Polymer single crystals, tensile properties

Several authors have analyzed the miscibility of iPP and PB-1, by means of different analytical approaches. Piloz et al. (16) found a single, composition-dependent, glass transition behavior for these blends, and concluded that they are compatible in the amorphous state. Sjegmann (17,18) reported that the composition dependence of tensile properties evidences a high degree of compatibility of iPP and PB-1 and observed a marked effect of the composition on the morphology of melt-crystallized samples. Conversely, the analysis of the crystallized blends indicates the presence of separated crystal phases of the two polymers, even if a mutual influence during the crystallization cannot be excluded. 

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

Secondly, the ultimate properties of polymers are of continuous interest. Ultimate properties are the properties of ideal, defect free, structures. So far, for polymer crystals the ultimate elastic modulus and the ultimate tensile strength have not been calculated at an appropriate level. In particular, convergence as a function of basis set size has not been demonstrated, and most calculations have been applied to a single isolated chain rather than a three-dimensional polymer crystal. Using the Car-Parrinello method, we have been able to achieve basis set convergence for the elastic modulus of a three-dimensional infinite polyethylene crystal. These results will also be fliscussed. 

In a very extensive study of both stress relaxation and dynamic mechanical properties in simple extension, on single crystal mats of fractions of linear polyethylene, Takayanagi and collaborators were able to combine data at different temperatures by reduced variables over most of the range from 16°C up to the temperature of crystallization and also to show that the dynamic and transient data corresponded fairly closely, provided the latter were corrected for nonlinear behavior by an extrapolation procedure to zero strain. It is characteristic of crystalline polymers that departures from linear viscoelastic behavior appear at very small strains, and are sometimes significant in stress relaxation even at a tensile strain of = 0.001. In dynamic measurements, the strains are usually small enough to fall within the linear range. 

---