Influence of pressure on yield and fracture in polymers

Some new data on the effects of pressure on a poly(p-oxybenzoyl) polymer are presented and existing data on various other polymers are reviewed and analyzed. It is demonstrated that the effect of pressure on the elastic response of a polymer depends on the location of T_g relative to room temperature, and that the modulus-pressure data can be used to estimate the pressure shift of T_g. Also, the pressure coefficient of the modulus increase can be deduced from considerations of finite strain elasticity theory. There is a marked increase of tensile and compressive yield strengths with pressure and this can be interpreted in terms of a Mohr-Coulomb type of yield criterion. In some polymers, hydrostatic pressure inhibits cold drawing and reduces the elongation to fracture. However, in other polymers which at atmospheric pressure fracture prior to yielding, increasing the pressure above some critical value can cause significant increases in ductility. This effect is utilized to show that even rigid high-temperature polymers, like polymide, can be successfully cold extruded at room temperature if a proper high-pressure environment is present. The nature of the changes occurring near the brittle-ductile transition pressure have been investigated by use of the scanning electron microscope and the possible influence of the pressure medium has been examined. SEM pictures of fracture surfaces and fracture modes of various polymers will be presented.     

Crazes and fracture in polymers

A review of crazes in glassy thermoplastic polymers is presented with particular emphasis on those aspects of craze properties that influence and control fracture behavior. Both crazes as they normally occur, and crazes at the top of cracks are covered. The occurrence of crazes, their microstructure, the stress distribution within them and the nature of craze fibrils are discussed. Theoretical treatments of the effect of crazes on polymer fracture are reviewed.     

Possible manifestations of the quantum effect (Tunneling) in elementary events in the fracture kinetics of polymers

An elementary event in the kinetics of fracture of polymers, i.e., breaking of a stressed skeletal bond in a chain molecule, has been simulated by the decay of a loaded quantum anharmonic oscillator. The probability and the average time of expectation of the escape of a particle from the potential well in the Morse potential under the action of a tensile force have been calculated over a wide range of temperatures. It has been demonstrated that the escape of the particle occurs predominantly through the tunneling mechanism at low and medium temperatures and through a combination of the tunneling (under-barrier) and over-barrier (thermal-fluctuation) mechanisms with comparable contributions at high temperatures. The calculations have revealed that the participation of the tunneling mechanism in the kinetics of fracture of polymers manifests itself in a low-temperature athermal plateau in the temperature dependence of the breaking strength. A comparison between the calculated and experimental temperature dependences of the breaking strength for the oriented polymer polycaproamide has shown that the calculated and experimental results are in qualitative and quantitative agreement, which allows the conclusion that the tunneling mechanism can contribute to the fracture of polymers.     

Intrinsic route to melt fracture in polymer extrusion: a weakly nonlinear subcritical instability of viscoelastic poiseuille flow

As is well known, the extrusion rate of polymers from a cylindrical tube or slit (a "die") is in practice limited by the appearance of "melt fracture" instabilities which give rise to unwanted distortions or even fracture of the extrudate. We present the results of a weakly nonlinear analysis which gives evidence for an intrinsic generic route to melt fracture via a weakly nonlinear subcritical instability of viscoelastic Poiseuille flow. This instability and the onset of associated melt fracture phenomena appear at a well-defined ratio of the elastic stresses to viscous stresses of the polymer solution.     

Strengthening of highly heat-resistant liquid-crystal polymer fibers upon heat treatment

Fibers of fully aromatic thermotropic liquid-crystal polymers containing mesogenic groups in the backbone are prepared on laboratory machines that make it possible to vary the conditions of jet spinning and subsequent heat treatment. The thermal and mechanical properties of the polymer fibers are investigated using differential thermal and thermomechanical analyses and strength measurements. It is found that, after spinning under optimum conditions, the strength of the fibers at room temperature is approximately equal to 1 GPa. After heat treatment under specially chosen conditions, the strength increases by a factor of two or three due to an increase in the activation energy of fracture. The role of crystallization and cross-linking in strengthening of liquid-crystal polymer fibers is elucidated.     

On the mechanical behaviours of a craze in particulate–polymer composites

In polymeric composites, well-defined inclusions are incorporated into the polymer matrix to alleviate the brittleness of polymers. When a craze is initiated in such a composite, the interaction between the craze and the surrounding inclusions will greatly affect the composite’s mechanical behaviours and toughness. To the best knowledge of the authors, only little research work has been found so far on the interaction between a craze and the near-by inclusions in particulate–polymer composites. In the current study, the first time, the influences of the surrounding inclusions on the craze are investigated in particulate–polymer composites. The three-phase model is adopted to study the fracture behaviours of the craze affected by multiple inclusions. An iterative procedure is proposed to solve the stress intensity factors. Parametric studies are performed to investigate the influences of the reinforcing particle volume fraction and the shear modulus ratio on fracture behaviours of particulate–polymer composites.     

Buckling Behavior of Carbon Nanotubes Functionalized with Carbene under Physical Adsorption of Polymer Chains: a Molecular Dynamics Study

The buckling analysis of functionalized carbon nanotubes (CNTs) is of great importance for the better understanding of mechanical behavior of nanocomposites. The buckling behavior of carbene-functionalized CNTs (cfCNTs) under physical adsorption of polymer chains (cfCNTs/polymers) is studied in this paper by the classical molecular dynamics (MD) simulations. In this regard, to investigate the interactions between non-covalent polymer chains and cfCNTs, two different non-covalent functional groups, i.e. polycarbonate (PC) and polypropylene (PP), are selected. The findings are compared with those of pure CNTs under the physical adsorption of polymer chains (pCNTs/polymers). The obtained results show that at a given weight percentage of non-covalent functional groups, the gyration radius of cfCNTs/polymers is higher than that of pCNTs/polymers. Furthermore, an increase in the critical buckling force of cfCNTs/polymers is dependent on the type of non-covalent polymer chains. For cfCNTs/PC and cfCNTs/PP, the critical buckling force is respectively lower and higher than that of pCNTs/polymers for the similar weight percentage of non-covalent functional groups. In addition, it is found that the critical buckling strain of cfCNTs/polymers is smaller than that of pCNTs/polymers for the same weight percentage of non-covalent polymer chains.     

Amorphous solidification in polymer-platelet nanocomposites

Computer simulations are used to understand the molecular basis of the rheology changes in polymer melts when loaded with platelet filler particles, specifically when the polymer and nanofiller interact attractively. With decreasing temperature, there is increasing aggregation between chains and filler and an increase in the polymer matrix structural relaxation time. These lifetimes are predicted to diverge at an extrapolated temperature, which we identify with the emergence of an amorphous solid state. Our findings suggest that filled polymers are phenomenologically similar to solutions of associating polymers and to supercooled liquids near their glass transition.     

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 2. Grafting and crosslinking of interphase materials

_—This paper presents an interphase engineering technique suitable for grafting silane-modified polymers onto glass fibres to be used in composites with enhanced impact tolerance. The silane-modified polymers include ethylene polymers grafted with γ-methacryloxypropyltrimethoxysilane (MPS) and a copolymer of butyl acrylate (BuA) and MPS. The grafting of functionalized interphase materials onto glass fibres is performed in solution. By changing the concentrations of the solutions, different amounts of polymer can be deposited on the fibres. Water crosslinking of the polymer gives the possibility of producing stabilised interfacial polymer coatings over a range of thicknesses. It is concluded that acidic conditions (1) promote the grafting of silane-modified polymers on glass fibres and (2) for a given reaction time, increase the amount of crosslinked polymer in the interphase, i.e. yield more stable interphases. It is also likely that preserving acidic conditions at the fibre/polymer interface is important for maintaining bonding across the interface. It is shown that polystyrene/glass-fibre composites having SEBS at the interface are promising candidates for high-impact-tolerance composites.     

Micromechanics of polymers

Polymers possess a very large inherent capacity for property modifications. The bridge between structure or morphology and mechanical properties is created by the micromechanical processes of deformation and fracture, the “micromechanics.” Developments mainly in electron microscopy (EM) (scanning, transmission, and high-voltage electron microscopy) and scanning force microscopy (SFM) opened up a wide range of experiments previously impossible, including the in situ study of micromechanical processes. These new techniques are reviewed and used to study micromechanical properties of amorphous and semi-crystalline polymers and several toughened polymers. On the basis of the detailed knowledge of micromechanical mechanisms, a new method of polymer modification becomes a realistic possibility, a method of micromechanical construction of new polymeric systems.     

Solid-like rheological response of non-entangled polymers in the molten state

We show that non-entangled polymers display an elastic-like behaviour at a macroscopic scale (probed at some 0.100 mm thickness) up to at least hundred degrees above the glass transition temperature. This observation, found under non-slippage conditions, both for side-chain liquid crystalline polymers and ordinary polymers, is in contradiction with the typically found flow behaviour of polymer melt. Our measurements were carried out with a conventional rheometer at thicknesses of several tenths millimetres. Thus, we were probing bulk properties. The observed elasticity supposedly implies that even in the melt the chains experience a cohesive effect of macroscopic distances, involving collective motions over time scales longer than the individual relaxation time of an individual polymer chain. The detection of such a solid-like property of molten non-entangled polymers is of considerable importance for a better understanding of the polymer dynamics.     

导电聚合物自组装纳米器件

导电聚合物自20世纪70年代以来得到了广泛的研究．然而，关于聚合物纳米器件的研究则鲜有报导．从纳米尺度上研究导电聚合物，不仅有利于从更小的尺度上解析聚合物的光电性能、电荷传输机理，也可以将导电聚合物和纳米电子学有机地结合起来，发展聚合物纳米电子学的研究．文章介绍了最近由胡文平等采用自组装的方法构筑的聚合物纳米器件和在纳米器件中观察到的一些有趣的现象．     

Nanocellulose enhanced interfaces in truly green unidirectional fibre reinforced composites

One of the main problems in fabricating natural fibre reinforced polymers is the poor adhesion between intrinsically polar plant fibres and non-polar polymer matrices. We have developed a truly green technique of modifying natural fibre (hemp and sisal) surfaces to improve the interaction between the fibres and polymers by attaching nano-scale bacterial cellulose to the fibre surfaces. These modified natural fibres were then incorporated into the renewable polymers cellulose acetate butyrate (CAB) and poly-L-lactic acid (PLLA). Unidirectional natural fibre reinforced composites were manufactured to investigate the impact of the surface modification on the fibre and interface dominated composite properties. Both the tensile strength parallel as well as perpendicular to the fibres of the composites reinforced by bacterial cellulose modified natural fibres were found to increase significantly, especially in the case of a PLLA matrix. In case of modified sisal reinforced PLLA the parallel strength increases by 44% and the off-axis composite strength by 68%. Scanning electron microscopy observations of the composite fracture surfaces confirm the improved interaction between the fibre and the polymer matrix.     

The fracture mechanics of glassy polymers

The application of fracture mechanics to glassy polymers, in particular crack growth in PMMA, is discussed. Particular attention is paid to two processes which modulate the energy supply to the crack tip: viscoelastic dissipation at slow crack speeds and specimen inertia at large crack speeds. The relation between fracture energy and crack speed is reviewed, and, where possible, fracture surface observations are correlated with dynamic behavior.     

Novel properties of nanoscale organic–inorganic systems and photonic crystals — conducting polymers in nanoscale periodic structures,microcavities and photonic crystals

Conducting polymer/C₆₀and C₆₀doped conducting polymer/C₆₀heterojunctions have been fabricated and found to exhibit remarkably enhanced photoresponse due to the highly effective photoinduced charge transfer at the interface. In conducting polymer/C₆₀alkali metal nanoscale composite systems, multiphase superconductivity has been clarified and explained by taking the coupling of nanoscale grains by Josephson junctions into consideration. As examples of intramolecular organic-inorganic combined systems, unique electrical and optical characteristics have been revealed in oligosilanylene oligophenylene polymers. Electroluminescence has been demonstrated in organic-inorganic junction devices such as conducting polymer/porous Si and conducting polymer/diamond junctions. Conducting, polymer-based nanoscale multilayer systems have been studied utilizing molecular self-assembly method and novel photosensitive characteristics have been revealed. \indent Novel optical and electrical properties of conducting polymers infiltrated in a photonic crystal, synthetic opal made of SiO₂spheres of several hundred nm in diameter, and also a conducting polymer replica have been revealed. A clear diffraction pattern was observed in a photonic crystal infiltrated with conducting polymers, and transmission spectra are dependent on various ambient conditions. Their photoluminescence (PL) spectra, spectral narrowing of PL and lasing characteristics at relatively low optical excitation have also been clarified. Novel conducting characteristics of conducting polymers in a photonic crystal that was prepared by pyrolysis of a polymer replica of opal have also been observed.     

Hydrophilic surface formation on polymers and its applications

A new surface modification technique, so-called ion assisted reaction (IAR) has been developed at the Korea Institute of Science and Technology (KIST) Ion Beam Laboratory while modifying the surface of polymer results in many of industrial applications. The IAR, in which a keV ion beam is irradiated on the surface of polymer in reactive gases environment, has been developed for improving wettability of polymer surface and enhancing adhesion of other materials. The contact angles of water drops with modified polymers were significantly reduced by Ar⁺ ion irradiation with flowing oxygen gas environment than without flowing oxygen gas. Change of contact angles for the modified polymers was explained by a two-step chemical reaction among polymer matrix, energetic ions and oxygen gas. X-ray photoelectron analysis showed that hydrophilic groups were formed on the surface of polymers by chemical reaction between the unstable chains induced by ion irradiation and the oxygen gas, and the hydrophilic groups were identified as –(C–O)–, –(CO)– and –(CO)–O– bonds. The enhanced adhesion between metal and modified polymers was explained by the formation of charge transfer complex in polymer and electron donors in metal. Possible industrial applications of the IAR are to be discussed.     

The utility of resonant soft x-ray scattering and reflectivity for the nanoscale characterization of polymers

The utility of resonant soft x-ray scattering (RSoXS) and reflectivity (RSoXR) is extended and exemplified through the characterization of thin films of polymers relevant to organic solar cells and of dilute polymer solutions. RSoXS and RSoXR are methods that utilize anomalous scattering principles at soft x-ray energies. Soft X-rays cover the carbon, nitrogen and oxygen absorption edges, elements very relevant for polymers and colloids. The rapid changes of optical properties near these absorption edges provide selectivity to specific moieties and high contrast. RSoXR is shown to be a powerful tool for the characterization of bilayers of conducting polymers. The RSoXR results point to an interesting strategy that will allow the chemical interdiffusion and physical roughness at a buried polymer/polymer interface to be determined independently. The high scattering cross sections also allows the investigation of thin films of conjugated polymer blends in transmission at thicknesses for which hard X-rays or neutrons would yield relatively little scattering. By scattering at photon energies that provide strong scattering contrast, even very dilute polymeric solutions yield a useable signal.     

Refractive index dispersion measurement on nonlinear optical polymer using V-prism refractometer

By using V-prism refractometer, the refractive indices of a polyetherketone (PEK-c) guest–host polymer system were measured with the polymer in solutions. The Lorenz–Lorentz local field formalism was used in the calculation of the refractive indices of the polymers from the measured indices of the polymer solutions and the pure solvent by using V-prism refractometer. The refractive index dispersions of the polymers were obtained by fitting the measured indices of the polymers to Sellmeyer equation. The method allows for an accuracy in index of 0.7% in the determination of the polymer indices. In addition, a large difference between the indices of the polymer and the solvent, and a higher polymer volume fraction in the measured polymer solution are favorable for a high accuracy.     

Laser Raman spectroscopic investigations of biodegradable vehicle of active agents eluting LVM 316 stainless steel cardiovascular stents for in vivo degradation characteristics

Laser Raman spectroscopy is an effective tool for the study of biodegradable polymers, which play a vital role in the new developments in coronary implants such as stents. There is much excitement around the potential capabilities of synthetic biodegradable polymers and the effect they will have on the design and function of implanted devices. In the present investigation, heparin‐conjugated biodegradable copolymers were evaluated for their durability as drug‐eluting stent coatings. Laser Raman spectroscopic studies were carried out and spectra recorded and analyzed of explanted stents coated with different amounts of polymer alone, showing the existence of different levels at different quantities of polymer. The polymer was detected on every stent analyzed. On the stents coated with a thick layer of polymer, a firm layer of polymer still existed on the stent. In contrast, this layer was degraded and spread around on the stents coated with only a thin layer of the polymer. This indicates that the polymers used in the stents in the present investigation exhibit acceptable biodegradability. Such polymers can be used as efficient drug carriers, as these materials show good degradation after the stipulated period. Copyright © 2009 John Wiley & Sons, Ltd.     

Morphology,miscibility and mechanical properties of PMMA/PC blends

This study deals with some results on morphology, miscibility and mechanical properties for polymethyl methacrylate/polycarbonate (PMMA/PC) polymer blends prepared by solution casting method at different concentration between 0 and 100 wt%. Dynamic storage modulus and tan δ were measured in a temperature range from 30 to 180°C using dynamical mechanical analyzer (DMA). The value of the storage modulus was found to increase with the addition of the PC in the matrix. Transition temperature of pure PMMA and pure PC is found to be 83.8 and 150°C, respectively. The result shows that the two polymers are miscible for whole concentration of PC in PMMA. The distribution of the phases in the blends was studied through scanning electron microscopy (SEM). Also the mechanical properties like elongation at break and fracture energy of the PMMA/PC blends increase with the increase in concentration of PC in PMMA.