Thermochromic Phenomena in Polymers
Structured by thermochromic material type, this review examines the state-of-the-art of existing as well as potential future applications based on specific material classes.
TABLE OF CONTENTS Thermochromic Phenomena in Polymers
Chapter 5: Thermotropic Polymer Systems
The term thermotropic denotes a change of the light-scattering properties of a system with temperature. This subclass of thermochromic materials changes transparency but not colour with temperature. Depending on temperature, switching between a highly transparent and a light-scattering milky white state takes place. Thermotropic effects can be caused by a phase separation process, by a phase transition between an isotropic and an anisotropic (liquid crystalline) state or by a different temperature dependency of the diffractive indices of a two-phase guest host system. In recent years the thermotropic properties of polymer blends, polymer hydrogels and casting resins doped with an additive which forms a separate phase have been extensively studied.
5.1 Polymer Blends
An uninterrupted miscibility of binary polymer systems is quite unusual. Miscibility gaps occur in most phase diagrams. Certain binary polymer systems exhibit critical solution temperatures, whereby with increasing temperature an increase or a decrease of the miscibility can occur. Polymer systems with an upper critical solution temperature (UCST) are uninterrupted miscible with one another above the critical solution temperature and possess a miscibility gap below the UCST. A lower critical solution temperature (LCST), in contrast, denotes an uninterrupted miscibility at low temperatures and a miscibility gap above the LCST. Within wide concentration ranges the polymer blends of such systems are thermotropic. On heating a thermotropic polymer blend of a polymer system with a LCST a transition from a transparent to a light-scattering milky white state occurs. Thermotropic polymer blends of systems with an UCST switch in the opposite manner.
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