Designed-in molecular interactions and cross-linking interface for superior nanocomposites: a multi-scale insight
Özden Yenigün, Elif (2013) Designed-in molecular interactions and cross-linking interface for superior nanocomposites: a multi-scale insight. [Thesis]
A defining feature of polymer nanocomposites is the nano-scale of fillers leading to dramatic increase in interfacial area and associated sensitivity of properties to the fillermatrix interface. Stronger/attractive interfacial region helps to prevent early failure and facilitates enhanced mechanical behavior of nanocomposites. This thesis is an effort to address how interface characteristic can impact dominated physical mechanisms and under which circumstances improve particularly mechanical and thermo-mechanical properties of nanofiber reinforced nanocomposite. The hypothesis is that incorporation of electrospun surface modified/reactive polystyrene-co-glycidyl methacrylate P(St-co-GMA) nanofibers with epoxide functional groups into the epoxy resin results in significant improvements in the mechanical properties. Several mechanical and thermo-mechanical tests demonstrate significant increase in the mechanical response. Given the choices of the fiber material under consideration, the enhancement is attributed to the combined effect of the two factors: the inherent cross-linked fiber structure and the surface chemistry of the electrospun fibers leading to cross-linked polymer matrix-nanofiber interfacial bonding. Multi walled carbon nanotubes (MWCNTs) can also be embedded into entangled nanofiber network during the electro-spinning process to improve composite strength, durability, and impact resistance. The enhancement by the nano-scale fibrous reinforcement with designed interface can be further propagated into structural composites. It was shown that structural integrity of the electrospun P(St-co-GMA) based nanofibers with/without MWCNTs as interlayers in conventional carbon fiber/epoxy prepreg result in increased resistance to transverse matrix cracking and delamination at macro scale without weight penalty. Consecutively, this thesis traces the effect of the nanofiller chemistry and crosslinking on mechanical behavior of thermoset polymer matrix nanocomposites via numerical simulations. Multi-scale simulations including molecular dynamics and dissipative particle dynamics are employed to address the reinforcing function in nanocomposites at nanoscale. Coupled with focused experimental study on the interface, our novel modeling efforts are helping to elucidate the physical mechanisms that underlie nanocomposite bulk performance and ultimately enable efficient design of nanocomposites. Overall, the idea of chemistry specific design of interface in nanofibrous matrix composites is significantly effective. The experimental results show that the given the knowledge of the matrix system, smart choice of fiber polymer provides stronger interfacial bonding and improved mechanical properties. Simulation tools, on the other hand can trace the signatures of these improvements, and promise an efficient assessment methodology for interface design which can be help to optimize also the experimental efforts.
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