Mikrophasenseparation von photo-vernetzbaren Blockcopolymeren in dünnen Filmen

Abstract

The micro-phase behavior of diblock copolymers with integrated UV-crosslinkable units has been investigated by atomic force microscopy (AFM) as well as their mechanical properties. Further, the influence of crosslinkers in the self-assembly in block copolymers was studied. The phase separating block copolymer systems were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. One component of the block copolymers were decorated with coumarin derivatives as UV crosslinkable units. In the first investigation of RAFT copolymerization of coumarin derivatives and various monomers, the combination of methyl methacrylate (MMA) and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin (CMA) turned out to be the most successful type due to the molar mass control. To favor the micro-phase separation (MPS), the resulting copolymer (macroRAFT agent) was copolymerized with n butylacrylate (BA) or butyl methacrylate (BMA) to synthesize block copolymer systems consisting of blocks with contrarily properties and also varying block lengths and molecular weights. The micro-phase separation of block copolymer systems was investigated by AFM. The versatile morphologies like spheres, cylinders and perforated lamellas depend on the composition of the block copolymers. The deciding annealing process for the formation of the equilibrium morphology takes at least 12 h at a temperature of 160 °C. The results of dynamic mechanical analysis (DMA) provide further indication for phase separation of the polymers whereby for the respective block copolymers two glass transition temperatures were determined. For the local and global elastic moduli, which were determined by AFM and DMA, a good agreement was found. The investigation of the MPS of the block copolymers with larger molecular weights by in situ AFM measurements at temperatures up to 210 °C showed a stable phase separated state of the system. In contrast, for the block copolymers with smaller molecular weights and same composition an order disorder transition at temperatures T > 140 °C has been found. The reversible crosslinking of the block copolymers is carried out by UV irradiation of the incorporated coumarin units that undergo a [2+2]-cycloaddition. The successful dimerization and cleavage was investigated by AFM and UV/Vis spectroscopy. The UV irradiation of the block copolymers showed a high efficiency for both reactions at a temperature of 125 °C. The crosslinking of the block copolymers leads to an increase in the moduli of elasticity with increasing irradiation time. Additionally, it could be shown that the self-assembly is prevented in the block copolymers by the crosslinking reaction and thereby the system can be transferred in a non-equilibrium state.