Interplay of Chain Orientation and Bond Length in Size Dependency of Mechanical Properties in Polystyrene Nanofibers

Single amorphous polymeric nanofibers exhibit tunable properties when their sizes drop down below a specific onset value, including increases in modulus and strength. Understanding the detailed mechanism and corresponding microstructure change at the molecular level is key for reaching the long-term goal of controlling their properties and targeting applicable designs. In particular, the experimental study of polymeric chains and covalent bonds in amorphous nanofibers has been proven extremely challenging due to the scale limitation. Here, we investigate the role of chain alignment and backbone bond length on the diameter dependence of individual amorphous polystyrene (PS) by molecular dynamics (MD) simulation. For the first time, the diameter of the modeled nanofibers in MD can be directly comparable to the experimental scale. We observed a linear increase of ultimate strength and an exponential increase of Young's modulus when reducing the nanofiber diameter to a certain onset value. The increase of ultimate strength is found to be more related to the linearly increased chain alignment while the variation in Young's modulus is directly attributed to the exponentially increased backbone bond length. The parameter normalized diameter is also introduced to evaluate the extent of chain confinement in nanofibers. Finally, MD simulation is validated by comparing with experimental results, and a model demonstrating the evolution of molecular structure with diameter is proposed.