INTELLECTUAL MERIT: The objectives of this proposal are to elucidate general principles for how molecular architecture (size, shape), molecular properties (flexibility, charge), molecular interactions (attraction, repulsion), and suspension composition influence phase behavior. Molecular shape is a fundamental property governing phase behavior. The concept of excluded volume describes a region of space that one molecule prevents another from entering. Reducing excluded volume gives molecules more freedom of motion and thus increases entropy. Consequently molecules are influenced to undergo phase transitions that have molecular configurations which minimize excluded volume. In order to reveal the role of entropy in phase transitions the PI will create shape amphiphiles or entropic surfactants, molecules composed of two parts that separately have a tendency to phase separate, but which are bound together forming a block co-polymer. Two experimental systems will be created. The first colloid will be formed of blocks of the filamentous virus fd, a long, thin, semi-flexible polymer that forms liquid crystals and DNA, a polymer too flexible to form liquid crystals. The second system will be PRINT particles ? nanosized colloids produced by a high throughput molding technology developed by collaborator, Joseph. DeSimone. The phase behavior of these systems will be studied, as well as the phase behavior of mixtures of the shape amphiphiles with the individual components. Entropy is the dominant feature controlling the phase behavior of these charge stabilized colloids. Because of the size, shape, and simplicity of the interparticle interactions this system can be theoretically modeled and simulated with high precision. To study the phase behavior of shape amphiphiles the PI will employ a microfluidic device, the PhaseChip, which can precisely meter, mix, and store sub-nanoliter amounts of sample, solvent, and other reagents. Tens of thousands of individual mixtures can be stored on a chip in individual wells. Each well is in contact with a reservoir through a membrane through which only water can pass, but not salt, polymer, or amphiphile. This enables the concentration of all solutes in a sample to be reversibly, rapidly, and precisely varied.
BROADER IMPACTS: This project has important scientific implications in terms of what will be learned about the factors that drive self assembly and determine the ultimate architecture of the assemblies. The proposal describes an already highly developed program for interdisciplinary training of Brandeis life science and biophysics graduate students, including an IGERT in Quantitative Biology. Undergraduate participation in research is actively encouraged, and both graduate and undergraduate students learn to incorporate microfluidics technology into life sciences research. The PI is involved in K-12 outreach through the Brandeis POSSE program designed to bring inner city students to Brandeis and to encourage and support their success in the sciences.
Intellectual Merit. The long term goals of this research program are to understand the phase behavior of macromolecular suspensions that are either partially or totally composed of liquid crystalline forming molecules and to forge connections between the microscopic scale of molecular structure, mesoscopic scale of interparticle correlations, and macroscopic scale of bulk material properties. The objectives of this proposal are to elucidate general principles for how molecular architecture (size, shape), molecular properties (flexibility, charge), molecular interactions (attraction, repulsion), and suspension composition influence phase behavior. Experimental Systems. We created two experimental systems. The first colloid is the filamentous virus fd, a long, thin, semi-flexible polymer that forms liquid crystals to which we grafted ssDNA, shown in Figure 1. Adding complementary ssDNA linkers leads to selective binding of specific viruses, providing a precise way to build up macroscopic structures from the molecular level. The second system is PRINT particles – nanosized colloids produced by a high throughput molding technology developed by our collaborator, Prof. DeSimone, of UNC Chapel Hill. The colloidal stability of these systems has studied, as a prelude to measuring the phase behavior. Entropy is the dominant feature controlling the phase behavior of these charge stabilized colloids and because of the size, shape, and simplicity of the interparticle interactions this system can be theoretically modeled and simulated with high precision. Experimental Methods. To study the phase behavior we developed a microfluidic device, the PhaseChip, that can precisely meter, mix, and store sub-nanoliter amounts of sample, solvent, and other reagents. Thousands of individual mixtures can be stored on a chip in individual wells. Furthermore, each well is in contact with a reservoir through a membrane through which only water can pass, but not salt, polymer, or amphiphile. This enables the concentration of all solutes in a sample to be reversibly, rapidly, and precisely varied, as shown in Figure 2. The PhaseChip uses five orders of magnitude less material than current phase diagram measurement techniques. As the materials made in this study are too costly to use with conventional methods, this vast reduction in material is necessary. Broader Impact. The shape amphiphiles in this study share features with three important classes of materials; surfactants, block co-polymers, and liquid crystals, which are all subjects of fields of research in their own right. All three systems show a tendency to microphase separation and to self-assemble into rich and complex structures. Understanding the self-organization of these materials is a basic goal of nanotechnology and complex fluids research. Because all molecules share the entropic contribution to the free energy that is explicitly manifested by the shape amphiphiles this work will serve as a foundation for theory, simulation, and experimental study for microphase separation in general. Students (Figure 3) in this project received a broad, interdisciplinary training in biomaterials, PRINT technology, microfluidics, computer simulations, and theory; in other words the training needed to be a successful material scientist in the 21st century.
