Influence of Network Design Parameters on the Morphology Evolution in Diels-Alder Blends via Thermodynamics and Kinetics Control

Abstract

Reversible polymer network blends leverage the advantageous properties of immiscible polymer backbones. Previous work showed that the phase morphology of blends of a hydrophilic poly(propylene oxide) (PPO) and hydrophobic polydimethylsiloxane (PDMS) cured by the reversible Diels-Alder reaction depends on the mass ratio of the two polymers and the maleimide-to-furan ratio used for the reversible network polymerization. This work studies the competition between the reversible Diels-Alder reaction and the phase separation kinetics and thermodynamics to control the phase formation. A furan-functionalized PPO with a molar mass of 4546 g mol-1 was blended with furan-functionalized PDMS with different molar masses, mass ratios of the polymers, and stoichiometric ratios. At the highest molar mass of 4961 g mol-1, the PDMS and PPO separated quickly into separate layers, creating a barrier against both water and oxygen, respectively. The thickness, morphology, and composition of the layers depend on the composition of the blend. At a lower molar mass of the PDMS, the chemistry of the furan end groups becomes more pronounced, which increases the compatibility of the two polymers, reducing the thermodynamic driving force for phase separation. In addition, the increased concentration of furan and maleimide groups increases the Diels-Alder reaction rates and leads to more cross-linked network blends. Mastering the interplay between the thermodynamics of the blends and the kinetics of the network formation and phase separation by judicious combinations of the network design parameters leads to final blend morphologies ranging from kinetically trapped uniform microstructures to almost completely phase-segregated morphologies. Finally, the solvent extraction time was used as a process parameter of the wet blending process. Slow evaporation of the solvent over the course of 1 week resulted in a near-equilibrium separation of the two immiscible polymers into separate layers with perfect interfacial bonding by the same Diels-Alder chemistry. Manipulation of these factors enables the development of Diels-Alder network blends with a wide range of properties that are suitable for a wide variety of applications. The fastest and most efficient autonomous healing is achieved at higher PPO contents and for the highest PDMS molar masses, while the best barriers against water and oxygen are obtained at the highest cross-link densities.