The gel‐phase modulus of high‐impact polystyrene from dynamic mechanical spectroscopy

High‐impact polystyrene (HIPS) constitutes a mechanically attractive composite, consisting of a glassy matrix and a rubberlike particle phase (gel phase). Dynamic mechanical spectroscopy was performed for the polystyrene matrix for three different types of HIPS as well as for the concentrated gel‐phase material, at the vicinity of the respective glass‐transition temperatures ( T g ). An approximate estimation of the gel‐phase modulus was attempted by using known mechanical models. A comparison with experiments was also made. The modulus of the composite was found to be lower than the theoretical lower bound for particulate composites. This was attributed to a separate phase between gel particles and the matrix. A diffusion‐type variation of the modulus of this mesophase layer was estimated, and a correlation between calculated fitting parametric exponents and impact behavior of HIPS was found. Moreover, the T g s of the materials under investigation were also measured with two independent methods. It was found that all types of HIPS presented higher T g s than the pure matrix by 5 to 10°C with the highest T g found being that of the gel‐enriched material. The shift of T g s to higher temperatures was attributed to an eventual increase of the effective cross‐link density of the matrix because of grafting.