Structural studies of PVC gels by Raman spectroscopy

In this thesis the first comprehensive study of the structure of poly(vinyl chloride) (PVC) gels by vibrational spectroscopy is presented. Raman spectroscopy is the principal experimental technique employed, and additional information is obtained by high resolution 13C nuclear magnetic resonance (NMR). Raman spectra of PVC gels were obtained at a range of concentrations and temperatures. A detailed curve fitting analysis was used to resolve the carbon-chlorine stretching region into its constituent peaks. The peak shape used for this fitting is a Lorentzian corrected by convolution with a triangular function for instrumental broadening caused by a finite slitwidth. In general the dependence of the intensities of the resolved peaks on temperature and gel concentration confirms their previous assignments to various stereoregular structures in amorphous and crystalline material. When this is not the case an attempt is made to suggest alternative assignments. Of the ten peaks fitted, two are attributable to vibrations in crystalline material, the remaining eight being assigned to amorphous conformations. Based on these assignments the data is interpreted in terms of the structure of PVC gels and the nature of the crystalline component. Corroborative evidence is found for the previously suggested fringed micellar and lamellar morphologies of the two crystal types found in PVC gels. It is shown that the relative intensities of the two crystal peaks and their depolarisation ratios depend strongly on the relative concentration of these two crystal types, and on the degree of development of the fringed micellar crystallites. The long term ageing of PVC gels is also investigated by Raman spectroscopy. It has been previously reported that short term ageing is accompanied by an increase in crystallinity. For the longer ageing times, however, no measurable change in the crystallinity or conformational content of the gels could be found. 13C NMR is used to study molecular motion in PVC gels. It is shown that the nuclear Overhauser effect can be used to measure the temperature range over which crystallite dissolution takes place. Correlation between this temperature range and the gel-sol transition temperature confirms the crystalline nature of the crosslinks. The activation energy for the motions responsible for spin-lattice relaxation is calculated, and found to compare well with the values reported for other polymers. (author)