Examples of the chain extender include glycols such as ethylene glycol, 1,4-butanediol and diethylene glycol, amines such as diethanolamine, triethanolamine, tolylenediamine and hexamethylenediamine, polyisocyanates such as TDI (tolylenediisocyanate) adduct of trimethylolpropane and triphenylmethanetriisocyanate, bis(2-hydroxyethyl)hydroquinone, a reaction product of bisphenol A with ethylene oxid