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

The dynamic viscoelastic behavior of Poly(l-lactic acid) (PLLA), with molec ular weights ranging from 2,000 to 360,000, have been studied over a broad range of reduced frequencies (approximately 1 X 10(-3) s(-1) to 1 X 10(3) s (-1)), using time-tempera ture superposition principle. Melts are shown to have a critical molecular weight, M-c, of approximately 16,000 g/mol, and a n entanglement density of 0.16 mmol/cm(3) (at 25 degrees C), PLLA polymers are noted to require substantially larger molecular weights in order to dis play similar melt viscoelastic behavior, at a given temperature, as that fo r conventional non-biodegradable polymers such as polystyrene. The reason f or this deviation is suspected to be due to steric hindrance, resulting fro m excessive coil expansion or other tertiary chain interactions. PLLA melts show a dependence of eta(0) on chain length to the 4.0 power (M-W(4.0)), w hilst J(e)(0) is independent of M-W in the terminal region. Low molecular w eight PLLA (similar to 40,000) shows Newtonian-like behavior at shear rates typical of those achieved during film extrusion. (C) 1999 John Wiley & Son s, Inc.