High temperature processable nanofibrous interlayers for composite structures
Ürkmez, Ayça (2015) High temperature processable nanofibrous interlayers for composite structures. [Thesis]
Nano-engineering of composite materials is an expanding research field, thanks to emerging manufacturing techniques and intriguing properties of nano-scale materials. It requires both "multi-disciplinary" and "multi-scaled" research insight for achieving the ultimate goal of superior material properties preferably with multifunctionality. Enhancing the mechanical properties such as toughening is arguably the most common interest. Interlayer toughening of structural composite materials is one of the several toughening mechanisms where interlaminar region, being one of the weakest links in composite structures, is at focus for the material solution developed here. Nanointerlayer toughening strategy thus aims to integrate nano-scaled reinforcements to interlaminar regions in order to improve the mechanical performance with minimum weight addition. Following this strategy, this thesis work firstly investigates the effect of glass transition temperature on the morphology of electrospun P(St-co-GMA) nanofibers which are proven to be a potential candidate for interlayer toughening in composite materials thanks to their epoxy compatibility. Secondly it offers a unique way to undisrupted electrospinning of these nanofibers in the presence of crosslinking agents. The goal is to achieve in-situ crosslinking at heat stimuli consistent with typical cure cycles of advanced polymeric composites. The thesis work is divided into two subsections: Heat Stimuli Self Crosslinking of Electrospun Nanofibers: Stimuli-Self – Crosslinking ability is introduced to P(St-co-GMA) nanofibers by the addition of Phtalic Anhydride (PA) as cross-linking agent and tributylamine (TBA) as the catalyst. Heat activated crosslinking procedure enables the manufacturing of cross-linkable nanofibers through electrospinning at room temperature without any rheological problems. A complete cross-linking event is characterized by co-use of FT-IR analysis focusing the consumption of PA and disappearance of available active sites in copolymer and swelling tests. Glass transition temperature of self-cross-linked copolymers increases by 30ºC without any post chemical treatments required, elevated temperature effect on the nanofiber morphology change before and after crosslinking is determined by SEM analysis. In-situ crosslinakable nanofibers for structural composites: The crosslinking recipe optimized in the first part is offered for the incorporation of polymeric nanofibrous interlayers into structural composites where high temperature curing cycle is needed. The hypothesis is that heat stimuli-self crosslinking enables a homogenous crosslinking regime both for nanofibers and epoxy matrix itself during curing which results in better mechanical performance. Following this motivation an example case is demonstrated where stimuli-self-crosslinkable P(St-co-GMA)/PA-TBA nanofibrous interlayers are added to carbon/epoxy prepreg composites cured at 135°C. Interlayered laminates are subjected to three-point bending and mode II fracture toughness tests (end-notched flexure-ENF). Mechanical test results are accompanied by cross-sectional and fracture surface microscopy analysis through Scanning Electron Microscopy (SEM). As a result of mechanical tests a significant increase in resistance against mode II delamination (80%) and flexural strength (15%) with precisely no weight penalty was observed.
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