CHAIN MOBILITY IN POLYMER SYSTEM - ON THE BORDERLINE BETWEEN SOLID AND MELT - 1 - LAMELLAR DOUBLING DURING ANNEALING OF POLYETHYLENE

In-situ small-angle X-ray scattering (SAXS) and low-frequency Raman sp ectroscopy experiments have been performed to study the mobility and m otion of polymer chains upon annealing in a temperature range dose to but below the melting temperature. Ultrahigh molecular weight polyethy lene, UHMW-PE, was taken as the model polymer, which was crystallized from solution. Regularly stacked lamellar crystals could be obtained a fter drying the solution-crystallized films. SAXS studies revealed tha t, upon heating above 110 degrees C, the lamellar thickness (long peri od) increases to twice the initial value. This quantum increase was co nfirmed by transmission electron microscopy (TEM). By means of in-situ low-frequency Raman spectroscopy it was observed that during heating a shift occurs in the longitudinal acoustic (LA) Raman frequency to lo wer values, indicative of an increase in the thickness of the crystall ine core. The combined experimental observations indicate that thicken ing occurs via a mutual chain rearrangement between the adjacent lamel lae involving a sliding motion along the chain axis to a doubling of t he lamellar thickness. A model to explain the quantum increase, i.e., doubling, is proposed.