Characterization of EO‐PO Block Copolymers by Liquid Chromatography Under Critical Conditions

Abstract

Summary: Block copolymers of ethylene oxide (EO) and propylene oxide (PO) are characterized by liquid chromatography under critical conditions (LCCC) for EO. At the critical adsorption point (CAP) for one structural unit, the non-critical block can elute in size exclusion (SEC) or adsorption (LAC) mode. Depending on the molar mass and architecture of the polymers, different strategies are applied. For samples with a higher molar mass, the SEC separation is the method of choice, while lower molar masses also allow a LAC separation. Examples for both situations are given, which show, that these approaches yield different information. In the SEC mode, homopolymers and diblocks can be separated from the triblocks. In LAC mode, a baseline resolution of individual oligomers can be achieved, in which homopolymers, diblocks and triblocks with the same number of repeat units of the non-critical block have the same elution volume.

Full separation of oligomers in block copolymers of ethylene oxide and propylene oxide

“…In the literature, critical conditions of PO have been described on C18 columns and EO elutes in exclusion regime in almost all cases [24,25,48]. Critical conditions for EO have been observed on RP columns in methanol -water (with 80 -90 wt% methanol) [5,49] and acetone -water (with 25 -30 wt% acetone) [31,32].…”

“…Critical conditions for EO have been observed on RP columns in methanol -water (with 80 -90 wt% methanol) [5,49] and acetone -water (with 25 -30 wt% acetone) [31,32]. In such mobile phases, PO elutes in the LAC mode [17,30]. In acetone -water one may separate only very low oligomers of PO, as the interaction parameter of PO is too large.…”

“…At the mobile phase composition corresponding to the CAP for the first block retention may be too strong, if the adsorbed block is long. In mobile phases consisting of different solvents, the retention of one block at the CAP for the other one may be different [17]. A fine adjustment can be achieved by changing the temperature and the pore size of the column [17,27,28]: columns with the same chemical nature, but different pore size will have a CAP at the same mobile phase composition.…”

“…In mobile phases consisting of different solvents, the retention of one block at the CAP for the other one may be different [17]. A fine adjustment can be achieved by changing the temperature and the pore size of the column [17,27,28]: columns with the same chemical nature, but different pore size will have a CAP at the same mobile phase composition. As retention in LAC depends on the pore surface (and thus on the pore diameter), one may separate copolymers of different molar mass on columns with different porosity [27 -30].…”

Liquid chromatography at critical conditions in ternary mobile phases: Gradient elution along the critical line

“…earlier than the critical homopolymer) or in the LAC regime (with c B >0 and elution volumes much larger than the void volume, i.e. much later then the critical homopolymer) [27,[34][35][36]. In the following text, we will use the abbreviations LCCC-E (for c A 5 0, c B o0) and LCCC-A (for c A 5 0, c B 40).…”