Viscoelastic properties determination

The calendering process can be described by means of a combined drag and pressure flow. The rotating rolls of a calender drag the mbber through the calender nip. The clearance as well as viscoelastic properties determine the sheet thickness. 

Marxer, C.G. (2002), Protein and Cell Absorption Topographical Dependency and Adlayer Viscoelastic Properties Determined with Oscillation Amplitude of Quartz Resonator, Department of Physics, University of Fribourg, Fribourg, Switzerland. 

Figure 12. Viscoelastic properties determined by Dynamic Mechanical Analysis at different temperatures for two-component epoxy (black lines B, C, D, F, H, L, M, O and P) and polyurethane (gray lines R and S) adhesives.

Viscoelastic properties determined by dynamic mechanical analysis at different temperatures for three different CFRP strips. 

Perhaps it is more understandable if we correlate adhesive performance to viscoelastic properties determined at the adhesive testing temperature. The bond is formed and destroyed during testing using conditions which differ in stress level, deformation rate, and extent of deformation at room temperature. Therefore, the measurement of viscoelastic properties at room temperature may be pertinent. Viscoelastic properties at constant temperature are determined by dynamic mechanical measurement over a range of frequencies, for example, from 0.1 to 100 rad/sec. We see that the frequency scan curves are approximately a reciprocal... 

For the identification of space effects on polymers, numerous standard tests must be used, namely spectroscopy, thermal analysis, dielectric and conductivity measurements, mechanical and viscoelastic property determination, etc. In the late 1990s NASA decided to substitute polyurethane coatings (having carcinogenic) with another one. The 18 alternative candidates were subjected to 34 standard test procedures (Table 3.7). In the following report the number of candidates fell to eight, from which the best with the lowest cost-to-performance ratio will have to be selected. This procedure illustrates how involved the polymeric testing for space application is, even if the test procedures are quite standard. 

In principle, the relaxation spectrum H(r) describes the distribution of relaxation times which characterizes a sample. If such a distribution function can be determined from one type of deformation experiment, it can be used to evaluate the modulus or compliance in experiments involving other modes of deformation. In this sense it embodies the key features of the viscoelastic response of a spectrum. Methods for finding a function H(r) which is compatible with experimental results are discussed in Ferry s Viscoelastic Properties of Polymers. In Sec. 3.12 we shall see how a molecular model for viscoelasticity can be used as a source of information concerning the relaxation spectrum. 

Acoustic Measurements. Measurement of the propagation of ultrasonic acoustic waves has been found useful for determining the viscoelastic properties of thin films of adhesives. In this method, the specimen is clamped between transmitting and receiving transducers. The change in pulse shape between successive reverberation of the pulse is dependent on the viscoelastic properties of the transmitting material. Modulus values can be calculated (267,268). 

The transition to turbulent flow begins at Re R in the range of 2,000 to 2,500 (Metzuer and Reed, AIChE J., 1, 434 [1955]). For Bingham plastic materials, K and n must be evaluated for the condition in question in order to determine Re R and establish whether the flow is laminar. An alternative method for Bingham plastics is by Hanks (Hanks, AIChE J., 9, 306 [1963] 14, 691 [1968] Hanks and Pratt, Soc. Petrol. Engrs. J., 7, 342 [1967] and Govier and Aziz, pp. 213-215). The transition from laminar to turbulent flow is influenced by viscoelastic properties (Metzuer and Park, J. Fluid Mech., 20, 291 [1964]) with the critical value of Re R increased to beyond 10,000 for some materials. 

Thermal and thermomechanical analyses44 are very important for determining die upper and lower usage temperature of polymeric materials as well as showing how they behave between diose temperature extremes. An especially useful thermal technique for polyurethanes is dynamic mechanical analysis (DMA).45 Uiis is used to study dynamic viscoelastic properties and measures die ability to... 

Braithwaite, G. J. C. and Luckham, P. F. (1999). The simultaneous determination of the forces and viscoelastic properties of adsorbed polymer layers. /. Colloid Interface Sci. 218,97-111. 

The surface characteristics of these species are determined by the particulates and stress transfer across the membrane will tend to be low, reducing internal circulation within the drop. The structure of the interface surrounding the drop plays a significant role in determining the characteristics of the droplet behaviour. We can begin our consideration of emulsion systems by looking at the role of this layer in determining linear viscoelastic properties. This was undertaken by... 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

The understanding of the temperature and conversion dependence of the crosslinking kinetics is one of the prerequisites for understanding the changes in viscosity and viscoelastic properties as a function of reaction time and reaction temperature ( ). Three main factors determine these relations the reaction kinetics determined by temperature and conversion, the changes in structure determined primarily by conversion and the changes in Tg determined primarily also by conversion. 

Polymers are viscoelastic materials meaning they can act as liquids, the visco portion, and as solids, the elastic portion. Descriptions of the viscoelastic properties of materials generally falls within the area called rheology. Determination of the viscoelastic behavior of materials generally occurs through stress-strain and related measurements. Whether a material behaves as a viscous or elastic material depends on temperature, the particular polymer and its prior treatment, polymer structure, and the particular measurement or conditions applied to the material. The particular property demonstrated by a material under given conditions allows polymers to act as solid or viscous liquids, as plastics, elastomers, or fibers, etc. This chapter deals with the viscoelastic properties of polymers. 

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. 

The theory of viscoelastic properties based on the free-volume concept was developed in18, the main idea being that some structural elements are displaced when the free-volume fraction exceeds a critical value /c. The frequency v of the determining molecular process may be obtained from the equation... 

In free vibration methods, the rubber test piece, with or without an added mass, is allowed to oscillate at the natural frequency determined by the dimensions and viscoelastic properties of the rubber and by the total inertia. Due to damping in the rubber, the amplitude of oscillations will decay with time and, from the rate of decay and the frequency of oscillation, the dynamic properties of the test piece can be deduced. 

Any rubber test piece with or without added mass has a natural or resonant frequency of vibration determined by the dimensions and viscoelastic properties of the rubber, the total inertia of the system, and the mode of deformation. If constant force amplitude cycles are applied to the rubber and the frequency varied, the resulting deformation cycles will have a maximum value when the applied frequency equals the resonant frequency of the test piece system. 

Dynamic Measurements. A Vibron direct reading viscoelastometer (Toyo Measuring Instruments Co., Ltd., Tokyo, Japan) was used to determine viscoelastic properties. This instrument and its operation have been described in detail by Takayanagi and Yoshino (II, 14). All polymers and blends were examined at a standard frequency of 110 cps. In a few selected examples measurements were also made at 3.5 cps. 

The human erythrocyte possesses a characteristic biconcave shape and remarkable viscoelastic properties. Electron microscopy studies performed on red blood cells (RBC), ghosts, and skeletons revealed a two-dimensional lattice of cytoskeletal proteins. This meshwork of proteins was thought to determine the elastic properties of the RBC. This... 

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