Abstract
Directly starting from liquid phase L-lactic acid (LLA) or its oligomer, the condensed-phase plasma polymerized poly(L-lactic acid) (PLLA) was for the first time successfully synthesized via in-situ plasma in radio frequency (RF) at 13.56 MHz. The polymerization process is based on the permeability of the initially polymerized crosslinking film to liquid LLA oligomer. As swelling into the film, LLA oligomer bonded on the film surface as a result of direct plasma activation and terminated in macro-scale bulk. The chemical structure was characterized by fourier-transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance (solid state NMR). Based on this multitechnique analysis, it was determined that condensed-phase plasma polymerized PLLA retained original functionalities of LLA; moreover, it contained methylene and enone which were absent from linear PLLA. Those could be explained by crosslink and Norrish-type II reaction. Increasing molecular weight of oligomer reduced the crosslink density from 10-4 g/cm3 to 10-5 g/cm3; however, it not only took longer reaction time, but obstructed reaction with exceed high oligomer. Structure was more regular for treating at low power, when compared at completely solidified phase, and reaction time at 50W took 6 times than at 200W. Besides, 70% of condensed-phase plasma polymerized PLLA hydrolyzed in distilled water at 95℃ in a week, and the other 30% remained through crosslink. This novel one-step, solvent-free and low temperature method had the same reaction constant(10-5~10-3 L/sec*mole) as traditional one at high temperature, thus was an alternative route to the synthesis of bulk PLLA.