Effect of free-volume holes on mechanical properties of carbon-fiber-reinforced polymers (CFRPs) studied by positron annihilation age-momentum correlation spectroscopy

Abstract

To improve the mechanical properties of carbon-fiber-reinforced polymers (CFRPs), various factors of microstructure were investigated, but the effect of free-volume holes was rarely studied. In this work, a series of CFRPs which were prepared from DGEBA (diglycidyl ether of Bisphenol A) epoxy resin and treated carbon fibers (TCF), were characterized by positron annihilation age-momentum correlation (AMOC) spectroscopy experiments to reveal their free-volume hole properties. The results of the $S\left(t\right)$ parameter indicate that CFRPs and the DGEBA matrix have similar free-volume environments, which are primarily governed by the free-volume of DGEBA and the carbon fiber/matrix interface. From the positron annihilation lifetime (PAL) spectra derived from AMOC data, with increase of TCF content, CFRPs exhibit similar free-volume hole size, narrower hole size distribution, and lower fractional free-volume (FFV). To characterize the interfacial interaction between matrix and TCF for each sample, a parameter $\beta$ was calculated from PAL spectra results. Eventually, for TCF contents in the range from 39 to 58 wt%, five mechanical parameters (tensile strength, tensile modulus, flexural strength, flexural modulus, and shear modulus) are almost linearly and negatively correlated with both relative fractional free-volume (FFV_r) and interfacial interaction parameter ($\beta$). This work provides a specific free-volume perspective to understand the macroscopic mechanical properties of CFRPs.