Viscoelastic temperature dependence

Characterisation of the temperature dependent viscoelastic properties of the example encapsulation material was presented in [17], Now the question arises about the time and the temperature dependent fiacture properties. This is relevant for applications where thermomechanical loading occurs. At first, fiacture tests at isotherm temperature are conducted for a better understanding of the rate dependent fi acture toughness of the example material. The results are later related to thermomeclmical loading tests. 

Connolly JAD, Podladchikov YY (2000) Temperature-dependent viscoelastic compaction and compartmentalization in sedimentary basins. Tectonophys 324 137-168 Cook SJ (1992) Contact metamorphism surrounding the Alta stock, Little Cottonwood Canyon, Utah. PhD Dissertation, University of Utah, Salt Lake City... 

The torsional vibration test according to DIN 53445 is used to investigate temperature-dependent viscoelastic material behavior. This is a short-term test. The time-to-mpture test according to DIN 53444 registers the influence of stress duration, stress type, and temperature. The torsional vibration test provides additional information on the damping behavior of the plastics tested (mechanical loss or damping factor d). Fig. 13 [9]. 

As expected, test temperatin-e will have a dramatic effect on the FCP kinetics of polymers owing considerably to their temperature-dependent viscoelastic nature. Indeed, studies of polystyrene and acrylonitrile-butadiene-styrene (ABS) have shown that FCP rates for given AX level generally decrease with decreasing test temperature (80). By contrast, a minimum FCP resistance was noted in polycarbonate and polysulfone at intermediate test temperatures (81,82). A complex test-temperatin-e response was also noted in studies on the influence... 

Skrypnyk, ID., T.O. Vasyljkevych and J.L. Spoormaker, Modeling of the temperature dependent viscoelasticity in plastics, Technical Report K381 of Faculty of Design, Engineering and Production, TU Delft, the Netherlands (1998). 

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

The theory relating stress, strain, time and temperature of viscoelastic materials is complex. For many practical purposes it is often better to use an ad hoc system known as the pseudo-elastic design approach. This approach uses classical elastic analysis but employs time- and temperature-dependent data obtained from creep curves and their derivatives. In outline the procedure consists of the following steps ... 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

The viscoelastic nature of the matrix in many fibre reinforced plastics causes their properties to be time and temperature dependent. Under a constant stress they exhibit creep which will be more pronounced as the temperature increases. However, since fibres exhibit negligible creep, the time dependence of the properties of fibre reinforced plastics is very much less than that for the unreinforced matrix. 

The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena that are typical of these plastics. When the change is an unwanted limitation it is called creep. When the change is skillfully adapted to use in the overall design, it is referred to as plastic memory. 

Viscoelasticity A combination of viscous and elastic properties in a plastic with the relative contribution of each being dependent on time, temperature, stress, and strain rate. It relates to the mechanical behavior of plastics in which there is a time and temperature dependent relationship between stress and strain. A material having this property is considered to combine the features of a perfectly elastic solid and a perfect fluid. 

Investigation of the linear viscoelastic properties of SDIBS with branch MWs exceeding the critical entanglement MW of PIB (about -7000 g/mol ) revealed that both the viscosity and the length of the entanglement plateau scaled with B rather than with the length of the branches, a distinctively different behavior than that of star-branched PIBs. However, the magnitude of the plateau modulus and the temperature dependence of the terminal zone shift factors were found to... 

For the investigation of the time and the temperature dependence of the fibre strength it is necessary to have a theoretical description of the viscoelastic tensile behaviour of polymer fibres. Baltussen has shown that the yielding phenomenon, the viscoelastic and the plastic creep of a polymer fibre, can be described by the Eyring reduced time (ERT) model [10]. The shear deformation of a domain brings about a mutual displacement of adjacent chains, the... 

Sugiyama, M. and Norimoto, M. (1996). Temperature dependence of dynamic viscoelasticities of chemically treated woods. Mokuzai Gakkaishi, 42(11), 1049-1056. 

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

The total deformation in the four-element model consists of an instantaneous elastic deformation, delayed or retarded elastic deformation, and viscous flow. The first two deformations are recoverable upon removal of the load, and the third results in a permanent deformation in the material. Instantaneous elastic deformation is little affected by temperature as compared to retarded elastic deformation and viscous deformation, which are highly temperature-dependent. In Figure 5.62b, the total viscoelastic deformation is given by the curve OABDC. Upon unloading (dashed curve DFFG),... 

When a polymer is used as a structural material, it is important that it be capable of withstanding applied stresses and resultant strains over its useful service life. Polymers are viscoelastic materials, having the properties of solids and viscous liquids. These properties are time- and temperature-dependent. 

Figure 5. Temperature dependence of viscoelastic property of ozonized lignin/epoxy resins cured with hexamethylenediamine. Ozonized lignin content in DGEBA (1) 0 PHR (2) 20 PHR (3) 40 PHR (4) 80 PHR.

Although most physical properties (e.g., viscosity, density, heat conductivity and capacity, and surface tension) must be regarded as variable, it is of particular value that viscosity can be varied by many orders of magnitude under certain process conditions (5,11). In the following, dimensional analysis will be applied exemplarily to describe the temperature dependency of the viscosity and the viscosity of non-Newtonian fluids (pseudoplastic and viscoelastic, respectively) as influenced by the shear stress. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Spin-spin relaxation times (T2) in polymer systems range from about 10-5 s for the rigid lattice (glassy polymers) to a value greater than 10-3 s for the rubbery or viscoelastic state. In the temperature region below the glass transition, T2 is temperature independent and not sensitive to the motional processes, because of the static dipolar interactions. The temperature dependence of T2 above Tg and its sensitivity to low-frequency motions, which are strongly affected by the network formation, make spin-spin relaxation studies suitable for polymer network studies. 

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