Abstract
Hydrogels are attractive materials not only for their tremendous applications but also for theoretical studies as they provide macroscopic monitoring of the conformation change of polymer chains. The pioneering theoretical work of Dusek predicting the discontinuous volume phase transition in gels followed by the experimental observation of Tanaka opened up a new area, called smart hydrogels, in the gel science. Many ionic hydrogels exhibit a discontinuous volume phase transition due to the change of the polymer–solvent interaction parameter χ depending on the external stimuli such as temperature, pH, composition of the solvent, etc. The observation of a discontinuous volume phase transition in nonionic hydrogels or organogels is still a challenging task as it requires a polymer–solvent system with a strong polymer concentration dependent χ parameter. Such an observation may open up the use of organogels as smart and hydrophobic soft materials. The re-entrant phenomenon first observed by Tanaka is another characteristic of stimuli responsive hydrogels in which they are frustrated between the swollen and collapsed states in a given solvent mixture. Thus, the hydrogel first collapses and then reswells if an environmental parameter is continuously increased. The re-entrant phenomenon of hydrogels in water–cosolvent mixtures is due to the competitive hydrogen-bonding and hydrophobic interactions leading to flow-in and flow-out of the cosolvent molecules through the hydrogel moving boundary as the composition of the solvent mixture is varied. The experimental results reviewed here show that a re-entrant conformation transition in hydrogels requires a hydrophobically modified hydrophilic network, and a moderate hydrogen-bonding cosolvent having competitive attractions with water and polymer. The re-entrant phenomenon may widen the applications of the hydrogels in mechanochemical transducers, switches, memories, and sensors.
Highlights
Hydrogels are physically or chemically cross-linked hydrophilic polymers swollen in water without dissolving
Veq (= υ2o /υ2 ), which is the bases for the design of superabsorbent polymers (SAPs). These equations predict that weak polyelectrolyte hydrogels with a pH-dependent charge density, or hydrophobically modified hydrogels with a temperature-dependent χ parameter undergo reversible volume changes in response to pH or temperature, respectively, and they constitute the group of stimuli responsive hydrogels
The closeness to the critical point in PIB-toluene/methanol system suggests that such a transition can be observed using loosely cross-linked organogels prepared at a low polymer concentration by varying the χ parameter playing the role of the environmental variable [11]. Such an observation may open up the use of organogels as smart and hydrophobic soft materials
Summary
Hydrogels are physically or chemically cross-linked hydrophilic polymers swollen in water without dissolving. Gels 2021, 7, 98 small amount of divinylbenzene leads to the formation of a chemically cross-linked polymer network absorbing large quantities of good solvents [3]. Tanaka the in gels by Dusekmilestones [7], and the discovery of smart by Tanaka [8]was canalso be considered first to observe an unusual swelling feature of hydrogels called re-entrant phenomenon as significant milestones in the development of the hydrogel science. Tanaka was the at which they first collapse but reswell as an environmental parameter such as the first to observe an unusual swelling feature of hydrogels called re-entrant phenomenon at solvent composition or temperature is monotonically varied [9]. A unique characteristic of biological and synthetic hydrogels is their swelling when exposed to aqueous solutions during which their volume increases to assume an equilibrium
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