DSC, dielectric and dynamic mechanical behavior of two- and three-component ordered polyurethanes

Abstract

Liquid crystalline (LC) polyurethanes were made from two diisocyanates (flexible HMDI and stiff TDI) (DIs), mesogenic diol (D) and a polybutadiene-diol (B) with stoichiometric ratios of reactive hydroxy (OH) and isocyanate (NCO) groups ((NCO)DI/((OH)D+(OH)B)=1/1). Two- (B/DIs, D/DIs) and three-component ((D+B)/DIs, D/B=1/1 by weight) polymers were prepared and their dielectric, dynamic mechanical and DSC behavior was investigated. For neat B, the glass transition temperature TgB (∼−46°C) was detected. Two-component B/HMDI and B/TDI polymers have exhibited a homogeneous structure with the glass transition temperatures TgU∼−9 and 2°C. On the other hand, for D/DI polymers on cooling from the melt and subsequent heating the glass transitions at TgU∼41°C (D/HMDI) and 58°C (D/TDI) together with nematic and smectic mesophases were found. In three-component systems, additional glass transitions at TgB∼−41°C (B/D/HMDI) and −31°C (B/D/TDI) were observed. This means that the polymers exhibit a distinct two-phase structure with soft polybutadiene (B) and hard polyurethane (D/DI) phases. In hard polyurethane phase, the glass transitions at TgU and LC mesophases similar to those found in two-component D/DI polyurethanes were detected. Dielectric and dynamic mechanical results correlate well with DSC measurements.