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Composites made with high performance polymer fibers can achieve axial properties comparable to those which use inorganic reinforcements, but with somewhat inferior interfacial properties. A high degree of chain alignment in polyaramids, such as Kevlar, produces weak interactions between adjacent polymers, resulting in poor transverse strength in the fiber, and low interfacial shear strength in composite systems. The latter controls many composite properties, such as transverse, shear and flexural strengths. Also, by reducing the tendency to form weak interfacial boundary layers, good fiber-matrix adhesion can enhance environmental stability. For these reasons, modifications to reinforcement fiber surfaces that promote fiber-matrix adhesion are frequently used to improve the performance of composites.