Tuning the Size of Thermoresponsive Poly(N-Isopropyl Acrylamide) Grafted Silica Microgels.

Nils Nun,Dina Sheyfer,Martin A Schroer,Gerhard Grübel,Birgit Fischer,Stephan Hinrichs

doi:10.3390/gels3030034

Abstract

Core-shell microgels were synthesized via a free radical emulsion polymerization of thermoresponsive poly-(N-isopropyl acrylamide), pNipam, on the surface of silica nanoparticles. Pure pNipam microgels have a lower critical solution temperature (LCST) of about 32 °C. The LCST varies slightly with the crosslinker density used to stabilize the gel network. Including a silica core enhances the mechanical robustness. Here we show that by varying the concentration gradient of the crosslinker, the thermoresponsive behaviour of the core-shell microgels can be tuned. Three different temperature scenarios have been detected. First, the usual behaviour with a decrease in microgel size with increasing temperature exhibiting an LCST; second, an increase in microgel size with increasing temperature that resembles an upper critical solution temperature (UCST), and; third, a decrease with a subsequent increase of size reminiscent of the presence of both an LCST, and a UCST. However, since the chemical structure has not been changed, the LCST should only change slightly. Therefore we demonstrate how to tune the particle size independently of the LCST.

Highlights

There is a large number of hydrogels that responds to external stimuli like temperature and pH [1,2,3]
We show that depending on the growth process we can alter the thermo-responsive properties of the poly(N-isopropyl acrylamide) (pNipam) shell
Each system consists of a silica core and a pNipam shell

Summary

Introduction

There is a large number of hydrogels that responds to external stimuli like temperature and pH [1,2,3]. These so called stimuli-responsive hydrogels offer a broad range of applications in the biomedical (e.g., controlled drug delivery systems [4]) as well as technical fields (e.g., in catalysis [2,5]). One of the most investigated stimuli-responsive hydrogels is poly(N-isopropyl acrylamide) (pNipam) due to its lower critical solution temperature (LCST) in aqueous solution of about 32 ◦ C [2,6]. The change of the solution behaviour depends on the hydrophobic nature of the solvent

Methods

Discussion

Conclusion