Periodic Surface Pattern Induced by Crystallization of Polymer Brushes in Solvents

Abstract

Precise control of the crystal orientation and hierarchical crystalline morphology is essential for application of crystalline polymers, although a homogeneous morphology with a controlled crystal orientation is rarely formed in polymer crystallization. Here we report the formation of a homogeneous surface morphology with submicrometer-scale periodicity by simple crystallization of polymer brushes on a solid substrate in poor solvents. Unlike the complex morphology commonly observed for crystallization in air, a periodic line pattern spontaneously formed when a thin brush layer of poly(ε-caprolactone) (PCL) was isothermally crystallized in marginally poor solvents. The length scale of the periodic line pattern was ∼100 nm in pitch and a few nanometers in height. The structure was found to depend on the crystallization temperature, brush layer thickness, and affinity of the poor solvent to the polymer. Grazing-incidence X-ray scattering experiments revealed that the PCL crystals within the periodic line pattern were predominantly aligned in the flat-on orientation. A structure formation mechanism based on the kinetic balance between the secondary nucleation and lateral growth processes at the growth front of crystal lamellae was proposed. The poor solvents acted as a plasticizer that enhanced the mobility of brush chains and modulated the nucleation and growth rates, which led to a highly ordered morphology with little branching.