STUDIES ON HIGH-PERFORMANCE THERMOSETS AND THEIR INTERFACE AND INTERPHASE WITH CARBON-NANOTUBES
Kirmani, Mohammad Hamza Hamza
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Materials with higher strength to weight ratio than the current state of the art (SOA) carbon fiber reinforced plastics (CFRP) are desired by NASA to support affordable space exploration, including human travel to mars and beyond. The carbon nanotube-polymer (CNT-polymer) composites are expected to have significantly better mechanical properties than the current SOA CFRP and qualify as a potential system for achieving the target mechanical properties in materials required to support human travel to mars. CNT containing polymer composites, however, have some limitations, one of which is the load transfer at the CNT- polymer interface. The interface plays a critical role in determining the overall macroscale properties of the composite. While, significant attention has been directed to this end the CNTs in the composites have not yet reached their full potential. There are several aspects of the CNT-polymer composites which can help create the next generation of high strength and lightweight materials, to help support human travel to mars and beyond. These include, (a) improving the fracture toughness of the polymer resin, (b) understanding and optimizing the CNT-polymer interactions, (c) understanding the effects of CNTs on the polymer cure reactions, which consequently can alter the mechanical properties of the composite, (d) modifying the CNT-polymer interface-interphase through surface treatment and sizing, (e) understanding the effects of amorphous carbon on the CNT-polymer interface-interphase. Herein, the first part of the dissertation focuses on the effect of processing on the molecular structure and the properties of a multi-component aerospace grade bismaleimide (BMI) resin, containing no CNTs, and is discussed in Chapter 2. Materials in nature such as nacre that are made of mechanically inferior building blocks exhibit extreme toughness at the macro scale because of the geometry and arrangement of their constituents. Taking a cue from these systems, we have investigated whether the molecular rearrangement in a heterogeneous BMI system can alter toughness at the macro scale. To this end, a multicomponent BMI system is processed by using (a) a melt and cast (termed Melt) approach and (b) a dual asymmetric centrifuge based high-speed shear mixing (termed HSSM) approach to enforce molecular rearrangement. FTIR, Raman, and NMR spectroscopies have been used to study the molecular rearrangement upon HSSM processing. Small-angle X-ray scattering has been used to study the effect of processing on the molecular arrangement of the BMI. The second part of this dissertation focuses on the structure, process and properties of CNT modified BMI, with tailored interface-interphase and is discussed in Chapter 3. With the recent large-scale production and availability of the CNT macro-assemblies in the yarn, tape and sheet forms, CNT-polymer composites could now be prepared through conventional CFRP manufacturing techniques such as filament winding. It is however expected that the resin dominated properties, such as the inter and intra laminar fracture toughness in these CNT- polymer composites would still remain relatively weak, as they have been for the CFRPs. Modifying the resin with CNTs is an attractive route for further improving the resin properties. Herein, CNT- BMI nanocomposites using three different CNTs and via two different processing routes, have been prepared and studied. The third part of this dissertation focuses on the effects that the CNT have on the cure of the BMI, as well as the effects that the cure of BMI has on the CNTs, in the nanocomposites containing up to 40 wt% CNTs, and is discussed in Chapter 4. CNTs can interact with the BMI system through the NH-π, π-π, CH-π, and OH-π, non-covalent interactions. The individual components of the BMI however can have exclusive non-covalent interactions with the CNTs. For example, in a BMI system containing 4,4'- bismaleimidodiphenylmethane (BDM) and diallylbisphenol A (DABA) components, only the BDM component contains the maleimide functional group which can potentially interact with the CNTs through the NH-π bonding, while only the DABA component, containing the OH functional group can potentially interact with the CNTs through the OH-π interactions. The potential for the preferential stacking of the different BMI components around the CNTs, can have important implications on the cure behavior of the BMI in the nanocomposite and consequently on the overall mechanical properties of the nanocomposite. Herein, two different types of CNTs in the sheet form: unbaked and baked (termed as UB and B CNT), have been employed. The effects of the varying CNT content on the inter-CNT spacing, cure reactions of the BMI, compression of CNTs and the thermomechanical properties of the nanocomposites have been investigated. The thermomechanical results and the theoretical calculations have then been used to estimate the interphase thickness of the CNT- BMI nanocomposites. The fourth part of this dissertation focuses on sizing and tailoring the CNT- BMI interface - interphase using a carbon fiber sizing, and is discussed in Chapter 5. Sizing of carbon and glass fibers is a critical step in the manufacturing of their respective composites with polymers and has led to improved interfacial shear strength (IFSS), inter-laminar shear strength (ILSS) and fracture toughness of the composites. As the CNT-polymer composites could now be prepared through conventional CFRP manufacturing techniques such as filament winding, the question is, could we integrate another critical step of the conventional CFRP manufacturing, i.e., ‘sizing’, to the CNT-polymer composite preparation to tailor the CNT-polymer interface-interphase? To be able to answer that question, we first need to understand the sizing-CNT interactions and reactions. To this end, herein, the effects that (a) CNTs, (b) the degree of functionalization and defects (DOFD) in the CNTs and (c) the sizing content, have on the sizing cure reaction and cure kinetics have been evaluated. CNTs with three different DOFD have been employed. The sizing coated CNTs have then been used to prepare nanocomposite films with a high- performance aerospace grade bismaleimide (BMI) resin. Overall three different types of CNT- BMI interface-interphase have been prepared and studied in nanocomposites containing 60 wt% CNTs: (a) pristine CNT- BMI, (b) functionalized CNT- BMI, and (c) sizing coated functionalized CNT- BMI. The effect of CNT, CNT functionalization and sizing coated CNTs on the BMI cure reactions, thermomechanical properties and the molecular heterogeneity and hierarchy of the nanocomposites have been studied and discussed. Finally, CNTs may contain amorphous carbon, among other impurities which consequently could interfere with the interfacial interactions of the CNT and the polymer. While such impurities are expected to have a negative effect on the polymer-CNT interface, quantitative evidence of the extent of such negative effects is lacking. Herein, the effect that the amorphous carbon and the baking of CNTs to remove the amorphous carbon have, on the interfacial stress transfer with the polyurea matrix has been studied and discussed in Chapter 6. During CNT synthesis, by products such as amorphous carbon may be formed which consequently could interfere with the interfacial interactions of the CNT and the polymer. While such impurities are expected to have a negative effect on the polymer-CNT interface, quantitative evidence of the extent of such negative effects is lacking. Herein, the difference in interfacial straining has been studied in composites of polyurea with two types of CNT sheets: (a) sheets containing amorphous carbon (termed as unbaked CNT sheet) and (b) sheets that are thermally treated to remove amorphous carbon (termed as baked CNT sheet). The understanding of the effects of the amorphous carbon and the baking treatment, based on the CNT- polyurea system should be translatable to other CNT-polymer system, including the CNT-BMI system. It is expected that these studies will provide guidance for the manufacturing of CNT, or CNT and carbon fiber hybrid based laminates that will ultimately meet NASA mechanical property goals.