A very large percentage of studies seeking to improve fiber-matrix adhesion to alter composite properties are directed at the formation of primary chemical bonds between the reinforcement surface and the matrix. The dynamic events that occur when the fiber-matrix interface is formed lead to the creation of an interphase which can have properties quite different from the matrix in addition to any chemical bond formation.
This study has been directed at elucidating the role of these interphase properties themselves on fiber-matrix adhesion. A reinforcement (AS-4 carbon fiber) with a quantifiable surface chemistry and an epoxy matrix have been kept constant through a series of experiments in which the distance between crosslinks of the matrix has been changed. The wettability of the fiber and the degree of chemical bonding to the fiber have not changed but the interfacial shear strength measured for each of these systems has decreased with decreasing matrix modulus. It will be shown that the properties of the matrix and the residual stresses created during processing limit the maximum interfacial shear stress that can be supported by the fiber-matrix interphase.