Liquid Chromatography of Synthetic Polymers under Critical Conditions. The Case of Single Eluents and the Role of Θ Conditions

As a rule, critical conditions in liquid chromatography of synthetic macromolecules are adjusted by both mixed mobile phase composition and temperature. The application of single component eluents is attempted in the present study. The enthalpic interactions in the column are controlled exclusively by temperature. Poly(methyl methacrylate)s (PMMA), poly(n-butyl methacrylate)s (PnBMA), and polystyrenes (PS) in various solvents are investigated with bare silica gel and silica C18 bonded phases. Using a series of single eluents based on various esters, PMMA probes, and bare silica gel, it is shown that minute variations in eluent nature strongly affect polymer retention, and temperature represents a too weak parameter to adjust critical conditions in thermodynamically good solvents. The idea is proposed and tested that the sensitivity of polymer retention toward temperature variations increases in the vicinity of Θ conditions, where the structure of macromolecules in solution strongly depends on temperature. The critical conditions have been identified in some single eluents which are thermodynamicaly poor solvents for polymers investigated, namely, acetonitrile for PMMA at 66 °C and dimethylformamide for PnBMA at 154 °C and for PS at 95 °C&sbd;all with C18 bonded silica gel. A “critical-like” elution was also observed in cyclohexane for PS at 9 °C with silica gel C18 packing, which is well below the Θ temperature. It is demonstrated that poly(methyl methacrylate)s can be separated according to stereoregularity in the area of critical conditions using acetonitrile eluent and silica C18 column packing. The excessive polymer peak broadening and decreased recovery which were observed in some mixed mobile phases are diminished in the single component critical eluents studied, but they are not fully eliminated.