An analysis of poly(lactic acid) with varying regio regularity

Poly(L-lactic acid) samples of varying regio regularity were studied by static and dynamic laser light scattering in conjunction with Raman spectroscopy. Light scattering results allow an estimate of the characteristic ratio, C-infinity. Experimental values were compared to the characteristic ratio computed from structural parameters determined from Raman spectroscopy. C-infinity for poly(L-lactic acid) was determined to be 12.4, a value consistent with an inherently rigid chain. As expected, C-infinity declines as regio regularity decreases. The Raman spectrum of solid amorphous samples of poly(lactic acid) of varying D content was compared with the simulated Raman spectrum calculated from an ensemble of 1000 different chains. Structural parameters, force constants and intensity parameters were transferred from similar molecules. The results reinforce the concept that poly(lactic acid) forms stiff chains. Upon introduction of D units, the copolymerization is more consistent with Markovian rather than Bernoullian statistics. Rotational isomeric states associated with the D to L junctions were also analyzed. (C) 2003 American Institute of Physics.