New insight into the formation of structural defects in poly(vinyl chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both H-1 and C-13 NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects

STRUCTURAL STUDY OF LANGMUIR-BLODGETT MONOLAYERS AND MULTILAYERS OF POLY(BETA-HYDROXYBUTYRATE)

In this study poly(beta-hydroxybutyrate) (PHB) was used as a spreading material to form LB monolayers at the air-water interface. Isotherms were found to show a transition at about 14 mN/m that is argued to be associated with a phase transition in the monolayer. Stable monolayers could be obtained at various surface pressures before as well as beyond the transition. Multilayers on substrates were investigated with FT-infrared techniques and were shown to have crystalline characteristics. Hysteresis experiments confirm the occurrence of irreversible processes in the monolayer during compression. Transmission electron microscopy pictures clearly show the structural changes that appear with increasing stabilization surface pressure of the monolayer. At large areas PHB exhibits an expanded monolayer behavior. Under the influence of surface pressure PHB is argued to change into a crystalline structure and eventually to form a bilayer of helical molecules which is reflected in the shape of the isotherm. S-shaped stabilization curves are argued to be the result of an accelerated bilayer formation process, as is confirmed by TEM pictures of this layer. Infrared external reflection spectroscopy of a PHB monolayer on the water surface gives clear indications that PHB already crystallizes at the air-water interface during compression, thus confirming our theorem