LCSTUCST

LCSTUCST refers to polymer–solvent systems that display both a lower and an upper critical solution temperature. In such systems, the solution is miscible with the solvent only inside a finite temperature window, becoming phase-separated at temperatures below the UCST or above the LCST. The UCST marks the temperature at which demixing occurs when cooling, and the LCST marks the temperature at which demixing occurs when heating. When both transitions are present, the phase diagram often forms a closed loop, yielding a soluble region between UCST and LCST. This behavior is sometimes described as double critical solution temperature (DCST) or reentrant solubility.

The underlying mechanism reflects a balance between enthalpic and entropic contributions to polymer–solvent interactions. At low temperatures, certain interactions promote phase separation; as temperature rises toward the UCST, solubility can improve. Beyond the UCST and up to the LCST, the solvent quality supports dissolution, but further heating past the LCST reduces solubility due to increasing unfavorable hydrophobic effects or disruption of stabilizing interactions, causing re-precipitation. Factors such as polymer architecture (e.g., copolymer composition), molecular weight, solvent composition (often water with cosolvents or salts), pH, and temperature history strongly influence the position of UCST and LCST and the width of the soluble window.

Examples are reported in various polymer systems, including some aqueous polymer solutions where PNIPAm shows LCST behavior and other systems exhibit UCST or DCST depending on additives. Applications of LCSTUCST behavior include smart, temperature-responsive materials for controlled release, separations, and actuated gels. Measurement methods frequently involve turbidity or cloud-point analysis, differential scanning calorimetry, and construction of detailed phase diagrams.