Synthesis and Characterization of High-Molecular-Weight PLAs with a Well-Controlled Particle Morphology

The synthesis of high-/ultrahigh-molecular-weight poly­(l-lactide) (PLLA) having a controlled morphology remains a challenge because of the low crystallization rate of the material and the absence of a proper nucleation site during polymer synthesis. Using three different polymerization routes, the synthesis of high- and ultrahigh-molecular-weight PLLA (weight-average molecular weight up to 1.1 million g/mol) having a well-defined particle morphology is reported. A comparative study on the evolution of the structure, morphology, and crystallization behavior of PLLA, influenced by different nucleating agents during polymerization, is addressed. The structure and the morphology of the resulting polymers, crystallization kinetics, and thermal properties of the PLAs are investigated systematically by GPC, SEM, TEM, NMR, DSC, and XRD. In the presence of PDLA, with the formation of the PLLA/PDLA stereocomplex crystal, spherical particles having a flakelike micromorphology are formed. The flakelike structures are proven to be single crystals of the stereocomplex PLLA and PDLA. The nascent reactor powder, having single crystals, could be compressed below the melting point and can be drawn into uniaxial objects in the solid state without melting. The solid-state deformation circumvents the entropic relaxation that is detrimental to chain orientation and restricts thermal degradation experienced during melt processing. Moreover, the proposed routes for controlling the nascent reactor particle morphology open the path to PLA synthesis in a continuous process, avoiding any postpolymerization treatment of the synthesized polymer.