Use of Polarized Resonance Raman Spectroscopy of a Polyene Probe,and FT-IR Dichroism,to Probe Amorphous-Phase Orientation in Uniaxially Drawn Poly(Ethylene)

Abstract

The linear diphenyl polyene Φ–(C=C–)₄Φ has been investigated as a resonant Raman-active probe for molecular orientation, suitable for incorporation into polymers prior to melt extrusion and drawing. By examination of the polarization properties of the Raman scattered radiation, it is possible to determine the P₂₀₀ and P₄₀₀ orientation parameters for the dye molecule as a function of draw ratio. Assuming that the dye resides only in amorphous polymer regions, it is expected that the data will give insight into the orienting forces present in the amorphous phase during processing. Expressions were derived relating P₂₀₀ and P₄₀₀ to the polarized Raman intensities. An expression for Hermans' orientation parameter was also derived—it was shown that f was always slightly higher than P₂₀₀ when evaluated from the same Raman data. These Raman measurements were illustrated by using the example of uniaxially drawn poly(ethylene) (PE) tubing. As an alternative approach, FT-IR dichroic ratios of bands due to the PE itself were used to determine the orientation (P₂₀₀) of the crystalline phase and also of gauche conformers present only in the amorphous phase; these results were compared with the Raman measurements on the dye probe. The orientation was examined as a function of draw ratio and subsequent thermal annealing. In general, both crystalline and amorphous phases oriented increasingly with draw ratio, but whereas annealing the samples only slightly relaxed the crystal orientation, the amorphous orientation decreased dramatically when measured by using either the probe molecule or the gauche conformer as a probe of the amorphous phase. This result support the assumption that the dye is localized in amorphous regions. This work is important since it shows how a cheap, commercially available probe molecule may be used to probe the amorphous-phase orientation of a polymer without requiring any chemical synthesis; simple melt-mixing is sufficient to dope the amorphous network. This capability will be useful for systems where convenient vibrational band: specific to the polymer amorphous phase are absent.