On fold surfaces of polymer crystals

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Abstract
The positional ordering of chain-folds in three polymers is deduced from moiré patterns. In polyoxymethylene and poly(4-methylpentene-1), stacking is mixed, with adjacent re-entry. This is also one alternative for polyethylene, the other being ordered RGI (or RGI') stacking. In either case folds must have restricted geometries and be regular. The occurrence of isolated partial edge dislocations in polyethylene and poly(4-methylpentene-1) and of edge dislocations in polyoxymethylene can be explained as a consequence of fold-surface packing. This explanation requires substantial ordering in as-grown crystals of polyethylene and in crystals of all polymers after annealing. It is suggested that regular folding probably occurs generally in crystals grown from dilute solution but that it may not always be detectable in moiré patterns because of inadequate lamellar contact.