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

Polylactides (PLAs) have been known for several decades and have recently gained considerable commercial significance. This development makes it highly desirable to have the rheological properties of these materials well characterized and reduced to useable mathematical models. However, comprehensive rheological characterization of PLAs is not yet available from the literature. In this study, rheological and thermal measurements were made on a comprehensive and well-characterized set of homopolymers and copolymers spanning wide ranges of molecular mass and stereoisomer proportions (L content). For all compositions within the weight average molecular weight range of 10 5 – 10 6 ( g ∕ mol ) and a reference temperature of 180 °C, the zero shear viscosity is described by the relationship log ( η 0 ) = − 14.26 + 3.4 log ( M w ) , the plateau modulus is 1.0 ± 0.2 MPa , and average WLF parameters are c 1 = 3.24 1 ∕ K and c 2 = 164.9 K ; the later correspond to a Vogel temperature of 288.25 K. The values of the glass transition temperatures at infinite molecular weight for 100%, 80%, and 50% L content are 60.2, 56.4, and 54.6 °C, respectively. Based upon a chain packing model, molecular parameters determined include a packing length of 2.51 Å, a tube diameter of 47.7 Å, and a characteristic ratio of 6.5 ± 0.9 , independent of stereoisomeric composition. The critical molecular weight for entanglement, M c , is found to be near 9000 g ∕ mol while the molecular weight between entanglements, M e , is near 4000 g ∕ mol . Reasons for accepting these values over previous literature studies are given; the available data imply linear polylactides are typical glass-forming polymers.