Universal Suspension Hydrodynamics: Molecular Characterization of Hydroxypropyl Methylcellulose

Abstract

A study of the rheology of aqueous solutions of hydroxypropyl methylcellulose (HPMC) is presented with the aim of supporting the previously suggested physics, referred to as laminar dynamics, of the viscosity increasing effect per unit volume of particles with extended shape on a flowing suspension. Since it is essential that appropriate flow is employed in order to utilize the proposed model enabling absolute values of the particle's weight average axial ratio a w to be derived from the intrinsic viscosity [η], the shear requirement is studied extensively. Hence, flow curves (viscosity η versus shear rate) for a series of commercial HPMC viscosity grades (3 to 10,000 cP) of USP substitution type 2910 were measured under an extended range of concentrations c (g/dL) and shear rates D (1/s). The results indicate that [η] (dL/g) can be obtained by combining c and D in such a way that either c → 0 (at constant D > 0) or D > D* (at constant c > 0), where D* is a critical shear rate. It may hence be concluded that laminar dynamics is applicable at any constant D > 0 if c → 0, and it is proposed that such flow corresponds to a complete absence of particle–particle interactions leading to a flow in the vicinity of the particle parallel to its length axis. It is demonstrated that the particle–particle interaction tends to become negligible, i.e., the Huggins constant kH → 0 (for extended shape), at any concentration when D > D*. Assuming that the particle–particle interaction is entirely hydrodynamic, i.e., a negligible non-hydrodynamic interaction such as an electrostatic or chemical interaction, at any D, it is possible to derive a universal suspension hydrodynamics, solely determined by the particle shape and concentration under Newtonian conditions and general to any polymer suspension or solution, by combining stationary dynamics (shown empirically to be a universal relation; ηsp = f(c[η]), D → 0) with laminar dynamics (ηsp = c[η], D > D* or c → 0). It is demonstrated how these universal functions can be used for absolute determination of a w and subsequent calculation of such values as molecular weight, size, Mark-Houwink constant, critical (“overlap”) concentration c*, and radius of gyration Rg,w. In addition, a universal suspension characteristic termed critical specific viscosity ηsp* is identified.