COMPLICATIONS IN INVESTIGATIONS OF THE SWELLING OF HYDROGEL MATRICES DUE TO THE PRESENCE OF TRAPPED GAS

In recent studies, NMR imaging has been used to investigate the swelli ng of hydroxypropylmethylcellulose (HPMC) tablets and to determine pol ymer concentration distributions in the swollen matrix. The total amou nt of polymer in the system was computed from these distributions and was up to 35% greater than the known weight of HPMC in the tablet. Thi s deviation was traced to the presence of air bubbles in the swollen m atrix which occupied a significant volume previously assumed to be occ upied by polymer and water. When the air in the tablet was removed by vacuum, the swollen gel contained no evidence of air bubbles and the c alculated total polymer weights from the HPMC distributions were equal to the amount of HPMC in the tablet. (C) 1998 Elsevier Science B.V.