(60c) Complex Polymer Architectural Designs for Interfacial Engineering

Authors: 
Tilton, R. D., Carnegie Mellon University
Understanding the relationships between polymer architecture, composition and interfacial behaviors is a classic topic for fundamental and applied interface science. Today, the advent and widespread dissemination of controlled polymerization techniques has enabled a tremendous variety of complex polymer architectures that promise novel engineering applications. Just as the composition and sequential arrangement of block copolymers have long been engineered with self-assembly or interfacial conditioning applications in mind, now it is possible to envision polymeric structures with far more elaborate geometric presentations of distinct chemical functional groups. These structures may be entirely polymeric in nature or they may be nanoscale hybrids of polymeric and inorganic components. Examples include star polymers with homopolymer or block copolymer arms, miktoarm star polymers with multiple chemically distinct polymeric arms emanating from a core, molecular bottlebrushes with multiple side-chain compositions and inorganic nanoparticles with polymeric brushes grafted over a wide range of grafting densities. This presentation will focus on the interfacial properties of several such structures that have proven to be advantageous for potential engineering applications. These structures share the common feature of having many polymer chains emanating from a central core. For example, brush-grafted nanoparticles and multi-arm star polymers are extremely efficient emulsifiers, and when the polymer brushes are environmentally responsive, emulsions can be made environmentally responsive. Similar materials can be deployed as responsive surface conditioning agents with switchable friction and adhesion. These conditioned surfaces can be readily switched between lubricious and adhesive contact states.

This presentation will emphasize recent studies of enhanced interfacial engineering performance achieved with complex polymeric architectures. It will also bring to bear some long-standing research themes from this group concerning the fundamental and practical importance of multi-component interactions and persistent non-equilibrium states in complex fluid interfacial systems. These concepts that were established with simpler polymer architectures in the 1990’s and 2000’s will be shown to be quite useful with the novel complex architectures being developed now. Interfacial engineering via polymer adsorption often relies on the persistence of non-equilibrium states in adsorbed layers, where the adsorbed layer configuration is hysteretic with respect to changing bulk conditions. Recognizing this situation opens up new opportunities to improve engineering performance by processing adsorbed layers into appropriate non-equilibrium configurations. The switchable boundary lubrication application of adsorbed brush-grafted nanoparticles nicely illustrates new opportunities to achieve some performance attribute that has not previously been achieved with simpler polymer architectures. It furthermore illustrates how non-equilibrium layer state processing may be necessary to do so. It is also quite common that a desired performance characteristic cannot be achieved by interfacial engineering with just a single surface active species. Commercially important multi-component mixtures have long been designed to provide desirable product performance through judicious specification of components that form multispecies complexes. The advantages of multi-component complexation of brush grafted nanoparticles with surfactants or with star polymers will be illustrated here in the context of emulsification synergism and lubrication control.