Mechanical Deformation of Polymer Blend Thin Films Reinforced with Boron Nitride

Abstract

Understanding deformation of polymer thin films at various strains is crucial for wide applications, especially the growing organic semiconductor field. Thin films of immiscible polymer blends containing nanoparticles are particularly interesting as the properties (thermal, mechanical, optical) can be controlled by tuning the morphology. However, brittle fracture of these blend thin films with or without nanoparticles can hinder their fabrication, handling, and use. Herein, we fabricate polymer blend thin films with a pseudo-bicontinuous morphology of dispersed glassy polystyrene (PS) and continuous rubbery polyisoprene (PI). Two-dimensional (2D) boron nitride (BN) nanoplatelets are incorporated from 0 to 30 vol %. As-received BN was functionalized with polydopamine and (3-aminopropyl)triethoxysilane (APTES) to enhance dispersion and interfacial adhesion in the polymer matrix. Transmission electron microscopic images show a majority of polydopamine-silane BN (ds-BN) localized in the continuous rubbery PI and a small amount at the interface of PS and PI. Without functionalization, processing defects occurred at 15 vol % ds-BN, but this limit was increased to >25 vol % with functionalization. We then present the deformation of these thin films under uniaxial mechanical testing (tension, compression, and cyclic tests) at 10, 20, and 30% strains by coupling a micromechanical stage with an optical microscope. Most of the deformations observed were recoverable when returning back to 0% strain from the maximum applied strains. PI provides elastic recovery, PS maintains global stiffness, and ds-BN provides a reinforcement effect in the thin films. We found that polymer blend thin films loaded with notably high concentrations of functionalized ds-BN (up to 25 vol %) localized primarily within the continuous PI exhibited mostly recoverable mechanical performance. These findings are valuable in selecting glassy and rubbery polymer thin films loaded with nanoparticles to design resilient thin films for mechanical, thermal, or electrical applications.