Stress-activated chain scission

The final mechanism of stress relief is thermomechanically activated chain scission. Primary bond breakage can be homolytic, ionic or by a degrading chemical reaction. It is worthwhile to note that the relative slippage of chains, microfibrils and fibrils reduces or prevents the mechanical scission of chains in quasi-isotropic polymeric solids. In other words, chain scission is an important mode of fracture only in highly oriented thermoplastic fibers or in thermosets. 

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process B), in which a covalent bond along the polymer main chain is broken and a stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for example, homolytic carbon-carbon cleavage, have obviously high activation energies. The stress-free rates of these reactions are therefore typically extremely low. 

SAN of 0.040 Jm-2. This small value of r(I) is probably related to the fact that separation of chains at low stress levels occurs in the most favourable sites, the polymer behaves like a visco-elastic fluid. As Fig. 12 shows, with increasing stress more such sites are activated and the scattering vector increases. On the other hand, annealing leads to a deactivation of such sites and to a coarser structure of the formed fibrils [62]. It must be concluded that in this regime no chain scission or forced reptation occur. 

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]. 

U Zero stress limit of activation energy for molecular chain scission Y(X) True tensile plastic resistance of homopolymer at a tensile extension ratio of A, Y Athermal plastic resistance of homopolymer with sorbed PB Yq Athermal plastic resistance of pure homopolymer a Craze half length... 

Vq PB diluent concentration at the solubility limit under a standard state Vp Poisson s ratio of block copolymer composite Vp Atomic frequency factor in molecular chain scission Q Active craze front length per unit volume a Negative pressure (mean normal stress)... 

The approach of Zhurkov and Bueche is an activation-volume argument in which the presence of the applied stress lowers the activation energy for chain scission, so increasing the value of the rate coefficient, k. The effect is to increase the prohahility, P, of scission of the central hond from Pq to (Casale and Porter, 1978)... 

Straining the macromolecules result in generation of stresses that may activate some bonds. Mechanically induced chain scission has been explored for grafting polymers and rubbers. 

In general, a variety of mechanisms may contribute to the failure of actual components in service. These may include chemical degradation or oxidation a chemical mechanism that may induce cross-linking and chain-scission. Alternately, other physical processes may alter the state of the polsrmer (eg, surface active agents in the presence of stress may induce crazes due to local diflfiision of the agents near defects). These aspects are not discussed in this article. 

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