Microphase Separation and Rheological Properties of Polyurethane Melts. 3. Effect of Block Incompatibility on the Viscoelastic Properties

Abstract

The accompanying paper described the structural characterization of polyurethane multi- block copolymers that varied block incompatibility at fixed block length and composition. Their linear viscoelastic properties are presented in this paper. Dynamic mechanical experiments in temperature sweep mode confirmed that the materials ranged from almost homogeneous to highly microphase- separated. Dynamic mechanical frequency sweep experiments showed a Rouse-like frequency response in all materials at high temperatures, including those polyurethanes that were highly microphase- separated. This is in stark contrast to the numerous reports on microphase-separated block copolymers that show nonliquidlike terminal behavior at low frequencies. We attribute this homopolymer-like response of microphase-separated polyurethanes to a lack of long-range order in their microphase-separated structure and regard it to be a crucial feature of most commercial multiblock copolymers. The apparent activation energy for terminal flow was found to be insensitive to the extent of microphase separation, indicating that the thermodynamic penalty N expected for chain motion in block copolymers plays an insignificant role in the dynamics of the present materials. We demonstrate that bare effects (primarily proximity to the Tg) and dynamic asymmetry (friction in one block much larger than in the other) play a major role in the dynamics of polyurethanes. The latter is expected to be especially important in the dynamics of elastomeric block copolymers whose blocks usually have widely separated Tg's.