The reinforcement mechanisms in graphene-based nanocomposites have been studied in this project, which primarily consists of three parts: the size and orientation effects of the graphene-based nano-fillers and their interfacial adhesion with the matrix. Overall Raman spectroscopy has been demonstrated to be a powerful technique to study the graphene-based nanocomposites. The deformation of small size graphene has been followed and a new model has been established to consider both the non-uniformity of strain along the graphene and laser intensity within the laser spot, which interprets the observed unusual Raman band shift well. Additionally, the deformation of monolayer graphene oxide (GO) has been followed for the first time. It appears that continuum mechanics is still valid, and the approximately constant strain distribution along the GO flake suggests a better stress transfer efficiency of GO than that of graphene. The spatial orientation of graphene has been studied based on the Raman scattering obtained from transverse sections of graphene, where the Raman bands intensities show a strong polarization dependence. Based on this, a new model has been established to quantify the spatial orientation of graphene in terms of an orientation distribution function, and the spatial orientation of monolayer graphene has been further confirmed by its surface roughness. This model has been extended to a variety of graphene-based materials and nanocomposites. It is also shown how the spatial orientation of graphene-based fillers affects the mechanical properties of the nanocomposites, through the first determination of the Krenchel orientation factor for nanoplatelets. The findings on both the size and orientation effects have been employed to study the deformation mechanics of bulk GO reinforced nanocomposite films. It has been demonstrated for the first time that the effective modulus of GO can be estimated using the Raman D band shift rate, and this is in agreement with the value measured using conventional mechanical testing. The effective modulus of GO is found to be lower than its Young’s modulus, probably due to the mis-orientation, waviness, wrinkling and agglomeration of the GO fillers.

Composites based on binary polymer blends of polystyrene (PS)/poly(ethylene-co-vinyl alcohol) (EVOH) (70/30 wt%) containing natural Montmorillonite, Na-MMTs (Nanomer PGW or Cloisite Na+) and organically modified Montmorillonite clays, OMMTs (Nanomer I.30T, Cloisite 30B or Cloisite 10A) were prepared via melt compounding. The interactions between the polymers and clays were studied using flow micro-calorimetry (FMC). Data obtained from FMC indicated that the probe molecule mimicking EVOH (butan-2-ol) interacted with the MMTs and OMMTs much more strongly than PS. Scanning electron microscopy (SEM) revealed that composites based on binary blends had dispersed/continuous morphologies, in which EVOH was dispersed in a PS matrix. The size of the EVOH droplets in the PS matrix increased with increasing clay loading. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) were used to determine the extent of dispersion and location of clay in the PS/EVOH/clay composites. These techniques confirmed the formation of intercalated clay structures. As predicted by FMC, the clay platelets were selectively located in the EVOH phase, independent of the blending sequence and the type of organic modifier in the OMMT. Composites containing OMMTs showed better dispersion of platelets within the EVOH phase than those containing Na-MMTs. Differential scanning calorimetry (DSC); showed the crystallisation behaviour of EVOH to depend on the clay loading and the nature of the organic modifier in the OMMT. Nanomer PGW, Cloisite Na+ and Cloisite 30B acted as weak nucleating agents. In contrast, Nanomer I.30T and Cloisite 10A significantly hindered the crystallisation of EVOH in the blends due to the restriction of chain segment mobility. Dynamic mechanical thermal analysis (DMTA) confirmed that the presence of clay increases the storage modulus of the composites compared to an unfilled blend. In addition, the improvement in storage modulus reflected the dispersion state of the different clays and their interaction with the polymers of the blend. Ternary-blend based composites were formed by adding poly(styrene-co-acrylonitrile) (SAN) to the composites based on binary PS/EVOH blends. This resulted in a finer dispersion of the EVOH phase and the development of a core-shell morphology, in which SAN encapsulated and formed shells around EVOH droplets. In contrast to binary blend composites, the clay platelets were found at the interface between SAN and EVOH in the ternary blends.

Understanding the deformation mechanism in nanocomposites is critical to realizing a host of next-generation technologies like stretchable electronics, three-dimensional multifunctional surfaces, and nanoscale machines. Graphene’s unparalleled mechanical strength and stability – owing to its two-dimensional geometry, high intrinsic strength, and Young’s modulus – have opened up new opportunities to engineer composites of higher strength-to-weight ratios for various practical applications. The ability of graphene (Gr) to act as a strength enhancer depends on the interface interactions and the composite’s microstructure. Here we demonstrate a microstructure design of Cu-Gr-Cu nanolaminate that enhances the composite’s load-bearing capacity, improves the composite’s strength, and reduces its coefficient of friction. The mechanical and frictional properties of Cu-Gr-Cu nanolaminate were probed using the nanoindenter. A series of nanoindentations performed on Cu-Gr-Cu nanolaminate exhibit an effective yield strength of 320 MPa and effective flow strength of 0•.5 GPa. Scratch tests performed on the free surface of the Cu-Gr-Cu nanolaminate show a considerable decrease in the coefficient of friction from 0.3 to 0.2. The cantilever bending test performed on Cu-Gr-Cu nanolaminate showed an increase in flow strength and strain hardening compared to Cu-Cu. The enhancement in the mechanical and friction properties of Cu-Gr-Cu nanolaminate suggests a build-up of dislocations at the Cu-Graphene interface. FEA simulations of the nanoindentation on Cu-Gr-Cu nanolaminate confirm the effectiveness of graphene as a barrier to plastic deformation. The pile-up of dislocations at the Cu-Graphene interface implies large plastic strain gradients near the interface. We developed a strain gradient plasticity computational model of the beam bending experimental system based upon Gudmundson’s higher-order theory and implemented it as a user element in ABAQUS. A set of material parameters is identified that reproduce the experimental for

Nanomaterial composites hold improvement potential for many materials. Improvements arise through known material behaviors and unique nanoscale effects to improve performance in areas including elastic modulus and damping as well as various processes, and products. Review of research spurred development of a load-stage. The load stage could be used independently, or in conjunction with an AFM to investigate bulk and nanoscale material mechanics. The effect of MWCNT content on structural damping, elastic modulus, toughness, loss modulus, and glass transition temperature was investigated using the load stage, AMF, and DMA. Initial investigation showed elastic modulus increased 23% with 1wt.% MWCNT versus pure epoxy and in-situ imaging observed micro/nanoscale deformation. Dynamic capabilities of the load stage were investigated as a method to achieve higher stress than available through DMA. The system showed energy dissipation across all reinforce levels, with ~480% peak for the 1wt.% MWCNT material vs. the neat epoxy at 1Hz. / ND NASA EPSCoR FAR0017788 / NDSU Development Foundation FAR0017503 / National Science Foundation (NSF) Grant# HRD-0811239 to the NDSU Advance FORWARD Program

The addition of fillers to a polymer matrix to endow soft materials with desirable properties has been a focused area of study over many decades and composite materials based on this idea are being increasingly incorporated into several end use products. Yet, almost always, the focus is on maximizing a particular property set for a unique polymer/filler combination for a specific application, which might not necessarily be translatable into another application. To exploit possible synergies, there is a need to develop materials that have the potential to perform multiple functions at the same time rather than a singular function. In this vein of thought, materials constructed using only polymer grafted nanoparticles (GNPs) have the potential to be one such class of materials as they have been shown to display a whole host of unique property sets – ranging from improved mechanical strength, enhancements in the gas and condensable penetrant transport properties, improvements in thermal conductivity, tunability of impact mitigation to more exotic behavior related to development of phononic bandgaps and quasi-crystalline materials. This thesis explores some of the structure-dynamics-property relations of some of the unique property sets described above and aims to provide insights into the nanoscale properties that lead to the improvements observed in macroscopic properties. In the first 2 chapters, we study the effect of tethering polymer chains to a spherical surface on the segmental and local vibrational dynamics of grafted polymer chains in an ensemble of GNPs. In the field of gas transport, the hopping motion of gas molecules inside a non-porous polymer matrix is facilitated by the motion of polymer segments, yet the understanding between the coupling of the two is very poor. By utilizing GNPs in which the diffusivity of gases is controlled by varying graft chain molecular weights, we can show that segmental dynamics of the polymer chains operating on a length scale of ~ 1 nm are positively correlated with the observed enhancements in diffusivities observed previously. We also propose that the inefficient packing of polymer chains leads to a decrease in the barriers of motion of the polymer segments, which is ultimately responsible for allowing penetrant molecules to move through the polymer phase much faster than a corresponding homopolymer melt. By utilizing a similar time and length scale approach, we can also explain the observed increases in thermal conductivities through the vibrational motion of polymer chains. This reaffirms the important role nanoscale polymer dynamics plays in both mass and thermal transport. In the next few chapters, we switch gears and focus on the microscopic structure and dynamics of the nanoparticles and how they impact the mechanical properties in suspensions. By studying the translational and vibrational motion of the GNPs, we find that the vibrational amplitude of a singular GNP decreases with increasing chain length all while the motion of the NP becomes faster, a phenomenon that we can associate with unjamming of the GNPs. This transition from jamming to unjamming is also visible in the local and long wavelength structure of the GNPs as well as the sound velocity through the material. Through these observations we can show that there is an intricate link between the structure and the relevant mechanical properties. Lastly, by building on the understanding laid out in the first few chapters, we propose that static features measurable through scattering are indicators of the enhanced transport properties of GNP based membranes. This also provides structural insights into the correlation between the structure of the polymer phase and the transport of penetrants. Each of the chapters touch upon a unique aspect of the structure and dynamics of different components of a GNP at different time and length scales, and how they are possibly linked to the several different property sets or dynamic features exhibited by the constructs, while also providing possible microscopic explanations for the same.

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Alpha-alumina is known to exhibit photo-luminescent (PL) properties, mainly characteristic R-lines that shift according to applied stress. In addition to showing excellent PL properties, polymers with embedded alumina nanoparticles have been shown to improve the overall composite mechanical properties. While the use of the PL properties to develop stress-sensing materials using an alumina-epoxy material has been success- fully shown in compression, the properties have not been developed for tension. In this study, the PL response of variable volume fraction alumina-epoxy composites will be determined under tensile conditions. It is expected that increasing the volume fraction of alumina nanoparticles will increase the sensitivity of the particles PL emission shift to applied stress. Three tensile alumina-epoxy specimens of 21.0%, 31.2%, and 34.5% volume fractions were manufactured and tested under tensile static loads. The results of this experiment will determine the piezospectroscopic (PS) coeffi cient and calibration of bulk alumina nanocomposites in tension. A linear region was identified in the PS response of the nanocomposite to the applied tensile load. The PS coeffi cient of this linear region increased as the volume fraction of the nanocomposite increased. To demonstrate the application of structural composites with stress sensing capabilities, alumina nanoparticles were integrated in the manufacturing of a carbon fiber composite specimen. The results of the stress-sensing composite mechanical experiment showed that alumina nanoparticles were able to detect changes in stress. The results for both the bulk nanocomposite calibrations and the application of stress-sensing alumina nanoparticles in a carbon-fiber composite will advance the development of this novel stress-sensing method.