Irreversibility of structural changes induced by stretching in poly(vinyl chloride)

It has been shown that certain structural (X-ray diffraction) and physical (infrared dichroism, alpha-relaxation dynamics) characteristics of poly(vinyl chloride) undergo irreversible changes upon sample drawing. This behaviour reveals some non-equilibrium character of the polymer, even in tetrahydrofuran (THF) solution or in the rubbery state (at, typically, glass transition temperature T-g + 10 degrees C). Two tentative ways of explanation have been proposed. The first one would attribute the cause of this peculiar behaviour to physical crosslinks linked to strong interchain bonding. These bonds would resist to dissolution in THF at room temperature but would be broken during stretching. The second way of explanation involves the presence of a metastable GTTG(-)TT conformation at the end of an isotactic sequence in an mml chain segment. Like the above mentioned physical crosslinks, it would be transformed upon mechanical loading but not during dissolution in THF or heating to 90 degrees C. Both hypotheses are not necessarily incompatible: as a matter of fact, GTTG(-) conformations at the end of the isotactic triads could represent sites of especially high intermolecular interaction, distributed along the polymeric chain.