Emergence of Non‐Hexagonal Crystal Packing of Deswollen and Deformed Ultra‐Soft Microgels under Osmotic Pressure Control. Issue 20 (13th September 2021)

• Record Type: Journal Article
• Authors: Islam, Molla R.; Nguyen, Rachel; Lyon, Louis Andrew
• Is Part Of: Macromolecular rapid communications. Volume 42:Issue 20(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-13
• Subjects: cubic lattices -- non‐hexagonal packing -- osmotic pressures -- particle deformation -- ultra‐soft microgels; Macromolecules -- Periodicals; Polymers -- Periodicals; Chemistry -- Periodicals; 547.705
• DOI: 10.1002/marc.202100372
• Languages: English
• ISSNs: 1022-1336

Abstract

Abstract: Highly solvent swollen poly(N‐isopropylacrylamide‐co‐acrylic acid) microgels are synthesized without exogenous crosslinker, making them extremely soft and deformable. These ultralow crosslinked microgels (ULC) are incubated under controlled osmotic pressure to provide a slow (and presumably thermodynamically controlled) approach to higher packing densities. It is found that ULC microgels show stable colloidal packing over a very wide range of osmotic pressures and thus packing densities. Surprising observation of co‐existence between hexagonal and square lattices is also made over the lower range of studied osmotic pressures, with microgels apparently changing shape from spheres to cubes in defects or grain boundaries. It is proposed that the unusual packing behavior observed for ULC microgels is due to the extreme softness of these particles, where deswelling causes deformation and shrinking of the particles that result in unique packing states governed by contributions to the entropy at the colloidal system, single particle and ionic levels. These observations further suggest that more detailed experimental and theoretical studies of ultra‐soft microgels are required to obtain a complete understanding of their behavior in packed and confined geometries. Abstract : Highly solvent‐swollen, ultralow crosslinked microgels show stable colloidal packing over a wide range of applied osmotic pressures. Surprisingly, observed packings include the coexistence of hexagonal and square lattices over a narrow range of pressures, with the particle shape‐shifting from spherical (hexagonal lattices) to cubic (square lattices) in a dynamic fashion. This suggests significant complexity in the physics of ultra‐soft microgels in confinement, warranting deeper investigation.