Nucleation and anisotropic crystalline growth of polyethylene under shear
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The crystallization of polyethylene was observed during shear experiments in isothermal conditions. The nucleation and the crystalline growth rates were measured from a microscopic observation of the growing morphologies. An unusual formation and development of row nuclei was observed throughout the experiment, followed by anisotropic growth. Three main growth rates were measured with respect to the main directions of the process. All these growth rates are enhanced differently by the shear rate. This result is interpreted first by an increase of the equilibrium melting temperature emanating from an entropy loss due to the chain orientation, second the anisotropy of growth is discussed as an effect of chain orientation with respect to the growth front of lamellae and of a local flow different from macroscopic shear-flow. An overall kinetic equation with shear effect has been proposed for this polyethylene and tested in a stimulation of crystallization during injection moulding of a polyethylene disc. The shear effect on the crystallization is necessary to predict the crystallization temperature and the thickness of polymer crystallized during the filling stage.
KeywordsEntropy Crystallization Polyethylene Shear Rate Crystallization Temperature
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