Neutron scattering investigation of the deformation at molecular scales in polymer networks

Abstract

“Stress-optical law” which stipulates proportionality between the stress tensor and optical anisotropy in polymer solids and fluids is not a universal principle. Unique relation between stress and optical tensor is justified only for systems, for which stress, and birefringence are controlled by the same, configurational mechanism. Linear behaviour is confined to the range of small stress. Therefore, linear “stress-optical” relations are commonly observed in configuration-controlled solutions, melts and networks of flexible polymer chains, but fail for elasticity-, and dissipation-controlled solutions and suspensions of rigid particles. Comparison of birefringence of various materials in various deformation (flow) regimes reflects as much material properties, as deformation conditions. A special, simple kind of deformation - uniaxial extension (or steady extensional flow) - reduces relations between stress and birefringence (orientation) tensors to scalar functions. Such characteristics are usually unique, but non-linear, and play important role in the theory of formation highly ordered structures.