Melt rheology of high L-content poly(lactic acid)

The melt rheology of linear poly(lactic acid)s (PLA) characterized by a hig h content of the L-form of the monomer is comprehensively investigated. Mea surements of dynamic, steady, and transient shear viscosities are presented . Extensional data on PLA are presented for the first time and show a stron g strain hardening behavior. The Cox-Merz relationship is obeyed over a par ticularly wide range (roughly 3 decades of shear rate). Results for high mo lecular weight samples suggest that the plateau modulus is approximately 5 x 10(5) Pa. In addition, the zero shear viscosity, eta (0), for these mater ials is found to roughly scale with the expected 3.4 power vs molecular wei ght. The transient shear results are satisfactorily predicted using a trunc ated form of the K-BKZ constitutive equation and a set of Maxwell modes (G( k), lambda (k)) derived from the dynamic spectra. However, to capture the o bserved extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate that the mel t stability of the polymer precludes long time measurements. Chemical chang es manifest themselves in a lack of adherence to the principle of time-temp erature superposition. It is shown that stabilization of poly(lactic acid) using tris(nonylphenyl)phosphite is possible and leads to a material that i s thermorheologically simple within the experimentally assessable rates of deformation.