Pre-crystalline, high-entropy aggregates: A role in polymer crystallization?

We distinguish three main modes of crystallization for polymers with a relatively flexible main-chain, i.e., (i) usual lamellar crystallization occurring by cooling from the reference state (melt or solution) above the temperature T-0 down to T > T-g; (ii) crystallization from the glass; (iii) crystallization from a stable thermotropic mesophase. In all three cases we propose that structure development proceeds via high entropy pre-crystalline aggregates, which may influence features of the crystalline organization. Pre-crystalline structures characteristic of modes (i) and (ii) are identified with bundles, i.e., energy-driven hexagonal associations among chain segments. At T < T-0 the polymer solution is regarded as meta-stable, and in this state the bundle segments are essentially consecutive whereas in the melt and the glass bundles also comprise nonconsecutive chain segments. The fold thickness L observed in lamellar crystallization, resulting from bundle aggregation and rearrangement, is basically controlled by the average fold length in the consecutive chain portions within bundles. For small values of Delta T(= T-0 - T-crystaillization) we obtain L proportional to 1/Delta T, in agreement with experimental data from polyethylene as well as with several simulation results; the proportionality factor appears to be the same for the solution and the melt. The bundle model appears to be consistent with indirect evidence such as segregation of short chains in the crystallization process and clustering of segments belonging to the same chain in the crystal. In mode (ii) it is plausible, at least in certain instances, that crystallization is preceded by bundle aggregation leading to phase separation. in the case of crystallization in mode (iii) we can identify the pre-crystalline high entropy state with the thermotropic mesophase itself. Such phases involve large domains of parallel, hexagonally packed, conformationally disordered chains, with a high propensity to fully extended macroconformations. They occur with polymers with a large persistence length of entropic (i.e., elastic) origin, mainly due to conformational disorder of the side groups. Folds and hairpins in these mesophases are energetically disfavored because adequate compensatory inter-stem attractions are missing. Finally, it is shown that crystallization of helical non-chiral polymers into crystalline modifications comprising isochiral helices only, may in certain cases be accounted for on the basis of hexagonal pre-crystalline intermediates like bundles and mesophases discussed in the present contribution.