Origin and influence of interphase material property gradients in thermosetting composite

For over 30 years interactions between the fibers and matrix of a composite have been suspected to cause the formation of interphase regions adjacent to reinforcement surfaces which possess neither the properties of the reinforcement nor those of the neat matrix. Expectations have been that such interphases may be tailored to effectively control composite behavior characteristics. This work represents a major contribution to elucidating the origins of an interphase region in thermosetting systems and to fulfilling the expectations for such tailoring.;A methodology has been developed which allows composite behavior characteristics affected by interphase structure to be predicted as a function of control variables. It has been applied successfully to investigate the interphase behavior of epoxy-amine thermosetting composite systems. Three major steps comprise the methodology: (i) linking of control variables, such as processing conditions and material selection, to reinforcement-matrix interaction and to the associated chemical and physical processes which lead to the formation of interphases; (ii) relating interphase chemical structure to interphase material properties; (iii) linking predicted material property information to the micromechanical characteristics which affect overall composite behavior.;A thermodynamic theory to describe the equilibrium behavior of a binary mixture in contact with a surface was developed in order to relate control variables to interphase formation in epoxy-amine-graphite systems. Interphase compositions were predicted for available epoxy-amine-graphite fiber systems. Generally, amines preferentially adsorb onto graphite fiber surfaces. The composition profiles are significantly dependent on the epoxy-amine system selected and on the type of fiber or fiber surface treatment used. The size of the interphase depends on the miscibility of the epoxy and amine components.;An experimental procedure was developed for determining interphase material property profiles given interphase composition profiles. This required an extensive investigation of the effects of stoichiometry on the material properties, such as the glass transition temperature, modulus, and coefficient of thermal expansion, of the Epon-828/PACM-20 epoxy/amine system. It is predicted that the interphase layer for the unsized epoxy/amine systems should possess substantially lower glass transition temperatures than the bulk material. This has serious implications regarding mechanical performance at high temperatures. Furthermore, there now exists convincing evidence that the material properties of the interphase can be controlled by varying control variables. (Abstract shortened with permission of author.).