Crystal structures of poly(l-lactide)-CO₂ complex and its emptied form

In this study, the crystal polymorph of poly(l-lactide) (PLLA) formed under high-pressure CO₂ and its transition behavior with CO₂ desorption were examined using mainly wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. We demonstrated that PLLA forms the complex crystal with CO₂ under high-pressure CO₂ below room temperature, and the crystal transition to the α-form, which is accompanied with gradual changes in the packing and conformation of PLLA chains, occurs with CO₂ desorption and subsequent annealing in air (annealing was needed only for films with a draw ratio smaller than three). Compared with the α-form, the oriented CO₂ complex film showed shorter a-, longer b-, and shorter c-axis lengths, resulting in a slight increase in the unit cell volume. The hexagonal packing (a/b ≈ 1.73), which is seen in the α-form, no longer exists for the oriented CO₂ complex film (a/b ≈ 1.33). It was indicated that the chain helical conformation of PLLA in the CO₂ complex is similar but different to that in the α-form (10/7 helix for both forms), because of the interactions between PLLA and CO₂. It seems likely that CO ₂ molecules are encapsulated in the cavity surrounded by four PLLA chains. With CO₂ desorption, the a-axis length increased and the b-axis one decreased, so that a/b increased to 3^1/2 (hexagonal packing), keeping the orthorhombic system. It was proposed that the formation of α″-crystals results from PLLA being trapped in the quasi-stable state during the CO₂ complex-to-α-form transition, and the energy barrier between the α″- and α-forms can be overcome by only CO₂ desorption in the case of a draw ratio higher than two.