THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, October 16, 2020

11:00 AM

via

Bluejeans Video Conferencing

https://bluejeans.com/560429496

will be held the

DISSERTATION DEFENSE

for

Mohammad Hamza Kirmani

"Studies on High-Performance Thermosets and Their Interface and Interphase With Carbon-Nanotubes"

Committee Members:

Prof. Satish Kumar, Advisor, MSE

Prof. Karl Jacob, MSE

Prof. Kyriaki Kalaitzidou, ME

Prof. Naresh Thadhani, MSE

Prof. Seung Soon Jang, MSE

Abstract:

One of the many challenges involved in human travel to mars and beyond, is the lack of materials that fulfill the mechanical property requirements for these missions. NASA calls for advanced composite materials with quasi-isotropic, specific tensile strength and modulus of 3 and 150 GPa/g/cc, respectively, and an interlaminar fracture toughness of 0.3 N/mm. The carbon nanotube-polymer (CNT-polymer) composites are expected to have significantly better mechanical properties than the current state of the art (SOA) carbon fiber reinforced plastics (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 realize the mechanical property goals for the next generation composite materials, set out by NASA. 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, and (e) understanding the effects of amorphous carbon on the CNT-polymer interface-interphase.

Herein, the first part of the dissertation focuses on the effects of processing, on the molecular packing and the properties of a multi-component aerospace grade bismaleimide (BMI) resin, containing no CNTs. 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, and in the process of doing so increased the resin toughness by more than a factor of 4. 

The second part of the dissertation focuses on the structure, process and properties of CNT modified BMI, with tailored interface-interphase. 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 the 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. 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. 

The fourth part of the dissertation focuses on sizing and tailoring the CNT- BMI interface - interphase using a carbon fiber sizing. 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? 

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 of the amorphous carbon on the interfacial stress transfer with the polyurea matrix has been studied.

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.