Some solution properties of fractionated water‐soluble hydroxypropylcellulose

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AbstractA solution‐fractionation technique has been developed for water‐soluble (molar substitution, MS, ca. 4) hydroxypropylcellulose (HPC), utilizing mixtures of anhydrous ethanol (solvent) and n‐heptane (nonsolvent) of varying composition. Solubility is primarily an inverse function of chain length, species of highest molecular weight being soluble only in solvent mixtures richest in ethanol. Fractionation data indicate HPC substitution is fairly uniform, although there is a tendency for shorter chains to be more highly substituted than long ones. Some control over the degree of variation is possible by changes in processing conditions. Molecular weight and molecular weight distribution reflect the properties of the cellulose employed in sample preparation and the use or omission of deliberate degradation for viscosity control. A sample of HPC ([η] = 4.2; MS = 4.4) was separated into seven fractions of approximately equal weight and the average molecular weight (Mw), root mean square radius of gyration, and second virial coefficient of each of these fractions in ethanol were determined by light scattering, while molecular weight distributions were investigated by gel permeation chromatography in tetrahydrofuran. The relation between intrinsic viscosity and degree of polymerization (DP) was found to be [η] = 7.2×10−3DP. A comparison of hydrodynamic and configurational parameters for HPC in ethanol with those for hydroxyethylcellulose (HEC) in water indicates these two polymers behave very similarly in solution. In both systems the ratio of the mean square end‐to‐end unperturbed molecular chain length (R) to DP diminishes with increasing molecular weight, reaching a minimum at a DPw of approximately 3000. The ratio [(R)/(R)], where (R)½ is the root mean square end‐to‐end distance for 1,4′‐polysaccharides assuming free rotation of the chain units, also diminishes with increasing molecular weight, reaching a limiting value of 3.5. This relatively high ratio indicates considerable rigidity in the coiled molecule and is believed to be at least partially the result of intramolecular hydrogen bonding.