Effect of Crystallization on the Lamellar Orientation in Thin Films of Symmetric Poly(styrene)-b-poly(l-lactide) Diblock Copolymer

Abstract

The effect of crystallization on the lamellar orientation of poly(styrene)-b-poly(l-lactide) (PS−PLLA) semicrystalline diblock copolymer in thin films has been investigated by atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In the melt state, microphase separation leads to a symmetric wetting structure with PLLA blocks located at both polymer/substrate and polymer/air interfaces. The lamellar period is equal to the long period L in bulk determined by small-angle X-ray scattering (SAXS). Symmetric wetting structure formed in the melt state provides a model structure to study the crystallization of PLLA monolayer tethered on glassy (Tc < Tg,PS) or rubber (Tc > Tg,PS) PS substrate. In both cases, it is found that the crystallization of PLLA results in a “sandwich” structure with amorphous PS layer located at both folding surfaces. For Tc ≤ Tg,PS, the crystallization induces a transition of the lamellar orientation from parallel to perpendicular to substrate in between and front of the crystals. In addition, the depletion of materials around the crystals leads to the formation of holes of 1/2L, leaving the adsorbed monolayer exposure at the bottom of the holes. In contrast, the adsorbed monolayer on the substrate with thickness of 1/2L does not change in parallel orientation and thickness upon crystallization. When Tc > Tg,PS, no perpendicular lamellae and holes could be observed due to the high mobility of molecules.