Solution properties of amylose tris(n-butylcarbamate). Helical and global conformation in alcohols

Particle scattering functions P(k), z-average radii of gyration z1/2, intrinsic viscosities [η], infrared absorption (IR) spectra, and specific rotations have been determined by light and/or small-angle X-ray scattering, viscometry, IR, and polarimetry for narrow distribution samples of amylose tris(n-butylcarbamate) (ATBC) ranging in weight-average molecular weight from 1.7 × 104 to 1.7 × 106 in 2-propanol at 35 °C, 1-propanol at 40 (or 35) °C, 2-ethoxyethanol at 25 °C, and 2-butanol at 45 °C. The two propanols are found to attain the theta state at 35 °C. The number fraction f1698 of intramolecular hydrogen bonds between the CO and NH groups of the ATBC chain is obtained from IR spectra, and the helix pitch h (or the contour length) per residue and the Kuhn segment length λ−1 are estimated to be 0.25-0.29 nm and 20-40 nm, respectively, from analyses of P(k), z, and [η] data on the basis of the wormlike chain. The relationship among the three parameters (f1698, h, and λ−1) in the four alcohols and that among the previous estimates in tetrahydrofuran-methanol mixtures are explained in a unified manner by a two-state model, in which each chain consists of semiflexible (loosely helical) and rodlike (rigid helical) sequences. Namely, h increases when intramolecular hydrogen bonds are broken. This differs from the case of amylose tris(phenylcarbamate) for which the bulkiness of polar solvent molecules extends the helix of the polymer.