Chemical Stress Relaxation Research Articles

Mechanics and hydraulics of pollen tube growth.

All kingdoms of life have evolved tip-growing cells able to mine their environment or deliver cargo to remote targets. The basic cellular processes supporting these functions are understood in increasing detail, but the multiple interactions between them lead to complex responses that require quantitative models to be disentangled. Here, I review the equations that capture the fundamental interactions between wall mechanics and cell hydraulics starting with a detailed presentation of James Lockhart's seminal model. The homeostatic feedbacks needed to maintain a steady tip velocity are then shown to offer a credible explanation for the pulsatile growth observed in some tip-growing cells. Turgor pressure emerges as a central variable whose role in the morphogenetic process has been a source of controversy for more than 50yr. I argue that recasting Lockhart's work as a process of chemical stress relaxation can clarify how cells control tip growth and help us internalise the important but passive role played by turgor pressure in the morphogenetic process.

An approach to assess the thermal aging effects on the coupling between inelasticity and network alteration in filled rubbers

The development of a constitutive representation for predicting the durability of filled rubbers upon aggressive environmental conditions is crucial for numerous technical examples but still remains largely an open issue. This paper presents a constitutive model for the prediction of thermal aging effects on the inelastic response of filled rubbers. A network alteration theory is proposed for the thermally driven network degradation by considering a collection of chemical and physical changes occurring inside the rubber-filler material system. The network decomposition considers scission-rebound mechanisms of links (between filler aggregates and between elastically active cross-linked chains) and changes in movements of the free chains superimposed into the (newly born and original) chemically linked network. The material kinetics is designed using multi-step (physical) stress relaxation experiments performed at room temperature on a sulfur-vulcanized styrene–butadiene rubber containing different amounts of carbon-black fillers, and previously submitted to an accelerated chemical stress relaxation procedure, i.e. aging at constant stretch for various temperatures and exposure times. The time–temperature equivalence principle is used to construct master curves of the alteration kinetics for both the cross-linked chains and the superimposed free chains. The amplified effect of the fillers is also considered in our network decomposition to account for the cross-linked chains/fillers interactions and the free chains/fillers interactions. The capabilities of the proposed network alteration based constitutive model to describe and predict the material response evolution in terms of stiffening, hysteresis area increase and physical relaxation increase are shown. The chemically induced “plastic-like” deformation and stress relaxation are predicted using the developed model. The results show the importance of the inelastic effects on the prediction of long-term material response due to thermally activated degradation. From an applicative point of view, the model makes possible to estimate the operational life of pre-strained rubber mechanical components with respect to loss of pressure and flexibility.

Coupled thermal- and deformation-induced degradation in planar rubber membranes under radial loading

Elastomers at elevated temperatures can undergo microstructural changes owing to the scission, re-coiling, and re-crosslinking in their macromolecular network. These result in chemically based stress relaxation and permanent set. In previous work, a constitutive equation was developed for the thermo-mechanical response of such elastomers that generalized the Tobolsky two-network theory and incorporated results from an experimental program. One experimental result was that the scission process appeared to be unaffected by stretch, to within moderate stretches. There are indications that larger deformations affect the scission process. The purpose of the present work is to modify the previously developed constitutive theory to allow for this. The constitutive theory contains material properties, representing the chemical kinetics of scission, re-coiling, and re-crosslinking, that are expressed in terms of activation energies. The premise considered here, motivated by the mechano-chemical discussion in the literature, is that an increasing deformation decreases the activation energy and results in faster chemical stress relaxation. A method for accounting for this is introduced. The resultant constitutive theory is then used in the simulation of the radial planar deformation of a planar annular sheet by tractions at its outer boundary. This particular geometry is chosen for two reasons: (1) its convenience for experimental study; (2) the stretch concentration near the inner circular boundary will cause faster chemically based stress relaxation there, which could be readily observable.

Achieving high aspect ratio wrinkles by modifying material network stress.

Wrinkle aspect ratio, or the amplitude divided by the wavelength, is hindered by strain localization transitions when an increasing global compressive stress is applied to synthetic material systems. However, many examples from living organisms show extremely high aspect ratios, such as gut villi and flower petals. We use three experimental approaches to demonstrate that these high aspect ratio structures can be achieved by modifying the network stress in the wrinkle substrate. We modify the wrinkle stress and effectively delay the strain localization transition, such as folding, to larger aspect ratios by using a zero-stress initial wavy substrate, creating a secondary network with post-curing, or using chemical stress relaxation materials. A wrinkle aspect ratio as high as 0.85, almost three times higher than common values of synthetic wrinkles, is achieved, and a quantitative framework is presented to provide understanding the different strategies and predictions for future investigations.

Influence of grain size and training temperature on strain of polycrystalline Ni50Mn29Ga21 samples

The alloy Ni-Mn-Ga aroused great interest for application as a magnetic shape memory (MSM) material. This effect is caused by reorientation of twin variants by an external magnetic field. So far, most of the experiments were concentrated on single crystals. But, the MSM effect can also be realised in polycrystals which can be prepared much more efficiently. Here, polycrystalline samples were prepared by directional solidification with a <100> fibre texture of the high temperature cubic austenitic phase parallel to the heat flow. Afterwards, a heat treatment was applied for chemical homogenisation and stress relaxation in the austenitic state. Then the samples were heated up to the austenitic state and cooled down under load. The microstructure was analysed by Electron Back Scatter Diffraction (EBSD) before and after that treatment. Mechanical training at room temperature and 40°C was tracked by recording stress-strain curves. By increasing the number of training cycles the strain also increases. The influence of different training temperatures was investigated on samples with different grain sizes.

Residual Strain Measurement of Thin-Layer Cured Adhesives and Their Effects on Warpage in Electronic Packaging

Residual strains (stresses) of thin-layer adhesives are important and need be determined and further analyzed for better reliability design of the joints or bump joints in electronic or optoelectronic packaging. The purposes of this paper are to quantify residual strains of two adhesives: A (paste type) and B (film type) due to chemical shrinkage, stress relaxation, and thermal-loadings and moisture-loadings, and to investigate their effects on the warpage of die attachment assembly. At the beginning, the mechanical properties of the two adhesives in terms of temperatures are measured by dynamic mechanical analyzer (DMA) and thermomechanical analyzer (TMA). The residual strains are documented by testing fully cured adhesive/silicon bi-material plates under thermal and moisture loading using Twyman-Green (T/G) interferometry system associated with Timoshenko's bi-material theory and finite element method (FEM). Furthermore, the warpage of silicon/adhesive/Cu tri-material plates (similar to die attachment assembly) with both adhesives under thermal and moisture loading are also investigated by T/G experiments and confirmed by Suhir's solution and FEM results. For adhesive-A, the results of the bi-material plate suggest that the residual strains are only induced by mismatch of the coefficients of thermal expansion during thermal loading, rather than other factors, right after the fully cured adhesive cooling down to room temperature. On the other hand, for adhesive-B, it was found that the additional residual strain of this film adhesive caused by chemical shrinkage plus stress relaxation, is about 2.26 ? 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , which accounts for 85% of thermal-residual strains in the bi-material plate from curing temperature cooling down to room temperature. Experimental results also indicated that moisture-absorption expansion of the both adhesives is the major cause of decreasing warpage for the bi-material plates at 29?C/55% RH, other than stress relaxation. In contrast to the results of the bi-material plate, the warpages of the tri-material plates are found to be insensitive to the moisture-absorption, chemical, and thermal strains of adhesives, but not to elastic moduli of adhesives, due to their relatively thin-layer and compliant nature. This paper has successfully laid down an approach for determining residual strain of thin-layer adhesive and provided information about their effect on warpage of die attachment assembly.

Isothermal martensitic transformation in polycrystalline Ni 50Mn 29Ga 21

The isothermal transformation behaviour of polycrystalline Ni 50Mn 29Ga 21 magnetic shape memory alloy is investigated by measuring the saturation magnetisation as a function of time and temperature. The results are analysed within the framework of the Johnson–Mehl–Avrami–formalism. The samples were prepared by directional solidification with a 〈1 0 0〉 texture of the high temperature cubic phase parallel to the heat flow. The samples were heat treated for chemical homogenization and stress relaxation in the austenitic state. They show a thermal hysteresis of martensite to austenite and the reverse transformation. The hysteresis becomes smaller with increasing measurement time. Grain boundary nucleation is a time controlling mechanism during the isothermal transformation in a constant magnetic field in these alloys.

Analysis of Variant Orientation Before and After Compression in Polycrystalline Ni50Mn29Ga21 MSMA

This paper deals with the microstructure and plastic deformation of Ni50Mn29Ga21 ferromagnetic shape memory alloys. In contrast to conventional shape memory alloys, plastic deformation in the martensitic phase, which is due to twin boundary motion, may be caused not only by mechanical stress but also by an external magnetic field. The polycrystalline sample was prepared by directional solidification with a texture parallel to the heat flow. Afterwards, a heat treatment follows for chemical homogenization and stress relaxation in the austenitic state. The configuration of the twin boundaries was analyzed before and after compressing the samples. The microstructure after compression was related to the magnetic properties.

A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Magnetostrain is achieved by twin boundary motion in magnetic shape memory materials. A 1% strain could be achieved in textured polycrystals of Ni–Mn–Ga at room temperature. Ni 50Mn 29Ga 21 was directionally solidified in order to obtain a texture parallel to the heat flow. The alloy was heat treated for chemical homogenization and stress relaxation in the austenite state. Thermomechanical training was employed to decrease the twinning stress and to enhance the strain by simplification of the variant microstructure. Investigations at slightly elevated temperatures were performed to increase the magnetostrain to 1%.

Introduction to polymer viscoelasticity

Phenomenological Treatment of Viscoelasticity. Time--Temperature Correspondence. Transitions and Relaxation in Amorphous Polymers. Statistics of a Polymer Chain. Rubber Elasticity. Viscoelastic Models. Dielectric Relaxation. Chemical Stress Relaxation. Appendix. References. Answers to Problems. List of Major Symbols. Index.

Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings II. Effect of hindered amine light stabilizer and ultraviolet light absorber additives

The chemical stress relaxation behavior of two acrylic/melamine clearcoats has been investigated to determine the effect of hindered amine light stabilizers (HALS) and ultraviolet light absorbers (UVA) on the rates of photooxidation induced crosslink/chain scission and crosslink formation. The addition of a HALS to one of the clearcoats, Mel′-A, slows the rate of crosslink/chain scission and in high enough concentrations stops the formation of crosslinks. In the other clearcoat, Mel′-N, the addition of small amounts of either HALS or UVA slows the rate of photooxidation. The overall changes in crosslink density are the same for Mel′-A containing small amounts of HALS and for Mel′-A containing moderate amounts of both HALS and UVA, indicating that the same changes in crosslink density can be achieved by different degradation pathways. These findings indicate that small changes in clearcoat stabilization packages can have dramatic effects on long-term weathering performance of acrylic/melamine coatings.

Residual strain measurement in composites using the cure-referencing method

This paper describes the details of a novel procedure called the cure-referencing method (CRM) to measure the strains associated with residual stresses on the surface of composite panels. The CRM involves the replication of diffraction gratings onto the surface of composite specimens during the autoclave during cycle. Residual strains associated with the curing process are measured using moire interferometry at room temperature after the specimens have been taken out of the autoclave. The procedures for both the grating replication and the moire interferometry experiment are described in detail. A method of high-temperature moire interferometry was developed to resolve the residual strains due to thermal expansion from those due to chemical matrix shrinkage and stress relaxation. These procedures are demonstrated on unidirectional and multidirectional laminates and on woven textile composites.

Application of chemical stress relaxation to the degradation of elastomers by heat and radiation

The article studied the following fundamental problems on the degradation of elastomers by heat and/or radiation by using the measurement of chemical stress relaxation. (1) The efficiency of scission and crosslinking of elastomers by radiation. (2) The continuous and intermittent chemical stress relaxation of tetrafluoroethylene-propylene elastomer was measured at 100°C under the irradiation of 60Co γ rays. The G value of crosslinking (Gc) was found to be 0.88 and the G value of scission (Gs) was 1.89 by this method. On the other hand, Gc and Gs values of the polymer, as obtained from a Charlesby-Pinner plot (sol fraction method) were 1.22 and 2.54, respectively. (3) The method how to express the magnitude of synergism between heat and radiation: The rate of continuous chemical stress relaxation under heat and radiation Kh+r is expressed as Kh+r = Kh + Cc Iα exp(–E'/RT), where Kh is the rate of stress relaxation by heat alone, I is dose rate, E' is the coefficient of synergism, R is the gas constant, T is the temperature (in K), and Cc is the constant which refers to the sensitivity for radiation. The exponent α shows the dose rate dependence. (4) The difference between heat aging and radiation induced degradation of elastomers: Pure vulcanized ethylene-propylene copolymer was irradiated with 60Co γ rays in air at room temperature. The rate of chain scission of irradiated samples by heat was independent from the dose and was found to be ten times higher than that of non-irradiated samples. The results suggest that irradiation consumes the antioxidant agents added during the polymer production process by the energy transfer from polymer to antioxidant agents. On the other hand, the rate of stress decay of non-irradiated samples increased with the heat aging time at all temperatures. In this case, heat degrades polymer and antioxidant agent simultaneously. (5) Pure vulcanized tetrafluoroethylene-propylene elastomer was irradiated by 60Co γ rays in air at room temperature. The chemical stress relaxation of the samples was measured at various temperatures ranging from 100°C to 200°C. The rate of scission increased with increasing dose, but the rate leveled off at about 100 kGy. The increase in scission by irradiation might be due to the initiation of autoxidation by thermal decomposition of hydroperoxide accumulated during irradiation.

Application of chemical stress relaxation to the degradation of elastomers by heat and radiation

Application of chemical stress relaxation to the degradation of elastomers by heat and radiation

Longitudinal Cracking in a Carbon Black Filled Natural Rubber Vulcanizate during Chemical Stress Relaxation

Abstract Thin specimens of a black-filled, natural rubber vulcanizate have been held in uniaxial tension at 72°C and 200% elongation in a forced air oven. After substantial oxidative attack (inferred from stress relaxation), small edge cracks formed. Initially, these cracks grew perpendicular to the loading direction, but, upon reaching about 0.1 mm in depth, longitudinal crack growth commenced and fracture progressed by a kind of 0°-peel process with “splitting-off” of successive strands of rubber. This phenomenon is attributed to anisotropy in strength caused both by straining and by oxidative attack.

Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings—I

Chemical stress relaxation was used to measure the rates of crosslinking and chain scission during accelerated weathering in two acrylic/melamine clearcoats that photooxidize at different rates. The more photooxidation-resistant clearcoat undergoes an initial increase in crosslink density followed by a steady decrease, while the more photooxidation-prone clearcoat undergoes primarily chain scission until approximately 800 h of accelerated weathering, after which the rate of crosslink formation begins to exceed the rate of chain scission. The net changes in crosslink density mirror those found using dynamic mechanical analysis. Clearcoat films weathered under no stress displayed the same amount and type of chemical composition change as the films weathered under stress. Chemical stress relaxation appears to be a ready means of assessing the influence of the fate of free radicals on the physical properties of the clearcoats.

A Study on the Relationships between Network Structure and Mechanical Properties of Interpenetrating Polymer Networks-Part 2: The Comparison of the Theory of Viscoelasticity for Thermoplastic Polyurethane Elastomers with Experiments

The relationship of the elastic modulus and loss tangent to the compositions for two series of thermoplastic polyurethane elastomers (ET and ES types) was investigated by the uniaxial extension method and dynamic mechanic analysis. The structure parameters (C100, C020 and C200) were determined by multilinear regression. At the same time, the process of chemical stress relaxation was carried out under a range of temperatures and at a given strain. The molecular parameters a, k, k3, [UNKNOWN]3 and activation energy ΔE3 were determined by a linear regression method. It was found that the results obtained from the theoretical equations were in good agreement with the experimental data.

Chemical stress relaxation of polyglycolic acid suture.

Chemical stress relaxation methods are employed to study chemical and mechanical factors influencing the degradation of uncoated polyglycolic (PG) sutures. Specially constructed instrumentation is used to study the kinetics of the load bearing capability of PG (Dexon¿) 3-0 sutures in hydrolytic solution. The effects of pH, temperature, strain rate, and initial load on the rate of chemical stress relaxation are presented. Data show how mechanical factors such as the rate of loading (related to the speed of knot tying), as well as the final tension, are related to the rate of structural degradation. Maximum stability is observed at approximately 40 degrees C, with slower degradation both above and below this point. Results show that the slower and tighter the suture is pulled, the greater its ability to sustain tensile loads during hydrolysis.

Stress relaxation studies of a model for unfilled, anionically stabilized, curable silicone latexes

Stress relaxation behavior of a model for an unfilled, anionically stabilized silicone latex has been used to examine the chemistry of the cured silicone network. It has been shown that the cure catalyst produces siloxane bond interchange reactions which cause chemical stress relaxation. The relaxation rate is accelerated in the presence of water due to a second interchange reaction presumably involving silanols. The measurement of the rate of stress relaxation at various temperatures gave an apparent activation energy of 7·6 kcal/mol for the relaxation process in the presence of water.

Studies on a New Rapid and Simple Determination Method of Chain-Length Distribution of a Crosslinked Polymer by Chemorheology

It is still impossible to estimate the distribution of the molecular-weight (chain-length) between two crosslinks of a crosslinked polymer, because of its insolubility and infusibility. The distribution p (x) of chain-length between crosslinks x will be estimated by chemorheological means.If the scission rate of chain is proportional to x, the rate of stress relaxation due to the main chain scission is related to p (x) through Laplace transformation. p (x) thus obtained is likely to indicate the real distribution, because the same result can be obtained for a given crosslinked polymer from different chemical stress relaxation curves at different high temperatures.