Morphological control of BaSO₄ microstructures by double hydrophilic block copolymer mixtures

Abstract

A mixture of two double hydrophilic block copolymers (DHBCs) was used to control the crystallization and organization of BaSO₄ microstructures as a simple model system for the synergistic action of multiple polymers in biomineralization processes. Independently, poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMAA) or a partially monophosphonated derivative, PEO-b-PMAA-PO₃H₂(1%), yielded spherical/elongated particles, or fiber bundles/cones, respectively, when used as additives for BaSO₄ crystallization, whereas similar experiments with combinations of the two DHBCs at various w/w ratios produced modified forms of these complex morphologies. Depending on the total DHBC concentration, two different morphogenetic mechanisms were identified. At 1 mg mL⁻¹, nucleation and outgrowth of fiber bundles/cones from spherical precursor particles could be controlled by the polymer mixing ratio to produce materials with a shuttlecock-like microstructure. The results were consistent with a predominantly cumulative mechanism involving a time-dependent sequence during which each DHBC acts independently. In contrast, depending on the ratio of copolymers used, disk-based cones, banded cones or interconnecting sheets of co-aligned fiber bundles were produced at a total polymer concentration of 3 mg mL⁻¹ by a cooperative mechanism involving the combined interaction of both DHBCs with growing BaSO₄ crystals. The results reveal that the use of polymer mixtures as additives opens up new variables for crystal morphogenesis compared to systems involving single polymers, and indicate that it should be possible to develop this approach to model the complexity of a wide range of biomineralization processes.