Crystalline Structure and Morphology of Poly(l-lactide) Formed under High-Pressure CO2

Crystalline structure and morphology of poly(l-lactide) (PLLA) formed under high-pressure CO2 were studied by comparing the CO2-treated PLLA and the annealed one in terms of the crystallization behavior, crystalline forms, and crystalline superstructures. The crystallization temperature dependence of the diffraction peak position (2θ ≈ 16°) and crystallinity for the CO2-treated PLLA indicates that the crystal modification changes continuously from the disorder α (α′′) to α forms not through the α′ one with increasing temperature. By using light scattering technique, we clarified that the morphological transition from spherulites on a micrometer scale to rodlike crystalline superstructures on a nanometer scale occurs around 15 °C under 7−15 MPa CO2 and around 30 °C under 3 MPa CO2. By introducing the parameters η and Δ in the theoretical calculation for Vv light scattering from spherulites, it is indicated that there is a distinct difference in the arrangement of crystalline lamellae within spherulite between the CO2-treated PLLA and the annealed one.