Cross-linking inhomogeneity in nano-composite hydrogels can be observed as sharp peaks by SAXS experiments under elongation

TL;DR

This study uses SAXS experiments to reveal how inhomogeneous cross-linking in nano-composite hydrogels causes sharp peaks under elongation, highlighting differences based on polymer-nanoparticle interactions.

Contribution

It demonstrates that SAXS can detect cross-linking inhomogeneity in hydrogels, especially through sharp peaks indicating non-affine nanoparticle movement in weakly interacting systems.

Findings

Sharp peaks in SAXS indicate cross-linking inhomogeneity.

Nanoparticle movement depends on polymer-nanoparticle interaction.

Differences observed between PDAM and PAM gels under elongation.

Abstract

We introduced silica nanoparticles into poly ($N$,$N$-dimethylacrylamide) gel, (PDAM-NC gel), and poly (acrylamide) gel (PAM-NC gel), and carried out SAXS measurements under uniaxial elongation. It is well known that PDAM chains are strongly adsorbed onto silica nanoparticles while PAM chains are not. Interestingly, we found from SAXS measurements that scattering profiles depend on the polymer-nanoparticle interaction. A four-spot pattern was observed in the 2D structure factors of PDAM-NC gel, which was assigned to a movement of the nanoparticles affinely under elongation. However, unexpectedly, we observed sharp peaks in the 2D structure factors of PAM-NC gel in the parallel direction to the stretching. These peaks appeared in much lower-$q$ region than the theoretical prediction of affine deformation of nanoparticles. We conjecture that these peaks do not correspond to the correlation of individual particles but to the correlation of high cross-linking regions because the interaction between PAM chains and silica nanoparticles is relatively weak and the displacements of silica nanoparticles are sensitive to spatial cross-linking inhomogeneity.