Coalesced core/shell latex films under elongation imaged by atomic force microscopy

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.