Structural studies of pressure-crystallized polymers. I. Heat treatment of oriented polymers under high pressure

A convenient pressure apparatus was designed for crystallization of high polymers under hydrostatic pressure up to 5000 atm. Melt crystallization as well as heat treatment under various temperatures and pressures was carried out on several polymers, and the effects of pressure on the molecular and crystal structures of the samples are discussed. Heat treatment of syndiotactic polypropylene under high pressure yields a new crystal modification rather than the previously known helix and planar zigzag modifications. Of the three modifications of poly(vinylidene fluoride), modification III was found as a high-pressure phase for specimens in the unoriented state, while modification I was obtained as the most stable one on heat treatment of oriented specimens under high pressure. Heat treatment under high pressure converts ordinary isotactic poly-4-methylpentene-1 with a lower density than the noncrystalline value, to a new crystal modification with higher density. As is reasonable, the dense modification is stable in a high-pressure range. For these three cases, the orientation of specimens was found to remain unchanged during the transitions, which must therefore occur in the solid state.