Development of Micro-Debond Methods for Thermoplastics Including Applications to Liquid Crystalline Polymers

Micro-bead and related debond techniques were used to study adhesion of liquid crystalline copolyesters (LCPs) and other semi-crystalline thermoplastic polymers to glass fibers. For polymers with poor flow even at high temperatures, symmetric beads on fibers were difficult to prepare so an alternative sample preparation method was developed where glass fibers were inserted into thin sections of molten polymer. Glass fibers of widely-varying diameters were used in order to extend the dynamic range of the debond techniques in terms of debonding area, showing a significant improvement in precision over that demonstrated previously with micro debond techniques. The fibers were freshly prepared in our laboratory and silane coated when necessary, which allowed us to minimize fiber surface heterogeneity effects which are believed to influence strongly debond test results. It was found that chemical bonding of the LCPs was quite favorable as was indicated by fracture surface analysis and by comparison with the shear strength of the neat resins. The apparent poor interphase strength in fiber-reinforced LCP composites is proposed to be due to orientation of the LCP molecules near the fiber interface leading to a cohesively weak layer of LCP near the interface. Reactive silane coupling agents lead to no improvement in interface strength as compared with bare glass because chemical reaction occurs on both surfaces. This results in very strong interfaces leading to polymer cohesive failure near the interface of all thermoplastics studied here