Polymer under tension

In order to emphasize the difference between ideal and real chains, we compare their behaviour under tension. Consider a polymer containing A monomers of size b, under tension in two different solvents a (9-solvent with nearly ideal chain statistics and an athermal solvent with excluded volume An ideal chain under tension was alreacly discussed in 

Section 2.6.1 and is repeated for comparison with that of a swollen chain. The major difference between ideal and real chains is that in the latter there are excluded volume interactions between monomers that are far apart 

The end-to-end distances of the chains in the unperturbed state (with no applied external force) are given by Eqs (2.18) and (3.20) with v s6  

Since both ideal and real chaitis are self-similar fractals, the same scaling applies to subsections of the chains of size r containing n monomers  

Note that there are fewer monomers within the same distance r in the Teal chain case compared with the ideal chain because the real chain is swollen. 

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Pellethane is an extrudable thermoplastic polyurethane based on MDI and a hydrophobic polyol was used in this application. For the most part, the application was successful, but evidence of stress cracking became apparent. The combination of the hydrolytic environment and the polymer under tension caused failures that led to current leakage and ultimate failure of the device. As a result, softer grades of Pellethane and alternative annealing procedures were adopted and reduced the problem dramatically. 

The anisotropy itself may be linear or circular, or a combination of both. In linear anisotropy the refractive index depends on the direction of polarised light. It is found in solid polymers under tension and in viscous polymeric liquids during flow (shear and elongation). The refractive index can also depend on the chirality of polarised light in this case one speaks of circular or elliptic anisotropy. Thus the so-called "optical activity" is circular birefringence its extinction analogue is circular dichroism. 

Brown, D. and Clarke, J. H. R. (1991) Molecular dynamics simulation of an amorphous polymer under tension 1. Phenomenology, Macromolecules, 24, 2075-2082. 

Most hard amorphous polymers under tension show brittle fracture. The strength, ct, is determined under the Griffith equation (Equation (1.3)) by the fracture surface energy, y, which in turn mostly depends on the energy absorbed by the plastic deformation and void formation (crazing) that occurs immediately at the tip of the crack. E is the elastic modulus and c is the crack length ... 

Rgurel.18 Different deformation zones in polymers under tension loading, showing tensile bars after loading and the light optical appearance of the deformation area (left), as well as schematic illustrations of the micromechanical structures and macromolecular mechanisms (right)... 

Rgure 1.11 Comparison of crazes and shear bands in polymers under tension loading ... 

Although the results described above have been observed for thin films under tension and large-strain plastic deformation is usually observed only for localized regions - crazes or shear deformation zones - Meijer has demonstrated that macroscopic plastic deformation can be indeed observed for these amorphous polymers under tension, as far as the thickness of the deformed polymer is less than a critical value [3-8]. Thin... 

C. G Sell, J. M. Hiver, and A. Dahoun, Experimental Characterization of Deformation Damage in Solid Polymers under Tension, and Its Interrelation with Necking , Int. J. Solids. Struct. 39, 3857-3872 (2002). 

FIGURE 2.2 Different mechanical behavior of viscoelastic polymers under tension, (a) Stress-strain behavior of acetal copolymer, (b) normal fracture plane after 50% strain in acetal copolymer, (c) hysteresis in strain cycling acetal copolymer, and (d) longitudinal fracture of PE tube under tension to 400%. 

Elastomers. Elastomers is a generic name for polymers that exhibit rubberlike elasticity. Elastomers are soft yet sufficiently elastic that they can be stretched several hundred percent under tension. When the stretching force is removed, they retract rapidly and recover their original dimensions. 

The systematic study of piezochromism is a relatively new field. It is clear that, even within the restricted definition used here, many more systems win be found which exhibit piezochromic behavior. It is quite possible to find a variety of potential appUcations of this phenomenon. Many of them center around the estimation of the pressure or stress in some kind of restricted or localized geometry, eg, under a localized impact or shock in a crystal or polymer film, in such a film under tension or compression, or at the interface between bearings. More generally it conveys some basic information about inter- and intramolecular interactions that is useful in understanding processes at atmospheric pressure as well as under compression. 

In order to reduce the tendency of the film to shrink oriented film may be annealed at about 100°C whilst under tension immediately after drawing. The film is often coated with another polymer sueh as a vinylidene ehloride-based copolymer. This both improves the barrier properties and improves the heat scalability. 

Fig. 6. Variation of elasticity modulus (E) under tension and yield strain (es) of the polymer matrix (I, I ) and polyethylene-based composites polymerization filled with kaolin (2,20 in function of polymer MM [320], Kaolin content 30% by mass. The specimens were pressed 0.3-0.4mm thick blates stretching rate e = 0.67 min-1...

In polymers the time dependence of an modulus plays a more important role than in metals. If polymers are loaded with a constant stress they undergo a deformation e, which increases with time. This process is named creep. Conversely, if a test specimen is elongated to a certain amount and kept under tension, the initial stress s decreases with time. This decay is called stress relaxation. 

Another reason for deviations from Gaussian behaviour, even at large N, lies in the finite extensibility of polymer chains. To account for this, one utilizes the complete expression for the partition function qt of a chain of N chain elements, rather than just the Gaussian approximation to it. A very clear exposition of the statistical mechanics of a chain with length rt under tension, can be found in Hill s book (85) yielding... 

Hard-Elastic Fibers. Hard-clastic fibers arc prepared by annealing a moderately oriented spun yam at tiigli temperature under tension. They are prepared from a variety of olefin polymers, acetal copolymers, and polypivalolactone. 

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