Craze formation in amorphous polymers in relation to the flow and main transition

Abstract

Crazes were produced and analyzed in unnotched tensile bars of poly(styrene-co-acrylonitrile) (SAN) and polycarbonate (PC) in creep experiments above T_VF (the Vogel temperature). The craze microstructure was investigated as a function of temperature (T) and load (σ) by means of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Contrary to expectation, the scattering vector of maximum intensity (s_max), which is inversely proportional to the distance between the fibrils of crazes, was not linearly dependent on σ at constant temperature. At the highest stresses (regime III), s_max was independent of stress, and the average distance between the fibrils reaches a minimum value. At intermediate stresses (regime II), a strong increase of fibrillation energy Γ was detected, as the temperature was reduced. In the vicinity of T_VF, Γ reached values of the order of the polymer chain fracture energy. At the lowest stresses (regime I), the energy of fibril formation was independent of temperature and corresponded to the van der Waals surface energy. The molecular motions during fibril formation may be linked to local stress-induced flow processes of polymer chains (regime I) and α-relaxations (regime II). Increasing stress restricts the range of mobility of macromolecules to shorter and shorter units and a transition from the formation of fibrillated crazes to homogeneous crazes or shear deformation processes occurs at the highest stresses (regime III). A pressure—temperature diagram was constructed from the transition between the regimes, particularly at negative pressure.