Rational design of thermoresponsive polymers in aqueous solutions: A thermodynamics map

Abstract

Temperature variations may induce changes in solution properties for thermoresponsive polymers, which have drawn much research attention due to their potential applications in different areas. To meet the practical requirements, rational design of the polymers is important and necessary. We reviewed literature in this field and proposed a thermodynamic map to guide the design of such polymers. The map is drawn up based on the enthalpic and entropic contributions (ΔHm and ΔSm) to the free energy change of mixing (ΔGm). The map is divided into four zones: soluble, insoluble, upper critical solution temperature (UCST), and lower critical solution temperature (LCST), with two lines representing the freezing and boiling points of the solvent (water) as two boundaries for the UCST and LCST zones. With the help of such a map, we attempt to illustrate how the UCST and LCST are affected by the various interactions in polymer-water mixtures, such as hydrophobic and ionic interactions, and hydrogen bonding, so that the thermoresponsive properties of a polymer can be predicted according to their molecular structures. The critical temperatures UCST and/or LCST of a polymer can be obtained from the slope of the straight line that passes through the original point and the location of the polymer on the map. Examples in the literature are shown to fit in different zones of this map, and we also demonstrate how the variations of the chemical composition or the molecular structure of the polymers can change the intra- and inter-molecular interactions involved, which in turn change the solution properties of the polymers and induce changes in the critical solution temperatures. This map may be used in the design and preparation of thermoresponsive polymers with UCST and LCST on demand.