Linear polyethylene, relaxation behavior

The dynamic mechanical results of this study demonstrate that backbone crystallinity plays an important role in the properties of these materials. Moreover, it is observed that thermal history affects the properties of the materials investigated in a more complex manner than can be explained by simple changes in the degree of crystallinity. At low levels of sulfonation the materials generally behave very much like linear polyethylene, but this behavior is modified significantly as the level of sulfonation is increased. Evidence is clearly present for the existence of an ionic-phase relaxation that supports the proposed model for micro-phase-separated domains (5,6,7). However, owing to the effects of crystallinity the concentration at which the ionic-phase relaxation first... 

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 behavior of amorphous polymers was dominated by two processes, the glass-rubber transition and the terminal flow region, which are both characterized by a temperature dependence given by the WLF equation. For polyethylene, one cannot expect a flow transition because flow is suppressed by the crystallites in the sample. The fact is that for linear polyethylene, i.e., polyethylene with high crystallinity, there is no WLF-controlled process at all. The numerous measurements in the literature provide clear evidence that the two processes observed in linear polyethylene, a and 7, are both based on activated mechanisms obeying the Arrhenius law. The process... 

Division of the total tensile strain under conditions of F = const into several components 25,6R,69) produced interesting results (see Fig. 8). It has been found that the behavior of molten low-density polyethylene (Fig. 8a) is qualitatively different from polyisobutylene (Fig. 8 b) the extension of which was performed under temperature conditions where the high-elasticity modulus, relaxation time, and initial Newtonian viscosity practically coincided (in the linear range) in the compared polymers. Flow curves in the investigated range of strain velocities were also very close to one another (Fig. 21). It can be seen from the comparison of dependencies given in Fig. 8a,... 

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