Macromolecular displacements and entanglements of flexible chains in polymers on crystallization
Polimery 1997, No 7-8, 432
Description of nucleation and crystal growth of flexible-chained linear polymers requires macromolecular movements in the molten polymer to be duly accounted for. The easiest to perform are reptant movements, i.e., displacements along a virtual tube circum scribed around a macromolecular chain. Other movements are more difficult to occur because extensive cooperation is required from adjacent chains. Apart from friction against the tube, chain movements areobstructed by entanglements with neighboring chains. To overcome anentanglement knot requires a sequence of conformational changes to occur in the chain, which causes additional friction. On melt crystallization, entanglement knots obstruct, while crystallization forces promote, the transportation of chains after secondary nucleation has occurred on the crystal surface. Calculations showed that, with disentangled chains, crystallization forces overcome the friction of the chain against the tube. The time to reel out the PE macromolecule, Mw = 50,000 g/mol, from the melt was estimated at 20 ms. On such a time scale, thermal motions (reptation) of other macromolecules are slow enough to assume the chains to be motionless. As undercooling is augmented, secondary nucleation becomes increasingly intense. Competition of the rate of secondary nucleation with that of buildup of the crystal face layer allows to distinguish three crystallization regimes. In Regimes I and II macromolecules are primarily reeled out of the melt and crystal face layers are built up in a chain folded manner. High-pressure crystallization allows to have the chains disentangled and extended. Spherulite growth rates are faster in Regimes I and II in a polymer melt with disentangled chains. In Regime III, crystallization produces a polymer with strongly bonded neighboring crystals and one endowed with a high mechanical strength.