Steven Nutt

Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites

For developing high performance graphene-based nanocomposites, dispersal of graphene nanosheets in polymer hosts and precise interface control are challenging due to their strong interlayer cohesive energy and surface inertia. Here we report an efficient method to functionalize graphene nanosheets. The initiator molecules were covalently bonded to the graphene surface via a diazonium addition and the succeeding atom transfer radical polymerization linked polystyrene chains (82 wt% grafting efficiency) to the graphene nanosheets. The prominent confinement effect arising from nanosheets resulted in a 15 °C increase in the glass transition temperature of polystyrene compared to the pure polymer. The resulting polystyrene nanocomposites with 0.9 wt% graphene nanosheets revealed around 70% and 57% increases in tensile strength and Youngs modulus. The protocol is believed to offer possibilities for optimizing the processing properties and interface structure of graphene-polymer nanocomposites.

Single-layer graphene nanosheets with controlled grafting of polymer chains

Single-layer graphene nanosheets (SLGNs) are prepared by reduction of well-exfoliated graphite oxide aided by a surfactant (sodium dodecylbenzene sulfonate, SDBS). Grafting density and polystyrene (PS) chain lengths are controlled by modulating the concentrations of diazonium compound and monomer during the grafting reaction of the initiator and the succeeding atomic transfer radical polymerization (ATRP). Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectra and transmission electron microscopy (TEM) are used to confirm the single-layer structure of graphene sheets, covalent bonding at the interface, and distribution uniformity of grafting PS chains at the SLGN surface. Thermogravimetric analysis (TGA) is performed to assess the control of grafting density and chain length. PS chains grafted on the SLGN surface exhibited remarkably confined relaxation behavior. An increase in the glass transition temperature (Tg) of up to 18 °C is observed for high grafting density, low molecular weight polymer-grafted graphene samples. The low grafting density, high molecular weight sample shows an increase in Tg of ∼9 °C, which is attributed to superior heat conduction efficiency. The measured thermal conductivity for the PS composite film with 2.0 wt% SLGNs increase by a factor of 2.6 compared to that of the pure PS.

InGaN/GaN Multiple Quantum Wells Grown on Nonpolar Facets of Vertical GaN Nanorod Arrays

Uniform GaN nanorod arrays are grown vertically by selective area growth on (left angle bracket 0001 right angle bracket) substrates. The GaN nanorods present six nonpolar {1⁻100} facets, which serve as growth surfaces for InGaN-based light-emitting diode quantum well active regions. Compared to growth on the polar {0001} plane, the piezoelectric fields in the multiple quantum wells (MQWs) can be eliminated when they are grown on nonpolar planes. The capability of growing ordered GaN nanorod arrays with different rod densities is demonstrated. Light emission from InGaN/GaN MQWs grown on the nonpolar facets is investigated by photoluminescence. Local emission from MQWs grown on different regions of GaN nanorods is studied by cathodoluminescence (CL). The core-shell structure of MQWs grown on GaN nanorods is investigated by cross-sectional transmission electron microscopy in both axial and radial directions. The results show that the active MQWs are predominantly grown on nonpolar planes of GaN nanorods, consistent with the observations from CL. The results suggest that GaN nanorod arrays are suitable growth templates for efficient light-emitting diodes.

Mechanical characterization of short fiber reinforced phenolic foam

The mechanical performance of fiber-reinforced phenolic foam is characterized and compared with unreinforced foam in terms of friability, compression and shear properties, and flexural behavior of simple sandwich beams. Compared with conventional phenolic foam, foam reinforced with aramid fibers exhibits significantly lower friability, higher resistance to cracking, and more isotropic behavior, while glass fiber-reinforced foam is significantly stiffer and stronger. Sandwich structures with reinforced phenolic foam cores show unique failure behavior, in which catastrophic collapse of the structure is not only delayed, but avoided altogether. The findings in this paper, coupled with earlier results, demonstrate the potential of reinforced phenolic foam as a fire-resistant, tough and low-cost engineering material.

Modified Graphene/Polyimide Nanocomposites: Reinforcing and Tribological Effects

By taking advantage of design and construction of strong graphene-matrix interfaces, we have prepared modified graphene/polyimide (MG/PI) nanocomposites via a two-stage process consisting of (a) surface modification of graphene and (b) in situ polymerization. The 2 wt % MG/PI nanocomposites exhibited a 20-fold increase in wear resistance and a 12% reduction in friction coefficient, constituting a potential breakthrough for future tribological application. Simultaneously, MG also enhanced thermal stability, electrical conductivity, and mechanical properties, including tensile strength, Youngs modulus, storage modulus, and microhardness. Excellent thermal stability and compatibility of interface, strong covalent adhesion interaction and mechanical interlocking at the interface, as well as homogeneous and oriented dispersion of MG were achieved here, contributing to the enhanced properties observed here. The superior wear resistance is ascribed to (a) tribological effect of MG, including suppression effect of MG in the generation of wear debris and protective effect of MG against the friction force, and (b) the increase in mechanical properties. In light of the relatively low cost and the unique properties of graphene, the results of this study highlight a pathway to expand the engineering applications of graphene and solve wear-related mechanical failures of polymer parts.

Transmission loss and dynamic response of membrane-type locally resonant acoustic metamaterials

Membrane-type acoustic metamaterials were fabricated, characterized, and analyzed to understand their acoustic response. Thin plates which obey the acoustic mass law have low transmission loss (TL) at low frequencies. Acoustic metamaterials with negative dynamic mass density have been shown to demonstrate a significant (5×) increase in TL over mass law predictions for a narrow band (100 Hz) at low frequencies (100–1000 Hz). The peak TL frequency can be tuned to specific values by varying the membrane and mass properties. In this work, TL magnitude as a function of frequency was measured for variations in the mass magnitude and membrane tension using an impedance tube setup. The dynamic properties of membranes constructed from different materials were measured and compared to the results of coupled field acoustic-structural finite element analysis modeling to understand the role of tension and element quality factor. To better comprehend the mechanism(s) responsible for the TL peak, a laser vibrometer was ...

Bio-inspired impact-resistant composites

Through evolutionary processes, biological composites have been optimized to fulfil specific functions. This optimization is exemplified in the mineralized dactyl club of the smashing predator stomatopod (specifically, Odontodactylus scyllarus). This crustaceans club has been designed to withstand the thousands of high-velocity blows that it delivers to its prey. The endocuticle of this multiregional structure is characterized by a helicoidal arrangement of mineralized fiber layers, an architecture which results in impact resistance and energy absorbance. Here, we apply the helicoidal design strategy observed in the stomatopod club to the fabrication of high-performance carbon fiber-epoxy composites. Through experimental and computational methods, a helicoidal architecture is shown to reduce through-thickness damage propagation in a composite panel during an impact event and result in an increase in toughness. These findings have implications in the design of composite parts for aerospace, automotive and armor applications.

Membrane-type metamaterials: Transmission loss of multi- celled arrays

Acoustic metamaterials with negative dynamic mass density have been shown to demonstrate a five-fold increase in transmission loss (TL) over mass law predictions for a narrowband (100 Hz) at low frequencies (100–1000 Hz). The present work focuses on the scale-up of this effect by examining the behavior of multiple elements arranged in arrays. Single membranes were stretched over rigid frame supports and masses were attached to the center of each divided cell. The TL behavior was measured for multiple configurations with different magnitudes of mass distributed across each of the cell membranes in the array resulting in a multipeak TL profile. To better understand scale-up issues, the effect of the frame structure compliance was evaluated, and more compliant frames resulted in a reduction in the TL peak frequency bandwidth. In addition, displacement measurements of frames and membranes were performed using a laser vibrometer. Finally, the measured TL of the multi-celled structure was compared with the TL behavior predicted by finite element analysis to understand the role of nonuniform mass distribution and frame compliance.

Experimental and analytical study of nonlinear bending response of sandwich beams

The practical value of the geometrically nonlinear higher-order theory is demonstrated using four-point bend tests carried out on sandwich beam specimens comprised of aluminum face sheets and a PVC foam core. The experimental results were compared with the predictions of classical sandwich theory, and with linear and geometrically nonlinear higher-order sandwich panel theory. The analytical predictions based on the higher-order theory are in excellent agreement with the experimental results. Response parameters show fundamentally distinct behavior with increasing external load, both in the particular section and along the span. Considering the longitudinal displacements, there is a significant geometrically nonlinear stage of the response that precedes the appearance of the material nonlinearity. The peeling stresses also exhibit significant geometrical nonlinearity in the vicinity of the internal supports. The linear higher-order theory can be used efficiently to estimate the vertical displacements of the soft-core sandwich beams up to high load levels with a great accuracy. Premature failure of sandwich beam specimens with weak adhesive layers is caused by high peeling stresses in the upper interface layer at the ends of the specimen, and the loading capacity decreases by more than 40%.

Enhanced peel resistance of fiber reinforced phenolic foams

Abstract Short-fiber reinforced phenolic foams were synthesized and characterized by climbing drum peel tests and tensile tests. Significant improvements in mechanical properties were realized, including a multiple-fold increase in peel strength. Because peel strength is closely linked to the fracture toughness of foams, particular attention was focused on the mechanism responsible for the enhanced peel resistance. Scanning electron microscope observations revealed that phenolic foams reinforced with aramid fibers exhibited a unique ‘micro-peel’ process. This process was caused by a moderately weak interface between the flexible aramid fibers and the surrounding phenolic matrix, resulting in higher toughness relative to similar foams reinforced with stiffer glass fibers. In addition, a design-of-experiments analysis supported the expectation that fiber loading and length were primary factors contributing to the improved peel strength.