Mechanical Loss Maxima Research Articles

New Thermoplastic Damping Polymeric Materials Based on Ethylene-Vinyl Acetate

This article is devoted to the problem of working out of damping polymer materials which are effective in the wide temperature and frequency range. In the modern world, work is being carried out to create damping polymer composite materials (DPM) from which it is possible to manufacture protective elements and parts of engineering structures of reduced vibration excitability. Existing DPM have a narrow temperature range, within which effective vibration absorption is observed, moreover, most of them go through a vulcanization stage, which increases the cost of the final product, has a harmful effect on environment and allows limited recycling of waste. One of the ways to solve this problem is to replace traditional rubber vibration-absorbing materials with thermo-elastoplasts (TEP). The most promising polymer for TEP is ethylene vinyl acetate (EVA), which has high damping properties, oil resistance and relative incombustibility. In this regard, experimental studies were conducted to establish the patterns of influence of the type and concentration of structure-forming components (plasticizers, fillers, modifiers) on the dynamic mechanical properties of TEP based on EVA in order to develop a new DPM effective in a wide temperature range. The leading method to investigate this problem is a method of dynamic mechanical analysis which allows to get information about changes of mechanical characteristics under mechanical load and controlled temperature and frequency. With the help of detected patterns it was possible to determine type of plasticizer which significantly decreases glass temperature of EVA. The percentage ratio of EVA/plasticizer system is stated, and the type of plasticizer at which the maximum of mechanical losses takes over greater values is accordingly detected. It is revealed, that to work out DPM on EVA basis, which are effective in wide temperature range it is more preferable to add not less than 40 % on volume basis inert fillers, such as talc or mica with addition of 5-10 % of carbon as the hardening additive. The kind of resin improving damping properties and raising rigidity of composites on EVA basis is defined. On the basis of the research, a material was developed which has the following properties: the maximum value of tan δ is at least 0.45 at a temperature of plus 5�C (oscillation frequency 10 Hz); width of the temperature interval within which tan δ is not less than 0.3 from minus 40 to plus 50�С (oscillation frequency 10 Hz); conditional tensile strength of not less than 10 kg/cm2, cold resistance up to minus 50�C.

Relaxation phenomena in latex polymers modified with water-soluble phthalocyanine

The relaxation properties of polymer composites based on latex styrene-alkyl methacrylate polymers and water-soluble 2,3,9,10,16,17,23,24-octa[(3,5-biscarboxy)-phenoxy]phthalocyanine are studied in the temperature range from −150 to +250°C. Characteristic mechanical loss maxima are observed on the inner friction spectra of unmodified and modified polymers corresponding to various patterns of intermolecular mobility and local and segmental types of intermolecular interactions. The modification of the polymers is accompanied by decreases in their glass-transition temperatures. This effect may be explained by a decrease in the intermolecular interactions of copolymer units in the presence of phthalocyanine.

Maxwell equation for conductivity of dielectrics as the basis of direct relationship of ionic electrical conductivity and mechanical losses in glasses. New problems of physical chemistry of glass

Use of the Maxwell equation for the conductivity of dielectrics and the theory of mechanical losses in glasses caused by ionic shifts has made it possible to calculate directly the temperature of the "ionic" max� imum of mechanical losses using data on direct current conductivity. The Maxwell equation was used without changes, in the view accepted in electromagnetic theory. An intuitive physicochemical interpretation of the nature of ionic conductivity measured at constant voltage as a relaxation process was suggested. Verification of the suggestion equation's validity was performed for onealkali glasses in silicate, borosilicate, phosphate, borate, and germanate systems. The ratio of calculated and experimental values of the temperature of the "ionic" internalfriction maximum corresponds to 0.995 ± 0.034 and does not depend on the frequency of measurements of internal friction (it changed by four orders) or the parameters characterizing conductivity. These results also confirm the validity of the Maxwell equation for the conductivity of dielectrics. The possi� bility of describing the direct current ionic conductivity of glasses as a Debye relaxation transition proved in this work is a consequence of the constant voltage condition. The design of a theory for the condition of nat� ural decay of a given potential, when the Kohlrausch relaxation regularity becomes valid, represents a princi� pally important problem of physics and physical chemistry of disordered systems.

Quantifying thermomechanical fatigue of light-metal-matrix composites by mechanical spectroscopy

This article reviews recent progress in understanding the stress-relaxation mechanisms in metal-matrix composites (MMCs) subjected to thermomechanical fatigue. Mechanical loss, dynamic shear modulus, and permanent torsional-strain measurements have been performed with forced oscillations during thermal cycling. A transient mechanical-loss maximum, which is absent in the monolithic material, appears during cooling. It has been attributed to the development of plastic zones around the reinforcements by dislocation generation and motion, which result from the differential thermal contraction of the matrix and reinforcement. This damping maximum is strongly dependent on both measurement and material parameters. The reversible shear-modulus evolution during thermal cycling suggests that no interfacial debonding occurs. In unalloyed matrices, extended thermal-stress-induced pleasicity occurs, leading to a plateau in the shear modulus, which is recovered at low temperatures by plastic-zone overlapping and matrix strain hardening. Simultaneously measured strain-temperature loops exhibit both reversible and permanent plasticity during thermal cycling (strain ratcheting).

Mechanical spectroscopy of thermal stress relaxation at metal–ceramic interfaces in Aluminium-based composites

The micromechanisms of thermal stress relaxation in aluminum-based metal-matrix composites (MMCs) have been investigated by mechanical loss and dynamic shear modulus measurements during thermal cycling between 100 and 500 K. A transient mechanical loss maximum, which is absent in the monolithic material, appears during cooling. This damping maximum is strongly dependent on the measurement parameters: oscillation frequency, oscillation amplitude and cooling rate. In addition, it increases with the volumetric reinforcement content and decreases if the matrix strength is improved. The shear modulus evolution during thermal cycling shows that no detectable interfacial debonding occurs. Compared with alloyed MMCs, Al4N-based MMCs show the highest damping maximum simultaneously with a plateau in the elastic shear modulus. The mechanical loss maximum is attributed to dislocation generation and motion near the interfaces, resulting from the differential thermal contraction of matrix and reinforcement. A new model is proposed which describes this specific mechanical behavior of MMCs in terms of the development of microplastic zones in the matrix near the metal–ceramic interfaces.

Effect of compatibilizing compounds on the viscoelastic properties of interpenetrating polymer networks

The effect of various compatibilizing compounds on the viscoelastic behaviour of semi-interpenetrating polymer networks (IPNs) based on cross-linked polyurethane and poly(n-butyl methacrylate) was investigated. It was established that in many cases, because of the specific chemical structure of compatibilizing compounds, the modified system reveals only one maximum of mechanical losses instead of two for initial semi-IPNs. This effect may be called compatibilization throughout the whole volume of the initially phase separated system. The disappearance of two maxima, revealing only one shows that the segregation degree in the IPN diminishes and in some cases miscible systems are formed.

Mechanical Spectroscopy of Thermal Stress Relaxation in Aluminium Alloys Reinforced with Short Alumina Fibres

The mechanical behaviour under low temperature thermal cycling of aluminium-based composites reinforced with short Al 2 O 3 SAFFIL fibres has been investigated by mechanical spectroscopy (mechanical loss and elastic shear modulus measurements). A mechanical loss maximum has been observed during cooling which originates in the relaxation of thermal stresses at the interfaces due to the differential thermal expansion between matrix and reinforcement. The maximum height increases with the volumetric fibre content. In addition, if the matrix strength is increased by the appropriated choice of alloy and thermal treatment, the maximum diminishes and shifts to lower temperatures. No damage accumulation at the interfaces has been detected during long period thermal cycling in the range 100 to 500 K. A description of the damping behaviour is made in terms of the development of microplastic zones which surround the fibres.

Isothermal physical aging of a fully cured epoxy?amine thermosetting system

The rate and effects of isothermal physical aging of a fully cured epoxy—amine/glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (−180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, Tg, was 182°C (0.9 Hz), and the principal glassy-state secondary transition temperature, Tβ, was ≈ −30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (Ta) and that there are two maxima in the aging rate vs. Ta. A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about Ta. The majority of the isothermal aging effect can be eliminated by heating to temperatures above Ta, but below Tg. Theoretical and practical implications of this observation are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 55–67, 1997

Phason Relaxation Mechanisms in Al-Pd-Mn Single-Quasicrystals

Quasicrystals represent a new state of condensed matter which exhibits long-range quasiperiodic translational order and orientational order with noncrystallographic symmetry. Mechanical spectroscopy was applied for the study of defect properties in stable icosahedral single-quasicrystalline Al-Pd-Mn samples. Two mechanical loss maxima appear at 360 K and 870 K for a measuring frequency of f3 Hz. The relaxation parameters for both peaks were determined as H = 0.98 eV, T τ -1 = 4.5.10 -14 s -1 for the low temperature peak, and H = 4.0 eV, τ∞ -1 = 3.0.10 24 s -1 for the high temperature peak, respectively. Both peaks are related to phason defects which are characteristic for the quasicrystalline structure.

Low temperature internal friction spectrum of calcium doped polycrystalline YBa2Cu3O6+ x

Internal friction measurements (IF) in the kilohertz frequency range were performed on Ca doped polycrystalline YBa 2 Cu 3 O 6+ x . The samples were also characterized by DC electrical resistivity and X-ray measurements. The IF spectra obtained in the present investigation show an increase in the low temperature mechanical loss maxima at 95 K and 115 K with increasing Ca content. This could be interpreted as being due to an increase in the electrical charge density, the Y 3+-ions being replaced by the lower valent Ca 2+-ions, thus creating additional holes. Furthermore the fact that doping increases the electrical charge density is shown by a decrease in the electrical resistivity with increasing Ca content. Thus the low temperature IF peaks could be attributed to the hopping of electronic defects in the CuO 2 planes and in the CuO chains, whose density increase with Ca content.

A fatigue crack growth model for fibre-reinforced metal-matrix composites: Chan, K.S. Fatigue Fract. Eng. Mater. Struct. 1990 13, (2), 171–183

The micromechanisms of thermal stress relaxation in aluminum-based metal-matrix composites (MMCs) have been investigated by mechanical loss and dynamic shear modulus measurements during thermal cycling between 100 and 500 K. A transient mechanical loss maximum, which is absent in the monolithic material, appears during cooling. This damping maximum is strongly dependent on the measurement parameters: oscillation frequency, oscillation amplitude and cooling rate. In addition, it increases with the volumetric reinforcement content and decreases if the matrix strength is improved. The shear modulus evolution during thermal cycling shows that no detectable interfacial debonding occurs. Compared with alloyed MMCs, Al4N-based MMCs show the highest damping maximum simultaneously with a plateau in the elastic shear modulus. The mechanical loss maximum is attributed to dislocation generation and motion near the interfaces, resulting from the differential thermal contraction of matrix and reinforcement. A new model is proposed which describes this specific mechanical behavior of MMCs in terms of the development of microplastic zones in the matrix near the metal–ceramic interfaces.

Viscoelastic behaviour of polycarbonate plasticized with stilbene

New parameters are introduced, permitting characterization of the effects of plasticization and antiplasticization of polymers. It was found that by the introduction of the same amount of stilbene into PK, in the range of rubber elasticity the dynamical shear modulus decreases by the same amount, by which it increases in the glassy state. The mechanical loss maximum preceding the glass transition, increases with increasing concentration of the plasticizer. It is assumed that this loss maximum corresponds to the melting of segmental mobility of the non-cooperative type.

Evaluation of Branching in Polyvinyltrimethylsilane on the Basis of Analysis of the Temperature Dependence of Mechanical Losses

Abstract The relaxation transitions in polyvinyltrimethylsilane in the region of 60, 120, and 180°C have been studied by means of the dynamic method. We have investigated the effect of branching on the magnitude and position of the maximum of mechanical losses.

Dynamic properties of a molecular model of a polymeric system in the glass transition

The temperature-frequency dependence of visco-elastic properties in the region of the α-transition (the mechanical transition) is considered on the basis of a molecular model proposed previously for a polymeric system in the glass transition. A kinetic equation has been obtained that describes, with the linear reaction approximation, the dynamic behaviour of a chain located in an effective self consistent medium. The solution of this equation leads to an expression for the complex compliance which satisfies the temperature-frequency reduction principle and this has enabled the non-Arrhenius form of the temperature-frequency dependence of the mechanical (dielectric) loss maxima to be explained.

The nature of the ? transition in the temperature dependence of polyamide mechanical loss

The mechanical loss maximum in polycaprolactam in the vicinity of −50°C at a frequency of ∼10 Hz (β transition) was shown not to be related directly to the presence of moisture in the polymer. This transition is also observed in dried caprone (vacuum drying at 10−4 torr and 180°C for 30 h) after a few days following annealing. The β maximum was shown by IR spectroscopy to decrease and shift towards lower temperatures when hydrogen bonding is weakened as a result of prior heating or compression strain. A conclusion is drawn concerning the relationship between the β transition and hydrogen bonding which fixes the amide groups in the amorphous polymer phase.

Dielectric and ultrasonic studies of the δ‐relaxation of poly(ethyl methacrylate)

AbstractComplex dielectric constants at frequencies of 110 Hz, 1 kHz, and 10 kHz and complex elastic moduli at 34 kHz and 3 MHz were measured in the temperature region the of δ relaxation of poly(ethyl methacrylate) (PEMA) down to 4°K. The temperatures of the loss maximum correlate well with literature values of the temperatures of mechanical loss maxima and with T1 minima. The activation energy is 2.1 kcal/mole. The energy map for the internal rotation of the ester ethyl group of PEMA was calculated; the activation energy and the energy difference between the two energy minima (0.31 kcal/mole) were found to agree well with those determined from experiment. The strength of dielectric relaxation is interpreted on the basis of 0.11 D as the dipole moment of the ethyl group. The strength of mechanical relaxation obtained in the present study as well as the previously published values are discussed. The volume change arising from the internal rotation of an ethyl group is found to be comparable at low temperatures with the van der Waals volume of the ethyl group and to decrease with increasing temperature.

Nature and mechanism of friction of rubberlike polymers in different physical states

We have investigated the frictional properties of crosslinked butadiene-nitrile and butadiene-styrene copolymers and natural rubber in friction against polished steel under vacuum conditions in the temperature interval from −200 to +150° C, which embraces the glassy and high-elastic states, as well as the transition region between them. The temperature dependence of polymer friction is characterized by two maxima, a principal and a low-temperature maximum. The principal maximum, observed in the glass transition region, is not associated with the mechanical loss maximum observed in the polymers themselves. The temperature dependence of the force of friction is composed of three parts. In the high-elastic region there is an increase in the force of friction with fall in temperature, in accordance with the molecular-kinetic theory of friction of rubberlike polymers. In this region the nature of friction is associated with mechanical losses in the surface layer of polymer. The mechanical losses inside the polymer itself are unimportant. The deviation from the theoretical curve and the fall in the force of friction below a certain temperature in the transition region are chiefly associated with a decrease in the actual area of contact as the polymer passes into the glassy state. In the glassy region the friction is significantly determined by the mechanical losses in the polymer itself associated with the repeated elastic and forced-elastic deformation of the asperities in the layer of polymer in contact with the rigid surface. Therefore the low-temperature maximum is closely related to the mechanical loss maximum observed in the same temperature region in dynamic tests. Apart from this, the friction maximum is also associated with the increase in the forces of adhesion and the reduction of the actual area of contact at temperatures at which a forced-elastic mechanism of compression of the polymer asperities is not realized.

Thermomechanical Behavior of an Aromatic Polysulfone

Abstract The thermomechanical behavior (less than 1 Hz) of a newly available, high temperature, aromatic polysulfone has been investigated by the technique of lorsional braid analysis (TBA). Speculations concerning the synthesis and structure are discussed in terms of NMR and infrared spectroscopy, elemental analysis, and a review of the general literature on polysulfones. In addition to the glass transition at 287°C, a secondary relaxation was detected in the glassy state at −100°C by a mechanical loss maximum and change in modulus. The stability of the polysulfone at elevated temperatures in air and nitrogen as monitored by TBA, DTA, and TGA is reported. The influence of heat to 500°C, high doses of ionizing radiation, and water on the transitions of the polymer is discussed.

Relaxation phenomena in high polymers

AbstractThe principles of the nuclear magnetic resonance, NMR, and dynamic mechanical methods and their use for the investigation of segmental motion in high polymers are briefly discussed. Mechanical loss‐temperature data have been obtained from 4.2°K. at audiofrequencies for various addition type high polymers along with NMR line‐width or second moment and NMR spin‐lattice relaxation time, T1, data from 77°K. The examples include a linear polymer, polyethylene, PE, four polymers with mono‐substituted repeat units, polypropylene, poly(4 methyl pentene‐1), poly(ethylacrylate) and polystyrene (PS), and three polymers with disubstituted repeat units, poly(methyl methitcrylate), poly(ethyl methacrylate) and poly(alpha methyl styrene). For all but PE and PS NMR line narrowing processes and T1 minima, attributed to methyl group reorientations, are found at low temperatures. Molecular motion, usually occurring above room temperature as a consequence of the glass to rubber transformation, leads to the appearance of a mechanical loss maximum, further NMR line narrowing, and a T1 minimum. Mechanical loss maxima, possibly due in some cases to side‐chain (other than methyl) reorientations and in other cases to main chain oscillations are also evident.

Proton spin-lattice relaxation measurements on some high polymers of differing structure and morphology: Polyethylene, poly-4-methyl-pentene-1, poly- l-leucine, poly-phenyl- l-alanine, polystyrene and poly-α-methyl-styrene

Proton spin-lattice relaxation times (T 1 ) have been measured at a resonant frequency of 21.5 Mc/s in the temperature range from 77° to 520°K for polyethylene (Rigidex 35) crystals and poly-4-methyl-pentene-1 (P4MP1) crystals grown from dilute solution as well as for melt-formed samples. Exponential decay of magnetization was observed for the polyethylene pellets at all temperatures. On the other hand decay of magnetization describable by two exponentials (two T 1 s) was found at low temperatures for isothermally grown polyethylene crystals, polyethylene dendrites, P4MP1 crystals and melt-formed P4MP1. All three polyethylene samples showed a T 1 minimum at about 250°K which is believed to be related to the γ mechanical loss maximum found for partially crystalline samples, formerly attributed to motions of three or more CH 2 units in the amorphous regions. The position of another minimum in the melt was strongly dependent on sample history and is believed to be a consequence of the increased chain mobility in the crystalline areas. Two T 1 minima were observed for P4MP1, one at about 150°K and another weaker one at 430° to 470°K, the latter being dependent on the sample history. The 150°K minimum is attributed to methyl group reorientation while the higher temperature process is believed to be associated with the glass-rubber transition or possibly a combination of this and a change in the crystalline regions. Spin-lattice relaxation time measurements have also been made for two synthetic polypeptides, poly- l-leucine (PLL) and poly-phenyl- l-alanine (PPLA). For PLL two T 1 values were found at low temperatures. The shorter T 1 has a minimum value at 174°K similar to the P4MP1 crystals which is attributed to methyl group reorientation. A second minimum at about 290°K is attributed to isobutyl or isopropyl group motion. For PPLA a minimum at 200°K and another wide shallow one at 330° to 400°K are found. The first is believed to be due to phenyl motion while the second is believed to be caused by oscillations of the benzyl group with an amplitude which increases with increasing temperature. We have also studied polystyrene and poly-α-methyl-styrene. The higher temperature T 1 minimum is related to the glass-rubber transition. Polystyrene has a minimum at 260°K and a plateau in the T 1 temperature curve at 120° to 150°K. Poly-α-methyl-styrene has a broad minimum at 300° to 340°K due, at least partially, to methyl group reorientation. There is also an inflection in the T 1 temperature curve at 100° to 140°K. The possible relation of these relaxation processes to mechanical loss maxima at low temperatures is discussed.