57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The scientific projects funded by the ACS Petroleum Research Grant over the last year focused on packing problems of nanoparticles. Specifically our research in this second year has been centered around the problem of packing of ultra-soft nano-components with implicit and explicit interactions and on phase separation of immiscible polymers on curved templates. In the former case we explored how relaxing the excluded volume interactions between nanoparticles affects the structural properties of the crystalline condensed phases formed at large densities. In the latter case, we uncovered the physical mechanisms behind the micro-phase separation of immiscible polymers or ligands on curved templates.

(1) Understanding how nanocomponents spontaneously organize into complex macroscopic structures is one of the great challenges in the field of soft matter today. Despite much effort, the question of how to design shape and interactions of nanocomponents for a much desired bottom-up approach to structure formation, remains unanswered.

Recently, it has been recognized that soft/deformable mesoparticles such as charged or neutral star polymers, dendrimers or micro-gels, have the potential of generating a plethora of novel crystalline phases previously inaccessible to spherical components interacting with hard-core (steep) potentials. Unfortunately, because of the internal degrees of freedom associated with this class of soft components, extracting pair potentials that accurately describe the interactions between them at high densities is very complicated as it involves density dependent many-body interactions. Because of this, we studied several phase diagrams of classes of soft pair potentials to understand how the structure of the crystalline phases depends on the details of the interactions. What we found was very insightful. (a) The overall structure of the phase diagrams is very much independent of the pair potential, i.e. all potentials explored presented an upper freezing point, and a re-entrant melting transition at large densities. However, (b) the symmetry and the number of the crystalline phases are extremely sensitive to the shape of the interactions. Our study therefore suggest that a great effort in finding accurate potentials between soft-nanocomponents must be pursued as the structure formation in these systems is very much dependent on the details of the interactions. Finally, we are performing an analysis of the phase behavior of star polymers with an explicit representation system to check the limits of applicability of pair interactions.

(2) Polymer brushes are dense systems of polymer chains tethered to a non-adsorbing surface. They are used in a large number of technological applications, including colloidal stabilization, lubrication, adhesives, and drug-biocompatibility enhancers. It was recently shown that a mixture of immiscible ligands or polymers having different length, spontaneously phase separate into striped phases when reorganizing over a spherical template (nanoparticle). Understanding how the width of the stripes depends on the difference in length of the polymers is a question of fundamental importance for controlling the morphologies of the self-assembled structures. We carried out extensive numerical simulations on these systems and found simple scaling laws governing the observed phenomenological behavior.

Overall I believe that the PRF grant was greatly beneficial for my career and for that of the two (half-time) graduate students that it supported. Our research produced several publications that have been accepted in competitive journals. The PRF grant allowed us to perform some highly exploratory research. Specifically, our work on ultra-soft nanoparticles and on phase separation on curved templates is very relevant as the link between structure and pair interactions is a crucial factor in the phase behavior of these systems. In the near future we will keep on exploring the features of these phase diagrams to better understand the limit of validity of these pair potentials when describing such complex mesoparticles with internal degrees of freedom, and eventually develop systematic coarse-graining strategies to better describe their physical properties. The PRF fund enabled us to setup the groundwork for a whole new line of research in my group.

The graduate students working on these projects have mastered the most advanced techniques in computer simulations for free energy calculations of complex fluids, and have been exposed to some very intriguing scientific puzzles whose realm goes well beyond the standard studies of a physical chemistry graduate program. They both attended international summer school on related over the summer.