Birefringence of polyethylene melt in transient elongational flow at constant strain rate

Simultaneous birefringence and elongational viscosity measurements were carried out on molten commercial grade, low-density polyethylenes during simple elongational flow at constant strain rate and constant temperature. The birefringence increased with time during constant strain rate elongation. The increase in birefringence was a linear function of elongational stress throughout whole elongation, but the elongational viscosity increased in two stages. The increase in elongational viscosity can be divided into linear viscoelastic and nonlnear viscoelastic regions. The linear region appeared at small strain and the nonlinear region appeared at strain greater than 0.7. The elongational viscosity in the nonlinear region increased much more rapidly than that in the linear region. The Gaussian approximation, which is commonly used in molecular models, could be used for the transient elongational flow. A constant stress-optical coefficient C = 1.3 × 10^?10 cm²/dyn was obtained for all the elongational experiments, independent of strain rate (0.002-0.2s^?1). The stress-optical coefficients were weakly dependent on temperature, as predicted by the theory of rubber elasticity.     

Evaluation of the relaxation modules from the response to a constant rate of strain followed by a constant strain

The stress response σ(t) to a constant rate of strain $ \dot \varepsilon $ ε during the period 0 < t ≤ t^* and to the constant strain ε^* $ ( = \dot \varepsilon t*) $ thereafter is considered in terms of the Boltzmann superposition principle. When t ≤ t^*, the data directly give the constant-rate modulus F (t) ≡ σ(t)/ε(t), which can be converted straightforwardly into the relaxation modulus E(t). Results from illustrative calculations show that a reduction in the relaxation rate effects a decrease in [σ(t^*)/ε^*]/E(t^*) and also in the time at which [σ(t)/ε^*]/E(t) becomes essentially unity. To evaluate E(t) at t > t^*, F(t) is first obtained from σ(t) and F(t ? t^*) by using a derived equation similar to that presented by Meissner. Thereafter, F(t) is transformed into E(t). For illustration, E(t) for a rubbery solid is evaluated over some 2.5 decades of time from its response to a strain rate of 0.25 min^?1 for 0.40 min and thereafter to the attained strain of 0.10 for 5.4 min.     

Characterization of filled elastomers by dynamic strain amplification at high frequencies

A high-frequency dynamic mechanical tester was used to characterize gum elastomers and their carbon black-filled counterparts. The objectives were to test a model for strain amplification in tensile deformation dynamically at high frequencies, to test further a previously proposed mechanism for elastomer reinforcement in shear and tensile deformation at high frequencies, and to evaluate the overall performance of a high-frequency dynamic mechanical analyzer. Differences in tensile and shear behavior of the compounds were found to be consistent with previous results, as were the effects of strain amplification, which were greater at higher frequencies and lower strains.     

Combined constant strain rate and stress relaxation test for linear viscoelastic studies

The strain history of the combined test is formed by a constant strain rate during the period 0 <t < t₁ followed by a constant strain for t > t₁. This test includes as limiting cases the relaxation test (t₁ → 0), the constant strain rate or stress growth experiment (t₁ → ∞), and the stress relaxation after cessation of constant strain rate flow (t₁ sufficiently large). The constant t₁ can be chosen arbitrarily such that the test conditions of the combined test can be adjusted according to the problem investigated and to the capability of the testing device. From this, practical advantages follow for linear viscoelastic studies of polymeric solids and liquids, e.g., the zero shear viscosity η₀ for unstable polymer melts can be determined at least approximately. In order to obtain linear viscoelastic material functions from the measured stress record, a simple procedure is given which avoids the difficulties with the “factor-of-10 rule” for short relaxation times.     

Rate of physical aging of polycarbonate at a constant tensile strain

A study was made of the physical aging of an annealed polycarbonate film at a constant extension of 2.6% at 5 temperatures from 30 to 110°C. During stress relaxation at the constant extension, the storage modulus in tension, E′, was determined by imposing a sinusoidal strain of small amplitude at frequencies up to 25 Hz during an aging period, commonly of 5 h. Plots of log E′ against log f, where f is the frequency, gave parallel straight lines, each at a different aging (elapsed) time t_e. These lines were superposed by shifts along the abscissa. The obtained shift factors log a varied linearly with log t_e, the slope being the aging rate μ, a quantity introduced by Struik. The results show that μ is about 1.37 at 30°C and that it increases progressively with the temperature and becomes approximately 2.13 at 110°C. Another measure of the aging rate is the rate of increase of E′ with t_e, expressed as the percent increase per decade of the aging time. This quantity also increases progressively with the temperature from about 2.6% at 30°C to nearly 3.5% per decade of time at 110°C.     

Dynamic bulk modulus of various elastomers

The dynamic bulk modulus of elasticity has been measured for 14 different rubbery elastomers: three natural rubbers, five neoprenes, three polyurethanes, and one each of butyl, nitrile, and butadiene types. The measurements ranged in temperature from ?10 to +40°C, at frequencies from 5 to 3000 Hz, but mostly in the range 100–1000 Hz, at 2.5 MPa pressure. Values of the real (storage) part of the modulus fell within 35% of the mean value of 2.9 GPa for all elastomers, whereas loss moduli were a few percent of the storage moduli. Master curves were obtained for two neoprenes, a polyurethane, and a butyl rubber. These were fitted by hyperbolic functions with four adjustable parameters. Effects of room-temperature aging in artificial sea water were also studied. Aging versus time profiles fell into two distinct forms. Natural rubbers were least stable, neoprenes were intermediate, and urethanes proved most stable in bulk modulus.     

Mechanisms of large actuation strain in dielectric elastomers

Subject to a voltage, a dielectric elastomer (DE) deforms. Voltage‐induced strains of above 100% have been observed when DEs are prestretched, and for DEs of certain network structures. Understanding mechanisms of large actuation strains is an active area of research. We propose that the voltage‐stretch response of DEs may be modified by prestretch, or by using polymers with “short” chains. This modification results in suppression or elimination of electromechanical instability, leading to large actuation strains. We propose a method to select and design a DE, such that the actuation strain is maximized. The theoretical predictions agree well with existing experimental data. The theory may contribute to the development of DEs with exceptional performance. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Primary radical termination rate constant at high conversion in radical polymerization

A primary radical termination rate constant given by: k_ti = A_1iD_i, where A_1i is a constant and D_i is the diffusion constant of the primary radical, was examined on the basis of the variation of conversion. It was proved that this rate constant is correct at high conversion. A relationship between primary radical termination rate constant and conversion was derived. The effect of variation of conversion on the gel effect is discussed.     

Precise dipolar coupling constant distribution analysis in proton multiple-quantum NMR of elastomers

In this work we present an improved approach for the analysis of (1)H double-quantum nuclear magnetic resonance build-up data, mainly for the determination of residual dipolar coupling constants and distributions thereof in polymer gels and elastomers, yielding information on crosslink density and potential spatial inhomogeneities. We introduce a new generic build-up function, for use as component fitting function in linear superpositions, or as kernel function in fast Tikhonov regularization (ftikreg). As opposed to the previously used inverted Gaussian build-up function based on a second-moment approximation, this method yields faithful coupling constant distributions, as limitations on the fitting limit are now lifted. A robust method for the proper estimation of the error parameter used for the regularization is established, and the approach is demonstrated for different inhomogeneous elastomers with coupling constant distributions.     

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at T_g – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1957–1967     

Phase separation in dilute polymer solutions at high‐rate extension

In this article the demixing instability and phase segregation in unentangled polymer solutions of semiflexible chains at high‐rate uniaxial extension above the coil to stretched coil transition was studied. Orientation of the stretched chains was described in terms of an effective potential field. Based on the free energy analysis it was shown that the flow‐induced orientation of polymer segments could drastically reduce the energy of their steric repulsion. As a result attraction between the chains gain more importance, and this effect lead to the demixing process and eventual segregation of polymer from the solvent if the strain rate exceeds some critical value. A mean‐field theory was developed to study this flow‐induced phase separation effect. The phase diagrams of the system showing the spinodal and binodal transitions at different extension rates were calculated and discussed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1066–1073     

Effect of viscosity on termination rate constant in radical polymerization at low conversion

By using the expression, k_t = A₁D_s for the chain termination rate constant (where A₁ is a constant and D_s is the diffusion constant of radical chain end), a familiar chain termination rate constant, k_t = A₂/η_s (where A₂ is a constant and η_s is solvent viscosity) was examined with variation of conversion x. It was found that the proportionality of chain termination rate constant and solution viscosity is a valid relation at conversion 0 but is approximate at conversion x_c ≥ x > 0. Here x_c denotes a critical conversion under the average distance around spherical polymers formed in polymerization solution is zero. At conversions above x_c, the inverse relation between chain termination rate constant and solution viscosity is not correct.     

Thermal aspects of constant rate transformation

A mathematical model for the conjugate formulation of the working conditions of the Q-derivatograph is created. There are no simplifying assumptions about the heat transfer processes for reacting active media and for their thermal interaction with the inert surroundings in this model. The energetic significance of the key components of the transfer and the redistribution of the fluxes is calculated by means of special balance relations. The influence of the kinetic and thermal reaction parameters on the nature of the establishment of the constant transformation rate is demonstrated. The accompanying heat phenomena are studied in detail. The essential differences in the behaviour of the TA system during endo and exothermal transformations are discovered. The exothermal reactions introduce significant irregularities, up to sharp oscillations.

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Zusammenfassung Für die Arbeitsbedingungen des Q-Derivatographen wird ein mathematisches Modell aufgestellt. Dabei werden keine vereinfachenden Annahmen über die Wärmetransportprozesse für die reagierenden Stoffe und über ihre thermische Wechselwirkung mit der inerten Umgebung gemacht. Die Energien der entscheidenden Beiträge des Transports und der Verteilung der Ströme werden mit speziellen Bilanzbeziehungen berechnet. Der Einfluss der kinetischen und thermischen Reaktionsparameter auf die Einstellung einer konstanten Umwandlungsgeschwindigkeit wird gezeigt, die entsprechenden thermischen Vorgänge werden im einzelnen untersucht. Die wesentlichen Unterschiede im Verhalten des Systems bei endo- und exothermen Vorgängen werden aufgezeigt, exotherme Reaktionen bewirken erhebliche Schwankungen bis zu deutlichen Oszillationen.

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The rate constant for the recombination of trifluoromethyl radicals at T = 296 K

The high-pressure rate constant of the CF₃ + CF₃ → C₂F₆ reaction at T = 296 K was measured in the pulse photolysis (λ = 694.3 nm, ruby laser) of CF₃NO in the presence of NO by means of the time-resolved detection of CF₃NO by the intracavity absorption of He(SINGLE BOND)Ne laser radiation (λ = 632.8 nm). The obtained value is k_2∞ = (3.9 ± 1.3) × 10⁻¹² cm³/s. © 1996 John Wiley & Sons, Inc.     

On the measurement of true fracture strain of thermoplastics materials

In tensile testing, engineering thermoplastic materials can sustain large deformation after necking. This property can be advantageous to the manufacturing process and mechanical performance. However, it causes difficulties in strain measurement and in the determination of true stress and true strain, particularly at higher strain rates. This paper discussed the measurement of true fracture strain. The true fracture strain was estimated using three different methods: the area reduction, elongation over the necked zone, and elongation over the gage length. With strain localization, strain measured by an extensometer becomes gage length dependent. To address the issue, an algorithm was formulated that estimates the average fracture strain over an arbitrary gage length from the fracture strain measured over the necked zone. The proposed algorithm provides a way to estimate the mesh dependence of the fracture strain for finite element (FE) analysis. This paper also presents an analysis-based two-step correction scheme for the crosshead extension to be used to supplement the strain measurement from the extensometer.     

线形低密度聚乙烯／废胶粉热塑弹性体动态硫化性能研究

利用动态硫化法制备了线形低密度聚乙烯(LLDPE)/废胶粉(GTR)热塑弹性体。重点研究了两种交联剂:硫和过氧化二异丙苯(DCP)对共混物性能的影响。加入一定量的苯乙烯-丁二烯-苯乙烯(SBS)共聚物作为增容剂。结果表明,经过DCP动态硫化后的共混物的力学性能比简单共混的共混物有明显的提高,而加入硫磺体系对共混物力学性能影响不大甚至有所下降。通过红外光谱、热分析(DSC)和扫描电镜(SEM)对共混物的热行为和表面形态研究表明,加入DCP交联剂使LLDPE、SBS和胶粉之间发生了交联反应,从而增加了胶粉颗粒与LLDPE间的界面相容性,使其热塑性弹性体的力学性能得以提高。     

Effect of uniaxial strain on a molecule-based ferrimagnet with crystal chirality

Bimetallic ferrimagnet [Cr(CN)₆][Mn(R)-pnH(H₂O)]H₂O with crystallographical chirality of space group P2₁2₁2₁ exhibits a giant third-order harmonic susceptibility at around 38 K. This anomaly appears as precursory magnetic phenomenon of the magnetic ordering. In order to investigate the effect of lattice distortion on this giant nonlinear magnetic response (NLMR) and finally elucidate the correlation between the structural chirality and the giant NLMR, we performed the ac susceptibility measurement for the single crystal in the situation of bringing about lattice distortion along the direction parallel to the a-axis. This giant NLMR was drastically suppressed even by slight strain, whereas the traces could be confirmed in the region up to 3.7 kbar. We conclude that the appearance of the giant NLMR requires crystal chirality well-regulated over the crystal.