Geometrical confining effects in compression molding of co-continuous polymer blends

Polymer blending is a versatile method for production of co-continuous porous materials. While numerous studies have been performed to elucidate the thermal annealing effects on the bulk structure, the effect of geometrical confinement is little understood. In the present work, possible effects from geometrical confinement during in-mold annealing were explored. A 50/50 wt.% poly (lactic acid)/polystyrene (PLA/PS) blend was compression molded and annealed between two parallel plates without being confined circumferentially during the compression molding process. Different conditions for geometrical confinement, including varied gap size and compression ratio (initial to final thickness ratio), as well as modified surface properties, were employed. The experimental results indicated that the gap size played a profound role in affecting the structural development; the phase size near the mold surface was smaller than away from the surface. The actual phase structure and the resulting gradient in pore size were further affected by the mold surface properties. Additionally, the compression ratio was found to affect the morphological development especially near the mold surface. At a high compression ratio, a thin layer of PLA was formed immediately during compression. The thickness of this layer either grew or reduced in size depending on the mold surface properties. Understanding of these geometrical confining effects and implementing them in processing may lead to the development of innovative porous materials.