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Formation of Periodically-Ordered Calcium Phosphate Nanostructures by Block Copolymer-Directed Self-Assembly

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journal contribution
posted on 09.02.2016, 00:00 by Rui-Qi Song, Tobias N. Hoheisel, Hiroaki Sai, Zihui Li, Joseph D. Carloni, Suntao Wang, Randall E. Youngman, Shefford P. Baker, Sol M. Gruner, Ulrich Wiesner, Lara A. Estroff
Structuring ionic solids at the nanoscale with block copolymers (BCPs) is notoriously difficult due to solvent incompatibilities and strong driving forces for crystallization of the inorganic material. Here, we demonstrate that elucidating pathway complexity in the BCP-directed self-assembly of an ionic solid, amorphous calcium phosphate (ACP), is a key component in obtaining nanostructured, bulk composite materials in which the nanostructure is the result of thermodynamically controlled BCP self-assembly, i.e., exhibiting sequences of bulk morphologies as known from typical equilibrium BCP phase diagrams. Specifically, we identify three critical pathway “decision points” for the evaporation-induced self-assembly of composites from ultrasmall, organosilicate-modified amorphous calcium phosphate nanoparticles (osm-ACP-NPs) and poly­(isoprene)-block-poly­(2-(dimethylamino)­ethyl methacrylate) (PI-b-PDMAEMA) block copolymers. Using this strategy enabled us to obtain composites with hexagonal, cubic network, and lamellar BCP morphologies, in addition to mesoporous, cellular materials and macrophase separated materials. The osm-ACP-NPs are synthesized via a two-step sol–gel process in which (3-glycidyloxypropyl)­trimethoxysilane (GLYMO) quenches the reaction, limits the particle size, and functionalizes the NP surface. Dynamic light scattering evidences a transition from BCP unimers to micellar aggregates with increasing amounts of sol solution, which is reflected by a corresponding switch from BCP-type morphologies to micellar/cellular morphologies of the nanocomposites. Nanostructured organic–inorganic composites with a continuous osm-ACP-NP matrix phase have indentation moduli (measured by nanoindentation) that are an order of magnitude larger than unstructured composites with similar compositions. Insights provided by this study have relevance to understanding the effects of pathway complexity in the assembly of organic–inorganic composites and may enable access to a broad range of hybrid nanostructures with potential applications in areas including dental repair and hard tissue engineering.