Correlated disorder in random block-copolymers

TL;DR

This paper investigates how quenched disorder affects microphase separation in symmetric diblock copolymers, revealing a transition from lamellar order to a glassy state influenced by chain length and disorder strength.

Contribution

It introduces a renormalization group approach combined with the replica trick to analyze disorder effects on copolymer phase transitions, showing a disorder-induced change in transition nature.

Findings

Disorder can induce a glassy state over the lamellar phase.

The transition character changes beyond a critical disorder strength.

Chain length influences the impact of disorder on the phase transition.

Abstract

We study the effect of a random Flory-Huggins parameter in a symmetric diblock copolymer melt which is expected to occur in a copolymer where one block is near its structural glass transition. In the clean limit the microphase segregation between the two blocks causes a weak, fluctuation induced first order transition to a lamellar state. Using a renormalization group approach combined with the replica trick to treat the quenched disorder, we show that beyond a critical disorder strength, that depends on the length of the polymer chain, the character of the transition is changed. The system becomes dominated by strong randomness and a glassy rather than an ordered lamellar state occurs. A renormalization of the effective disorder distribution leads to nonlocal disorder correlations that reflect strong compositional fluctuation on the scale of the radius of gyration of the polymer chains. The reason for this behavior is shown to be the chain length dependent role of critical fluctuations, which are less important for shorter chains and become increasingly more relevant as the polymer length increases and the clean first order transition becomes weaker.