The bundle theory for polymer crystallisation

The bundle theory of polymer crystallisation expounded by Allegra (J. Chem. Phys., 1977, 66, 5453; Ferroelectrics, 1980, 30, 195) is revisited as new experimental and simulation results are considered. We propose a framework by which recent evidence of structure formation prior to crystallisation ma y be understood at the molecular level. We suggest that polymer crystallisa tion is controlled by the metastable equilibrium whereby van der Waals intr amolecular associations (bundles) form among parallel stems with a length o f a few chain atoms. Bundle aggregation may develop into pre-crystalline st ructures. The similarity between initial fold lengths observed from solutio n and from bulk crystallisation appears to support the assumption that thei r value is controlled by intramolecular bundle statistics with an equilibri um character. Secondary, i.e. surface, nucleation is preceded by chain adso rption on the growing crystal. Chain segregation is determined by the unfav ourable adsorption equilibrium of shorter chains. Three models of bundle fo rmation are considered, differing in the length distribution of the crystal line stems and in the topology of bundle association. For moderate undercoo lings (Delta T less than or similar to 50 degrees C for polyethylene), cons idering bundle aggregates with more complex topologies rather than simple b undles hardly affects the predicted fold length. Conversely, allowing for s tems with different lengths within the same bundles improves the agreement with experimental data. The mechanism by which bundle equilibrium may influ ence current interpretations of the crystal growth rates in different regim es remains to be assessed.