Size‐exclusion chromatographic NMR of polymer mixtures

Abstract

The use of chromatographic stationary phases or solvent modifiers to modulate diffusion properties in NMR experiments is now well established. Their use can be to improve resolution in the diffusion domain or to provide an insight into analytemodifier interactions and hence the chromatography process. Here we extend previous work using size-exclusion chromatographic (SEC) stationary phases to the investigation of polymer mixtures. We demonstrate that similar diffusion modulation behaviour is observed with an SEC stationary phase which can be understood in terms of size-exclusion behaviour.3

Size‐exclusion chromatographic NMR under HR‐MAS

“…This change is an unexpected increase in the diffusion coefficient. Previous experiments with size‐exclusion phases resulted in a decrease in measured diffusion coefficient due to molecules entering the pores of the stationary phase and being retarded, with the degree of retardation being dependent on both the molecular weight and pore size . Chromatographic NMR of benzene and silica stationary phases has shown an increase in diffusion coefficient upon addition of the stationary phase.…”

“…The trends in the measured diffusion coefficients are, however, similar between the spinning and static cases. Fitting these data to the phenomenological equation, similar to that obtained by Anderson and Stoddart and Determann and Michel, used previously, results in the parameters given in Table . The similarity of the parameters for the spinning and static samples suggests that the polymer reference standards are behaving in a comparable manner under both sets of experimental conditions albeit with different measured diffusion coefficients.…”

“…The size‐exclusion stationary phase, when present, will also be exposed to the same physical forces and having a much larger effective molecular weight due to cross‐linking, therefore likely to be predominantly found in the outer portions of the rotor. The result of this sedimentation effect is that the loading of the polymer solution into the stationary phase will be considerably different compared with the static case reported previously . The loading of the stationary phase, i.e.…”

“…Recently, we have extended the concept of chromatographic NMR to the use of size‐exclusion stationary phases . These phases comprise a porous material, typically based on cross‐linked dextran, with pore sizes on the order of 10–100 nm.…”

“…[14] Recently, we have extended the concept of chromatographic NMR to the use of size-exclusion stationary phases. [24][25][26] These phases comprise a porous material, typically based on cross-linked dextran, with pore sizes on the order of 10-100 nm. Chromatographic separation depends on the time different analytes spend exploring the pores, with smaller molecules typically spending longer inside the pores than larger molecules.…”