Rheological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(l-lactic acid)

Abstract

The dynamic viscoelastic behavior of Poly(l-lactic acid) (PLLA), with molecular weights ranging from 2,000 to 360,000, have been studied over a broad range of reduced frequencies (approximately 1 × 10−3 s−1 to 1 × 103 s−1), using time–temperature superposition principle. Melts are shown to have a critical molecular weight, Mc, of approximately 16,000 g/mol, and an entanglement density of 0.16 mmol/cm3 (at 25°C). PLLA polymers are noted to require substantially larger molecular weights in order to display similar melt viscoelastic behavior, at a given temperature, as that for conventional non-biodegradable polymers such as polystyrene. The reason for this deviation is suspected to be due to steric hindrance, resulting from excessive coil expansion or other tertiary chain interactions. PLLA melts show a dependence of η0 on chain length to the 4.0 power (M), whilst J is independent of MW in the terminal region. Low molecular weight PLLA (∼ 40,000) shows Newtonian-like behavior at shear rates typical of those achieved during film extrusion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1803–1814, 1999