Liquid chromatography under limiting conditions of adsorption and limiting conditions of desorption for separation of complex polymers. The role of “flower-like” interactions of macromolecules

Abstract

Liquid chromatography of polymers under limiting conditions (LC-LC) of enthalpic interactions employs different velocities of small (eluent or auxiliary liquid) molecules and large (sample) molecules within a HPLC column. Slowly moving small molecules of a liquid act as a "barrier" hindering fast progression of large molecules during with exclusion retention mechanism. Under specific ("limiting") conditions the low molar mass barrier enables a molar mass independent polymer elution. Depending on the applied barrier mechanism (adsorption, partition, phase separation) and on the experimental arrangement (eluent as a continuous barrier or narrow zone of auxiliary liquid as a local barrier) six different modes of LC-LC can be created. Here, we deal with the limiting conditions of adsorption (LC-LCA-eluent as a barrier) and the limiting conditions of desorption (LC-LCD -the barrier created by a zone of an auxiliary liquid), which utilize adsorption barrier mechanism.Reduced sample recovery was repeatedly observed in many LC-LCA systems, especiallywhen working with the narrow -pore column packing. The fraction of macromolecules retained within the LC-LCA column raised with the decreasing pore size of the column packing and with the increasing molar mass of the polymer samples. With the same both polymers and column packing, LC-LCD yielded full sample recovery. This finding is explained by the difference in the liquid layer composition on the outer surface of the packing particles and on the walls of broader pores compared to that within the narrow pores. Local equilibrium within the liquid phase seems not to be reached in the narrow packing pores. This means that adsorption promoting state is preserved within narrow pores of the LC-LCA column, which may lead to the "flower-like" adsorption of macromolecules. Polymer species penetrate ("reptate') into these narrow packing pores to be strongly adsorbed by a multiple attachment. The desorption of the flower-like adsorbed macromolecules is slow and, consequently irregularities of chromatographic bands and reduced sample recovery may appean In contrast, the desorption promoting state remains in the narrow pores of the LC-LCD packing and it prevents entrapment of macromolecules. The flower-like adsorption of macromolecules is not feasible in LC-LCD and this results in high sample recovery. Our results indicate an important advantage of the LC-LCD over the LC-LCA method and may also help explaining some irregularities observed in liquid chromatography under critical conditions of adsorption.

Separation of Parent Homopolymers from Diblock Copolymers by Liquid Chromatography under Limiting Conditions of Desorption, 2 ‐ Optimization of Experimental Arrangement

“…Moreover, LC LCD tolerates high polymer sample concentrations; (2) overall experimental simplicity, because the elution of sample is isocratic and usually only the barrier composition but not the eluent composition is to be adjusted; (3) robustness, that is, low sensitivity toward variations in experimental conditions; (4) ability to separate more than two polymers of different nature in one single experiment by means of multiple barriers; (5) potentially high sample recovery. [32] The diblock copolymers of poly(X)-block-poly(Y) that contain both parent homopolymers poly(X) and poly(Y) represent a ternary polymer system. Therefore two barriers of different composition and consequently of different adsorption promoting efficiency are to be introduced into the LC LCD column to simultaneously separate both parent homopolymers from the diblock copolymers.…”

Enthalpy assisted size exclusion chromatography Part 2. Adsorption retention mechanism

“…This tendency is typical for molar masses which would be excluded from the packing pores in the absence of enthalpic interactions, that is under "ideal SEC" conditions. It is expected that both solubility differences in the stationary and mobile phases, as well as "flower-like" enthalpic interactions [4,5] are responsible for this phenomenon. In the course of "flower-like" adsorption, only a part of the polymer chain de-coils and "reptates" into the pore.…”

Adsorption and enthalpic partition retention mechanisms in liquid chromatography of non-charged synthetic polymers

“…Elution of the flower-like polymer species, which are attached to the pore walls in the form of long trains may be a slow process. It needs a prolonged application of a very strong solvent in the case of adsorption [46,47]. The manifestation of the flowerlike adsorption of large macromolecules at a given e value seems to rise with the molar mass of polymer species and with the decreasing pore size [35].…”