Shuhui Kang

Raman characterization of orientation in poly(lactic acid) films

Poly(lactic acid) is a new biopolymer material which is marketed by Cargill Dow Polymers under the tradename Nature Works*. One major application for this material is biaxially oriented films for food packaging because it possesses excellent barrier for flavor constituents, deadfold and heat sealability. Shrinkage must be minimized when the film is heat sealed for these applications and, therefore, characterization of the orientation of the amorphous phase of PLA films is necessary. Raman spectroscopy methodology has been developed to quantify orientation in PLA films. Bands were assigned to crystalline and amorphous phases of PLA such that orientation in both phases could be monitored. Raman depolarization ratios were used to characterize uniaxial systems but were insufficient for most biaxial draws. A new phenomenon for oriented films involving Raman band shifts was observed in these systems, and was shown to be capable of determining orientation, even for symmetrical biaxially drawn films. The origin of these shifts, as well as their use for the quantification of orientation will be discussed. Further, since the line widths of the bands could be used to quantify crystallinity, both crystallinity and orientation could be determined with one measurement.

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∞. Experimental values were compared to the characteristic ratio computed from structural parameters determined from Raman spectroscopy. C∞ for poly(L-lactic acid) was determined to be 12.4, a value consistent with an inherently rigid chain. As expected, C∞ 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. Rot...