Craze stability

At higher temperatures, fibrils fail by chain slippage, which is a MW-dependent process. Indeed, even for CSCs, the MW of the chains within the fibrils increases with the initial MW of the sample. Thus, the craze stability is directly related to the polymer MW. When dealing with CDCs, the break of fibril by chain slippage is a direct function of the material MW. 

An increase in molecular weight leads to significant improvements in the fatigue life of both amorphous and crystalline polymers. This improvement is attributed primarily to an increase in craze stability and to an improvement in the fibril density, contributing to greater resistance to liquid transport. As the fraction of low-molecular-weight molecules cannot contribute to ESCR, it is found that for similar molecular weights narrow MWDs are clearly superior. 

An increase of the craze stability of tensile-induced crazes with rising M has been noted in various glassy polymers. In a study made on thin PS films by Berger and Kramer the craze extension ratio was determined at various temperatures for polymers of varying M. For measurements made at room temperature, X varied little with molecular weight but, as the temperature was raised, and chain disentanglement processes became more probable, the situation changed. For example, at 50 °C,... 

The influence of M on craze stability is also evident from studies made on polymer blends. In a study by Donald and Kramer PPO (M = 35,000) was blended with either a HMW PS (M = 300,000) or a LMW PS (M = 4,000). Over a wide range of compositions, the X values were considerably higher, and craze stability poorer, for the blends made with the LMW polymer. In another study, blends were prepared from two different molecular weight grades of PPO with a grade of PS having M = 4,000. The crazes formed in the HMW PPO (M - 197,000) were long and stable while those formed in the LMW PPO (M = 35,000) were short and weak and readily fractured . 

The dramatic enhancement of fatigue performance in glassy polymers with increase of molecular weight is attributed primarily to an increase in craze stability and, in some polymers, there is evidence that it also results from a delay in the craze initiation process. 

The other important influence of the bulk properties of the homopolymers is on the fibril stress, i.e. the craze stability. Even if, as pointed out above, the crazing stress for a glassy polymer does not vary much, it is well known that the molecular weight of the polymer has a profound effect on its fracture toughness [58]. 

Tres is a measure of the time the strand spends within the active zone and tdis is the time for disentanglement, and is a function of strand molecular weight, the molecular weight between entanglements, the force on the strand, and the monomeric friction coefficient. Though the Tres term is a fitting parameter, the model captures many of the features observed with craze stability such as the molecular weight dependence (117). 

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