Investigation of composite interphase using dynamic mechanical analysis: Artifacts and reality

Abstract The fiber‐matrix interface is an important factor determining the overall mechanical properties of composites. This interface is no longer regarded as a sharp boundary, but is now considered to be an „interphase,”︁ i.e., a region surrounding the fiber where properties differ from those of the bulk matrix. Although the concept of the interphase is rapidly gaining acceptance, its in situ detection and characterization remains a largely unsolved problem. Dynamic mechanical analysis (DMA) is a technique known for its sensitivity in the detection of inhomogeneity in polymer morphology. Recent publications have claimed that the interphase in unidirectional fiber‐reinforced composites is detectable using DMA. We have been evaluating a Du Pont 982 DMA on its uses in characterization of composites and their individual components. We have found that using heating rates higher than 2°C/min produces an artificial peak in the DMA loss spectrum of glass‐fiberreinforced epoxy composites at temperatures above the matrix glass‐transition temperature ( T g ). This peak was not present in the data from the unreinforced matrix nor in data from carbon‐fiber‐reinforced samples. This artifact could be interpreted as evidence of an interphase. However, our investigations revealed that it is in fact due to a complex interaction of the instrument, the thermal conductivity of the sample, the heating rate, and the sample modulus above T g . Despite this artifact, the high sensitivity of the Du Pont 982 DMA enables detection of inhomogeneities in the composite matrix that are attributable to an interphase.