Development and evaluation of surface treatments to enhance the fiber-matrix adhesion in PAN-based carbon fiber/liquid crystal polymer composites

Various amine and electrochemical polymerization treatments were applied to comingled as-received AS4 carbon fiber/Liquid Crystal Polymer (LCP) fibers, and unidirectional composite plates were fabricated. Short beam shear (SBS) tests were conducted to determine the level of AS4 carbon fiber-LCP matrix adhesion present. The shear strength of the as-received AS4/LCP system was approximately 49 MPa (7.1 ksi). The as-received AS4 fiber-LCP matrix adhesion was determined to be reasonably good, at approximately 85 to 90 percent of a perfect bond as determined by micromechanics analyses.; Several di/triamine treatments yielded approximately a 10 to 15 percent increase in fiber-matrix adhesion relative to the as-received AS4/LCP system. These treatments included a 1 weight percent ethanol solution of 4-aminophenyl sulfone, a 10 weight percent aqueous solution of diaminopiperazine hydrochloride, a 10 weight percent 2-propanol solution of diethylenetriamine, and a 5 weight percent methanol solution of 1,12 diaminododecane. ESCA surface analysis showed that amines reacted with carboxylic acids on the AS4 surface to form covalent amide bonds. Furthermore, FTIR spectroscopy revealed that amines reacted with LCP to form covalent amide bonds. Therefore, the di/triamine treatments improve the adhesion of LCP to AS4 by the formation of covalent amide bonds. As a result, the di/triamine provides a "bridge" between the AS4 carbon fiber and the LCP matrix material.; The electropolymerization of a polyphenylene oxide (PPO) coating on the AS4 surface enhanced the fiber-matrix adhesion by 14 percent relative to the as-received AS4/LCP system. Apparently, a good bond was established between the AS4 fiber, the PPO coating, and the LCP matrix.; After these treatments, SEM observations of the shear failure surfaces indicated that a perfect fiber-matrix bond was obtained. That is, the failure location was in the LCP matrix material. A two-dimensional finite element analysis also supported this result. Thus, in order to improve the shear strength of the AS4/LCP system further, the shear strength of the LCP matrix material must be increased.