Chain Conformation of Poly(<scp>d</scp>-lactide) in Tetrahydrofuran by Static Light Scattering, Small-Angle X-ray Scattering, and Intrinsic Viscosity
Yoshinori Suzuki, Takahiro Watanabe, Hiroki Kosugi, Kayo Ueda, Moriya Kikuchi, Atsushi Narumi, Seigou Kawaguchi
Abstract
The dimensional features of an isotactic poly(lactic acid), poly(d-lactide) (PDLA), chain with a weight-averaged molar mass (Mw) ranging from 9.84 × 102 to 4.44 × 105 g mol–1 are studied in tetrahydrofuran (THF) at 25 °C using static light and small-angle X-ray scattering and the intrinsic viscosity ([η]). Sixteen PDLA samples with polydispersity indices (Mw/Mn) ranging from 1.04 to 1.15 are prepared by ring-opening polymerization of d-lactide, followed by fractionation using recycling preparative size-exclusion chromatography. The Mw dependence of the z-averaged root-mean-square radius of gyration (⟨S2⟩z1/2) and [η] is tabulated and compared with that of atactic poly(lactic acid) (PDL50, a random copolymer with a 1:1 molar ratio of d-lactide to l-lactide). The Mw dependence of ⟨S2⟩z1/2 and [η] and the scattering form factor [P(q)] for the PDLA chain are consistently and quantitatively described by a cylindrical wormlike model with a stiffness parameter (λ–1 = 6.5 nm), molar mass per unit contour length (ML = 350 g mol–1 nm–1), protruding effects at both ends (δ = 0.4 nm), bead diameter (dB = 1.06 nm), and excluded volume strength (B = 0.50 nm). The results demonstrate that the isotactic PDLA chain in THF at 25 °C possesses a considerably different conformation from that reported to date, that is, 103 or 31 helices in the crystalline state of poly(l-lactide) and the conformation of the atactic PDL50 chain (λ–1 = 2.9 nm, ML = 270 g mol–1 nm–1, and dB = 0.62 nm) in THF at 25 °C. It is concluded that the stereoregularity has a significant effect on the conformation, and the PDLA chain has, in solution, a helical-like conformation which is stiffer and thicker with a larger radius of curvature and lower pitch than the PDL50 chain.