Shear induced finite orientational order in urethane/urea elastomers

Abstract

New experimental results on urethane/urea free standing thin films (60 μm) prepared from a polypropylene oxide isocyanate terminated triol prepolymer (PU) and polybutadiene diol (PBDO) in the amount of 60% (PU/60PBDO) by weight of polybutadiene, which exhibit periodic patterns upon the application of a mechanical field are used to critically analyse these systems. We focus in this work on the analysis of the casting conditions and on the solid film characterization before and after UV irradiation. The casting of the different solutions was observed by polarizing optical microscopy (POM) coupled to a shearing system. The stress–strain and swelling data collected for the film before UV irradiation allowed the detection of an orientational order induced by the solution casting conditions, involving shear and evaporation of the solvent, which manifests itself in the anisotropy of the stress–strain and swelling film's properties in toluene. After UV irradiation of the elastomeric film, the interlinking of PBDO chains and amines linkages enhances the orientational order enabling instabilities to appear and the mesophase to become evident in the system as shown by POM, stress–strain and small angle light scattering data. Segregated soft and hard parts of the copolymer are thought to constitute the material unit, which is anisotropically distributed. The results here presented reinforce the earlier made claim that these systems do have an anisotropic structure akin to liquid crystallinity.