Thermal expansion coefficient and bulk modulus of polyethylene closed‐cell foams

A regular Kelvin foam model was used to predict the linear thermal expansion coefficient and bulk modulus of crosslinked, closed‐cell, low‐density polyethylene (LDPE) foams from the polymer and gas properties. The materials used for the experimental measurements were crosslinked, had a uniform cell size, and were nearly isotropic. Young's modulus of biaxially oriented polyethylene was used for modeling the cell faces. The model underestimated the foam linear thermal expansion coefficient because it assumed that the cell faces were flat. However, scanning electron microscopy showed that some cell faces were crumpled as a result of foam processing. The measured bulk modulus, which was considerably smaller than the theoretical value, was used to estimate the linear thermal expansion coefficient of the LDPE foams. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3741–3749, 2004     

A strip resonance technique for measuring the ultrasonic viscoelastic parameters of polymeric sheets with application to cellulose

An acoustic strip resonance apparatus that measures the viscoelastic response of polymeric sheets over the temperature range of 100–400°K and at frequencies from 20 to 100 kHz is described. This is a computer-controlled instrument which uses the resonant frequency and the quality factor of standing waves in strips to calculate the ratio of complex Young's modulus to density. Results are presented for cellophane and paper samples. These are consistent with measurements reported by others at lower frequencies. That is, (1) a single predominant secondary relaxation process which corresponds to the γ relaxation occurring at 200°K and 1 Hz is found in dry samples at 260°K and 60 kHz; (2) as moisture is added, the γ relaxation weakens and a higher temperature, β, relaxation appears; and (3) at low temperature, moisture addition initially increases the mass specific Young's modulus. The effects of methanol on the viscoelastic parameters of cellulose were also measured and found to be similar to those of water.     

Complementary DSC and dilatometric investigation of M-PTFE pyrotechnic compositions

Results of high thermal resolution microcalorimetric and dilatometric measurements performed on reducer — polytetrafluoroethylene (M-PTFE) pyrotechnic compositions have been discussed. The materials were selected for the study because of their behaviour in combustion tests. Two complementary thermal properties, i.e. the specific heat and coefficient of linear thermal expansion (CLTE), have been analysed in detail. The specific heat was obtained from DSC measurements performed from −20 to 375°C. Measurements of CLTE and linear expansion were carried out from −40 to 270°C. In both cases the measurements were performed on thermocycling with the high thermal resolution preserved. A special attention has been paid to a two-stage phase transition occurring just below the room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

The structural,electronic, elastic,vibration and thermodynamic properties of GdMg

We have investigated the structural, elastic, electronic, vibration and thermodynamic properties of GdMg alloy using the methods of density functional theory within the generalized gradient approximation (GGA) for the exchange-correlation functional. We have presented the results on the basic physical parameters, such as the lattice constant, bulk modulus, pressure derivative of bulk modulus with and without spin-polarization (SP), second-order elastic constants, Zener anisotropy factor, Poisson's ratio, Young's modulus, and isotropic shear modulus. The thermodynamic properties of the considered compound are obtained through the quasi-harmonic Debye model. In order to obtain further information, we have also studied the pressure and temperature-dependent behavior of the volume, bulk modulus, thermal expansion coefficient, heat capacity, and Debye temperature in a wide temperature range of 0–1200 K. We have also calculated phonon frequencies and one-phonon density of states for B2 structure of GdMg compound. The temperature-dependent behavior of heat capacity and entropy obtained from phonon density of states for GdMg compound in B2 phase is also presented.     

Quantitative determination of cristobalite by thermal methods

Cristobalite was quantitatively determined by quartz samples fired between 1350 and 1490°C by thermal methods and specific gravity measurements. The amount of cristobalite was calculated from the thermal expansion coefficient, the area of the differential thermal expansion together with specific heat measurements and specific gravity determinations. The calculated values from the expansion coefficient and specific gravity are misleaing due to the presence of tridymite and glass in the samples. The results of differential thermal expansion compare well with those of specific heat measurements. Therefore, this method could be taken as a very precise measure for the quantitative determination of cristobalite. Also, the temperature of the peak indicates whether ordered or disordered cristobalite is present.     

Thermal expansion and Young's modulus of uniaxially drawn poly(tetrafluoroethylene) in the temperature range from 100 to 400 K

PTFE specimens with a crystallinity of 42 % were uniaxially stretched at 473 K resulting in draw ratios between 1. and 4. The degree of molecular orientation was obtained by X-ray wide angle and birefringence measurements. The thermal expansion coefficients and the Young's moduli both parallel and perpendicular to the draw direction were measured in the temperature range from 100 to 400 K. The thermal expansion behaviour turned out to be dependent upon the molecular orientation in a sensitive manner and could be explained by a simple model of structure changes caused by the deformation processes.     

Characterization of the matrix of polybutene-1 films containing needle crystals

The microstructure and the mechanical properties of polybutene-1 films containing needle crystals are described. AboveT _g, the Young's modulus is nearly temperature independent over a temperature range of 100 °C. Mechanical and thermal measurements indicate that the end and side surfaces of the needle crystals act as physical tie points for the amorphous molecules.     

Transitions and relaxations in poly(1,4-phenylene ether)

Transitions and relaxation phenomena in poly(1,4-phenylene ether) were studied over temperature range from 100 to 800°K by applying a combination of calorimetric, dilatometric, dynamic mechanical, and dielectric techniques. Amorphous polymer, exhibiting no x-ray crystallinity, is obtained only by quenching molten samples at extremely fast cooling rates (ca. 1000°C/sec) and by minimizing thermal gradients within specimens. A weakly active mechanical relaxation region with a loss maximum at 155°K of unknown origin was observed. The glass transition interval of completely amorphous polymer is characterized by a discontinuous jump in heat capacity of 2.76 cal/deg per chain segment occurring at 363°K (corrected for kinetic effects), and a fourfold increase in the coefficient of linear thermal expansion. Strongly active, dynamic mechanical relaxations occur in the T_g interval with a loss maximum at 371°K (f = 110 cps) and resulting in a drop in the dynamic storage modulus from 10¹¹ to 10⁹ dyne/cm². Cold crystallization takes place just above T_g, to yield a polymer with an x-ray crystallinity of 0.7 and a heat of crystallization of 270 cal/mole. The crystalline polymer shows a complex melt structure. Depending upon the thermal history, multiple endothermic peaks indicative of structural reorganizations occur just prior to fusion. Very high dielectric losses with a wide distribution of relaxation times were observed in the melt interval. The mechanical relaxation spectrum in this region is typical of viscous flow behavior.     

Elastic modulus and thermal conductivity of ultradrawn polyethylene

The axial and transverse Young's modulus and thermal conductivity of gel and single crystal mat polyethylene with draw ratios λ = 1–350 have been measured from 160 to 360 K. The axial Young's modulus increases sharply with increasing λ, whereas the transverse modulus shows a slight decrease. The thermal conductivity exhibits a similar behavior. At λ = 350, the axial Young's modulus and thermal conductivity are, respectively, 20% and three times higher than those of steel. For this ultradrawn material both the magnitude and the temperature dependence of the axial Young's modulus are close to those of polyethylene crystal. The high values of the axial Young's modulus and thermal conductivity arise from the presence of a large percentage (∼85%) of long needle crystals. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3359–3367, 1999     

Ab initio Predictions of Structural and Thermodynamic Properties of Zr2AlC Under High Pressure and High Temperature

The structural and thermodynamic properties of Zr₂AlC at high pressure and high temperature are investigated by first principles density functional theory method. The calculated lattice parameters of Zr₂AlC are in good agreement with the available theoretical data. The pressure dependences of the elastic constants, bulk modulus, shear modulus, Young's modulus, and Vickers hardness of Zr₂AlC are successfully obtained. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. By using the quasiharmonic Debye model, the thermodynamic properties including the Debye temperature, heat capacity, volume thermal expansion coefficient, and Grüneisen parameter at high pressure and temperature are predicted for the first time.     

Structural,elastic, electronic and thermal properties of the cubic perovskite-type BaSnO3

First-principles calculations are performed to investigate the structural, elastic, electronic and thermal properties of the cubic perovskite-type BaSnO₃. The ground-state properties are in agreement with experimental data. The independent elastic constants, C₁₁, C₁₂ and C₄₄, are calculated from direct computation of stresses generated by small strains. A linear pressure dependence of the elastic stiffnesses is found. From the theoretical elastic constants, we have computed the elastic wave velocities along [100], [110] and [111] directions. The shear modulus, Young's modulus, Poisson's ratio, Lamé’s coefficients, average sound velocity and Debye temperature are estimated in the framework of the Voigt-Reuss-Hill approximation for ideal polycrystalline BaSnO₃ aggregate. Using the sX-LDA for the exchange-correlation potential, the calculated indirect fundamental band gap value is in very good agreement with the measured one. The analysis of the site-projected l-decomposed density of states, charge transfer and charge density shows that the bonding is of ionic nature. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the temperature effect on the lattice constant, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature is calculated.     

Styrenated phenol modified nanosilica for improved thermo-oxidative and mechanical properties of natural rubber

The present work aims to prepare thermal and oxidation resistant Natural Rubber (NR) composites using antioxidant-modified nanosilica (MNS). The thermo-oxidative aging performance of the composites was evaluated by the variations in mechanical properties after aging at 100 °C for 24 h. The performance was further monitored through Scanning Electron Microscopy, Fourier Transform Infrared spectroscopy, Thermogravimetric Analysis, and Dynamic Mechanical Analysis. NR nanocomposite with 1–7.5 phr nanosilica (NS) and 3 phr MNS were prepared and its rheological properties were studied. A comparative study of the theoretical models yielded that modified Guth-Gold equation predicted Young's modulus better than other models. Thermal stability of natural rubber MNS composite was improved by 10 °C with pre-eminent mechanical properties like tensile strength and heat build-up. A linear relationship of compression set with modulus of all composites were also established. Equilibrium swelling test revealed improved crosslink density in NR MNS composite. The strong interaction between antioxidant and nanosilica enabled low migration of antioxidant in NR MNS composite. Hence its protective function after aging showed more effective than NR NS composites. These versatile functional properties of NR MNS composite suggest its potential application in electrical, electronic and high performance rubber products.     

Hydrostatic extrusion of polyoxymethylene

The hydrostatic extrusion behavior of polyoxymethylene (POM) is described. Extrusions were performed at 164°C for a range of different molecular weight grades. Excellent unflawed lengths of extrudate were obtained with axial Young's moduli, measured at room temperature, reaching values as high as 24 GPa. The extrusion characteristics are discussed in terms of the very strong dependence of the flow stress of POM on strain and strain rate and pressure. In addition to measurements of Young's modulus over a wide temperature range, data for shear modulus and transvers modulus are also presented. A limited amount of other structural measurements is presented.     

The thermoelastic effect in glassy polymers

The thermoelastic effect has been measured in compression on four glassy polymers; namely, polystyrene, poly(methyl methacrylate), polycarbonate, and epoxy resin. Quantitative results have been obtained for the first time on three of these polymers. It has been shown that by paying attention to specimen geometry and instrumentation results can be obtained to a high degree of accuracy (better than ±1.5% on a given set of measurements). The polymers are shown to obey the classical Thompson equation for thermoelasticity in solids over the temperature range studied (ca. 220–350°K). By inference such materials can be expected to behave classically in general. The results have been used, as first suggested by Trainor and Haward, to obtain values for the linear thermal expansion coefficient and the values so obtained are shown to be in excellent agreement, in general, with literature values obtained by more conventional methods. Results are given for a range of stress from 5 MN m^?2 to between 25 and 50 MN m^?2 according to ambient temperature. The method affords a measurement of parameters, in particular, linear thermal expansion coefficient. Values of specific heat for the four plastics have been measured by differential scanning calorimetry and the results compared with published data.     

Effects of humidity on Young's modulus in poly(methyl methacrylate)

Tensile tests on poly (methyl methacrylate) (PMMA) were conducted to clarify the effects of humidity and strain rate on tensile properties, particularly Young's modulus. Prior to the tensile tests, specimens were kept under various humidity conditions at 293 K, which were the same as the test conditions, for a few months to adjust the sorbed water content in the specimens. The tensile tests were performed under each humidity condition at three different strain rates (approximately 1.4 × 10^?3, 1.4 × 10^?4, and 1.4 × 10^?5 s^?1). Stress‐strain curves changed with humidity and strain rate. Young's moduli were also measured at small applied stresses (below 6.7 MPa) under various humidity conditions at 293 K. Young's modulus decreases linearly with increasing humidity and a decreasing logarithm of strain rate. These results suggest that Young's modulus of PMMA can be expressed as a function of two independent parameters that are humidity and strain rate. A constitutive equation for Young's modulus of PMMA was proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 460–465, 2002; DOI 10.1002/polb.10107     

Thermal Properties of Polymers at Low Temperatures

Abstract

The existing measurements and theories of the low-temperature thermal properties, heat capacity, and thermal conductivity of polymers are reviewed with particular attention paid to the differences between partly crystalline and amorphous polymers. The most striking feature of the low-temperature heat capacity of polymers is that in the liquid helium temperature range the heat capacity does not depend upon the cube of the temperature as for other solids. Further, only well below 1°K does the heat capacity approach the value predicted on the basis of the sound velocity. This behavior indicates the presence of a small number of low-frequency modes of vibration in the frequency spectrum. The fact that such anomalous behavior seems linearly related to the crystallinity implies that this behavior is associated with the amorphous structure, perhaps with motions of pendent groups within cavities formed in the amorphous structure. The thermal conductivity of semicrystalline and amorphous polymers differs considerably. Semicrystalline polymers display a temperature dependence of the thermal conductivity similar to that obtained from highly imperfect crystals, the thermal conductivity having a maximum in the temperature range near 100°K which moves to lower temperatures and higher thermal conductivities as the crystallinity is increased. Amorphous polymers display a temperature dependence similar to that obtained for glasses with no maximum but a significant plateau region in the range between 5 and 15°K. The theoretical interpretation of the thermal conductivity of these materials is considered.     

Heat capacity of molten polymers

Heat capacities of molten polyethylene, polypropylene, poly-1-butene, polystyrene, and poly(methyl methacrylate) were measured over a wide range of temperature by using a differential scanning calorimeter. The upper limit of temperature was established for each polymer at about 10°K below the beginning of thermal decomposition. For poly-1-butene and poly(methyl methacrylate) the solid-state heat capacity was also measured starting from room temperature. Several samples of each polymer were used so that average values of heat capacities could be established (reported in 10°K intervals). The data revealed for all polymers a nearly linear increase of heat capacity with increasing temperature over the whole temperature range investigated.     

Elastic Tensor and Thermodynamic Property of Magnesium Silicate Perovskite from First-principles Calculations

The thermodynamic and elastic properties of magnesium silicate (MgSiO₃) perovskite at high pressure are investigated with the quasi-harmonic Debye model and the first-principles method based on the density functional theory. The obtained equation of state is consis-tent with the available experimental data. The heat capacity and the thermal expansion coefficient agree with the observed values and other calculations at high pressures and tem-peratures. The elastic constants are calculated using the finite strain method. A complete elastic tensor of MgSiO₃ perovskite is determined in the wide pressure range. The geo-logically important quantities: Young's modulus, Poisson's ratio, Debye temperature, and crystal anisotropy, are derived from the calculated data.     

Heat capacity of poly(vinylidene fluoride) and polytetrafluoroethylene between 5 and 200°K

Heat capacities of poly(vinylidene fluoride) (PVF₂) and polytetrafluoroethylene (PTFE) have been measured between 5 and 100°K with an accuracy of (1–5)% by adiabatic calorimetry. Calculations based on contributions from known optical lines and the Tarasov continuum model are in good agreement with experimental results down to 30°K for PVF₂ and 10°K for PTFE, and yield characteristic temperatures θ₁ and θ₃ which are consistent with previous values determined from high-temperature (100—350°K) data. At lower temperature the measured heat capacity is significantly higher [(30–100)%] than the model prediction, and can be satisfactorily accounted for by the introduction of localized vibrators at a concentration of about 1% as compared to acoustical oscillators and at a characteristic temperature of about 20°K. Using established data on polyethylene for comparison, the principle of additivity for heat capacities is found to be valid down to at least 20°K, convering the region (<60°K) where interchain vibrations contribute significantly to the heat capacity. Possible reasons for this unexpected behavior are discussed.     

Mechanical properties of the novel organometallic polymer poly(ferrocenyldimethylsilane)

A study of the mechanical properties of poly(ferrocenyldimethylsilane) [Fe(η‐C₅H₄)₂SiMe₂]_n, 3 , a novel organometallic polymer, has been performed on thin films of this material. The Young's modulus and Poisson's ratio of film samples (15 × 1 × 1 mm) of 3 were measured in quasi‐static tension using a video extensometer. For 3 , the values of the Young's moduli (E) and Poisson's ratios (ν) were similar between axes in the plane and independent of the splicing direction used during sample preparation. The mean and standard deviation of the Young's modulus and Poisson's ratio were 0.78 ± 0.08 GPa and 0.37 ± 0.06 GPa, respectively. Thermomechanical analysis of 3 revealed a steady decrease of E from a room temperature value of approximately 0.70 GPa. Additionally, it was found that at 150 °C, 3 was unable to support even small stresses, consistent with the onset of a melt transition (ca. 135 °C). A mathematical model based on molecular geometry is developed to describe the results. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2280–2288, 2005