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Asymmetries of Habit in Polyethylene Crystals Grown from the Melt

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Growth habits have been studied in crystals of polyethylene grown at relatively high temperatures in thin films of melt supported on substrates. Two notable asymmetries have been found to be general in these highly elongated lamellae. First, lateral growth is slower on growth faces which, because of chain tilt, overhang the substrate and form reentrant angles with it. Secondly, as revealed by the Wittmann-Lotz decoration method, the structure of fold surfaces is different depending upon whether folding occurred at edges of crystals where these surfaces make acute or obtuse angles with tilted growth faces (this applies both to exposed upper fold surfaces and to fold surfaces in contact with substrates). These asymmetries are discussed in relation to twisting orientation in banded spherulities of polyethylene. Curved lamellar growth on substrates, as first noted by Labaig, has also been examined but continues to present difficulties in interpretation.