Solution-grown crystals of model ethylene copolymers

Studies on the crystal growth habit of model crystallizable polymers, the very narrow molecular weight distribution hydrogenated polybutadienes, are reported. These polymers are analogues for 4 mol % butene-1 copolymers with ethylene. Crystals have been grown from dilute solutions, either as suspensions or as epitaxial overgrowths on KCl cleavage surfaces. The growth faces of these crystals are highly irregular in shape, and the fold surfaces (imaged using dark field electron microscopy) are unusually thick. Both effects appear to be due to the process by which chains attach to the crystal. This process involves chain attachment to the growth face followed by a partitioning of its ethyl branch-rich segments to the fold surfaces and its ethyl branch-lean segments to the crystal cores. Thus, it must build crystal only locally, and at the same time it excludes other arriving chains from a region centered about its attachment site. Continued repetition of this process ultimately leads to irregularly shaped growth faces of the kinds observed.