Nano- and Micromechanics of Crystalline Polymers
Chapitre d'ouvrage scientifique
It is currently thought that crystalline polymers consists of lamellar crystals which are separated from each other by a layer of amorphous polymer and are held together by tie molecules through the amorphous phase [e.g. 1]. The lamellae are formed from mostly folded chains. The thickness of lamellae is determined by the parameters such as interfacial energies, glass transition temperature and melting temperature, undercooling, segmental diffusivity, etc. The thickness reported lies usually in a narrow range between 3 and 20 nm as obtained from observations in various types of microscopes or calculated from the degree of crystallinity and long period. It has been recognized that chain folding is not so regular as it was thought and molecular packing in lamellae is subject to considerable and irregularly distributed disorder depending on undercooling- regimes of crystallization. It has been demonstrated in various ways that the planar growth front will always break up into fibrous or cellular growth. Also crystallization of polymers leads to interface instability. More sophisticated treatment of the instabilities involve perturbation analyses of planar interfaces, correlating diffusion, temperature gradients along the interface, and interfacial energy with the size of the growing crystals.
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