Cold and Hot Gelling of Alginate-graft-PNIPAM: a Schizophrenic Behavior Induced by Potassium Salts

Abstract

Recently, alginates (ALG) characterized by high mannuronic content (M blocks) have been shown to undergo a reversible sol/gel transition during cooling in the presence of potassium salts. Cold gelling takes place at low temperatures, just below 0 °C for a KCl concentration of 0.3 mol/kg, but the aggregation process can be easily shifted to higher temperatures by increasing the salt concentration. In the present paper, we take advantage of this peculiar behavior to design a copolymer with schizophrenic gelling properties. For this purpose, side chains of poly(N-isopropylacrylamide) (PNIPAM), characterized by a Lower Critical Solution Temperature (LCST) in water, were grafted on the alginate backbone. Working in semidilute solutions, we show by coupling DSC and viscoelastic measurements that ALG-g-PNIPAM solutions are able to form gels either by cooling or heating depending on the ionic environment. As the aggregation process of ALG and PNIPAM depends mainly and respectively on the nature of the cations and anions, the choice of the salt is then critical to control the self-assembly behavior and the gel properties. Moreover, as the gelation process of alginates driven by the aggregation of mannuronic sequences is characterized by a large hysteresis of 20-30 °C between gelling and melting, both ALG and ALG-g-PNIPAM polymers offer a large versatility not only in terms of salt (nature and concentration) but also in preparation history as different states (sol or gel) can be obtained at room temperature.

Alginate-g-PNIPAM-Based Thermo/Shear-Responsive Injectable Hydrogels: Tailoring the Rheological Properties by Adjusting the LCST of the Grafting Chains

“…Among others, alginate-based graft copolymers [ 7 ] have been designed to form 3D networks through the association of the pendant grafting chains, responding to a trigger like a temperature. In this case, poly( N -isopropylacrylamide), PNIPAM, was mainly used as the type of grafting chain due to its appropriate lower critical solution temperature (LCST), appearing at about 32 °C, below the physiological temperature [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Above a percolation concentration of alginate-g-PNIPAM (gelator) in water, the graft copolymer forms a 3D network upon heating, above a critical temperature, T gel , exhibiting a sol-to-gel transition.…”

Thermoresponsive Hydrogels Based on Telechelic Polyelectrolytes: From Dynamic to “Frozen” Networks

“…For instance, in PNIPAM at 32 °C the hydrated monomeric units are still about 90% and becomes 40% at 55 °C. 47 To overcome this problem, many other LCST polymers have been studied as stickers. 48 Alternatively, statistical copolymers of NIPAM (or other monomers) with hydrophobic monomers exhibit lower LCST values and, in turn, lower sol−gel transition temperatures.…”

Topology-Specific Injectable Sticky Hydrogels

“…The heating rate was 2 °C·min –1 . In the case of PDMA- g -PNIPAM, the low-temperature behavior is characteristic of a viscous fluid ( G ″ > G ′) and the moduli follow an Arrhenius dependence with an activation energy of 26 kJ·mol –1 , typical of a nonassociating polymer aqueous solution in this range of concentrations . The activation energy was calculated from the slope of ln­(η*) versus 1/ T , where T is the temperature in Kelvin and η* is the complex viscosity , where ω is the angular frequency.…”