| Popis: |
The present research has investigated the presence of configurational defects in poly(vinyl fluoride). Both tacticity variations and head-to-head/tail-to-tail defects have been considered. Focus has been placed on the use of X-ray analysis and has been complimented by molecular modeling calculations and differential scanning calorimetry measurements. Configurational variations can exist in the crystalline regions of PVF due to the isomorphous nature of hydrogen and fluorine. However, configurational variations are expected to influence the conformation of the chain. In addition, the large electronegativity of the fluorine should affect the packing of the polymer. Molecular modeling calculations indicate a syndiotactic sequence is favored in a planar zigzag conformation, whereas for the possible HH arrangements, an extended conformation is very close to the minimum energy. For an isotactic sequence, there is a large energy penalty for the trans conformation. However, if a syndiotactic or meso HH/TT unit is placed in the middle of an isotactic sequence the energy of the structure decreases. Packing calculations indicate that for all systems studied the energy was negative, thus favoring crystallization. The melting point, as determined by DSC, has been used as a relative measure of the HH/TT content in PVF. The melting temperature of several samples was measured and correlated to the polymerization temperature. It was found that the melting temperature increased as the polymerization temperature was decreased. Three X-ray techniques were used in the study: a refinement of fiber data using the LALS program, a Rietveld analysis of powder data, and an analysis of the meridional intensities. The LALS refinement used data reported by Natta and Allegra and data collected for a commercial sample of PVF. No improvement was achieved in refining the data reported by Natta. The refinement of the commercial sample resulted in a system where the chains were packed in a statistical manner. The weighted residual for refinement was 0.10. The Rietveld analysis was applied to a perfect HT sample and appears to confirm the statistical packing of the chains. An analysis of the meridional intensities indicates that the ratio of the 001 to the 002 structure factor is a sensitive measure of the HH/TT content. Values were determined for several samples and correlated to the polymerization temperature. The results agree with those determined by DSC. |
