Chain scission craze

Under conditions where chain mobility is very low, such as at low temperatures, the loss of entanglements occurs by chain scission this is what happens for polystyrene air crazes. Of course, chain scission crazes (CSC) are MW-independent as soon as chains are long enough to be entangled (at too-low MWs, no craze can be formed, only cracks happen). 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

In order to understand fracture behaviour, it is important to analyse the types of deformation micromechanisms undergone under strain chain scission craze (CSC), shear deformation zone (SDZ), chain disentanglement craze (CDC) and the temperature range over which each one occurs. Furthermore, it is worth wondering whether these micromechanisms are related to /i transition motions. 

Chain scission crazes also exist in BPA-PC films, but they are observed only at temperatures below -120 °C. 

Characteristic stress associated with chain disentanglement craze Characteristic stress associated with chain scission craze Limit of purely elastic behaviour Plastic flow stress... 

Molar percent of the latter monomer CSC Chain scission craze... 

Fig. 2. This figure illustrates the Uansition from failure by simple ehain scission to failure by crazing with chain scission. In PS crazes the scission tends to occur not at the junction point but in the PS copolymer block.

The low temperature range is characterised by a toughness independent of MW. It extends to a temperature where all the /3 transition motions are not still active. Consequently, the lack of chain mobility within the craze fibrils avoids chain slippage and the craze fibrils fail by chain scission. 

The deformation of the polymer within a thin active zone was originally represented by a non-Newtonian fluid [31 ] from which a craze thickening rate is thought to be governed by the pressure gradient between the fibrils and the bulk [31,32], A preliminary finite element analysis of the fibrillation process, which uses a more realistic material constitutive law [36], is not fully consistent with this analysis. In particular, chain scission is more likely to occur at the top of the fibrils where the stress concentrates rather than at the top of the craze void as suggested in [32], A mechanism of local cavitation can also be invoked for cross-tie generation [37]. 

Craze breakdown is experimentally characterized by a critical craze thickness Acr which is primarily dependent (Eq. 20) on the craze stress ac, the force for chain scission, and the entangled chain density along the craze surface vs. The craze stress ac is assumed to be rate and temperature depen-... 

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