Reconsideration of the conformation of methyl cellulose and hydroxypropyl methyl cellulose ethers in aqueous solution

Abstract

The structure and conformation of methyl cellulose (MC) and hydroxypropyl methyl cellulose (HpMC) ether samples dissolved in dilute aqueous (D₂O) solutions at a temperature of 25 °C were reconsidered in detail based on the experimental results obtained using small- and wide-angle neutron scattering (S-WANS) techniques in a range of scattering vectors (q) from 0.05 to 100 nm⁻¹. MC samples exhibited an average degree of substitution (DS) by methyl groups per glucose unit of DS = 1.8 and the weight average molar mass of M_w = 37 × 10³ and 79 × 10³ g mol⁻¹. On the other hand, HpMC samples possessed the average molar substitution number (MS) by hydroxypropyl groups per glucose unit of MS = 0.25, DS = 1.9, and M_w = 50 × 10³ and 71 × 10³ g mol⁻¹. The concentration-reduced scattering intensity data gathered into a curve for the solutions of identical sample species clearly demonstrated the relationship I(q)c⁻¹ ∝ q⁻¹ in a q range from 0.05 to 2.0 nm⁻¹, and small interference peaks were found at q ∼ 7 and 17 nm⁻¹ for all examined sample solutions. These observations strongly revealed that form factors for both the MC and HpMC samples were perfectly described with that for long, rigid rod particles with average diameters of 0.8 and 0.9 nm, respectively, and with an inner structure with characteristic mean spacing distances of ca. 0.9 and 0.37 nm, respectively, regardless of the chemically modified conditions and molar masses. A rationally speculated structure model for the MC and HpMC samples dissolved in aqueous solution was proposed.