Mechanical, morphological, and thermal properties of poly(ethylene 2,6-naphthalate) and copolyester LCP blends

Binary blends of a liquid crystalline polymer (LCP) and poly(ethylene 2,6-n aphthalate) (PEN) were melt blended and injection molded. The mechanical pr operties were studied as a function of LCP content. Both the ultimate tensi le strength and Young's modulus are higher than the theoretical values pred icted by the rule of mixtures and they display a synergistic behavior at 70 wt % LCP content. However, the tensile strength decreases with LCP content and Young's modulus remained unchanged at lower LCP contents (10 to 30 wt %). The poor mechanical property is attributed to the immiscibility between PEN and LCP and the fibrillation behavior of LCP as revealed by differenti al scanning calorimetry (DSC) and scanning electron microscopy (SEM) result s. However, LCP and PEN are found to be partially miscible at higher LCP co ntent, ascertained by DSC and dynamic mechanical analysis (I)DMA.). This is attributed to the transesterification reaction between PEN and PET moiety in the LCP molecules. SEM micrographs reveal a skin/core morphology in the tensile bars, that is, the LCP is better oriented in the skin than in the c ore region. At lower LCP content, the dispersed LCP phase is spherical in t he core and ellipsoidal in the skin, with long axes oriented in the flow di rection. DSC studies show that the crystallization rate is significantly en hanced by the presence of LCP up to 50 wt %, where the LCP acts as a nuclea ting agent for PEN crystallization. The melting temperature decreases with LCP content, probably as a result of imperfect crystals formed in the prese nce of LCP heterogeneous nucleating centers and the increasing miscibility between LCP and PEN. (C) 2001 John Wiley & Sons, Inc.