The properties of segmented‐copolymer‐based H‐bonding and non‐H‐bonding crystallisable segments and poly(tetramethylene oxide) segments were studied. The crystallisable segments were monodisperse in length and the non‐hydrogen‐bonding segments were made of tetraamidepiperazineterephthalamide (TPTPT). The polymers were characterised by DSC, FT‐IR, SAXS and DMTA. The mechanical properties were studied by tensile, compression set and tensile set measurements. The TPTPT segmented copolymers displayed low glass transition temperatures (Tg, ?70 °C), good low‐temperature properties, moderate moduli (G′ ≈ 10–33 MPa) and high melting temperatures (185–220 °C). However, as compared to H‐bonded segments, both the modulus and the yield stress were relatively low.
The long‐term viscoelastic behavior of reinforced all‐poly(propylene) composites was studied by flexural creep tests. Both unidirectional and cross‐ply laminates were prepared from PURE® coextruded tapes by vacuum bag molding in an autoclave. The specimens were subjected to isothermal creep tests at different temperatures ranging from 20 to 80 °C under an applied load. The time‐temperature superposition principle was verified for the creep data. An Arrhenius type relationship was found to better describe the shift data obtained from the creep tests. The activation energies relating to the different reinforcement architecture and different relaxation process were calculated.
The tensile deformation of materials with Poisson's ratio smaller than 0.5 generates an additional free volume, which means that tensile creep under constant stress and temperature is a non‐iso‐free volume process. Fractional free volume rising proportionally to the creep strain accounts for a continuous shortening of retardation times. To account for this effect, “internal” time has been introduced which is related to a hypothetical pseudo iso‐free‐volume state. The shift factor along the time scale in the time‐strain superposition is not constant for an isothermal creep curve, but rises monotonically from point to point with the elapsed creep time. The reconstructed compliance dependencies obtained for various stresses approximately obey the time‐strain superposition thus forming a generalised creep curve. A routinely used empirical equation has been found suitable to describe the effects of time and stress on compliance of parent polymers and their blends. The previously proposed predictive format for the time‐dependent compliance of polymer blends has been found applicable also to poly(propylene) (PP)/cycloolefin copolymer (COC) blends with fibrous morphology. As COC shows a tendency to form fibres in a PP matrix, the mixing rule customarily used for fibre composites has been found more appropriate for injection moulded specimens than the equivalent box model for isotropic blends. The predicted compliance curve for a pseudo iso‐free‐volume state can be transformed into a “real” curve for a selected stress σ (in the interval up to the yield stress).
SEM microphotograph of the fractured surface (perpendicular to the injection direction) of the PP/COC blend 60/40. 相似文献
The most widespread application of polymers in structural applications is their use as pipe material for e.g., gas distribution systems. Pipes have a design lifetime of typically 50 years, which rules out real‐time lifetime assessment methods. Here, an engineering approach is presented, which makes it possible to predict long‐term ductile failure of loaded glassy polymers based on short‐term tests. The approach is based upon the hypothesis that failure is governed by accumulation of plastic deformation up to a critical strain. A pressure‐modified Eyring relation is employed to calculate the accumulation of plastic strain for any simple loading geometry. It is demonstrated that the approach can produce accurate quantitative time‐to‐failure predictions for loaded PC specimens and uPVC pipe segments.
The viscoelasticity of two thermally crosslinked polymer coatings was examined in terms of relaxation of the applied stress after a sudden strain. Two different transient methods were utilized: flat‐ended cylindrical indentation testing of a polymer film on a rigid substrate and tensile testing of a corresponding free‐standing polymer film. The correlation between tensile and indentation tests was studied. The mechanical response of a viscoelastic layer deposited on a rigid substrate was investigated as a function of indentation depth. There was good agreement between the results of the tensile and indentation tests for thick film layers at moderate indentation depths. The findings indicate that the substrate influences the coating performance by reducing the viscous contribution to the stress response and amplifying the magnitude of the equilibrium modulus for large indentation depths. The indentation method utilized here was shown to be a potentially suitable tool for the determination of Poisson's ratio of polymer films.
Soft coatings are widely used to tailor the surface chemistry of materials without altering their bulk properties. However, the strength of adhesion between the coating and the substrate must be high enough for long‐term applications. This has become a major challenge in the medical field, especially for polymer‐coated stents, mainly due to several coating failures reported after mechanical expansion during clinical implantation. In this work, the applicability of current polymer‐metal adhesion tests to polymer‐coated stents is discussed. The small punch test was proposed as an adhesion test that allows fundamental studies on the adhesion and coating properties. This adhesion test was applied to thin fluorocarbon coatings deposited by plasma on 316L stainless steel.
Summary: The success of the use of layered silicates in polymer nanocomposites, to improve physical and chemical properties is strictly related to a deeper knowledge of the mechanistic aspects on which the final features are grounded. This work shows the temperature induced structural rearrangements of nanocomposites based on poly[ethylene‐co‐(vinyl acetate)] (EVA) intercalated‐organomodified clay (at 3–30 wt.‐% silicate addition) which occur in the range between 75 and 350 °C. In situ high temperature X‐ray diffraction (HT‐XRD) studies have been performed under both nitrogen and air to monitor the modifications of the nanocomposite structure at increasing temperatures under inert/oxidative atmosphere. Heating between 75 and 225 °C, under nitrogen or air, causes the layered silicate to migrate towards the nanocomposite surface and to increase its interlayer distance. The degradation of both the clay organomodifier and the VA units of the EVA polymer seems to play a key role in driving the evolution of the silicate phase in the low temperature range. The structural modifications of the nanocomposites in the high temperature range (250–350 °C), depended on the atmosphere, either inert or oxidizing, in which the samples were heated. Heating under nitrogen led to deintercalation and thus a decrease of the silicate interlayer space, whereas exfoliation was the main process under air leading to an increase of the silicate interlayer space.
Heat induced structural modification of EVA‐clay nanocomposite under nitrogen and air. 相似文献
Electrically conducting films containing AgNws, hydrophilic and hydrophobic resins were prepared. FT‐IR reveals that the interface between the AgNws and epoxy could be successfully modified by APTES. XPS shows that the AgNws were attracted by hydrogen bonds of ? NH2 and ? NH? groups after APTES modification. SEM analysis shows that the AgNws were well dispersed in the resin. The AgNws were also blended with hydrophilic and acrylic resins, and the resulting blends were compared with AgNws/epoxy blends. Results show that AgNw/PVA‐resin films possess the lowest surface electrical resistance. The AgNw/PVA‐resin and silane‐modified AgNw/epoxy resin conductive films possess a similar electrical percolation threshold.
Summary: A solid lubricant composite material was prepared by compression molding PTFE and acid treated nano‐attapulgite. The friction and wear tests were performed on a block‐on‐ring wear tester. Scanning electron microscopy (SEM), energy‐dispersive X‐ray spectrometer (EDS) and DSC were utilized to investigate material microstructures and examine modes of failure. Experimental results showed that there was no significant change in coefficient of friction, but the wear rate of the PTFE composite was orders of magnitude less than that of pure PTFE. Acid treated nano‐attapulgite was superior to untreated nano‐attapulgite in enhancing the wear resistance of PTFE. Moreover, the wear resistance of the composite increased monotonically with increasing treated attapulgite concentration. Investigation of transfer film and analysis of debris for PTFE and its composite showed that acid treated nano‐attapulgite filled to PTFE could facilitate formation of transfer film on the steel ring surface and inhibit breakage of PTFE molecular chain. The PTFE composite with higher heat absorption capacity exhibited improved wear resistance. Furthermore, the steel ring counterface abrasion was not found.
Effect of load on the wear rate of PTFE and its composites. 相似文献
Elastomer materials are used in a wide application range and subjected to different loading from which failure of the material results. Because this failure is caused by initiation and propagation of cracks, the application of fracture mechanics methods for the assessment of the material is obvious. A short summary of the methods of technical fracture mechanics including possibilities of determination of crack resistance curves is given. Vulcanizates on the basis of SBR 1500 with various sulfur and carbon black contents were investigated. For describing the crack initiation and crack propagation behavior, several fracture mechanics examination methods were applied. Tear‐analyzer results were used to assess the crack propagation behavior under fatigue‐like loading conditions. Furthermore, for the characterization of the crack resistance of the materials under impact‐like loading conditions, instrumented tensile‐impact tests were performed. To obtain information about the initiation and propagation of a stable crack, quasi‐static fracture mechanics tests were applied. The results of the several tests are discussed in dependence on sulfur and carbon black contents. We found a non‐monotonous behavior of the toughness as a function of carbon black loading. An explanation is given in connection with a percolation‐like transition in filler morphology on larger length scales.
Schematic crack propagation curve for characterizing the fatigue behavior of the vulcanizates recorded in a TFA test. 相似文献
The objective of this work was to investigate the influence of irradiation conditions on grafting of styrene into tetrafluoroethylene‐hexafluoropropylene‐vinylidene fluoride (THV) terpolymer films. Stress–strain measurements, infrared spectroscopy and electron paramagnetic resonance spectroscopy have been used to characterize the pre‐irradiated polymer films regarding tensile strength, elongation at break, changes in the chemical structure and concentration of trapped radicals, respectively. Main‐chain scissions associated with the formation of carbonyl end groups and end‐chain double bond structures have been identified to be the reason for a moderate deterioration of the mechanical properties of the pre‐irradiated films. The yield of grafting has been found to be influenced by THV grade, irradiation temperature and concentration of cross‐linker.
A series of methyl, benzyl, and mixed polybenzimidazolium halides was synthesised and characterised by NMR spectroscopy. Membranes were formed and ion exchanged with hydroxides. These membranes are of interest for use in potentially platinum‐free anionic exchange membrane fuel cells. Crosslinked membranes were obtained by the addition of α,α′‐dibromo‐p‐xylene to the casting solution. The ion conductivity of membranes was determined by impedance spectroscopy. A hydroxide conductivity of 29 mS · cm?1 at 26 °C and 58 mS · cm?1 at 60 °C was obtained. The thermal and hydrolytic stability was investigated and a pathway for hydrolytic degradation proposed. Hydroxide ions react at the 2 position, the intermediate carbinol opens to the amine–amide, and further degrades under chain scission to diamine and carboxylic acid.
A special unilateral NMR sensor has been designed for investigations of thin samples with a thickness of less than 1 mm and of surface effects of polymers. For use with the bar‐magnet NMR‐MOUSE®, the so‐called “crazy coil” is introduced with a low penetration depth. It is a flat meander coil etched on a printed circuit board with wiggles in the conductors. The design of the new coil and FEM simulations of the B1 field are presented. Different applications are discussed by means of illustrative examples. They are the detection of surface damage in rubber samples, the swelling and drying of a latex membrane exposed to cyclohexane vapor mimicking a chemical sensor, and the drying of a thin sprayed adhesive layer.
Synthesis, structure, and properties of rigid‐rod polymers with special emphasis on poly(p‐phenylene benzobisoxazole) (PBO) and poly(p‐phenylene benzobisthiazole) (PBZT) have been reviewed. Recent studies on chemical modifications and molecular simulations have also been given. After nearly 20 years of research and development, PBO fiber was commercialized in the late 1990s. However, due to processing difficulties, the concept of the so called molecular composites has not been successful. Development of the high compressive strength M5 and dihydroxy‐PBI fibers clearly suggest that there is potential for further developing properties of this class of materials. Opto‐electronic properties have also been reviewed.
Summary: Compacted fiber composites offer unique properties due to their lack of an extraneous matrix. The conditions of processing ultra‐high molecular weight polyethylene (UHMWPE) fibers were simulated in a heated pressure cell. In situ X‐ray diffraction measurements were used to follow the relevant transitions and the changes in the degree of crystallinity during melting and crystallization. The results strongly support the suggestion that the hexagonal crystal phase, in which the chain conformation is extremely mobile on the segmental level, constitutes the physical basis of compaction technologies for processing UHMWPE fibers into a single‐polymer composite. This report suggests that using a pseudo‐phase diagram outlining the occurrence of different phases during slow heating and the degree of crystallinity can provide valuable insight into the technological parameters relevant for optimal processing conditions.
Degree of crystallinity as a function of pressure and temperature in a region relevant to compaction processes. 相似文献
Summary: Segmented block copolymers, consisting of non‐polar soft segments from hydroxyl‐terminated liquid natural rubber (HTNR) and polar hard segments from α,ω‐diisocyanato telechelics obtained by “criss‐cross”‐cycloaddition, have been synthesized. The block copolymer formation took place under relatively mild reaction conditions at 80 °C in dichloroethane in the presence of dibutyltin dilaurate as a catalyst. The resulting block copolymers were characterized by spectroscopic techniques (1H NMR, FTIR, UV‐vis spectroscopy) as well as GPC for molar mass determination. The block copolymers were compression molded in a hot stage press, and the resulting samples were characterized by DSC and stress‐strain measurement. The solubility and phase morphology of the materials have also been studied.
Segmented block copolymer from HTNR and α,ω‐diisocyanato telechelics 相似文献
Summary: The recycling of post‐consumer plastics leads, in general, to secondary materials having properties worse than those of the reclaimed material and certainly worse than those of the same virgin polymer. This is because of the degradation undergone by the objects during their use and because of the thermo‐mechanical degradation undergone during the reprocessing operations. The change of the molecular architecture is responsible for this worsening of properties. The use of stabilizing systems can slow the degradation during the melt processing but cannot give any improvement of the final properties of the material. In order to enhance the properties of the recycled plastics, some rebuilding of the molecular structure is necessary. The use of suitable additives can enlarge the molecular weight distribution or can create branching and cross‐linking during the melt processing of the photo‐oxidized PE. The processability in film blowing and the mechanical properties of these secondary materials are reported in this work. The rheological behavior, the filmability and most of the mechanical properties of the secondary PE with the rebuilt molecular structure are better than those of the post‐consumer material and similar to those of the virgin polymer.
TS in the machine and in the transverse direction for all the samples extruded at 50 rpm. 相似文献
Summary: A new strategy for the synthesis of composite polymers with larger volume fraction of aqueous inclusions less than 1 µm in diameter is presented. A water‐in‐oil miniemulsion of aqueous droplets in a continuous, cross‐linkable monomer phase is prepared. The addition of an organo‐gelator allows the immobilization of the droplets in a solid gel, thus avoiding the usual demixing upon polymerization of the continuous phase. This pregelled system is then converted into a composite polymer by photoinitiated free radical polymerization. Such coatings may be used for an improved climate control of buildings or as a deposit for the controlled release of actives from polar nano‐droplets.
SEM image of a cross‐linked composite polymer showing controlled droplet inclusions with a maximal diameter of 500 nm. 相似文献
A simple, easily accessible solvent‐free method for the dispersion of MWCNTs into PET is proposed, based on the preparation of a microparticulate polymer/nanotube masterbatch via cryogenic impact‐milling and its subsequent melt blending with the bulk polymer. Thermal and mechanical properties of nanocomposites prepared using this method were evaluated as a function of nanotube concentration. Thermal stability was improved, and superior crystallization behavior of PET in the nanocomposites was observed. Significant improvements of around 25% in tensile strength and tensile modulus of the nanocomposites was achieved using this strategy, with only 0.25 wt.‐% MWCNT, compared to previous literature data where 1 wt.‐% MWCNT was employed.
