Kinetics of HEX-BCC Transition in a Triblock Copolymer in a Selective Solvent: Time Resolved Small Angle X-ray Scattering Measurements and Model Calculations

TL;DR

This study investigates the kinetics of HEX to BCC phase transition in a triblock copolymer using time-resolved SAXS, revealing detailed mechanisms, metastability limits, and validating a geometrical fluctuation model with experimental data.

Contribution

It provides new insights into the phase transition mechanisms and develops a geometrical model that accurately predicts SAXS scattering patterns during the transition.

Findings

HEX to BCC transition occurs via nucleation and growth below the spinodal temperature.

Metastability limit of HEX in BCC identified at ~150°C.

Model predicts peak splitting due to incommensurate cylinder spacing.

Abstract

Time-resolved small angle x-ray scattering (SAXS) was used to examine the kinetics of the transition from HEX cylinders to BCC spheres at various temperatures in poly(styrene-b- ethylene-co-butylene-b-styrene) (SEBS) in mineral oil, a selective solvent for the middle EB block. Temperature-ramp SAXS and rheology measurements show the HEX to BCC order-order transition (OOT) at ~127 oC and order-disorder transition (ODT) at ~180 oC. We also observed the metastability limit of HEX in BCC with a spinodal temperature, Ts ~ 150 oC. The OOT exhibits 3 stages and occurs via a nucleation and growth mechanism when the final temperature Tf < Ts. Spinodal decomposition in a continuous ordering system was seen when Ts< Tf < TODT. We observed that HEX cylinders transform to disordered spheres via a transient BCC state. We develop a geometrical model of coupled anisotropic fluctuations and calculate the scattering which shows very good agreement with the SAXS data. The splitting of the primary peak into two peaks when the cylinder spacing and modulation wavelength are incommensurate predicted by the model is confirmed by analysis of the SAXS data.