Investigation of critical polymer properties for polymer release and swelling of HPMC matrix tablets

Four different HPMC batches were characterized to investigate properties related to critical functionality for their use in hydrophilic matrix tablets. In this study, the HPMC batches were chemically characterized and correlated to the behaviour of pure HPMC tablets. Parameters such as the molecular weight, viscosity, intrinsic viscosity and radius of gyration were kept in a rather limited range, which resulted in a weak correlation to polymer release and degree of swelling. The hydrophilic/hydrophobic character of the HPMC samples was elucidated by the degree of substitution and by the clouding behaviour, where an increased hydrophilicity increased the tablet swelling. This phenomenon was interpreted in a refined model for water transport into HPMC tablets. A five times slower polymer release and a considerably larger degree of swelling were found for one batch of HPMC tablets compared to the others, although the characterized average polymer parameters were in the same range. However, the conformation plot displayed a fraction with compact aggregates. In conclusion, the existence of aggregates in aqueous solution seems to perturb the functionality of HPMC tablets and it seems important to understand and characterize these aggregates to fully predict the polymer release and swelling of HPMC tablets.