Molecular Dynamics Simulation on the Heat Transfer in the Cross-Linked Poly(dimethylsiloxane).

In this work, the effect of cross-linking degree and stretching on the thermal conductivity of poly(dimethylsiloxane) (PDMS) is explored by performing a molecular dynamics simulation. Our results demonstrate that the thermal conductivity of PDMS exhibits a monotonous rise with an increase in the cross-linking degree. By decomposing the total heat flux into three microscopic heat transfer modes, the high cross-linking degree improves the contribution from bonding interactions to the heat transfer more than that from the nonbonding interactions. An analysis of the vibrational density of states shows a blue-shift of the vibrational modes at low frequencies, indicating a large phonon group velocity due to the strong interchain bonding interaction. From the spectral distribution of heat flux, the spectral contributions are shifted toward the higher frequencies with the increasing cross-linking degree, which reflects more contribution from the high-frequency modes to the heat transfer. Stretching can improve the thermal conductivity parallel to the tensile direction with the increase in strain. This is mainly due to the further improved contribution of bonding interactions or high-frequency modes to heat transfer. Interestingly, the anisotropy of the thermal conductivity first decreases and then increases with the increasing cross-linking degree. Our study conducts a detailed investigation of the thermal conductivity of cross-linked PDMS, providing guidance on the application of thermal interface materials.

Quantification Characterization of Hierarchical Structure of Polyurethane by Advanced AFM and X-ray Techniques.

Polyurethane (PU) with microphase separation has garnered significant attention due to its highly designable molecular structure and a wide range of adjustable properties. However, there is currently a lack of systematic approaches for quantifying PU's microphase separation. To address this research gap, we utilized an atomic force microscopy (AFM) nanomechanical mapping technique along with Gaussian fitting to recolor and quantitatively analyze the evolution of PU's microphase separation. By varying the ratios of the chain extender to cross-linking agent, we observed the changes in the hydrogen bonding between the soft and hard segments. As the ratio of the chain extender to cross-linking agent decreases, the strength of the hydrogen bonding weakens, resulting in a reduction in the quantity and phase percentage of hard segment (HS) domains. Consequently, the degree of microphase separation between the soft and hard segments decreases, leading to specific alterations in the material's mechanical properties and dynamic viscoelasticity. To further investigate the hierarchical structure of PU, we employed various techniques, such as X-ray analysis, transmission electron microscopy (TEM), and AFM-based infrared spectroscopy (AFM-IR). Our findings reveal a spherulite pattern composed of lamellae within the HS domains, with the cross-linking density gradually increasing from the center to the periphery. Overall, our comprehensive characterization of PU provides valuable insights into its hierarchical structure and establishes a quantitative framework to explore the intricate relationship between the structure and properties.

Manipulating the Thermal Conductivity of the Graphene/Poly(vinyl alcohol) Composite via Surface Functionalization: A Multiscale Simulation.

The reverse non-equilibrium molecular dynamics simulation is used to investigate the influence of functional groups (FGs) on the thermal conductivity of a graphene/poly(vinyl alcohol) (PVA) composite, which considers non-polar (methyl) and polar (hydroxyl, amino, and carboxyl) groups. First, the polar groups can be more effective to improve the interfacial thermal conductivity than the non-polar group. This can be explained well by characterizing the interfacial Coulombic energy, number and lifetime of hydrogen bonds, vibrational density of states, and integrated autocorrelation of the interfacial heat power. Moreover, the hydroxyl group can improve the interfacial thermal conductivity more than the other groups, which can be rationalized by analyzing the surface roughness of graphene and the radial distribution function of FGs and the PVA chains. However, the introduction of FGs destroys the graphene structure, which consequently reduces the intrinsic thermal conductivity. Furthermore, by adopting the effective medium approximation model and finite element method, there exists a critical graphene length where the overall thermal conductivities are equal for the functionalized and pristine graphene. Finally, the distribution state of graphene is emphasized to be more vital in determining the overall thermal conductivity than the generally accepted interfacial thermal conductivity.

Performance Enhancement of Polymerized, Functionalized Solution Styrene-Butadiene Rubber Composites Using Oligomeric Resin towards Extremely Safe and Energy-Saving Tires.

Polymerized, functionalized solution styrene-butadiene rubber (F-SSBR) is a new type of polymerized styrene-butadiene rubber solution containing specific terminal groups, which can be used in treads for high performances. However, the wet skid resistance related to safety, the rolling resistance to energy consumption, and the wear resistance to service life are often contradictory and form the performance "magic triangle". In this work, oligomeric resins, including Coumarone resin, C9 resin, C5/C9 resin and a styrene-α-methyl styrene copolymer (SSC), were used as tire functional additives and selected to replace treated distillate aromatic extract (TDAE) to improve the performances of silica-filled F-SSBR composites. The C9 resin, C5/C9 resin and SSC could enhance the modulus at 300% and tensile strength of the F-SSBR composite. The four resins could improve the wet skid resistance and wear resistance of the composites. However, Coumarone resin caused poor silica dispersion in the F-SSBR matrix and eventually, the lower modulus, higher loss factor at 60 °C and the higher heat buildup in the composite were comparative to the composite with TDAE. Furthermore, the synergistic effect of the C5/C9 resin and SSC was found to improve the mechanical performance of the composites and it resulted in higher tensile strength and modulus, and a lower heat buildup, compared to the case when only TDAE was used. It is noted that the properties "magic triangle" was broken by the C5/C9 resin and SSC, and the C5/C910T15 increased the wet skid resistance by 21.7%, fuel-saving rate by 2.3%, and wear resistance by 8.3%, while S20T5 increased the wet skid resistance by 30.4%, fuel-saving rate by 7%, and wear resistance by 25% compared with CG.

Electrospun nanofibers for manipulating soft tissue regeneration.

Soft tissue damage is a common clinical problem that affects the lives of a large number of patients all over the world. It is of great importance to develop functional scaffolds to manipulate and promote the repair and regeneration of soft tissues. Owing to their unique composition and structural properties, electrospun nanofibers have attracted much attention for soft tissue regeneration. Electrospun nanofibers can be easily constructed and functionally modified to regulate their composition, morphology, structure, three-dimensional architecture, and biological functions, as well as specific light/electric/magnetic properties. By integrating multiple types of guidance cues, such as topographical and biochemical cues and external stimuli, electrospun nanofiber scaffolds can be used to manipulate cell behaviors and thus facilitate tissue regeneration. In this review article, we have first described the construction of electrospun nanofibers with specific morphology and topography and their capability of modulating cell migration, cell morphology, and stem cell differentiation. We have then discussed the role of electrospun nanofiber scaffolds in promoting the regeneration of different types of soft tissues, including nerves, skin, heart, blood vessels, and cornea, from the point of view of the anatomical structures and physiological regeneration processes of tissues. By presenting and discussing the recent progress of electrospun nanofibers in manipulating soft tissue regeneration, we hope to provide a possible solution and reference for the repair of tissue damage in clinical practice.

Molecular imaging nanoprobes for theranostic applications.

As a non-invasive imaging monitoring method, molecular imaging can provide the location and expression level of disease signature biomolecules in vivo, leading to early diagnosis of relevant diseases, improved treatment strategies, and accurate assessment of treating efficacy. In recent years, a variety of nanosized imaging probes have been developed and intensively investigated in fundamental/translational research and clinical practice. Meanwhile, as an interdisciplinary discipline, this field combines many subjects of chemistry, medicine, biology, radiology, and material science, etc. The successful molecular imaging not only requires advanced imaging equipment, but also the synthesis of efficient imaging probes. However, limited summary has been reported for recent advances of nanoprobes. In this paper, we summarized the recent progress of three common and main types of nanosized molecular imaging probes, including ultrasound (US) imaging nanoprobes, magnetic resonance imaging (MRI) nanoprobes, and computed tomography (CT) imaging nanoprobes. The applications of molecular imaging nanoprobes were discussed in details. Finally, we provided an outlook on the development of next generation molecular imaging nanoprobes.

Novel Three-Dimensional-Printing Strategy Based on Dynamic Urea Bonds for Isotropy and Mechanical Robustness of Large-Scale Printed Products.

Additive manufacturing via fused deposition modeling (FDM) has become one of the most widely used technologies owing to its ease of operation and effective cost. However, the disappointing interlayer adhesion produced by FDM often results in inferior mechanical properties, which has become a technical bottleneck for industrial production. Herein, we demonstrate a facile and efficient printing strategy to enhance interlayer adhesion by introducing a self-healing mechanism into the printing material, thereby concurrently enhancing the mechanical properties and isotropy of the printed products. This strategy relies on the self-healing property of three-dimensional-printing materials. This self-healing property is endowed by introducing dynamic urea bonds on the thermoplastic polyurethane (TPU) molecular chains, and then, such dynamic bonds can be activated through thermal heating. Accordingly, the synthesized TPU reveals an efficient self-healing property and excellent printability owing to the existence of dynamic reversible covalent bonds. Moreover, objects with complex structures can be split and printed and then assembled using this strategy, avoiding the need for supporting structures and realizing the rapid prototyping of large-sized objects. The printing strategy proposed paves a candidate way to overcome the current challenges in obtaining high-quality products via FDM.

Fabrication of a High-Performance and Reusable Planar Face Mask in Response to the COVID-19 Pandemic.

The coronavirus disease 2019 (COVID-19) pandemic has caused a surge in demand for face masks, with the massive consumption of masks leading to an increase in resource-related and environmental concerns. In this work, we fabricated meltblown polypropylene (mb-PP)-based high-performance planar face masks and investigated the effects of six commonly used disinfection methods and various mask-wearing periods on the reusability of these masks. The results show that, after three cycles of treatment using hot water at 70 °C for 30 min, which is one of the most scalable, user-friendly methods for viral disinfection, the particle filtration efficiency (PFE) of the mask remained almost unchanged. After mask wearing for 24 h and subsequent disinfection using the same treatment procedures, the PFE decreased to 91.3%; the average number of bacterial and fungal colonies was assessed to be 9.2 and 51.6 colony-forming units per gram (CFU∙g- 1), respectively; and coliform and pyogenic bacteria were not detected. Both the PFE and the microbial indicators are well above the standard for reusable masks after disinfection. Schlieren photography was then used to assess the capabilities of used and disinfected masks during use; it showed that the masks exhibit a high performance in suppressing the spread of breathed air.

Influence of Surface Defects on the Thermal Conductivity of Hexagonal Boron Nitride/Poly(dimethylsiloxane) Nanocomposites: A Molecular Dynamics Simulation.

In this simulation, the reverse nonequilibrium molecular dynamics simulation is employed to explore how the surface defects in hexagonal boron nitride (h-BN) influence the thermal conductivity of poly(dimethylsiloxane) (PDMS)-based composites. First, the interfacial thermal conductivity and the intrinsic thermal conductivity of h-BN are obtained by tuning the defect density, the inhomogeneity of the defect distribution, and the number of h-BN layers. The defects enhance the interfacial thermal conductivity, especially for h-BNs with high inhomogeneity of the defect distribution and multilayer. However, the intrinsic thermal conductivity of h-BN is declined significantly by the defects. They can be explained well by the vibrational density of states of PDMS and h-BNs and their overlap. Then, by combining the effective medium approximation model with the simulation, the overall thermal conductivity of composites is obtained. It exhibits a gradual decrease with increasing defect density or reducing the inhomogeneity of the defect distribution. Meanwhile, the enhancement extent of the overall thermal conductivity by improving the concentration and size of h-BNs depends on the defect density and the defect distribution. Finally, the comparison between the simulation and experiment is discussed. In summary, our work provides some valuable insights into how the defect density, the defect distribution, and the number of layers influence the thermal conductivity of the PDMS-based composite.

Research of TPU Materials for 3D Printing Aiming at Non-Pneumatic Tires by FDM Method.

3D printing technology has been widely used in various fields, such as biomedicine, clothing design, and aerospace, due to its personalized customization, rapid prototyping of complex structures, and low cost. However, the application of 3D printing technology in the field of non-pneumatic tires has not been systematically studied. In this study, we evaluated the application of potential thermoplastic polyurethanes (TPU) materials based on FDM technology in the field of non-pneumatic tires. First, the printing process of TPU material based on fused deposition modeling (FDM) technology was studied through tensile testing and SEM observation. The results show that the optimal 3D printing temperature of the selected TPU material is 210 °C. FDM technology was successfully applied to 3D printed non-pneumatic tires based on TPU material. The study showed that the three-dimensional stiffness of 3D printed non-pneumatic tires is basically 50% of that obtained by simulation. To guarantee the prediction of the performance of 3D printed non-pneumatic tires, we suggest that the performance of these materials should be moderately reduced during the structural design for performance simulation.

Increasing the thermal conductivity of styrene butadiene rubber: insights from molecular dynamics simulation.

It is very important to improve the thermal conductivity of styrene butadiene rubber (SBR) which can widen its application. By employing reverse nonequilibrium molecular dynamics simulations in a full atomistic resolution, the effect of the composition ratio of styrene, temperature, and tensile strain on the thermal conductivity of SBR has been investigated in this work. The results indicate that the thermal conductivity of SBR gradually decreases with increasing composition ratio of styrene. This closely depends on the number of degrees of freedom and the diffusion coefficient of backbone atoms. Under the tensile field, the orientation of backbone bonds improves the thermal conductivity parallel to the tensile direction, but reduces the thermal conductivity perpendicular to it. Meanwhile, the thermal conductivity parallel to the tensile direction is enhanced with the strain rate while it is reduced with the composition ratio of styrene. Interestingly, there exists a linear relationship between the logarithm of anisotropy of the thermal conductivity and the orientation degree of bonds. Finally, the parallel thermal conductivity of the strained SBR first rises and then declines with temperature. This transition reflects a crossover from disorder to anharmonicity dominated phonon transport. Moreover, the transition temperature is gradually reduced with increasing strain which is attributed to the polymer orientation. In summary, this work provides some fundamental insights into the thermal transport processes in SBR with different composition ratios of styrene and temperature, especially under tensile strain.

The Effect of Epoxidation on Strain-Induced Crystallization of Epoxidized Natural Rubber.

The effect of epoxidation on strain-induced crystallization (SIC) of epoxidized natural rubber (ENR) and mechanism are studied with synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and polarized infrared spectroscopy (P-IR). WAXD results reveal that appropriate epoxidation, for example, ENR-25 epoxidized with ≈25% isoprene units, can unexpectedly enhance the SIC of natural rubber (NR), resulting in the improvement of tear resistance. On the other hand, exorbitant epoxidation, for example, ENR-40 epoxidized with ≈40% isoprene units, depresses the SIC and weakens the mechanical properties of NR remarkably. P-IR studies reveal that epoxidation can promote the orientation of chain segments along the stretching direction, which plays a determining role on SIC of NR. Accordingly, hierarchical multiscale schematic models are proposed. This insight into epoxidation on SIC of ENR strongly suggests that ENR with appropriate epoxidation degree is a promising candidate material for the fabrication of high-performance engineering rubber products.

A combined molecular dynamics simulation and experimental method to study the compatibility between elastomers and resins.

C5 and C9 petroleum resins are widely used in the rubber industry and their softening, tackifying and reinforcing effects highly depend on their compatibility and interaction strength with the rubber matrix. Herein, we chose five commercially used petroleum resins and two industrial solution polymerized styrene-butadiene rubbers (SSBR). By employing atomistic molecular dynamics (MD) simulation, the influence of resin composition on the compatibility was studied. Results show that different compatibility orders obtained from the solubility parameter (δ), binding energy (E binding), mean square displacement (MSD), and the related self-diffusion coefficient (D s) match well with each other, and are consistent with our experimental solubility parameter data. More importantly, by calculating the non-bond energy (E non-bond) between single resin chain and rubber units (styrene unit, trans-1,4 unit, cis-1,4 unit, and vinyl unit), it was found that the styrene unit has the strongest interaction with resins, while the cis-1,4 unit has the weakest, which fits well with the solubility parameter result that resins have better compatibility with SSBR than cis-polybutadiene rubber (cis-BR). This chain/unit level MD method saves much time compared to the traditional chain/chain level method. In general, by combining MD simulation and experiments, our work provides some guidance to a compatibility investigation between rubbers and resins, and may promote design and development of high-performance resins and other new materials.

Synchronously Tailoring Strain Sensitivity and Electrical Stability of Silicone Elastomer Composites by the Synergistic Effect of a Dual Conductive Network.

The use of conductive polymer composites (CPCs) as strain sensors has been widely investigated. A wide range of strain sensitivities and high repeatability are vital for different applications of CPCs. In this study, the relations of the conductive filler network and the strain-sensing behavior and electrical stability under fatigue cycles were studied systematically for the first time based on the conductive polymethylvinylsiloxane (PMVS) composites filled with both carbon nanotubes arrays (CNTAs) and carbon black (CB). It was proved that the composites could be fabricated with large strain-sensing capability and a wide range of strain sensitivities by controlling the volume ratio of CNTA/CB and their amounts. Additionally, the CNTA/CB/PMVS composite with 3 vol % content of fillers showed high sensitivity (GF is 10 at 60% strain), high repeatability (the relative standard deviation (RSD) of the max R/R0 value is 3.58%), and electrical stability under fatigue cycles (value range of R/R0 is 1.62 to 1.82) at the same time due to the synergistic effects of the dual conductive network of CNTAs and CB. This could not be achieved by relying on a single CNTA or CB conductive network. This study may provide guidance for the preparation of high performance CPCs for applications in strain sensors.

Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity.

A homogeneous dispersion of hexagonal boron nitride nanosheets (BNNSs) in elastomers is obtained by solution compounding methods, and a high orientation of BNNSs is achieved by strong shearing. The composites show high thermal conductivities, especially when BNNS loading exceeds 17.5 vol%, indicating that the material is promising for thermal-management applications which need high thermal conductivity, low dielectric constant, and adequate softness.

Constructing covalent interface in rubber/clay nanocomposite by combining structural modification and interlamellar silylation of montmorillonite.

Strong interfacial interaction and nanodispersion are necessary for polymer nanocomposites with expectations on mechanical performance. In this work, montmorillonite (MMT) was first structurally modified by acid treatment to produce more silanol groups on the layer surface. This was followed by chemical modification of γ-methacryloxy propyl trimethoxysilane molecule (KH570) through covalent grafting with the silanol groups. (29)Si and (27)Al magic angle spinning (MAS) NMR results revealed the microstructural changes of MMT after acid treatment and confirmed the increase of silanol groups on acid-treated MMT surfaces. Thermogravimetric analysis indicated an increase in the grafted amount of organosilane on the MMT surface. X-ray diffraction (XRD) showed that the functionalization process changed the highly ordered stacking structure of the MMT mineral into a highly disordered structure, indicating successful grafting of organosilane to the interlayer surface of the crystalline sheets. The styrene-butadiene rubber (SBR)/MMT nanocomposites were further prepared by co-coagulating with SBR latex and grafted-MMT aqueous suspension. During vulcanization, a covalent interface between modified MMT and rubber was established through peroxide-radical-initiated reactions, and layer aggregation was effectively prevented. The SBR/MMT nanocomposites had highly and uniformly dispersed MMT layers, and the covalent interfacial interaction was finally achieved and exhibited high performance.

A Raman-scattering study on the net orientation of biomacromolecules in the outer epidermal walls of mature wheat stems (Triticum aestivum).

BACKGROUND AND AIMS: Raman spectroscopy can be used to examine the orientation of biomacromolecules using relatively thick samples of material, whereas more traditional means of analysing molecular structure require prior isolation of the components, which often destroys morphological features. In this study, Raman spectroscopy was used to examine the outer epidermal cell walls of wheat stems. METHODS: Polarized Raman spectra from the epidermal cell walls of wheat stem were obtained using near-infrared-Fourier transform Raman scattering. By comparing spectra taken with Raman light polarized perpendicular or parallel to the longitudinal axis of the cell, the orientation of macromolecules in the cell wall was investigated. KEY RESULTS: The net orientation of macromolecules varies in the epidermal cell walls of the different components of wheat stem. The net orientation of cellulose is parallel to the longitudinal axis of the cells, whereas the xylan and the phenylpropane units of lignin tend to lie perpendicular to the longitudinal axis of the cells, i.e. perpendicular to the net orientation of cellulose in the epidermal cell walls. CONCLUSIONS: The results imply that cellulose, lignin and xylan form a relatively ordered network that defines the mechanical and structural properties of the cell wall. Such results are likely to have a significant impact on the formulation of definitive models for the static and growing cell wall.