Amorphous yield strength

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/in (55 MPa) and tension modulus values are of the order of 500 000 Ibf/in (3450 MPa). In the molecular weight range in which these materials are used differences in molecular weight have little effect. 

Figure 2 shows tensile-yield strengths for blends of the crystalline EPDM with various levels of LDPE. The curve increases monotonically as expected if no phase inversion occurs. Since amorphous LDPE has a glass-transition temperature near that of EPDM (7) and since the LPDE has only 27 % crystallinity, one should not expect a rubber-to-rigid phase transition. 

Supercritical or high pressure carbon dioxide can induce polymer crystallization and plasticize polymers. The systematic study on the interaction of carbon dioxide with twenty different crystalline and amorphous polymers has been performed and various influencing parameters have been determined. Through the examination, analysis and comparison of the yield strength, ultimate elongation and modulus both before and after treatments in supercritical carbon dioxide at 3000 psi and 70°C, it was found that two main factors, i.e., degree of crystallinity and the presence of a polar side chain group, e g., ester. 

Although polymer crystal structures are known, and some slip mechanisms (slip plane and slip direction) determined, these are less important than for metals. Firstly, the amorphous phase plays an important part in the mechanical properties. Secondly, polymer yield strengths are not determined by obstacles to dislocation movement. However, it is possible to fabricate highly anisotropic forms of semi-crystalline polymers, so crystal characterization and orientation are important. 

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. 

Z. H. Stachurski, Deformation Mechanisms and Yield Strength of Amorphous Polymers , Prog. Polym. Sci. 22, 407-474(1997). 

Table 11.1 (5) summarizes the tensile yield strength of the three subclasses of plastics amorphous, crystalUne, and thermoset. Often crystalline plastics have higher elongations to break than amorphous materials because the crystalline regions act as reinforcement. The thermosets are almost always amorphous. 

---