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Cocontinuous Polymer Blends: Controlling Morphology via Interfacial Modification and Rheology

Aaron Hedegaard Ph.D. Thesis Defense
aaronHedegaard.jpgChemical Engineering and Materials Science Department
IPrime Programs: Microstructured Polymers
March 7, 2014, University of Minnesota
Advisors: Chris Macosko (CEMS)

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Cocontinuous polymer blends are formed by melt blending two or more immiscible polymers to form multiple continuous interpenetrated networks. These are non-equilibrium structures where the morphology is determined by a combination of processing conditions, interfacial properties, and rheology. Thermodynamic instability causes the morphology to coarsen when heated above the glass transition temperatures of the components. Furthermore, due to the complexity of the mechanism of cocontinuity formation and the interdependence of the factors which determine cocontinuity, a thorough understanding of when and how cocontinuity forms has not been developed, and predictive models are empirical and frequently contradictory. This thesis seeks to advance the field of immiscible polymer blends by providing insight to two critical questions. First, can a better understanding of the role of interfacial stabilization on cocontinuity be developed? Second, can morphological predictions based on rheology be improved?

Concerning interfacial stabilization, this thesis approaches the problem via reactive blending and interfacially localized clay nanoparticles. The effectiveness of reactive blending was found to be heavily dependent on the molecular weight of the reactive polymers. Also, the formation of a copolymer brush at the interface was able to prevent coarsening due to a compression of that brush when interfacial area decreased. Nanoclays, when interfacially localized, were found to also prevent coarsening by jamming at the interface, and the combined compatibilization mechanism of reaction and clay was found to achieve the smallest phase sizes. As an application, these compatibilized blends were also tested as gas separation membranes.

Concerning the predictions of cocontinuity, various models from the literature were tested against experimental data to determine the center of the compositions that resulted in cocontinuity. It was found that models based on droplet packing worked best, though they gave no information concerning the range of cocontinuous compositions. Various mechanisms and rules of thumb were developed from the present work to provide much-needed insight for predicting the relative size of these ranges. This study also investigated the role of extensional viscosity on cocontinuity by blending with long-chain branched polymers, where it was found that strain hardening branched polymers significantly broadened the range of cocontinuity. This demonstrated a shortcoming in the existing predictive models, which only considered shear rheology when predicting cocontinuity.

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