Use of a compound or a combination of compounds for crystallization of polyhydroxy-alkanoate (PHA), poly-lactic acid (PLA) or combinations thereof, characterized in that each of said compounds comprises a core motif with two oxalamide motifs, flanked by two arms, wherein said core motif has the formula: R-NH-C(O)-C(O)-NH-(CH2)n-NH-C(O)-C(O)-NH-R', wherein n is between 1 and 10 and the arms R and R' are each independently of one another chosen from: (i) H; (ii) an alkyl group with a total number of carbon atoms between 1 and 20; (iii) an aromatic ring; or (iv) an ester group as e.g. - X-Ester-Y, or - X-Ester- X-Ester-Y, wherein X is a saturated aliphatic hydrocarbon group comprising 1 to 20 carbon atoms, Y is chosen from H, an alkyl group with a total number of carbon atoms between 1 and 20 or an aromatic ring and Ester is -C(O)-O- or -O-C(O)-
