Particles that are partially wetted by oil and water are well known to adsorb at oil/water interfaces. Such interfacially adsorbed particles can stabilize emulsions of oil and water, and these emulsions are called Pickering emulsions. We seek to transplant this idea of Pickering emulsions to polymeric systems: specifically to create stable morphologies composed of two immiscible homopolymer phases and interfacially adsorbed particles. Previously we have shown that particles readily adsorb at the interface between a variety of polymers, and that even a small fraction (well under 1 vol. %) of particles can greatly affect the morphology of droplet/matrix polymer blends. Furthermore, when particles are sufficiently crowded at the interface, they jam the interface into a solid like state that prevents interfacial tension driven changes of the morphology. Here we propose a comprehensive experimental study of the effects of interfacially active particles in immiscible polymer blends. We hypothesize that particles that adsorb at the interface between two immiscible homopolymers can be used to control the morphology of the blend, in particular they can control the sizescale of the morphology, create highly anisotropic (cylindrical or lamellar) morphologies, as well as generate a new type of layered structure.
Research Proposal: Experiments will be conducted on a model pair of homopolymers,(crosslinkable polyisoprene (PI) and polydimethylsiloxane (PDMS)), and silica particles that are surfacemodified to adsorb at the PI/PDMS interface. In situ flow visualization experiments will be conducted under shear flow conditions to characterize the morphology. Particle scale imaging will be performed ex situ by crosslinking the PI, washing away the uncrosslinked PDMS, and then conducting SEM of the particle laden interface. In initial research, we will devise flow protocols that induce the particles to rapidly adsorb at the interface, and induce them to crowd at the interface to cause interfacial jamming. We will also examine whether particles aggregate or spread on the interface, and whether they reduce the interfacial tension. The core of the research then is to test the above hypothesis about morphology control. The effect of particles on the flow induced morphology will be examined as a function of the relative volumes of the PI and PDMS, the particle loading, and the wettability of the particles. The ability of the particles to control the sizescale of the morphology and their ability to stabilize anisotropic morphologies by interfacial jamming will be examined. Finally, at high particle loadings when the particles tend to form layers in shear flow, we will examine whether these particle layers can template a PI/PDMS alternating lamellar morphology.
Intellectual merit: This proposal unites knowledge in three different areas: interfacially adsorbed particles in oil/water systems, polymer compatibilizers at polymer/polymer interfaces, and the flow induced morphology in polymer blends. These ideas are combined together to develop particulate compatibilizers particles that act as interfacial modifiers in polymeric systems in a fashion similar to block copolymer compatibilizers. Such particulate compatibilizers have the potential for achieving morphology control far beyond what is possible with conventional block copolymer compatibilizers. During the course of this research, we will also address several practical and fundamental issues such as how to rapidly adsorb the particles on the interface, and whether particles reduce the interfacial tension of polymeric interfaces. Finally, the PI has had success with transplanting several concepts from oil/water systems to polymeric systems, and this proposal goes considerably further in that direction.
Broader impact: This proposal will develop a new mechanism to precisely control the morphology of polymer blends, and develop a new type of particle templated morphology composed of alternating layers of two immiscible polymers with particles at each interface. One graduate student and several undergraduates will be trained. The PI will develop a new hour long undergraduate level module on wetting phenomena for engineers combining hands on experiments with theoretical discussion. This is a part of a series of modules on soft matter that will be made available publicly.
Blends of immiscible polymer are used in numerous applications, e.g. toughened plastics or diffusion barrier packaging. The blend properties depend strongly on the microstructure and hence there is much interest in controlling and tuning the microstructure of multiphase polymeric materials. The specific goal of this research was to examine how particles can be used to control the microstructure of immiscible polymer blends. Experiments were conducted using materials carefully chosen to highlight the phenomena of interest, viz. coupling between the effect of particle adsorption at the polymer/polymer interface and capillary phenomena associated with interfacial tension. We discovered that particles greatly affected blend microstructure and rheology (i.e. flow properties) even at volumetric loading as low as 0.1%. Notably the effects of adding particles depended – even qualitatively – on particle wettability. If particles were equally-wetted by both polymers, then the particles would adsorb at the interface at sufficiently high coverage to jam the interface resulting unusual morphologies, e.g. non-spherical drops. In contrast, if particles were preferentially-wetted by the continuous phase polymer, the particles would bridge drops together into a gel-like state with a many-fold increase in viscosity. In summary, the early portion of this research showed that adding a small amount of interfacially-active particles to an immiscible polymer blend is an effective way to control morphology. This research also addressed the opposite case when one starts with a concentrated dispersion of particles in one polymer and adds a small amount of another immiscible polymer. In this case as well, there are major structural consequences: the initially-free-flowing dispersion turns into a gel or into a suspension of pasty aggregates (depending on how much of the second polymer is added). The flow properties of the gels were found to be very similar to conventional colloidal gels. We have also conducted experiments on similar oil-water-particle systems across a broader range of compositions and shown numerous structural transitions which are analogous to similar transitions in wet granular materials. The intellectual merit of this project was to draw from three well-studied areas of research: (1) the structure-flow relationships in polymer blends, (2) the role of compatibilizers in tuning blend structure and properties, (3) the interfacial activity of particles at the interface of two liquids. We successfully united knowledge from all three areas to show how interfacially-adsorbed particles can be used to tune the structure of polymer blends. This project points to how the coupling between capillary phenomena, wetting phenomena, and mixing can be exploited to realize the desired morphologies in polymer blends. It also shows how three-phase systems can be used for templating the synthesis of new materials. Additional broader impact of this project was in training two post-doctoral fellows and several undergraduate researchers. The principal investigator conducted outreach at undergraduate and pre-college level to bring aspects of this research to lay audiences and to encourage pre-college students to consider engineering careers.
