Effect of nanofiber on material properties of vapor-grown carbon nanofiber reinforced thermoplastic polyurethane (TPU/CNF) nanocomposites prepared by melt compounding

The present work deals with the preparation of the CNF based TPU nanocomposites by melt blending to explore the effect of state of dispersion and wt.% loading of CNF on material properties. In addition, the morphology, mechanical, thermal, rheological, and electrical properties of the nanocomposites have been evaluated through various characterization techniques with an aim to find the suitability of the nanocomposites for industrial applications. Transmission electron microscopy (TEM) study reveals that the CNFs exhibited a uniformly dispersed in TPU matrix. The thermal stability of the TPU evaluated by thermogravimetric analysis (TGA) showed significant increase with increased CNF content. It is observed that storage modulus (E′) and glass transition temperature (T_g) of the TPU matrix increases by the incorporation of CNF. The melting point (T_m) and the T_g of soft segments observed from the differential scanning calorimetry (DSC) were found to shift towards higher temperature with the inclusion of CNF.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Mechanical property degradation of a CuZr-based bulk metallic glass composite induced by sub-Tg annealing

The annealing effect at temperatures below glass transition temperature (T_g) on the mechanical behavior of a ductile Cu₄₈Zr₄₈Al₄ bulk metallic glass composite (BMGC) containing a B2-CuZr phase was investigated. It was found that remarkable plasticity and fracture strength degradations of the CuZr-based BMGC occurred with increasing the annealing temperature in the condition without significantly changing the crystalline volume fraction in the BMGC, which were correlated with the annealing-induced microstructure variations. It is expected that the free volume in the glassy matrix of the BMGC still played an important role for its deformation behavior, in despite of the existence of the B2-CuZr phase. The sub-T_g annealing-induced free volume annihilation depressed the shear band generation in the glassy matrix, reduced the synchronous contribution of the “blocking effect” and “deformation-induced martensitic transformation effect” of the B2-CuZr phase to the multiplication of shear bands, resultantly caused the plasticity degradation. The annealing-induced martensitic transformation of the B2-CuZr phase at the temperature close to T_g would further expand the plasticity degradation due to the absence of the “deformation-induced martensitic transformation effect”. Furthermore, the plasticity degradation simultaneously resulted in the fracture strength reduction of the BMGC because its work-hardening-like behavior was conditioned by the plastic deformation ability. The present results indicate that the ductile CuZr-based BMGC reinforced by the B2-CuZr phase similarly suffers from sub-T_g annealing-induced embrittlement, as is the case for most monolithic BMGs.     

The influence of hydrothermal conditioning on the Mode-I,thermal and flexural properties of Carbon/Benzoxazine composites with a thermoplastic toughening interlayer

Carbon/Benzoxazine laminates with and without non-woven thermoplastic fibrous polyamide (PA) veils at the interlaminar regions were manufactured using Vacuum Assisted Resin Transfer Moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-I strain energy release rate (G_IC), flexural stiffness, glass transition temperature (T_g) and water absorption behaviour was determined using two commercially available Benzoxazine resins. Despite an increase in the maximum moisture content, the veils greatly enhanced G_IC by an increase in fibre bridging of PA fibres, with concurrent reductions in flexural stiffness. Water ingress resulted in large reductions in the T_g, although no significant change was observed due to the PA interlayers. Fibre bridging and fibre pull-out were the main mechanisms by which the veils assisted in resisting delamination. The presence of the water was observed to degrade mechanical properties due to a reduction in fibre/matrix interfacial strength, molecular degradation and plasticisation of the matrix.     

Interface modification of clay and graphene platelets reinforced epoxy nanocomposites: a comparative study

The interface between the matrix phase and dispersed phase of a composite plays a critical role in influencing its properties. However, the intricate mechanisms of interface are not fully understood, and polymer nanocomposites are no exception. This study compares the fabrication, morphology, and mechanical and thermal properties of epoxy nanocomposites tuned by clay layers (denoted as m-clay) and graphene platelets (denoted as m-GP). It was found that a chemical modification, layer expansion and dispersion of filler within the epoxy matrix resulted in an improved interface between the filler material and epoxy matrix. This was confirmed by Fourier transform infrared spectroscopy and transmission electron microscope. The enhanced interface led to improved mechanical properties (i.e. stiffness modulus, fracture toughness) and higher glass transition temperatures (T _g) compared with neat epoxy. At 4 wt% m-GP, the critical strain energy release rate G _1c of neat epoxy improved by 240 % from 179.1 to 608.6 J/m² and T _g increased from 93.7 to 106.4 °C. In contrast to m-clay, which at 4 wt%, only improved the G _1c by 45 % and T _g by 7.1 %. The higher level of improvement offered by m-GP is attributed to the strong interaction of graphene sheets with epoxy because the covalent bonds between the carbon atoms of graphene sheets are much stronger than silicon-based clay.     

Multiscale molecular modeling of SWCNTs/epoxy resin composites mechanical behaviour

Atomistic and mesoscale simulations were conducted to estimate the effect of the diameter and weight fraction of single walled carbon nanotubes (SWCNTs) on mechanical behaviour and glass transition temperature (T_g) of SWCNTs reinforced epoxy resin composites. Atomistic periodic systems of epoxy resin and epoxy resin/SWCNTs were built with different weight ratios and were subject of an extensive multistage equilibration procedure. Molecular dynamics simulations were used to estimate glass transition temperature, Young modulus and solubility parameter of epoxy resin and epoxy resin/SWCNTs composites. Dissipative particle dynamics method and Flory–Huggins theory was employed to predict epoxy resin/SWCNTs morphologies. The results show that incorporation of SWCNTs with diameters ranging from 10 to 14 Ǻ has beneficial effect on mechanical integrity and T_g. Overall, the agreement between predicted material properties and experimental data in the literature is very satisfactory.     

Multiscale molecular modeling of SWCNTs/epoxy resin composites mechanical behaviour

Atomistic and mesoscale simulations were conducted to estimate the effect of the diameter and weight fraction of single walled carbon nanotubes (SWCNTs) on mechanical behaviour and glass transition temperature (T_g) of SWCNTs reinforced epoxy resin composites. Atomistic periodic systems of epoxy resin and epoxy resin/SWCNTs were built with different weight ratios and were subject of an extensive multistage equilibration procedure. Molecular dynamics simulations were used to estimate glass transition temperature, Young modulus and solubility parameter of epoxy resin and epoxy resin/SWCNTs composites. Dissipative particle dynamics method and Flory–Huggins theory was employed to predict epoxy resin/SWCNTs morphologies. The results show that incorporation of SWCNTs with diameters ranging from 10 to 14 ? has beneficial effect on mechanical integrity and T_g. Overall, the agreement between predicted material properties and experimental data in the literature is very satisfactory.     

Discontinuities in Amplitudes of Particles Micromotions Characterizing Phase Transitions in Liquid State

The behavior of amorphous solids below Vogel's (T _V ) or the glass transition (T _g ) temperature, as well as the solid-liquid transition, have been analyzed taking into consideration the anharmonicity of motion of microparticles forming the amorphous bodies. The T _g transition is explained within the logical association of this transition with the higher-temperature transitions, which can eventually involve the process of particle release into the gas phase through the process of a sudden vibrational amplitude growth. It follows from the mathematical solution of the anharmonicity problems that the pulses and the double amplitudes will always be present in aliquid matrix. The T _g temperature is considered as the boundary point for the liquid state at which the dynamical microcracks of a solid state matrix start to proceed. The processes at T _g are accompanied by appearance of new, highly agitated spots and the first microcracks (vacancies) filled up with the ‘semi-evaporated’ particles. In the mechanical sense, these vacancies form a new particle species characterized by quite different properties (different thermal expansion coefficient) as compared with the particles of the original matrix. It is assumed that a number of new mechanical units are growing up to the critical temperature when the original liquid frame, bonding the particles to lower amplitudes, is completely destroyed. The approach proposed does not contradict the traditional views reflected in the famous Adam-Gibbs-Di Marzio or WLF approaches, but allows a different approach to these theories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanical characterization of steel/CFRP double strap joints at elevated temperatures

This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (T_g, 42 °C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20 °C and 60 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached T_g. In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than T_g, while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements.     

Dynamic mechanical analysis of the seawater treated glass/polyester composites

Glass fibre/unsaturated polyester composites were produced by using the VARI (vacuum assisted resin infusion) technique. The fabricated four-layer specimens were treated by seawater for various periods of time. A decreased trend of the tensile strength with prolonged treating time was noticed. DMA (dynamic mechanical analysis) technique was employed to investigate the matrix degradation and interfacial debonding of the seawater treated specimens. The gradually decreased T_g (glass transition temperature), E′ (storage modulus), and the increased tan δ (energy dissipation) with extended treating time indicated that both the matrix and the interface had been deteriorated by the seawater.     

Impact of mechanical stress on barium titanate-based positive temperature coefficient resistive material

The sensitivity toward mechanical stress of barium titanate-based positive temperature coefficient resistor material was investigated by determining the resistance change with application of uniaxial stress from room temperature to 200 °C, which is well above the Curie temperature T_C. Using the Landau–Ginsburg–Devonshire theory the resistance increases in the paraelectric state, the negligible impact of stress close to T_C and the observed increase in T_C with increasing stress could be rationalized. For the ferroelectric state, the stress-related resistance increase was attributed to ferroelasticity, a change in bulk permittivity and interfacial stress inducing a piezoelectric potential. The obtained results are also discussed with respect to recent endeavors to tune properties of potential barriers in piezoelectric semiconductors by mechanical stress.     

Service life assessment and moisture influence on bio-based thermosetting resins

In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, T _g. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and T _g data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3–10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. T _g measurements show that moisture inside the specimen decreases T _g; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites.     

Effect of CO2 fixation reaction on the thermal and dynamic mechanical properties of tetrafunctional epoxy resin

A tetrafunctional epoxy resin was modified using CO₂ fixation process in the presence of tetra-n-butyl ammonium bromide as catalyst. The unmodified tetrafunctional epoxy resin (UMTE) and CO₂ fixated modified tetrafunctional epoxy resin (CFMTE) were cured by diethylenetriamine. A bifunctional glycidyl ether compound was used as a reactive diluent to control the viscosity of CFMTE. The activation energy of curing reaction was computed using the advanced integral isoconversional method. The activation energy, which depends on the conversion, was considerably changed due to the CO₂ fixation process. The thermal stability parameters including the initial degradation temperature, the temperature at the maximum rate of weight loss (T _max), and the decomposition activation energy (E _d) were determined by thermal gravimetry. Dynamic mechanical thermal analysis measurements showed that the CO₂ fixation decreases the T _g of the epoxy resin. The surface morphology of UMTE and CFMTE were determined by scanning electron microscope. It is concluded that CO₂ fixation reaction improves the properties of tetrafunctional epoxy resin.     

Cu-based metallic glass particle additions to significantly improve overall compressive properties of an Al alloy

We report the development of a novel light-weight Al (520) alloy-based composite reinforced with particles of a Cu-based (Cu₅₄Zr₃₆Ti₁₀) metallic glass by mechanical milling followed by induction heated sintering. The consolidation of the composite is performed at a temperature in the super-cooled liquid region of the metallic glass just above its glass-transition temperature (T_g). Metallic glasses are a promising alternative reinforcement material for metal-matrix composites capable of producing significant strengthening along with a «friendly» sintering behavior. The mechanical milling procedures were properly established to allow reduction of the size of the metallic glass particles and their uniform distribution in the matrix. Microstructural observation of the composite did not reveal any porosity. The interface between the glassy particles and the matrix remained free of such defects. The fully dense consolidated composite showed a drastic gain in specific yield strength under compression relative to the matrix alloy and appreciable plasticity at fracture.     

Surface glass transition temperatures of monodisperse polystyrene films by scanning force microscopy

Surface molecular motion of monodisperse polystyrene (PS) films was examined by scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus, E′, and loss tangent, tan δ, at a PS film surface with a smaller number-average molecular weight, M_n, than 40k were found to be smaller and larger than those for the bulk sample even at room temperature, meaning that the PS surface is in a glass–rubber transition state or a fully rubbery one at this temperature if the M_n, is small. In order to elucidate quantitatively how vigorous the molecular motion at the PS surface is, SVM and LFM measurements were made at various temperatures. The glass transition temperature, T_g, at the surface was discerned to be markedly lower than its bulk T_g, and the discrepancy of T_g between surface and bulk becomes larger with the decreasing M_n. Such an intensive activation of thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinity of the film surface induced by the preferential segregation of chain end groups.     

Natural fiber-reinforced thermoplastic starch composites obtained by melt processing

Thermoplastic starch (TPS) from industrial non-modified corn starch was obtained and reinforced with natural strands. The influence of the reinforcement on physical–chemical properties of the composites obtained by melt processing has been analyzed. For this purpose, composites reinforced with different amounts of either sisal or hemp strands have been prepared and evaluated in terms of crystallinity, water sorption, thermal and mechanical properties. The results showed that the incorporation of sisal or hemp strands caused an increase in the glass transition temperature (T_g) of the TPS as determined by DMTA. The reinforcement also increased the stiffness of the material, as reflected in both the storage modulus and the Young’s modulus. Intrinsic mechanical properties of the reinforcing fibers showed a lower effect on the final mechanical properties of the materials than their homogeneity and distribution within the matrix. Additionally, the addition of a natural latex plasticizer to the composite decreased the water absorption kinetics without affecting significantly the thermal and mechanical properties of the material.     

Effect of water absorption on the dynamic mechanical properties of composites used for windmill blades

Glass fiber/unsaturated polyester composites (with and without nanofiller) were produced by using the VARI (vacuum assisted resin infusion) technique. These materials will be used in windmill blades and, therefore, they are expected to be exposure to high humidity environments. The fabricated specimens were immersed in water at 80 °C for different periods of time. DMA (dynamic mechanical analysis) technique was employed to investigate the matrix degradation and interfacial debonding of the aged specimens. The gradually decreased T_g (glass transition temperature), E′ (storage modulus), and the increased tan δ (energy dissipation) with extended exposure time indicated that both the matrix and the interface had been deteriorated by the water. While matrix degradation occurs in a short period of time, composites degradation takes place gradually, showing that the degradation of the interphase in composites is the limiting step for the whole degradation process. Nanoclays were incorporated to the UP (unsaturated polyester) matrix showing a detrimental effect in degradation resistance, probably because of the degree of hydrophilicity of the selected clay, which produces a weak interphase with the polymeric matrix.