CRAZE FORMATION IN AMORPHOUS POLYMERS IN RELATION TO THE FLOW AND MAIN TRANSITION

Crazes were produced and analyzed in unnotched tensile bars of poly(st yrene-co-acrylonitrile) (SAN) and polycarbonate (PC) in creep experime nts above T-VF (the Vogel temperature). The craze microstructure was i nvestigated as a function of temperature (T) and load (sigma) by means of small angle X-ray scattering (SAXS) and transmission electron micr oscopy (TEM). Contrary to expectation, the scattering vector of maximu m intensity (s(max)), which is inversely proportional to the distance between the fibrils of crazes, was not linearly dependent on sigma at constant temperature. At the highest stresses (regime III), s(max) was independent of stress, and the average distance between the fibrils r eaches a minimum value. At intermediate stresses (regime II), a strong increase of fibrillation energy Gamma was detected, as the temperatur e was reduced. In the vicinity of T-VF, Gamma reached values of the or der of the polymer chain fracture energy. At the lowest stresses (regi me I), the energy of fibril formation was independent of temperature a nd corresponded to the van der Waals surface energy. The molecular mot ions during fibril formation may be linked to local stress-induced flo w processes of polymer chains (regime I) and a-relaxations (regime II) . Increasing stress restricts the range of mobility of macromolecules to shorter and shorter units and a transition from the formation of fi brillated crazes to homogeneous crazes or shear deformation processes occurs at the highest stresses (regime III). A pressure-temperature di agram was constructed from the transition between the regimes, particu larly at negative pressure.