Characterisation of hydroxypropylmethylcellulose phthalate (HPMCP) by GPC using a modified organic solvent

Hydroxypropylmethylcellulose phthalate (HPMCP) is readily soluble in a number of organic solvents which could be used as eluent in gel permeation chromatography (GPC) for the determination of polymer molecular weight distribution (MWD). GPC analysis performed using acetone as the mobile phase indicated a strong interaction between the polymer and the column packing material. GPC analysis performed using tetrahydrofuran (THF) as the mobile phase also indicated that there was some sample to column interaction although it was much weaker than that observed with acetone. It was found that the anomalous behaviour of HPMCP in THF could be suppressed by the addition of 5% acetic acid. This modified eluent system was used to characterise the MWD of a number of different batches of HPMCP by GPC with multiple detection. A good correlation was observed between the viscosity values quoted by the supplier and the average molecular weight values determined by GPC, although the GPC analysis revealed some differences in polydispersity. The effect of storage conditions on the MWD of HPMCP was also studied.