Effect of Free-Volume Holes on Dynamic Mechanical Properties of Epoxy Resins for Carbon-Fiber-Reinforced Polymers

Six types of matrices of carbon-fiber-reinforced polymers were prepared from different epoxies, amines, and thermoplastics at different curing conditions. Dynamic mechanical analysis measurements were performed to investigate the dynamic mechanical properties of storage modulus E′, loss modulus E″, damping factor tan δ, and complex viscosity |η*|. Positron annihilation lifetime (PAL) spectroscopy was carried out to evaluate each size and fraction of free-volume holes in the sample. The correlations between the dynamic mechanical properties and relative free-volume fraction were studied by using the Williams–Landel–Ferry equation. With increasing relative free-volume fraction, regular changes of dynamic mechanical properties are revealed: log[E′(T)] and log[|η*|(T)] decrease linearly in the temperature range of Tg(PAL) < T < Trub(E′) (Tg(PAL) is the glass transition temperature determined by PAL measurements; Trub(E′) is the lowest temperature where E′(T) curve coincides with its fitting line in the rubber...