Abstract
The main objective of this work has been to develop some rational hypothesis for the toughness of rubber polyblends as defined by the energy to rupture in a tensile measurement. It is shown, both by induction and by direct experimental evidence, that the S/AN rigid phase yields and cold draws. It is thereby established that the toughness of a polyblend is a result of the large energy absorption involved in the cold drawing of the matrix. The presence of inherent flaws or cracks limits the strength and ductility of some glassy polymers, whereas others are known to cold-draw. It is believed, therefore, that the rubber phase in a polyblend acts principally to induce a yielding in the S/AN matrix. There are at least two factors which contribute to this yielding. (a) By limiting the crack growth, the rubber phase effectively strengthens the rigid phase, prevents premature fracture, and permits yielding. In this connection, it is believed that under a tensile strain, the difference in the Poisson's ratio of the rubber and matrix phases results in a triaxial stress field in the environment of the rubber particle. Under such a triaxial stress field, the rubber particle possesses a strength which is greater than in tension and which is sufficient to prevent crack propagation. (b) Secondly, the triaxial field will result in an increase in the free volume of the SAN matrix adjacent to the rubber particle. This, combined with other factors, causes an increase in free volume sufficient to lower the glass transition temperature of S/AN to the point where large-scale molecular motion or cold drawing is further favored.
Interrelation between mechanical response, strain field, and local free volume evolution in glassy polymers: Seeking the atomistic origin of post-yield softening