Browsing by Author "Wang, Shiren"
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Item 3D Printing of Frontal-polymerized Multiscale Epoxy Thermoset and Composites(2022) Zhang, Zimeng; Gao, Chongjie; Liu, Ruochen; Qiu, Jingjing (TTU); Pei, Zhijian(ZJ) J.; Wang, ShirenEpoxy thermosets and their composites demonstrate an intensive application due to their high strength, chemical resistance, and lightweight while the conventional manufacturing methods are very difficult to fabricate complex geometry and also time-consuming and energy-intensive. Frontal polymerization based in-situ curing provides an alternative efficient way in the additive manufacturing of epoxy thermosets with both design and manufacturing freedom. However, low reactivity of epoxy resins makes it difficult to integrate frontal polymerization of epoxy resin into energy-efficient additive manufacturing. Carbon nanotubes (CNTs) catalyzed frontal polymerization of epoxy resins afforded a new way to free-form manufacturing of epoxy thermosets with low energy consumption and design flexibility. In this paper, frontal curing of epoxy resins was tuned by CNT catalysation and then integrated to fast printing of multiscale epoxy composites. Specifically, discontinuous CNTs and continuous carbon fibers (c-CF) were integrated to print frontal-cured multiscale epoxy composites. The mechanical tests indicated that the CNTs/c-CF/epoxy thermosets composites demonstrated a tensile strength of 1.2GPa while the CNTs/epoxy thermosets only showed a tensile strength of around 50 MPa. The young's modulus of the CNTs/c-CF/Epoxy thermosets reached 95 GPa, around 9-fold higher than that of the CNTs/Epoxy thermosets. This emerging technology provides a new direction for thermosets and composites manufacturing.Item Architecture effects on the bulk and shear rheology and PVT behavior of polymers(2011-08) Guo, Jiaxi; Simon, Sindee L.; McKenna, Gregory B.; Hedden, Ronald C.; Quitevis, Edward L.; Wang, ShirenThe viscoelastic bulk modulus [K(t)] plays an important role in residual stress development during polymer and composite processing and application, and in developing relationships among the four fundamental material functions, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio. In addition, the origins of viscoelastic bulk and shear moduli are still unresolved. However, while the viscoelastic shear modulus has been widely studied, only a handful of investigations can be found in the literature concerning the viscoelastic bulk response. In order to investigate the viscoelastic bulk modulus, pressure relaxation responses were measured in a custom-built pressurizable dilatometer capable of making K(t) and pressure-volume-temperature (PVT) behavior measurements. The architectural effects on the bulk and shear relaxation responses of two polycyanurate networks have been studied and suggest that the shift factors used to construct the reduced curves are identical in the liquid states. Furthermore, comparisons of retardation time spectra indicates that bulk and shear responses have similar underlying molecular mechanisms at short times since the slopes are similar for the spectra; however, long-time mechanisms that are available to the shear are not available to the bulk. In addition, the architectures are found to have negligible effects on the bulk response; on the other hand, the relaxation/retardation time distributions for the shear are observed to increase with decreasing the crosslink density. The architecture effects were also studied on the bulk and shear responses for linear and star shape polystyrenes. The shift factors are also found to be identical for the bulk and shear responses of the two polymers in the liquid state; moreover, by comparing the bulk and shear retardation time spectra, shear deformations are found to have long-time mechanisms that are not available for the bulk. The pressure-volume-temperature (PVT) behavior of the thermosetting networks is studied to investigate the pressure-dependent glass transition temperature (Tg) and the architecture effects on the PVT behavior. The results show that although the Tg values are different, the two networks have similar values of dTg/dP. By comparing the PVT data calculated from Tait equation with best fits to the experimental data for the two networks, the most important variable governing the PVT behavior of the thermosetting materials is found to be the glass transition temperature, which strongly depends on crosslink density. Finally, the temperature- and pressure-dependent shift factors which are related to the relaxation times are reduced using a thermodynamic scaling, where Tau= ƒ(T^-1V^-gamma), and compared the results to the T – Tg scaling, where Tau = ƒ(T – Tg). The thermodynamic scaling law successfully reduces the data for all of the samples; however, polymers with similar structures, but with different Tg and PVT behavior, i.e., the two polycyanurates, cannot be superposed unless the scaling law is normalized by TgVg^gamma. On the other hand, the T – Tg scaling successfully reduced the polymers having similar microstructures.Item Branch-and-cut for complementarity-constrained linear programs(2015-05) Ding, Li; de Farias, Ismael R., Jr.; Cross, Jennifer A.; Howle, Victoria E.; Wang, Shiren; Smith, Philip W.A Complementarity-Constrained Linear Program (CCLP) is a linear program which has additional constraints called complementarity constraints. Which are such that, for some sets of variables, at most one variable can be positive. Numerous real world problems, such as resource allocation problems, can be mapped into CCLP. Traditionally, these problems are formulated as conventional MIP problems by introducing additional binary variables and adding new constraints. In the alternative approach we proposed in this dissertation, continuous variables are kept in the model. We will derive valid inequalities (called cutting planes) for the convex hull of the feasible set, which are then added to the cut pool in a branch-and-cut algorithm. We will present three families of valid inequalities, two of which are obtained by lifting valid inequalities of the projected polytope.Item Dynamics and thermodynamics of small molecule glass formers, polymers and organic crystals(2012-12) Xu, Ben; McKenna, Gregory B.; Weeks, Brandon L.; Vanapalli, Siva A.; Wang, ShirenDynamics and thermodynamics of small molecule glass formers, polymers and organic crystals are addressed in this thesis. Dynamics of glass formers in the vicinity of glass transition are investigated in the dynamic part, while the thermodynamics of rubber swelling and the melting of the energetic materials PETN are discussed in the thermodynamics section. The glass transition, where liquids transform into metastable, non-crystallized solids below the melting temperature, is a subject of intense research despite decades of investigation. Among all the problems in glass transition, the dynamics of glass-forming systems in the vicinity of glass transition, which affects the viscoelastic properties of glass formers, is of great significance and has drawn considerable attention. The dynamic divergence, where the dynamics of the glass formers, e.g. viscosity and diffusion coefficient, goes to infinity at finite temperature, is conventially depicted by the Vogel-Fulcher-Tammann (VFT) or Williams-Landel-Ferry (WLF) expressions. However, recent study suggests that the dynamic divergence may not exist. Furthmore, there is no solid-therotical foundation to explain the reason of the dynamic divergence. Both of these two factors inspire us to find a new model that does not have dynamic divergence to study the dynamics response near Tg. In this dissertation, the Dyre shoving model, an elastic model which does not have dynamic divergence was evaluated. The dynamic response near Tg of small molecule glass formers were found to deviate from the dynamic divergence and can be described by the Dyre shoving model, which provides a new perspective to the study of the dynamics of glass formers. Investigations of the dynamics of a rubbery polymer in the glass-rubber transition region are present in Chapter 3. There is usually one shoulder in the dynamic data in terms of relaxation spectrum H(τ) and loss tangent (tanδ) for glass formers. In the dynamic mechanical measurements in the glass-rubber transition zone of polyisobutylene (PIB), an additional shoulder beside the major shoulder was observed in the relaxation spectrum H(τ) and loss tangent (tanδ). We attributed this additional shoulder to the sub-Rouse modes. This interpretation was further tested by studying the change of the mechanics by adding plasticizers to PIB, which should disrupt the effective chain packing of undiluted PIB. It was found by adding plasticizers, the softening dispersion becomes narrower, and the additional shoulder disappears eventually, confirming our assumption that the additional shoulder in tanδ is due to the sub-Rouse mode of motion. Detailed study of the thermodynamics of rubber swelling, especially the explanation of the nonexistence of the peak in the swelling activity parameters, are presented in Chapter 4. This problem has puzzled the rubber community for half a century. A new avenue of thought combined with simulated results were used to investigate the swelling activity parameter. It was found that the calculation method used by the previous rubber researchers exaggerate the experimental uncertainties and trigger the confusion concerning the purported peak in S. Swelling activity parameter obtained by our new calculation method shows consistent trend with changing temperature, crosslinking density, and more importantly, the prediction of classic Freckle-Flory-Renner (FFR) theory. The fifth part of this dissertation is a continuation of the fourth part. The new methodology developed in the second part to calculate S was used to analyze the swelling data for PDMS and polyisoprene networks. Consistent correlations between S and temperature and crosslinking density were found for the first time. Meanwhile, the thermodynamics of the melting behavior of nanocrystals of energetic materials were investigated systematically in this work. Controlled pore glasses were used to control the size of the nanocrystals. It was found that the melting temperature of nanocrystals depcreases as the pore size of the controlled pore glasses decreases, which is consistent with the prediction of Gibbs-Thomson equation. The value of liquid-solid interface energy, calculated according to the Gibbs-Thomson equation, is also reasonably consistent with the estimation from the empirical Turnbull equation.Item Dynamics below the glass transition temperature and viscoelastic and calorimetric investigation of different fossil resins(2014-05) Zhao, Jing; McKenna, Gregory B.; Simon, Sindee L.; Khare, Rajesh; Quitevis, Edward L.; Wang, ShirenThe dynamics of glass-forming systems below the glass transition temperature (Tg) have become of special interest. The classic Vogel-Fulcher-Tammann (VFT) and Williams, Landel, and Ferry (WLF) equations predict the dynamic divergence behavior that the equilibrium relaxation time and viscosity of glass-forming liquid become infinite at a finite temperature above the absolute zero. However, such VFT behavior is challenged by an increasing amount of evidence from both theoretical and experimental works. In order to determine the dynamic response of glass-forming liquids, both dielectric and mechanical experiments were performed in poly(vinyl acetate) (PVAc) glass former in this work. The temperature dependence of the equilibrium shift factors at below Tg shows an apparent Arrhenius behavior rather than the classic VFT response. Besides, the dielectric and mechanical responses of PVAc seem to be probing the glass state differently as the former response reaches its equilibrium behavior much faster than the latter. In addition, a 20 million old Dominican amber was used to investigate the dynamics of glass-forming systems far below Tg. The upper bound to the equilibrium relaxation time shows that the dynamic response deviates dramatically from the VFT extrapolation as much as 44 K below Tg. Finally, 12 pieces of fossil resin from different locations and ages ranging from approximately 100 years to 230 million years have been studied using differential scanning calorimetry. The results show that different resins have different glass transition temperatures and thermal stabilities, though the Tg did not correlate with the age of the amber. Furthermore, both the apparent activation energy and dynamic fragility of amber increase as glass transition temperature increases. The Tg dependence of fragility for amber shows a similar trend with temperature as do the metallic glass formers and aromatic polymers with phenyl rings on the side chain.Item Effects of electrical fields in aligning SWNTs during production of polymer/SWNTs nanocomposites(2008-08) Hernandez, Rocio; Rivero, Iris V.; Kobza, John E.; Wang, ShirenSingle Wall carbon nanotubes (SWNTs) are distinguished by their exceptional mechanical strength and electrical properties. However, these properties can only be inherited by nanocomposites if SWNTs are strongly bonded, uniformly distributed, and aligned within the composite matrix during their manufacturing processing. Conversely, the limited availability and economical constraints presented by bulk manufacturing process of nanocomposite materials have been hindering factors in devising their utilization for structural applications and thus their commercialization. This study is focused on the application of an electrical field to align SWNTs during the casting process of polymer nanocomposites where the experimental methodology is comprised of preparing samples exhibiting various percent weights of SWNTs in the polymer matrix while applying distinct electrostatic field strengths. Specifically, SWNTs are dispersed into a semi-crystalline polymer, poly (ethylene-co-vinyl alcohol) (EVOH) matrix. The resulting nanocomposites were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and tensile testing to evaluate their physical, thermal, and mechanical properties. The highest electrostatic field strength used in this study (204V/mm, 60Hz) aligned low concentrations of SWNTs through the polymer matrix. Unfortunately, it also formed large agglomerations when using higher concentrations of SWNTs, and affected the tensile strength of the nanocomposite properties. The electrical field did not affect thermal characteristics, but the addition of SWNTs to the polymer matrix accelerated the melting of small crystals.Item Exceptional thermoelectric properties of flexible organic−inorganic hybrids with monodispersed and periodic nanophase(2018) Wang, Liming; Zhang, Zimeng; Liu, Yuchen; Wang, Biran; Fang, Lei; Qiu, Jingjing (TTU); Zhang, Kun; Wang, ShirenFlexible organic−inorganic hybrids are promising thermoelectric materials to recycle waste heat in versatile formats. However, current organic/inorganic hybrids suffer from inferior thermoelectric properties due to aggregate nanostructures. Here we demonstrate flexible organic−inorganic hybrids where size-tunable Bi2Te3 nanoparticles are discontinuously monodispersed in the continuous conductive polymer phase, completely distinct from traditional bi-continuous hybrids. Periodic nanofillers significantly scatter phonons while continuous conducting polymer phase provides favored electronic transport, resulting in ultrahigh power factor of ~1350 μW m−1 K−2 and ultralow in-plane thermal conductivity of ~0.7 W m−1 K−1. Consequently, figure-of-merit (ZT) of 0.58 is obtained at room temperature, outperforming all reported organic materials and organic−inorganic hybrids. Thermoelectric properties of as-fabricated hybrids show negligible change for bending 100 cycles, indicating superior mechanical flexibility. These findings provide significant scientific foundation for shaping flexible thermoelectric functionality via synergistic integration of organic and inorganic components.Item Fabrication and characterization of hierarchical nanostructure materials(2013-05) Zhang, Yue; Wang, Shiren; Farias, Ismael R. d.; Hope-Weeks, Louisa J.; Qiu, Jingjing; Zhang, Hong-ChaoGraphene, a one-atom-thick planar sheet of graphite, has demonstrated high electrical conductivity, charge carrier mobility, specific surface area, as well as excellent mechanical and thermal properties. The graphene nanosheets have been integrated with various nanoparticles (NPs) to form NPs/graphene hierarchical nanostructure materials. The hierarchical nanostructure materials exhibit excellent properties and new functionalities due to the synergetic effects between graphene nanosheets and the nanoparticles, and are regarded as one of the most promising materials for energy storage and conversion. However, to date, the wide application of NPs/graphene nanostructures has been hindered due to several technical challenges. The first challenge involves the effective functionalization of graphene with nanoparticles. The second one is a lack of the understanding of the processing-structure-property relationship of the hierarchical nanostructure. In this dissertation, two types of nanoparticles (metallic silver nanoparticles and non-metallic C60 nanoparticles) were functionalized onto graphene nanosheets. The processing-structure relationship of the hierarchical structures was well studied and analyzed. Moreover, the specific capacitance of the silver NPs/graphene hierarchical nanostructures and the power factor of the C60/graphene/polymer nanocomposites were quantitatively compared and analyzed. First, the processing-structure-property relationship of silver nanoparticles (Ag NPs)/graphene nanostructure was studied. Silver nanoparticles were deposited onto graphene nanosheets through electrostatic attraction and subsequent reduction. Characterizations by X-ray diffraction and transmission electron microscopy (TEM) have confirmed the formation of Ag NPs/graphene nanostructure. The TEM images and TGA test results showed that the concentration of the silver salt solution can effectively tune the graft density and morphology of the silver nanoparticles on graphene. The nitrogen adsorption/desorption tests indicated that the Ag NPs prevented the restacking of graphene sheets, resulting in larger surface area. The electrical conductivity and the specific capacitance of silver-deposited graphene were improved by adjusting the concentration of the silver salt solution. Particularly, the electrical conductivity and capacitance increased by 3 times and 2 times, respectively, when compared with the as-fabricated graphene nanosheets. Secondly, the processing-structure relationship of C60/graphene nanostructure was investigated. In this study, a novel covalent method and a unique non-covalent method were presented to synthesize the nanostructure. The resultant materials were characterized by FT-IR spectroscopy, UV-Vis spectroscopy, atomic force microscopy, and TEM. The characterization results confirmed that fullerene was successfully attached onto graphene nanosheets through Fisher esterification reaction and liquid-liquid interfacial precipitation, respectively. Through particle analysis, it was found that the particle size (from 5 to 85 nm), size distribution, and morphology can be tuned by adjusting the concentration of the C60 solution. Finally, the enhancement effect of C60/graphene hierarchical structure on the power factor of composites was investigated. The C60/graphene hierarchical materials were incorporated into epoxy resin and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PSS). The electrical conductivity of epoxy resin was significantly increased by incorporating the hierarchical structures. An optimal power factor of 1.1 μW/K2m was achieved with the addition of 45 wt% C60/graphene hierarchical nanostructure. When incorporated the hierarchical materials with the PEDOT:PSS, the Seebeck coefficient was increased through an energy-filtering effect between the hierarchical nanostructure and the PEDOT:PSS. A power factor of 32.4 μW/mK2, which was ~50% higher than the value of pure PEDOT:PSS, was achieved by properly adjusting the weight ratio between the C60, graphene and PEDOT:PSS. This dissertation developed three novel methods to functionalize graphene with nanoparticles. The processing-structure relationship of Ag NPs/graphene and C60/graphene hierarchical nanostructures was well studied and analyzed. Through these investigations, the specific capacitance and power factor of the hierarchical nanostructures and their composites were improved. This dissertation will provide guidance for design and fabrication of the NPs/graphene hierarchical nanostructures to achieve a better performance on energy storage and conversion.Item Fabrication and characterization of micro- or nano-scale organic energetic materials(2014-12) Zhang, Xin; Weeks, Brandon L.; Gill, Harvinder Singh; Khare, Rajesh; Wang, ShirenOrganic energetic material (OEM) research has attracted a lot of attention due to its wide application in mining, demolition, military weaponry, and aeronautical engineering. OEMs often exhibit a tradeoff between properties, associated with the energy content of the materials, and the potential for inadvertent ignition to detonation. Microstructures have been shown to have a significant impact on properties of OEMs. Thus, properties of OEMs can be tailored by manipulating their microstructures. In this dissertation, several techniques are developed for fabricating micro- or nano-scale OEMs. The thermal stability, burn rate, and laser ignition properties of some prepared micro- or nano-scale OEMs are also investigated. In addition, the prepared highly nanoporous graphene oxide (GO)-nitrocellulose (NC) films are used to synthesize three dimension (3D) graphene networks and nitrogen-doped (N-doped) 3D graphene networks. In chapter I, the author provides a comprehensive description of the progress and development of microfabrication techniques in OEMs and the motivation of the dissertation. The developed techniques are divided into two categories: (1) fabricating micro- or nano-scale void structures on the surface or inside of OEM films or bulk materials; and (2) controlling the crystalline structure and particle size of OEMs. In addition, this chapter will discuss properties of micro- or nano-scale OEMs. In chapter II, purification methods of various organic materials, preparation methods of various substrate and organic thin films, and procedures of different patterning methods are addressed. The author also demonstrates the procedures to fabricate pentaerythritol tetranitrate (PETN) micro-crystals, graphene oxide (GO)-PETN micro-composites, and various nanostructured nitrocellulose films. In chapter III, two novel techniques, adhesive reagent assisted lift-off lithography and tip induced crystallization lithography (TICL), are developed to pattern OEM thin films. For the adhesive reagent assisted lift-off lithography, an adhesive agent is coated on a PDMS stamp to increase the adhesion force between the stamp and the organic thin film. In comparison to traditional lift-off lithography techniques, the adhesive reagent assisted lift-off lithography does not require high contact pressures or external heating, which potentially allowing for patterning of a wide range of thermally sensitive compounds. The TICL technique depends on coating an amorphous organic thin film on a substrate and then inducing crystallization of the thin film using an AFM tip. After removing the non-crystalline materials from the substrate, the organic crystal arrays can be obtained on the substrate. In chapter IV, two novel techniques are developed to manipulate the crystalline structure of PETN. Firstly, the author investigates the fabrication of PETN thin films through spin coating. The crystalline structure of PETN is found to have a strong dependence on the size of non-crystalline PETN particles in amorphous thin films. The non-crystalline PETN particles can be controlled by adjusting the spin coater rotational speed and solution concentration. The author also sets up a model to describe the relationship between the non-crystalline particle size, spin coater rotational speed and solution concentration. Secondly, the author indicates that GO can be used to manipulate the crystalline structure of PETN. The irregular PETN micro-crystals can be fabricated by using a solvent/non-solvent method; however, after introducing 0.5% or more GO, the PETN formed regular microrods. In comparison to pure PETN micro-crystals, GO-PETN micro-composites have higher thermal stability and slower sublimation rate and vapor pressure at temperatures ranging from 105 oC to 135 oC. In chapter V, nanostructured pure NC and GO-NC films are fabricated via using an evaporating method and a mixing method, respectively. The morphology of pure NC films can be controlled by the solvent and growth temperature. Using dimethylformamide (DMF) at a growth temperature was 5 oC, reproducibly yielded spherical NC particles. And the final diameter of the prepared NC particles can be further tuned by solution concentration. The highly nanoporous NC films can be prepared via introducing 0.5% to 3% GO. In comparison to the bulk NC film, the pure NC films with sub-micro spherical particles and nanoporous GO-NC films have faster burn rate. The thermal stability and NIR laser ignition properties of NC films are also obviously improved when doped with 0.5% or more GO. Furthermore, the high-quality and uniform 3D graphene networks and N-doped 3D graphene networks can be prepared through thermal decomposition and combustion of GO-NC composites, respectively. In chapter VI, the author summarizes the dissertation and presents the future works.Item Fabrication of biodegradable biopolymer composites for orthopedic applications(2014-05) Swain Spearman, Shayla; Rivero, Iris V.; Zhang, Hong-Chao; Wang, Shiren; Green, Micah; Abidi, Noureddine; Harrysson, OlaNew materials for the manufacture of orthopedic devices are needed in order to aid with the alleviation of problems associated with the materials currently in production. The desire for new materials is due to the disparity between the mechanical properties of metal materials and surrounding tissues, along with the necessity of a second surgery for removal of temporary devices. Orthopedic devices are used to repair problems related to the musculoskeletal system. These devices can be used for permanent applications, such as total knee replacements where the device is essential for the lifetime of the patient, or for temporary applications, such as bone fractures where the device is no longer needed once the patient has healed. This research will introduce a novel composite material design for the fabrication of temporary implanted orthopedic devices. The use of biopolymers for fabrication of orthopedic devices has gained much attention because of the biopolymer’s ability to biodegrade and be replaced by natural tissues. The new composite material design combines the biopolymers polycaprolactone (PCL) and polyglycolide (PGA) to produce miscible 50/50 PCL-PGA blended electrospun fibers with a PCL matrix. Single-walled carbon nanotubes (SWNTs) were purified and wrapped with double stranded deoxyribonucleic acid (dsDNA) and introduced in the fibers to further increase strength. This design utilizes the long degradation rate of PCL while acquiring the strength of PGA. The PCL-PGA blended fibers will increase the interfacial bonding between the fibers and matrix while the dsDNA will aid in improving dispersion of the SWNTs in the fibers, thereby increasing the mechanical properties of the composite. The PCL, PGA, PCL-PGA, and PCL-PGA/dsDNA-SWNT fibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) so that the integrity of the fibers could be assessed. Bulk PCL was compression molded to encapsulate the PCL-PGA blended fibers as well as the PCL-PGA/dsDNA-SWNT fibers. Mechanical testing of the composites included tensile testing and three-point bend testing to determine if load transfer occurred. The enzymatic and thermal degradation of the composites was studied using SEM, DSC, and x-ray diffraction (XRD). Incorporation of the PCL-PGA fibers was able to increase the tensile yield strength and Young’s modulus over that of the bulk PCL, while decreasing the percent elongation at break. The incorporation of the PCL-PGA/dsDNA-SWNT fibers was able to increase the bending strength and modulus over that of the bulk PCL and PCL-PGA fibers alone. The use of blended fibers allowed load transfer from dsDNA-SWNT to PCL matrix, thereby creating a stronger biodegradable polymer. The goal of creating a stronger, biocompatible, fully biodegradable composite for use in implanted orthopedic applications was achieved with PCL, PGA, and dsDNA-SWNTs.Item Friction study of alkanethiol self-assembled monolayers: Influence of heat and humidity(2012-08) Liao, Yen-Chih; Weeks, Brandon L.; Khare, Rajesh; Vaughn, Mark W.; Wang, ShirenSelf-assembled monolayers (SAMs) are well-organized molecular layer formed by the adsorption of molecules on the surface of a suitable substrate. Thermal stability of SAMs is important for applications in various surface science applications. As a model material, 16-mercaptohexadecanoic acid (MHA) on template stripped gold surfaces was investigated to determine the effect of temperature on the change of lateral force signal using atomic force microscopy (AFM). Friction force signals were obtained at various temperatures in order to determine whether it was possible to correlate the friction signal with desorption of the thiol molecules from the surface. Samples were heated for up to 10 h ranging from 40 to 80 °C in air and scanned every hour. A kinetic model was introduced to correlate the lateral force signal to the activation energy of desorption of the SAM from gold surface with heating. The activation energy of the detachment using this technique is 25.4 kcal/mol, which is consistent with other more complex techniques. The effect of cyclic heating on the lateral force of MHA SAMs patterned on gold surfaces was also investigated by AFM. The results show a distinctive pattern for the response of lateral force dependent on the heating temperature. Two distinct phases are observed and are postulated to correspond to molecular rearrangement of the SAM surface. The friction properties of two distinctive alkanethiol SAMs, MHA and 1-octadecanethiol (ODT) on gold substrates in various humidity conditions were examined by AFM. The results suggest that hydrophobic ODT SAM is insensitive to humidity. The difference of lateral force signal is within ±10% regardless of humidity. The lateral force signal of hydrophilic MHA SAMs has a significant decrease in signal in humid environments. The influence of bulk water was also investigated by LFM. By imaging under water, the capillary force is eliminated on ODT SAMs, which leads to a lower lateral force. However, the lateral force image was reversed on MHA SAMs, which suggested that hydrophobic forces dominated in water.Item Hierarchical nanostructure enabled high performance supercapacitor for energy storage(2014-05) Li, Li; Wang, Shiren; de Farias, Ismael R., Jr.; Fan, Zhaoyang; Qiu, Jingjing; Zhang , Hong-ChaoAs one of the next generation energy-storage devices, supercapacitors play an important role in future vehicles and microelectronic devices because of their stable cycling life, fast charging–discharging rate, and intrinsically ultra-high power density. The performance of the device depends intimately on the properties and structures of their materials. Conjugated polymers have been shown as a promising electrode material because of the reversible oxidation-reduction activity which enables a high energy density. Also, the structural design provides a promising approach to enhance energy density without sacrificing power density because of the regular nanostructures. In this thesis, three novel, facile, and scalable approaches are presented to control the geometries and structures of the graphene inter-linked aligned PANI nanofibers to achieve optimal supercapacitive performance. By carefully selecting the synthesis approaches and parameters, PANI nanofibers with uniform alignment and narrow size distribution were realized. In the first work, PANI nanowire-pillared graphene was fabricated in the bulk solution by a template-free method with melamine initialized nucleation. PANI nanowire morphology was readily controlled by tailoring the amount of melamine. The PANI nanowires prepared at optimized conditions were vertically aligned on graphene sheets, and showed uniform diameter, length, and inter-nanowire spacing. The nanowire diameter was as small as ∼20 nm. The resultant specific capacitance of PANI nanowire–pillared graphene electrode reached a maximum of 625 F/g. In the second work, vertically aligned 3D network electrodes consisting of PANI array/graphene-film/ PANI array were assembled via a multistep nanowire growth. The acidic doping in the synthesis process played a critical role in the morphologies, structures, and electrochemical performance of 3D network. The 2M HClO4 doped 3D electrode showed optimal geometry and performance. The electrochemical impedance results indicated that the charge transfer resistance was ignorable, and the ion diffusion resistance was 0.22 Ω due to well-defined nanostructures. In order to further decrease the charge transfer resistance, an all electrochemical deposited GO-linked multiple PANI forests 3D electrode was prepared in the third work. When compared to the graphene paper used in the second work, the electrochemical deposited monolayer GO sheets were as thin as 0.88 nm. These sheets served as the transition nodes for the neighboring nanowire arrays after in-situ reduction during the further growth of PANI. The PANI nanofibers were highly oriented with diameters of around 20–30 nm. This carefully defined and sophisticate structure produced an optimal electrochemical performance because of the optimized ion diffusion and charge transferring. The energy density of as-produced supercapacitors was as high as 137 Wh/Kg while the power density was 1980 W/Kg in aqueous electrolyte. In summary, this dissertation points out the simple pathways to tailor electrode architecture for supercapacitors with both high energy density and high power density. Also, a theoretical model has been established to quantify the influences of various factors on the supercapacitor performance in both aqueous and organic electrolytes. Thus, it provides both experimental and theoretical approaches for constructing high energy density and high power density supercapacitors.Item Laser surface cleaning-based method for electric vehicle battery remanufacturing(2013-12) Ramoni, Olalekan O; Zhang, Hong-Chao; Ghebrab, Tewodros; Wang, Shiren; Matis, Timothy I.; Rahnama, MashaThe global impact of carbonaceous emissions from the internal combustion engine has created a huge incentive for the auto-industry and the general public to move from the internal combustion engines to the electric vehicles. The auto-industry has responded positively to the challenge by producing electric vehicles such as Nissan Leaf and Chevy Volt. However, the market penetration of these vehicles has been very dismal. The actual sales of the vehicles have been very far below the projected sales. Cost and periodic battery replacement have been indicated in many reports as the issues hampering widespread adoption of the electric vehicles. Experimental studies and theoretical modeling have shown that formation of passive solid electrolyte interface (SEI) layers on battery electrodes causes the fast capacity loss in the EV battery. This research study developed process to recover battery electrodes from the degraded EV battery for purpose of remanufacturing of the battery. Understanding of the capacity loss is essential for the development of EV battery remanufacturing. The study provided detailed degradation processes of the batteries, and made use of laser surface cleaning method to remove the passive SEI layers from the surfaces of the battery electrodes. The laser ablation of SEI was carried out in order to recover the battery electrodes. The electrode surfaces were characterized by analytical tools before and after the laser ablation of SEI to provide information about the morphological and structural changes. Electrochemical measurements were carried out to determine functional properties and performances of the recovered EV battery electrodesItem Multi-trigger mechanism with shape memory polymer nanocomposite(2012-05) Carrell, John; Zhang, Hong-Chao; Wang, Shiren; Dai, Lenore L.; Rivero, Iris V.; Tate, DerrickShape memory polymers (SMPs) and their composites are set of smart materials that exhibit a special ability to recover a trained shape deformation upon activation of a single environmental trigger. This ability has made SMPs an emerging technology and has provided novel solutions for applications such as actively expandable vascular stents, deployable space structures, and releasable fasteners. However, the single trigger can be problematic in applications where an ambient field can accidentally trigger the SMP or where direct mechanical access is not available for training of SMP. This research has thus investigated a novel multi-trigger SMP nanocomposite. This SMP nanocomposite is sensitive to thermal and magnetic fields and requires both fields to be applied for shape deformation. SMP nanocomposites were manufactured using a commercially available SMP and magnetite nanoparticles at varying weights (5, 10, 15, 20, and 25 wt.% magnetite). Basic thermomechanical testing of the SMP nanocomposites at ambient conditions and transition conditions along with specially created thermomechanical-magnetic tests have been performed and have shown the multiple sensitivities of the SMP nanocomposites. Further, the varying addition of the magnetite nanoparticles and/or the applied magnetic field to the SMP nanocomposite shows results with higher magnetic sensitivity as well as larger shape deformations. Based on this special behavior, a constitutive model has been presented for the SMP nanocomposites. This constitutive model considers four specific phases of the SMP nanocomposites that are defined by the initial/final configurations of the SMP nanocomposite following and during the application of transition state environmental conditions. This model has been used within LS Dyna FEA to simulate the multi-trigger behavior of the SMP nanocomposite. The simulations have matched closely the actual test case scenario and have validated the developed constitutive model. Further, simulations have been performed to test the SMP nanocomposite in the application of active disassembly. From these simulations, a definite case can be made for the SMP nanocomposite over its SMP counterpart per reduced processing time. This research has exhibited novelty through the documentation of the multiple field sensitivities of the SMP nanocomposite smart material. Smart material research, especially with SMPs, has focused on shape memory actuation on application of a single trigger (i.e. heat, light, chemical, electrical, magnetic, etc.). This research looks to expand on this research by analyzing and understanding the special behavior of a developed multi-trigger SMP nanocomposite. This research documents the multiple field properties and actuation strategies for SMP nanocomposites. The development and study of these materials can create a transformative new dimension for smart material research that will provide additional avenues for manufacturers in smart material applications. Furthermore, this research can create a number of opportunities that extend beyond the technical contributions produced. The key will be in the use of the derived constitutive model as a design tool, which could be used in a number of fields.Item Multilayer 3D Chiral Folding Polymers and Their Asymmetric Catalytic Assembly(2022) Tang, Yao; Jin, Shengzhou; Zhang, Sai; Wu, Guan-Zhao; Wang, Jia-Yin; Xu, Ting; Wang, Yu; Unruh, Daniel; Surowiec, Kazimierz; Ma, Yanzhang; Wang, Shiren; Katz, Courtney; Liang, Hongjun; Li, Yunze; Cong, Weilong; Li, GuigenA novel class of polymers and oligomers of chiral folding chirality has been designed and synthesized, showing structurally compacted triple-column/multiple-layer frameworks. Both uniformed and differentiated aromatic chromophoric units were successfully constructed between naphthyl piers of this framework. Screening monomers, catalysts, and catalytic systems led to the success of asymmetric catalytic Suzuki-Miyaura polycouplings. Enantio- and diastereochemistry were unambiguously determined by X-ray structural analysis and concurrently by comparison with a similar asymmetric induction by the same catalyst in the asymmetric synthesis of a chiral three-layered product. The resulting chiral polymers exhibit intense fluorescence activity in a solid form and solution under specific wavelength irradiation.Item Nanoconfined MMA Polymerization and Structural Recovery in Ge-Se Glasses(2014-12-09) Zhao, Haoyu; Simon, Sindee L.; McKenna, Gregory B.; Hedden, Ronald C.; Quitevis, Edward L.; Wang, ShirenThe effect of nanoconfinement on the free radical polymerization of methyl methacrylate (MMA) is investigated using differential scanning calorimetry, gel permeation chromatography, and 1H nuclear magnetic resonance. Both hydrophobic and hydrophilic controlled pore glass (CPG) with pore diameters of 13 to 110 nm are used for polymerization under nanoconfinement. The effective reaction rates are unchanged in hydrophobic pores but significantly increased in hydrophilic pores. For both pore surfaces, the time required to reach autoacceleration decreases with decreasing pore size, with the effect much more pronounced in the hydrophilic pores. These results are quantitatively described by a model incorporated with nanoconfinement effects using free volume theory. For the PMMA synthesized under CPG, the number-average and weight-average molecular weights increase because the onset of autoacceleration shifts to shorter times, whereas the polydispersity index at full conversion decreases relative to the bulk value. The higher percentages of isotactic-rich triads in hydrophilic pores are observed and the dependence of tacticity on temperature is predicted by the first-order Markov model. In addition, the glass transition temperature increases for both pore surfaces, but the increase in hydrophilic pores is more pronounced. For the high temperature equilibrium polymerization, the ceiling temperature is shifted to lower temperatures in 13 nm diameter nanopores, with pore surface chemistries showing no significant effects. The observed lower equilibrium conversion and ceiling temperatures are attributed to the larger negative change in entropy on propagation, which is a constant for the bulk equilibrium polymerization but changes within increasing polymerization temperature in nanopores presumably due to the reduced chain length at high temperature. The kinetics associated with the glass transition is investigated using conventional DSC for germanium selenide glasses with Ge content ranging from 0 to 30 atom %. As Ge content increases, the glass transition region broadens and the step change in heat capacity at Tg decreases. The change in enthalpy linearly increases with the logarithm of aging time and then levels off at an equilibrium value that increases with decreasing aging temperature. The time required to reach equilibrium increases with decreasing aging temperature and, at a given distance from Tg, the time increases with decreasing germanium content. The results indicate that all samples show expected structural recovery, and no evidence is found for an intermediate phase characterized by high stability and absence of physical aging.Item New techniques to use dates palm fronds in architectural and product design applications(2014-08) Alquimi, Majed; Flueckiger, Urs Peter; Haq, Saif; Wang, ShirenEvery year in the Middle East, date palm farmers cut thousands of palm fronds as a kind of maintenance. As a result, most of these fronds are disposed either by burning or shredding. However, before the discovery of oil, the fronds were one of the most important raw materials used in house construction and furniture manufacturing. In fact, date palm fronds have many unique characteristics in terms of rigidity and flexibility, and are advantageous in their availability and economically, which make them a reliable raw material. For these reasons, this research will look into date palm history and impact on culture, society, and religions to build fundamental knowledge about the date palm. In addition, the agricultural side of the date palm will be explored in detail and we will focus on both its traditional and contemporary uses as well as the specific techniques employed when working with them. Finally, the palm frond’s behavior will be analyzed through several fabrication experiments that apply contemporary and digital tools and fabrication methods to rejuvenate the use of date palm fronds in contemporary architecture and product design.Item A novel approach to fabricating interconnected porous PCL-based biodegradable scaffolds for articular cartilage tissue engineering(2011-05) Allaf, Rula Marwan; Rivero, Iris V.; Grimson, Mark; Hope-Weeks, Louisa J.; Kobza, John E.; Wang, ShirenTissue engineering has recently attracted great attention in science, engineering, and medicine as a promising strategy to fabricate tissue by combining cells and bioactive agents in a scaffold aimed at replacing diseased and/or damaged tissue. This study presents and investigates a novel fabrication approach aimed at developing interconnected porous scaffolds for cartilage tissue engineering. The main objective was to develop a versatile and simple approach to fabricating interconnected porous scaffolds without the use of potentially harmful solvents. The approach consists of three major steps: cryomilling, compression molding, and porogen leaching for the preparation of interconnected porous scaffolds by selective leaching of porogen(s) from blends and composites created by combining cryomilling with conventional compression molding. It was hypothesized that cryomilling would create homogeneous blends and composites and produce co-continuous binary blend morphologies to fabricate a wide range of interconnected porous scaffolds by selectively leaching one continuous phase from the co-continuous structure. Poly(ε-caprolactone) (PCL) was chosen as the base material with poly(ethylene oxide) (PEO), which is immiscible with PCL, as a water soluble biodegradable porogen. Polyglycolide (PGA) and multi-walled carbon nanotubes (MWCNTs) were also incorporated as potential additives used to manipulate the degradability, hydrophilicity, stiffness, strength, and cell stimulation of the biodegradable PCL scaffold. The ultimate goal was to create scaffolds that could potentially mimic the 25.5 MPa, and compressive strength at 10% strain from ~1.2 MPa to 1.8 MPa. Furthermore, those scaffolds had a remarkably fast degradation compared to the PCL polymer slow degradation rate. Addition of MWCNTs to PCL and PCL/PGA resulted in significant changes to scaffold morphology in spite of the persistent interconnected porosity. Partially continuous structures exhibiting rough textures were observed. Mean pore sizes were estimated in the range of ~3 μm to ~5 μm. MWCNTs were occasionally seen on wall surfaces creating rough and nanotextured surfaces. Other nanocomposite scaffolds properties include: water uptake in the range from ~79% to 81%, compressive modulus from ~29 MPa to 65 MPa, and compressive strength at 10% strain from ~1.6 MPa to 3.2 MPa. The fabricated PCL-based scaffolds properties imply that they could be interesting candidates for cartilage tissue engineering. This research confirmed the preparation of blends possessing highly continuous or co-continuous morphologies using solid-state blending followed by melt molding. The study has demonstrated the potential of this approach as a route to obtain interconnected porosity in PCL scaffolds. Once the co-continuous blend has been prepared, extraction of the porogen yielded an interconnected porous scaffold. The research showed that control of pore size, size distribution, and porosity can be effectively obtained. In summary, the results of this research provide significant insight into an original scaffold fabrication approach for the tissue engineering of articular cartilage that will lead to new composites and blends in scaffold manufacturing.Item Synthesis, growth mechanism and optical properties of YBO3-based LEDs phosphors(2014-12) Zhang, Xianwen; Chaudhuri, Jharna; Wang, Shiren; Kim, Jungkyu; Yeo, Changdong; Weeks, Brandon L.A family of monodisperse YBO3: Eu3+ 3D microstructure with nine morphologies were firstly synthesized under hydrothermal conditions. Different microstructures were controllably obtained through adjusting the molar ratio of Y: B (Yttrium: Boron) and solvent. Photoluminescence (PL) of nine samples were investigated and demonstrated that under the excitation of 254 and 363 nm honeycomb-like YBO3: Eu3+ spheres had the highest Red/Orange ratio as potential red phosphor for applications, such as PDPs (plasma display panels) and LEDs (light emitting diodes). Subsequently, the morphology of YBO3: Eu3+ is further controlled by changing the borate starting material and pH values. Moreover, related photoluminescence of YBO3: Eu3+ with various morphologies was compared. There is the graduate increase of luminescence intensity of Eu3+ on annealing the YBO3: Eu3+ microflowers at 400, 600, and 800 °C. The growth process of the YBO3 sparse and dense flowers was explored based on the time-dependent experiments and the results showed that the growth mechanism follows an in-situ growth through an initial nucleating, localized self-assembly, and Ostwald ripening process rather than self-assembly process as reported previously. Photoluminescence of white LEDs phosphors YBO3: Tb3+, Eu3+ was systematically studied demonstrating that under the excitation of 365 nm ultraviolet (UV) light. Tunable emission by varying the relative doping ratios were demonstrated, and eventually YBO3: Tb3+ (12.5%), Eu3+ (2.5%) exhibits a white light. It includes three emissions: a blue band attributed to self-trapped exciton, a green band due to the Tb3+ transition of 5D4 −7Fj (j = 6, 5, 4, 3), and a red band due to the Eu3+ transition of 5D0 −7Fj (j = 0, 1, 2, 3, 4). Energy transfers from host YBO3 to Tb3+, and Eu3+ and Tb3+ to Eu3+, as well as tunable emission by varying the relative doping ratios were identified through experimental strategies. At last, the combination of blue emission from self-trapped exciton with green and red emissions from activators was firstly used to fabricate white light emitting diodes by coating YBO3: Tb3+ (12.5%), Eu3+ (2.5%) phosphors on the commercial UVLED. Corresponding CIE coordinate, electroluminescence, color temperature, luminous efficiency, etc. were measured for the assessment of application.Item Thermal-recoverable tough hydrogels enhanced by porphyrin decorated graphene oxide(2019) Wang, Jilong; Wei, Junhua (TTU); Su, Siheng; Qiu, Jingjing (TTU); Hu, Zhonglue; Hasan, Molla; Vargas, Evan (TTU); Pantoya, Michelle (TTU); Wang, ShirenArtificial tissue materials usually suffer properties and structure loss over time. As a usual strategy, a new substitution is required to replace the worn one to maintain the functions. Although several approaches have been developed to restore the mechanical properties of hydrogels, they require direct heating or touching, which cannot be processed within the body. In this manuscript, a photothermal method was developed to restore the mechanical properties of the tough hydrogels by using near infrared (NIR) laser irradiation. By adding the porphyrin decorated graphene oxide (PGO) as the nanoreinforcer and photothermal agent into carrageenan/polyacrylamide double network hydrogels (PDN), the compressive strength of the PDN was greatly improved by 104%. Under a short time of NIR laser irradiation, the PGO effectively converts light energy to thermal energy to heat the PDN hydrogels. The damaged carrageenan network was rebuilt, and a 90% compressive strength recovery was achieved. The PGO not only significantly improves the mechanical performance of PDN, but also restores the compressive property of PDN via a photothermal method. These tough hydrogels with superior photothermal recovery may work as promising substitutes for load-bearing tissues.