Temperature-Dependent Recrystallization Morphologies of Carbon Coated Isotactic Polypropylene Highly Oriented Thin Films

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Abstract:Temperature-dependent recrystallization morphologies of carbon-coated iPP oriented thin films were studied by electron microscopy combined with electron diffraction. It was found that the vacuum evaporated carbon layer always results in oriented melt-recrystallization of the preoriented iPP thin films. The morphology of the recrystallized carbon-coated iPP thin films is, however, dependent on both melting and crystallization temperatures. At low melting temperatures, e.g., lower than 200 degrees C, parallel-aligned edge-on lamellae with the same molecular chain orientation as the prepared samples were always generated regardless of crystallization temperature. When melting at 300 degrees C and crystallization at low temperature, e.g., 120 degrees C, both edge-on and flat-on lamellae were observed. At elevated crystallization temperature, e.g., 150 degrees C, the flat-on iPP crystals show more or less a regular lath shape. At the same time, quadrites caused by wide-angle lamellar branching originating from homoepitaxy of alpha-iPP were seen. It was further found that all of the three kinds of iPP crystals have a fixed mutual orientation. While a portion of the lamellae in the quadrites possess the same molecular chain orientation as the parallel-aligned edge-on lamellae, the flat-on iPP crystals arranged with their a-axes along the molecular chain direction of the parallel-aligned edge-on lamellae. This suggests that both the quadrites and flat-on crystals of iPP are initiated by the parallel-aligned edge-on lamellae. These results indicate unambiguously the strong fixing effect of vacuum evaporated carbon layer on the iPP oriented thin film. The fixing effect is, however, inhomogeneous. Raman spectroscopy study indicates the existence of chemical bonding between the carbon layer and the iPP film. It is those molecular chains fixed to the carbon layer by chemical bonds that produce always oriented edge-on lamellae. The molecular chains fixed weakly to the carbon layer can be released from the carbon layer at high temperature and generate flat-on lamellar crystals by crystallization at high temperatures.

Volume:50

Issue:9

Translation or Not:no