Preparation of porous poly(L-lactic acid) honeycomb monolith structure by phase separation and unidirectional freezing.

A honeycomb monolith structure with micro/nanoscale porous walls was successfully fabricated in poly(l-lactic acid) (PLLA) by integrating polymer-solvent and polymer-polymer phase separations induced during a pseudosteady-state unidirectional freezing process. Poly(ethylene glycol) (PEG) and PLLA were dissolved in 1,4-dioxane to prepare a single phase polymer solution. The direction of freezing created a honeycomb monolith structure of PLLA/PEG polymers. Crystallization of the solvent reduced the solvent concentration and induced liquid-liquid phase separation during the unidirectional freezing. A sea-and-island morphology, where PEG domains were dispersed in the PLLA matrix, was developed, and pores were created in the channel walls of the honeycomb monolith structure by leaching out the PEG domain. The effects of the PEG molecular weight and the PLLA/PEG weight ratio on the aligned honeycomb structure and the pores in the channel walls were investigated. A ternary phase diagram for PLLA, PEG, and 1,4-dioxane was created from cloud point temperature measurements. Based on this phase diagram, hypotheses for the mechanism of the cellular-dendritic transition and the formation mechanism of the pores in the channel walls are proposed.