EFFECT OF MICRODOMAIN STRUCTURE ON THE ORDER-DISORDER TRANSITION-TEMPERATURE OF POLYSTYRENE-BLOCK-POLYISOPRENE-BLOCK-POLYSTYRENE COPOLYMERS

The effect of microdomain structure on the order-disorder transition temperature (Tr) of polystyrene-block-polyisoprene- block -polystyrene (SIS) copolymers was investigated. For the study, we synthesized, via anionic polymerization, three SIS block copolymers with lamellar, cylindrical, and spherical microdomain structures. The SIS block copolymers were determined to have the following segment molecular weights: (a) 7400S-99000I-7400S for sample A, (b) 5850S-27300I-5850S for sample B, and (c) 7300S-13400I-7300S for sample C. Transmission electron microscopy has identified that sample A has a spherical microdomain structure, sample B has a cylindrical microdomain structure, and sample C has a lamellar microdomain structure. Using logarithmic plots of the storage modulus G’ versus the loss modulus G”, obtained by dynamic viscoelastic measurements with a Model R16 Weissenberg rheogoniometer, we found that sample A has a Tr of 180 °C, sample B has a Tr of 90 °C, and sample C has a Tr of 105 °C. The Tr of sample A was also determined by small-angle X-ray scattering by Professor T. Hashimoto at Kyoto University and was found to be about 180 °C, which is in remarkably good agreement with that determined by the log G’ versus log G” plots. Also, we found that the plot of log G’ versus temperature at an angular frequency of 0.15 rad/s shows an abrupt decrease in G’ at about 105 °C for sample C, but no such trend was observed for samples A and B. This observation was confirmed by independent measurements with a Rheometrics dynamic spectrometer, i.e., by temperature scans of storage modulus G’ at angular frequencies of 0.15 and 0.05 rad/s. This had led us to conclude that temperature scans of G’ do not allow us to determine the Tr of samples A and B, whereas log G’ versus log G” plots do. Finally, the values of Tr determined from log G’ versus log G” plots for each of the SIS block copolymers are compared with theoretical predictions by the currently held theories of Helfand-Wasserman and Leibler. It is found that the accuracy of the theoretical predictions of Tr depends very much on the accuracy of the tempera’ture dependency of the interaction parameter χ for a given polymer pair. © 1990, American Chemical Society. All rights reserved.