Mechanical Deformation of Polymer Blend Thin Films Reinforced with Boron Nitride

Understanding deformation of polymer thin films under various strains at the microscale is important for designing high performance polymer nanocomposites. In this study, we fabricated immiscible polymer blend thin films, with a pseudo-bicontinuous morphology of discontinuous glassy polystyrene (PS) and continuous rubbery polyisoprene (PI). The phase separated morphology of PS and PI domains kinetically lock during spin coating as solvent evaporates. The interfacial instability of wetting bilayers of PS and PI forming on the substrate surface leads to phase separation. Flory-Huggins theory explains stable or phase separated polymer blends based on the free energy of mixing. Functionalized 2D boron nitride (BN) nanoplatelets were incorporated from 0 – 30 vol.%. The majority of the BN localized in the continuous rubbery PI with some at the interface of the two phases. The thin films were deformed under uniaxial mechanical strains in tension, compression, and cyclic modes with strains up to 30%. Deformation visualization was accomplished by coupling a micro-mechanical stage with an optical microscope. Most of the deformations observed in the thin films with functionalized BN concentrations up to 25 vol.% were recoverable. PI provided elastic recovery, PS maintained materials stiffness, and BN provided a reinforcement effect in the thin films. These findings are valuable in designing resilient thin films for flexible electronics, multifunctional coatings, and free-standing composite materials.