Surface Topology of Redox- and Thermoresponsive Nanogel Droplets.

Hydrogels are usually depicted as a homogenous polymer block with a distinct surface. While defects in the polymer structure are looked into frequently, structural irregularities on the hydrogel surface are often neglected. In this work, thin hydrogel layers of about 100 nm thickness (nanogels) have been synthesised and characterised for their structural irregularities, as they represent the surface of macrogels. The nanogels contain a main-chain responsiveness (thermo responsive) and a responsiveness in the cross-linking points (redox responsive). In combining data from ellipsometry using box-model and two-segment-model analysis, as well as atomic force microscopy, a more defined model of the nanogel surface could be developed. Starting with a more densely cross-linked network at the silica wafer surface, density of cross-linking gradually decreased towards the hydrogel-solvent interface. Thermo-responsive behaviour of the main chain affected the entire network equally as all chain segments changed solubility. Cross-linker based redox-responsiveness, on the other hand, was only governed by the inner, more cross-linked layers of the network. Such dual responsive nanogels hence allowed for developing a more detailed model of a hydrogel surface from free radical polymerisation. It provides a better understanding of structural defects in hydrogels and how they are affected by responsive functionalities. This article is protected by copyright. All rights reserved.