Microphase Separation and Rheological Properties of Polyurethane Melts. 2. Effect of Block Incompatibility on the Microstructure

This second paper in this series regarding the relationship between structure and viscoelastic properties of multiblock polyurethane elastomers presents the results of varying segmental incompatibility at fixed segment length and composition. Three series of amorphous polyurethanes with pendant trimethylammonium groups were synthesized. Quaternization of these groups with iodomethane caused segmental incompatibility to increase in a controlled fashion without significantly affecting the block length, composition, or molecular weight. This gave three series of polyurethanes with varying incompatibility; their microstructures have been characterized in this paper and their viscoelastic properties, in the accompanying paper. DSC experiments showed that increasing incompatibility caused microphase separation and that the polyurethanes ranged from almost homogeneous to highly microphase-separated. All polyurethanes showed a peak in their SAXS data. The peak intensity was independent of temperature for all materials studied except one, which showed a decrease in intensity with increasing temperature. This was found to be in accordance with a mean-field-like approach to a spinodal temperature that was well below room temperature, although this polyurethane showed strong microphase separation even at room temperature. In accordance with past research on polyurethanes, there was no evidence for long-range lamellar order in the microphase-separated structure for any polyurethane studied. This paper demonstrates that in such a situation rigorously distinguishing between the homogeneous and the microphase-separated states is a difficult task, and the concept of a binodal separating these two states is highly ambiguous.