Upper critical solution temperature
The upper critical solution temperature (UCST) or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word upper indicates that the UCST is an upper bound
Upper bound
In mathematics, especially in order theory, an upper bound of a subset S of some partially ordered set is an element of P which is greater than or equal to every element of S. The term lower bound is defined dually as an element of P which is lesser than or equal to every element of S...

 to a temperature range of partial miscibility, or miscibility for certain compositions only. For example, hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...

Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...

 mixtures have a UCST of 19°C, so that these two substances are miscible in all proportions above 19°C but not at lower temperatures. Examples at higher temperatures are the aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

-water system at 168°C (at pressures high enough for liquid water to exist at that temperature), and the lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 system at 798°C (a temperature where both metals are liquid).

A solid state example is the palladium-hydrogen system which has a solid solution phase (H2 in Pd) in equilibrium with a hydride phase (PdHn) below the UCST at 300°C. Above this temperature there is a single solid solution phase.

In the phase diagram of the mixture components, the UCST is the shared maximum of the concave down spinodal
In thermodynamics, the spinodal is the limit of stability of a solution, denoting the boundary of absolute instability of a solution to decomposition into multiple phases. Within this curve, infinitesimally small fluctuations in composition and density will lead to phase separation via spinodal...

 and binodal
In thermodynamics, the binodal, also known as the coexistence curve or binodal curve, denotes the condition at which two distinct phases may coexist...

 (or coexistence) curves. The UCST is in general dependent on pressure.

The phase separation at the UCST is in general driven by unfavorable energetics; in particular, interactions between components favor a partially demixed state.

Polymer-solvent mixtures

Some polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 solutions also have a lower critical solution temperature
Lower critical solution temperature
The lower critical solution temperature or lower consolute temperature is the critical temperature below which the components of a mixture are miscible for all compositions...

 (LCST) or lower bound to a temperature range of partial miscibility. As shown in the diagram, for polymer solutions the LCST is higher than the UCST, so that there is a temperature interval of complete miscibility, with partial miscibility at both higher and lower temperatures.

The UCST and LCST of polymer mixtures in general depend on polymer degree of polymerization
Degree of polymerization
The degree of polymerization, or DP, is usually defined as the number of monomeric units in a macromolecule or polymer or oligomer molecule.For a homopolymer, there is only one type of monomeric unit andthe number-average degree of polymerization is given by...

 and polydispersity.

The seminal statistical mechanical
Statistical mechanics
Statistical mechanics or statistical thermodynamicsThe terms statistical mechanics and statistical thermodynamics are used interchangeably...

 model for polymer UCST and LCST is the Flory–Huggins solution theory.
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