Phenomenological viscoelasticity of a heterogeneous pharmaceutical semisolid.

This study presents the results of an investigation of the viscoelastic properties of anhydrous lanolin USP, as determined by dynamic mechanical testing. The elastic shear modulus (G'), viscous shear modulus (G"), and loss tangent (tan delta) were determined as a function of shear frequency, nu, (0.01--10.0 Hz) and temperature, T, (0--30 degrees). These viscoelastic parameters were found to be temperature and shear frequency dependent. Up to 100-fold changes in shear moduli and tan delta values were observed with appropriate changes in T and nu. Many of the observed properties are also characteristic of high molecular weight polymers and can be attributed to a high degree of molecular structure. It was found that dynamic mechanical testing was a sensitive tool for measuring structural changes, and was especially useful in detecting a major structural transition well below the accepted melting temperature of anhydrous lanolin.