Yield strength and modulus

On comparison of the yield strengths and elastic moduli of amorphous polymers well below their glass transition temperature it is observed that the differences between polymers are quite small. Yield strengths are of the order of 8000 Ibf n (55 MPa) and tension modulus values are of the order of 5000001bf/in (3450 MPa). In the molecular weight range in which these materials are used differences in molecular weight have little effect. 

In the case of commercial crystalline polymers wider differences are to be noted. Many polyethylenes have a yield strength below 20001bf/in (14 MPa) whilst the nylons may have a value of 12 000 Ibf/in (83 MPa). In these polymers the intermolecular attraction, the molecular weight and the type and amount of crystalline structure all influence the mechanical properties. 

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The color of the material changed from translucent white to translucent light yellow after treatment with carbon dioxide. The SCF carbon dioxide has plasticized the material as noted by a decrease in both yield strength and modulus. It is not certain whether carbon dioxide has induced further crystallization in the samples, since the ultimate elongation remains high and no increase in modulus after treatment in either condition is noted. 

PVC test specimens became lightly distorted after treatments at both conditions. Yield strength and modulus significantly decreased after treatments at both conditions indicating the occurrence of a high extent of plasticization. 

POM has high values in yield strength and modulus and a low value in ultimate elongation owing to the high degree of crystallinity... 

The increase in both yield strength and modulus, and the decrease of ultimate elongation suggest that the degree of crystallinity of Nylon 66 has been increased during the treatments at both conditions. No significant difference has been found between two treatment conditions in terms of these three mechanical properties. 

Other estimates of the ultimate shear strength of amorphous polymers have been made by a number of authors and generally all fall within a factor of 2 of each other (38,77,78). Stachurski (79) has expressed doubt as to the validity of the concept of an intrinsic shear strength based on the value of the shear modulus, G, for an amorphous solid. He questions which modulus is the correct value to use— the initial small strain value or the value at higher strain (the yield point or the ultimate extension). Further, the temperature and strain-rate dependence of both the yield strength and modulus (however defined) suggests that perhaps the ratio of yield strength to modulus is not a true intrinsic material property. We remark however that the temperature and strain-rate dependence of both the yield stress and the shear modulus are often similar. 

Figure 10 Mechanical properties of a CTBN-modified epoxy (E)GEB A/piperidine). Rubber modification has its drawbacks, both yield strength and modulus are reduced.

The mechanical properties as well as in-vitro testing of cylindrical dumbbells were also studied (Tables 4-6 and 8-14). As can be seen from the tables, the yield strength and modulus of the aromatic poly(anhydride)s developed by the methods described herein are similar to or greater than poly(p-dioxanone), an absorbable polyester used extensively for medical devices, and poly(anhydride)s described by other researchers. This is another indication that the aromatic poly(anhydride)s have the high molecular weights (I.V. > 1.0 dl/g), and consequently, the high strengths required in wound closure devices. 

Like the 1,6 PA, poly[l,4-bis(p-carboxyphenoxy)butane anhydride] (1,4 PA) was also molded into cylindrical dumbbells, and baseline as well in-vitro physical properties were determined (Tables 5, 6 and 8-10). As can be seen from these tables, the yield strength and modulus is greater than that of 1,6 PA. This is as expected, since the 1,4 PA contains two less methylene groups per repeat unit. This leads to a polymeric chain, which is slightly stiffer and, therefore, causes a corresponding increase in yield strength and modulus. 

Flexural strength at break, flexural yield strength, and modulus of elasticity of rigid cellular plastics can be determined by the procedures described in Chapter 2. 

Sauer et al. observed that both the compressive and tensile yield strength and modulus changed as a function of hydrostatic pressure. When the compressive and tensile strengths are determined at a given hydrostatic pressure, the result is incorporated into the experimentally determined failure envelope. However, when the strengths are determined at atmospheric or hydrostatic pressures different from the use of pressure, they must be correct for the difference in pressure. The relationships for tensile, Oyt, and compressive, yield stress are a simple linear function of hydrostatic pressure. 

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