher-molecular-weight poly(viny1 alcohol). Compared with emulsifier-free seeded emulsion polymerization, the reaction rate increased when the higher-molecular-weight poly(viny1 alcohol) was used. However, the polymerization rate decreased when the lower-molecular-weight poly(viny1 alcohol) was applied. It was also shown that for seeded emulsion polymerization of vinyl acetate in the absence of poly(viny1 alcohol), the kinetics follows Smith-Ewart case 111 theory. These observations are discussed and a theory is proposed to explain the effect of macromolecular emulsifier on the nucleation of particles in seeded emulsion polymerization. This theory involves nucleation of new particles via precipitation of "copolymer" poly(viny1 alcohol)-poly(viny1 acetate) radicals in the water phase.

Uniaxial deformation at finite strain of coalesced core/shell (hard core/film-forming shell) latex films is investigated by means of micromechanical calculations. Elongation ratios, strain rate, and energy density distribution within the film are presented and confirm the strain amplification phenomenon well-known in the filled elastomer area. The important role of the core-shell interphase on the overall film mechanical behavior is stressed by the presented results. Moreover, strain-stress curves have been calculated without adjustable parameters and compared to experimental ones in order to gain substantial information about the deformation mechanism. It is then proposed that uniaxial deformation of coalesced core/shell latex films proceed through two simultaneous and/or successive mechanisms: isotropic matrix deformation and geometrical core rearrangement within the film. The stiffening of coalesced core/shell latex films appears therefore to be mainly due to mechanical effects.