Mechanical Properties of Polymers and Plastics

In the last section we showed the fundamentals of rheology, the theory that interrelates stress and strain, flow and deformation. This section deals with the practical behavior of solid polymers, namely, the mechanical properties. One has initially to differentiate between properties that are measured over short periods and those measured over long periods. Needless to say, the rate of measurement and the temperature dictate the type of performance and the magnitude of resulting data. 

Elongation is measured by the distance between two control points in the 

However, if the real (smaller) cross-section after stretching is considered (A), the resultant tme stress, S, will actually occur. 

Whenever the material retains its volume during stretching (incompressibility may occur with ductile or rubber-like materials), one derives a simplified correlation  

The very last portion of the stress-strain curve indicates strain hardening, induced mainly by further chain orientation. In semi-crystalline polymers, one detects an increase in crystallinity and orientation—both leading to an increase of stress. In these cases, Sb Sy. 

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Figure 13.24 Effect of plasticization or copolymerization on the modulus-temperature curve. The curves correspond fo different copciymer ccmpositions. (B) Unplasticized homopolymer (A) either a second homopolymer or plasticized B. (From Nielsen, L.E., Mechanical Properties of Polymers and Composites, Vci. 2, Marcel Dekker, New York, 1974. With permission.)... 

Having presented a general discussion of physical and chemical properties of polymers and plastics, it is appropriate to analyze the effect of structure on mechanical properties. The chemical and steric structures determine the strength of primary and secondary bonds, the location of transition temperatures, as well as the morphology. These act in addition to the effects of chain dimensions—molecular weights and their distribution. 

In the processing of plastic masses the destruction is traditionally considered as a negative factor deteriorating physical and mechanical properties of products and manufacturers try to avoid it in every possible way. Mechanical destruction of molten polymers takes place, primarily, under the action of shear strains effectuating the tension of macromolecules 65-661 in this case, molecules with a high molecular mass... 

This study was therefore undertaken to prepare and evaluate acrylonitrile—butadiene-styrene (ABS) and methyl methacrylate-butadiene-styrene (MBS) polymers under similar conditions to determine whether replacement of acrylonitrile by methyl methacrylate could improve color stability during ultraviolet light aging, without detracting seriously from the good mechanical and thermal-mechanical properties of conventional ABS plastics. For purposes of control, the study also included briefer evaluation of commercial ABS, MBS, and acrylonitrile-butyl acrylate-styrene plastics. 

The mechanical properties of polymers are not single-valued functions of the chemical nature of the macromolecules. They will vary also with molecular weight, branching, cross-linking, crystallinity, plasticizers, fillers and other additives, orientation, and other consequences of processing history and sometimes with the thermal history of the particular sample. 

Organic polymers that possess the electronic, magnetic, and optical properties of metals are known as conductive polymers (CPs). Because of their conjugated u electron backbones, they can be oxidized or reduced more easily and more reversibly than conventional polymers with charge-transfer agents, also commonly called dopants, a term borrowed from condensed matter physics. While retaining some of the mechanical properties of polymers, they do not melt or dissolve in common organic solvents, a major impediment to their widespread commercialization in the same manner as traditional plastics. The same electronic structure that confers electrical conductivity to these polymers also contributes to their intractability and instability. 

The viscoelastic response of polymeric materials is a subject which has undergone extensive development over the past twenty years and still accounts for a major portion of the research effort expended. It is not difficult to understand the reason for this emphasis in view of the vast quantities of polymeric substances which find applications as engineering plastics and the still greater volume which are utilized as elastomers. The central importance of the time and temperature dependence of the mechanical properties of polymers lies in the large magnitudes of these dependencies when compared to other structural materials such as metals. Thus an understanding of viscoelastic behavior is fundamental for the proper utilization of polymers. 

Variations in the physical-mechanical properties of polymers are very often non-monotonous because of ambiguous changes in their structure on plastification [3]. More fundamental changes in the concentration dependencies of some physical-mechanical and structural parameters of PE plasticized by mineral oil take place at 10-30 wt% content (to a less degree at 50-60 wt%) (Fig. 2.40) [3,107]. Maximum values of deformation and strength characteristics of PE-based films plasticized by mineral oil and incorporating contact Cl (Vital and GRM) are within similar concentration limits of PI 60 wt% of PE, 20-30 wt% of oil and 10-20 wt% of the inhibitor (Fig. 2.41) [117]. 

As reported above, the addition of plasticizers is considered a relatively simple route to modify the thermal and mechanical properties of polymers. Blending polymers with plasticizers may modify the physical properties of polymers and a decrease in processing temperature can be achieved. Thus, PHB is commonly blended with plasticizers and nucleation agents that lead to a lower glass temperature and lower crystallinity due to the formation of numerous, small, and imperfect crystallites. 

Boyer (85) argued in 1968 that polymer scientists must understand mechanical properties of polymers in terms of the molecular structure and molecular motions in order to be able to design better plastics. The same may be said of imderstanding physical, electrical, and transport properties of glassy polymers, and the chapters contained in this book present current research efforts toward that end. 

The compatibility of polymers and plasticizers determines the choice of components for the plasticized material. Compatibility is the ability of a plasticizer to form a homogenous system with polymer. During mixing in roll mixers or in an extruder the plasticizer is dispersed in polymer as a result of expenditure of mechanical energy. But if the initial emulsion is thermodynamically unstable, the system becomes stratified. The maximum amount of a plasticizer incorporated into a polymer and retained by it without exudation during storage is popularly accepted as the limit of compatibility. The external attributes of incompatibility are whitening, tackiness, or exudation of the plasticizer. The internal attribute is a decrease in mechanical properties due to incompatibility. 

V. Tanrattanakul and P. Saithai, Mechanical properties of bioplastics and bio-plastic-organoclay nanocomposites prepared from epoxidized soybean oil with different epoxide contents , /AppZ Polym Sci, 2009,114, iC61 (n. 

As seen in Table 5.1, the mechanical properties of polymer composites compare very well with those of other materials, and further substitution for at least some of the metal in an automobile is feasible. Functionally equivalent plastic components that can replace metal counterparts weigh 50-75% less. Typically, a 10% reduction in vehicle weight is estimated to reduce its fuel consumption by about 5-8%. Therefore, this weight advantage translates into very significant improved fuel efficiency, fossil fuel conservation, and avoided carbon emissions, given the size of the world fleet of vehicles. 

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