Molecular weight studies on hydroxypropyl methylcellulose II. Intrinsic viscosity

In order to obtain absolute molecular weight information for hydroxypropyl methylcellulose HPMC from viscosity measurements, the physics of the viscosity-increasing effect of particles with extended shape on a flowing suspension has been elaborated. It is suggested that the phenomenon causes orientation of the particles producing complete alignment of the longer axis with the flow direction at sufficiently high shear rate, and that the viscosity increment per volume unit of particles, that is, the unitless laminar intrinsic viscosity [eta](phi), approaches the relative axial ratio a of the particle as compared to the liquid constituents with increasing particle length if the particle is a sufficiently large object. Hence, provided that the required liquid dynamic conditions are fulfilled, corresponding to laminar Newtonian flow, viscosimetry can be used as an absolute method for determination of a. In the case of fully extended molecules, the weight-average molecular weight M-w can then be estimated as M-w = ([eta]100 rho - 1.5) M(u)a(l)/a(u), where [eta] is in dL/g, rho (conversion factor into volume fraction) in g/mL, M-u the molecular weight of the repeating unit (g/mol), a(u) its axial ratio including solvation and a(i) is the axial ratio of the liquid constituents. The theory is used to calculate M-w of various commercial HPMC viscosity grades (3-10,000 cP) of USP substitution type 2910 from capillary viscosimetry assuming complete extension as deduced from supplementing information obtained previously by osmometry. The value of rho (g dry polymer/mL solvated polymer) is determined by a novel method based on the temperature influence on the specific viscosity under conditions of constant extension assuming that the solvation becomes negligible at a critical solution temperature T-theta, coinciding with phase separation. Furthermore, the proposed model for the laminar dynamics of suspensions appears to be generally applicable to polymers; the constant K of the empirical relation [eta] = KMalpha, usually referred to as the Mark-Houwink equation, is derived as K-w = a(u)M(u)(-alpha)/(a(l)100 rho), where K-w is in dL g(-1) (g/mol)(-alpha).