Compression-Molded Polyurethane Block Copolymers. 1. Microdomain Morphology and Thermomechanical Properties

The microdomain structure and thermomechanical properties of a series of compressionmolded segmented polyurethane block copolymers are investigated as a function of hard-segment content. The crystallizable hard segments form from the reaction of 4,4′-diphenylmethane diisocyanate (MDI) with 1,4-butanediol (BDO). Soft segments are poly(oxypropylene) end-capped with poly (6oxyethylene). Polyurethanes containing 40% or less by weight of hard segment exhibit a discrete hard-segment morphology. Materials with higher amounts of hard segment show hard-segment microdomain structures consistent with the lamellar model of Koberstein and Stein. Melting behavior is found to be rate dependent, reflecting kinetic effects on microdomain structure during thermal cycling. At high heating rates, DSC thermograms exhibit a single high-temperature endotherm corresponding to the melting of an extended form of a MDI/BDO crystal. Multiple endotherms are observed at low DSC scan rates. For high hard-segment content specimens, catastrophic softening is coincident with the onset of the first high-temperature endotherm. The primary softening point for materials with discrete hard microdomains occurs at the soft microphase T_g. The soft microphase T_g is a minimum for a hard-segment content of 50%. The apparent hard microdomain T_g decreases monotonically with increasing hard-segment content and is consistent with heat capacity data for the soft microphase T_g which indicates increased incorporation of soft-segment material into the hard microdomain for higher hard-segment content materials. Small-angle X-ray scattering measurements of the specific interfacial area and estimates of the lamellar thickness suggest that the critical hard-segment length corresponds to a chain with 3-4 MDI residues. In this model, hard segments that are shorter than the critical length are assumed to be dissolved within the soft microphase. The degree of microphase separation generally improves with an increase in the hard-segment content, correlating well with the corresponding decrease in the fraction of hard segments of length below the critical length. Intersegmental mixing occurs primarily within the microphase; only a minor fraction of the mixing occurs within the diffuse microphase interphase.