ABA Type Triblock Copolymer Based on Mesogen-Jacketed Liquid Crystalline Polymer: Design, Synthesis, and Potential as Thermoplastic Elastomer

Abstract

We have designed and successfully prepared a well-defined ABA type triblock copolymer consisting of poly(n-butyl acrylate) as soft B block and poly{2,5-bis((4-methoxyphenyl)oxycarbonyl)styrene} (PMPCS) as hard A block with potential as liquid crystalline thermoplastic elastomer (LCTPE). The synthetic strategy is to obtain a difunctional poly(n-butyl acrylate) first and then use it as a macroinitiator for chain extension. Both steps employ atom transfer radical polymerization (ATRP). Kinetic study of the chain-extension polymerization demonstrates a well-controlled process. The resulted triblock copolymers have high molecular weights (Mn > 70 000) and low molecular weight distributions (Mw/Mn < 1.2). Their chemical structures are confirmed by nuclear magnetic resonance. Curves of differential scanning calorimetry indicate the immiscibility of the two components. Morphological investigations were made with transmission electrical microscopy and gave direct evidence of the existence of nanophase- separated structures with long-range order. After examinations by polarized optical microscopy, we found the triblock copolymers are LC only if the molecular weight of PMPCS block exceeds 20 000. Wide-angle X-ray diffraction measurements indicate a very stable nematic phase between glass transition of PMPCS block and decomposition. We found three samples are elastomeric. Preliminary mechanical tests were done on their films. Typical dynamic mechanical analysis and tensile tests prove that the sample has the potential as thermoplastic elastomers (TPEs). According to our best knowledge, the unique triblock copolymer is the first reported well-defined ABA type TPE imparting liquid crystallinity to the hard domains which serve as physical cross-links. Since the novel LC block copolymer has the rod-coil-rod conformation, it is also a new type of elastic rod-coil block copolymer.