Crystal thickening

IC Sanchez. Theory of polymer crystal thickening during annealing. J Appl Phys 45 4216-4219, 1974. 

The lamellar thickening proceeds through many metastable states, each metastable state corresponding to a particular number of folds per chain, as illustrated in Fig. 8. In the original simulations of [22], Kg was monitored. Rg is actually very close to the lamellar thickness due to the asymmetric shape of the lamella. The number of folds indicated in Fig. 8 were identified by inspection of the coordinates of the united atoms. This quantization of the number of folds has been observed in experiments [50], as already mentioned. The process by which a state with p folds changes into a state with p - 1 folds is highly cooperative. The precursor lives in a quiescent state for a substantial time and suddenly it converts into the next state. By a succession of such processes, crystals thicken. If the simulation is run for a reasonably long time, the lamella settles down to the equilibrium number of folds per chain. 

Fontaine et al. [81] concluded that the increase in crystallinity by further heating material, crystallized at 200 °C, to 215 °C involves a crystal (lamellae) thickening process which is probably due to crystal perfection at the boundary layers. Further annealing of this material at temperatures above 215 °C led to a melting temperature increase that was attributed to crystal perfection alone and not to crystal thickening. 

Polyethylene crystals change shape with time at temperatures between the crystallisation temperature and the final melting point [42]. The process is commonly referred to as crystal thickening because the major effect is that the crystals increase their dimensions along the c axis. These changes may occur without any change in crystal volume, i.e. crystal thickening is associated with a lateral contraction of the crystals (Fig. 9). A second option is that... 

Fig. 9a, b Schematic description of crystal thickening options, a Crystal thickening at constant crystal volume, b Crystal thickening accompanied by an increase in crystal volume... 

The old work on crystal thickening was confined to medium to high molar mass polyethylenes with molar mass polydispersity, and the results obtained were in some respects different from the more recent findings obtained on strictly monodisperse w-alkanes. 

Fig. 10 Schematic representation of crystal thickening of C294H590. Drawn after findings of Hobbs et al. [51]...

Reorganization of the metastable crystals formed during heating resulting in crystal perfection and/or crystal thickening. 

Here, the intramolecular crystal nucleation has been discussed mainly in association with the properties of critical free-energy barrier and in comparison with some basic experimental observations. The contributions of other terms in the rate equation, especially the prefactor, can be further considered to explain more experimental facts. Even those subsequent processes following crystal nucleation, such as crystal thickening, perfection, recrystalUzation, and transient mesophase transitions should be further considered to cover the complete picture of the microscopic mechanism of polymer crystalUzation. Some discussions on these processes can be found in other contributions of this book. 

---