Lamella Thickening

The lamella thickening depends sensitively on the initial lamella thickness as well as on the annealing temperature. We first considered the thickening of very thin lamella of about 18-bonds thick this thickness nearly corresponds to that of the lamella crystallized at 0 K (Fig. 7). The temperature of annealing Ta was taken between 20 K and 150 K a quick jump to a higher temperature resulted in partial melting and re-crystallization and a continuous thickening process could not be observed. 

Three-Dimensional Crystallization of a Single Chain from Vapor 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

With progress of adsorption, the molecular order in the adsorbed layer gradually grows. The chain entanglements, however, hinder the development of order, and cause the persistent amorphous overlayers. At low temperatures, the molecule does not completely spread over the surface leaving a large 

We also found that the average stem length after a sufficiently long time, which corresponds to the thickness of the adsorbed lamella, shows a pronounced dependence on crystallization temperature (Fig. 15). Though the detailed molecular processes involved are quite different from those in our previous 2D simulation, the tendency for thicker lamella at higher Tc is here again reproduced. 

We have so far considered the substrate to be infinitely extended both in the x- and in the y-axis directions. Actually, polymers form thin lamellae and the crystallization takes place on the narrow side surface of the lamellae. 

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Fig. 37 Three dimensional pictures of growing lamellae of Ciooo at 370 K 3 at 0.128 ns, b at 6.4 ns, c at 12.8 ns, and d at 19.2 ns. We again see the tapered growth fronts and their advances in the normal (the z-axis) direction together with the lamella thickening along the chain axis (the y-axis direction)...

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. 

The addition of a new chain at the growth front is not hindered by a barrier, in contradiction with the underlying assumptions of the LH theory. Simultaneous to the addition of new chains at the growth front, chains inside the lamella move cooperatively. The center of mass of the lamella diffuses in space while the lamella thickens by a process of internal rearrangements for details, see [30]. The mean squared displacement of a labeled monomer varies with the elapsed time, t, with an effective power law of t0Ji by shuffling back and forth between the lamellar and amorphous regions. 

There has been much confusion about the mechanism of formation of extended chains. The most probable conception is that formation direct from the melt is the dominant mechanism unfolding and subsequent lamella-thickening may also take place, especially at lower temperatures and long annealing times. 

Lamellae thickening mechanism of polyethylene with various morphologies... 

Finally, for the kinetics of lamella thickening it is essential to know the lattice resistance to the motion of dispiration loops. This has been estimated by Reneker and Mazur (1983) to be describable by a diffusion constant 1 x 10 cm /s at 343 K. They consider that dispiration loops are likely to have a thermal equilibrium concentration of roughly one defect per molecule stem in a lamella, or a linear concentration of roughly 10 m ... 

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