Conformation and Structure of Hydroxyethyl Cellulose Ether with a Wide Range of Average Molar Masses in Aqueous...

The solution properties of a water-soluble chemically modified cellulose ether, hydroxyethyl cellulose (HeC), were examined using static light scattering (SLS), dynamic light scattering (DLS), small-to-wide-angle neutron scattering (S-WANS), small-to-wide-angle X-ray scattering (S-WAXS) and viscometric techniques at 25 °C. The examined HeC samples had average molar substitution numbers ranging from 2.36 to 2.41 and weight average molar masses (M_w) that fell within a wide range from 87 to 1500 kg mol^-1. Although the relationship between the determined radius of gyration (R_g) and M_w was described as R_g ∝ M_w^~0.6, as is observed usually in flexible polymer solutions in good solvents, the observed scattering vector (q) dependencies of excess Rayleigh ratios were well interpreted using a rigid rod particle model, even in high-M_w samples. Moreover, the ratios of the formed particle length (L) evaluated assuming the model for rigid rods to the determined R_g showed the relationship LR_g^-1 ~ 3.5 irrespective of M_w and were close to those theoretically predicted for rigid rod particle systems, i.e., LR_g^-1 = 12. The observed SLS behavior suggested that HeC molecules behave just like rigid rods in aqueous solution. As the L values were not simply proportional to the average molecular contour length calculated from the M_w, the chain conformation or structure of the formed particles by HeC molecules in aqueous solution changed with increasing M_w. The q dependencies of excess scattering intensities observed using the S-WANS and S-WAXS experiments demonstrated that HeC molecules with M_w less than 200 kg mol^-1 have a diameter of ~1.4 nm and possess an extended rigid rod-like local structure, the size of which increases gradually with increasing M_w. The observed M_w dependencies of the translational and rotational diffusion coefficients and the intrinsic viscosity of the particle suspensions strongly support the idea that the HeC molecules behave as rigid rod particles irrespective of their M_w.