Reversible Stress Whitening in PTMG / PLLA ‐Based Polyurethanes

ABSTRACT Reversible stress whitening was observed in polymers such as natural rubber and is generally attributed to strain‐induced crystallization (SIC). Under mechanical deformation, polymer chains become highly oriented, leading to nonequilibrium crystallization, which can subsequently revert to an amorphous state upon the release of strain. In this work, reversible stress whitening in poly( l ‐lactide) (PLLA)‐poly(tetramethylene glycol) (PTMG) mixed‐soft‐segment polyurethanes was investigated through tensile testing, optical observation, and in situ synchrotron small‐ and wide‐angle X‐ray scattering (SAXS/WAXS). The PLLA content in the mixed soft segments was varied from 0 to 41 wt%. A second yielding point accompanied by stress whitening was observed when the PLLA content exceeded 23 wt%. Under large deformation, the 2D SAXS patterns of 23%‐PU, 34%‐PU, and 41%‐PU exhibited pronounced anisotropy, with a significant increase in scattering intensity occurring near the onset of stress whitening. WAXS measurements revealed that the diffraction peaks of PLLA crystal gradually disappeared with the increasing strain. When the degree of orientation exceeds 0.17–0.18, the cavitation phenomenon appeared. Upon unloading, both SAXS and WAXS profiles recovered to their original states. These results probably reveal a reversible strain‐induced cavitation behavior of PLLA segments confined within polyurethane soft matrix.