Preparation of poly( L-lactic acid) honeycomb monolith structure by unidirectional freezing and freeze-drying

Honeycomb monolith structured porous poly( L-lactic acid) (PLLA) was successfully fabricated by combining pseudo steady state unidirectional freezing and freeze-drying techniques. Dehydrated 1,4-dioxane was used as a solvent for PLLA and the single-phase mixture was unidirectionally frozen by lowering the sample tube into liquid nitrogen at a constant rate. The 1,4-dioxane crystal was nucleated in the solution and grown in the freezing direction. Using the crystalline structure as a template, the PLLA was solidified and structured. Then, the crystal was allowed to sublimate by freeze-drying and the aligned porous structure, which has either smooth wall microtubes or ladder-like microtubes aligned along the freezing direction and a honeycomb structure in its vertical direction was prepared. The effects of polymer concentration, sample tube lowering rate and water contents on the aligned porous structure were thoroughly investigated. We found that the smoothness of the microtube wall and its interconnectivity could be controlled by the polymer concentration and water content in solution.