Dynamic Covalent Bond Exchange at Block Copolymer Junctions Impacts Self-Assembly Kinetics

Abstract

Methods to control the self-assembly kinetics of block copolymers are needed for their use in various applications. In this work, vinylogous urethane (VU) dynamic covalent bonds were incorporated at the junction of poly(dimethylsiloxane)-block-poly(ethylene oxide) (PDMS–PEO) copolymers, and the self-assembly kinetics were investigated through a combined experimental and simulation study to understand how dynamic covalent chemistry impacts ordering. Dynamic bond exchange was facilitated through the addition of both excess nucleophiles (amine groups) and salt, and the ordering kinetics were studied using time-dependent X-ray scattering and dynamic self-consistent field theory. The addition of excess homopolymer with amine groups had a greater impact on ordering kinetics than the addition of a catalyst at 70 °C where bond exchange is slow, and a transition between regimes controlled by chain diffusion and bond exchange was observed. A nonmonotonic trend in the incubation time for microphase separation and the growth rate of ordered structures is observed with added amine groups. At 120 °C, both chain diffusion and bond exchange are facilitated, significantly reducing the time required for assembly. Even as VU bond exchange is accelerated, chain diffusion contributes more to the overall ordering process. Simulations demonstrate the same nonmonotonic trend in the onset of microphase separation and provide further insights into the second stage of ordering, which is not resolved in experiments. PEO and PDMS vitrimers were also synthesized to understand the role of excess nucleophiles and salt in homopolymer networks in comparison to linear polymers, using shear rheology. A large difference in relaxation times between PEO and PDMS vitrimers is observed, attributed to the distribution of the salt catalyst in the polymer matrix or at the dynamic bond, respectively. In the dynamic network, the addition of salt has a significantly larger role in decreasing the relaxation times compared to excess amines. This work provides new insights into the effect of dynamic covalent bonds on the kinetics of the self-assembly ordering process in BCPs.