The effect of thermal annealing on the thermal properties and molecular weight of a segmented polyurethane copolymer

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Abstract
The influence of thermal annealing on molecular weight, microphase mixing, and multiple melting behavior of a segmented block copolyurethane is reported. The material studied contained 55% of hard segment consisting of 4,4'‐diphenylmethane diisocyanate and butanediol, and a poly (propylene oxide) diol of molecular weight 2000 as the soft segment. The thermal stability of the materials was influenced greatly by the order‐disorder transition, estimated to occur at ca. 191°C. Upon annealing above this temperature, molecular weight increased rapidly as a result of chain branching reactions. Microphase separation increased under these conditions, while the degree of hard segment crystallinity decreased. Annealing below the order‐disorder transition temperature resulted in relatively small molecular weight increases for short annealing times, but large increases for annealing times greater than one hour. Glass transition temperature data for these thermal treatments was consistent with upper critical solution temperature behavior and selective solubility by hard segment sequence length according to the Koberstein‐Stein hard microdomain model. The critical hard segment sequence for segregation was estimated (for 30 min annealing) to contain ca. 5 diisocyanate residues at 80°C, ca. 8 residues at 185°C, and increased slowly with annealing time. © 1994 John Wiley & Sons, Inc.