Three-dimensional model for twinning in polymer single crystals with inclined chains

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Abstract
A model describing the three-dimensional structure of twinned crystals of poly(paraxylylene) (PPX) in the aL form is proposed. PPX crystals were grown at 218°C in 1-methylnaphthalene at a concentration of 0.005% using the self-seeding technique. A large number of twinned crystals are obtained, corresponding to two basic modes: aLta and aLtb, involving respectively {110} and {130} twinning planes. Lattice images of the boundary in αta crystals occasionally show 110 lines. Since chains are inclined to the surface normal in untwinned crystals, the presence of 110 lattice fringes at the boundary is explained by the collapse onto the carbon substrate of a three-dimensional roof-like structure consisting of two lamellar twin components. This ‘nonplanar twin’ model, in a sense that the two lamellar components do not lie in the same plane, is extended to the crystals with aLtb twins and further demonstrated by electron diffraction tilt experiments. Multiple twins as well as coalesced α–β crystals are also described with the help of the basic twin modes and the roof-like model. These results clearly demonstrate the concept that c axis-invariant twinning in inclined chain lamellae leads to nonplanar twins. This principle is further discussed in relation to previous three-dimensional models proposed for polyethylene and its twins.