Complex Crystal Formation of Poly(l-lactide) with Solvent Molecules

Abstract

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 <i>N</i>,<i>N</i>-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 10₇ (left-handed 10₃) helical conformation and are packed in the orthorhombic lattice (<i>a</i> = 1.5–1.6 nm, <i>b</i> = 1.2–1.3 nm, <i>c</i> = 2.8–2.9 nm, and α = β = γ = 90°). Based on <i>R</i>-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