wherein n is from 2 to 12; R3 and R4 are each an alkyl group or a hydrogen, and at least one of R3 or R4 is an alkyl group having from about 1 to 13 carbon atoms; R2 is an alkyl group having from about 1 to 13 carbon atoms; and the combined number of carbon atoms (N) in the spacer R1 is at least n+1.