Adsorption and Enthalpic Partition in Liquid Chromatography of Non‐Charged Synthetic Polymers, 3

Macro Chemistry & Physics

2006

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Liquid chromatography under limiting conditions of insolubility (LC LCI) is a novel member of the group of liquid chromatographic methods working under limiting conditions of enthalpic interactions (LC LC). These employ differences in the velocity of small, pore permeating molecules (eluent components, auxilliary substances) and large, excluded macromolecules (sample components) in the course of their transport along an LC column packed with porous particles. A ''barrier'' of slowly moving small molecules of an appropriate nature selectively decelerates certain kinds of macromolecules due to their enthalpic interactions either with the column packing mediated by the barrier or with the barrier itself. The retained macromolecules accumulate on the barrier edge and elute from the column independently of their molar mass. Other kind(s) of macromolecules may be inert towards the barrier effect and elute in the exclusion mode. In this way, polymer species of different natures can be easily and efficiently separated. In LC LCI, a barrier impermeable to macromolecules consists of a narrow zone of non-solvent, which is injected into the column immediately before the sample solution. The effects of basic experimental variables on the LC LCI elution of selected model polymer species were studied in the present work. These included both the polarity and pore size of the column packing, the nature and composition of the eluent, sample concentration and volume and the non-solvent barrier width, as well as temperature. The general LC LCI behavior of macromolecules was confirmed. It was only slightly affected by the experimental conditions mentioned above, as long as the precipitation power of the non-solvent barrier remained high enough. It has been demonstrated that LC LCI operates over a very broad polymer molar mass range, from a few thousand g Á mol À1 up to several million g Á mol À1 . Under well chosen experimental conditions, LC LCI produces well focused polymer peaks, even for extremely large sample volumes reaching 16% of the total column volume. Narrow pore column packings are preferred because wide pore materials may produce skewed or even split LC LCI peaks. Chromatographically strong eluents and barrier components prevent adsorption of the sample on the column packing so that the mixed retention mechanism is avoided and the elution behavior of macromolecules is controlled exclusively by their solubility. In conclusion, LC LCI is a robust, userfriendly technique which can help solve numerous separation problems in polymer science. For example, LC LCI enables fast and efficient separation of polymer species, which differ in their chemical composition.Fast and efficient separation of the LC LCI eluted polystyrene from the SEC eluted poly(methyl methacrylate) of similar molar mass.

Section: Experimental Partmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

Section: Experimental Partmentioning

confidence: 99%

Section: Experimental Partmentioning

confidence: 99%

Section: Experimental Partmentioning

confidence: 99%

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Liquid Chromatography Under Limiting Conditions of Insolubility, II

Macro Chemistry & Physics

2006

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Liquid Chromatography under Limiting Conditions of Enthalpic Interactions: Enthalpic Partition Retention Mechanism

Macro Chemistry & Physics

Capek

Mendichi

et al. 2006

Summary: Applicability of the enthalpic partition (absorption) retention mechanism was evaluated in liquid chromatography under limiting conditions of enthalpic interactions (LC LC). It was shown that the barrier principle of LC LC also held in the case of enthalpic partition retention mechanism. LC columns packed with porous silica C‐18 bonded phase in combination with low‐polarity PS and PnBMA were employed. The partition‐promoting solvent was DMF and the partition‐preventing solvent was THF. Pore‐permeating molecules of DMF moved slowly along column and acted as a barrier, which hindered fast progression of pore excluded macromolecules. The barrier was either the eluent itself, or a narrow zone of DMF injected immediately before polymer solution. In the former case, the eluent contained a high concentration of DMF and would not allow polymer elution. However, the sample was injected in pure THF and traveled within its zone. Being decelerated by the DMF barrier, both PS and PnBMA eluted independently of their molar mass in total volume of column liquid. They could be efficiently and independently of their molar masses separated from the medium‐ and high‐polarity polymers, which did not exhibit enthalpic partition and eluted in the size exclusion mode. In contrast to recently evaluated adsorption based LC LC procedures, enthalpic partition produced broader, skewed, and often split peaks. This is assumed due to slow establishment of enthalpic partition equilibrium, possible presence of micropores in the C‐18 bonded phase, and/or due to deformation of shape of barrier edge. Therefore, limiting conditions of enthalpic partition are to be applied preferably to characterization of polar SEC eluted polymers after their discrimination from the low‐polarity macromolecules eluting in the LC LC mode.Fast and efficient separation of polar minor (1%) polymer components from major (99%) polymer component.magnified imageFast and efficient separation of polar minor (1%) polymer components from major (99%) polymer component.

Liquid Chromatography of Synthetic Polymers under Limiting Conditions of Insolubility III

Macromolecular Symposia

2007

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Summary Performance was evaluated of silica based commercial monolithic rod-like columns in liquid chromatography of synthetic polymers under limiting conditions of enthalpic interactions (LC LC). LC LC employs the barrier effect of the pore permeating and therefore slowly eluting small molecules toward the pore excluded, fast eluting macromolecules. Phase separation (precipitation) barrier action was applied in present study. The barrier was created either by the narrow pulse of an appropriate nonsolvent injected into the column just before the sample solution (LC LC of insolubility -LC LCI) or by the eluent itself. In the latter case, the polymer sample was dissolved and injected in a good solvent (LC LC of solubility -LC LCS). In LC LCI, polymer species cannot break thru the nonsolvent zone while in LC LCS they cannot enter eluent, which is their precipitant. Therefore, polymer species keep moving in the zone of their original solvent. Macromolecules eluting under the LC LC mechanism leave the column in the retention volume (V R ) roughly corresponding to V R of the low molar mass substances and can be efficiently separated from the polymer species non-hindered by the barrier action. The known advantages of monoliths were confirmed. From the point of view of LC LCI and LC LCS the most important quality of monolithic columns represents their excellent permeability, which allows both working at high flow rates and injecting very high (in the range of 5%) sample concentrations. Monolithic column tolerate also extremely high molar mass samples (M>10,000 kg Á mol À1). On the other hand, the mesopores (separation pores) of the tested monoliths exhibited rather small volume and wide size distribution. These shortcomings partially impair the permeability advantage of monoliths because in order to obtain high LC LC separation selectivity a tandem of several monolithic columns must be applied. Presence of large mesopores also reduces applicability of monolithic columns for molar masses below about 50 kg Á mol À1 because V R s of polymers eluted behind the barrier are similar to that of freely eluting species. The non-negligible break-thru phenomenon was observed for the very high polymer molar masses largely eluting behind the barrier. It is assumed that the fraction of very large mesopores present in the monoliths or association/microphase separation of macromolecules may be responsible for this phenomenon. This is why the presently marketed SiO 2 monolithic columns are mainly suitable for the fast purification of the LC LC eluting macromolecules from the polymeric admixtures non-hindered by the barrier-forming liquid. Still, monolithic columns have large potential in the LC LCI and LC LCS procedures provided size (effective diameter) of the mesopores can be reduced and their volume increased.