Factors influencing orientations of covalently-attached and doped aromatic groups in stretched polyethylene films

Linear polarizations have been measured for covalently attached and doped 9-anthryl and 1-pyrenyl groups residing in interior sites of stretched polyolefinic films. The influences of polymer crystallinity, the concentration of aromatic groups, and the length of the substituents attached to doped molecules or of the tethers to polymer chains of covalently attached species on the degree of polarization have been explored. The results demonstrate the utility of comparing orientational parameters from doped and covalently attached groups in analyzing the factors responsible for stretch-induced orientation. The anthryl and pyrenyl groups prefer to reside in interfacial regions more than amorphous regions even before film stretching, and the specificity of their orientations is determined by the nature of interactions with surrounding polymer chains. The magnitudes of orientation factors are dependent on polymer crystallinity and substituent or tether length, but are independent of aromatic group concentrations as long as they are low. There are significant differences between the orientations of doped and covalently attached groups of the same type due to the inability of the latter to translocate between site types during film stretching. The results, as interpreted in the context of current theories, demonstrate the necessity of crystallite surfaces (i.e., interfacial sites), but not stretching-induced translocation, for selective orientation of aromatic groups along the axis of stretching.