Determination of initial and long-term microstructure changes in ultrahigh molecular weight polyethylene induced by drawing neat and pyrenyl modified films.
Deformation processes in gel-crystallized ultrahigh molecular weight polyethylene (UHMWPE) films with draw ratios (DR) as high as 96 have been investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and positron annihilation lifetime spectroscopy (PALS). In addition, low concentrations of pyrene molecules have been introduced at the time of film preparation from the gels or afterward by sorption after film preparation, and the polarization of their electronic absorption and fluorescence spectra at different draw ratios has been measured over a large temperature range extending to below the glass transition. The pyrene-doped films have been irradiated to introduce covalently attached 1-pyrenyl groups, and these films at two draw ratios have been employed to investigate over large temperature ranges (1) the steady-state fluorescence intensity and (2) the rates of diffusion of N,N-dimethylaniline (DMA). These data have been correlated with the XRD, DSC, and PALS information obtained on the unmodified films. On the basis of analyses of this body of information, a novel deformation model that explains the decreased crystallinity and increased mean free volumes in gel-crystallized UHMWPE at low draw ratios is proposed. It involves "stretch" and "flip" motions of microfibrils present in the undrawn films. The high crystallinity content and stiffer chains due to drawing UHMWPE films result in weak alpha- and beta-relaxation processes, slower diffusion of DMA than in undrawn films, and orientation factors for doped pyrene molecules that are constant over a large temperature range. The overall picture that emerges allows several aspects of the morphology of UHMWPE, a polymer of fundamental importance in materials research, to be understood.