| Abstract: | The tear strength of several thermoplastic polymers filled with colloidal silica has been determined as a function of polymer molecular weight. There were two regimes of behaviour. Low molecular weight materials were embrittled by a small amount, around 10% volume fraction, of filler. High molecular weight polymers, on the other hand, were not degraded at low filler concentrations, and some thermoplastics were actually reinforced, that is toughened, by the colloidal particles with a peak in crack resistance at 12% volume fraction. There was found to be a characteristic transition in toughness, similar to that observed with environmental stress cracking, between the low and high molecular weight regimes. It was concluded that a polymer molecular weight above the transition value was a necessary but not sufficient condition for colloidal reinforcement. |
