Influence of chemical treatments on the interfacial adhesion between sisal fibre and different biodegradable polymers

The influence of chemical treatments on the interfacial adhesion of sisal fibres and biodegradable matrices were studied in the present work. For that purpose, four different polymers were used: polycaprolactone (PCL), cellulose acetate, MaterBi Z (a commercial starch/polycaprolactone blend) and MaterBi Y (a commercial starch/cellulose derivatives blend). Alkaline and acetylation treatments were performed on sisal fibres. Properties were determined by means of tensile tests, adhesion measurements and contact angle determination. The interfacial shear strength was correlated with the hydrophilic character of the material.     

强激光辐照下纯铝的力学响应和层裂的实验测量与分析

 采用速度干涉（VISAR）测试技术,对强激光辐照下纯铝的动态力学响应和层裂特性进行了实验测量和分析。样品厚度分别为200 μm 和485 μm,激光脉冲的半高宽约为10 ns,功率密度变化范围为10¹⁰~10¹¹ W·cm^-2。实测了样品自由面速度波形,反映了强激光加载作用下材料损伤演化过程以及损伤对材料动态响应的影响。计算得到了冲击波强度(2.0~13.4 GPa) 和不同拉伸应变率下铝的层裂强度(1.6~2.3 GPa)。在所采用的实验条件和1维近似下,激光辐照产生的冲击波强度与激光功率密度之间成线性关系。最后讨论了层裂强度与拉伸应变率之间的关系,显示层裂强度随着拉伸应变率的增加而增大。     

Determination of SWCNT diameters from the Raman response of the radial breathing mode

We report on the evaluation of the distribution of diameters for nanotube samples with a wide variation of mean diameters. Such results were obtained from a detailed analysis of the radial breathing mode Raman response and compared to results obtained from an evaluation of optical spectra and X-ray diffraction pattern. The evaluation of the Raman data needs a well refined analysis as the experimental analysis exhibits a rather complicated and oscillating relation between response and exciting laser. Both, an exact calculation where the density of states was considered explicitly and an approximate calculation were applied. Both models used for the analysis are able to explain several unexpected results from the experiment such as the oscillating behavior of the spectral moments, unusual discontinuities in the first moments of the Raman response for excitation in the IR, a fine structure for the response in optics and Raman, and an up shift of the RBM frequency as compared to qualified ab initio calculations. In detail the first moment and the variance of the spectra were used for the evaluation of the diameter distribution. To obtain good results between experimental and theoretical oscillation pattern the transition energy between the first two van Hove singularities had to be scaled up which is considered as a result from coulomb interaction of the electrons in the tubular material. On the other hand the analysis does not only allow to determine the mean value and the width of the diameter distribution but yields also a value for the average bundle diameters or, alternatively, the strength of the tube-tube interaction. The model used for the analysis of the Raman data is also appropriate to analyze the optical response, at least for the spectral range from 0.5 eV to 3.5 eV. The fine structure in the response for the transitions between the three lowest van Hove singularities is well reproduced and the mean tube diameters and their distribution is obtained in very good agreement with the results from the Raman analysis. From the X-ray analysis the same mean values and comparable distributions for the tube diameters were received whereas the bundle diameters could not be retained with high precision in this case. Received 18 February 2001 and Received in final form 3 April 2001     

Characterization of structural modifications in poly-tetra-fluoroethylene induced by electron beam irradiation

The effects of electron beam irradiation doses on the poly-tetra-fluoroethylene (PTFE) have been studied. Several techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), mechanical properties and Fourier transform infrared spectroscopy (FTIR) were applied to characterize the PTFE samples and to study the radiation effects on the crystal structure of the polymer.The irradiation dose up to 150 kGy showed an increase in the crystallinity degree of PTFE, which has been observed and confirmed during the DSC and XRD measurements. The increase in crystallinity was attributed to the scissions of the chain in the amorphous region. Moreover, the number-average molecular weights were estimated from the heat of crystallization measured by DSC technique. The results indicated that the molecular weights were decreased by increasing the heat of crystallization due to irradiation with doses up to 150 kGy. Radiation resistance of the irradiated and non-irradiated PTFE was investigated during its mechanical properties at room temperature. The dose at half value of the elongation at break is about 3.10 kGy while the dose at half value of the tensile strength is about 1.70 kGy.     

The effect of α, β crystalline structure on the mechanical properties of polypropylene

In order to study the effect of the α,β crystalline structure of polypropylene (PP) on its mechanical properties, it is necessary to prepare samples with variable α/β-phase content but with constant crystallinity and constant spherulite size. With this objective, heat treatment was first defined to be applied to an isotactic PP containing a β nucleating agent in order to achieve these conditions. Then study of the effect of the β-phase content on the tensile properties and fracture behaviour has been done at room temperature. The mechanical properties at fracture were assessed by three-point bending tests and were analysed on the basis of the “Essential Work of Fracture” (EWF). The results show that the elongation at fracture under tensile stress and the “near” Plane-Strain Essential Work of Fracture, w _Ie, increase substantially with the β-phase content. Besides, Young's modulus and the yield stress in tensile tests decrease slowly with the β-phase content. Finally, these results are analysed taking account the differences in structure of the α and β spherulites. Received 18 September 2000 and Received in final form 19 December 2000     

Stretching dynamics of semiflexible polymers

We analyze the nonequilibrium dynamics of single inextensible semiflexible biopolymers as stretching forces are applied at the ends. Based on different (contradicting) heuristic arguments, various scaling laws have been proposed for the propagation speed of the backbone tension which is induced in response to stretching. Here, we employ a newly developed unified theory to systematically substantiate, restrict, and extend these approaches. Introducing the practically relevant scenario of a chain equilibrated under some prestretching force f _pre that is suddenly exposed to a different external force f _ext at the ends, we give a concise physical explanation of the underlying relaxation processes by means of an intuitive blob picture. We discuss the corresponding intermediate asymptotics, derive results for experimentally relevant observables, and support our conclusions by numerical solutions of the coarse-grained equations of motion for the tension.     

Influence of molecular relaxation processes on mechanical properties of polyvinyl chloride in tensile experiments

The relation between the molecular relaxation processes and the results of tensile experiments has been studied for hard and toughened polyvinyl chloride (PVC). The relaxation spectra have been determined between 10^?4 and 10⁺⁴ sec by means of stress relaxation, flexural vibration experiments, and tensile tests. Yield stress, yield strain, rupture stress, rupture strain, and rupture energy have been determined at 23° and 50° C as functions of yield time or rupture time, respectively, over a time-range from 10^?2 to 10⁺⁴ sec approximately. It is shown that the molecular relaxation processes observed in the range of small deformations also influence the behavior in the nonlinear range, especially the elongation at rupture in tensile experiments. In addition, effects of the molecular processes on the yield stress, the rupture stress, and, consequently, the rupture energy have been observed. These effects are due to the time- and temperature-dependent decrease of the macroscopic stress and of the microscopic notch stresses by certain relaxation mechanisms. The most suitable characteristic parameters in tensile experiments are the yield stress and the rupture strain. It is pointed out that the usefulness of a single value of a mechanical property as determined in a standard “one-point” test is very restricted.     

聚氨酯改性TDE-85/ MeTHPA环氧树脂的力学性能研究

 采用自制的聚氨酯预聚体(PUP)制备聚氨酯(PU)改性TDE-85/ MeTHPA环氧树脂体系，探讨了聚醚二元醇(PPG)分子量的大小、PUP加入量等因素对PU改性TDE-85/ MeTHPA环氧树脂体系力学性能的影响。研究结果表明，PU改性TDE-85/ MeTHPA环氧树脂的拉伸强度和冲击强度随着合成的PUP加入量的增加先呈上升趋势，达到最大值后又开始下降。采用的PPG分子量不同，得到的改性材料的力学性能相差悬殊。当PPG分子量为1 000，PUP质量分数为15%时，改性材料的拉伸强度达到69.39 MPa，冲击强度达到23.56 kJ/m²，力学综合性能显著提高。     

Frequency and temperature amplitude dependence of complex heat capacity in the melting region of polymers

Amplitude and frequency dependence of reversible melting of polycaprolactone (PCL) and an ethylene octene copolymer (EOM) were studied using temperature-modulated differential scanning calorimetry (TMDSC) (2?10^?1 Hz <f < 0.05 Hz) and shear spectroscopy (dynamic mechanical analysis, DMA) (5?10^?4 Hz < f < 100 Hz). It was found that the excess heat capacity of PCL is constant for temperature amplitudes in the range 5 mK < A_T < 2 K. The excess heat capacity decreases with frequency of temperature perturbation and tends to zero at about 0.1 Hz and 100 Hz for PCL and EOM, respectively. The constant excess heat capacity and the frequency dependence support the idea that reversible melting is related to a relaxation process on the surface of the polymer crystals. The occurrence of such a relaxation process was shown by shear modulus measurements in the same frequency and temperature region. The relaxation process is, in the melting region, much slower than main relaxation (glass transition). At low temperatures, a crossover can be seen, indicating the independence of both processes because of spatial separation. The main relaxation is related to the melt, while the other is related to the crystal surface.     

Fiber modification by steam-explosion: 13C NMR and dynamic mechanical analysis studies of co-refined wood and polypropylene

Wood-plastic material from a novel reactive processing method (co-refining by steam-explosion) was investigated by ¹³C solid state nuclear magnetic resonance (NMR) and dynamic mechanical analysis (DMA). NMR spectra indicated chemistry of the material changed as a result from co-steam-explosion. It was also observed from NMR analysis that the crystallinity of the cellulose increased in the presence of iPP during steam-explosion co-processing. By using variable contact time cross pulse experiments, the relaxation parameters (T _CH and T _1ρ ) for the constitutive components were evaluated to reveal the level of interactions. T _1ρ values for steam-exploded wood had values similar to those published in literature, which are independent relaxation values for lignin and cellulose. However, for co-steam-exploded wood and iPP, the independent value of lignin relative to the amorphous cellulose was absent. It is proposed that lignin adopts an alternate arrangement during co-steam-explosion processing. This arrangement is transient because the independent relaxation of lignin is recovered after the application of heat during compression molding. DMA demonstrated a mechanical reinforcing effect of the steam-exploded wood without influencing the glass transition of polypropylene for the compression molded co-steam-exploded sample. The paper concludes by describing a hypothetical scheme for a meta-stable interaction of wood bio-polymers and iPP.     

Effect of hot acid etching on the mechanical strength of ground YAG laser elements

High-power pumped Nd:YAG elements may exceed their tensile strength under high thermally induced stress. Providing extra strength to such rods is essential for their employment in high-power lasers. The tensile strength of YAG elements was increased by chemical etching in concentrated phosphoric acid. The highest tensile strength was achieved by etching of fine-ground YAG components: an average for slabs, and for rods, which are 3.6 times and 5 times higher than those of non-etched elements, respectively. The measurements were carried out by four-point flexure strength test. We have established a dependency among the micro-roughness of YAG elements, the surface morphology obtained by etching, and the tensile strength: the tensile strength of the etched element improves for finer after-etch surface texture, which is obtained for finer initial micro-roughness.

To assure the withstanding of Nd:YAG rods under high thermal gradients, a new approach was employed, namely, increasing the pump-power applied to the Nd:YAG rod till fracture. Our results show an increase by more than 2.7 times in tensile strength of etched Nd:YAG rods as compared to standard commercial rods, which corresponds to a thermal loading of excess of 434 W/cm.     

Investigation of Ti-6Al-4V alloy acoustic softening

High power ultrasonic vibration is widely used for improving manufacturing processes such as machining and metal forming. High frequency mechanical vibration affects material properties and friction forces in contacting surfaces. Flow stress reduction under superimposed ultrasonic vibration is called as acoustic softening. The amount of this parameter should be determined for ultrasonic assisted metal forming processes. For determination of this parameter for workhorse Ti-6Al-4V alloy, experimental setup was designed and fabricated. Then tensile test under longitudinal ultrasonic vibration was performed for different ultrasonic powers. Results show that ultrasonic vibration has considerable effect on plastic behavior of the alloy and decreases flow stress. Also, increasing ultrasonic power leads to higher acoustic softening. Yield stress reduction up to 9.52%, ultimate stress reduction up to 4.55% and elongation up to 13% were obtained at 340 W ultrasonic power. After applying ultrasonic vibrations and its termination, hardness of specimens were measured in which increase up to 9% was observed.     

Ac conductivity study of lithium and potassium diphosphate LiK3P2O7 using impedance spectroscopy

In this work, a LiK₃P₂O₇ ceramic material was prepared by the solid-state reaction method and identified by X-ray diffractometry. The dielectric properties, impedance characteristics, and modulus were studied over a range of frequency (200 Hz to 5 MHz) and temperature (615–708 K). The frequency and temperature dependence of dielectric permittivity, dielectric loss, and electric modulus is studied. The frequency analysis of modulus properties showed a distribution of relaxation times. Conductivity plots against frequency at a higher frequency suggested the response obeying the universal power law. The temperature behavior of the frequency exponents shows that the correlated barrier hopping CBH model is well adapted to this material. The activation energy associated with the impedance relaxation and the electric modulus spectra is close to the activation energy for dc conductivity indicating the similar nature of relaxation and conductivity. Thermodynamic parameters such as free energy of activation, enthalpy, and entropy have been calculated.     

Room temperature mechanical behaviour of a Ni-Fe multilayered material with modulated grain size distribution

To gain fundamental insight into the relationship between length scales and mechanical behaviour, Ni-Fe multilayered materials with a 5-μm-layer thickness and a modulated grain size distribution have been synthesized by pulsed electrodeposition. Microstructural studies by SEM and TEM reveal the alternating growth of well-defined layers with either nano (d = 16 nm) or coarse grains (d ≥ 500 nm). Room temperature tensile tests have been performed to investigate the mechanical response and understand the underlying deformation mechanisms. Tensile test results and fractographic studies demonstrate that the overall room temperature mechanical behaviour of the multilayered material, i.e. strength and ductility, is governed primarily by the layers containing nanocrystalline grains. The measured properties have been discussed in the context of modulated grain structure of the multilayered sample and contribution of each grain size regime to the overall strength and ductility.     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H _ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to k _B T) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity H _C(H _C < H _ani), - a soft dipole regime (anisotropy energy of the order of k _B T) for maghemite nanoparticles with a relaxation time independent of the field intensity H _C(H _C > H _ani). Received 5 June 2000 and Received in final form 8 January 2001     

Preparation and characterization of carbon nanotubes/epoxy resin nano-prepreg for nanocomposites

The surface carbon nanotubes (CNTs) were modified to generate functional reactors by using the sonicication method to distribute CNTs evenly among epoxy resin, which was prepared into nano-prepreg with carbon fibers. Additionally, based on various proportions of modified and unmodified CNTs, the mechanical properties and conductivities of the composite, as well as, the characteristics of material subjected to various temperature conditions were investigated. Experimental results indicate that increasing CNT content enhances the mechanical strength and electrical properties. At various temperatures, the mechanical strength drops with increase in temperature because different expansion coefficients differ between fiber and epoxy resin. Finally, the failure surface of nanocomposite was examined using scanning electron microscopy (SEM). Finally we provide a discussion of the failure mechanism of the material.     

Radiation effects on room temperature epoxy adhesive molecular structure: Mechanical tests and correlation with calorimetric and outgassing analyses

In applications like space satellites, high-energy physics experiments, and nuclear power stations, epoxy structural adhesives are normally used in an ionizing radiation environment. To check the effects of γ-irradiation on room temperature epoxy adhesives, mechanical measurements were undertaken for three different resins up to the dose of 3 MGy. Both dumbbell and single-lap shear tests were performed. To correlate the measured radiation effects on these mechanical properties with the molecular modifications of the resins, outgassing and calorimetric tests were performed on one of the tested adhesives. As a result of these tests, the mechanical modifications were associated with the combination of reticulation, network scission, and production of low-weight molecules due to radiation. Differences in shear and tensile strength behaviors were associated with the presence of the adhesive-adherend interface.