Tensile stress polymers

H. Nakamura, T. Nakamura, T. Noguchi, and K. Imagawa. Photodegradation of PEEK sheets under tensile stress. Polym. Degrad. Stabil, 91(4) 740-746, April 2006. 

The phenomenon of crazing has been observed in many glassy polymers, and also in some crystalline polymers, when subjected to tensile stress. Polymer crazes (Fig. 9.8a) in their appearance are similar to the very fine cracks known for a long time to occur on the surfaces of inorganic materials such as ceramics. However, there is a difference between the crazes and the cracks in that crazes have a continuity of material across the craze plane (Figs. 9.8b, 9.9—9.11) whereas cracks do not possess any continuity. Consequently crazed zones are capable of bearing loads as opposed to cracked ones. 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Extensional Viscosity. In addition to the shear viscosity Tj, two other rheological constants can be defined for fluids the bulk viscosity, iC, and the extensional or elongational viscosity, Tj (34,49,100—107). The bulk viscosity relates the hydrostatic pressure to the rate of deformation of volume, whereas the extensional viscosity relates the tensile stress to the rate of extensional deformation of the fluid. Extensional viscosity is important in a number of industrial processes and problems (34,100,108—110). Shear properties alone are insufficient for the characterization of many fluids, particularly polymer melts (101,107,111,112). 

Cracks grow in the way shown in Fig. 15.9. In a pure metal or polymer (left-hand diagram), the tensile stress produces a plastic zone (Chapter 14) which makes the crack... 

Below about 0.75 T, polymers are brittle (Fig. 23.9). Unless special care is taken to avoid it, a polymer sample has small surface cracks (depth c) left by machining or abrasion, or caused by environmental attack. Then a tensile stress tr will cause brittle failure if... 

Figure 10.6. Effect of temperature on the tensile stress-strain curve for polyethylene. (Low-density polymer -0.92g/cm . MFI = 2.) Rate of extension 190% per minute ...

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

In many practical situations involving the flow of polymer melts through dies and along channels, the cross-sections are tapered. In these circumstances, tensile stresses will be set up in the fluid and their effects superimposed on the effects due to shear stresses as analysed above. Cogswell has analysed this problem for the flow of a power law fluid along coni-cylindrical and wedge channels. The flow in these sections is influenced by three factors ... 

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

In semi-cristalline polymers, rate-enhancement under stress has been frequently observed, e.g. in UV-photooxidation of Kapron, natural silk [80], polycaprolactam and polyethylene terephthalate [81]. Quantitative interpretation is, however, difficult in these systems although the overall rate is determined by the level of applied stress, other stress-dependent factors like the rate of oxygen diffusion or change in polymer morphology could occur concurrently and supersede the elementary molecular steps [82, 83], Similar experiments in the fluid state showed unequivocally that flow-induced stresses can accelerate several types of reactions, the best studied being the hydrolysis of DNA [84] and of polyacrylamide [85]. In these examples, hydrolysis involves breaking of the ester O —PO and the amide N —CO bonds. The tensile stress stretches the chain, and therefore, facilitates the... 

In the identification of different polymorphs in polymers the FTIR technique presents, with respect to the diffraction techniques, the advantage of easier and more rapid measurements. In particular, the high speed of the measurements allows to study the polymorphic behavior under dynamic conditions. As an example let us recall the study of the transition from the a toward the P form of PBT induced by tensile stresses, evaluated by quantitative analysis of the infrared spectra [83],... 

FIGURE 3.6 Tensile stress-strain plots of acrylic mbber (ACM)-silica hybrid nanocomposites using different tetraethoxysilane (TEOS) concentrations. The number in the legends indicates wt% TEOS concentrations. (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl. Polym. Sci., 93, 2579, 2004. Courtesy of Wiley InterScience.)... 

FIGURE 3.13 Tensile stress-strain plots for acrylic rubber (ACM)-siUca and epoxidized natural rubber (ENR)-sibca hybrid composites synthesized from various solvents (a) ACM-siUca and (b) ENR-siUca. The letters after the rubbers in the legend indicate solvents used T = THF, M = methyl ethyl ketone (MEK), D = DME, E = EAc, CH = CHCl3, CC CCLj. (From Bandyopadhyay, A., De Sarkar, M., and Bhowmick, A.K., J. Appl. Polym. Sci., 95, 1418, 2005 and Bandyopadhyay, A., De Sarkar, M., and Bhowmick, A.K., J. Mater. Sci., 40, 53, 2005. Courtesy of Wiley InterScience and Springer, respectively.)... 

FIGURE 4.8 Comparative tensile stress-strain plot of polychloroprene-ordinary zinc oxide (ZnO) and poly-chloroprene-nano-ZnO system. (From Sahoo, S., Kar, S., Ganguly, A., Maiti, M., and Bhowmick, A.K., Polym. Polym. Compos., 2007 (in press). Courtesy of Smithers Rapra Technology Ltd.)... 

FIGURE 11.18 Tensile stress-strain responses of polypropylene/styrene-butadiene rubber (PP-SBR) blends at several ratios (where LL is linear low molecular weight LH is linear high molecular weight BL is branched low molecular weight and BH is branched high molecular weight). (From Cook, R.F., Koester, K.J., Macosko, C.W., and Ajbani, M., Polym. Eng. Sci., 45, 1487, 2005.)... 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

For a polymer fibre with a single orientation angle the modulus, E, or the slope at each point of the tensile curve, is a function of the tensile stress and given by... 

A substantial number of definitions in the terminology section are either of physical quantities or are expressed mathematically. In such cases, there are recommended symbols for the quantities and, when appropriate, corresponding SI units. Other terms have eommon abbreviations. The following format is used to indicate these essential eharaeteristics name of term (abbreviation), symbol, SI unit unit. Typical examples are tensile stress, interpenetrating polymer network (IPN). If there are any, alternative names or synonyms follow on the next line, and the definition on the sueeeeding lines. 

If tan 5 is high, so are the tensile stresses when the foam face polymer is subjected to biaxial extension. The foam faces may fracture if the tensile stress exceeds a critical level. The consequence of such fractures is the growth of abnormally large cells, with more than the usual 14 or 15 faces (Figure 5). 

---