Kinetics of phase transition from lamellar to hexagonally packed cylinders for a triblock copolymer in a selective solvent

TL;DR

This study investigates the phase transition kinetics of a triblock copolymer from lamellar to hexagonally packed cylinders in a selective solvent, revealing a one-step continuous transformation at deep quenches and a two-step nucleation at shallow quenches.

Contribution

It provides the first detailed kinetic analysis distinguishing between continuous and nucleation mechanisms during the lamellar to hexagonal phase transition.

Findings

Deep quenches lead to a one-step continuous transformation.

Shallow quenches exhibit a two-step nucleation and growth process.

Geometric modeling supports the continuous transformation mechanism.

Abstract

We examined the kinetics of the transformation from the lamellar (LAM) to the hexagonally packed cylinder (HEX) phase for the triblock copolymer, polystyrene-b-poly (ethylene-co-butylene)-b-polystyrene (SEBS) in dibutyl phthalate (DBP), a selective solvent for polystyrene (PS), using time-resolved small angle x-ray scattering (SAXS). We observe the HEX phase with the EB block in the cores at a lower temperature than the LAM phase due to the solvent selectivity of DBP for the PS block. Analysis of the SAXS data for a deep temperature quench well below the LAM-HEX transition shows that the transformation occurs in a one-step process. We calculate the scattering using a geometric model of rippled layers with adjacent layers totally out of phase during the transformation. The agreement of the calculations with the data further supports the continuous transformation mechanism from the LAM to HEX for a deep quench. In contrast, for a shallow quench close to the OOT we find agreement with a two-step nucleation and growth mechanism.