Martin's rule for high-performance liquid chromatography retention of polystyrene oligomers

The liquid chromatographic retention of polystyrene (PS) oligomer was examined taking the column hold-up volume as an adjustable parameter based on the assumption that Martin's rule is valid, i.e., ln k (retention factor) is proportional to the degree of polymerization (DP). In both bare silica and C18-bonded silica columns, thus determined column hold-up volume is systematically smaller than the elution volume of the injection solvent. When the column hold-up volume was taken as an adjustable parameter, the retention of PS oligomer in organic mobile phase follows Martin's rule (ln k proportional, variant DP) near perfectly. In addition, the enthalpy and entropy changes associated with the solute transfer, determined from the temperature dependence of the PS oligomer retention, also show excellent linear dependence on the degree of polymerization if the adjustable column hold-up volume was used. When we compare the retention behavior of PS and hydroxyl terminated PS, the repeating (styrene) unit exhibits the same enthalpy and entropy change while the end group shows the corresponding difference. All observations conform to Martin's rule regardless of the columns used, but the physical nature of the adjustable column hold-up volume remains to be elucidated.