The Double-Faced Electrostatic Behavior of PNIPAm Microgels

Abstract

PNIPAm microgels synthesized via free radical polymerization (FRP) are often considered as neutral colloids in aqueous media, although it is well known, since the pioneering works of Pelton and coworkers, that the vanishing electrophoretic mobility characterizing swollen microgels largely increases above the lower critical solution temperature (LCST) of PNIPAm, at which microgels partially collapse. The presence of an electric charge has been attributed to the ionic initiators that are employed when FRP is performed in water and that stay anchored to microgel particles. Combining dynamic light scattering (DLS), electrophoresis, transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments, we show that collapsed ionic PNIPAm microgels undergo large mobility reversal and reentrant condensation when they are co-suspended with oppositely charged polyelectrolytes (PE) or nanoparticles (NP), while their stability remains unaffected by PE or NP addition at lower temperatures, where microgels are swollen and their charge density is low. Our results highlight a somehow double-faced electrostatic behavior of PNIPAm microgels due to their tunable charge density: they behave as quasi-neutral colloids at temperature below LCST, while they strongly interact with oppositely charged species when they are in their collapsed state. The very similar phenomenology encountered when microgels are surrounded by polylysine chains and silica nanoparticles points to the general character of this twofold behavior of PNIPAm-based colloids in water.

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

“…Such a role, not often recognized in the literature, appears to be important to properly capture the internal modifications of the microgels upon collapse, as also confirmed by electrophoretic mobility measurements across the VPT for a subset of the samples investigated in this work ( SI Appendix , Fig. S2 ) ( 44 , 47 ).…”

“…In a previous work by some of us ( 44 ), the minimum was tentatively associated to the presence of initiator charges, manifesting their effect under collapsed conditions even for simple pNIPAM microgels. This conjecture was supported by electrophoretic mobility measurements also displaying a rapid increase of the signal for temperatures above the VPT, thus providing evidence that, in the high temperature regime, pNIPAM microgels have to be considered as charged micro-objects ( 29 , 47 , 48 ). However, a legitimate doubt concerned whether the minimum in would be solely due to charges or could be attributed to other ingredients, such as the inhomogeneous core–corona topology of pNIPAM microgels synthesized by precipitation polymerization methods.…”

Complexation of DNA with Thermoresponsive Charged Microgels: Role of Swelling State and Electrostatics

“…Likewise, Kleinen and Richtering also provide electrophoresis measurements to investigate the charge inversion of polyelectrolyte-microgel complexes formed by cationic poly(diallyldimethylammonium chloride), PDADMAC, and anionic PNIPAM microgels [ 54 , 55 ]. More recently, Sennato et al have shown that when PNIPAM microgels are collapsed, they strongly interact with oppositely charged polyelectrolytes and a large charge inversion is observed [ 56 ]. In this sense, coarse-grained Monte Carlo simulations have recently been applied to predict the charge inversion previously detected through µ e for nanogel-polyelectrolyte complexes [ 57 ].…”

The role of charge content in the two-step deswelling of Poly(N-isopropylacrylamide)-based microgels

“…2,[11][12][13] Among polymeric microgels, those made of poly(N-isopropylacrylamide) (PNIPAm) are particularly interesting, since they undergo a Volume Phase Transition (VPT) at ambient temperature: below a characteristic temperature, T c ≈ 32 • C, these colloidal networks exist in a swollen, fully hydrated state, while above T c they collapse due to their increased hydrophobicity. PNIPAm microgels can be synthesized using standard emulsion polymerization in aqueous media 14 and their size, shape and structure have been investigated using various methods including x-ray, light and neutron scattering, [15][16][17][18] and electron or atomic force microscopy. 18 The combination of many different experimental techniques to investigate the VPT of PNIPAm microgels made it possible to grasp a lot of information on this phenomenon, which is one of the most studied in soft condensed matter nowadays.…”