Isothermal crystallization kinetics, subsequent melting behavior, and the crystal morphology of short carbon fiber and poly(ethylene 2,6-naphthalate) composites (SCF/PEN) were investigated by using differential scanning calorimetry and polarized optical microscopy. The crystal morphology of the composites isothermally crystallized at T c 5 2208C is predominantly banded spherulites observed under polarizing micrographs, while the pattern of banded spherulites changed from ring to serration as the SCF content added into the PEN. Moreover, nonbanded spherulites formed at T c 5 2108C. The commonly used Avrami equation was used to fit the primary stage of the isothermal crystallization. The Avrami exponents n are evaluated to be 2.6-3.0 for the neat PEN and 3.7-4.0 for SCF/ PEN composites, and the SCF acting as nucleation agents in composites accelerates the crystallization rate with decreasing the half-time of crystallization, and the sample with SCF component of 2% has the fastest crystallization rate. The crystallization activation energy calculated from the Arrhenius formula suggests that the adding SCF component improved the crystallization ability of the PEN matrix greatly, and the sample with of 2% SCF component has the most crystallization ability. Subsequent melting scans of the isothermally crystallized composites exhibited triple melting endotherms, in which the more the component of SCF, the lower temperature of the melting peak, indicating the less perfect crystallites formed in those composites. Furthermore, the melting peaks of the same sample are shifted to higher temperature with increasing T c , suggesting the more perfect crystallites formed at higher T c .
Short carbon fiber reinforced poly(ethylene 2,6-naphthalate) composites (PEN/SCF) were prepared by twin-screw extruder. The structure, mechanical, rheological properties, and nonisothermal crystallization kinetics of the composites were investigated by scanning electron microscope, universal tester, and differential scanning calorimetry. The results suggest that there is better interaction between SCF and PEN matrix, which leads to an increase in the tensile strength, Young's modulus, and impact strength of the composites with proper contents of SCF. Rheological behavior of the PEN/SCF composites melt is complicated, combining a dilate fluid at lower shear rate and a pseudoplastic fluid at higher shear rate. Moreover, the flow activation energy of the composites suggests that the melt with more SCF has higher sensitivity to the processing temperature. In conclusion, the composite with 5-10 wt % content of SCF has better properties. The Avrami equation modified by Jeziorny and Ozawa theory was used, respectively, to fit the primary stage of nonisothermal crystallization of various composites. The Avrami exponents n are evaluated to be 2.6-3.1 for the neat PEN and 3.4-4.8 for PEN/SCF composites, and the SCF served as nucleation agent accelerates the crystallization rate of the composites, and more the content of SCF faster the crystallization rate.
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