Crystal lamella melt-grown

The next thermal analyses are for solution-grown dendrites, hedrites, and melt-grown lamellae, represented by Eigs. 6.93 and 6.94. The dendrites melt at much lower temperatures than the extended-chain crystals and show multiple melting peaks. 

Keller has shown that a terrace-like structure is observed reflecting the stacking of the lamellae structure in a melt-grown crystal (Figure 5.6). 

For melt-grown polymers the amorphous density is 10-20% lower that the crystal density. Flory argued that all the chains entering the amorphous phase would take a random walk before re-entering the crystal lamellae. A significant fraction of the chains must, however, be fold directly back into the crystal in order to account for the observed low amorphous density. 

The earliest stages of melt growth are a rewarding area of study. They show, for example, features related to nucleation and to the question of whether melt-grown lamellae are chain folded. The lamellae which form have habits which have become central to recent discussions of polymeric crystallization. One can also test directly to see whether... 

FIGURE 4 Random stacks of liquid and crystal lamellae for a sample of PVDF grown from a melt (upper) and after drawing the film several times its original length (bottom). (From Ref. 43.)... 

CRYSTALS GROWN FROM THE MELT AND THE CRYSTAL LAMELLA STACK... 

Melt-grown crystals have the same shape as the solution-grown single crystals in most respects. They are lamella-shaped with a thickness-to-width ratio of 0.01-0.001. A typical feature of melt-crystallized samples, shown in Fig. 7.27, is the crystal stack. The lamellae of the stack are almost parallel and the amorphous component is located in the space between the crystal lamellae. The number of lamellae in a stack varies considerably between different samples. Linear polyethylenes of low or intermediate molar mass form stacks with a great many lamellae, whereas branched and/or high molar mass samples exhibit stacks containing only a few crystal lamellae (Fig. 7.27). 

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