Thermomechanical and adhesive properties of radiation-modified polymer composites for thermosetting products

Gamma-irradiated blends of polyethylene (PE) with ethylene-propylene-diene copolymer (EPDM) and a thermotropic liquid-crystalline polymer (LCP) are investigated at absorbed radiation doses not exceeding 150 kGy (10 kGy=1 Mrad). The temperature dependences of elastic moduli, tension diagrams at a temperature above the melting point of the crystalline phase of PE, and long-term strain recovery curves for oriented test specimens are presented. The kinetics of thermal relaxation and shrinkage stresses in previously oriented composite specimens upon their heating and cooling is investigated. Data on the influence of LCP additions on the adhesive interaction of the compositions with steel are obtained. The peculiarities of thermomechanical and adhesion properties of these composites are discussed taking into account the morphologic and calorimetric data obtained. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 379–394, May–June, 2000.     

Features of thermomechanical properties of radiation-modified blends of high-density polyethylene with liquid-crystalline copolyester

The present paper surveys the investigation results of gamma-irradiated blends of high-density polyethylene (PE) with thermotropic liquid-crystalline polymer (LCP). The LCP used was a liquid crystalline copolyester of 40% poly(ethyleneterephthalate) with 60% p-(hydroxybenzoic acid). The LCP content in the blends was 0,5, and 10 wt.%. The constituents were blended with the use of a single-screw extruder. The samples were prepared by compression molding and irradiated by a Co⁶⁰ -radiation source in an inert atmosphere (argon) to relatively low absorbed doses (up to 200 kGy; 1 Mrad=10 kGy). Morphological, thermal, and mechanical properties in a wide temperature range were investigated for the irradiated and nonirradiated samples. The effects of gamma-irradiation and LCP addition on the thermomechanical behavior of PE are discussed. It was found that the LCP addition affected significantly the stress-strain behavior of PE at temperatures above the melting point. The features of thermorelaxation properties of the PE/LCP blends previously irradiated and oriented, particularly thermorelaxation and residual shrinkage stresses at isometric heating and cooling, were also established. The results obtained testify that the LCP addition makes it possible to improve considerably the thermosetting properties of irradiated PE.Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 1, pp. 124–139, January–February, 1998.     

Comparison of modified and standard methods of constructing thermomechanical curves with reference to the plasticization of cellulose triacetate films

With reference to the analysis of a thermomechanical curve obtained in the usual way it is shown that by virtue of its sharply expressed dependence on time in the region of the glass-transition point the strain is not a sufficiently convenient criterion for estimating the glass-transition temperature of a polymer. A modified method of constructing the thermomechanical curve, which takes this factor into account, is proposed. The possibility of using this method to investigate the plasticization of cellulose triacetate films is demonstrated.Mekhanika Polimerov, Vol. 4, No. 3, pp. 555–558, 1968     

Variation of the deformation properties and internal stresses during the heating of polyacrylonitrile fiber on the temperature interval 20–400°C

The thermomechanical and isometric heating methods have been used to study the deformation properties and internal stresses of oriented polyacrylonitrile fiber heated in air under a specified initial load on the temperature interval 20–400°C. The stress growth kinetics have been investigated. The effect of heat-treatment temperature on the isometric heating diagrams is considered.     

Modified method of constructing the thermomechanical curves of polymer films

It is shown that because of its sharply expressed dependence on time in the region of the glass-transition point, deformation is not a sufficiently convenient criterion for evaluating the glass-transition temperature of polymers. A modified method of constructing the thermomechanical curve that takes this factor into account is proposed. It is shown that this method can be used to investigate the plasticization effect in cellulose triacetate films.Mekhanika Polimerov, Vol. 3, No. 5, pp. 940–943, 1967     

Self-reinforcement of liquid-crystal polymers at static and dynamic loading

The self-reinforcement effect of a solid uniaxially oriented SVM-K liquid-crystal polyamide and a copolyester of hydroxybenzoic and hydroxynaphthoic acids has been investigated by tensile-strength, stress-relaxation, and dynamic methods. The samples were prepared by spinning from lyotropic solution (SVM-K) and from a thermotropic melt (polyester). The tensile-strength and stress-relaxation tests were performed on complex fibers and the dynamic test on single fibers. The set of stress-strain curves, changing from a convex shape with two linear sections (at room temperature) to a concave shape (at high temperatures) is shown for both materials in Fig. 1. There is a pronounced difference between the deformation mechanisms at low and high strains in the stability of rigidity. At high temperatures the rigidity becomes less than the initial one during deformation and the current modulus at high strains has the same value within large ranges of temperatures and strains (Fig. 2). A low-deformation transition of another physical parameter than the yield-stress has been found. The stress-strain diagram for both investigated polymers has been generalized by using the constant value of the current modulus for the normalization of the stress value (Fig. 3). The stress-relaxation phenomena are shown to be anomalous. At high temperatures the stress-relaxation intensity decreases with increasing deformation, i.e., after deformation the polymer is characterized by a stability of rigidity which is higher than the initial value (Fig. 4). The dynamic modulus appears to increase with increasing deformation rate (Fig. 5). Due to these peculiarities the liquid-crystal polymers must be considered not only as normal high-modulus reinforcements for composite materials but also as materials, self-reinforcing under loading.Translated from Mekhanika Kompozitnykh Materialov, Vol. 30, No. 4, pp. 435–441, July–August, 1994.     

Uniaxial deformation of partially crystalline oriented polymers (fibers)

The uniaxial deformation of partially crystalline oriented polymers is examined under the following assumptions: the volume of the specimen is constant, deformation is affine, stable and reversible, and the crystallites are only slightly deformable as compared with the deformation of the amorphous part. A general expression is obtained for the applied load as a function of the crystallinity, anisotropy factor, and relative elongation. The following cases are considered: isotropic, completely amorphous specimen; isotropic, partially crystalline specimen; and highly oriented, partially crystalline specimen. The results obtained are compared with experimental data.Mekhanika Polimerov, Vol. 3, No. 1, pp. 24–28, 1967     

Effect of gamma radiation on the mechanical properties of polyvinylidene fluoride

The variation of the strength and relative elongation at break with radiation dose has been investigated for unoriented block specimens of PVF₂ and for oriented specimens with different crystalline structure ( and ). A certain difference is observed in the behavior of the irradiated oriented specimens and an important difference in the behavior of oriented and unoriented specimens of PVF₂ with the same crystalline structure (). On the basis of the data obtained and the results of a study of the crystalline and supermolecular structure of the irradiated PVF₂ specimens it is concluded that there is a difference in the state of the amorphous regions in the oriented and unoriented PVF₂.Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 146–149, January–February, 1973.     

Thermomechanical method for determining the molecular weights of amorphous linear polymers

A thermomechanical investigation of polyisobutylene specimens under various time and stress conditions enabled us to extend the applicability of this method of determining the molecular weight of an undissolved polymer. The previously proposed mechanical model of a polymer, which is the basis of the relationship between the molecular weight of the polymer and the difference between the yield and glass-transition temperatures, must be refined to make allowance for the supramolecular structure.Mekhanika Polimerov, Vol. 3, No. 4, pp. 586–590, 1967     

Time and temperature dependent deformation of poly(ether ether ketone) (PEEK)

The stress-strain behavior in tension and the effect of temperature on the creep of poly(ether ether ketone) (PEEK) have been studied. At room temperature, 130° below the glass-transition temperature, the material does not become brittle, and the specimens show necking in tension over a wide range of elongation rates. The stress and strain at yield and the strain at break are almost linear functions of the logarithmic elongation rate. The values of stress and strain at yield increase slightly with increasing elongation rate, while the strain at break decreases markedly. The short-term creep tests were conducted at temperatures extending from 20 to 200°C. The glass-transition temperature was found to be about 155°C. The creep of PEEK is greatest at temperatures above 130°C. In the glass region the time dependence of the deformation is much weaker. It has been found that the time-temperature relation for PEEK corresponds well with its thermorheological simplicity in the temperature range investigated. The data on the temperature shift factor below and above the glass-transition temperature may be fitted separately to the Arrhenius and Williams-Landel-Ferry (WLF) equations, respectively. The long-term creep tests show that PEEK has excellent creep resistance at room temperature. After 14-month tests at a stress level of 30 MPa the total strain exceeds the instantaneous elastic strain only by a factor of 1.15.Institute of Polymer Mechanics. Latvian Academy of Sciences, 23 Aizkraukles St., LV-1006 Riga. Latvia. Department of Polymeric Materials, Chalmers University of Technology. S-412 96 Gothenburg, Sweden. Published in Mekhanika Kompozitnykh Materialov, No. 6, pp. 734–746, November–December, 1997.     

Polytetramethylene glycol-modified polycyanurate matrices reinforced with nanoclays: synthesis and thermomechanical performance

The outstanding improvement in the physical properties of cyanate esters (CEs) compared with those of competitor resins, such as epoxies, has attracted appreciable attention recently. Cyanate esters undergo thermal polycyclotrimerization to give polycyanurates (PCNs). However, like most thermo setting resins, the main draw back of CEs is brittleness. To over come this disadvan tage, CEs can be toughened by the introduction of polytetramethylene glycol (PTMG), a hydroxyl-terminated polyether. How ever, PTMG has a detrimental impact on Young’s modulus. To simultaneously enhance both the ductility and the stiffness of CE, we added PTMG and an organoclay (mont morillonite, MMT) to it. A series of PCN/PTMG/MMT nanocomposites with a constant PTMG weight ratio was pre pared, and the resulting nanophase morphology, i.e., the degree of filler dispersion and distribution in the composite and the thermomechanical properties, in terms of glass-transition behaviour, Young’s modulus, tensile strength, and elongation at break, were examined using the scanning elec tron micros copy (SEM), a dynamic mechanical analysis (DMA), and stress–strain measurements, re spectively. It was found that, at a content of MMT below 2 wt.%, MMT nanoparticles were distributed uniformly in the matrix, suggesting a lower degree of agglomeration for these materials. In the glassy state, the significant increase in the storage modulus revealed a great stiffening effect of MMT due to its high Young’s modulus. The modification with PTMG led to a 233% greater elongation at break compared with that of neat PCN. The nanocomposites exhibited an invariably higher Young’s modulus than PCN/PTMG for all the volume factors of organoclay examined, with the 2 wt.% material displaying the most pronounced in crease in the modulus, in agreement with micros copy results. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 2, pp. 255–268, March–April, 2009.     

A mathematical model for induction hardening including mechanical effects

We investigate a mathematical model for induction hardening of steel. It accounts for electromagnetic effects that lead to the heating of the workpiece as well as thermomechanical effects that cause the hardening of the workpiece. The new contribution of this paper is that we put a special emphasis on the thermomechanical effects caused by the phase transitions. We take care of effects like transformation strain and transformation plasticity induced by the phase transitions and allow for physical parameters depending on the respective phase volume fractions.The coupling between the electromagnetic and the thermomechanical part of the model is given through the temperature-dependent electric conductivity on the one hand and through the Joule heating term on the other hand, which appears in the energy balance and leads to the rise in temperature. Owing to the quadratic Joule heat term and a quadratic mechanical dissipation term in the energy balance, we obtain a parabolic equation with L¹ data. We prove existence of a weak solution to the complete system using a truncation argument.     

Structure and properties of injection molded blends of liquid crystal polymer (40 PET/60 PHB) with poly(butylene terephthalate)

Blends of a poly(butylene terephthalate) (PBT) and a liquid crystalline polymer (LCP) were prepared in a single screw extruder. The LCP used was a liquid crystalline copolyester of 40% polyethylene terephthalate with 60% p-hydroxybenzoic acid (40 PET/60 PHB). Specimens for mechanical testing were prepared by injection molding. Rheological, morphological, microstructural, and mechanical properties were investigated by capillary rheometry, polarized light microscopy, x-ray diffractometry, and a tensile tester, respectively. For the LCP content higher than 5 wt. % a significant decrease of the viscosity was noted. The morphology of the dispersed LCP phase varies between droplets and oriented fibrils. The degree of cystallinity of PBT and the crystal sizes were found to be affected by LCP addition. The tensile mechanical behavior of the PBT + LCP blends was also found to be affected by their compositions. The elasticity modulus and the parameter of elasticity anisotropy monotonically increase with the LCP content. The strain at break shows a drastic decrease for the blends containing more than 10 wt. % LCP. The long-term creep test shows that the creep compliance decreases noticeably when the LCP content increases. Institute of Polymer Mechanics of the Latvian Academy of Science, Riga, LV-1006, Latvia. Ecole Europeénne de Chimie des Polymeres et des Matériaux, Université Louis Pasteur, F-67000 Strasbourg, France. Poznan University of Technology, Laboratory of Crystallography and Polymer Physics, PL-60-965 Poznan, Poland. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 5, pp. 676–689, September–October, 1996.     

Automated thermomechanograph for investigating polymer microsamples

An instrument has been developed for recording the thermomechanical curves of microsamples of polymers (30 mg) by the penetration method in the continuous heating regime. The thermomechanical curves are recorded by a PDS-021 two-coordinate recorder; the effective temperature range is 20–400°C.Institute of High-Molecular Compounds, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 746–749, July–August, 1968.     

Rheological and dynamic thermomechanical propert ies of epoxy composites reinforced with single- and multiwalled carbon nanotubes

An epoxy resin (Epon 828) was filled with single- and multiwalled carbon nanotubes (SWCNT and MWCNT) in two steps by using the high shear mixing and ultrasonication techniques. The melt flow of the composites was characterized in a plate/plate rheometer. The thermomechanical properties of the composites were determined in dynamic mechanical analysis tests performed at various frequencies and temperatures. It was found that the incorporation of SWCNT or MWCNT increased the viscosity and stiffness of epoxy above its glass-transition temperature. The time-temperature superposition principle was employed to estimate the storage modulus of the composites as a function of frequency (f = 10^–33–10³ Hz) in the form of master curves.     

Molecular mechanism of oriented crystalline polymers

It is shown with reference to linear polyethylene that the deformation of an oriented crystalline polymer takes place as a result of elongation of the coiled parts of the macromolecules in the amorphous zones owing to conversion of gauche into extended trans isomers. The decrease in coiled isomer content when the specimen is deformed by approximately 5% is accompanied by a small number of chemical bond breakages. Repeat deformation by the same amount does not result in any additional breakage of polymer chains.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of The USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 584–588, July–August, 1973.     

Effect of moisture on the viscoelastic properties of an epoxy-clay nanocomposite

The results of a complex study on the viscoelastic behavior of an epoxy-clay nanocomposite after a long-term exposure to moisture are presented. The main laws of variation in the glass-transition temperature of the nanocomposite in relation to the different content of filler and absorbed moisture were determined by using a thermomechanical analysis. The loading levels in creep experiments were chosen according to the results of quasi-static tensile tests. The sets of creep and creep recovery curves obtained were approximated by the Boltzmann–Volterra linear integral equation with account of the principle of moisture-time analogy. The variation in the spectrum of retardation time of the epoxy resin with introduction of the nanofiller was estimated. It is shown that the moisture-time reduction function correlates with changes in the forced rubber-like elasticity and the volume of nanocomposite specimens upon their moistening.     

Relaxation mechanisms in polymers

The various types of molecular-kinetic effects are examined and the place of relaxation processes among them is determined. The transitions in noncrystalline and crystalline polymers are classified in accordance with physical principles. The experimental results of a study of molecular mobility on a broad temperature interval are analyzed for bulk polymers with different effective chain stiffnesses. The importance of taking into account the various relaxation processes that develop in polymers used in different branches of technology is noted.     

Vibrations and heating of inelastic solids under harmonic loading

Two problem statements for the investigation of the thermomechanical behavior of inelastic materials under harmonic loading are developed. The “complete” problem statement consists of universal balance equations of thermodynamics and constitutive equations derived from the general thermodynamic theory of viscoplastic solids with internal state variables. An approximate problem statement is derived by the application of modified harmonic linearizing technique to the complete system of equations. Thereby the subsystem of mechanical equations is formulated in the complex-value form and material properties are described by means of amplitude-dependent complex-value moduli. The problems of vibration and dissipative heating of physically nonlinear solids are formulated. The main thermomechanical characteristics are analyzed for some classes of problems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Static and dynamic elasticity of some thermotropic liquid-crystalline copolyesters

Two series of thermotropic liquid crystalline copolyesters were studied in static and dynamic tensile deformation. Static loading generally does not lead to any peculiar phenomena: the stress-strain diagrams are linear or convex, except one copolyester fibre, the stress-strain diagram of which is concave at high temperatures. On the contrary, in dynamic loading the elasticity modulus increased during deformation for all the samples investigated. So it can be assumed that formation of a more rigid and stable structure takes place due to the dynamic deformation.Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 704–709, September–October, 1997.