Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Arun Shukla

Abstract

The importance of nanotechnology in the world has dramatically increased in the recent years as miniaturization has become more important in areas such as computing, sensing, biomedicine and many others. Advancements in these disciplines depend largely on the ability to synthesize nanoparticles of various shapes and sizes, as well as to assemble them efficiently into complex architectures. Currently, materials reinforced with nanoparticles have an enormous range of applications owing to their superb mechanical and physical properties. More specifically, recent progress has shown that using inorganic nanomaterials as fillers in polymer/inorganic composites has tremendous application potential in automotive, aerospace, construction and electronics industries. If properly incorporated into a polymeric matrix, conductive nanofillers such as carbon nanotubes (CNTs) and graphene can be assembled into a three-dimensional electrical network. Furthermore, these highly intricate networks can be utilized as an internal sensory mechanism capable of detecting information such as material deformation and various forms of damage. Fundamental investigation into the mechanical and electrical properties of nanomaterial-based polymer composites when subjected to loading is paramount before they can be incorporated into high performance applications. For this purpose, a comprehensive study was conducted to understand the electro-mechanical behavior of both CNT-based and graphene-based composites under static and dynamic loading conditions. Moreover, novel strategies were developed to produce designer graphene-based composites that possess tailored transport and mechanical properties. A series of dynamic compressive experiments were performed to experimentally investigate the electrical response of multi-wall carbon nanotube (MWCNT) reinforced epoxy nanocomposites subjected to split Hopkinson pressure bar (SHPB) loading. Low-resistance CNT/epoxy specimens were fabricated using a combination of shear mixing and ultrasonication. Utilizing the carbon nanotube network within, the electrical resistance of the nanocomposites was monitored using a high-resolution four-point probe method during each compressive loading event. In addition, real-time deformation images were captured using high-speed photography. The percent change in resistance was correlated to both strain and real-time damage. The results were then compared to previous work conducted by the authors (quasi-static and drop-weight impact) in order to elucidate the strain rate sensitivity on the electrical behavior of the material. In addition, the percent change in conductivity was determined using a Taylor expansion model to investigate the electrical response based upon both dimensional change as well as resistivity change during mechanical loading within the elastic regime. Experimental findings indicate that the electrical resistance is a function of both the strain and deformation mechanisms induced by the loading. The bulk electrical resistance of the nanocomposites exhibited an overall decrease of 40- 65% for quasi-static and drop-weight experiments and 65-85% for SHPB experiments. An experimental investigation was conducted to understand the electromechanical response of graphene reinforced polystyrene composites under static and dynamic loading. Graphene-polystyrene composites were fabricated using a solution mixing approach followed by hot-pressing. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. A modified split Hopkinson (Kolsky) pressure bar apparatus, capable of simultaneous mechanical and electrical characterization, was developed and implemented to investigate the dynamic electro-mechanical response of the composites. In addition to measuring the change in electrical resistance as well as the dynamic constitutive behavior, real-time surface damage and global deformation was captured using high-speed photography. The real-time damage was correlated to both stress-strain and percent change in resistance profiles. The experimental findings indicate that the bulk electrical resistance of the composite increased significantly due to the brittle nature of the polystyrene matrix and the presence of relative agglomerations of graphene platelets which resulted in micro-crack formations. A novel capillary-driven particle-level templating technique along with hot melt pressing was developed and utilized to disperse few-layer graphene (FLG) flakes within a polystyrene matrix to enhance the electrical conductivity of the polymer. The conducting pathways provided by the graphene located at the particle surfaces through contact of the bounding surfaces allow percolation at a loading of less than 0.01% by volume. This method of distributing graphene within a matrix overcomes the need to disperse the sheet-like conducting fillers isotropically within the polymer, and can be scaled up easily. The novel capillary-driven particle-level templating technique was then extended to allow for distribution of conductive sheet-like particles, such as graphite nanoplatelets (GNPs) into specially constructed architectures throughout a polystyrene matrix to form multi-functional composites with tailored electro-mechanical properties. By precisely controlling the temperature and pressure during a melt compression process, highly conductive segregated composites were formed using very low loadings of graphene particles. Since the graphene flakes form a honeycomb percolating network along the boundaries between the polymer matrix particles, the composites show very high electrical conductivity but poor mechanical strength. To improve the mechanical properties, a new processing technique was developed that uses rotary shear through fixed angles to gradually evolve the honeycomb graphene network into a concentric band structure over the dimensions of the sample. An experimental investigation was conducted to understand the effect of GNP loading as well as the rotary shear angle on the mechanical strength and electrical conductivity of the composites. The experimental results show that both the electrical and mechanical properties of the composites are significantly altered using this very simple technique, which allows rational co-optimization of competing mechanical and electrical performance as appropriate for a given target application . Flexible multi-functional composites with tailored electro-mechanical properties were produced using a modified capillary-driven particle-level templating technique. A fixed-angle rotary shear technique was utilized during the melt compression process to distribute GNPs into specially constructed architectures throughout a styrenebutadiene matrix. An experimental investigation was conducted to understand the effect of GNP loading as well as rotary shear angle on the mechanical strength and electrical conductivity of the composites. The experimental results show that this technique can be used to produce flexible composites that possess exceptional conductivity while still maintaining the salient mechanical characteristics the copolymer has to offer.

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