Elution behavior of poly(lactide-co-succinimide) copolymers studied by SEC-MALS

We synthesized poly(lactide-co-succinimide) (PLS) copolymers with the ratio of lactide to succinimide units of 3:1 and 6.5:1 and studied their elution behavior by size exclusion chromatography with an on-line light-scattering detection. Since the copolymers contain a certain amount of carboxyl groups, they behave as ionomers in N,N-dimethylacetamide (DMAc) and show a typical polyelectrolyte (PE) effect. The PE effect was eliminated by the addition of simple electrolyte like LiBr, H(3)PO(4), or both in DMAc. The efficiency of the additive decreases in the order: LiBr > LiBr + H(3)PO(4) > H(3)PO(4). The ionic strength of the 0.1 M LiBr/DMAc was high enough for the onset of hydrophobic interactions of PLS lactic acid segments intermolecularly as well as with the column packing material. The drawback of the LiBr + H(3)PO(4)/DMAc solvent system is a rather high intensity of the system peaks, which are imposed on the right side of the copolymer signal. System peaks strongly influence the determination of number and to a lesser extent the weight average molar masses of PLS copolymers. An addition of only H(3)PO(4) in high enough concentration to DMAc (0.05 and 0.1 M) successfully eliminated the PE effect of the 6.5:1 PLS copolymer. On the contrary, the PE effect of the 3:1 PLS copolymer having higher charge density compared to 6.5:1 PLS copolymer cannot be entirely canceled out in any of the H(3)PO(4)/DMAc solutions examined.