Melt Rheology of High l-Content Poly(lactic acid)

Abstract

The melt rheology of linear poly(lactic acid)s (PLA) characterized by a high content of the l -form of the monomer is comprehensively investigated. Measurements of dynamic, steady, and transient shear viscosities are presented. Extensional data on PLA are presented for the first time and show a strong strain hardening behavior. The Cox−Merz relationship is obeyed over a particularly wide range (roughly 3 decades of shear rate). Results for high molecular weight samples suggest that the plateau modulus is approximately 5 × 10 5 Pa. In addition, the zero shear viscosity, η 0, for these materials is found to roughly scale with the expected 3.4 power vs molecular weight. The transient shear results are satisfactorily predicted using a truncated form of the K−BKZ constitutive equation and a set of Maxwell modes ( G k, λ k ) derived from the dynamic spectra. However, to capture the observed extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate that the melt stability of the polymer precludes long time measurements. Chemical changes manifest themselves in a lack of adherence to the principle of time−temperature superposition. It is shown that stabilization of poly(lactic acid) using tris(nonylphenyl)phosphite is possible and leads to a material that is thermorheologically simple within the experimentally assessable rates of deformation.