Thermoresponsive Nanoparticles with Cyclic-Polymer-Grafted Shells Are More Stable than with Linear-Polymer-Grafted Shells: Effect of Polymer Topology, Molecular Weight, and Core Size.

Max Willinger,Erik Reimhult

doi:10.1021/acs.jpcb.1c00142

Abstract

Polymer brush-grafted superparamagnetic iron oxide nanoparticles can change their aggregation state in response to temperature and are potential smart materials for many applications. Recently, the shell morphology imposed by grafting to a nanoparticle core was shown to strongly influence the thermoresponsiveness through a coupling of intrashell solubility transitions and nanoparticle aggregation. We investigate how a change from linear to cyclic polymer topology affects the thermoresponsiveness of poly(2-isopropyl-2-oxazoline) brush-grafted superparamagnetic iron oxide nanoparticles. Linear and cyclic polymers with three different molecular weights (7, 18, and 24.5 kg mol–1) on two different core sizes (3.7 and 9.2 nm) and as free polymer were investigated. We observed the critical flocculation temperature (CFT) during temperature cycling dynamic light scattering experiments, the critical solution temperature (CST), and the transition enthalpy per monomer during differential scanning calorimetry measurements. When all conditions are identical, cyclic polymers increase the colloidal stability and the critical flocculation temperature compared to their linear counterparts. Furthermore, the cyclic polymer shows only one uniform transition, while we observe multiple transitions for the linear polymer shells. We link the single transition and higher colloidal stability to the absence in cyclic PiPrOx shells of a dilute outer part where the particle shells can interdigitate.

Highlights

Thermoresponsive colloidal smart materials received tremendous attention in recent years
We investigate the influence of cyclic polymer topology on the critical solution temperature (CST) of the stabilizing shells and the critical flocculation temperature (CFT) of nanoparticle dispersions of superparamagnetic iron oxide cores grafted with dense poly(2-isopropyl-2-oxazoline) (PiPrOx) brush shells
With the results from thermal gravimetric analysis (TGA), gel permeation chromatography (GPC), and Transmission electron microscopy (TEM), the grafting density (σ) was calculated using the formula σ = (% w/w)shell ρiron oxideVcoreNA (% w/w)coreMpolymerAcore where (% w/w)shell was the percentage of mass loss in TGA for the organic fraction corresponding to the polymer grafted onto the iron oxide core, ρiron oxide was the density of iron oxide, and NA was the Avogadro constant

Summary

Introduction

Thermoresponsive colloidal smart materials received tremendous attention in recent years. Many parameters influence the critical solution temperature (CST) of the particles’ polymer coating, which controls the thermoresponsive behavior in all such applications. These parameters include the concentration,[8] the end group,[9] the monomer composition,[10] the ionic strength and type of the aqueous surrounding,[8] and, as recently demonstrated, the local monomer concentration determined by the polymer shell morphology,[8,11] which is influenced by the nanoparticle curvature.[12,13]. Colloidal nanoparticles investigated for thermoresponsive applications include hydrogels,[14] micelles,[15,16] vesicles,[17,18] as well as inorganic-polymer hybrid particles.[19−21] Inorganicpolymer hybrid particles are interesting. They can combine the unique optical, magnetic, and electric properties of inorganic nanomaterials with the thermally induced phase transitions of hydrated polymers.[22,23] The most well-defined inorganic-polymer hybrid nanoparticle is the core−shell nanoparticle, comprising an inorganic core with a grafted thermoresponsive polymer shell.[24−27]

Methods

Results

Conclusion