Melting Temperatures

Abstract

Melting temperature, T_m, is defined as a temperature at which the temperature-induced change of an experimental quantity is 50% complete; it is the equilibrium temperature of transition reactions which are alternatively referred to as, e.g., ordered state ⇋ disordered state; helix ⇋ coil; structured form ⇋ melted form; stacked unit ⇋ unstacked unit; or complex ⇋ components. For analysis of melting curves, see ref. 208. Percent thermal hyperchromicity (hper.) accompanying the structured form ⇋ melted form transition is defined as hper. = [A_m(λ)/A_s(λ) − 1] • 100, where A_m (λ)and A_s (λ) are absorbances at the wavelength λ of the melted and structured forms of the polymer, respectively. The A_m and A_s values are usually obtained from melting profiles. The melting profiles generally show a sigmoid curve which tends to level off at sufficiently high and low temperatures, where the absorbances are considered to be those of the melted and structured forms, respectively. The temperature interval in which the hyperchromicity was determined may be given in parentheses, e.g., hper.17% (20–90°), particularly in cases where the temperatures of the melted and structured forms cannot be unambiguously determined due to poor definition of the melting profile. Thermal transition, t, usually refers to the steep part of the melting profile; individual steps in multiple transitions are identified by a subscript (e.g., • t₁, t₂,...), where the lower number refers to the lower-temperature transition. For transitions which are incomplete below certain temperatures, such temperatures may be given (e.g., t.in-compl. 90°). Broad thermal transition (b.t.) is usually indicative of a noncooperative transition.