Structure and Conformation of Hydroxypropylmethyl Cellulose with a Wide Range of Molar Masses in Aqueous Solution─Effects of Hydroxypropyl Group Addition

The structure of hydroxypropylmethyl cellulose (HpMC) samples with a wide range of weight average molar masses (M_w) from 23 to 5000 kg mol^-1, a controlled degree of substitution (DS) of 1.9 by methyl groups, and a molar substitution number (MS) of 0.25 by hydroxypropyl groups dissolved in aqueous solution was examined using static light scattering (SLS), dynamic light scattering (DLS), small-to-wide angle neutron scattering (S-WANS) techniques, and intrinsic viscosity ([η]) measurements. The determined M_w and the radius of gyration (R_g) showed the relationships R_g ∝ M_w^1.0 and [η] ∝ M_w^1.7 in a range of M_w < 100 kg mol^-1, similar to rigid rod molecules in solution. However, exponents in the relationships decreased gradually with increasing M_w and reached ∼0.5 in a high M_w region, which is a typical value of flexible chain molecules for both R_g and [η]. These observations suggest that the HpMC samples behave as semiflexible rods with a certain persistence length (l_p). The ratios of the hydrodynamic radius via DLS measurements to R_g also supported semiflexible rod behavior. Particle form factors and the average lengths (L) resulting from SLS and S-WANS experiments are well described with rigid rod particles in the range of M_w < 100 kg mol^-1 and semiflexible rods with l_p ∼ 100 nm in M_w > 100 kg mol^-1. Because the average contour length (l_c) calculated from M_w is approximately twice as long as L in the M_w range < 100 kg mol^-1, the formed HpMC particles possess a folded hairpin-like elongated rigid rod structure. However, the l_c/L value increases gradually in the range M_w > 200 kg mol^-1, where the formed HpMC particles behave as semiflexible rods. The formed particle structure was substantially different from that found in methyl cellulose samples with a similar DS value, which showed rod-like behavior over a wide M_w range. The addition of hydroxypropyl groups only at MS = 0.25 effectively changed the formed particle structure.