Experimental study on interfacial shear transfer in partially-debonded aluminum/epoxy joint

A six-step phase-shifting method is applied to calculate the whole-field shear stress of an adhesively bonded aluminum alloy-to-epoxy joint containing an initially debonded interfacial crack for studying shear transfer behavior. For a well-bonded interface, the isochromatic fringe order and the interfacial shear stress (ISS) distribute continuously and increase with compression. The epoxy corner formed at the right-angle edges of the bonded interface during specimen preparation exhibits more than eight orders of dense fringes and a maximum shear stress of 1.4 MPa under a load of 3.0 kg. The load is transferred by a shear band that connects the bonding interface to the support.A denser isochromatic pattern occurs at the crack-tip than at the right-angle edge from the partially debonded interface. The crack-tip displays about seven fringe orders, 1.3 MPa of ISS under a load of 3.0 kg, and an increase of about 3 mm in the interfacial crack length. The shear stress decreases rapidly at the debonded interface but takes 26% of the shear force of the entire interface, indicating that the debonded interface obstructs and decreases the load transfer capability from the bonding interface to the support.As the curing temperature decreases to 20 °C, a thermal residual shear stress appears on the interface because of the discrepancy in the coefficients of thermal expansion between the aluminum alloy and the epoxy. The residual shear stress redistributes on the bonded and debonded interfaces due to the formation of the initial crack induced by an external load. The calculated effective stress intensity factors (SIFs) of the interface crack are identical to theoretical prediction.     

Influence of nanoparticle-polymer interactions on the apparent migration behaviour of carbon nanotubes in an immiscible polymer blend

We investigate the influence of nanoparticle-polymer interactions on the apparent migration behavior of multiwall carbon nanotubes (CNTs) in an immiscible polymer blend of ethylene-acrylate copolymer (EA) and polyamide 12 (PA). The polymer-CNTs interaction is tuned by using different surface modification strategies, comprising grafting and coating. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) are chosen as surface modifiers. The nanocomposite materials are prepared by melt-blending polymer-modified-CNTs in EA and PA. Polymer-grafted-CNTs tend to concentrate at the PA/EA interface, even if predispersed in PA, as opposed to pristine CNTs, which stay inside PA under the same circumstances. This new behavior is consistent with the morphology of PA/EA/(PMMA or PS) ternary blends and suggest a dominance of interfacial thermodynamics on CNTs localization. If we use polymer-coated-CNTs instead, the behavior depends on molar mass of the coating polymer. For low molar mass, it is similar to that of pristine CNTs and indicates desorption of the coating, owing to the weak interaction with the CNTs surface. Interestingly, we observe that long PS chains do not desorb and can drive the CNTs to the interface of the PA/EA blend. Moreover, the influence of kinetics is clearly observed through the dependence of CNTs interfacial confinement on dispersed droplet size.     

玻璃纤维增强改性聚苯醚的性能研究

通过熔融挤出的方式制备了玻璃纤维增强改性聚苯醚材料(mPPE)。探讨了自制界面改性剂LJ01含量对mPPE的力学性能和热氧老化性能的影响。研究发现:LJ01能明显改善mPPE中树脂材料与玻璃纤维的界面结合能力,mPPE的机械性能和热氧老化性能均得到了显著的改善,当LJ01用量达到3 phr时,热氧老化后的缺口冲击强度保持率在92%以上。     

Acid-functionalized fullerenes used as interfacial layer materials in inverted polymer solar cells

Two types of carboxylic acid functionalized fullerence derivatives, 4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (p-EHO-PCBA) and bis-4-(2-ethylhexyloxy)-[6,6]-phenyl C₆₁-butyric acid (bis-p-EHO-PCBA), were synthesized and investigated as an interfacial layer for inverted polymer solar cells (iPSCs). The –COOH groups on the PCBAs chemisorb to inorganic metal oxide (TiO_X), generating fullerene-based self-assembled monolayers (FSAMs). The devices with the mono- and bis-FSAMs exhibited substantially lower series resistance (R_S) values of 2.10 Ω cm² and 1.46 Ω cm², compared to that (4.15 Ω cm²) of the unmodified device. The TiO_X films modified with mono- and bis-FSAMs showed higher contact angles of 50° and 91°, respectively, than that of the pristine TiO_X film (33°). The increased contact angles were attributed to the enhanced hydrophobicity, improving the wetting properties with the organic photoactive layer. In addition, a comparison of device characteristics with electroactive FSAMs and non-electroactive benzoic acid SAMs clearly indicates that the FSAMs may suggest an additional pathway for photo-induced charge transfer and charge collection to ITO. After surface modification with FSAMs, the short-circuit current density (J_SC) and fill factor (FF) values increased substantially. The iPSCs based on poly(5,6-bis(octyloxy)-4-(thiophen-2-l)benzo[c][1,2,5]thiadiazole) (PTBT) and [6,6]phenyl-C₆₁-butyric acid methyl ester (PCBM) as an active layer showed remarkably improved power conversion efficiency up to 5.13% through incorporation of the FSAMs-based interfacial layer.     

Controllable threshold voltage of a pentacene field-effect transistor based on a double-dielectric structure

The authors report controllable threshold voltage (V_th) in a pentacene field-effect transistor based on a double-dielectric structure of poly(perfluoroalkenyl vinyl ether) (CYTOP) and SiO₂. When a positive switching voltage is applied to the gate electrode of the transistor, electrons traverse through the pentacene and CYTOP layers and subsequently trapped at the CYTOP/SiO₂ interface. The trapped electrons induce accumulation of additional holes in the pentacene conducting channel, resulting in a large V_th shift from ?4.4 to +4.6 V. By applying a negative switching voltage, the trapped electrons are removed from the CYTOP/SiO₂ interface, resulting in V_th returning to an initial value. The V_th shift caused by this floating gate-like effect is reversible and very time-stable allowing the transistor to be applicable to a nonvolatile memory that has excellent retention stability of stored data.     

Probing the Physics of Slip–Stick Friction using a Bowed String

Slip–stick vibration driven by friction is important in many applications, and to model it well enough to make reliable predictions requires detailed information about the underlying physical mechanisms of friction. To characterize the frictional behavior of an interface in the stick–slip regime requires measurements that themselves operate in the stick–slip regime. A novel methodology for measurements of this kind is presented, based on the excitation of a stretched string “bowed” with a rod that is coated with the friction material to be investigated. Measurements of the motion of the string allow the friction force and the velocity waveform at the contact point to be determined by inverse calculation. These friction results can be correlated with microscopic analysis of the wear track left in the coated surface. Results are presented using rosin as a friction material. These show that “sticking” involves some temperature-dependent shear flow in the friction material, and that the exact definition of the states of “sticking” and “slipping” is by no means clear-cut. Friction force during slipping shows complex behavior, not well correlated with variations in sliding speed, so that other state variables such as temperature near the interface must play a crucial role. A new constitutive model for rosin friction, based on the repeated formation and healing of unstable shear bands, is suggested.     

ON THERMODYNAMICS OF THIN FILMS: THE MECHANICAL EQUILIBRIUM CONDITION AND CONTACT ANGLES

A thermodynamic model for plane parallel thin liquid films applicable to solid-liquid-vapor systems was presented using the detailed method. The film was modeled as a bulk phase bound by two dividing surfaces. The thermodynamic thickness of the film was established as well as excess properties such as film tension. The analysis using this model yielded disjoining pressure definition identical to the literature reports. The effect of definition for contact angle on the resulting mechanical equilibrium condition was also demonstrated. It was concluded that from a theoretical perspective it is important to clearly define contact angles as the angle a sessile drop makes with either the solid phase or the thin film. However, on a practical level for most cases, the difference between using either of the two mechanical equilibrium conditions to determine film tension or contact angle will be minimal (ascertained by an order of magnitude analysis). The attempt was also made to bring about clarity concerning some of the questions in the literature regarding the thermodynamic model for thin films presented by Li and Neumann.     

Metalizable Polymer Thin Films in Supercritical Carbon Dioxide

We report an environmentally “green” method to improve adhesion at a polymer/metal interface by using supercritical carbon dioxide (scCO₂). Spun-cast polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films on cleaned Si wafers were used for this study. Film thicknesses of both polymer films were prepared in the range of 100 Å to 1600 Å. We exposed the films to scCO₂ in the pressure-temperature (P–T) range corresponding to the density-fluctuation ridge, where the excess swelling of both polymer films occurred, and then froze the swollen structures by quick evaporation of CO₂. A chromium (Cr) layer with film thickness of 300–400 Å was deposited onto the exposed film by using an E-beam evaporator. X-ray reflectivity (XR) measurements showed that the interfacial width between the Cr and exposed polymer layers increased by a factor of about two compared with that without exposure to scCO₂. In addition, the large interfacial broadening was found to occur irrespective of the thickness of both polymer films. After the XR measurements, the dewetting structures of the PS/Cr films induced by additional annealing were characterized by using atomic force microscopy, showing improved surface morphology in the exposed films. Contact angle measurements showed that a decrease in interfacial tension with exposure to scCO₂ accompanied the increase in interfacial width.     

Reactions and Intermediates at the Metal-Polymer Interface as Observed by XPS and NEXAFS Spectroscopy

Potassium or chromium were evaporated by means of a Knudsen effusion cell under ultra-high vacuum conditions onto a number of common polymers, prepared as stretched foils and spin-coated films. The metal-polymer interface was studied by X-ray Absorption and X-ray Photoelectron Spectroscopy. Evaporated samples were analyzed without exposure to the atmosphere. Different general types of reactions of the metal atoms with the polymers were observed. With deposited K and Cr a redox process including the transfer of substrate oxygen atoms across the interface was found. The formation of π-electron complexes and covalent metal-carbon bonds were obtained exclusively with chromium. Aromatic rings, carbonyl groups and, to a lesser extent, ether linkages are scissioned by metal-polymer interactions. The fourfold substitution of aromatic rings and the exclusive existence of C ─ O ─ C structures within polyphenylene ether (PPE) make this polymer most stable toward reactions with chromium. In contrast, bisphenol-A polycarbonate is susceptible to redox reactions with the carbonate group and to forming Cr complexes with aromatic rings using the chromium 3d orbitals. The result is a succession of complex formation, followed by Cr substitution onto aromatic rings and destruction of rings with the formation of chromium carbides.     

Conjugated polyelectrolytes: A new class of semiconducting material for organic electronic devices

This feature article presents a short review of the recent developments in the synthesis of conjugated polyelectrolytes (CPEs) along with their applications in organic optoelectronic devices with particular focus on the molecular structures of CPEs with ionic functionality, synthetic approaches, and their utilization as an interfacial layer. The orthogonal solubility of the CPEs allows the simple preparation of multilayer organic devices by solution casting on top of a nonpolar organic photoactive layer without disturbing the interfaces, showing their effectiveness in tuning the electronic structures at the interfaces for improving the charge carrier transport and resulting device properties. These achievements highlight the dynamic nature of CPEs and their applicability to a wide range of optoelectronic devices.