Surface melting and lamella rearrangement process in linear low density polyethylene

The surface melting and lamella rearrangement process that happen in commercially available linear low-density polyethylene (LLDPE) were studied with special emphasis on conformational and crystal structural changes during the process. The thermal melting behavior of the specimens was examined by using differential scanning calorimetry and the occurrence of the surface melting was strongly suggested in low-crystallinity LLDPE (LLDPE-L) in the temperature range of 90-120 degrees C at the heating rate of 2 degrees C/min. The structural and conformational disordering process during heating was studied by IR spectroscopy combined with two-dimensional spectral correlation analysis. The formation of conformational defects has been determined to proceed prior to the overall disordering of orthorhombic crystalline structure in all the PE specimens except for LLDPE-L. During the heating process, conformational defects are preferably developed in side chains that are excluded from tightly packed orthorhombic crystal in PEs other than in LLDPE-L. The temperature dependency in the thickness of lamella layers of LLDPE-L examined by small-angle X-ray scattering indicates the occurrence of the lamella thickening in the temperature range where surface melting has been observed. In this way, the formation of new orthorhombic cells due to the lamella rearrangement dominates over the formation of conformational defects in LLDPE-L while other specimens indicate opposite. Wide-angle X-ray scattering shows significantly suppressed dilation of the unit cell in LLDPE-L, especially toward orthorhombic unit cell parameter a in the temperature range where lamella thickening process undergoes. It is implied that the lamella thickening happens not only by rearranging the chains originally exist in the interfacial region but also it incorporates those originally existing in the orthorhombic crystal phase in LLDPE-L. The heat-induced morphological changes are explored by means of polarized light microscope with liquid crystal universal compensator and circular polarizer (LC-PolScope). Unlike other PE specimen, high-crystallinity LLDPE (LLDPE-H) shows the retardance histogram with multiple peaks, indicating the presence of several types of higher order structure, such as aggregation within the specimen.