Crystals Grown from the Melt and Lamellae Stacks

4 Crystals Grown from the Melt and Lamellae Stacks 

Polymers are always polydisperse with a distribution in molar mass and often contain chain branches, either introduced specifically during synthesis or as a consequence of synthetic defects, and both these effects will influence the observed morphology. As we shall see later, copolymers are a special case however, the introduction of low levels of comonomers can lead to behaviour which is rather like that of random branched chains. Different molecular species crystallize in different stages indicating the thermodynamic control on the overall process, i.e. they are incorporated into the crystal structure at different temperatures and times. The intermediate and high molar mass component crystallizes early in the stacks of thick dominant crystals. Small pockets of rejected molten low molar mass material remain after crystallization 

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 number of chain segments tr) in a straight chain, stem, is given by 

The ratio of segments in the two phases will be in the ratio of the amorphous (Pa) to crystalline density (pc)  

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