Does plastic deformation proceed near thermodynamic equilibrium? The case made for shear-strained lamellar diblock copolymers

Observations on kink bands in lamellar diblock copolymers (SEP 40-70), caus ed by unidirectional or oscillatory shear strain, are interpreted in terms of the low-energy structure (LES) hypothesis, to wit: "In a material subjec t to mechanical stresses, that structure will be approached which has the l owest free energy among all structures which are in equilibrium with the tr actions and are accessible to the system." This is the generalization of th e low-energy dislocation structure (LEDS) hypothesis applicable to dislocat ion structures in crystalline materials. In agreement with the LES hypothes is, moderate fatigue cycling of initially disordered material establishes a n order such that the plane of the lamellae is parallel to the plane of she ar stress application, being the orientation of lowest shear modulus and, h ence, for fixed fatigue amplitude, of lowest strain energy. At fatigue stra in amplitudes above about 40% the material develops kink bands on account o f the compressive stress along the body diagonal of the samples. The geomet ry of these kink bands shows that the plane parallel to the lamellae serves as preferred slip plane with the lowest resistance against sliding among a ll possible directions. Also the kink band morphology conforms with the LES hypothesis. Specifically, on average the ratio of kink band length (L) to the square of kink band width (W), i.e., L/W-2, is nearly constant as expec ted from the minimization of kink band boundary energy and the elastic stra in energy on account of the strain discontinuity at the ends of the bands. Subsequent experiments on a different copolymer in a range of temperatures additionally verify the LES hypothesis through establishing that, throughou t, large-amplitude cycling causes the lamella orientation of lowest shear m odulus. (C) 1999 American Institute of Physics. [S0021-8979(99)02809-1].