Complex crystal formation of poly(l -lactide) with solvent molecules

By screening examinations for a wide variety of organic solvents, we found that poly(l-lactide) (PLLA) forms the crystalline complex (ε-form) with the specific organic solvents such as tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) below room temperature. It was revealed that PLLA has high selectivity for low molecular weight compounds to form the ε-crystals. By fiber diagram analyses for the ε-forms, it was found that PLLA chains take the 10 ₇ (left-handed 10 ₃) helical conformation and are packed in the orthorhombic lattice (a = 1.5-1.6 nm, b = 1.2-1.3 nm, c = 2.8-2.9 nm, and α = β = γ = 90°). Based on R-factor and packing energy calculations, the plausible crystal structure of PLLA-DMF complex was proposed, in which four PLLA chains and eight guest solvents are packed in the unit cell.