Direction of Temperature Gradient and Retention Mechanism in Normal-Phase Temperature Gradient Interaction Chromatography for Poly(methyl methacrylate) Fractionation

Abstract

ABSTRACT:Temperature gradient interaction chromatography (TGIC) is a powerful technique for molecular characterization of polymers, in which the interaction strength is controlled by varying the column temperature. In the present paper, the direction of temperature gradient for normal-phase mode TGIC in poly(methyl methacrylate) (PMMA) fractionation was studied by using System I (column: cyano-modified silica == eluent: 1,4-dioxane/n-hexane) and System II (bare silica == 1,4-dioxane/CH 2 Cl 2 ), respectively. It was found that PMMA samples were separated by positive and negative temperature gradients using the respective systems. The results were also discussed theoretically. 1,2 TGIC separates polymer components in the order of molecular weight with a resolution superior to size exclusion chromatography (SEC). TGIC separation is driven mainly by adsorptive interaction between solutes and stationary phase and the strength of the interaction is controlled by varying the column temperature, to offset the molecular weight dependency of the adsorption strength. During the sample elution, the column temperature is varied continuously by positive (rising) or negative (lowering) temperature gradient. In many studies of TGIC, reversed-phase mode TGIC (RP-TGIC) has been applied to the separation of various polymers. In almost all cases of RP-TGIC, the positive temperature gradient was applied. The exception reported so far was a case where poly(ethylene oxide) (PEO) was fractionated using C18 bonded silica column and a mixture of acetonitrile and water by negative temperature gradient.2-4 On the other hand, the results in the normal-phase mode TGIC (NP-TGIC) reported are not so many. However both temperature gradients were reported in NP-TGIC. That is, polystyrene (PS), 5,6 polyisoprene (PI), 5 PEO 4 and poly(2-vinylpyridine) (P2VP) 7 were separated by the positive temperature gradient, whereas PS samples were separated also by the negative temperature gradient using the respective columns of bare silica and NH 2 -modified silica packing materials and the eluent mixtures of chloroform and n-hexane (nHx) at the respective compositions in our previous work. 6 It was reported that the positive temperature dependencies of the retention were obtained for PS 8 and poly(styrene-co-methyl methacrylate) [P(S-co-MMA)] 9 in common organic eluents and also poly(ethylene glycol) in aqueous eluents.10 If these systems were used for NP-TGIC, the negative temperature gradient should be effective. Then, it is very interesting whether not only low polar polymer PS but also high polar polymer poly(methyl methacrylate) (PMMA) can be separated by both of positive and negative temperature gradients in NP-TGIC. All cases of TGIC separation of PMMA samples reported so far were carried out by RP-mode using positive temperature gradient, as far as we know. However, PMMA samples and block copolymer samples involving PMMA block were separated mainly by NP-mode in the liquid chromatography at the critical condition (LCCC), 11,12 and the above mentio...

Temperature Gradient Interaction Chromatography: A Perspective

“…In a typical TGIC experiment, the temperature is raised to gradually increase polymer desorption and promote elution, given that adsorption of polymer is generally an exothermic process [1]. However, it should be noted that in a few rare cases polymer-eluent-column systems have been reported with an inverse temperature dependence [15][16][17].…”