Effect of Block Copolymer Architecture on the Thermally Induced Swelling of Micelle-Containing Multilayer Thin Films
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We investigate the effect of block copolymer (BCP) molecular weight and connectivity on the morphology and time-temperature dependent swelling of thin film hydrogels created through layer-by-layer (LbL) assembly of BCP micelles with poly(acrylic acid). BCPs of poly(N,N-dimethylaminoethyl methacrylate) (D) and poly(propylene oxide) (P), a P-D diblock, a long D-P-D triblock, and a short D-P-D triblock copolymer, were compared in terms of their temperature response in solution and within micelle-polyelectrolyte multilayers (mPEMs). The critical micellization concentration and micellization temperature of the BCPs in solution, as well as the swelling transition temperature, Tstt, of the mPEMs, decreased with increasing P block length. AFM imaging of dry mPEMs shows regular dimpled surface structures that arise from surface relaxation of micelles. When the mPEMs are cooled below Tstt in water, the thin 200 nm films can swell reversibly between 3 and 6 times their dry thicknesses within 2 min. The degree of swelling ( = wet thickness/dry thickness) increases with undercooling (T = Tstt - T) and shows time dependencies related to T and the constituent BCP connectivity. While the diblock films swell uncontrollably and lose integrity within 30 min at T 6 C, the triblock copolymer multilayers are able to sustain steady values (in the range of 4-10) under equivalent conditions. The differences in dynamic swelling behavior originating from BCP architecture have important implications in their utility as temperature responsive surfaces. 2011 American Chemical Society.
