MOLECULAR MECHANISM OF NECK FORMATION IN UNIAXIALLY STRETCHED POLY(ETHYLENE NAPHTHALATE) FILMS

When amorphous poly(ethylene-2,6-naphthalate) (PEN) sheets are stretch ed between the glass transition temperature and the cold crystallizati on temperature, they exhibit necking. The occurrence of this unusual n eck formation above the glass transition temperature causes large loca l fluctuations in the thickness of samples. However, uniformity of sam ple thickness can be re-established when the films are stretched beyon d the critical strain at which stress hardening due to crystallization starts to occur. To elucidate the mechanism of neck formation in film s stretched up to 20-30 degrees C above the glass transition temperatu re, a series of samples were prepared under selected conditions and we re sliced in the machine direction-normal direction (MD-ND) plane. The structural variations along the necked region were analysed using opt ical retardation, infra-red dichroism and micro-beam X-ray diffraction techniques. The results obtained from these techniques indicate that, during the course of deformation, the naphthalene groups, which are l arge and highly planar, rapidly align their broad faces parallel to th e surface of the flat him. This behaviour resembles a disorder-nematic order transition and occurs at highly localized regions in the sample , thereby manifesting itself macroscopically as a neck. The free-width uniaxially stretched films were found to exhibit uniplanar axial text ure instead of the expected transverse isotropy as a result of this pr eferential orientation of the naphthalene planes.