Ultimate Strength of Polymers

Graph adapted from S.J. Krause etal.. Polymer, 1354 (1988) 

PPTA poly(p-phenyleneterephthalamide) 0 50 100 150 200 250300 350 PBT poly(p-phenylene benzobisthiazole) Specific Tensiie Moduius 

The discussion in this chapter has dealt in Sects. 5.1 to 5.3 with the description of structure and properties of crystals, perhaps the best-known branch of material science of polymers. Complications arise, however, since most crystallizing polymers do not reach equilibrium. The metastable, multiphase sttuctures will be discussed in Chaps. 6 and 7. They represent the most complex structure and need for their description all tools of thermal analysis of polymeric materials. 

In this section, the upper temperature limit of the crystalline state is explored on the basis of experimental data on the thermodynamics of melting, extrapolated to equilibrium. The more common nonequilibrium melting will see its final discussion in Sects. 6.2 and 7.2. The other condensed states of macromolecules, the mesophases, glasses, and melts are treated in Sects. 5.5 and 5.6. Much less is known about them than about the crystals. 

Next is the exothermic crystallization transition at T. Such crystallization on heating above the glass ttansition temperature is called cold crystallization [37]. It contrasts to crystallization on cooling from the melt, see also Figs. 4.122,4.136, and 4.138. After cold crystallization, the sample is semicrystalline with about 40% 

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Secondly, the ultimate properties of polymers are of continuous interest. Ultimate properties are the properties of ideal, defect free, structures. So far, for polymer crystals the ultimate elastic modulus and the ultimate tensile strength have not been calculated at an appropriate level. In particular, convergence as a function of basis set size has not been demonstrated, and most calculations have been applied to a single isolated chain rather than a three-dimensional polymer crystal. Using the Car-Parrinello method, we have been able to achieve basis set convergence for the elastic modulus of a three-dimensional infinite polyethylene crystal. These results will also be fliscussed. 

By using a similar procedure for the preparation of hybrids of silica, hybrids materials consisting of other metal oxides were also prepared by the group of Wilkes [15]. For example, titania was incorporated into organic polymers by using the chemically controlled condensation (CCC) method for the preparation of poly(tetramethylene oxide)-silica or poly(dimethylsiloxane)-silica hybrids. Especially, in the case of the hybrid with poly (tetramethylene oxide), the modulus or ultimate strength of the hybrid increased in the presence of titania component, as shown in Table 3. This phenomenon was explained by the catalytic ability of... 

The preceding models permit an evaluation of the ultimate strength of some simple polymers under mechanical stress. Under specific circumstances, it is important to predict the average lifetime of a bond stressed at a level which is below its... 

In the previous section we discussed the ultimate strength of a polymer fibre o0. This value corresponds to the stress at which all secondary bonds in the fibre are broken. Due to the presence of the chain orientation distribution alone, it follows that even the strength of a polymer fibre without any flaws will never attain this value. Yet, fracture in a real fibre may not always initiate in the most disoriented domains. If there are inhomogeneities that lead to stress concentrations, fracture can also occur in domains at a smaller angle to the fibre axis. 

As will be shown in Sect. 3, Yoon derived the following expression for the ultimate strength of a polymer fibre with long and parallel oriented chains of finite length... 

For the transformation of the macrocomposite model to a molecular composite model for the ultimate strength of the fibre the following assumptions are made (1) the rods in the macrocomposite are replaced by the parallel-oriented polymer chains or by larger entities like bundles of chains forming fibrils and (2) the function of the matrix in the composite, in particular the rod-matrix interface, is taken over by the intermolecular bonds between the chains or fibrils. In order to evaluate the effect of the chain length distribution on the ultimate strength the monodisperse distribution, the Flory distribution, the half-Gauss and the uniform distribution are considered. 

Carswell and Nason assigned five classifications to polymers (Figure 14.8). It must be remembered that the ultimate strength of each of these is the total area under the curve before breaking. The soft and weak class, such as PIB, is characterized by a low modulus of elasticity, low yield (stress) point, and moderate time-dependent elongation. The Poisson ratio, i.e., ratio of contraction to elongation, for soft and weak polymers is about 0.5, which is similar to that found for many liquids. 

They are capable of providing insight into the presence or absences of transfer (wear), the adhesive strength of polymer to metal, amount of transfer, bond scission, mechanical effects such as loading of surfaces together, chemical effects on bonding and surface energetics. The field ion microscope coupled with the atom probe is the ultimate tool for the study of polymer wear because it allows... 

The size and number of sphemhtes in the polymer tends to affects the physical properties. Thus, the impact strength of polymer hhns or their flex life usually increases as the sphemlite size decreases. On the other hand, there does not appear to be any correlation between the yield stress and ultimate elongation and the size of the sphemhtes. 

Multi-walled nanotubes can be brittle, with an ultimate elongation of about 1.5%, and can produce unwanted conductive dust, but single-walled ones form continuous ropes that tend not to break easily, and they can have an elongation in double figures. Nanotubes do not degrade the strength of polymers as much as carbon black does. 

Aerospace composite adhesives. In addition to molded parts, polyamide-imides are used as the high-strength adhesives and polymer matrix in aerospace composites. One particular area is honeycomb composites [19]. The adhesives and polymer matrix are critical because the ultimate strength of the honeycomb is often directly dependent on the strength of the bond between the honeycomb layers. The polyamide-imide resin is dissolved in an appropriate solvent and applied to the honeycomb fabric, using a bonding technique to achieve the desired... 

The ultimate strength of a composite of fibre and polymer is basically dependent upon the stress-strain relationship of its components. Any fibre whose strength and modulus of elasticity is greater than a polymer is capable of theoretically providing reinforcement to that polymer. Practically, however, the difference in properties has to be considerable to obtain efficient reinforcement and the effectiveness of the reinforcement is dependent upon the degree of adhesion which can be achieved. Other factors which also influence the properties of a composite are relative volumes of reinforcement and matrix, their physical and chemical properties, the temperature resistance and, of very considerable importance, the fibre length which is being used. 

It is useful, therefore, to distinguish between brittle strength, yield strength, and ultimate strength of a polymer. 

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