AbstractThe force versus temperature behavior (at constant length) of poly(p‐phenylene benzobisthiazole) fibers (PBT) and Kevlar fibers has been investigated. Thermal expansion coefficients are evaluated for heat treated PBT (PBT‐HT) and Kevlar 49, to be −1.1 × 10−6/°C and −3.2 × 10−6/°C, respectively. Kevlar 49, PBT‐HT, and steel fibers exhibit a linear thermal elastic behavior whereas Kevlar 29 and as‐spun PBT do not. Material changes which occur during heat treatment processing are detected in the force‐temperature profile. A correspondence between the thermal and mechanical energy input which induces these material c
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