Lamellar thickening

From the experimental fact that the lamellar thickening growth rate (17) of ECSC of PE is independent of l [8], we can regard ve on the end surface of the nucleus as constant and independent of l. For simplicity, we assume that the shape of ECSC is rectangular parallelepiped (Fig. 28). ve is given by the ratio of the volume of the crystal to the surface area of the crystal as shown below,... 

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. 

Fig. 8 Quantized lamellar thickening. Rg is the radius of gyration of the lamella [22] Lamellar Thickness and Quench Depth... 

We now consider the role of lamellar thickening on the relation between Tc and Tm- At Tc, the initial thickness of the virgin lamella is given by Eq. (1.32) as... 

The lamellar thickening proceeds through many metastable states, each metastable state corresponding to a particular number of folds per chain, as illustrated in Figure 1.25. This quantization of number of folds has been observed in experiments [25], as already mentioned. The process by which a state with p folds changes into a state with p + I folds is highly cooperative. The... 

Annealing of the nascent powder sample passed through Process 1, and above 90 °C, dynamic molecular motion started, as defined by Process 2. This critical temperature is slightly higher than that of the solution-crystallised sample. This difference indicates the restricted crystalline chain motion for the domain network structure crystallised during polymerisation. In Process 2, the crystallinity remained at a constant level for the nascent powder sample. This shows that the lamellar thickening is limited for the nascent powder morphology. 

Well-annealed polymeric solids tend to be stiffer and more brittle than unannealed solids. For crystalline polymers annealing increases the degree of crystallinity and crystallite sizes (lamellar thickening). These changes are defined and measured thermodynamically as the decrease in enthalpy and entropy. 

This shows that by means of synchrotron radiation, for the first time the primary lamellar thickening on melt crystallization in polyethylene could be observed. This enabled also the determination of the true crystal thickness as a function of supercooling AT. Most satisfyingly it was found to be closely identical to that obtained in solution crystallization were no isothermal thickening takes place. In conclusion, the hitherto existing gap between melt and solution crystallization has been removed and the corresponding f versus AT curves brought in coincidence. 

Tc, is increased the temperature of peak L is found to increase in direct proportion while that of peak H increases only slightly. As Tc is further increased, the L endotherm Increases in size while the H endotherm diminishes and eventually disappears. These trends generally confirm those reported previously (2 ). and suggest that the multiple peaks are most likely the result of lamellar thickening processes, perhaps requiring melting and recrystallization (23). 

Crystallization results basically in the succession of two events the primary nucleation of a new phase and then the three-dimensional growth of lamellae these steps can be followed by lamellar thickening, fold surface smoothing, or reorganization into more perfect crystals. As reported by others, with increasing the annealing temperature in PVDF/PMMA blend film, the amount of a phase increases due to easier local internal chain rotation at higher temperature and most of p and y form convert to a form. 

M. Hikosaka, K. Amano, S. Rastogi, and A. Keller. Lamellar thickening growth of an extended chain single crystal of polyethylene. 1. pointers to a new crystallization mechanism of polymers. Macromolecules, 30(7) 2067-2074, March 1997. 

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