Oriented Single-Crystal Mats

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. 

The relaxation spectrum H, reduced to 16°C, is plotted with a linear ordinate 

A decr se in molecular weight, with consequent increase in nmlecular ends and hence lattice irregularities, causes the maximum in H to broaden and shift to shorter times.  

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Figure 4.3. Schematics of an oriented single-crystal mat of a polymer as grown from solution. (From Ingram PeterUn, 1968.)...

M. I. Weaver, M. J. Kaufman, and R. D. Noeb, The Effects of Alloy Purity on the Mechanical Behavior of Soft Oriented NiAl Single Crystals, Scripta Met. Mat., 29,1113 (1993). 

Single polyethylene crystals (79), mats of single crystals (51, 69, 76), and b axis-oriented melt-formed samples (77) all show an a mechanical loss process which is broader for the dilute solution-grown samples ( 70° to... 

Early electron diffraction data published by Lieser et al. [67], for both unoriented and flow-oriented samples, disagree altogether with later studies, and are not contained in Table 1.2. Another report of deviating cell data, also omitted from this table, has been published by Cao et al. [68]. They use a Nd-based catalyst system, and And an entangled fibrillar mat containing regions that appear as single crystals in electron diffraction. 

These earlier papers did not take into account the fact that the oriented polymers were partially oriented. It is therefore now necessary to extend the previous discussion of the aggregate model to the situation where molecular motion occurs. This has been done independently by Olf and Peterlin, McBrierty and Douglass, and Folkes and Ward. McBrierty and Douglass paper gives a formal theoretical treatment, as part of a general discussion of the influence of molecular motions on Tj, Tz and the other papers are particularly concerned with an analysis of the effects of molecular motion on the second moment anisotropy but nevertheless provide explicit results for special types of motion. The results are then applied to oriented mats of polyethylene single crystals and to... 

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