Dynamics of Disordered PI-PtBS Diblock Copolymer

Viscoelastic ($G^*$) and dielectric ($\varepsilon''$) data were examined for a LCST-type diblock copolymer composed of polyisoprene (PI; M = 53K) and poly(\textit{p}-\textit{tert}- butyl styrene) (PtBS; M = 42K) blocks disordered at $T \quad \le 120 \textrm{C}^{\circ}$. Only PI had the type-A dipole parallel along the chain backbone. Thus, the $\varepsilon''$ data reflected the global motion of the PI block, while the $G^*$ data detected the motion of the copolymer chain as a whole. Comparison of these data indicated that the PI block relaxed much faster than the PtBS block at low $T$ and the dynamic heterogeneity due to PtBS was effectively quenched to give a frictional nonuniformity for the PI block relaxation. The $\varepsilon''$ data were thermo-rheologically complex at low $T$, partly due to this nonuniformity. However, the block connectivity could have also led to the complexity. For testing this effect, the $\varepsilon''$ data were reduced at the iso- frictional state defined with respect to bulk PI. In this state, the $\varepsilon''$ data of the copolymer at low and high $T$, respectively, were close to the data for the star-branched and linear bulk PI. Thus, the PI block appeared to be effectively tethered in space at low $T$ thereby behaving similarly to the star arm while the PI block tended to move cooperatively with the PtBS block at high $T$ to behave similarly to the linear PI, which led to the complexity of the $\varepsilon''$ data. The PtBS block also exhibited the complexity (noted from the $G^*$ data), which was well correlated with the complexity of the PI block.