Mechanical properties of polymers

The mechanical behavior of polymers is quite different from metals and ceramics and depends greatly on their structure and operating temperature. Below their glass transition temperature, Tg (the temperature at which their covalent bonded chains can no longer move relative to one another), they are quite brittle and exhibit glass-like behavior. Above their Tg, they behave plastically. In this sense, they are similar to metals that exhibit ductile-to-brittle transitions, but for entirely different reasons. 

This chapter covers some of the methods and instruments used to determine the mechanical properties of polymers. Examples of instrument designs and typical data generated in these measurements will be introduced. In particular, automated axial tensiometers (to find elastic modulus, yield stress, and ultimate stress), dynamic mechanical analyzers (to determine storage and loss moduli), and rheometers (to measure flow viscosity) will be introduced. This chapter considers the principles behind the devices used to establish and measure the properties of viscometric flows. One of the common techniques used to determine viscous flow properties, PoisueiUe (laminar) flow in cylindrical tubes, is also important in technical applications, as polymer melts and solutions are often transported and processed in this manner. The time-temperature superposition principle is also covered as a way to predict polymer behavior over long timescales by testing materials across a range of temperatures. 

A number of physical parameters can be measured as a way to characterize polymers. The specific tests that are done are normally dictated by the end use of a particular part. We have defined many of these parameters in the previous few chapters, such as elastic (or Young s) modulus, E, shear (or Hooke s) modulus, G, storage modulus (E or G ), loss modulus (E or G ), tan 6, which all apply to solids, as well as the viscosity used for polymer melts (and solutions) as described with polymer rheology. In addition to these terms, you may also run across hardness (how resistant the polymer is to penetration by a needle), toughness 

Fundamental Principles of Polymeric Materials, Third Edition. Christopher S. Brazel and Stephen L. Rosen. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

---

Regarding mechanical properties of polymers, the efficiency of the Car-Parrinello approach has enabled us to evaluate the ultimate Young s modulus of orthorhombic polyethylene, and demonstrate basis set convergence for that property. 

In this section we resume our examination of the equivalency of time and temperature in the determination of the mechanical properties of polymers. In the last chapter we had several occasions to mention this equivalency, but never developed it in detail. In examining this, we shall not only acquire some practical knowledge for the collection and representation of experimental data, but also shall gain additional insight into the free-volume aspect of the glass transition. 

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. 

Glass Transition. The glass-transition temperature T reflects the mechanical properties of polymers over a specified temperature range. 

Molecular Weight. The values of the mechanical properties of polymers increase as the molecular weight increases. However, beyond some critical molecular weight, often about 100,000 to 200,000 for amorphous polymers, the increase in property values is slight and levels off asymptotically. As an example, the glass-transition temperature of a polymer usually follows the relationship... 

L. E. Nielsen, Mechanical Properties of Polymers and Composites, Vol. 1, Marcel Dekker, New York, 1974. 

And we are still learning how best to fabricate and use them. As emphasised in the last chapter, the mechanical properties of polymers differ in certain fundamental ways from those of metals and ceramics, and the methods used to design with them (Chapter 27) differ accordingly. Their special properties also need special methods of fabrication. This chapter outlines how polymers are fabricated and joined. To understand this, we must first look, in slightly more detail, at their synthesis. 

In general, physico-mechanical properties of polymers depend on the molecular weight. However, the physico-mechanical properties of PSs decreased in the presence of cationic catalysis, but increased in the case of the binding of functional groups to the aromatic ring in spite of the destruction of PS. Therefore, new properties such as adhesion and photosensitive capability increase... 

N. Bikales (ed.). Mechanical Properties of Polymer, Wiley Interscience, New York (1971). 

The mechanical properties of polymers are of interest in all applications where they are used as structural materials. The analysis of the mechanical behavior involves the deformation of a material under the influence of applied forces, and the most important and characteristic mechanical property is the modulus. A modulus is the ratio between the applied stress and the corresponding deformation, the nature of the modulus depending on that of the deformation. Polymers are viscoelastic materials and the high frequencies of most adiabatic techniques do not allow equilibrium to be reached in viscoelastic materials. Therefore, values of moduli obtained by different techniques do not always agree in the literature. 

The opportunity to synthesize new conjugated polymers with improved properties began to attract the attention of a larger number of synthetic chemists in the 1980s. Equally important was the subsequent development of stable, processible metallic polymers. As a result of these efforts, we now have a class of materials which exhibit a unique combination of properties the electronic and optical properties of metals and semiconductors in combination with the processing advantages and mechanical properties of polymers. 

Nielsen LE (1974) Mechanical properties of polymers and composites, Marcel Dekker, New York, Ch 7... 

Siesler, H. W. Rheo-Optical Fourier-TranformInfrared Spectroscopy Vibrational Spectra and Mechanical Properties of Polymers. Vol. 65, pp. 1 —78. 

Usually, crystallization of flexible-chain polymers from undeformed solutions and melts involves chain folding. Spherulite structures without a preferred orientation are generally formed. The structure of the sample as a whole is isotropic it is a system with a large number of folded-chain crystals distributed in an amorphous matrix and connected by a small number of tie chains (and an even smaller number of strained chains called loaded chains). In this case, the mechanical properties of polymer materials are determined by the small number of these ties and, hence, the tensile strength and elastic moduli of these polymers are not high. 

The mechanical properties of polymers also depend on the extent of crosslinking. Uncrosslinked or lightly crosslinked materials tend to be soft and reasonably flexible, particularly above the glass transition temperature. 

Most suggestions for further reading are given in full in the Bibliography at the end of this book. However, in view of the specialised nature of the topics discussed in this chapter it is worth citing a list of further reading which concentrates on mechanical properties of polymers. They are ... 

---