Studies on the Mechanism of Craze Formation Process in Glassy Polymers

It is generally recognized that craze cracks are not mere voids, but are filled with molecules oriented to certain microvoids. However, there has been scarcely any precise quantitative studies hitherto made on the mechanism of the formation of crazes. This is partly due to the statistical nature of crazing phenomena.In this paper, what is the statistical nature of the crazing phenomena in polymethyl methacrylate and polycarbonate has been made clear. Fluctuation of time required for the initiation of crazes after loading were analyzed on the basis of a stochastic theory. The results show that the frequency distribution curves of the time for craze initiation are markedly a skew. This fact indicate that the fluctuation that is observed has not been produced by an experimental error nor by the inhomogeneity of the specimens. The cumulative frequency can be expressed by P=exp(mt), where m is probability of the craze initiation. A slight delay in time for m to approach the constant value is recognized. The relation between m and applied stress is expressed by m=A exp(Bσ), where σ is applied stress, A and B are constants independent of the stress. This expression is in accordance with the Eyring equation dependent on stress. Therefore, the crazing phenomena are explained as a kind of rate process. The value of 16kcal/mol has been obtained as an activation energy for both cases with and without kerosene. This value is much lower than that of macroscopic yielding behaviors. This difference means, therefore, that crazing is not regarded as a local yielding or flow of polymers.