Block copolymers in selective solvents exhibit simultaneous change in the shape of self-assembled nanoscale domains along with a change in the symmetry of the ordered phases. While there are many studies of the equilibrium phase diagrams and thermodynamics of solvent mediated interactions in block copolymer systems, the dynamics and kinetics of transformations are less well understood. In particular, not much is known about the pressure dependence of transformation kinetics. The primary goal of the research proposed here is (i) to examine the pressure dependence of phase transition mechanisms and the kinetics of order-disorder and order-order phase transitions in block copolymers in selective solvents, using time-resolved synchrotron based small angle x-ray scattering, and (ii) the internal dynamics of the block copolymer chains in the nanoscale domains using X-ray Intensity Fluctuation Spectroscopy.
The x-ray scattering studies will be complemented by other experimental techniques, including small angle neutron scattering, atomic force microscopy, dynamic light scattering and rheology. Pressure jump measurements are particularly suitable for examining the early stages of the transformation processes. To obtain detailed insight into the underlying physical mechanisms, Molecular Dynamics simulations and models for analyzing the scattering data will be developed.
NON-TECHNICAL SUMMARY:
Solutions and gels made up of block copolymers that inseparably combine two or more chemically distinct polymers have broad range of applications from consumer products to novel materials for pharamaceutical and other applications. The research proposed here has potential applications in the use of block copolymer materials at high ambient pressure, such as those involved in oil drilling. Pressure can also be used as a means to control fabrication processes in the manufacture of block copolymer based products.
The project will provide interdisciplinary, hands-on training to graduate and undergraduate students, and thus contribute to the development of future scientists. The students will also learn to use state-of-the art synchrotron based x-ray instrumentation at National Laboratories. Special emphasis will be made to incorporate best practices to recruit, train and mentor women students at all levels by interacting closely with Women in Science and Engineering (WISE) group at Boston University. The senior faculty will present the work to a broader audience, and participate in outreach programs designed to attract and mentor high school and college students to pursue science and engineering careers. An international collaboration with researchers in Czech Republic is an integral part of this program. This collaboration provides opportunities to US students and senior researchers to benefit from the expertise that is being developed in Europe, particularly in the area of synchrotron- based soft matter and polymer physics.
Multiblock copolymers are used in numerous applications, ranging from detergents and stabilizers for food, medicine and cosmetic products to automobile tires and novel high density memory for computers. These materials self assemble to form materials with nanoscale structure and their properties depend on their structure that can be controlled by changing external variables such as temperature, pressure or solvent. The Bansil research group has focused on understanding how the structural properties of block copolymer solutions evolve with time following a change in temperature or pressure. For these studies they examine the block copolymer samples by shining an x-ray beam and measuring how the x-ray beam is deflected using high speed cameras and advanced technologies available at facilities called Synchrotrons at National Laboratories. Through the course of this project they have had several key outcomes. They have shown the details of how block copolymers change from multilayered sheets to cylinders arranged in a hexagonal array to interconnected polymer tubes and spheres stacked in a cubical arrangement. Their experiments and simulations identify several stages in these processes, suggesting that materials with different material structure and properties can be generated by arresting the phase separation process at different times after a sudden change in temperature or pressure. For example a disordered liquid-like solution can transform to an ordered soft-solid by rapidly changing pressure. During the course of this research they have developed novel instrumentation as well as computer simulation and image analysis programs that have a wide range of applicability. As an example of the applicability of their techniques they collaborated with researchers at an industrial laboratory to investigate the aggregation of tar. Additionally this project helped support the educational and outreach activities of the Bansil group. Research training was provided to undergraduate students, graduate students pursuing Ph. D, and a postdoctoral researcher. The computer programs developed in this research were used as instructional materials for graduate level courses. Outreach activities included presentations to high school students and at international scientific meetings.
