Star-like thermoresponsive microgels: a new class of soft nanocolloids

TL;DR

This paper introduces a new class of thermoresponsive star-like microgels synthesized via a novel method, combining experimental and simulation evidence to reveal their unique internal structure and sharp volume phase transition.

Contribution

It presents a new synthesis route for star-like microgels with thermoresponsive properties, bridging star polymer and microgel fields, and provides detailed experimental and simulation characterization.

Findings

Microgels have a star-like internal structure with a core and arms.

They exhibit a sharp volume phase transition at 32°C.

The new synthesis method is simple and reproducible.

Abstract

We provide experimental and numerical evidence of a new class of soft nanocolloids: star-like microgels with thermoresponsive character. This is achieved by using the standard precipitation polymerization synthesis of poly(N-isopropylacrylamide) (PNIPAM) microgels and replacing the usually employed crosslinking agent, N,N'-methylenebis(acrylamide) (BIS), with ethylene glycol dimethacrylate (EGDMA). The fast reactivity of EGDMA combined with its strong tendency to self-bind produces colloidal networks with a central, crosslinker-rich core, surrounded by a corona of long, crosslinker-free arms. These novel star-like microgels fully retain PNIPAM thermoresponsivity and undergo a volume phase transition at a temperature of 32{\deg}C that is very sharp as compared to standard PNIPAM-BIS microgels, independently of crosslinker content. Dynamic light scattering and small angle X-ray scattering experiments are compared to extensive simulation results, based on ideal star polymers as well as on state-of-the-art monomer-resolved simulations, offering a microscopic evidence of the star-like internal structure of PNIPAM-EGDMA microgels. This can be described by a novel model for the form factors combining star and microgel features. The present work thus bridges the fields of star polymers and microgels, providing the former with the ability to respond to temperature via a facile synthetic route that can be routinely employed, opening the way to exploit these soft particles for a variety of fundamental studies and applicative purposes.