Solid state properties===
Various specialized properties of [[Molecular solid|molecular crystals]] and [[Polymer|organic polymers]] with [[conjugated system]]s are of interest depending on applications, e.g. thermo-mechanical and electro-mechanical such as [[piezoelectricity]], electrical conductivity (see [[conductive polymer]]s and [[organic semiconductor]]s), and electro-optical (e.g. [[non-linear optics]]) properties. For historical reasons, such properties are mainly the subjects of the areas of [[polymer science]] and [[materials science]].