Moduli Stabilization Research Articles

Design and construction of PTFE porous membranes with high modulus and structural stability via in-situ interlocking and anchoring strategy

PTFE nanofiber membrane, as an effective supplement to biaxial stretched PTFE membrane, suffers from low modulus and poor structural stability. In this study, the PTFE porous membrane with high modulus and structural stability was designed and achieved via the in-situ interlocking and anchoring strategy by co-electrospinning polyacrylonitrile (PAN) into PTFE matrix. During the sintering process, the continuous flexible PTFE nanofibers formed, while PAN nanofibers was pre-oxidized to create a stable trapezoidal structure. The effects of pre-oxidized PAN (OPAN) nanofibers' content and diameter on the morphology and structure of the PTFE/OPAN composite nanofiber membrane were investigated in detail. Results showed that the introduction of OPAN nanofiber effectively improve the modulus and structural stability of PTFE nanofiber membrane. The Young's modulus and tensile strength of the composite nanofiber membrane were over 50 MPa and 2 MPa, which were 1166% and 80% higher than that of pure PTFE nanofiber membrane, respectively. These results of this work would contribute to solve the defects of PTFE nanofiber membrane's low modulus and poor structural stability, and then expand its application scope.

Large-scale fabrication and performance improvement of polyvinylidene fluoride piezoelectric composite films

Sensing applications such as wearable electronics, electronic skin, and soft robotics using flexible piezoelectric films have received extensive attention. In this study, we fabricate large-scale 0–3 type composite films of polyvinylidene fluoride (PVDF)-doped modified lead zirconate titanate (PZT) particles (PZT@UP) through an extrusion-casting process to enhance piezoelectric properties and explain the piezoelectric performance mechanism. The introduction of PZT@UP particles improved the dielectric permittivity of the composite films and promoted the polarization of PVDF, which positively affected the piezoelectric properties of PVDF/PZT@UP films. The composite film with 25 wt% PZT@UP possessed an excellent piezoelectric coefficient (d33) of 29 pC/N, which was 93% higher than that of neat PVDF (15 pC/N). Moreover, the addition of PZT@UP particles effectively inhibited the movement of PVDF molecular chains and improved the mechanical modulus stability of the composite films. The law of electrostrictive effect of the PVDF/PZT@UP composite films is clarified from the aspects of the movement of PVDF molecular chains, the principle of electric field distribution, and the construction of the internal electric field. Further, the similarities and differences between the piezoelectric and electrostrictive mechanisms of the PVDF/PZT@UP composite films are summarized. The extrusion-casting process combined with modified fillers shows promising potential in improving wear and tear of and electricity generation from PVDF-based composite films, leading to a profound impact on the development of large-scale fabrication of piezoelectric sensors.

Effects of chain composition of PBAT on the supercritical CO2 foaming and degradation behavior

Poly (butylene adipate-co-butylene terephthalate) (PBAT) with different copolymer compositions and molecular weights exhibits unique foaming behavior and dimensional stability when supercritical CO2 is used as a blowing agent. With an increase in the content of butylene glycol terephthalate (BT) from 43% to 61%, the melting point and crystallinity of PBAT increase. Consequently, the foaming temperature range of PBAT shifts toward a narrower, higher temperature zone, i.e., from 70 – 120 °C to 135 –160 °C under 13 MPa CO2 pressure. Additionally, CO2 solubility and desorption rate decrease, while PBAT exhibits a higher compression modulus and better foam dimensional stability. For similar BT contents, an increase in the molecular weight of PBAT widened the foaming temperature window and increased the cell density of the foam. It is noteworthy that increasing the BT content of PBAT to 52% not only ensures good dimensional stability of the PBAT foams, but also good biodegradability of PBAT. In addition, the effect of foam structure on degradation behavior was investigated. It was found that the PBAT foams with a higher volume expansion, larger cell size, and lower cell density degrade faster.

Inverse-perovskites Sc3GaX (X = B, C, N): A comprehensive theoretical investigation at ambient and elevated pressures

We have made a comprehensive theoretical investigation of structural, mechanical, thermophysical, electronic, optical, thermodynamical and vibrational properties of inverse-perovskites Sc3GaX (X = B, C, N) as a function of pressure using density functional theory (DFT). Pressure dependent elastic constants are studied as regards mechanical stability, ductility, machinability and anisotropy of moduli. At ambient pressure, the density of states (DOS) at Fermi level of Sc3GaB is almost twice than that of Sc3GaC or Sc3GaN. This explains the predicted superconductivity of Sc3GaB. The nature of chemical bonding is studied via charge density distribution map. Calculated phonon dispersions exhibit dynamic stability of these compounds. The temperature and pressure dependent thermodynamic properties of Sc3GaB and Sc3GaN are investigated for the first time using quasi-harmonic Debye model. Debye and melting temperatures increase with increasing pressure which indicate that our studied compounds could be used for possible devices in harsh environments. Calculated minimum thermal conductivities of the compounds are much smaller than the optimum value (1.25 Wm−1K−1) up to 20 GPa. Hence the compounds are promising TBC materials over the entire pressure range considered. Optical properties of the compounds indicated several practical usefulness including the potential to be employed as coating materials to prevent solar heating.

Impediment effect of two-dimensional graphene on gaseous fluorine permeation during direct fluorination of polyurethane

Direct fluorination with gaseous fluorine (F2) is a simple way to prepare fluorine-containing polymers, however, partial hydrolysis of CONH- group during fluorination process often reduces the bulk properties of fluorinated polyurethane (PU). Here, two-dimensional graphene (GE) as barrier layer was introduced into the thermoplastic polyurethane (TPU) matrix followed by fluorination treatment by F2 gas. The conjugated π structure of graphene limited excessive fluoridation of the GE/TPU sample, while the large aspect ratio restrained the diffusion and penetration of F2 gas in the bulk. As a result, rich F elements were accumulated on the surface with the slightly-fluorinated bulk. The fluorinated GE/TPU sample integrated the excellent mechanical properties of TPU with friction behaviors of fluorine-containing polymers. The initial friction coefficient was 0.5, 50% lower than the original GE/TPU while maintaining the unchanged mechanical modulus and thermal stability. This research may effectively solve the issue on performance degradation caused by partial hydrolysis of polymer materials containing CONH- group during fluorination process, providing a simple and effective surface modification strategy for preparing high-performance fluorine-containing TPU.

Development of epoxy composites with graphene nanoplatelets and micro-sized carbon foam: Morphology and thermal, mechanical and tribological properties

The effect of graphene nanoplatelets (GNPs) and carbon foam (CF) micrograins on the morphology and thermomechanical and tribological properties of epoxy composites was determined. The binary composites containing separately CF grains or GNPs and multiphase composites with CF particles and GNPs together obtained by two procedures were studied. In the first method GNPs were dispersed initially in epoxy resin and then CF particles were introduced. In the second procedure, conversely, CF particles were distributed in polymer before GNPs addition. CF simultaneously applied with GNPs strongly improves storage modulus, hardness and thermal stability of composites and reduces the friction coefficient and the wear rate compared to epoxy matrix. The composite fabricated by the first method exhibited better tribological properties, especially considering the higher wear load.

Compatibilized bio-based polybutylene-succinate blended composites filled with surface modified Si3N4 for improved rigidity and thermal performance

In this study, polybutylene succinate was melt blended with polypropylene (PP) to fabricate partially bio-based and miscible composites filled with various loading silicon nitride (Si3N4) particles. Oleic acid and PP-grafted maleic anhydride were used as a functionalizing agent for Si3N4 and as a compatibilizer for polymer blends, respectively. The surface functionalization of the Si3N4 particles enhanced the interfacial adhesion between the filler and the blended polymers, affecting the thermal and mechanical properties of the composites. The thermal conductivity of the composite filled with only 18.6 vol% surface-treated Si3N4 increased by 342% compared to that of the blended polymer. In addition, the surface treatment of Si3N4 significantly improved the Young’s modulus, tensile strength, and thermal stability of the corresponding composites. However, the temperature-dependent storage modulus was unaffected by the surface treatment of Si3N4 and increased as the loading content of the Si3N4 particles increased.

Constructing Fe2O3 particles uniformly grown on graphene oxide as additives for ethylene vinyl acetate copolymer thermal stability, thermal conductivity and anti‐static performance enhancement

AbstractEthylene‐vinyl acetate copolymer (EVA) has been widely used for packaging materials for decades. However, EVA was highly restricted due to the poor thermal stability and anti‐static performance. For enhancing thermal stability and anti‐static performances, a large number of additives were added to EVA, which seriously prejudice the processability and mechanical properties of the polymer. To address this problem, N‐doped reduced graphene oxide@Fe2O3 (RGF) was constructed as a heat and electronic conduction actor in EVA matrix in this work. As as‐prepared RGF composites shown, Fe2O3 particles well dispersed on the surface of RGO sheets, forming a point‐plane three‐dimensional structure. Therefore, the thermal conductivity and volume resistivity of the EVA composites reached 0.58 W/mK and 1.7 × 1010 ohm∙cm at 1.0 wt.% of RGF addition, which shows 284% folds increasement and 7 orders of magnitude decrement than neat EVA. Moreover, the storage modulus and thermal stability of EVA composites at 180°C were also improved. In sum, all those enhancements were attributed to the Fe2O3 particles well dispersed on RGO sheets, and amino groups from RGF provided mass of hydrogen bonds with EVA chains which provide more sites and path for heat and electronic conduction. In this work, RGF composites and its synthesis strategy show potential promising in the highly effective thermal and electrical exchange materials.

Steel slag-KNO3 phase change composites for thermal storage at medium-high temperature and solid waste recycling

Phase change composites (PCCs) have broad applications in thermal energy storage systems. In this work, we fabricated a series of steel slag-KNO3 PCCs by mixed sintering, which can be used in the medium-high temperature TES system and have the advantages of solid waste resource utilization. The composite's properties were characterized by determining their compressive strength, Young's modulus, latent heat, thermogravimetric performance, thermal conductivity, and cyclic thermal stability. The results suggested that the steel slag-KNO3 PCCs possessed good mechanical and thermal properties. When the KNO3 contents ranged from 38 wt% to 50 wt%, the composites maintained high compressive strength (30–35 MPa) and Young's modulus (1600–1700 MPa). The composites with 50 wt% KNO3 exhibited a latent heat of 43 J/g in 300–350 °C and a thermal conductivity of about 0.68 W m−1 K−1. The steel slag-KNO3 PCCs also presented excellent cyclic thermal stability. After 48 thermal cycles, the composite's latent heat decreased slightly by 2.5–3.7 J/g, and the thermal conductivity increased by 16%–25% compared with that of composites before thermal cycling. This study has important guiding significance for developing medium-high temperature PCCs with low cost and high performance used in TES systems, and may provide a new path for the utilization of solid waste resources.

Preparation and Characterization of Nanocomposite Hydrogels Based on Self-Assembling Collagen and Cellulose Nanocrystals.

Collagen (Col) hydrogels are an important biomaterial with many applications in the biomedical sector. However, deficiencies, including insufficient mechanical properties and a rapid rate of biodegradation, hamper their application. In this work, nanocomposite hydrogels were prepared by combining a cellulose nanocrystal (CNC) with Col without any chemical modification. The high-pressure, homogenized CNC matrix acts as nuclei in the collagen's self-aggregation process. The obtained CNC/Col hydrogels were characterized in terms of their morphology, mechanical and thermal properties and structure by SEM, rotational rheometer, DSC and FTIR, respectively. Ultraviolet-visible spectroscopy was used to characterize the self-assembling phase behavior of the CNC/Col hydrogels. The results showed an accelerated assembling rate with the increasing loading of CNC. The triple-helix structure of the collagen was preserved with a dosage of CNC of up to 15 wt%. The CNC/Col hydrogels demonstrated an improvement in both the storage modulus and thermal stability which is attributed to the interaction between the CNC and collagen by the hydrogen bonds.

The rheological performance of shear-thickening fluids based on carbon fiber and silica nanocomposite

Current available shear-thickening fluid (STFs) may suffer from issues such as unsatisfactory energy dissipation performance and unstable dynamic stability for practical engineering applications. This paper investigates the innovated compounded STFs which are fabricated by mixing needlelike carbon fiber powder (CFP) and silicon dioxide (SiO2) into polyethylene glycol (PEG) under proper synthesis conditions. The microstructure and rheological properties of the compounded STFs, namely, CFP-SiO2/PEG, are investigated. The interaction between CFP and SiO2 and the shear-induced microstructure are analyzed using scanning electron microscopy. Steady-state rheological tests reveal that compounded STFs with different mass ratios exhibit significant rheological behavior and shear-thickening effects. The peak viscosity is demonstrated to be increased from 51.59 (monodispersed STFs) to 574.74 Pa s (compounded STFs), and the critical shear rate decreased from 79.42 to 10.00 s−1 when the mass fraction of CFP is set at 0.2%. The peak viscosity of the compounded STFs is shown to be increased by 313.96% when the plate spacing is increased from 0.25 to 1.00 mm. The dynamic rheological analysis shows that the compounded STFs exhibit excellent energy dissipation capacity at different stages. More importantly, the modulus instability and shear-thinning problems of monodispersed STFs could be significantly improved. According to the results, the key performance index of the CFP/SiO2-PEG compounded STFs is demonstrated to be improved by ten times or even higher. This work presents a novel type of STFs with high energy dissipation capacity and high dynamic stability for the application of shear-thickening fluids composite in engineering practice.

Nanoscale Structure–Property Relations in Self-Regulated Polymer-Grafted Nanoparticle Composite Structures

Using a model system of poly(methyl methacrylate)-grafted silica nanoparticles (PMMA-NP) and poly(styrene-ran-acrylonitrile) (SAN), we generate unique polymer nanocomposite (PNC) morphologies by balancing the degree of surface enrichment, phase separation, and wetting within the films. Depending on the annealing temperature and time, thin films undergo different stages of phase evolution, resulting in homogeneously dispersed systems at low temperatures, enriched PMMA-NP layers at the PNC interfaces at intermediate temperatures, and three-dimensional bicontinuous structures of PMMA-NP pillars sandwiched between two PMMA-NP wetting layers at high temperatures. Using a combination of atomic force microscopy (AFM), AFM nanoindentation, contact angle goniometry, and optical microscopy, we show that these self-regulated structures lead to nanocomposites with increased elastic modulus, hardness, and thermal stability compared to analogous PMMA/SAN blends. These studies demonstrate the ability to reliably control the size and spatial correlations of both the surface-enriched and phase-separated nanocomposite microstructures, which have attractive technological applications where properties such as wettability, toughness, and wear resistance are important. In addition, these morphologies lend themselves to substantially broader applications, including: (1) structural color applications, (2) tuning optical adsorption, and (3) barrier coatings.

Engineering small flux superpotentials and mass hierarchies

We study the stabilization of complex structure moduli in Type IIB flux compactifications by using recent general results about the form of the superpotential and Kähler potential near the boundaries of the moduli space. In this process we show how vacua with an exponentially small vacuum superpotential can be realized systematically and understood conceptually within asymptotic Hodge theory. We distinguish two types of vacua realizing such superpotentials that differ by the mass scales of the stabilized moduli. Masses polynomially depending on the moduli arise if the superpotential contains exponential corrections whose existence is required to ensure the non-degeneracy of the moduli space metric. We use the fact that such essential corrections are controlled by asymptotic Hodge theory and have recently been constructed for all one- and two-moduli asymptotic regimes. These insights allow us to obtain new vacua near boundaries in complex structure moduli space that include Seiberg-Witten points. In these examples we find that the scale of the vacuum superpotential can be bounded from below through the exponential of the negative D3-brane tadpole.

Preparation and Characterization of Polypropylene/Sepiolite Nanocomposites for Potential Application in Automotive Lightweight Materials.

Polypropylene (PP)/sepiolite nanocomposites were prepared using the melt blending technique. The effects of nano-sepiolite content on the mechanical property, thermal property, crystallinity, morphology and rheological property of PP/sepiolite nanocomposites were investigated. The organic modified sepiolites (OSep) were dispersed evenly in PP matrix after surface treatment. The addition of OSep improved the storage modulus and thermal stability, showing a strong interaction between OSep and PP matrix. With the increase of OSep content, the fluidity of PP/OSep composites first increased due to the lubrication of surface modifiers and then decreased due to the interaction between OSep and PP. The size of the toughening agent elastomer first increased and then decreased, and the impact notched strength of PP/Osep composites first decreased and then increased. The loading of OSep also reduced the crystallinity and shrinkage rate of PP. PP/OSep nanocomposites have potential applications in high-performance automotive lightweight materials.

The mechanical properties, tribological behaviors and color stability of a feldspar nanoceramics strengthening extrinsic stain for high-translucent zirconia

The present work is aimed to explore the mechanical properties, tribological behaviors and color stability of nanoceramics and microceramics strengthened extrinsic stain coatings (NS and MS) upon high-translucent zirconia (TZ). The Na-rich feldspar ceramics component, microstructure and particle size of NS and MS were verified. The mechanical properties including elastic modulus and hardness of NS were enhanced compared to MS. Reciprocating wear tests under a ball-on-plate configuration manifested that the reduced coefficient of friction, wear depth and wear volume loss of NS was evaluated after 1 × 10 4 cycles and the wear scar morphology of NS characterized by microcracks while MS featured more delamination and wear debris. Post toothbrushing simulation revealed that the color stability of extrinsic stain coatings was elevated with the addition of feldspar nanoceramics. The feldspar nanoceramics strengthening extrinsic stain exhibited enhanced elastic modulus, hardness, wear resistance and color stability, especially for TZ.

Joint statistics of cosmological constant and SUSY breaking in flux vacua with nilpotent Goldstino

We obtain the joint distribution of the gravitino mass and the cosmological constant in KKLT and LVS models with anti-D3 brane uplifting described via the nilpotent goldstino formalism. Moduli stabilisation (of both complex structure and Kähler moduli) is incorporated so that we sample only over points corresponding to vacua. Our key inputs are the distributions of the flux superpotential, the string coupling and the hierarchies of warped throats. In the limit of zero cosmological constant, we find that both in KKLT and LVS the distributions are tilted favourably towards lower scales of supersymmetry breaking.

Combined digestion and bleaching of New Zealand flax /harakeke fibre and its effects on the mechanical, thermal, and dynamic mechanical properties of poly(lactic) acid matrix composites

In this study, New Zealand flax (harakeke) fibre was initially modified through digestion in an alkali solution followed by bleaching with hydrogen peroxide and sodium silicate with the aim of improving thermal and mechanical performance of its composites, through increased interfacial bonding. X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy and lignin analysis showed that the combination of bleaching and alkali treatment resulted in a higher cellulose content than digestion alone. Fibre inclusion was found to increase the crystallinity of PLA, likely due to heterogeneous nucleation on the treated fibres, which in turn helped to improve the composite strength. The highest tensile strength, tensile modulus and thermal stability were achieved with the bleached fibre which is believed to be due to better fibre distribution and stronger interfacial interaction. This was supported by the adhesion factor and effectiveness coefficient calculated using the data obtained from dynamic mechanical analysis.

Mechanical properties and injection molding processability of glass fiber modified polylactic acid composites

In order to promote the development and application of environment-friendly plastics, a glass fiber-modified starch/polylactic acid composite was prepared by the melt extrusion method. The influence of glass fiber content on the mechanical and thermal properties of the composite was studied, and multi-objective optimization of the injection molding process was carried out with consideration of the influence of material properties. The results show that with the increase of glass fiber content from 0 to 5%, the tensile strength of the composite decreases first and then increases to an average of 60Mpa, which is about 30% higher than that of pure polylactic acid. The elastic modulus and thermal stability increase, while the elongation at break and flow rate decrease with the increase of glass fiber content. In addition, the optimized injection parameters are obtained, which can effectively reduce the warping deformation, volume shrinkage, and residual stress of injection parts. Glass fiber modified starch/polylactic acid composites show great potential in engineering application, and may provide a reference for the development and application of high-performance and green degradable materials.

Moduli stabilization with bulk scalar in nested doubly warped braneworld model

We examine the modulus stabilization mechanism of a warped geometry model with nested warping. Such a model with multiple moduli is known to offer a possible resolution of the fermion mass hierarchy problem in the Standard Model. A six dimensional doubly warped braneworld model under consideration admits two distinct moduli, with the associated warp factors dynamically generating different physical mass scales on four 3-branes. In order to address the hierarchy problem related to the Higgs mass, both moduli need to be stabilized around their desired values without any extreme fine tuning of parameters. We show that it is possible to stabilize them simultaneously due to the appearence of an effective 4D moduli potential, which is generated by a single massive bulk scalar field having non-zero VEVs frozen on the 3-branes. This gives rise to two scalar radions, one of which has mass slightly below {mathcal {O}}(TeV) and couplings to SM fields proportional to the inverse of its {mathcal {O}}(TeV) VEV, and the other has nearly {mathcal {O}}(M_{Pl}) mass and interactions with SM fields suppressed by the Planck scale. We also discuss how the entire mechanism can possibly be understood from a purely gravitational point of view, with higher curvature f(R) contributions in the bulk automatically providing a scalar degree of freedom that can serve as the stabilizing field in the Einstein frame.

Breakage-resistant hydrogel electrode enables ultrahigh mechanical reliability for triboelectric nanogenerators

Hydrogel based triboelectric nanogenerators (H-TENGs) are deemed as one of most promising flexible energy harvesting and self-powered systems due to its multifunctionality, but the poor mechanical properties of hydrogel electrode bring a major risk of shortening the service life of H-TENGs in case of huge physical impact. Herein, a durable conductive hydrogel electrode with breakage-resistant capacity is developed utilizing the Hofmeister effect on starch polymers through solvent-exchange strategy. Owing to bundled starch chains within hydrogel, the breakage-resistant hydrogel electrode possesses excellent mechanical reliability, including outstanding modulus, fracture energy, anti-puncture capacity and long-term stability. Furthermore, the reliable hydrogel electrode endows triboelectric nanogenerator with good electrical output performances and superb damage immunity, prolonging its service life under accidental physical impact. More significantly, this fabricated triboelectric nanogenerator shows great potential in high-impact application. This investigation offers a versatile and effective way to design next generation hydrogel-based triboelectric nanogenerator with superior damage immunity and long-service life.