MEMORY EFFECTS IN POLYMERS .5. PROCESSING HISTORY VERSUS THERMALLY-INDUCED SELF-ORIENTATION OF UNORIENTED POLY(CHLOROTRIFLUOROETHYLENE) FILMS

A peculiar phenomenon is reported whereby a melt-extruded, low-crystallinity, unoriented film of poly(chlorotrifluoroethylene) upon unconstrained thermal treatment, self-extends in the machine direction (MD) while shrinking along the transverse (TD) and normal/thickness (ND) directions. In addition to the expected increase in crystallinity, the annealing process leads to an unexpected development of crystalline orientation along the MD. This phenomenon is an example of ''processing-induced memory effects'' since it depends on the processing history of the starting film, e.g., melt-extrusion leads to the subject behavior whereas compression molding does not. We must mention that the melt-extruded films of poly( chlorotrifluoroethylene) are isotropic to start with, that is, MD and TD are indistinguishable prior to the annealing process. Furthermore, this phenomenon has not been observed for any other semicrystalline polymer and is believed to be the first citation for poly(chlorotrifluoroethylene) since its commercialization in 1957. Thermomechanical analysis (TMA) is the analytical technique that led to this novel phenomenon which was later substantiated by x-ray diffraction (XRD). (C) 1995 John Wiley & Sons, Inc.