Ozonolysis of polybutadienes with different microstructure in solution

The reaction of ozone with 1,4‐cis‐polybutadiene and polybutadiene having the following linking of the butadiene units: 1,4‐cis (47%), 1,4‐trans (42%), 1,2 (11%) was investigated in CCl₄ solution. It was found out by means of IR‐spectroscopy and ¹H‐NMR spectroscopy that the basic ozonolysis products of both elastomers are ozonides and aldehydes. The aldehyde:ozonide ratio was 11 : 89 and 27 : 73 for E‐BR and BR, respectively. In addition, epoxide groups were detected, only in the case of BR, and their yield was about 10% of that of the aldehydes. On the basis of BR ozonolysis it was established that the ozonide yield from 1,4‐trans units is about 50%. By using the aldehyde yields, an evaluation was made of the efficiency of ozone degradation of the two polybutadienes, according to which the respective value of BR is considerably higher than that of E‐BR. A reaction mechanism is proposed, which explains the formation of the identified functional groups and the differences in the ozone degradation of the studied elastomers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 427–433, 2007     

Temperature dependence of the properties of low-density polyethylene

The dynamic mechanical properties of low-density polyethylene melts were measured as a function of frequency and temperature using the Orthogonal Rheometer. These results were expressed in terms of the components G′ and G″ of the dynamic modulus and the components η′ and η″ of the dynamic viscosity. The functions J′, J″, η*, and G* were also calculated from the results. The method of reduced variables or time-temperature superpositions was attempted on the results. The classical method was found to require modification to be applied to these low-density polyethylenes. From this modified form of the reduced variables technique, the temperature dependence of the elastic and viscous parts of the response could be separated. The experimentally determined temperature dependence of the elastic part of the response was found not to be in accord with the accepted theory of rubber elasticity. The temperature dependence of the viscous part of the response is discussed in terms of the concept of flow activation energy, and clarification of this term is explored. It is concluded that the temperature dependent properties of polymer melts are best compared at equivalent time scales of response in the non-Newtonian region. In order to do this the temperature dependence of the elastic part of the response must be included explicitly in the reduction scheme.     

A study of miscibility of blends of polybutadienes of varying microstructure

The miscibility of various amorphous polybutadienes with mixed microstructures of 1,4 addition units (cis, 1,4 and trans 1,4) and 1,2 addition units have been investigated. The studies here involved optical transparency, differential scanning calorimetry, and small angle light scattering. It was found that a 90 percent (cis) 1, 4 addition polybutadiene was immiscible with high (91 percent) 1,2 addition polybutadiene. Reduction of the 1,2 content to 71 percent induced an upper critical solution temperature (UCST) with the cis 1,4 polymer. Polybutadienes with 50 percent and 10 percent 1,2 contents were miscible above the crystalline melting temperature of the cis 1,4 polybutadiene. Immiscibility of the 91 percent 1,2 addition polymer was also found with a 10 percent 1,2 polybutadiene. The latter polymer also exhibits an UCST with the 71 percent 1,2 polymer. The results are used to interpret the characteristics of blends of polybutadienes of varying microstructure.     

Synthesis and characterization of (deuterated) polybutadienes of well-defined molecular weight and microstructure

Summary In order to attain reliable information on the structuration and organization of macromolecules participating to interphases in multicomponent materials, it is necessary to use polymers of well defined characteristics in terms of molecular weight, molecular weight distribution and microstructure. This paper describes the synthesis and characterization of a series of polybutadienes, using deuterated monomers in particular. The obtaining of the latter from 2,5-dihydrothiophene-1,1-dioxide and their subsequent polymerization will be described together with some of the characteristics of the resulting polymers. Received: 3 March 2001/Revised version: 15 May 2001/Accepted: 15 May 2001     

Temperature dependence of mechanical properties of aluminum titanate ceramics

Aluminum titanate (Al₂TiO₅: AT) is a synthetic ceramic material of potential interest for many structural applications. A critical feature, which greatly limits the mechanical properties of polycrystalline AT, is the considerable intergranular microcracking, which occurs due to the large thermal anisotropy of individual grains during cooling after sintering. This study discusses the temperature dependence of mechanical properties, and presents observations on the microstructure morphology. Both the fracture strength and fracture toughness increased considerably with increasing temperature. The critical frontal process zone (FPZ) size was estimated from the mechanical properties. A decrease in FPZ size was observed with increasing temperature in the first thermal treatment. These phenomena were explained on the basis of the stress redistribution and unique microscopic feature on the fracture surface of AT ceramics. The experimental results revealed that further thermal treatment increased the fracture strength and reduced the fracture toughness, while it had almost no effect on the FPZ size.     

Temperature dependence of the tensile properties of lignin/paper composites

The mechanical properties of dioxane lignin (DL)/paper and kraft lignin (KL)/paper composites were investigated as functions of temperature and strain rate. The tensile properties of the lignin/paper composites were governed by the viscoelastic properties of lignins used as a matrix. In the temperature dependence of the tensile properties, the strength of DL/paper composite decreased at 70° and 130°C at which the elongation had maxima. This behaviour was caused by the viscoelastic properties of DL having two relaxations in the primary dispersion region at 120° and 160°C. In the case of KL/paper composite, a drastic decrease in the strength and maximum of elongation were observed at the glass transition of KL (140°C). The strain rate dependence of the strength of DL/paper composite showed behaviour typical of viscoelastic materials. The strength increased with increase of the strain rate and then decreased after reaching a maximum, which showed a transition from a brittle to a ductile type of fracture.     

Temperature dependence of the electrical transport properties in few-layer graphene interconnects

We report a systematic investigation of the temperature dependence of electrical resistance behaviours in tri- and four-layer graphene interconnects. Nonlinear current–voltage characteristics were observed at different temperatures, which are attributed to the heating effect. With the resistance curve derivative analysis method, our experimental results suggest that Coulomb interactions play an essential role in our devices. The room temperature measurements further indicate that the graphene layers exhibit the characteristics of semiconductors mainly due to the Coulomb scattering effects. By combining the Coulomb and short-range scattering theory, we derive an analytical model to explain the temperature dependence of the resistance, which agrees well with the experimental results.     

Temperature dependence piezoelectric properties of 0.78PMN-0.32PT ceramics

The temperature dependence piezoelectric properties of 0.78PMN-0.32PT ceramics were studied. The results show that the planar coupling coefficient Kp keeps almost unchanged from room temperature to 90°C, and drops rapidly at temperature regions from 100°C to 130°C. It drops slowly at 130-150°C. Qm reaches minimum values at about 90-100°C, and maximum value at about 140-150°C. This phenomenon can be attributed to the phase transition in this composition. The rhombohedra1 phase changes to tetragonal phase at about 90°C, and tetragonal phase changes to cubic at about 150°C.     

Viscoelastic behavior of polybutadienes with high 1,4‐microstructure and narrow molecular weight distribution

Polybutadienes (PBs) of narrow molecular weight distributions with high 1,4‐microstructure were synthesized by sec‐butyllithium‐initiated, living polymerization. The shoulder‐like anomalous behavior of the storage modulus G′(ω) in the terminal zone for PBs was studied using size exclusion chromatography (SEC) and rheometric mechanical spectrometry (RMS). It was found that a high‐molecular‐weight fraction in the PBs was the main reason for the occurrence of the shoulders. The linear viscoelastic spectra were successfully obtained by the Pade–Laplace method from experimental dynamic data, using either the dynamic storage, G′(ω), or loss, G"(ω), modulus. Both the dynamic moduli and stress relaxation in the linear regime were then calculated with reasonable precision. The feasibility of time–temperature–molecular weight superposition for PBs using this method was demonstrated. The dependencies of relaxation spectra on temperature and molecular weight were obtained by a fitting procedure. Thus, with a known reference state, the relaxation spectrum for a given molecular weight PB at a given temperature can be predicted. The predicted dynamic moduli are in good agreement with experimental data. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1315–1324, 1999     

Temperature dependence of mechanical properties of alumina up to the onset of creep

Mechanical properties of a commercial polycrystalline alumina are evaluated as a function of temperature up to the onset of creep. Hertzian indentation tests in combination with finite element modelling (FEM) are used to determine its elastic and plastic properties from the corresponding indentation stress–strain curves at prescribed temperatures up to 1200 °C. The results reveal a strong decline in the Young's modulus and the yield stress of the material at about 600 °C that is attributed to grain boundary degradation occurring around that temperature. The results are compared with dynamic Young's modulus and Vickers hardness measurements from the literature. The practical interest of measuring the mechanical properties of ceramics at intermediate temperatures is discussed and a general guideline to retain optimum elastic and plastic properties at intermediate temperatures is outlined.     

On the miscibility of liquid polybutadienes

The miscibility of a new type of liquid polybutadiene and dihydroxypolybutadiene with various organic compounds was examined. The aim was to collect data which could be used in the evaluation of the role of miscibility in polyurethanes prepared from this new polymer. Qualitative tests using the cloud‐point method were made in the temperature range from laboratory conditions up to 140°C. Results were put on a common basis by the solvation equation and some characteristics derived from the electrostatic potential. Marked differences in the miscibility behavior between pure and hydroxyl‐terminated polybutadiene with hydroxyl compounds were revealed. The presence of only two hydroxyl groups in a relatively long hydrocarbon chain may thus affect properties of the molecule. Polar molecules are not, in general, miscible with polybutadienes. Polarity may be balanced and miscibility enhanced by supporting π‐electron‐interaction capabilities, H‐bond basicity, or a sufficiently long low‐polar part of a partner molecule. Other ways of improving miscibility is the nonsymmetrical placement of polar groups in the molecule structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1628–1635, 2000     

Rheo-optical properties of polymers: Relaxation and recovery during absorption and desorption of organic liquids

As part of a study in elucidating the mechanisms by which crazing is initiated in polymers, we have measured the force and birefringence decay which occurs in several polymers when placed in contact with methanol. In all cases, the force decay can be used to determine the rate of bulk diffusion of the liquid into the polymer. In contrast, the birefringence decay appears to be related to shear relaxation mechanisms which are highly dependent on the type of material. For example, in the case of simple rubbers such as polyisoprene, the rates of decay of force and birefringence are identical within experimental error. In contrast, in amorphous thermoplastics the force decay is typically an order of magnitude faster than the birefringence decay. These results are discussed in terms of distortional and orientational birefringence and with respect to the morphology of the glassy state.     

Temperature and stress dependence of electrical and mechanical properties of Green River oil shale

Simultaneous measurements of the variation in the electrical and mechanical properties with stress and temperature have been carried out on Green River oil shale. The results of the present study reveal the presence of a structural transition at 380 °C; this temperature also corresponds to the structural yield point of the shale. Changes in the mechanical strength of the shale at temperatures corresponding to its yield point are accompanied by corresponding sharp variations in its electrical resistivity. The loss in mechanical strength at the yield point is found to be directly proportional to the amount of organic matter in the shale. The structural transition at the yield point is absent in the second heating cycle; electrical and mechanical measurements on reheated shale samples show clearly the irreversible nature of this transition. Elastic rebound effects are, however, considerable, especially for the lean shales. The high resistivities characteristic of Green River oil shale and the trends in its temperature-dependent resistivity behaviour indicate a predominantly ionic conduction mechanism. The structural yield point at 380 °C also represents the temperature below which electrical resistivity of the shale is structure-sensitive; the region of temperatures above 380 °C represents an intrinsicconductivity domain with an activation energy of 1.8 eV. The close correlation observed in the present study between electrical and mechanical behaviour, underlines the complementary nature of the information gained from simultaneous parameter measurements.     

Temperature dependence of gel properties of two‐component physical gels

The cascade model for mixed gels developed by the author in a previous work is extended to describe the temperature‐dependent gel properties. The equilibrium constant of the association between component polymers is assumed to depend on temperature via a van't Hoff‐type equation. The temperature variation of the network structure and gel modulus is presented and discussed at different parameters such as enthalpy change per crosslink ΔH°, entropy change per crosslink ΔS°, functionality ratio s, and concentration ratio r. It is demonstrated that the model agrees reasonably well with the experimental data obtained from the rheological gelling for galactomannan/xanthan and glucomannan/xanthan mixed gels. However, the resulting model parameters are not consistent with those obtained from the concentration dependence study. A further investigation on the calorimetric thermogram of the glucomannan/xanthan mixed gel reveals that the gelling process involves an association reaction followed by a structural rearrangement, which is beyond the scope of this work. Finally, the cascade model is shown to be consistent with the Eldridge–Ferry equation. It is also demonstrated that the sol–gel behavior of the galactomannan/xanthan mixed gel follows the Eldridge–Ferry relationship, but the calculated melting enthalpy is composition‐dependent, contrary to the assumption made in the cascade model. This discrepancy is due to the self‐association of xanthan when xanthan is present in excess amounts. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 663–673, 2006     

Temperature dependence of the electrical properties of Poly(ethylene oxide)/salt complex composite

The electrical properties of Poly(ethylene)/Salt complex membrane containing 50 wt.% natural salts evaporated from the Dead Sea water were studied as a function of temperature in the range 25-55°C through the impedance technique. It was found that the impedance decreases and the AC ionic conductivity increases with increasing temperature. The variation in the dielectric constants, relaxation time and activation energy with temperature are discussed. It was found that the observed relaxation peak is shifted to higher frequency as the temperature is increased. Thus, it was seen that addition of the Dead Sea salt complex can improve the electrical conduction behavior of the prepared composite.     

Mechanical and thermal properties of polyurethane elastomers based on hydroxyl‐terminated polybutadienes and biopitch

Biopitch is a renewable source of polyol obtained from Eucalyptus tar distillation, which was studied as an active component of polyurethane (PU). The polymerization occurred in one step, with a mixture of biopitch and hydroxyl‐terminated polybutadiene polyols reacted with 4‐4′‐diphenyl methane diisocyanate in the presence of dibutyltin dilaurate. Solid‐state ¹³C‐NMR, IR spectroscopy, elemental analysis, and thermal analysis [thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)] were used to characterize the biopitch. The biopitch sample showed an aromatic and oxygenated structure with great thermal stability at high temperatures. Multiphasic PUs were synthesized and characterized by IR spectroscopy (attenuated total reflectance), elemental analysis, thermal analysis (TGA and DSC), mechanical assays (tensile strength, elongation at break, toughness, hardness, and resilience), and water absorption resistance (ASTM D 570‐81). In a comparative study of the synthesized elastomers, biopitch content increased tensile strength and hardness and decreased thermal stability, elongation at break, and water absorption. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 759–766, 2003     

Temperature dependence of chain dimensions

The dimensions of linear atactic polystyrene ($\text{M}\text{?}{}_{\mathrm{w}}\text{= 75 700}$) in cyclohexane have been determined at a series of temperatures using small-angle neutron scattering. Three solutions were examined: dilute (2% polymer), semi-dilute (19% polymer) and concentrated (47% polymer). End-to-end distances obtained from the data were compared with current theories of polymer solutions. For the semi-dilute solution results agreed with scaling law predictions, whereas results from the concentrated solution agreed with the formula obtained by Edwards. Furthermore, the latter results gave a characteristic ratio (C_∞) of 9.5 ± 0.7 for polystyrene.