Correlation between the free volume and the metoprolol tartrate release of Metolose patches

The correlation between the release characteristics of metoprolol tartrate and the free volumes of Metolose patches containing Metolose 90SH 100.000SR (hypromellose) and Metolose SM 4000 (methylcellulose) of various proportions was studied. Positron annihilation lifetime spectroscopy (PALS) measurements were performed in parallel with the metoprolol tartrate release study to track the free volume changes of patches. Second-order polynomial relationship was found, with good correlation, between the metoprolol tartrate released at the 6th hour, the o-positronium lifetime, indicating the free volume of the polymer, and the Metolose SM 4000 relative proportions. The main reason for this correlation is the change in the free volume size of the polymer patches embedding metoprolol tartrate as a function of the relative proportion of Metolose 90SH 100.000SR containing hydroxypropoxy substitution. Since there were no significant changes between the free volumes of Metolose polymers at any ratio of the constituents in the case of lack of metoprolol tartrate, the free volume changes refer to a possible intermolecular interaction between the polymer and the active agent.