Comparing the Structural, Thermal, and Rheological Properties of Poly(meso-lactide) to Poly(l-lactide) and Poly(rac-lactide)

The melt rheological behavior of two highly stereo irregular chain configurations of PLA is compared to stereo regular PLLA using small amplitude dynamic frequency response in parallel plate flow geometry. The degree of heterotactic bonds along the polymer backbone is related to changes in the critical molecular weight, entanglement molecular weight, plateau modulus, and characteristic ratio. The melt rheology of PMLA was further characterized for temperature and frequency response. The mole fraction of syndiotactic lactic acid monomer transitions was estimated to be 0.01, 0.22, and 0.79 for PLLA, PRLA, and PMLA respectively. The highly disrupted stereo configuration of PMLA led to a more flexible chain in the melt than PLLA, with PMLA C-infinity = 5.9 vs that of PLLA where C-infinity = 7.5. PRLA exhibited chain flexibility behavior between that of PMLA and PLLA, with C-infinity = 6.9. The greater chain flexibility of PMLA in the melt resulted in lower flow activation energy temperature dependence, lower zero shear viscosity for a given molecular weight, and lower frequency at the onset of shear thinning than PLLA. The critical molecular weight and entanglement molecular weight of PMLA were estimated to be 1.75x and 2.05x that of PLLA. Plateau moduli of PMLA and PRLA were estimated from the crossover frequency using models of Wu and Nobile-Cocchini to enable direct comparison to PLLA and calculation of entanglement characteristics, packing length, and characteristic ratio. Frequency dependences of the dynamic moduli and complex viscosity were modeled using generalized Maxwell, Bird-Carreau, and Havriliak-Negami models.