The product comprises at least two diamide compounds selected from the diamide compounds of following formulae (Ia), (Ib), and (Ic): R2R3NOC-Aa-CONR4R5&emsp；&emsp；(Ia) R2R3NOC-Ab-CONR4R5&emsp；&emsp；(Ib) R2R3NOC-Ac-CONR4R5&emsp；&emsp；(Ic) wherein: R2, R3, R4, and R5, either identical or different, are groups selected from saturated or unsaturated, linear or branched, optionally cyclic, optionally aromatic, optionally substituted, hydrocarbon groups comprising an average number of carbon atoms ranging from 1 to 36, R2 and R3 on the one hand and R4 and R5 on the other hand may optionally form together a ring, optionally substituted and/or optionally comprising a heteroatom, and Aa, Ab, and Ac are linear divalent alkyl groups, each comprising a different number of carbon atoms.
