(173e) What Is the Origin of “Strain Hardening”?

In uniaxial compress, polymer glasses show a sequence of events as a function of strain in the order of elastic deformation, yielding, plastic flow and “strain hardening”.  Plastic flow corresponds to the regime where the stress is observed to drop and level off.  The so called strain hardening occurs when further straining beyond the plastic flow regime produces rising stress.  Since the stress level and the corresponding “modulus” G_R is 100 times the elastic plateau modulus measured in the melt state, the origin of stresses has become a subject of intensive investigation.  Modeling this strain hardening behavior using the formula of the classical rubber elasticity theory is clearly fruitless and misleading as remarked by Kramer [1].  But in polymer physics only two types of force are known to be prevalent: viscous forces as seen in stead shear flow of entangled polymeric liquids and elastic forces of entropic origin as seen in vulcanized rubbers.  Can we explain the strain hardening in terms of a combined of these forces?

[1] Kramer, E. J. J Polym Sci Part B: Polym Phys 2005, 43, 3369–3371.