(357b) Rheological Studies of Poly (DL-lactic acid) Solutions and Melts

The processing and use of poly (DL-lactic acid) (PDLLA) for biomedical, agricultural and packaging applications is sensitive to its rheological properties in the solution and melt states, but these properties have not yet been systemically studied. In this work, we measured the intrinsic viscosities of PDLLA samples of molecular weights ranging from 28,000 to 349,000 (determined by Zimm plots of light-scattering data) and obtain the Mark-Houwink constants for PDLLA in the solvent dimethyl sulfoxide (DMSO) at 25°C. We also performed steady-shear and linear viscoelastic measurements on one of the samples at different solution concentrations, and on the melt. The entanglement molecular weight for PDLLA in the melt state was found to be 5100g/mol, using the standard tube-model formula, where G_N⁰ is the melt-state plateau modulus, obtained with the help of time-temperature superposition. We plot zero shear viscosities versus concentration for semi-dilute solutions of PDLLA on axes rescaled in accordance with the “blob theory” of de Gennes for semi-dilute solutions and find that a previously determined “universal curve” valid for other semi-dilute polymer solutions does not seem to apply to PDLLA polymers, when we use the entanglement molecular weight for solutions derived by “blob” scaling laws from the entanglement molecular weight for the melt. We also find anomalously high Huggins coefficients as the dilute state is approached. These results suggest that PDLLA might partially aggregate in solution in DMSO.