Spectroscopic and thermal analyses of α′ and α crystalline forms of poly(l-lactic acid)

Poly(lactic acid) samples rich in α′ or α crystals have been characterized using spectroscopic and thermal methods. Cryogenic infrared and Raman spectroscopy were used to probe the differences in chain conformation and packing. Compared to the α crystal, the α′ crystal has weakened specific carbonyl and methyl interactions. Experimental spectroscopic analysis in conjunction with simulation studies have shown that the α′ crystal has uniform conformational disorder in the Cα–C torsion angle. This disorder in chain conformation and packing leads to different crystalline forms with different stabilities. The difference in thermal stability was quantified by measuring enthalpic change at melting for both crystalline forms. Significantly different values for the two crystalline forms were obtained ( Δ H m o ( α ′ ) = 57 ± 3 J g − 1 and Δ H m o ( α ) = 96 ± 3 J g − 1 ). The transformation from the less stable α′ to the more stable α phase has been characterized. This analysis provides an explanation to the double melting peaks usually found in PLLA samples.