Heat capacity study of linear polyurethanes

Abstract

A r~ARG~, group amongst the considerable number of urethane-type polymers is formed by the linear polyurethanes (PI~), which can crystallize. The X-ray studies of the formation and decomposition of the crystal structure of the PU based on hexamethylene diisocyanate and various low molecular weight (m.w.) glycols led to the conclusion that the main feature of the crystal structure is the distinct anisotropy of the crystal lattice, which is due to the considerable number of hydrogen bonds present [1-5]. The crystal structure therefore consists of a number of fiat lattices, in which adjacent macromolecules are regularly linked by hydrogen bridges, while those superimposed on each other form a three-dimensional network linked by Van I)er Waals forces. The different nature of the bonds inside the lattices and between them results in an anisotropic decomposition of the crystal lattice, in which the Van Der Waals forces are gradually disrupted first; this is followed by a relatively quick disruption of the hydrogen bonds. The same investigations also established tha t linear PU easily form pseudo-crystalline structures which can rearrange themselves, under certain conditions, into a three-dimensional network. On this basis it is thought that the main features of the melting and crystallization processes of linear PU are determined by the hydrogen bonds. These should naturally also affect the nature of the changes of thermodynamic properties during the formation and decomposition of crystal lattices, and in particular the temperature dependence of heat capacity, which is the subject of this investigation. Two linear P U of the following structures were used in our work: