Fundamental solution and single-chain properties of polylactides

Polylactides (PLA) have been known for several decades and are recently of considerable commercial significance. However, the literature on basic chain properties and solution characterization is divided and inconsistent. In this study, a comprehensive and well-controlled set of experiments is combined with rigorous quantitative analysis to resolve existing apparent contradictions. Homopolymers and copolymers spanning wide ranges of molecular weight and stereoisomer proportions were prepared by ring-opening polymerizations Of L- and D-lactides using stannous octanoate as the catalyst. Samples were characterized by means of. (1) dilute-solution viscometry in three different solvents; (2) size exclusion chromatography in tetrahydrofuran (THF) with light scattering detection; (3) static multiangle light scattering in a mixed acetonitrile-dichloromethane solvent; (4) variable-angle spectroscopic ellipsometry; and (5) melt rheology. The data imply that PLA are typical linear flexible polymers; unperturbed PLA chain dimensions are describable in terms of a characteristic ratio of 6.5 +/- 0.9, regardless of stereoisomer content. The Schulz-Blaschke and Mark-Houwink constants for dilute PLA solutions in chloroform and in THF are determined. For chloroform at 30 degrees C, the correct values are k(SB) = 0.302, K = 0.0131 (mL/g), and a = 0.759, while for THF at 30 degrees C, the correct values are k(SB) = 0.289, K = 0.0174 (mL/g), and a = 0.736. (c) 2005 Wiley Periodicals, Inc.