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 may be understood at the molecular level. We suggest that polymer crystallisation is controlled by the metastable equilibrium whereby van der Waals intramolecular associations (bundles) form among parallel stems with a length of a few chain atoms. Bundle aggregation may develop into pre-crystalline structures. The similarity between initial fold lengths observed from solution and from bulk crystallisation appears to support the assumption that their value is controlled by intramolecular bundle statistics with an equilibrium character. Secondary, i.e. surface, nucleation is preceded by chain adsorption on the growing crystal. Chain segregation is determined by the unfavourable adsorption equilibrium of shorter chains. Three models of bundle formation are considered, differing in the length distribution of the crystalline stems and in the topology of bundle association. For moderate undercoolings (Delta T less than or similar to 50 degrees C for polyethylene), considering bundle aggregates with more complex topologies rather than simple bundles hardly affects the predicted fold length. Conversely, allowing for stems with different lengths within the same bundles improves the agreement with experimental data. The mechanism by which bundle equilibrium may influence current interpretations of the crystal growth rates in different regimes remains to be assessed.