Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels

Abstract

The present study addresses the multiresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this so-called cononsolvency effect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specific surface details, as well as the sample preparation. Thereby, the microgels are deposited on two different kinds of surfaces: on gold surface and on polycation (PAH) coating which show different interactions with the microgels in terms of electrostatic interaction and wettability. In addition, the microgels were deposited from different solvent mixtures. This influences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T > VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

Numerical modelling of non-ionic microgels: an overview

“…By contrast, when methanol is added in excess the microgel re-swells in force of a favourable entropic contribution 31. Recently, detailed models have been used to quantitatively understand the origin of the co-non-solvency experienced by polymers dispersed in water/alcohol mixtures in both good and bad solvents,32 providing an additional tool to control the responsiveness of microgels 33…”

Nanomechanics and Nanorheology of Microgels at Interfaces

“…The Poisson ratio is the ratio of transverse strain to axial strain. In studies on microgels it is usually assumed to be 0.5, which is the value for materials, where the increase in volume caused by stretching (axial strain) is compensated by the decrease in volume due to the transverse diminution (transverse strain) [16,21,25,26,27,28,29,30]. While this is valid for many samples, there is evidence of soft materials with lower Poisson ratios.…”

“…This method has been first used in biophysics, where it was applied to cells [36,37,38]. After studies on both non-cross-linked and cross-linked polyelectrolyte multilayers [39] it has also been used for measurements on microgels [29,30].…”

Microgels at fluid-fluid interfaces for food and drinks

“…However, experimental measurements on such small particles are obviously not easy. A number of studies (Backes et al, 2017); (Backes and von Klitzing, 2018); (Aufderhorst-Roberts et al, 2018) have used atomic force microscopy (AFM) techniques to try and measure this directly for individual particles. Microgel mechanical properties can also be inferred from calorimetry (Aangenendt et al, 2017), compression isotherms of microgel monolayers at the A/W interface (Garbin et al, 2015) or O/W (Murphy et al, 2016) interface, or bulk rheological measurement of close packed microgel dispersions (Mattsson et al, 2009).…”