Properties dynamic mechanical

The dynamic mechanical tests over a wide temperature range are very sensitive to the physical and chemical structure of polymers and composites. They allow the study of glass transitions or secondary transitions and yield information about the morphology of polymers. Experimental results of dynamic tensile tests (DMTA) conducted on nanocomposites are shown in Table 2 for selected temperatures (20 °C, 100 °C, 220 °C, and glass transition, T ). 

The rigid micro- and nanofillers push the complex modulus of epoxy to a higher level. The modulus decreases with rising temperature. At elevated temperatures, in the rubbery plateau well above Tg, the nanocomposites retain higher modulus and damping than the neat epoxy, which probably indicates a higher network density. With rising filler content, the Tg of nanocomposites increases 

It has been found that steel fibres are particularly effective In concretes exposed to dynamic loading of any kind, such as fatigue, Impact and blast. 

Steel fibres have been found to improve the fatigue performance of pavements [97], particularly when high performance steel fibre concretes are used [98]. They also appear to improve the fatigue properties of conventionally reinforced structural elements [99-101]. 

While the results of these earlier studies are still valid, more recent work has provided much more detail about the impact behaviour of SFRC. Pacios and Shah [110] showed that at higher rates of loading, the fibres deveioped more pull-out resistance and slip at the peak load. However, if the impact velocity is too great, the mode of failure of the fibres may change from pull-out to fracture, leading to a considerable drop in fracture energy [111,112]. This suggests that the fibre 

Turning to more structural applications, Yan and Mindess [119-121] showed that steel fibres could considerably improve the bond between the matrix and conventional reinforcing bars under impact, primarily by inhibiting the growth of cracks emanating from the deformations (lugs) on the reinforcing bars. Also, Mindess etai [122] showed that steel fibres could considerably improve the impact performance of prestressed concrete railroad ties. 

The sinusoidally varying stress, o(f), in a dynamic mechanical test can be written as  

This indicates the energy consumed during the dynamic mechanical test is completely stored as the internal energy by the ideal elastic deformation, and the energy can be completely recovered. 

If the sinusoidally varying stress is applied on an ideal viscous material, the strain rate can be written as  

Since the viscous deformation of an ideal viscous material is unrecoverable, the energy consumed in a cycle is released in the form of heat. 

Polymer fibers are viscoelastic. If the sinusoidally varying stress is apphed on a viscoelastic polymer fiber, the resultant strain can be given as  

As discussed in Section 12.4 of Chapter 12, dynamic mechanical testing is conducted by subjecting a polymer sample to a sinusoidal strain of amplitude yo and fi equency co. Because the strain amphtude is usually of infinitesimal magnitude, we can legitimately apply the Boltzmann superposition principle to this flow situation. Before doing this, though, we change the independent variable in Eq. (14.6.3) from s to t, where l = t — s. Thus, we have 

Example 14.5 How do the storage modulus and loss modulus vary with fi-equency when co 0  

Equations (14.7.5) and (14.7.6) are vahd for all materials at low enough frequencies. However, we may not always observe this behavior due to the inabihty to accurately measure small stresses at very low frequencies. The limiting behavior at very high frequencies can also be obtained (see Problem 14.17). The final expressions are as follows  

Although a knowledge of the stress-relaxation modulus allows us to calculate G and G via Eqs. (14.7.3) and (14.7.4), in practice we find that the experimental measurement of G (cai) or G (q)) (using a cone-and-plate viseo-meter, for example) is much more accurate than the measurement of G(f). Consequently, we measure G (co) and G (co) and use these data to compute G(f). The computed value of G(t ) can, in turn, be used to calculate any other linear viscoelastic fimction through the use of Eq. (14.7.1). 

In order to accomplish these objectives, we need methods of interrelating the various linear viscoelastic functions. The general techniques of obtaining one fimction from another have been discussed by Ferry [17]. In the present case, Baumgaertel and Winter have proposed a particularly simple method [18]. If we introduce Eq. (14.6.10) into Eqs. (14.7.3) and (14.7.4) and carry out the integrations, we get (see also Example 12.2). 

5 Thermoplastic Copolyester Elastomers with Different Chemical Composition 

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The dynamic mechanical properties of PTFE have been measured at frequencies from 0.033 to 90 Uz. Abmpt changes in the distribution of relaxation times are associated with the crystalline transitions at 19 and 30°C (75). The activation energies are 102.5 kj/mol (24.5 kcal/mol) below 19°C, 510.4 kJ/mol (122 kcal/mol) between the transitions, and 31.4 kJ/mol (7.5 kcal/mol) above 30°C. 

Fatty Acid Process. When free fatty acids are used instead of oil as the starting component, the alcoholysis step is avoided. AH of the ingredients can therefore be charged into the reactor to start a batch. The reactants are heated together, under agitation and an inert gas blanket, until the desired endpoint is reached. Alkyds prepared by the fatty acid process have narrower molecular weight distribution and give films with better dynamic mechanical properties (34). 

Determining and Reporting Dynamic Mechanical Properties of Plastics... 

Using physical properties relating to performance parameters leads to the development of algorithms for predicting performance for laboratory screening of potential improvements. Many of these algorithms have been estabUshed. The two main categories of measurement criteria are quasi static and dynamic mechanical properties. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

Measurement of dynamic mechanical properties was carried out under tension mode using a viscoelasto-meter, (Rheovibron DDV-III-EP, M/s, Orientec Corp., Tokyo, Japan). Sample size was 3.5 cm x 6.5 mm x 2 mm. Testing was carried out at a low amplitude, 0.025 mm, over a temperature range of - 100°C to +200°C. Heating rate was TC/min and frequency of oscillation was 3.5 Hz or 110 Hz. 

Torsion property As noted, the shear modulus is usually obtained by using pendulum and oscillatory rheometer techniques. The torsional pendulum (ASTM D 2236 Dynamic Mechanical Properties of Plastics by Means of a Torsional Pendulum Test Procedure) is a popular test, since it is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial processes or easily cut from fabricated products. 

Interest in the use of syntactic foam as a shock attenuator led to studies of its static and dynamic mechanical properties. Particularly important is the influence of loading rate on stiffness and crushing strength, since oversensitivity of either of these parameters can complicate the prediction of the effectiveness of a foam system as an energy absorber. 

Finer dispersion of silica improves the mechanical and dynamic mechanical properties of the resultant composites. Figure 3.11a and b compares the tensile properties of the acrylic copolymer and terpolymers in the uncross-hnked and cross-linked states, respectively. 

Adsorption of rubber over the nanosilica particles alters the viscoelastic responses. Analysis of dynamic mechanical properties therefore provides a direct clue of the mbber-silica interaction. Figure 3.22 shows the variation in storage modulus (log scale) and tan 8 against temperature for ACM-silica, ENR-silica, and in situ acrylic copolymer and terpolymer-silica hybrid nanocomposites. 

Recently Sahoo and Bhowmick [75] synthesized hydroxyl-terminated POSS in their laboratory starting from (3-aminopropyl) triethoxysilane (APS) and phenylglycidylether (PGE) and used it as a curative in carboxylated nitrile mbber (XNBR). This has been a newer class of material where the nanofiller simultaneously cures the mbber and promotes solvent resistance, as well as mechanical and dynamic mechanical properties. Table 3.3 illustrates some of these findings. 

Dynamic mechanical properties of the nanocomposites are shown in Figure 4.6. There is 10% improvement of the storage modulus at 20°C by incorporating only 4 wt% of the nanombe. 

ZnO nanoparticles possess greater surface/volume ratio. When used in carboxylated nitrile rubber as curative, ZnO nanoparticles show excellent mechanical and dynamic mechanical properties [41]. The ultimate tensile strength increases from 6.8 MPa in ordinary rabber grade ZnO-carboxylated nitrile rubber system to 14.9 MPa in nanosized ZnO-carboxylated nitrile mbber without sacrificing the elongation at failure values. Table 4.1 compares these mechanical properties of ordinary and nano-ZnO-carboxylated nitrile rubbers, where the latter system is superior due to more rubber-ZnO interaction at the nanolevel. 

Zhou L.L. and Eisenberg A., lonomeric blends. II. Compatibility and dynamic mechanical properties of sulfonated cis-l,4-polyisoprenes and styrene/4-vinylpyridine copolymer blends, J. Polym. Sci., Polym. Phy., 21, 595, 1983. 

Research concerning nylon-elastomer blends has mostly focused on the improvement of mechanical and thermal properties. Their dynamic mechanical properties are quite important both for processing and engineering applications. Wang and Zheng have smdied the influence of grafting on the dynamic mechanical properties of a blend based on nylon 1212 and a graft... 

The formation of PPD groups on the polymer backbone provides a mechanism to improve the polymer-filler interactions. The nitrogen-hydrogen bonds are capable of hydrogen bonding with polar groups on the surface of the filler. This enhanced interaction provides for somewhat unique dynamic mechanical properties. Under ideal conditions rolling resistance improves when QDI is used in the mix. Also, abrasion characteristics are maintained and in some cases even modest improvements occur. 

Mooney Viscometer studies at 100°C and 120°C show lower viscosity of the Al-hlled gums. Lower viscosity compounds require less energy input for extrusion. Comparative results on the dynamic mechanical properties, measured using a rubber process analyzer (RPA), show that at the... 

The B-series of silica samples were also blended with rubber and the compound formulation is shown in Table 17.6. The uncured gums were then tested according to ISO 5794-2 1998. The uncured samples were tested using a Mooney viscometer and an RPA, which measures the dynamic mechanical properties as the samples cure. Figure 17.7 shows the results of these two tests for the Mooney viscosity at 100°C, storage modulus, loss modulus, and tan 8. 

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