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Physically crosslinking PVA polymers using the freeze-thaw method (Peppas 1975) results in regions of interconnected PVA polymers through crystalline regions formed when frozen and which persist after thawing. These physical crosslinks are sufficient to produce a homogenous hydrogel of predictable bulk characteristics. The formation of Jello gelatin uses a similar phenomenon.

My question is: Are these crystalline regions only available to form at the free ends of the polymer chains, or can these lamellae form along any portion of the polymer chain?

If crystallization were only able to occur at free ends of the chain, this would result in a massively long polymer chain (assuming tip to tail occurred for many polymer chains in the solution) and as such the bulk polymer might be defined as a tangled network.

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The physical alignments can occur anywhere. Keep in mind that these regions are both inter- and intra- molecular. (CH4 can't align with itself, but CH3(CH2)1000CH3 can and does.)

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