PAA-b-PPO-b-PAA triblock copolymers with enhanced phase separation and inverse order-to-order phase transition upon increasing temperature

Enhanced phase separation and inverse order-to-order phase transition with increasing temperature was observed for a series of poly (acrylic acid)-b-poly (propylene oxide)-b-poly (acrylic acid) triblock copolymers with various block lengths by temperature-variable synchrotron small-angle X-ray scattering (SAXS). This abnormal phase behavior was attributed to the change of hydrogen (H)-bonding interaction in block copolymers upon increasing temperature and theoretically explained from thermodynamic viewpoint. The temperature-dependent FTIR analysis revealed that increasing temperature led to the weakening of overall H-bonding interactions and the increase of free C-O and C-O-C groups. The Flory-Huggins interaction parameter chi of PAA and PPO blocks was positive and increased with increasing temperature as determined from the SAXS profiles of disordered block copolymer. The thermodynamic calculation indicated that the increase of Delta H was larger than the decrease of -T Delta S with increasing temperature because of the relatively large value of chi, leading to the increase of overall Delta G(mix) and hence the enhanced phase separation at higher temperature.