Physical and chemical stress relaxation of elastomers

In this paper we have developed a method whereby physical and chemical relaxation processes can be distinguished, using stress relaxation experiments as a function of temperature. We assumed that there exists some temperature range above the glass transition temperature over which the chemical effects can be neglected for the time scale of the experiments. The data in this low temperature range were then used to determine the WLF constants and other physical relaxation parameters. The physical component of the stress relaxation could then be subtracted from high temperature experiments in order to extract chemical kinetic information. Based on certain reasonable assumptions, an equation was developed for the relaxation modulus of a chemically reacting system. This equation could be used to determine the time dependence of the crosslink density, or conversely could be used to predict the long term relaxation modulus from an assumed kinetic mechanism. These calculations were demonstrated for ethylene propylene and butyl elastomers.     

On the separation of physical and chemical component of stress relaxation

This paper proposes a new method to separate physical and chemical components of stress relaxation. The stress relaxation measurements of tetrafluoroethylene-propylene rubber were carried out at various temperatures ranging from 200° to 310°C. A physical stress relaxation master curve could be generated from data of early period of time by the time-temperature superposition principle. The rate of physical stress decay at given temperatures was calculated from the master curve. The rate of chemical stress relaxation was given by subtracting the rate of physical decay from experimentally obtained rate at the corresponding temperatures. The activation energy was found to be 8.4 kcal mol^?1 for the rates of the calculated chemical stress relaxation, while it was found to be 5.7 kcal mol^?1 for the rates which were obtained in air. The results show that the physical component of stress decay should be subtracted from the measured stress relaxation curve to obtain the rate of chemical stress decay, especially at the low temperature.     

Stress–relaxation properties of segmented polyurethane rubbers

Stress–relaxation behavior of polyurethane elastomers based on two hydroxyl-terminated polyesters: poly(ethylene adipate) and poly(ethylene maleate) was studied. In addition, a polyester-diol consisting of poly(ethylene adipate) and oligo(ethylene terephthalate) blocks, a number of low-molecular-weight diols as chain extenders, and 4,4′-diphenylmethane diisocyanate (MDI) were determined. The elastomers were crosslinked by an excess of MDI and had stiff segments of differing chemical structure and length. Stress–relaxation properties of the elastomers conformed with the three-component Maxwell model, with negligibly small contribution from the fastest relaxation process. The influence of crosslinking density, chemical structure, and stiff segment content on the relative relaxation speed and the parameters of the slow and fast relaxation processes, was examined. The elucidation of the results was based on the morphological models of segmented polyurethanes.     

Influence of physical relaxation on chemical stress–relaxation of natural rubber vulcanizates

Networks were prepared in the swollen and dry states to investigate the influence of physical relaxation on chemical stress–relaxation. The stress–relaxation behavior of solution-cured samples was different significantly from that of the samples crosslinked conventionally. The same result was also observed in the number of chain scission for both kinds of samples. On the other hand, the number of chain scission estimated by using the swelling method for samples crosslinked conventionally was in good agreement with that by the chemical stress–relaxation for solution-cured samples. It was found that there is little or no influence of the physical relaxation caused by entanglements, and no effect of dangling chains arising from scission in the equilibrium swelling. The relative change of network chain density determined by means of the swelling method was also consistent with that by sol fraction determination. These results indicate that the swelling method can be used as a measure of a degree of degradation on chemorheology. Taking into account the influence of physical relaxation on chemical stress–relaxation, a new relationship between the relative stress decay and the relative network chain density was experimentally proposed.     

Dielectric spectroscopic results and chemical accessibility of sulfite pulps

The relation between the chemical reactivity of a dissolving pulp and the molecular mobility of cellulose was investigated by an unconventional method for these problems: dielectric relaxation spectroscopy (DRS). Carefully dried pulps show better reactivity in comparison to a mill dried sample. A specific deformation of the local polymeric backbone dynamics (β‐relaxation) was observed in the former samples. The intensity of the dielectric β‐relaxation correlates with the acetylation velocity in an excellent manner. A similar correlation was found between the relaxation strength and the water retention capacity of pulps. All experimental results support the explanation that hydroxyl groups of those anhydroglucose units which contribute to the low frequency unstructured underground relaxation (here named δ‐relaxation) outside of the real β‐peak in the dielectric spectrum are preferably substituted. The results of the investigation indicate that dielectric spectroscopy can provide a new, additional, alternative tool to characterize the accessibility and chemical reactivity of cellulose.     

Local Dynamics of Bismaleimide Adhesives in an Aggressive Environment

Molecular aspects of chemical and physical changes in bismaleimide (BMI) adhesive joints caused by absorbed moisture were investigated. The focus was on the early (pre-damage) stage that precedes the formation of voids and microcracks. Local dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and the changes in the chemical state of the matter were monitored by Fourier transform infrared spectroscopy (FTIR). Absorbed water interacts with the BMI network and gives rise to a fast relaxation process (termed γ*), characterized by an increase in the dielectric relaxation strength, an Arrhenius temperature dependence of the average relaxation time, and an activation energy of 50 kJ/mol. The γ* dynamics are slower than the relaxation of bulk liquid water because of the interactions between the absorbed water and various sites on the network (the ether oxygen, the hydroxyl group, the carbonyl group, and the tertiary amine nitrogen). One particularly significant finding is that the average relaxation time for the γ* process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as a non-destructive inspection (NDI) tool for adhesive joints. FTIR spectra reveal the presence of non hydrogen-bonded water and hydrogen-bonded water, the latter bonded to one and/or two sites on the BMI network. A good agreement was found between the calculated ratio of non hydrogen-bonded to total absorbed water from DRS and FTIR data.     

Local Dynamics of Bismaleimide Adhesives in an Aggressive Environment

Molecular aspects of chemical and physical changes in bismaleimide (BMI) adhesive joints caused by absorbed moisture were investigated. The focus was on the early (pre-damage) stage that precedes the formation of voids and microcracks. Local dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and the changes in the chemical state of the matter were monitored by Fourier transform infrared spectroscopy (FTIR). Absorbed water interacts with the BMI network and gives rise to a fast relaxation process (termed γ*), characterized by an increase in the dielectric relaxation strength, an Arrhenius temperature dependence of the average relaxation time, and an activation energy of 50 kJ/mol. The γ* dynamics are slower than the relaxation of bulk liquid water because of the interactions between the absorbed water and various sites on the network (the ether oxygen, the hydroxyl group, the carbonyl group, and the tertiary amine nitrogen). One particularly significant finding is that the average relaxation time for the γ* process above 20°C is of the order of nanoseconds or less and, hence, the detection and monitoring of this process hinges upon the ability to perform high precision DRS at frequencies above 1 MHz. This is an important consideration in the ongoing efforts aimed at the implementation of DRS as a non-destructive inspection (NDI) tool for adhesive joints. FTIR spectra reveal the presence of non hydrogen-bonded water and hydrogen-bonded water, the latter bonded to one and/or two sites on the BMI network. A good agreement was found between the calculated ratio of non hydrogen-bonded to total absorbed water from DRS and FTIR data.     

Mechanical relaxation processes of wood in the low-temperature range

The dynamic viscoelastic properties of untreated and chemically modified wood specimens were determined in the temperature range 123 to 293 K and at constant frequencies. Absolutely dry wood specimens exhibited one relaxation process labeled γ at around 180 K, being attributed to the motions of methylol groups in the amorphous region of the wood constituents. The changes in the γ process due to the chemical modifications were explained by the reduction in the original γ loss peak due to the decrease of methylol groups, and an additional relaxation induced by the other groups introduced. With moisture adsorption, an additional relaxation labeled β was induced at 220–240 K. It appeared only when the wood adsorbed moisture irrespective of chemical modifications, and its characteristics were not affected by the formaldehyde and PEG treatments involving the remarkable changes in the mobility of amorphous molecules. These results suggested that the dominant mechanism of β relaxation was not the segmental motions of the main chain, but the motion of the adsorbed water molecules. The positive activation entropy of the β relaxation was interpreted to reflect rearrangement of the adsorption sites required for the rotation of the adsorbed water molecules. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3338–3347, 2001     

橡胶大气老化应力松弛的评价

对８种不同类型的硫化橡胶在大气老化试验时的应力松弛变化进行了探讨。研究结果表明，橡胶的大气老化是一个复杂的化学过程，硫化橡胶的化学应力松弛变化能反映出这种过程的矛盾变化。各种橡胶的应力松弛速率的不同，反映出其耐老化性能的不同，因而硫化橡胶的化学应力松弛可以用来评价橡胶的耐候性。     

STAGE SEPARATION WITH CHEMICAL REACTION STEADY STATE COUNTERCURRENT EQUILIBRIUM STAGE SEPARATION WITH CHEMICAL REACTION BY RELAXATION METHOD

The relaxation method is used to calculate mole fractions and temperature profiles in a rectification column where a chemical reaction takes place. The method is general and the non-ideal vapor-liquid equilibria can be easily incorporated. The computer time is very low and the danger of divergence can be alleviated by an appropriate guess of relaxation factor. Calculated problems involving esterification reactions are presented.     

STAGE SEPARATION WITH CHEMICAL REACTION STEADY STATE COUNTERCURRENT EQUILIBRIUM STAGE SEPARATION WITH CHEMICAL REACTION BY RELAXATION METHOD

The relaxation method is used to calculate mole fractions and temperature profiles in a rectification column where a chemical reaction takes place. The method is general and the non-ideal vapor-liquid equilibria can be easily incorporated. The computer time is very low and the danger of divergence can be alleviated by an appropriate guess of relaxation factor. Calculated problems involving esterification reactions are presented.     

Orthonormal Chemical Reactions and Chemical Relaxation Part 4. Reactions in Solution with Concomitant Entropy and Volume Changes

The method described previously [2, 3] for obtaining the relaxation times, normal coordinates and normal modes of relaxation of a chemical system is extended to the case in which heat transfer and volume changes occur at rates too slow to maintain isothermal and isobaric conditions. The four types of chemical relaxation treated previously (isothermal and adiabatic relaxation at constant pressure and at constant volume) form limiting cases of this problem whose solution thus provides a generalization and unification of the concept of orthonormal chemical reactions.     

Molecular relaxation phenomena during accelerated weathering of a polyurethane coating

Results from a polyester-urethane (PU) coating system under accelerated weathering showed that crosslink density, obtained from high-temperature modulus data, diminished due to chain scission. However, T _g and room-temperature tensile modulus both increased with weathering. Molecular relaxation phenomena in polymers have long been investigated to explore changes occurring in a polymer at temperatures below its glass transition and were explored for an explanation to supplement chemical degradation for these observations. Relaxation was quantified using “enthalpy recovery” which first increased with exposure, then diminished. The concurrent physical and chemical aging effects were characterized by tracking nonexponentiality in the spectrum of relaxation times, and the size of “co-operatively relaxing regions” deduced from relaxation around the glass transition. Mechanical relaxation in this coating extended longer than cycle periods typical of accelerated weathering, suggesting that frequency effects might be important when comparing accelerated to natural weathering. This paper was awarded Third Place in the John A. Gordon Best Paper competition, presented as part of 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006, in New Orleans, LA.     

Limiting Cases of the Equations of the Wave Model of Longitudinal Dispersion Accompanied by a Chemical Reaction

The relationship between the wave (relaxation) and diffusion models of longitudinal dispersion accompanied by a chemical reaction in an apparatus is analyzed. Of greatest interest is the limiting form of the wave-model equations for a case where the relaxation time is relatively short as compared to the time of mass (heat) transfer through the apparatus. The limiting equation of the wave model corresponds to the equation of the diffusion model if the dispersion coefficient depends on the concentration of the desired substance in a certain way. Comparison is performed with other models and the exact (numerical) solution to the problem of the dispersion of the substance throughout a round pipe in which there is a Poiseuille flow accompanied by a first-order chemical reaction.     

Limits in the evaluation of closely spaced chemical relaxation processes

Earlier studies on fundamental errors in the evaluation of chemical relaxation parameters from curves of closely spaced relaxation processes are reinvestigated with rigorous mathematical tools. Very precise data referring to two neighboring relaxation processes are produced by non-linear equations and subsequently evaluated by non-linear least squares analysis, assuming two exponential terms. Previously, linear equations were used in the generation of precise data and then evaluated by graphical means. Relaxation processes are evaluated with ratios of two consecutive relaxation times of up to 2/3. The enzyme catalyzed isomerization is used as underlying chemical mechanism with most thermodynamic parameters fixed. The analyses showed that derived signal amplitudes should be used with great care, if relaxation times exceed ratios of around 1/10. If these amplitudes have the same sign, the relaxation times are generally quite precise, as long as the overall amplitudes are sufficiently large. If amplitudes become comparatively small in selected regions, evaluated amplitudes may deviate considerably from computed (theoretical) ones. The experimentalist has to be aware of this problem even if high signal-to-noise ratios are maintained (S/N ? 1000).     

Monitoring of photopolymerization kinetics and network formation by combined real-time near-infrared spectroscopy and ultrasonic reflectometry

For in situ monitoring of fast changes of shear modulus and chemical conversion during UV radiation curing an ultrasonic (US) reflection method was combined with real-time near infrared (NIR) spectroscopy. The combined setup has been applied to study photopolymerization of different resins as acrylates, epoxy acrylates, acrylated polyurethanes and cationic epoxy resins in order to achieve a deeper understanding of the interdependence of reaction kinetics and changes of mechanical/rheological properties. The simultaneously recorded conversion–time and modulus–time curves allow differentiating between mass and diffusion controlled polymerization regime. Light curing and dark curing phases are indicated by two distinct regions in the conversion–time curves. By rescaling the curing time by chemical conversion modulus–conversion curves were constructed, which are described by combining a viscoelastic relaxation model with the conversion dependence of relaxation times. The NIR-US setup was used to study the influence of chemical composition and curing conditions on the polymer network formation.     

A modified balance-type stress relaxation apparatus for chemical relaxation under liquid phase

A relatively simple and inexpensive automatic apparatus for chemical relaxation giving good accuracy has been developed for measurement of chemical relaxation in the gaseous phase in high polymers. The apparatus is based on a previously described balance-type stress relaxation apparatus intended for automatic operation.     

高分子聚合物松弛行为及模型的研究进展

高分子聚合物材料作为一种使用频率高且应用范围广的材料,其松弛行为对于其加工制品的精度具有重要的影响,且高分子化学合成的结果会最终影响高分子聚合物的松弛行为。由此总结了高分子聚合物松弛行为的特点,整理了其力学松弛及模型、松弛时间的研究进展。并由此提出现阶段高分子聚合物的松弛行为研究的局限性,并指出未来的研究方向。     

含物理交联高吸油树脂的合成和吸油性能

提出在单一化学交联吸油树脂中引入物理交联的设想，并采用悬浮聚合法全盛了含部分物理交联和单一化学交联的高吸油聚丙烯酸酯。研究了聚合温度、反应时间以及引发剂、分散剂用量和交联剂形式及用量对树脂形态、凝胶分率和吸三氯乙烯、苯等的影响。结果表明物理交联和化学交联方式对树脂的吸油性能影响较大。     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.