Bioinspired Polymers: Self-Assembling Hydroxyapatite-Polymer Nanocomposites

Program Scope: Synthesis and characterization of novel bioinspired polymeric materials that mimic living systemsin their abilities to switch among several states in response to the environment and self-assemble hierarchically. Use of these polymers as templates to direct biomineralization processes, and to facilitate a bottom-up approach to materials design. Understanding guiding mechanisms of assembly across multiple length scales through combination of experiment and theory. Advanced solid-state NMR techniques for investigating interactions of the polymeric materials with inorganic components and biomolecules.

Major Program Achievements: We have designed and synthesized novel bioinspired pH and temperature-sensitive block copolymers with various cationic blocks that exhibit hierarchical self-assembly from nanoscale micelles to macroscale gels and solids. Small angle neutron scattering and cryo-transmission electron microscopy studies of these polymers in solution indicate a switch from spherical to cylindrical or thread-like micelles upon increasing temperature and pH. These self-assembly processes and spherical to cylindrical transitions were also predicted by molecular dynamics simulations, where the effect of changes in temperature and pH were modeled by changing the relative hydrophobicities of the different blocks in the multiblock copolymer chains. Small angle X-ray scattering studies show that the self-assembled solids exhibit a hexagonal structure formed by the assembly of the cylindrical micelles, which is distinct from the lamellar structure seen in the melt. Studies Different calcium phosphate phases, both hydroxyapatite and brushite, were formed on these polymer micelle templates in solution. Since the self-assembly process is driven by the relative hydrophobicities of the different polymer blocks, these nanoscale micelles coated with thin inorganic shells exhibited self-assembly into gels and solids, just as in the case of the pure polymer. New advanced solid-state NMR techniques were developed to investigate the hybrid organic-inorganic structures formed by self-assembly. These NMR techniques provide the thickness of the inorganic layer formed (of the order of a few nm) as well as compositional information about the nanoscale inorganic layer at the organic-inorganic interface, which is very difficult to obtain using other established techniques. In addition, small angle X-ray scattering studies of these hybrid gels and solids at Argonne indicated that the hexagonal structure of the polymer gels was preserved even in the presence of the inorganic coatings on the micelles and did not alter the self-assembly process of the underlying polymer template. The chemical structure of the polymer template was seen to have a significant role on the mineralization process and the thickness and composition of the calcium phosphate layer formed.

Program Impact: Self-assembly across multiple length scales is key to the development of a “bottom-up” approach to materials design and our research has generated a new class of hierarchically self-assembled nanocomposites. We have developed NMR techniques that are being used worldwide by other groups.

Contact Information:
Surya Mallapragada

Selected Publications

Patents