Preparation and characterization of linear and star-​shaped poly L-​lactide blends

A series of star-shaped poly(L-lactide) (sPLLA) with different branch length was synthesized by ring-opening polymn. of L-lactide using pentaerythritol as initiator and stannous-octoate as catalyst. The structures and properties of the sPLLAs were characterized by 1H-NMR, rheol., differential scanning calorimetry, and size-exclusion chromatog. The star-shaped polymers were blended with a linear PLLA resin (3051D; NatureWorks) and studied for their melt flow and thermal properties. Blends contg. 10 wt % of sPLLA displayed equiv. zero shear viscosities to PLLA 3051D, however displaying stronger shear thinning and increased melt elasticity. The blends exhibited also similar thermal and crystn. properties as the PLLA 3051D. A higher ratio of low mol. wt. sPLLA (30 wt %; Mwtotal: 2500 and 15,000 g mol-1) lowered the zero viscosity and increased shear thinning of PLLA 3051D. Crystn. of PLLA 3051D appears to increase at 30 wt % blends with the higher mol. wt. sPLLA, while the star-shaped structure acts as a nucleating agent.