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 CO2 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 31/2 (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.