Mechanical property measurement polymers

In describing the various mechanical properties of polymers in the last chapter, we took the attitude that we could make measurements on any time scale we chose, however long or short, and that such measurements were made in isothermal experiments. Most of the experimental results presented in Chap. 3 are representations of this sort. In that chapter we remarked several times that these figures were actually the result of reductions of data collected at different temperatures. Now let us discuss this technique our perspective, however, will be from the opposite direction taking an isothermal plot apart. 

All VGCF was graphitized prior to composite consolidation. Composites were molded in steel molds lined with fiberglass reinforced, non-porous Teflon release sheets. The finished composite panels were trimmed of resin flash and weighed to determine the fiber fraction. Thermal conductivity and thermal expansion measurements of the various polymer matrix composites are given in Table 6. Table 7 gives results from mechanical property measurements. 

In this paper, we examine the relationship between radiation resistance and polymer structure using volatile product and mechanical property measurements. 

A torsional pendulum (Figure 5.80) is often used to determine dynamic properties. The lower end of the specimen is clamped rigidly and the upper clamp is attached to the inertia arm. By moving the masses of the inertia arm, the rotational momentum of inertia can be adjusted so as to obtain the required frequency of rotational oscillation. The dynamic shear modulus, G, can be measured in this manner. A related device is the dynamic mechanical analyzer (DMA), which is commonly used to evaluate the dynamic mechanical properties of polymers at temperatures down to cryogenic temperatures. 

Molecular Orientation Characterization of molecular orientation is important as many physical and mechanical properties of polymers depend on the extent and uniformity of the orientation [2,4,25]. Orientation can be measured by using a variety of techniques [2,4,25,33,34]. IR spectroscopy not only allows the characterization of amorphous and crystalline phases separately, it also provides morphological data and can be used to map orientation with high spatial resolution [35]. 

Figure 1. Methods of Measuring Dynamic Mechanical Properties of Polymers. Commercially available instruments are listed by schemes of measurement.

The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stress-strain curves changes with the strain rate and temperature. Figure 14.1 illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. 14.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-... 

This equation was used to estimate the interfacial adhesion in comparison with the acid-base properties of glass fibers in LDPE. The effect of surface treatment of glass beads on their interfacial adhesion to PET was also estimated from a mechanical property measurement. A mathematical model describing the adsorption of polymers on filler surfaces related coupling density to the average area available for coupling between rubber and filler surface. ... 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

The low-speed mechanical properties of polymer blends have been frequently used to discriminate between different formulations or methods of preparation. These tests have been often described in the literature. Examples of the results can be found in the references listed in Table 12.9. Measurements of tensile stress-strain behavior of polymer blends is essential [Borders et al., 1946 Satake, 1970 Holden et al., 1969 Charrier and Ranchouse, 1971]. The mbber-modified polymer absorbs considerably more energy, thus higher extension to break can be achieved. By contrast, an addition of rigid resin to ductile polymer enhances the modulus and the heat deflection temperature. These effects are best determined measuring the stress-strain dependence. 

Mechanical properties are discussed here only in an introductory manner in order to understand the main applications of polymers. An extended discussion of the mechanical properties of polymers and their measurement can be found in Chapter 21. 

Polymers are also unique in their viscoelastic nature, a behavior that is situated between that of a pure elastic solid and that of a pure viscous liquid-like material their mechanical properties present a strong dependence on time and temperature. Given all the factors that have to be taken into account to determine the mechanical properties of polymers, their measurement would appear to be very complex. However, there is a series of general principles that determine the different mechanical properties and that give a general idea of the expected results in different mechanical tests. These principles can be organized in a systematic manner to determine the interrelation of polymer structure and the observed mechanical properties, using equations and characteristic parameters of polymeric materials. 

With regard to the mechanical reactirai of a polymer network to a stress applied, it is important that loose ends of macromolecules in a network structure are as shmrt as possible and/or their concentration is low. As these ends mostly extend out of the lamellas of crystallites then, while crossUnking is taking place in an amorphous phase and with the simultaneous presence of crystallites, a network with small loose ends should be formed. The crosslink junctions stabilize the natural molecular network (entanglements and crystallites), and every chain in the system is potentially elastically operative and can contribute to the stress in a tensile experiment [33]. The stabilization effect of chemical crosslinks on entanglements and crystallites may be the direct cause of observed differences in the determination of the amount of chemical crosslinks from mechanical property measurements and sol-gel analysis of the cross-linked polymer. 

TABLE 52.15. Mechanical properties measured at200 °C of an ECTFE polymer after y-irradiation. 

In virtually all applications, polymer materials have to be subjected to a loading force in some way or other. Hence, the assessment of mechanical properties is very important for the design of polymer materials for various applications. Polymer materials are more sensitive to the service temperature and other environmental effects compared with conventional materials. Hence, the data for mechanical properties, measured using conditions similar to a service environment, should be used for design rather than standard data available in the literature. 

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