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 delta, and complex viscosity vertical bar eta*vertical bar. 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[vertical bar eta*vertical bar(T)] decrease linearly in the temperature range of T-g(PAL) < T < T-rub(E') (T-g(PAL) is the glass transition temperature determined by PAL measurements; T-rub(E') is the lowest temperature where E(T) curve coincides with its fitting line in the rubbery status stage) and then remain nearly unchanged at T > Trub(r); log[E ''(T)] and log[tan delta(T)] increase linearly at T-g(PAL) < T < T-g(tan delta max)) (Tg(tan.6..) is the temperature of maximum tan delta) and then decrease linearly at T > T-g(tan delta max). In the present work, PAL spectroscopy is demonstrated to be a reliable experimental technique to provide precious quantitative information on free-volume holes in polymers.