Especially preferred are diesters of dicarboxylic acids in which at least one of R1 and R2 is an alkyl group having at least about 2 carbon atoms. [0031] Specific examples of polycarboxylic acid esters include aliphatic polycarboxylic acid esters such as diethyl succinate, dibutyl succinate, diethyl methylsuccinate, diisobutyl alpha-methylglutarate, dibutyl malonate, diethyl methylmalonate, diethy