Stress-strain behavior polymers

These differences on the stress-strain behavior of P7MB and PDTMB show the marked influence of the mesomorphic state on the mechanical properties of a polymer. When increasing the drawing temperatures and simultaneously decreasing the strain rate, PDTMB exhibits a behavior nearly elastomeric with relatively low modulus and high draw ratios. On the contrary, P7MB displays the mechanical behavior typical of a semicrystalline polymer. 

Copolymerization of ethylene and styrene by the INSITE technology from Dow generates a new family of ethylene-styrene interpolymers. Polymers with up to 50-wt% styrene are semicrystalline. The stress-strain behavior of the low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress-strain curve at the higher strain and the fast instantaneous recovery [67], Similarly, ethylene-butylene copolymers may also be prepared. 

LDPE affect the dynamic mechanical, as well as other material properties of these polymers. The similarity of the temperature dependence of E between our toluene cast HB film and the quenched LDPE (both of 40% crystallinity) in Figure 14A as compared to our quenched HB film (% crystallinity 30%) is another indication of the importance of the level of crystallinity on properties. (This topic has already been discussed in some length in the section on stress-strain behavior). 

Penumadu, D., Yamamuro, Abrantes, A.E., and Campbell, G.A., Stress-Strain Behavior of Polymer Pellets, SPEANTEC Tech. Papers, 43, 224 (1997)... 

A number of physical tests emphasizing stress-strain behavior will be covered in Chapter 14. Here, we will concentrate on other areas of testing, emphasizing thermal and electrical properties and on the characterization of polymers by spectral means. Spectroscopic characterization generally concentrates on the structural identification of materials. Most of these techniques, and those given in Chapter 14, can also be directly applied to nonpolymeric materials such as small organic molecules, inorganic compounds, and metals. 

For perspective. Figure 14.6 contains general ranges for the three major polymer groupings with respect to simple stress-strain behavior. 

Several stress-strain plots are shown in Fig. 1-10. Four important quantities characterize the stress-strain behavior of a polymer ... 

The hardness of a polymer can also be estimated from the modulus of elasticity E (high E modulus indicates high hardness). The advantage here is that every region of elasticity and every degree of hardness can be detected with a single kind of measurement (determination of stress-strain-behavior or torsional oscillation). 

Figure 5.58 The stress-strain behavior of brittle polymer (curve A), ductile polymer (curve B), and highly elastic polymer (curve C). Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 475. Copyright 2000 by John Wiley Sons, Inc.

Figure 5.59 Stress-strain behavior of (a) spring of modulus E and (b) dashpot of viscosity rj. Reprinted, by permission, from J. M. G. Cowie, Polymers Chemistry Physics of Modern Materials, p. 277, 2nd ed. Copyright 1991 by J. M. G. Cowie.

For optimum cross-linking efficiency, a combination of accelerators is used. Table 6 shows the increase in load-bearing capability of vulcanizates based on different rubbers as the ratio of two accelerators is changed (14). The term 300% modulus represents the strain at 300% stress and is not a true modulus because rubber gives nonlinear stress—strain behavior. For polymers with primary allylic carbon atoms, the use of two accelerators gives significandy higher 300% modulus than either accelerator used alone. When the mbber polymer consists of secondary allylic carbon atoms, the modulus is level until the sulfenamide OBTS becomes the principle accelerator. 

Typical amorphous polymers can exhibit each of these types of stress-strain behavior when the temperature is changed from below to above the Tg of the polymer. 

A number of attempts have been made to describe the behavior of polymers in terms of models. Following is a description of some of the simpler models employed in the description of the stress-strain behavior of polymeric... 

Price,C., Allen,G., de Candia,F., Kirkham,M.C., Subramaniam,A. Stress-strain behavior of natural rubber vulcanized in the swollen state. Polymer (London) 11, 486-491 (1970). 

Bristow, G.M. Relation between stress-strain behavior and equilibrium volume swelling for peroxide vulcanizates of natural rubber and cis-1,4-polyisoprene. J. Appl. Polymer Sci. 9, 1571-1578 (1965). 

Fig. 11. Stress-strain behavior of a) polypyrrole doped with p-toluene sul-phonate and b) the same polymer plasticised with acetonitrile. Reproduced with permission from Wynne KJ, Street GB (1985) Macromolecules 18 2361...

Some interesting results have already been obtained (JO, 11) on these polymers, where the effect of the above molecular parameters on the mechanical properties has been studied. Thus, Figure 11 shows the effect of variations in block length and styrene content on the stress-strain behavior of styrene-butadiene-styrene (SBS) polymers. As expected, the stress levels increase with increasing styrene ( filler ) content but are independent of the block lengths. In other words, the center block size does not exert the same influence as the molecular weight between cross-... 

Fig. 10.60 Compressive stress-strain behavior of PS and LLDPE at 25°C and crosshead speed of 25.4 mm/min. At a compressive stress level of 20 MPa the deformation of the soft LLDPE is large, in the dissipative region and nearly twenty times the PS deformation, which is of the order of 0.04, in the elastic nondissipative range. [Reprinted by permission from B. Qian, D. B. Todd, and C. G. Gogos, Plastic Energy Dissipation (PED) and its Role in Heating/Melting of Single Component Polymers and Multi-component Polymer Blends, Adv. Polym. Techn., 22, 85-95 (2003).]...

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