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The effects of flow induced domain orientation on the rheological response of triblock copolymers
Abstract
Block copolymers comprised of incompatible blocks form ordered microphase separated domains. The domains orient within a microscopic granular structure. But, the director of the grains varies randomly from grain to grain, giving a macroscopically isotropic structure. However, application of flow orients the domains, producing a macroscopically anisotropic structure that dictates the final properties. The effects of flow on domain orientation and the resulting anisotropic properties of triblock copolymers were investigated. Domain orientation effects were best seen in the most ideal structure where all the domains are aligned uniformly throughout the sample, known as 'single crystal' structure. Appropriate deformation conditions to produce single crystal in microphase separated triblock copolymers were developed. The flow induced morphologies were studied by transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). The linear viscoelastic properties of the anisotropic morphologies were investigated as a function of domain orientation with small amplitude oscillatory shear flow. Two triblock copolymer materials were studied: polystyrene-polybutadiene-polystyrene (SBS81) and polystyrene-polyisoprene-polystyrene (SIS56). Both materials have an equilibrium morphology of hexagonally packed cylinders of polystyrene in a rubbery matrix. However, the phase behavior of these two materials is quite different. SBS81 remains phase separated at all accessible temperatures while SIS56 has an accessible homogeneous phase. SBS81 samples were subjected to planar extension to produce single crystal structure. Optimum extension criteria were defined by following domain orientation as a function of extension conditions. Development of a novel sample preparation technique allowed linear viscoelastic property measurements with respect to the domain orientation. Dynamic shear moduli revealed the flow mechanism for domain orientation is dependent on the domain orientation direction. Large strain behavior of single crystal structure in combination with SAXS provided information about the flow processes during domain orientation. The second material studied has an accessible homogeneous phase, where the transition to the homogeneous phase was characterized with rheology and SAXS. These two experimental techniques were in good agreement. The ordering kinetics of SIS56 were exploited to produce single crystal structure with large amplitude oscillatory shear during phase separation. The linear viscoelastic properties of SIS56 single crystal were found to be influenced by domain orientation.
Subject Area
Chemical engineering|Polymer chemistry
Recommended Citation
Scott, Diane Brooks, "The effects of flow induced domain orientation on the rheological response of triblock copolymers" (1992). Doctoral Dissertations Available from Proquest. AAI9219495.
https://scholarworks.umass.edu/dissertations/AAI9219495