Thermoresponsive Core-Shell Nanoparticles: Does Core Size Matter?

Martina Schroffenegger,Erik Reimhult

doi:10.3390/ma11091654

Martina Schroffenegger, Erik Reimhult

Open Access

https://doi.org/10.3390/ma11091654

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Abstract

Nanoparticles grafted with a dense brush of hydrophilic polymers exhibit high colloidal stability. However, reversible aggregation can be triggered by an increase in temperature if the polymer is thermoresponsive, as the polymer shell partly loses its hydration. We investigate the role of nanoparticle curvature on the critical solution temperature (CST) of grafted poly(2-isopropyl-2-oxazoline) (PiPOx) and critical flocculation temperature (CFT) of the core-shell nanoparticle dispersion. Cores with diameters ranging from 5 to 21 nm were studied by temperature-cycled dynamic light scattering and differential scanning calorimetry over a large range of concentrations. We show that core size and curvature only have a minor influence on particle aggregation (CFT and cluster size), while they have major influence on the CST of the polymer shell. The densely grafted shells exhibit three distinct solvation transitions, the relative contributions of each is controlled by the core curvature. We link these transitions to different polymer density regimes within the spherical brush and demonstrate that the CST of the innermost part of the brush coincides with the CFT of the particle dispersion.

Highlights

Nanoparticles with inorganic cores and polymer shells are an interesting class of composite materials
We investigate for the first time if the core diameter has a strong influence on the critical solution temperature (CST) of the shell
For the first time, presented a comprehensive study on the thermoresponsiveness of iron oxide core-shell nanoparticles grafted with poly(2-isopropyl-2-oxazoline) as function of core size

Summary

Introduction

Nanoparticles with inorganic cores and polymer shells are an interesting class of composite materials They combine the properties of both the core and the shell. Such core-shell nanoparticles dispersed in a liquid can show very complex behaviour due to the interplay between the properties of the core, polymer and continuous phase. This enables the creation of so-called smart multi-functional materials. A solvated shell provides steric-osmotic repulsion that keeps the particle cores from aggregating In polar solvents, such as water, dispersible polymers often show a lower critical solution temperature (LCST).

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