Crystal Structure of Stereocomplex of Poly(L-lactide) and Poly(D-lactide)

An equimolar mixture of poly (L-lactide) and poly(D-lactide) was crystallized into a stereocomplex whose crystal system is triclinic (P1) with cell dimensions: a = 0.916 nm, b = 0.916 mm, c (chain axis) = 0.870 nm, a = 109.2°, β = 109.2°, and y = 109.8°. In the unit cell, a poly(L-lactide) segment and a poly (D-lactide) segment are contained as a pair and packed laterally in parallel fashion. The L- and D-poly(Iactides) in the complex take a 3₁helical conformation, which is a little extended from a 103 helix in the homopolymer crystal with the α-form. Homopolymers are also able to take the 3₁helical conformation and form the β-form crystal. The 3₁helix in the homopolymer crystal is less stable than the 10₃one, and hence the β-form is easily transformed to the a-form by annealing.