Amorphous nylon failure

Confirmation of the general fatigue superiority of semicrystalline polymers is implicit in studies of a-N behavior at frequencies low enough to minimize temperature rises (169). While the ratios of the endurance limit to tensile strength for typical amorphous polymers (polystyrene, PMMA, and cellulose acetate) are approximately 0.2, values for nylon-6,6, acetal resin, and polyterafluoroethylene are 0.3, 0.5, and 0.5, respectively. At a low frequency, polyethylene (Mw > 50,000) exhibits no failure after 5 x 10 cycles, even at the relatively high alternating stress... 

Polyesters provide the option of nanocomposites prepared with crystalline and amorphous continuous phases. The crystalline polymers behave in a similar way to the polyamides (nylons) as regards structure-property relationships. The amorphous polymers are a different matter. What is unexplained by first-principle arguments is the dramatic increase in percent elongation to failure [39,45,59] observed with amorphous polymer-montmorillonite composites. This phenomenon also applies to elastomers. 

The mechanical properties of crystalline materials can be viewed from two extreme positions. Materials of low crystallinity may be pictured as essentially amorphous polymers with the crystallites acting as massive cross-links, about 5-50 nm in diameter. The cross-links restrain the movement of the amorphous network just as covalent cross-links would. However, unlike the covalent bonds, the crystal crosslinks can be melted or mechanically stressed beyond a rather low yield point. At the opposite end of the crystallinity spectrum, one can regard a highly crystalline material as a pure crystal that contains numerous defects such as chain ends, branches, folds, and foreign impurities. Mechanical failure of highly crystalline nylon, for example, bears a great resemblance to that of some metals, with deformation bands rather than the ragged failure typical of amorphous polymers. 

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