Structure of Surfaces and Interfaces of Poly( N,N -dimethylacrylamide) Hydrogels

TL;DR

This study reveals a gradient in cross-linking at the surface of PDMA hydrogels, influenced by mold surface hydrophobicity, affecting their surface properties and potential applications.

Contribution

It provides detailed characterization of the surface structure and cross-linking gradient of PDMA hydrogels using rheology and neutron reflectivity, highlighting the influence of mold surface properties.

Findings

Existence of a less cross-linked surface layer at multiple length scales.

Surface composition depends on mold surface hydrophobicity.

Surface treatment affects submicrometric surface structure.

Abstract

We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion.