An environmental instrument for measuring creep and stress relaxation in polymers

The construction and operation of an instrument for measuring tensile stress relaxation and creep, particularly of polymers, is described. The instrument is comparatively inexpensive to build and enables measurements to be carried out in vacuo or in a controlled atmosphere of gas or vapor. The design is based on principles used for some earlier stress relaxometers modified to enable characterization of samples having a very wide range of moduli either as stress relaxation or, additionally, as creep measurements. The instrument can therefore be used to evaluate material properties of hard plastics or of soft rubbers when exposed to selected environments.     

Crazing of fibrous polymers

Highly oriented filaments of nylon 6 and polypropylene (PP) are shown to craze when deformed slowly at room temperature. A marked yield point occurs at 20-40 percent elongation. After this point extensive voiding occurs until at fracture the overall density is reduced by ∽25 percent for PP and ∽15 percent for nylon 6. SAXS analysis indicates that for PP the voids are elongated in the fiber axis while for nylon 6 voiding occurs both parallel and perpendicular to the fiber axis. Optical and electron microscopy show typical craze structures. The use of etching demonstrates that crazing occurs across the whole filament cross section. This yielding behavior is shown to be associated with the use of a low temperature of drawing during fiber production and becomes less important when a higher temperature of drawing or a separate annealing step are used. The crazing mechanism is thought to involve progressive separation of interfibril boundaries with the production of elongated voids.     

Crazing and the stress dependence of creep in two glassy polymers: Polystyrene and a poly(styrene–acrylonitrile) copolymer

The stress dependences of crazing and of tensile creep were studied at 30.5° and 80°C in polystyrene (M _v = 2.7×10⁵) and in a poly(styrene–acrylonitrile) copolymer (73.5% styrene, M _v = 2.35×10⁵) at four stresses in the interval from 0.6 to 1.5×10⁸ dynes/cm². Material failure was observed in all cases for the polystyrene and in no cases for the copolymer. Crazing was found to occur at all stresses for polystyrene, the spacing between craze lines decreasing with increasing stress and temperature. A much higher stress level for the onset of crazing was found for the copolymer. An inverse stress dependence of the compliance was observed for polystyrene at 30.5°C, i.e., the compliance decreased with increasing stress. This behavior was partially reversed at 80°C below 10² sec and became a positive stress dependence at long times. The stress dependence of the compliance for the SAN copolymer was partially reversed at 30.5°C. At 80°C, the stress dependence was positive for stresses ≥0.9×10⁸ dynes/cm². The present results suggest that in the copolymer there may exist an enhanced local mobility which alters the stress dependence observed in pure polystyrene and which enhances the ability of the material to deform without failure. This concept is discussed further in light of the stress dependence of the compliance and of crazing in these materials and appears to be consistent with our previous studies of the stress dependence of creep and of the stress dependence of whitening in ABS systems.     

Crazing and the creep behaviour of PMMA in methanol

A considerable increase in strain was observed when crazes formed during tensile creep tests on poly(methyl methacrylate) (PMMA) specimens immersed in methanol. A model of craze growth from surface flaws is proposed which is based on craze growth kinetics, and which shows good correlation with the measured time and stress dependent behaviour.     

Environmental stress relaxation studies of polymers

Polyethylene films were irradiated by ⁶⁰Co gamma-rays, and the effect of ionizing radiation on polyethylene — ketone system was studied by the method of stress relaxation. An empirical equation for the rate constant of penetrant-enhanced relaxation in the polyethylene — ketone system was obtained.     

Quantitative correlation of creep and stress relaxation in synthetic fibres

The quantitative correlation of creep and stress relaxation was determined. The reliability of the indicial equation and calculated deformation characteristics for describing the different conditions with a given law of deformation was confirmed.All-Russian Scientific-Research Institute of Technical Fabrics, Yaroslavl'. Institute of Textile and Light Industry, St. Petersburg. Translated from Khimicheskie Volokna, No. 6, pp. 41–42, November–December, 1992.     

Non-linear creep of polymers under superimposed static and dynamic stress

The non-linear creep of polymeric materials under super-imposed static and dynamic stress is considered theoretically. The equation of state due to Green, Rivlin, and Spencer to-gether with the power law of time dependence for the kernel functions as suggested by Nakada is assumed to characterize the non-linear materials. Expressions for creep strain under constant static, oscillatory dynamic and superimposed static and dynamic stress are obtained in terms of the material constants and time dependent functions, called dynamic creep functions. It is shown that the creep strain due to dynamic stressing is damped as the number of stress cycles is increased. Damping is faster if the power law of time dependence is high. Expressions for the cumulative creep strain after multiple stress cycles are also obtained in terms of cumulative strain functions. All these functions are evaluated numerically at one thousand stress cycles. Finally, a special case of stress history is considered where the stress periodically reaches zero. It is shown that the ratio of the strains due to dynamic and static stressing can be characterized by the power law parameter when the mean stress is either very high or very low. Due to the slow damping when the power law parameter is small, the decrease of the strain ratio with number of cycles is slow compared to higher power law parameters.     

Energetics of stress relaxation of glassy polymers at high pressure

Kinetic changes of relaxation of the shift stress of glassy poly(methyl methacrylate) were studied when temperatures and pressures were changed in isobaric, isothermal, and isochoric conditions. In isochoric conditions the relaxation rate depends slightly on temperature. The volume of the polymer is the main parameter for the rate of stress relaxation. Isobaric and isochoric activation energies and activation vohime of stress relaxation were determined. The results are shown to satisfy the thermodynamic relation of the activation parameters of the relaxation process. The dimensions of elementary kinetic unit of the stress relaxation process were estimated.     

Crazing in glassy polymers: Studies on polymer-glass bead composites

Stress-strain curves of glass bead-filled glassy polymers show a slope discontinuity related to crazing in the matrix. This discontinuity serves as basis for a simple method for determining ε_p′ the critical strain in the polymer to initiate crazing. The stress at the discontinuity (σ_D) is governed by the nature of the dispersed particles acting as stress concentrators. The two parameters, ε_p′ and σ_D, (neither of which is a function of the filler content) have the following values in ascending order σ = 1540, 1750, 1880, 2390 and 3300 psi for polystyrene, ABS, poly(vinyl chloride), SAN and poly(phenylene oxide) respectively; ε_p = 0.63, 0.72, 0.81, 1.10 and 1.67 percent for poly(vinyl chloride), polystyrene, SAN, ABS and poly(phenylene oxide) respectively. In coupled particulate systems, (σ_D) is increased and apparently vaies with the efficiency of the coupling agent. The modulus of a particulate system can be predicted by a one-parameter equation.     

Mechanical spectroscopy connected to creep and stress relaxation in a high resistant silicon nitride

Silicon nitride processed by gas pressure sintering contains a very small amount of glassy phase and consequently exhibits a strong resistance to deformation until 1450 °C. Above this temperature, both relaxation kinetics and creep rate rapidly increase. To explain such a behaviour, the formation of a liquid phase by dissolution of YSiAlON phases was proposed. The present paper shows that mechanical spectroscopy argues for the existence of such a liquid phase at high temperature. The mechanical loss is very low in the as-sintered material. Nevertheless, the internal friction peak generally observed in silicon nitride, and attributed to the glass transition in the glassy pockets, is also observed in the gas pressure sintered silicon nitride. Moreover, the peak is much higher in annealed and “quenched” specimens and it increases with annealing time. These results show that the annealed and “quenched” material contains much more glassy phase and so argues for the dissolution of crystalline phases at high temperature.     

Experimental and constitutive analyses of the stress relaxation behavior of glassy polymers

The physical mechanism underlying the mechanical behavior of glassy polymers has been studied over decades but remains a long-standing issue. A consensus view achieved is that the yield, flow, and stress relaxation behaviors are due to structural relaxation in the polymer mainly caused by chain conformation transitions. This is the key physical idea behind the many existing elastic–plastic constitutive models for glassy polymers. In this paper, such a constitutive model was employed for predicting and analyzing the stress relaxation of a glassy polymer. It is found that the model works well in predicting the pre-yield stress relaxation but significantly underestimates the post-yield stress relaxation. As considering the chain conformation transition alone leads to a dilemma for the model to concurrently represent the yield/flow and stress relaxation behaviors, the model was extended to incorporate an additional structural relaxation mechanism assumed to originate from the dissociation of weak linkages in the chain network. The extended model succeeds in concurrently representing the yield/flow, and stress relaxation behaviors in the whole deformation region, of which the reasons were analyzed. The knowledge revealed in this paper is instructive and may shed new light on understanding the structural relaxation and mechanical behavior of glassy polymers.     

Creep and stress relaxation in methacrylate polymers: Two mechanisms of relaxation behavior across the glass transition region

This study explores the long‐term behavior of poly(methyl methacrylate), poly(ethyl methacrylate), and poly(2, 2, 2‐trifluoroethyl methacrylate) by creep and stress relaxation (SR) experiments. Dielectric analysis (DEA) and dynamic mechanical analysis (DMA) are used to measure and compare the structural relaxations associated with side group motion and with the glass transition, T_g, region. An analysis of shift factor data for creep and SR experiments reveals two modes of molecular motion in the glass‐rubber relaxation region, but the modes are less discernible than those reported previously for a series of styrene polymers. The high temperature side exhibits WLF behavior while the low temperature side exhibits Arrhenius behavior. The extent of definition of the two modes of deformation is related to the effect of cooperative α dynamics on the β relaxation process. Some discussion is presented concerning the magnitude of the activation energy for both processes and the sharpness of the transition betwen the two modes.     

Tensile fatigue,stress relaxation,and creep behaviors of worsted core spun yarns

The objective of this research was to investigate the time‐dependent behaviors of yarns, which have significant bearings upon the properties of stretch fabrics made from them. In this study, 100% wool, wool–lycra (W‐L) (97 : 3), and polyester–wool–lycra (P‐W‐L) (52 : 45 : 3) blended yarns were considered. These yarns were subjected to the tensile fatigue failure, stress relaxation, and creep experiments. The findings showed that of these three yarns, P‐W‐L blend exhibited maximum fatigue lifetime, stress retention, and creep recovery, the pure wool yarn followed suit albeit to a lesser extent, whereas the W‐L blend made it least. The investigation suggests that wool alone with lycra as a core component is not sufficient to impart the expected properties upon stretch fabrics, rather a blend of polyester and wool with lycra as in the former would definitely make a worthwhile product. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Polydisperse polymers: Relations among molecular weight distribution,stress relaxation,and distribution of relaxation times

Relaxation times in polydisperse polymers were calculated on the basis of more realistic viscosity mixing rules than have previously been used. These relaxation times and mixing rules are in turn used to calculate viscoelastic functions such as stress relaxation following sudden straining or steady shearing. Inversion of these functions provides an accurate way to estimate the molecular weight distribution. This method is useful for insoluble or otherwise intractable polymers.     

Thermal stress relaxation creep and microstructural evolutions of nanostructured SiC ceramics by liquid phase sintering

We explored the thermal relaxation creep characteristics of nanostructured SiC ceramics by bend stress relaxation (BSR) method. The effects of the differences in microstructure and secondary phases by liquid phase sintering at 1800 or 1900 °C were especially discussed, based on microstructural evolutions during the creep. The creep was characterized by the BSR ratio (m) of ～0.80 up to 1200 °C, and the proportion of amorphous phase as a secondary phase was related to the creep resistance at 1300 °C. The microstructural evolutions during the creep consisted firstly in the re-distribution of amorphous phase, probably as a consequence of its viscous flow, and secondly in an extensive nucleation and growth of cavities. Furthermore, the former enhanced inter-diffusion of Al–Y among intergranular areas above the ternary eutectic temperature, which caused the significantly reduced creep resistance, and the latter reflected the crystalline YAG decomposition as another secondary phase near 1500 °C.     

Environmental stress relaxation studies of polymers: Effect of alcohols on polyethylene

The method of stress relaxation has been used to study stress decay behavior in polyethylene–alcohol systems. The behavior of stress decay in polyethylene under alcohols was observed and the respective activity forward stress decay of alcohols on polyethylene was obtained.     

Cooperative relaxation processes in polymers

The basic mode of relaxation in polymer molecules involves the rotation of a conformer, with a time scale of the order of picoseconds. This fast relaxation process, however, cannot take place easily in the condensed state crowded by densely packed conformers, necessitating the intermolecular cooperativity among them. The domain of cooperativity grows at lower temperatures, towards the infinite size at the Kauzman zero entropy temperature, though the system deviates from the equilibrium as the glass transition intervenes at about 50°C above that temperature. From the temperature dependence of the domain size, the well-known Vogel equation is derived, which we consider is the basic origin of the empirical WLF and free volume equations. The molar volume is a crucial factor in determining molar free volume. The molecular weight of a conformer with a density correction, therefore, can be used as a parameter in determining the T_g of liquids and amorphous polymers. A larger size conformer means a higher glass transition temperature. A conformer at the chain end, on the other hand, has a higher enthalpy, i.e., a smaller effective size for that conformer. If a conformer is reacted trifunctionally, the resulting conformer is a combination of the two conformers and T_g increases, but a further addition of another conformer to that branch point reduces the average size of the conformers, so T_g decreases. The model for cooperative relaxation can be directly applied to predicting T_gs from the chemical structure of polymers, the kinetics and T_gs of thermosets during the crosslinking reaction, the distribution of relaxation times from the domain size distribution at a given temperature, the dynamics of the physical aging process, and other complex behaviors of polymers and liquids near the glass transition temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 77–93, 1997