Dissolution of Polydisperse Polymers in Water

Abstract: The different steps in the dissolution process of polydisperse polymers in water have been studied. To that end, poly(ethylene oxide), dextran and hydroxypropyl methylcellulose tablets varying widely in polydispersity have been produced by mixing low molecular weight and high molecular weight samples in different proportions. The individual release of the low and high molecular weight fractions has been monitored and the swelling of the so-called gel layer, as well as the disappearance of the solid core, of selected tablet compositions have been followed during the dissolution process. Moreover, the release rates of the mixed tablets have been compared to the release rates of less polydisperse tablets. Even though the swelling and the release rate varied dramatically between the tablets of different polymer type, they all shared the same release behaviour; They all developed a gel layer, they all showed smooth release curves with similar shapes and the low and the high molecular weight fractions were released at the same rate for all tablets. Furthermore, corresponding tablets, that is tablets with the same release rate differing widely in polydispersity, showed similar swelling behaviours and shared the same viscosity versus concentration behaviour. Based on these results a physical picture is presented of the polymer dissolution process where the polymer release is described in terms of a threshold viscosity at the gel layer surface. Furthermore, the polymer release from the tablets is shown to correlate with the inverse of the intrinsic viscosity of the polymer in the tablet.