In situ crosslinking of nanostructured block copolymer microparticles in supercritical carbon dioxide

Abstract

We report a novel and facile approach to “fix” the internal nanostructure of block copolymer (BCP) microparticles via in situ crosslinking copolymerisation in dispersion in supercritical CO₂ (scCO₂). By delaying the addition of the crosslinker and a portion of the second monomer, polymerisation induced microphase separation (PIMS) within the microparticles is well preserved, while the growing chains of the precursor poly(methyl methacrylate)-block-poly(4-vinyl pyridine) (PMMA-b-P4VP) or poly(methyl methacrylate)-block-poly(benzyl methacrylate) (PMMA-b-PBzMA) microparticles are crosslinked. The unique structure of the as-synthesised crosslinked microparticles was fully characterised using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, the swelling and solubility behaviour of the crosslinked PMMA-b-P4VP microparticles was investigated. Notably, the porosity generated by swelling in ethanol can be well controlled by the quantity of crosslinker incorporated. Macropores > 100 nm–∼20 nm, sub-10 nm mesopores, and non-porous microparticles were all achieved by varying the crosslinker incorporation from 0, 0.5, 1, to 4 wt%, respectively. In situ AFM nano-mapping of the crosslinked P4VP domains in 80% humidity revealed that microparticles with a high degree of crosslinking (8 wt% divinylbenzene) are highly resistant to swelling in humidity, by contrast to their non-crosslinked counterparts. This versatile approach further expands the available repertoire for fabricating porous BCP microparticles with tunable physico-chemical properties, morphologies and pore sizes, greatly broadening their application potential to more diverse fields.