Controlling the structure of star-like copolymer microgels through the monomer chain length to modulate softness and deswelling

Hypothesis. Softness and stimulus responsivity of deformable polymeric particles are at the core of their properties and behaviors in suspension or when interacting with interfaces. Star-like poly(N-isopropylacrylamide) (PNIPAM) microgels crosslinked with ethylene glycol dimethacrylate (EGDMA) are novel soft colloids which present a density profile characterized by a densely crosslinked core surrounded by a fuzzy shell with star-like arms. We propose copolymerization with oligo(ethylene-glycol) methyl ether methacrylates (OEGMA) of varying chain lengths as a tool to finely tune particle softness and thermal response, and thus their phase behavior in dense states. Experiments. By integrating light scattering with a recently introduced method that combines neutron scattering experiments and variation of the suspension osmotic pressure, we linked osmotic compression and particle bulk modulus to temperature deswelling, elucidating the complex interplay between network conformation and mechanical properties in response to different stimuli. Findings. We demonstrate that the incorporation of OEGMA chains alters the internal polymer distribution in a complex way that depends on the chain length: short OEGMA chains mitigate the internal inhomogeneities between core and shell of PNIPAM-EGDMA microgels, resulting in particles that are overall softer. For longer OEGMA the morphology changes to an even more compliant star-like structure that does not collapse into a compact sphere even at high temperature. These results show how the internal microstructure and physico-chemical properties of soft colloids can be modulated in great details in copolymer networks, in light of applications exploiting their softness for the fabrication of functional materials.