Influence of Dopamine Methacrylamide on swelling behaviour and nanomechanical properties of PNIPAM microgels

TL;DR

This study investigates how dopamine methacrylamide (DMA) affects the swelling behavior and nanomechanical properties of PNIPAM microgels, revealing that DMA influences phase transition temperatures, swelling ratios, and stiffness.

Contribution

It provides the first detailed analysis of DMA's impact on PNIPAM microgels, linking chemical composition to swelling and mechanical properties through experimental techniques.

Findings

DMA decreases VPTT and VPT CT due to hydrophobicity.

DMA increases microgel stiffness and protrusion from surfaces.

High DMA content hinders microgel shrinking while maintaining stiffness.

Abstract

The combination of the catechol-containing co-monomer dopamine methacrylamide (DMA) with stimuli-responsive microgels such as poly(N-isopropylacrylamide) (PNIPAM) bears a huge potential in research and for applications due to the versatile properties of catechols. This research gives first detailed insights into the influence of DMA on the swelling of PNIPAM microgels and the correlation with their nanomechanical properties. Dynamic light scattering (DLS) was used to analyse the swelling behaviour of microgels in bulk solution. The incorporation of DMA decreases the volume phase transition temperature (VPTT) and completion temperature (VPT CT) due to its higher hydrophobicity when compared to NIPAM, while sharpening the transition. The cross-linking ability of DMA decreases swelling ratios and mesh sizes of the microgels. Microgels adsorbed at the solid surface are characterised by atomic force microscopy (AFM): Scanning provides information about the microgel's shape on the surface and force spectroscopy measurements determines their nanomechanical properties ($E$ modulus). As the DMA content increases, microgels protrude more from the surface, correlating with an increase of $E$ modulus and a stiffening of the microgels - confirming the cross-linking ability of DMA. Force spectroscopy measurements below and above the VPTT display a stiffening of the microgels with the incorporation of DMA and upon heating across it's entire cross-section. The affine network factor $\beta$, derived from the Flory-Rehner theory describing the elasticity and swelling of the microgel network, is linearly correlated with the $E$ moduli of the microgels for both - pure PNIPAM and P(NIPAM-co-DMA) microgels. However, for large amounts of DMA, DMA appears to hinder the microgel shrinking, while still ensuring mechanical stiffness, possibly due to catechol interactions within the microgel network.