Amorphous ultimate tensile strength

The four protein conformations that provide mechanical stability to cells, tissues, and organs include the random coil or amorphous structure that characterizes a part of the structure of elastin, the a helix, which is represented by the keratin molecule, the collagen triple helix, and the p structure of silk. In humans the P structure is found only in short sequences connecting parts of other structures such as the a helix, but serves as an example of the relationship between protein structure and properties. The ultimate tensile strength and modulus of each structure differs as discussed below. 

Polymer composites have a fracture toughness, ultimate tensile strength and ultimate elongation that can be varied by changing the glass transition temperature of the composite. The thermodynamic properties of the amorphous matrix change at Tg. 

Under ordinary conditions natural rubber is an amorphous material. When frozen or stretched, it crystallizes in the cis form. It is crystallization that gives rubber its self-reinforcing effect. As the rubber is stretched, crystallization increases, and this raises the ultimate tensile strength. Shipments of rubber when held for extended periods of time at cool temperatures — say 5 °C — can become frozen. In this state, rubber is rock hard and impossible to plasticize mechanically. Luckily, the crystallization process is completely reversible. At plants, the rubber can be stored in a hot room with temperatures over 45 °C and its original softness brought back. The time required for this is largely dependent upon how well the bales are separated, for natural rubber is a poor conductor of heat. 

PP thus strength and heat resistance depend on the PE content [63,64]. Blends of PE and PP were immiscible in either the amorphous or the crystalline phase [65]. The two polymers tend to form mixtures of crystal structures, and each affects the crystallization of the other [66-70]. Studies on mechanical properties gave mixed results. Improvements in modulus, ultimate tensile strength, and heat deflection temperature [71] suggested good binding in the amorphous interphase [72] but use of HDPE to improve low temperature impact strength and environmental stress rack resistance required a compatibilizer such as 5% of ethylene-propylene rubber [42]. 

Ultimate properties, tensile strength, and ultimate strain typically decrease with the addition of HPL however, combinations of molecules with strong interaction between amorphous components may also exhibit enhanced ultimate properties. Injection molding produces superior material properties. 

The influence of strain rate and temperature on the tensile properties of elastomers and amorphous polymers has been studied extensively, particularly by Smith and co-workers [154-156], who measured the variation of tensile strength and ultimate stain as a function of strain rate for a number of elastomers. The results for different temperatures could be superimposed, by shifts along the strain rate axis, to give master curves for tensile strength and ultimate strain as a function of strain rate. Results of this nature are shown in... 

Figure 13.43, which summarises Smith s data for an unfilled styrene-butadiene mbber. Remarkably, the shift factors obtained from superposition of both tensile strength and ultimate strain took the form predicted by the WLF equation (see Section 6.3.2) for the superposition of low-strain linear viscoelastic behaviour of amorphous polymers (Figure 13.44). The actual value for Tg agreed well with that obtained from dilatometric measurements.

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