ALIGNED SOLUTES IN STRETCHED POLYETHYLENE - UNIAXIALITY AND TEMPERATURE EFFECTS

Samples of aligned molecules may be produced by using stretched polyme rs as anisotropic solvents. Although the molecular alignment is rarely perfect and the orientation distribution function is not known, an ex act, simple, and useful mathematical description of the partial alignm ent is possible. The aligned samples may be studied by optical spectro scopy with linearly polarized light in order to gain information on th e properties of the molecular alignment, on the structure of the solut e molecules and their interaction with the solvent, or on spectral ass ignments of electronic and vibrational transitions in the solute molec ules. It is usually assumed that the alignment of solute molecules in stretched polymer sheets is uniaxial around the stretching direction, even in thin polyethylene sheets. The validity of this assumption was investigated and confirmed through a series of measurements, using dif ferent angles between the direction of the linearly polarized light be am and the plane of the stretched sheet, The degree of alignment of so lutes in stretched polyethylene is known to increase when the temperat ure is lowered from room to liquid nitrogen temperature. A systematica l study was carried out of the alignment of a solute molecule in stret ched polyethylene as a function of temperature. It showed that the mai n change in alignment takes place at temperatures around -10 degrees C . The change was associated with large improvements in alignment withi n the crystalline regions of polyethylene. As a practical consequence, the improved alignment, which was previously obtained by cooling to l iquid nitrogen temperature, may be easily available at more convenient temperatures. (C) 1997 Elsevier Science B.V.