Yielding Robertson theory

In another molecular approach Argon (28) has proposed a theory of yielding for glassy polymers based on the concept that deformation at molecular level consists in the formation of a pair of molecular kinks. The resistance to double kink formation is considered to arise from the elastic interactions between chain molecule and its neighbors, ie, from intermolecular forces. This is in contrast to the Robertson theory, where intramolecular forces are of primary consideration. We need to recall that the intramolecular forces are by several orders of magnitude stronger than the intermolecular ones—except for entanglements which operate as if they were primary chemical bonds. 

Different approaches were used to describe the yielding of polymers quantitatively. Some theories took into account the free volume fraction. Eyring considered thermally activated mechanisms, and Robertson s model was based on changes of chain conformations. Argon s and Bowden s models were based on a metallurgical approach and a dislocation theory. A brief summary of the existing yielding theories is presented. 

Klemperer. Calculations by Coulson and Robertson on other hydrogen bonding systems yielded times of -10 s. While these discrepancies between theory and experiment are probably not a record, they give one pause to wonder about the factors which determine the efficiency of the vibrational predissociation process. 

Molecular Theories of Yielding. Several models of yielding of amorphous polymers are based on continuum mechanics invoking the concepts of free volume, springs and dashpots, and viscosity. Some of the theories take molecular point of view. Robertson (26,27) assumed chain bending as the fundamental mechanism for yielding. He postulated that the 3rield point corresponds to the state where the amount of bends produced by the stress were equivalent to the number of bends that occur at Tg. 

---