Kinetics of the deformation induced memory effect in polyamide-6

Nascent polyamide-6 shows a peculiar and irreversible effect; the quiescent crystallization kinetics on cooling are accelerated upon deformation in the melt, even after full relaxation of the melt. This phenomenon, known as the orientation (or better, deformation) induced memory effect of polyamide materials, is explored in a step-by-step manner using in-situ wide-angle X-ray diffraction. For this purpose, unique polyamide-6 samples were used, which were created by compression moulding the virgin pellets just below the average melting temperature, defined by the peak position in a DSC-curve. The experimental data shows that only the imposed strain level controls the irreversible change in the quiescent crystallization kinetics. During cooling of the undeformed melt, an unusual crystallization behaviour is observed, i.e. the stable αα-phase converges into the less stable β-mesophase. Possible causes for the memory-effect are discussed and related to hypotheses from the literature.