Polyethylene activation energy

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

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. 

As part of a multi-technique investigation (see also discussion under mid-infrared spectroscopy later), Corrales et al. [13] plotted the carbonyl index for films prepared from three grades of polyethylenes a high-density PE (HDPE), a linear low-density PE (LLDPE) and a metallocene PE (mPE) (see Figure 5). In this study, the data trend shown in Figure 5 correlated well with activation energies derived from the thermal analysis, which showed that the thermal-oxidative stability followed the order LLDPE > mPE > HDPE, whereas the trend... 

Table IV contains some comparative data regarding the electrical conductivity of some polychelates based on Fe3+ and Mn2+. The data dealing with electrical conductivity of polychelates, the starting polymers (for polyethylene terephthalate,

Molecular Weights, Flow Activation Energies and Type of Catalyst for a Series of Polyethylenes Examined for Long Chain Branching. 

Figure 3.13 shows the shift factors aT determined from time-temperature superposition as a function of temperature for melts of two semi-crystalline thermoplastics as well as the Arrhenius plot. For the two polyethylenes (HDPE, LDPE), the progression of log ax can be described with the Arrhenius equation. The activation energies can be determined from the slope as Ea(LDPE) 60 kj/mol and Ea(HDPE) 28 kj/mol. Along with polyethylenes (HDPE, LDPE, LLDPE), other significant semi-crystalline polymers are polypropylene (PP), polytetrafluoroethylene (PTFE) and polyamide (PA). 

FIG. 18.3 Activation energy of diffusion as a function of Tg for 21 different polymers from low to high temperatures, ( ) odd numbers (O) even numbers 1. Silicone rubber 2. Butadiene rubber 3. Hydropol (hydrogenated polybutadiene = amorphous polyethylene) 4. Styrene/butadiene rubber 5. Natural rubber 6. Butadiene/acrylonitrile rubber (80/20) 7. Butyl rubber 8. Ethylene/propylene rubber 9. Chloro-prene rubber (neoprene) 10. Poly(oxy methylene) 11. Butadiene/acrylonitrile rubber (60/40) 12. Polypropylene 13. Methyl rubber 14. Poly(viny[ acetate) 15. Nylon-11 16. Poly(ethyl methacrylate) 17. Polyethylene terephthalate) 18. Poly(vinyl chloride) 19. Polystyrene 20. Poly (bisphenol A carbonate) 21. Poly(2,6 dimethyl-p.phenylene oxide). 

In polyethylene, the values for k2A are in the range of 10 s and those for k A are about one order of magnitude lower, ca. 10 s rate constants at the lowest and highest temperatures examined and activation energies ( 2a and 4a) for the radical parr combinations from irradiations of lb in an LDPE and an HDPE film are collected in Table 13.5. The preexponential factors associated with the activation energies were not reported because the rate constants were not as precise as is necessary for the relatively small temperature ranges over which the data were collected. 

TABLE 13.5 Rate Constants and Activation Energies for Combination of 2-phe-nylpropanoyl/l-naphthoxy Radical Pairs and Relative Rate Constants (at 295 K) for Combination of 1-phenylethyl/l-naphthoxy Radical Pairs from Irradiation of lb in Unstretched (u) and Stretched (s) Polyethylene Films... 

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

The second step (6.23) involves the breaking of a carbon-hydrogen bond and has a higher activation energy. The rate of this reaction is therefore much slower, and the rate of this step in the chain reaction generally determines the rate of oxidation, once it has started. Oxidative chain scission also occurs subsequently, according as the following reactions for polyethylene ... 

Strength of the specific interaction. An example of this is shown in Fig. 9 for blends of poly(butyl acrylate) with chlorinated polyethylene. In this case the blend requires a higher activation energy than its additivity value in the form of heat to allow chain movements. A review of this subject and of the relations between and chemical structure of blends has been given by Cowie For miscible blends many attempts have been made to correlate the with the blend composition as is frequently done with random copolymers. Several miscible blends studied by Hammer and Hichman and Ikeda exhibit a composition dependence of which can be described by the simple Fox relationship. 

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