Adsorption and enthalpic partition in liquid chromatography of non-charged synthetic polymers, 3 - Interphase adsorption versus enthalpic partition of polystyrene in alkane-bonded silica gel

摘要

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 the 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 trimethyl-silylated 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 -CH2- groups plus the methyl end-group take part in the enthalpic partition process. the 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.