Dimensional characterization of isolated ethylcellulose in tetrahydrofuran

Abstract The dimensional characterization of ethylcellulose (cellulose ethyl ether, EC x , where x is the degree of substitution based on the number of hydroxyl groups in a repeating unit (in this study, x = 3.0 and 2.5)) with a weight-averaged molar mass ( M w ) ranging from 6.32 × 10 3 to 3.83 × 10 5 g mol –1 and a relatively narrow molar mass distribution ( M w / M n < 1.2) was studied in tetrahydrofuran (THF) at 25 °C using static light and small-angle X-ray scattering and intrinsic viscosity ([ η ]) measurements. Eleven fully substituted EC 3.0 samples with M w / M n values ranging from 1.05 to 1.22 were prepared by reacting commercially available EC 2.5 with ethyl iodide in THF at 55 °C in the presence of sodium hydride, followed by fractionation using recycling preparative size exclusion chromatography (SEC) in CHCl 3 . Furthermore, eight EC 2.5 samples with M w / M n values ranging from 1.04 to 1.19 were obtained by applying the same fractionation technique to EC 2.5 . Afterward, the z-averaged root-mean-squared radius of gyration (< S 2 > z 1/2 ) and [ η ] for the isolated EC 3.0 and EC 2.5 chains were measured and tabulated as functions of M w . Furthermore, their M w dependencies were analyzed using cylindrical wormlike chain and wormlike touched-bead models. The chain stiffness parameter (Kuhn segment length, λ –1 ), molar mass per unit contour length ( M L ), and hydrodynamic bead diameter ( d B ) were determined to be 23.1 nm, 491 g mol –1 nm –1 , and 1.8 nm for EC 3.0 and 16.5 nm, 467 g mol –1 nm –1 , and 1.1 nm for EC 2.5 , respectively. These results indicate that both EC 3.0 and EC 2.5 form semiflexible chains with moderate stiffness, primarily because of steric hindrance arising from the ethyl groups in the cellulose backbone. The monomer counter length ( l M ) was estimated to be 0.50 nm for both EC 3.0 and EC 2.5 , suggesting that the local conformation of the EC chain remains largely unaffected by x between 2.5 and 3.0. In addition, the l M value was almost equal to that (0.51–0.52 nm) of crystalline cellulose but considerably greater than that (0.33 nm) of α-1,4-linked amylose derivatives. In contrast, λ –1 and d B were influenced by x , likely because of greater steric hindrance in the main chain and desolvation around the hydroxyl groups.