Polyethylene mechanical relaxation

Figure 10. 5 The o-Ps lifetime (x3) and intensity (I3) for polyethylene as a function of temperature. The temperature ranges corresponding to mechanical relaxation processes (a,P and y) are shown in the figure [44].

Y Relaxation. Unlike the other dynamic mechanical relaxations observed in this study, the Y relaxation does not have an analog in the dynamic mechanical behavior of polyethylene, hydrogenated PP s, or other ionomer systems. In addition, it displays no definite trends in changing temperature or magnitude as the level of sulfonation and thermal history are altered. Coupled with the fact that these systems are known to contain water as well as nitrogen, it is not possible to assign this relaxation to any specific phase or mechanism. Additional studies are necessary before this task can be approached adequately. 

Figure 9.4 shows a comparison of the dielectric and mechanical relaxation spectra of various forms of polyethylene. The most obvious feature is that the main relaxations, here the a, (3 and y relaxations, occur at approximately the same temperatures in both spectra, although their relative relaxation strengths in the two spectra are diiferent. This is a feature common to the spectra of many polymers. The peaks are not, however, in exactly the same positions in the two spectra for the same type of polyethylene. In addition to the possible reasons for this described above, the frequencies of measurement are diiferent. The dielectric measurements were made at a much higher frequency than that used for the mechanical measurements, as is usual. Molecular motions are faster at higher temperatures, so this factor alone would lead to the expectation that the dielectric peaks would occur at a higher temperature than the mechanical peaks. The y peak, which is assigned to a localised motion in the amorphous material and is in approximately the same place for all samples, behaves in accord with this expectation. 

Fig. 9.4 Dielectric and mechanical relaxation in various forms of polyethylene. The vertical lines are simply a guide to the eye in comparing the positions of the major relaxations. See the text for discussion. (Adapted by permission from Akademiai Kiado, Budapest.)...

The mechanical relaxations occurring in unoriented low density polyethylene are indicated schematically in Fig. 7(a). Studies on unoriented... 

The initial published reports on high density polyethylene were dynamic mechanical studies, but before considering them it is necessary to compare the mechanical relaxations in isotropic material with those observed in unoriented low density polyethylene. From the schematic curve of tan S v. temperature f Fig. 7(b)] it can be seen that the p relaxation, which was ascribed to branch point mobility, is not present, and that the high temperature relaxation is frequently resolvable into a and a peaks. 

R.G. Matthews, I.M. Ward, and G. Capaccio, Relationship between the dynamic mechanical relaxations and the tensile deformation behaviour of polyethylene, Journal of Materials Science, 34 (12), 2781-2787,1999. 

Crystalline polymers exhibit more mechanical relaxations than amorphous polymers. It is not an overstatement to remark that the greater number of mechanical relaxations in crystalline polymers is the cause of the substantial difference in properties between crystalline and amorphous polymers (4.N.4). For example in linear (LPE) and branched (BPE) polyethylene at temperatures above — 200 "C, there is a sequence of relaxations (see Fig. 4.12). In branched PE the processes are y-relaxation at — 120 C, -relaxation at — lO C, and a-relaxation at 70 C. The presence of a relaxation is detected most easily by the peak in A this is one reason why this parameter is of value. The relaxation observed in creep in linear PE at room temperature and above (shown in Fig. 4.4) is the a-process. The torsion pendulum is a useful tool for yielding quickly a description of temperature regions where creep or stress-relaxation processes are to be expected. In addition, the relaxation temperatures often mark transitions in ductility the polymer becomes increasingly brittle as it is eooled. 

Leung, W.P., Chen, F.C., Choy, C.L. et al. (1984) Ultrasonic measurements of the mechanical relaxations and complex stiffnesses in oriented linear polyethylene. Polymer, 25, 447. 

Figure 10.10 Temperature dependence of tan S in a cold-drawn and annealed HDPE sheet in different directions at 50 Hz. (Reproduced from Stachurski, Z.H. and Ward, I.M. (1969) Mechanical relaxations in polyethylene. J. Macromol. Sci. Phys. B, 3, 445. Copyright (1969) Taylor and Francis.)...

As we have seen, the strain rate dependence does suggest that yield behaviour often indicates the presence of two thermally activated processes, as discussed above. In some cases, notably polyethylene, a double yield point is observed. Ward and co-workers [64], Seguala and Darras [65] and Gupta and Rose [66] concur that these two deformation processes are essentially interlamellar shear and intra lamellar shear (or c-slip). They are akin to the dynamic mechanical relaxation processes identified in Chapter 10.7.1 for the specially oriented PE sheets, and Seguala and Darras have related them to the a and o 2 transitions reported by Takayanagi [67]. This establishes a direct link between yield and viscoelastic behaviour. 

How can these different facts be reconciled and brought together in one common picture The answer is The a-process in polyethylene has a composite nature. The mechanical relaxation indeed originates firom an additional shear of the amorphous regions. The prerequisite for this shear, however, is a chain transport through the crystallites. By this intracrystalline motion the... 

Mechanical properties of polyethylenes vary with density and melt index. Low-density polyethylenes are flexible and tough high-density products arc quite rigid and have creep resistance under load. Toughness is the primary mechanical property affected by melt index, with lower-melt-index polyethylenes having greater toughness. Under loads, polyethylene is subject to creep, stress relaxation, or a combination of both,... 

In conclusion, one cannot but state, that the present-day knowledge of the mechanism of the low-temperature relaxation of polyethylene remains limited and qualitative, even though theoreticians have mainly studied this kind of molecular motion. The low-temperature relaxations of the other polymers without side chains are ascribed to analogous types of motion because the existing experimental data do not allow a better founded interpretation. 

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