Honeycomb structured porous films from amphiphilic block copolymers prepared via RAFT polymerization

Abstract

The synthesis of polystyrene-block-poly(N,N-dimethylacrylamide) (PS-b-PDMA) via RAFT polymerization was investigated in detail. Two different RAFT agents – benzyl dithiobenzoate and 3-(benzylsulfanylthiocarbonylsufanyl) propionic acid, were employed to prepare polystyrene macroRAFT agents with molecular weights varying between 3000gmol−1 and 62,000gmol−1 and polydispersities between 1.1 and 1.4. Chain extensions with N,N-dimethylacrylamide (DMA) were carried out using a constant monomer to RAFT agent concentration ([DMA]/[RAFT]=500), to compare the rate of polymerization in dependency of the polystyrene chain length. A decreasing rate of polymerization with increasing block length was observed. Depending on the sizes of the first block and type of RAFT agents used, chain extension polymerization with DMA was found to be incomplete, leading to significant low molecular weight tailing in the GPC analyses. Block copolymers prepared using 3-(benzylsulfanylthiocarbonylsufanyl) propionic acid, followed the expected molecular weight evolutions with polydispersity indices of 1.2–1.4. In contrast, block copolymers using benzyl dithiobenzoate clearly showed bimodal molecular weight distributions, especially when the longest PS macroRAFT agent with a molecular weight of 38,000gmol−1 was employed. These amphiphilic block copolymers were used to fabricate honeycomb structured porous films using the breath figure technique. The regularity of the film was considerably influenced by the humidity of the environment, which could be controlled by the rate of the airflow or the humidity in the casting chamber. The interaction between the hydrophilic block copolymer and the humidity was found responsible for the delicate equilibrium during the casting process, which prevented high pores regularity at very low (below 50%) and at elevated (above 80%) humidity. The interactions of the hydrophilic block with the humidity were observed to superimpose an additional nano-scaled order onto the hexagonal micron-sized porous array. Pores, which are created by encapsulation of water droplets, were found to be more hydrophilic than the surface. Confocal microscopy studies were employed to locate hydrophilic blocks within the film using a fluorescence labeled PDMA polymer.