Controlling microgel morphology and swelling behavior by copolymerization

TL;DR

This review discusses recent advances in controlling microgel morphology and thermosensitive swelling behavior through copolymerization, enabling external modulation of phase transition temperatures and particle structures.

Contribution

It highlights new methods for tuning microgel properties via copolymerization and external stimuli, emphasizing the diversity of achievable morphologies and thermosensitivity control.

Findings

Multiple external controls of VPTT are now commonly achieved.

Different microgel morphologies can be synthesized, such as core-shell and patchy structures.

Copolymerization enables precise tuning of microgel thermosensitivity.

Abstract

The thermosensitive behavior of microgel particles suspended in solvents, i.e. their temperature-dependent swelling properties, has triggered ongoing interest in industry and academia over the past forty years. The most-studied polymer is poly(N-isopropylacrylamide) - PNIPAM -, where the volume phase transition temperature is well known to depend on the detailed molecular architecture of the monomers. In this article, we focus on publications mostly of the past five years in chemical synthesis, aiming at shifting or controlling the volume phase transition temperature (VPTT) of such polymers by copolymerization of a main monomer - often from the PNIPAM family - with either monomers of different hydrophobicity, or with ones bearing ionizable groups. In some cases, hydrophobicity may be modulated by light as external switching parameter, whereas ionic strength or pH may act on the thermosensitivity of the microgels containing charged groups. Due to either differences in reactivity, or specific synthesis routes, particular microgel morphologies, such as molecular gradient, core-shell, interpenetrated, or patchy (multi-lobular) structures may be generated. They may give rise to spatial modulations of thermosensitivity within particles and are highlighted in this review. Our short overview shows that multiple external control of VPTT and morphology is commonly achieved nowadays.