Conformational, Dimensional, and Hydrodynamic Properties of Amylose Tris(n-butylcarbamate) in Tetrahydrofuran, Methanol, and Their Mixtures

Abstract

Twelve amylose tris(n-butylcarbamate) (ATBC) samples ranging in weight-average molecular weight from 1.7 × 104 to 1.7 × 106 have been prepared and studied by light and small-angle X-ray scattering, sedimentation equilibrium, viscosity, infrared absorption (IR), and optical rotation in methanol (MeOH), tetrahydrofuran (THF), and their mixtures at 25 °C (or 20−25 °C for IR). Data for the mean-square radius of gyration, the particle scattering function, and the intrinsic viscosity are analyzed on the basis of the wormlike chain to yield h (contour length per residue) = 0.32 nm and λ−1 (Kuhn’s segment length) = 11 nm in MeOH and h = 0.26 nm and λ−1 = 75 nm in THF. The high stiffness in THF indicated by λ−1 = 75 nm is most likely due to the intramolecular hydrogen bonding (between C═O and NH groups of ATBC) observed as the splitting amide I band in the IR spectra. Furthermore, the h value in this solvent is considerably smaller than the helix pitches per residue of 0.37−0.40 nm for amylose triesters in the crystalline state and those of 0.32−0.42 nm for semiflexible amylose tris(phenylcarbamate) in various solvents (λ−1 = 15−24 nm), indicating that the ATBC chain forms a tightly wound helix in THF. As the number of intramolecular hydrogen bonds decreases, i.e., as the MeOH content increases in THF−MeOH mixed solvents, λ−1 decreases while h increases. These relationships are successfully explained by a two-state model in which each chain consists of randomly distributed semiflexible (loosely helical) and rodlike (rigid helical) sequences. The resultant h values for the rodlike and semiflexible portions are 0.25−0.26 and 0.32 nm, respectively.