Dilute solution properties of poly(d,l-lactide) by static light scattering, SAXS, and intrinsic viscosity

Abstract

The dilute solution properties of poly(d,l-lactide)s (PDL50) with a weight-averaged molar mass (M_w) ranging from 0.154 × 10⁴ to 75.7 × 10⁴ g mol⁻¹ are thoroughly studied in tetrahydrofuran at 25 °C by static light and small-angle X-ray scattering and intrinsic viscosity ([η]) measurements. Fourteen PDL50 samples with a narrow molar mass distribution are synthesized at 150 °C by ring-opening copolymerization of a 1:1 mixture of d-lactide and l-lactide with benzyl alcohol as an initiator and tin(II) dichloride dihydrate as the catalyst, followed by fractionation using recycling preparative size exclusion chromatography. The M_w dependences of the z-averaged root-mean-square radius of gyration (〈S²〉_z^1/2) and [η] are rationalized and analyzed based on the cylindrical wormlike chain model. The experimental M_w dependence of 〈S²〉_z^1/2 is quantitatively described by the wormlike cylinder with stiffness parameter λ⁻¹ = 2.9 nm, molar mass per unit contour length M_L = 270 ± 20 g mol⁻¹ nm⁻¹, protruding effects at both ends δ = 0.4 nm, and excluded-volume strength B = 0.31 ± 0.07 nm. The experimental M_w dependence of [η] and the scattering form factor P(q) are also consistently expressed by the current theories with the same model parameters. The results indicate that the PDL50 chain behaves as a typical flexible polymer but is essentially 1.6−2.1 times stiffer than the representative vinyl polymers, polystyrene (λ⁻¹ = 1.8 nm) and poly(methyl methacrylate) (λ⁻¹ = 1.4 nm), most likely due to the planar feature of the ester linkage, which partially constrains the free rotation of the main chain.