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Lamella thickness/thickening

Fig. 7 Lamella thickness 2.5 ns after quench to each crystallization temperature ( ), and that after annealing 6.4 ns (o). At 250 K and 300 K, the lamella is still thickening even after 6.4 ns by an appreciable rate... Fig. 7 Lamella thickness 2.5 ns after quench to each crystallization temperature ( ), and that after annealing 6.4 ns (o). At 250 K and 300 K, the lamella is still thickening even after 6.4 ns by an appreciable rate...
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. [Pg.50]

Independent of crystallization conditions, whether from solutions or melt, the polymer molecules crystallize into thin lamellae. The lamellar thickness is about 10 nm, about two orders of magnitude smaller than values allowed by existing equilibrium considerations. This is in contrast to the case of crystallized short alkanes, where the lamellar thickness is proportional to the length of the molecules. Clearly the chains in the case of polymers should fold back and forth in the lamellae to support the experimentally observed lamellar thickness. It is believed in the literature [3-9] that the lamellar thickness is kinetically selected and that if enough time is permissible, the lamella would thicken to extended chain crystal dimension. What determines the spontaneous selection of lamellar thickness ... [Pg.3]

Because a thicker layer is more stable than a thinner one, it is always thermodynamically advantageous for a lamella to thicken irreversibly. If the lamellar thickness equals that of the secondary nucleus, ie = 0, then a lamella will melt at its crystallization temperature, as substitution into the two previous equations... [Pg.4945]

Annealing of semicrystalline polymers is a difficult process to understand. Both microscopic and macroscopic defects are reduced upon sample exposme to temperatures somewhat below the crystalline temperature. Polymer crystals are metastable, ie the lamellae thickness and lateral size are generally determined by the degree of supercooling balanced by the side and end free energies. Therefore, as a function of time, chain-folded polymer crystals thicken on annealing to minimize the number of folds. Although the exact mechanism has yet to be determined... [Pg.8785]

In general, with high molecular weight polymers, it is well established that the lamellae thickness increases with temperature of crystallization, and a process of thickening occurs on heating above or close to the crystallization temperature. [Pg.270]

There is a linear relationship between the crystallization temperatures and the inverse lamella thicknesses, which is quite in accordance with Gibbs-Thomson equation. There is also a linear relationship between the melting temperatures and the inverse lamella thicknesses. Crossover of these two linear curves is considered to be the triple point of mesophase transition. Recently, the crossover was reproduced in the molecular simulations of lattice polymers, and the interpretation was updated to an uplimit of instant thickening at the lateral growth front of lamellar crystals (Jiang et al. 2016). [Pg.134]

The logarithmic-time dependence of crystal thickness can be deduced easily. By assuming a frictional barrier (AfJ for chain-sliding diffusion proportional to the lamella thickness (/), thus the thickening rate of monolayer lamellar crystal under a certain temperature is... [Pg.137]

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. [Pg.250]

Fig. 8 Quantized lamellar thickening. Rg is the radius of gyration of the lamella [22] Lamellar Thickness and Quench Depth... [Pg.251]

When the lamellae are annealed at a given temperature, they thicken with time. The thickening is usually continuous and L increases logarithmically with time. However, there are several examples, where the lamellar thickness increases in a stepwise manner. For example, the initial lamella may contain chains each with four folds (five stems). As thickening process continues, the lamellar thickness jumps discontinuously to three folds, and so on. This phenomenon is referred to as quantized thickening [25]. [Pg.6]

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... [Pg.17]


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See also in sourсe #XX -- [ Pg.53 , Pg.80 ]




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