Surface modification of poly(vinyl alcohol) fibers to control the fiber-matrix interaction in composites

Polymeric fibers with varied acid/base behavior and wettability were prepared to control fiber-matrix interactions in fiber-reinforced composites. Water-insoluble high-performance poly(vinyl alcohol) fibers were equipped with different surface functionalities on a molecular level by chemical bonding of aldehydes and adsorption of acidic and alkaline polyelectrolytes. The fibers were characterized by surface-sensitive methods, such as X-ray photoelectron spectroscopy, zeta potential, and contact angle measurements. The modification resulted in stable thin nonpolar layers or polar acidic, alkaline, and amphoteric surface functionalities on the fiber surface, with advancing contact angles of deionized water between 30° and 90°. Fiber-matrix interactions were probed by pullout tests of single fibers embedded in a cementitious matrix and subsequent morphological analysis of the fibers. Polar surface functionalities caused a strong fiber-matrix adhesion while nonpolar, hydrophobic surface layers decreased the adhesion dramatically. The surface charge and acid/base behavior of the fibers had no significant influence.