Melt rheology of poly(lactic acid): Entanglement and chain architecture effects

Poly(lactic acids) (PLAs) are a family of polyesters available via fermenta tion from renewable resources and are the subject of considerable recent co mmercial attention. In this study, the melt rheological properties of a fam ily of poly(lactic acid) stars are investigated and compared to the propert ies of the linear material. For polymers made from a 98:2 ratio of the L to D enantiomeric monomers it is found that the entanglement molecular weight is in the range of 9000 g per mole (M-e approximate to 8700 g/mol) while t he molecular weight for branch entanglement is inferred to be approximately 3500 g per mole (M-b approximate to 34 600 g/mol). In addition, the zero s hear viscosity of the linear material increases with the 4.6 power of molec ular weight. These results may suggest that PLA is a semistiff polymer in a ccordance with other recent findings. The increase in zero shear viscosity for the branched materials is measured and quantified in terms of appropria te enhancement factors. Relaxation spectra show that the transition zone fo r the linear and branched materials an nearly indistinguishable, while the star polymers have greater contributions to the terminal regime. The effect s of chain architecture on the flow activation are found to be modest, impl ying that small scale motions in PLA homopolymers largely control this phen omenon. Good agreement is found between the dynamic data and many aspects o f the theory of star polymers, however, a dependence of the zero shear visc osity on the number of arms is observed. (C) 1999 The Society of Rheology. [S0148-6055(99)01305-X].