Early Stage Interplay of Microphase Separation and Crystallization in Crystalline−Coil Poly(l-lactic acid)-block-polystyrene Thin Films

The interplay of microphase separation and crystallization has been investigated at the early stage of annealing the amorphous thin films of the crystalline−coil poly(l-lactic acid)-block-polystyrene (PLLA-b-PS) diblock copolymer. The homogeneous and heterogeneous films were annealed at the temperatures between the glass transition temperature of PS (TgPS) and the melting point of PLLA (TmPLLA) so that the microphase separation and crystallization were simultaneously possible. (1) The homogeneous films formed spinodal-like pattern, with the amplitude amplified to the lamellar spacing (L0), indicating the microphase separation and perpendicular orientation of the copolymer chains with respect to the surface and substrate. Afterward, abnormal relief structures (domain II, ∼30 nm in height) started and grew laterally within the original relief patterns (domain I). The surface wavevector q decayed with the time t as qt -1/3, while the growth of domain II could be described by the Avrami equation with the exponent of 1.0. It was hypothesized that the PLLA may crystallize within the mesophase, increasing the lamellar spacing. (2) Using THF/CS2 mixtures as solvent, semicrystalline nuclei were integrated to the spin-coated PLLA-b-PS films. These nuclei initiated the crystallization of the thin films. The crystallization kinetics follows the Avrami equation with the exponent of 2.6. The crystallization induced cracks in the films, which nucleated holes at the surface. The coarsening kinetics of the holes is an Ostwald ripening type. Comparison of the crystallization halftimes of the homogeneous and heterogeneous films suggests that the crystallization of these films may be a relaxation-controlled process.