Crystal Structure and Morphology of Poly( l -lactide-b- d -lactide) Diblock Copolymers

The isothermal crystallization behavior of low-molecular-weight (DP ≅ 50) poly( l -lactide- b - d -lactide) diblock copolymers is reported, as studied by small- and wide-angle X-ray scattering. The symmetric diblock copolymer crystallizes in the stereocomplex form for the whole range of crystallization temperatures, while the one with a block ratio of 80/20 grows into homocrystallites. For an intermediate block ratio (66/33), the stereocomplex form is obtained at high crystallization temperatures while a coexistence region with homocrystal phase exists at low temperatures. In addition to the transition enthalpy, size effects and kinetic routes play a role in the competition between the stereocomplex and the homocrystalline forms. The lamellar spacing (long period and crystal thickness) shows a different temperature dependence for the three diblock copolymers, which is discussed in relation to the specific morphology and molecular constitution.