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.