Abstract
We investigated the formation of cocontinuous structures in polymer blends. These polymeric bijels (bicontinuous interfacially jammed emulsion gels) were composed of polystyrene oligomer, polybutene and fluorescent hydrophobic silica nanoparticles. A micron-sized cocontinuous morphology was stabilized by a monolayer of silica nanoparticles at the interface. Real-time observation of coalescence dynamics in co-continuous polymer blends stabilized by interfacial particles was for the first time achieved via laser scanning confocal microscopy. We demonstrated that suppression of coalescence arises from coverage of interfaces by nanoparticles. Furthermore, by combining confocal microscopy with rheology, we correlated the rheological response of a cocontinuous structure with its morphology change. We found that the rheological behavior can be attributed to competition between interface shrinkage and particle network formation. In addition, we showed that a particle scaffold is maintained even after the remixing of two polymer phases above the spinodal point. Finally, we also discussed differences between the shear response of the particle-stabilized cocontinuous structure and normal colloidal gels: the former one is more fragile than the latter under shear.
Introduction
Cocontinuous polymer blends are composed of two immiscible or partially miscible polymers in two interpenetrating domains.1 In contrast to the more accessible droplet-matrix morphology, cocontinuous morphology can impart superior material properties to composites such as enhanced mechanical modulus, impact and electrical conductivity.1 Moreover, after extracting one phase, cocontinuous polymer blends become porous membranes, which have been used as lithium battery separators2, supporting substrates for catalysts3 and scaffolds in tissue engineering4,5. Given their importance, it is not surprising that there is growing interest in research of cocontinuous polymer blends 1,6,7.
For two immiscible polymers, cocontinuous blends can be achieved by mechanically melt mixing.1,7 Partially miscible polymer blends with a miscible region can also form the cocontinuous morphology via the demixing process from spinodal decomposition.8,9 Regardless the specific route, however, the resulting cocontinuous morphology is intrinsically unstable due to its non-equilibrium nature. Left alone, interfaces between the two polymers will coalesce and the cocontinuous structure will reduce to droplet-matrix morphology.6,10 To stabilize cocontinuous morphologies, a creative strategy exploiting nanoparticles has been proposed. Nanoparticles trapped at the interface by capillary forces can effectively suppress the coarsening of polymer phases during annealing and stabilize the cocontinuous morphology. For example, Jerome and co-workers added oxidized carbon black (CB) in immiscible polymer blends such as polyethylene/polystyrene (PE/PS)11 and polystyrene/poly(methyl methacrylate) (PS/PMMA)12 during melt mixing. Thermodynamic stabilization of cocontinuous morphologies of these polymer blends was achieved thanks to selective localization of CB at the blend interface. Composto et al. trapped PMMA grafted silica nanoparticles (SNP) at the interface of a partially miscible polymer blend, poly(methyl methacrylate)/poly(styrene-co-acrylonitrile)(PMMA/SAN), which led to cocontinuous
morphology after spinodal decomposition with particles jammed at the interface.13,14,15
Although a few particle-stabilized cocontinuous polymer blends have been achieved in experiments, to our knowledge no studies have been conducted to systematically investigate dynamics by which particles inhibit the coalescence of cocontinuous polymer structure.16,13 In contrast, the dynamics of cocontinuous morphology has been recently investigated in the so-called “bi-continuous interfacially jammed emulsion gel” or “bijel”. Bijels are composed of two interpenetrating low-viscosity fluids in a cocontinuous structure, which is stabilized by an interfacial colloidal monolayer through spinodal decompostion.16,3,17 The low viscosity of fluid phases in a bijel enables direct study of the dynamics of interfacial particles during coalescence and the formation of cocontinuous structure.16,3,17 Here, to exploit the unique feature of bijel systems in the study of polymeric cocontinuous structure, we bridged the separate studies from polymer and colloid fields by introducing a novel polymeric bijel composed of two low molecular weight polymers or oligomers. The system allows us for the first time to investigate the microscopic dynamics of interface coalescence in polymer blends and access the change of particle interfacial coverage in real time.
Specifically, we used a polystyrene oligomer (PS) and a low molecular weight polybutene (PB), which show an UCST phase separation and have a viscosity low enough at room temperature to be suitable for direct imaging. We introduce hydrophobic silica nanoparticles (B-SNP) to stabilize cocontinuous morphology through spinodal decomposition. The new PS/PB/B-SNP bijel system has several unique advantages: First, as already explained, when compared with high molecular weight cocontinuous polymer blends, PS and PB are viscous liquids at room temperature, thus facilitating real time observation of coalescence under confocal microscopy. Second, compared with conventional bijels, the PS/PB/B-SNP system has higher viscosity at room temperature (~10-102 Pa·s), and its coalescence dynamics are directly relevant to that of high molecular weight cocontinuous polymer blends (Fig. 1). Third, different from bijels 3
where highly polar organic solvents have been used such as water/2,6-lutidine (W/L)19 and nitromethane/ethylene glycol (NM/EG)20, PS/PB used in our polymeric bijel, like most high molecular weight polymeric blends formed at high temperature, are much less polar.21 Such a difference imparts unique particle-matrix and inter-particle interactions, as we show later. Thus, our study can be seen as the first attempt to bridge two separate fields, which may stimulate further investigations on the universal properties of the cocontinuous blend in widely different systems.
