The influence of interphase regions on local thermal displacements in composites

Although the exact physical and chemical mechanisms are not clearly understood, it is widely believed that an interphase region with properties that differ from those of the plain matrix is developed near fiber surfaces in polymer matrix composites. The current study involves experimental investigation and theoretical modeling of the influence of the interphase on local thermal displacements. Experimental studies have centered on the development of a scanning microinterferometer for in-situ measurements of thermal displacements in the interphase. Thermal displacement measurements have been successfully made for specimens containing a single carbon fiber embedded in an epoxy matrix. A three-phase composite cylinder model is adopted to predict the thermal displacements of the single fiber specimen. Comparison of the theoretical displacement predictions with the experimental profiles measured by the interferometer indicate that the value of the matrix properties near the fiber surface differs from the value in the bulk resin. The data provide evidence of the existence of a lower glass transition temperature in the interphase.