This proposal seeks to contribute fundamental understanding regarding the thermodynamics and kinetics of self-assembly of functional rod-coil block copolymer systems. This work is of particular importance since rod-coil block copolymers have been suggested in the optimization of organic optoelectronic as well as biological applications, and a predictive understanding of their self-assembling properties is a necessary step towards application. For instance, recent breakthroughs in the device physics of these materials have demonstrated that the nanometer scale structure of interfaces between two conducting organics of different work function play an integral role in the separation of electrons and holes to harvest voltage in a photovoltaic cell or conversely in the recombination necessary to generate light. Direct application of block copolymer templating techniques is complicated by the fact that classical studies employed model polymers with Gaussian chain shape that did not participate in liquid crystalline interactions. While a number of novel structures have been observed in rod-coil block copolymer systems, the thermodynamics which control self-assembly are currently unclear. Preliminarily, the Segalman group has demonstrated a weakly segregated rod-coil block copolymer system which transitions from lamellar to nematic to isotropic phases with increasing temperature while following polymeric scaling relationships. This unique model system provides an exceptional opportunity to probe equilibrium thermodynamics. The thermodynamic parameters which control this self-assembly including the block copolymer segregation strength, rod-rod interaction, and molecular geometry will be investigated. The thin film architecture is the most technologically relevant, and an understanding of the effects of thin film confinement on rod-coil block copolymers will be sought. The CAREER research goals are to (i) understand the effect of a rod-block on the thermodynamics of block copolymer self-assembly, (ii) develop methods for controlling thin film self-assembly of these technologically important materials, and (iii) to foster an interest in polymer science in a broader population by integrating the proposed research with ongoing educational activities for students at various levels.
NON-TECHNICAL SUMMARY: Rod-coil block copolymers play a central role in many recent efforts to optimize organic optoelectronic devices, biological membranes, and drug delivery applications. Critical to all of these efforts is an understanding of the thermodynamics that control nanometer-scale self-assembly in polymers with non-classical interactions. This work is expected to have a significant impact on this broad range of applications and communities as control is gained over the nanoscale patterning of functional block copolymers. Furthermore, structure-property relationships are at the core of how children develop an understanding of their surroundings. A comprehensive educational plan will harness this innate curiosity to introduce a broad spectrum of students to polymer science and to the research discussed above. A high school physics teacher and the principal investigator will develop a set of hands-on exploratory modules that will help high school freshmen understand the interdisciplinary nature of science. These modules will then be disseminated to a broader, younger group of students through workshops with the Exploratorium Teacher Institute which trains a national pool of teachers. The research results will also be employed to improve undergraduate polymer education and to expose both undergraduate and graduate students to research involving self-assembling polymers. International students will also routinely visit the laboratory to expose the group to the cross-disciplinary and international nature of modern research.
Block polymers are molecules composed of two or more materials which, when processed correctly, can assemble into well-defined structures on the nanometer length scale. It has been proposed that such nanoscale patterning may be able to improve the performance of biologically or electronically active materials by compartmentalizing functionality at a domain size only one or two orders of magnitude larger than individual molecules. However, while many of these biological or semiconducting molecules possess molecular structures which impart their functionality, when these components are included in block polymers this structure also complicates the self-assembly process. Our group has focused on understanding the effect that this molecular-scale structure has on the assembly of the larger domains by studying the self-assembly of semiconducting polymers which have molecular shapes that are rod-like. In the past, we have published papers which have displayed an ability to study the thermodynamics of this self-assembly process with a model system, poly(2,5-di(2’-ethylhexyloxy)-1,4-phenylenevinylene)-b-polyisoprene (PPV-PI) [1,2]. Recently we have shifted our focus to a different semiconducting polymer, poly(3-ethylhexylthiophene) (P3EHT), which is chemically similar to that of the most studied semiconducting material in solar cell applications, poly(3-hexylthiophene). In doing so, we have presented pioneering work on controlling the self-assembly of polythiophene-based materials in the solid state. With knowledge of the relevant parameters which result in self-assembly, multiple systems with a variety of chemical structures and functionalities have been synthesized. In particular, polylactide, a biodegradeable and selectively-etchable polymer, has been shown to form alternating lamellae and cylinders in a P3EHT matrix (Figure 1) [3]. These polylactide cylindrical domains can be removed with a mild chemical wash, resulting in a semiconducting sample with identically sized pores that are evenly spaced throughout the material. These pores could potentially be filled with another functional component, or simply left as air or vacuum gaps in the material to lower the thermal conductivity. Over the course of this CAREER award, we have presented rules regarding the self-assembly of block polymers with rigid molecular structure and applied this fundamental work to a novel semiconducting polymer system. We propose that this knowledge can be further extended to other semiconducting or biologically active systems with the potential to improve performance of these functional materials through nanopatterning. References 1. Olsen, B.D. & Segalman, R.A., Structure and thermodynamics of weakly segregated rod-coil block copolymers. Macromolecules 38 (24), 10127-10137 (2005). 2. Olsen, B.D., Shah, M., Ganesan, V., & Segalman, R.A., Universalization of the phase diagram for a model rod-coil diblock copolymer. Macromolecules 41 (18), 6809-6817 (2008). 3. Ho, V. Boudouris, B.W., McCulloch, B., Shuttle, C., Burkhardt, M., Chabinyc, M., & Segalman, R. A., Poly(3-alkylthiophene) diblock copolymers with ordered microstructures and continuous semiconducting pathways. J. Am. Chem. Soc. 133(24), 9270-9273 (2011).
