Elsevier

Polymer

Volume 25, Issue 10, October 1984, Pages 1446-1452
Polymer

Polymer paper
A model for chain folding in polymer crystals: rough growth faces are consistent with the observed growth rates

https://doi.org/10.1016/0032-3861(84)90108-3Get rights and content

Abstract

The hypothesis that growth surfaces of polymer crystals can be molecularly rough has been tested using models designed to predict lamellar thicknesses and crystal growth rates. Monte Carlo methods used for crystallization of atomic growth units have been adapted to take account of the connectedness along the chains by including two types of restrictions for adding units to the crystal. Firstly new units cannot be added to an established fold surface. Secondly the effect of loops created at the growth surface is simulated by ‘pinning’ points generated at random along the crystalline stem, corresponding to stems growing from chains which are attached elsewhere on the growing face. No interruptions are allowed along crystalline stems. The trends in the results do not depend on the precise nature of the restrictions, and agree with the experimental ones. Crystal thick nesses vary approximately inversely with supercooling at small supercooling, and then level off to an approximately constant ‘plateau’ (no ‘catastrophic’ increase). They are smaller in magnitude than experimental ones since high fold energies are not included. The growth rates obey equations derived for secondary nucleation, though nucleation is certainly not present. The explanation is related to the high free energy (low entropy) states that are required for overall advance of the crystal. The low entropy depends on the crystal thickness as does the high enthalpy states in secondary nucleation. These results resolve the problem that both the existence of lamellae and the growth rate curves were explained on the assumption of molecularly smooth growth surfaces, whereas polymer crystals are often curved.

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