a detailed investigation of the microscopic mechanisms of tensile stress deformation was performed. The experiments involved two different thermoplastic materials: high impact polystyrene (HIPS! and a novel polystyrene CPS)-polybutadiene (PB) blend, at temperatures ranging from 30 to 70 degrees C, deformed at a constant uniaxial strain rate. Real-time measurements of the applied stress, X-ray absorption by the sample, and small-angle scattering were used to identify the presence of shear and craze deformation stages. Craze fibril surface energies were determined from calculations of the fibril diameter in order to distinguish between craze growth by the competing mechanisms of chain scission and forced reptation.