Effects of Molecular Weight and Block Length Ratio on the Rheological Behavior of Low Molecular Weight Polystyrene-block-polyisoprene Copolymers in the Disordered State

Abstract

Effects of molecular weight (M) and block length ratio (Φ) on the rheological behavior of low molecular weight polystyrene-block-polyisoprene (SI diblock) copolymers in the disordered state were investigated. Here Φ is defined by N PS /(N PS + N PI ), where N PS and N PI are polymerization indices for PS and PI blocks, respectively. For the study, a series of SI diblock copolymers were synthesized via anionic polymerization, and their dynamic storage and loss moduli (G' and G) were measured as functions of angular frequency at various temperatures. The disordered state of each block copolymer was ascertained by observing that plots of log G' versus log G were independent of temperature and had a slope of 2 in the terminal region. Using log G' versus log G plots for the block copolymers in the disordered state, we were able to investigate the effects of M and Φ on their linear viscoelastic properties. We found that the melt elasticity, as judged from the values of G' in the log G' versus log G plots, increased with increasing M but decreased with increasing Φ. Reduced plots for dynamic storage modulus and dynamic viscosity were prepared with reference temperature, which was chosen by two different ways : (1) by choosing the temperature at an equal distance from the glass temperature of component polystyrene in each SI diblock copolymer and (2) by choosing the temperature at an equal distance from the order-disorder transition temperature of each SI diblock copolymer. We point out an urgent need for developing a comprehensive molecular theory predicting the rheological behavior of block copolymers.