Colloidal Polymer Chains: Construction, Statics and Dynamics

PROPOSAL NO.: CBET - 0730780

PRINCIPAL INVESTIGATOR: COLBY, RALPH H

INSTITUTION: PENNSYLVANIA STATE UNIV UNIVERSITY PARK

COLLOIDAL POLYMER CHAINS: CONSTRUCTION, STATICS AND DYNAMICS

Freely moving mammalian joints are lubricated by synovial fluid, consisting of 3 mg/mL hyaluronic acid (a high molar mass polyelectrolyte) and 20 mg/mL globular proteins. Natural synovial fluid has a typical friction coefficient of 0.002 to 0.010, compared with 0.010 to 0.050 for motor oil or physiological water without proteins. Recently, the PI's group has identified self-assembly of the globular proteins into chain-like aggregates as the source of considerable viscoelastic character in synovial fluid, which may account for the superb lubrication properties. The PIs plan to study such self-assembly in detail with biomimetic lubricating fluids made from self-assembled colloids with tunable interactions. In this way we seek to develop superior lubricants and other pragmatic fluids, while systematically studying the connections between interparticle interactions, aggregate structure and viscoelasticity. We plan to develop an 'artificial synovial fluid' (ASF) based on flexible string-like assemblies of 'colloidal atoms'. Atomic processes such as crystallization have been studied rigorously in the literature using colloidal particles, with the advantage of slower time scales and easier experimental observation. Fractal aggregates of colloids have also been studied in detail. However, because there has been no bottom-up assembly method to form long, flexible chains of colloids having rotatable 'bonds' between them, macromolecular systems have not been studied with 'colloidal polymers'. These are easily visualized and characterized chains of colloidal particles and will be constructed for studying macromolecular properties. In the depletion assembly method, depletion forces are used to hold colloids (e.g., polystyrene, PMMA) together. However, the key to the PIs technique is that two small flat regions are formed onto spherical colloids, at some specified angle. Since the region of depletant exclusion is much larger in the flat region than near the curved regions of the particles, the depletion force can be 10-100 times stronger when the flat regions face each other. The interaction strength along the polymer colloid chain (intra-'polymer' interactions) are thus mostly controlled in situ by changing the depletant concentration and flat region size, while the inter-polymer interactions are primarily controlled by only the depletant concentration. Thus, colloidal particles will be formed into chains with easily-tunable intra- and inter-polymer interactions. Moreover, depletion bonds are rotatable like true carbon-carbon bonds and reversible by changing solution depletant concentration. The fabrication of 'colloidal polymers' requires development of depletion assembly methods, and will enable us to examine many problems in polymer science with the assumptions made quite explicit. This research has three objectives. First, the PIs will construct ?colloidal polymers?. Spherical colloids with two flat regions will be prepared in the Penn State Nanofabrication Facility, and depletion assembly will be used to form flexible chains of colloidal particles, with controlled intra and inter-polymer interactions. They will, secondly, examine static properties. Chain size and conformation will be measured with video and confocal microscopy, and compared with predictions from classical polymer theory. Finally, they will examine dynamic properties. Rheology of the colloidal polymer chains will be studied as a function of chain length, concentration, intra- and inter-polymer interactions. Broader impact: The ability to have experimentally observable 'polymers' will expose assumptions in the study of polymer systems, and open new classes of observable phenomena. In addition, we will host a web site that shows photos and video of static and dynamic properties of colloidal polymers, for instructional purposes at both the collegiate and high school levels. Seeing the properties by eye will facilitate learning. The PhD student on this project will learn techniques from both polymer and colloid science, and assist with disseminating the information in conferences, in journals, and with pictures and movies on the website.