Evidence of a Rod-like Structure for Hydroxypropyl Cellulose Samples in Aqueous Solution

Because hydroxypropyl cellulose (HpC) is a popular polymeric material that forms a liquid crystalline phase in solutions with various kinds of solvents, including water, it is commonly thought that HpC has a typical rod-like structure in solution. In this study, the structures of commercial HpC samples in aqueous solution with average molar substitution numbers (MS) ranging from 3.6 to 3.9 and weight-average molar masses (Mw) ranging from 36 to 740 kg mol–1were investigated in detail. We first used multiple techniques, including standard static and dynamic light scattering (SLS and DLS), neutron and X-ray scattering experiments, and viscometric measurements, to obtain clear evidence of rod-like structures quantitatively. The dependence of excess scattering intensities for HpC samples under dilute conditions on the magnitude of the scattering vector over a wide range from 8.9 × 10–3to 3.0 × 10 nm–1was reasonably described by the form factor of rod particles with length (L) and diameter (d). Although the determined Lvalue was close to the contour length (lc) calculated from the Mwvalues in the lower Mwrange, Lbecame obviously less than lcwith increasing Mw. The radius of gyration (Rg) determined via SLS measurements was proportional to Lby a factor of approximately 3.5 ∼ √12 over the Mwrange examined. These observations revealed that the conformation of HpC molecules changes from an elongated single chain to a certain folded structure, maintaining the shape of the rod-shaped particles. Moreover, the Mwdependencies of the intrinsic viscosities and translational diffusion coefficients of the HpC samples resulting from DLS measurements were reasonably described with a theoretical rod-like particle model, assuming that Land dare identical to those resulting from the scattering behaviors.