Ultrasonic degradation for molecular weight reduction of pharmaceutical cellulose ethers

Several pharmaceutical grade cellulose ethers, hydroxypropyl methyl cellulose (HPMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC) and ethylhydroxyethyl cellulose (EHEC) have been degraded to different molecular weights by exposing either a 1.5 or 4.0 wt.% aqueous solution of the polymer to varying periods of ultrasonic radiation over 24 h. The resulting solutions were characterised with respect to molecular weight and molecular weight distribution of the polymer using capillary viscometry, size exclusion (SEC) and in the case of HPC, dynamic and static light scattering. Depending upon the ultrasonication time, a range of polymer solutions were produced over a range of molecular weights in a suitable form for further experimental use. In all cases a limiting polymer molecular weight was reached after 24 h of ultrasonic degradation; the limiting polymer molecular weight was dependent upon the nature of the polymer and not the concentration of the polymer solution. From measurements of intrinsic viscosity using capillary viscometry and molecular weight using static light scattering, the Mark-Houwink parameters of HPC were found to be K = 6.25 × 10−5 and a = 0.84 over the molecular weight range 139,000–58,000 relative molecular mass.