Studies of Structural Dynamics in Polymer Blends Using Rheology and Small-Angle X-Ray Scattering Techniques

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Structural dynamics in three polymer blend systems, differing largely with respect to morphology, have been investigated. The first system, composed of two immiscible homopolymers, exhibits a microstructure of micron-sized droplets dispersed in a matrix phase. We have examined small-angle x-ray scattering (SAXS) as a new approach for conducting in situ studies of flow-induced structural changes in these types of blends. This approach relies on Porod scattering, which is related to the interfacial properties in two-phase systems. Indeed, we have successfully performed in situ SAXS measurements of an immiscible blend in response to applied shear, allowing for the observation of deformation and relaxation of interface with time. The second blend system, a bicontinuous microemulsion (BmE), consists of two cocontinuous domains of immiscible homopolymers compatibilized by diblock copolymer at the interface. The equilibrium dynamics were characterized via x-ray photon correlation spectroscopy. These measurements were used to test rheological predictions for bicontinuous microemulsions by P├Ątzold and Dawson. Although the predictions describe the shape of the relaxation spectrum fairly well, the theory fails to predict absolute values of the rheological properties. These results highlight a need for the development of more sophisticated theory to describe the dynamics of bicontinuous microemulsions. The shear-induced dynamics of the BmE microstructure were interrogated using rheology, in situ SAXS, and optical microscopy. Optical microscopy revealed micron-sized phase-separated structures coexisting with the nano-scale BmE phase at equilibrium. Direct comparison of this multi-phase system to a previously documented BmE system by Bates, Lodge, and coworkers strongly indicates that the microemulsion morphology dominates the rheology and scattering behavior at linear to moderately non-linear shear conditions, whereas the phase-separated structures govern the response under more severe shear conditions. The third system is a sponge phase, characterized by two domains of a single homopolymer separated by a continuous, multiply-connected bilayer membrane, which is formed by triblock copolymer. The flow behavior of polymer-based sponge phases has not been previously documented. Rheology and in situ SAXS measurements of our sponge system indicate very slow topological dynamics, which appear to give rise to unexpected, persisting anisotropy in the sponge structure that does not relax under quiescent conditions

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  • 08/01/2018
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