BEHAVIOR OF POLYMERS

The mechanical properties of polymers are specified with many of the same parameters that are used for metals—that is, modulus of elasticity and yield and tensile strengths. For many polymeric materials, the simple stress-strain test is used to characterize some of these mechanical parameters. The mechanical characteristics of polymers, for the most part, are highly sensitive to the rate of deformation (strain rate), the temperature, and the chemical nature of the environment (the presence of water, oxygen, organic solvents, etc.). Some modifications of the testing techniques and specimen configurations used for metals (Chapter 6) are necessary with polymers, especially for highly elastic materials, such as rubbers. 

Material Specific Gravity Tensile Modulus [GPa (ksi)] Tensile Strength MPa (ksi) Yield Strength [MPa (ksi)] Elongation at Break (%) 

Source Modern Plastics Encyclopedia 96. Copyright 1995,The McGraw-Hill Companies. Reprinted with permission. 

The influence of strain rate on the mechanical behavior may also be important. In general, decreasing the rate of deformation has the same inflnence on the stress-strain characteristics as increasing the temperature that is, the material becomes softer and more ductile. 

Figpre 15.3 The influence of temperature on the stress-strain characteristics of poly(methyl methacrylate). 

In this chapter we will discuss extensively the behavior of polymers and their response to stress. This will allow us to better understand the physical and thermal properties—relating structure and performance. 

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Deutsoh H-P and Binder K 1993 Mean-field to Ising orossover in the oritioal behavior of polymer mixtures—a finite size sealing analysis of Monte Carlo simulations J. Physique II 3 1049... 

In this chapter we examine the elastic behavior of polymers. We shall see that this behavior is quite different from the elasticity displayed by metals and substances composed of small molecules. This is a direct consequence of the chain structure of the polymer molecules. In many polymers elasticity does not occur alone, but coupled with viscous phenomena. The combination of these effects is called viscoelasticity. We shall examine this behavior as well. 

We have relied heavily on the use of models in discussing the viscoelastic behavior of polymers in the transient and dynamic experiments of the last few sections. The models were mechanical, however, and while they provide a way for understanding the phenomena involved, they do not explicitly relate these phenomena to molecular characteristics. To establish this connection is the objective of this section. 

The preceding example of superpositioning is an illustration of the principle of time-temperature equivalency. We referred to this in the last chapter in connection with the mechanical behavior of polymer samples and shall take up the... 

This chapter contains one of the more diverse assortments of topics of any chapter in the volume. In it we discuss the viscosity of polymer solutions, especially the intrinsic viscosity the diffusion and sedimentation behavior of polymers, including the equilibrium between the two and the analysis of polymers by gel permeation chromatography (GPC). At first glance these seem to be rather unrelated topics, but features they all share are a dependence on the spatial extension of the molecules in solution and applicability to molecular weight determination. 

The most subjective of the words which (1 hope) describe this book is interesting. The fascinating behavior of polymers themselves, the clever experiments of laboratory researchers, and the elegant work of the theoreticians add up to an interesting total. 1 have tried to tell about these topics with clarity and enthusiasm, and in such a way as to make them intelligible to students. 1 can only hope that the reader agrees with my assessment of what is interesting. 

A number of theoretical models have been proposed to describe the phase behavior of polymer—supercritical fluid systems, eg, the SAET and LEHB equations of state, and mean-field lattice gas models (67—69). Many examples of polymer—supercritical fluid systems are discussed ia the Hterature (1,3). 

The mechanical behavior of polymers, as well as many other topics in polymer engineering, are presented in an up-to-date way in a book by McCrum et al. (1998). 

In Section 4.2.2 the central role of atomic diffusion in many aspects of materials science was underlined. This is equally true for polymers, but the nature of diffusion is quite different in these materials, because polymer chains get mutually entangled and one chain cannot cross another. An important aspect of viscoelastic behavior of polymer melts is memory such a material can be deformed by hundreds of per cent and still recover its original shape almost completely if the stress is removed after a short time (Ferry 1980). This underlies the use of shrink-fit cling-film in supermarkets. On the other hand, because of diffusion, if the original stress is maintained for a long time, the memory of the original shape fades. 

Molecular dynamics, in contrast to MC simulations, is a typical model in which hydrodynamic effects are incorporated in the behavior of polymer solutions and may be properly accounted for. In the so-called nonequilibrium molecular dynamics method [54], Newton s equations of a (classical) many-particle problem are iteratively solved whereby quantities of both macroscopic and microscopic interest are expressed in terms of the configurational quantities such as the space coordinates or velocities of all particles. In addition, shear flow may be imposed by the homogeneous shear flow algorithm of Evans [56]. 

The main predictions of the scaling theory [40], concerning the dynamics behavior of polymer chains in tubes, deal with a number of characteristic times the smallest time rtube measures the interval of essentially Rouse relaxation before the monomers feel the tube constraints significantly, 1 < Wt < Wrtube = and diffusion of an inner monomer is... 

PEs, as other polymers, exhibit nonlinear behavior in their viscous and elastic properties under practical processing conditions, i.e., at high-shear stresses. The MFI value is, therefore, of little importance in polymer processing as it is determined at a fixed low-shear rate and does not provide information on melt elasticity [38,39]. In order to understand the processing behavior of polymers, studies on melt viscosity are done in the high-shear rate range viz. 100-1000 s . Additionally, it is important to measure the elastic property of a polymer under similar conditions to achieve consistent product quality in terms of residual stress and/or dimensional accuracy of the processed product. 

Flow Behavior of Polymer Blends as Affected by Interchain Crosslinking... 

Therefore in many cases, where the behavior of polymer compositions under real technological conditions is of practical interest, i.e. in the region of high deformation rates, the yield stress and the region near it may be neglected and only upper branches... 

The above considerations illustrate the difficulties of trying to formulate equations descriptive of rheological behavior of polymer melts with gas bubbles. An optimistic approach to the solution of this task is contained in [60, 61]. The content of these works is revealed by their titles On the Use of the Theory of Viscoelasticity for Describing of the Behaviour of Porous Material and for the Calculation of construction... 

A specialized computer program has been developed at PLASTEC to provide rapid access to data describing the influence of propints and expls on the behavior of polymers. Termed COMPAT for short — it is the only known central source of this type information, obtained from tests conducted at ARRADCOM and a wide assortment of data published in the open literature... 

One of the hallmarks of the special behavior of polymers is their tendency toward phase separation. Two phenomena come immediately to mind the demixing of... 

Some aspects of the polymorphic behavior of polymers, with particular reference to the structural organization at the molecular level are reviewed. 

Changes in the Polymorphic Behavior of Polymers 4.1 Polymorphism and Comonomeric Units... 

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are also very useful tools for the characterization of polymers. TGA and DSC provide die information about polymer stability upon heating and thermal behaviors of polymers. Most of the polymers syndiesized via transition metal coupling are conjugated polymers. They are relatively stable upon heating and have higher Tgs. 

Shiga, T., Deformation and viscoelastic behavior of polymer gels in an electric fields, Adv. Polym. Set, 134, 131, 1997. 

Flory-Huggins model for polymer solutions, based on statistical thermodynamics, is often used for illustrating the behavior of polymer blends [6,7]. The expression for the free energy change... 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

Kammer, H. W., Kressler, H. and Kummerioewe, C Phase Behavior of Polymer Blends - Effects of Thermodynamics and Rheology. Vol. 106, pp, 31-86. 

Shiga, T. Deformation and Viscoelastic Behavior of Polymer Gels in Electric Fields. Vol. 134, pp. 131-164. 

We have investigated the photochemical behavior of polymer 1 under various conditions in air and found that UV irradiation of the thin liquid films with a thickness of less than 10 pm, indeed produced transparent solid films. However, when the films with a thickness of 100 pm were irradiated with a mercury lamp, cross-linking leading to the solid films occurred only on the surface of the films, but inside remained as liquid after prolonged irradiation. In thses cases, tha surface of the films was found to be slightly opaque. Therefore, most of the light would not be transmitted to the inside of the films. 

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