Controlling Cocontinuous Polymer Blends with Nanofillers Jammed at the Interface

Abstract

Cocontinuous polymer blends are composed of two or more immiscible or partially miscible polymers coexisted within the same volume in multiple interpenetrated networks. They can be created by either melt compounding of immiscible polymers or phase separation of partially miscible polymer pairs via spinodal decomposition. The polymer blends with cocontinuous structure have significantly improved mechanical properties and they have applications in conductive plastics, porous membranes for filtration and tissue scaffolds for drug delivery devices. As the cocontinuous morphology is in a non-equilibrium state, the thermodynamic instability causes the morphology to coarsen during post-mixing processing, which is a major drawback for applications. In order to control and optimize the phase morphology, nanofillers have been localized and jammed at the interface as an effective method to suppress the coarsening and stabilize the cocontinuous structure during annealing. However, the mechanisms involved in the morphology stabilization by interfacial nanofillers are not yet fully understood. This thesis seeks to systemically study the structure-processing-properties relationships of nanofiller stabilized cocontinuous polymer blends by providing insight to these three questions: (1) how do thermodynamic factors determine nanofiller localization and their morphology stabilization ability? (2) How do kinetic factors affect nanofiller migration during melt compounding and coarsening suppression during annealing? (3) How is the morphology dynamics of nanofiller stabilized polymer blends connected to their rheology response during annealing? Concerning the thermodynamic factors, this thesis approaches the problem via incorporating nanofillers with different surface properties into cocontinuous polymer blends. The different hydrophobicities of silica nanoparticles and the different polarities of graphene nanoplates determine the different localization of these nanofillers in the polymer blends. Nanofiller localization in one polymer phase or at the interface is explained by the system’s tendency to minimize its free energy. Wetting coefficients, which are derived from the Young’s equation and calculated based on the surface energies of nanofillers and two polymer components, have been applied to predict the nanofillers localization in the polymer blends. Concerning kinetic factors, different processing parameters during melt compounding were systemically investigated to study their effect on the migration and localization of nanofillers and their corresponding morphology stabilization ability during annealing. The proper sequence of addition of components is crucially significant to achieve the interfacial localization: nanofillers were generally premixed with the thermodynamically less favorable phase, and then melt compounded with the thermodynamically more favorable phase to enable nanofillers to migrate from the premixed phase to the interface. The effect of different melt compounding time is also systemically studied in the cocontinuous polymer blends stabilized by graphene nanoplates, and we found blends with short melt compounding time have more nanofillers jammed at the interface and more effective coarsening suppression ability during annealing. In order to correlate the morphology dynamics with rheology, we combined rheology time sweeps with morphology information from confocal microscopy, scanning electron microscopy and transmission electron microscopy. We found that morphology coarsening results in shrinkage of interfacial area and jamming of interfacial nanofillers. The nanofillers jammed at the interface contributed to stabilization of the cocontinuous morphology and formation of a 3D nanofiller network. The nanofiller network gave rise to the increase of storage modulus during annealing and the typical gel-like behavior in rheology frequency sweeps.