Relaxations of polyethylene

The y relaxation takes place at the lowest temperature, overlaps with the )3 relaxation (Fig. 15), and coincides in location and activation energy with the typical y relaxation of polyethylene [35,36], and also of polyethers [37], and polyesters [38] with three or more consecutive methylene units. It appears, for 3 Hz and tan6 basis, at - 120°C (P7MB) and - 126°C (P8MB), and its location and activation energy (35-45 kJ mol ) agree with the values of a similar relaxation associated with kink motions of polymethylenic sequences. 

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. 

The y relaxation of polyethylene, expressed in terms of tan 5, is centered in the vicinity of — 120°C at 1 Hz. This relaxation is believed to be caused by molecular motions occurring in the amorphous phase as indicated by the fact that the relaxation is very weak in highly crystalline polyethylene crystallized from dilute solutions (39,40). The relatively high intensity and universality of the y relaxation in polyethylenes, independent of whether they do or do not have branches in their structure, seems to suggest that the y relaxation may be associated with the glass transition. According to this interpretation, the glass transition temperature of polyethylene would be located in the vicinity of — 120°C. 

FIGURE 11.7 TSC relaxations of polyethylene glycol/oleic acid dispersion. 

The potential of ROFTIR has been exploited in a wide variety of polymer systems. These include the orientation processes during elongation and relaxation of polyethylene [299-301, 312] uniaxial deformation phenomena in polypropylene [302, 303, 309-311], PET [304, 305] and polystyrene [306] hard and soft segmental orientations during stretching of polyurethanes [307, 308] and the stress-induced reversible a-P phase transition in crystalline PBT [312-314] and uniaxial deformation of amorphous PBT [315]. 

Popli R, Glotin M, Mandelkern L and Benson R S (1984) Dynamic mechanical studies of alpha-relaxation and beta-relaxation of polyethylene, J Polym Sci Polym Phys Ed 22 407-448. 

Folland R, Charlesby A. Effect of previous history on the NMR relaxation of polyethylene. Eur Polym J 1979 15 953. 

Dielectric relaxation measurements of polyethylene grafted with acrylic acid(AA), 2-hydroxyethyl methacrylate (HEMA) and their binary mixture were carried out in a trial to explore the molecular dynamics of the grafted samples [125]. Such measurements provide information about their molecular packing and interaction. It was possible to predict that the binary mixture used yields a random copolymer PE—g—P(AA/HEMA), which is greatly enriched with HEMA. This method of characterization is very interesting and is going to be developed in different polymer/monomer systems. 

Proton NMR relaxation parameters have also been determined for polyethylene ( ) and polyethylene oxide (39) in the melting region. The apparent contradiction between the proton spin-lattice relaxation parameter for a high molecular weight linear polyethylene sample at its melting point, with the relaxation measurements, has previously been pointed out. (17) This discrepancy is still maintained with the more detailed results reported here for both types of polyethylene. For the proton relaxation a small, but distinct, discontinuity is reported at the melting teirperature. (38)... 

On the other hand, Connor and Hartland ( ) have reported results of a proton NMR study for a series of polyethylene oxide samples by rotating frame proton relaxation time T p meastirements. Ti was also determined. For their lowest molecular weight sam-... 

The products of thermal oxidation of polyethylene films can be characterized by C FTNMR furthermore, using the spin-lattice relaxation technique, quantitative estimates can be made of the oxidized functional groups. Observation of the development progress of the various functional groups leads to the postulation of hydroperoxides as the primary oxidation products, which undergo further transformations to the other derivatives in a complex scheme . 

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 the copolyamides under consideration, the dipoles that are responsible for the dielectric relaxations are associated with the C = 0 groups of the amide functions. Due to the quasi-conjugated character of the CO - NH bond, the amide group takes on a rigid plane conformation in such a way that the dielectric relaxations of copolyamides should correspond to motional modes that involve amide groups and not only the carbonyls, in contrast to what happens with the ester groups encountered in polyethylene fere-phthalalc (Sect. 4.1.2). 

Using the method isoclinics, Checker and coworkers [116] studied the flow of polyethylene and polypropylene melts through two dimensional contractions and used birefringence measurements to determine the relaxation behavior of these materials subject to complex flows. 

The recombination between two alkyl radicals is believed to be the main source of radiation-induced crosslinks in polyethylene [21], so that it is important to study the mechanism for formation and reaction of the alkyl radical. We have applied the ESE method to elucidate the paramagnetic relaxation mechanism and the spatial distribution of the alkyl radical, in order to get further insight into the radiation-chemical reactions of polyethylene resulting in the formation of crosslinks. 

The longitudinal relaxation of the alkyl radical in polyethylene as demonstrated in Fig. 7 can be expressed by the following equation ... 

Many aH NMR analyses have been carried out for molten- or solution-state of polyethylene to characterise the random-coil molecular chain.14,17,21 36-39 The relaxation evaluation for solid-state, amorphous polyethylene chains remain ambiguous because of the strong crystalline component that obscures the entanglement component in the amorphous phase. Perfect FID fitting in combination with TEM and SAXS was applied to evaluate the effects of prior concentration on amorphous chain characteristics. 

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