Evaluation of liquid chromatography column retentivity using macromolecular probes

Macro Chemistry & Physics

2005

Self Cite

Summary: Two kinds of retention mechanisms of polystyrene (PS) macromolecules are considered in liquid chromatography (LC) applying alkane-bonded silica gel phases as column packings. They are based on non-polar interactions between polymer segments and alkane groups, which lead to adsorption of polymer species on the surface situated between bonded alkane groups and eluent on the one hand (interphase adsorption) and to partition of macromolecules between bonded phase volume and eluent (enthalpic partition) on the other hand. The roles of both retention mechanisms were assessed for PS species with different molar masses and several series of binary eluents. Bare silica and C-1, C-4, C-8, as well as C-18-bonded phases were compared. The results demonstrated that the eluent had to be a thermodynamically poor solvent for macromolecules to push them onto/into the bonded phase. However, no quantitative dependence between the eluent's thermodynamic quality for macromolecules and the extent of retention of polymer species was found. Similarly, no correlation was identified between eluent quality and prevailing retention mechanism -either interphase adsorption on the trimethylsilylated silica gel or interphase adsorption plus enthalpic partition in the case of C-4, C-8 and C-18-bonded phases. The explanation is to be sought in the actual ''composition'' of the preferentially solvated macromolecules and bonded alkane groups. The bonded phase probably has to be a slightly better ''solvent'' than the eluent for a polymer species to cause their retention. Enthalpic partition of PS plays an especially important role in DMF/THF mixed eluents. The results indicate, however, that only three to five terminal -CH 2 -groups plus the methyl end-group take part in the enthalpic partition process. This means that silica gel bonded with shorter alkane groups may be advantageous compared to the C-18 packings in some coupled high performance liquid chromatography methods for synthetic polymers, combining entropic and enthalpic retention mechanisms.log V h vs. V R for bare Kromasil (&), as well as for C-18 (^), C-8 (!), C-4 (~) and C-1 (*) phases.

Section: Resultssupporting

confidence: 77%

Section: Experimental Partmentioning

confidence: 76%

Section: Experimental Partmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

Macro Chemistry & Physics

2005

Self Cite

Critical assessment of “critical” liquid chromatography of block copolymers

J of Separation Science

2016

Self Cite

Brief elucidation is presented of potential difficulties the researcher can face in the course of the molecular characterization of block copolymers with help of liquid chromatography under critical conditions. The impediments include the demanding identification of critical conditions, the limited area of applicable polymer molar mass, the high sensitivity of critical conditions toward minute changes in experimental conditions including changes in the interactivity of the column packing, extensive band broadening, limited sample recovery, pressure effects, detection problems, coelution of block copolymers with their noninteractive parent homopolymers, possible effect of the noninteracting chains of block copolymers on the behavior of the interacting blocks, and role of preferential solvation of macromolecules in mixed solvents. These matters may complicate proper data evaluation and challenge the exactness of results obtained. Special attention is paid to the so far neglected phenomena of preferential solvation, which may affect not only detection of block copolymers but also their retention. The latter occurrence is demonstrated by the behavior of both poly(methyl methacrylate)s eluted from bare silica gel in a mixed eluent tetrahydrofuran plus toluene and polystyrenes eluted from silica gel C18 bonded phase with the mobile phase of tetrahydrofuran/n-hexane.

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.