The effects of high-shear processing and thermal exposure on the molecular weight and solution viscosity stability of selected polymers were studied. Aqueous solutions of two molecular weight grades of hydroxyethyl cellulose (HEC) were subjected to high-shear rates using a high-shear mixer and two colloid mills. Extremely rigorous shearing conditions were required to alter the viscosity. The viscosity stability of the two grades of HEC was studied under prolonged thermal treatment at 70'. Viscosity degradation was found to follow a first-order reaction rate, the viscosity half-life for the higher grade being considerably less than that for the lower molecular weight grade. Number-average molecular weights for the HEC's were determined by the osmotic pressure technique and related to viscosityaverage molecular weights through intrinsic viscosity data. The correlation between the two types of molecular weights was high, implying that bond-scissions and hydrolysis were the cause of viscosity degradation.
HE PHYSICAL nature of most pharmaceutical 'dispersed sy-ctems ncccssitates the use of viscosity imparting agents or dispersing aids to reduce phase separation tendencies, thereby enhancing medicament distribution. Virtually all such agents arc polynieric in structure and fall under the general categories of natural gums, resins, and synthetic polymers. The latter group finds the most application in present-day suspension technology. The viscosity imparted to a system by the presence of these solvated polymeric agents is dependent on their spacial configurations in colloidal solution which is primarily a function of molecular weight, and to some extent, the degree of substitution or branching along the polymer chains.
Various physicocheniical forces are capable of randomly rupturing the long chain structure of high molecular weight polymcrs (1). Two physical forces which may initiate depolymcrization and subsequent viscosity loss in high polymer sys- tems are mechanical shear and thermal energy. Studies ( 2 4 ) have shown that mechanical depolymerization of polymers in solution may occur and that this phenomenon is difficult to quantify. In most instances, the degradation reactions approximate the first-order rate law. Factors contributing to this mechanical phenomenon include ( a ) shear rate, (b) the avcragc molecular weight of the polymer, ( 6 ) polymer-solvcnt interactions, and (d) the stereochemistry of the polymer and solvent. With polymer systems containing suspended particles, the concentration and particle size characteristics of the suspended material