ABSTRACT
Polyureas and polythioureas fall in the class of amino resins, which have been known from over 100 years ago. Amino resins, which are thermosetting polymers, are the products of polycondensation of aldehydes, commonly formaldehyde, and diamino compounds such as urea, thiourea, melamine, guanidine, and other related diamino compounds. Amino resins have been used commercially in molding, adhesives, laminating, textile finishing, protective coatings, paper manufacture, stoving, lacquers, enamels, fillers, varnishes, binders for inorganic fibers and foundry core sand, graphite resistors, foam structures, ion-exchange resins, leather tanning, plywood bonding, and electrical devices [1-4]. In the past two decades, very significant progress in the chemistry and technology of polyureas has been made by synthesizing a wide variety of aliphatic as well as aromatic polyureas from different diamines and diisocyantes [5]. The polar structures of polyureas lead to higher melting points than those of the corresponding polyimides, due to the greater molecule cohesion, though aliphatic polyureas have lower thermal stability than their aromatic counterparts. Thermal stability of polyureas is higher than polyurethanes but lower than polyamides. Polyurea fibers have tensile strengths between those of polyamides and polyester fibers. Polyureas have been used for cable insulation, foams, moldings, lubricants, and biomedical applications. Thermosetting polymers of polyurea and polythiourea were long considered suitable only as insulating materials, and the study of electrical properties of polyureas and polythioureas is of recent origin. The first attempt in this direction was made by Nalwa et al. in 1978 [6], who proposed to study the electrical behavior of thiourea when incorporated as a part of a polymer strand, because thiourea is known for its very interesting dielectric, pyroelectric, piezoelectric, and ferroelectric properties. The influence of thiourea structure on thermally stimulated depolarization effects, as well as dielectric and pyroelectric properties of a variety of poly-
thioureas, was investigated [6,7]. Pyroelectricity in a polythiourea was also first reported in 1979 by the same research team [8]. There was no report on the electrical properties of polythioureas and polyureas until that time.
