Trans-vinylene groups

Since BG-LDPEs contain mainly trans-vinylene group and vinyl one, the reactions (7-1), (7-2), (8-1) and (8-2) take place in the irradiation of these polymers. While, the reactions (7-2), (7-3), (8-2) and (8-3) take place in the irradiation of IG-LDPE containing vinyl and vinylidene groups. The lower rate of grafting onto IG-LDPE may be caused by the lower reactivity of... 

To use the absorbance of the band at 967 cm. 1 to determine trans-vinylene groups in the high cis-polymer, it was necessary first to correct for the overlap from the wing of the cts-vinylene band at 998 cm. 1... 

Concentrations are expressed in moles of olefin groups per liter and absorptivities in liters per mole-cm. The concentration of the side vinyl group in the sodium polymer is cvi and for the trans-vinylene group, 15.3 — Cvi similarly for the emulsion polymer, cv> is vinyl, and 15.8 — cv2 is trans concentration. Solving these equations gave molar absorptivities and concentrations from which calibration curves were constructed, relating corrected absorbance at 910 and 967 cm."1 to concentration of side vinyl and trans groups, respectively. 

Initial yields for conversion to the trans-vinylene isomer can be derived by adding to the apparent yield the rate of destruction calculated from the initial concentration of trans-vinylene groups and the first-order rate constants determined for the amorphous trans polymer. The initial yield of trans-vinylene groups is 6.8 for gamma radiation and 4.4 for the reactor. The value for gamma radiation is similar to Golub s value of 7.2 for both gamma and electrons. 

The first-order rate constants for terminal vinyl groups and for trans-vinylene groups in linear polyethylene were observed by Dole and Williams (8) to be 20 to 30 times higher than our corresponding rates in the amorphous trans- and vinyl polybutadienes (Table III). On the other hand, the initial yields were lower than our G0 values because of the... 

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

The band at 1018 cm 1 is due to the presence of occulted oxygen in PE film (thickness, 22-24 pm). The bands at 452 and 456 cm"1 appear only for LDPE (thickness, 22-24 pm) which are attributed to the P (C=C-H). The disappearance of a small band at 888 cm"1 is possibly due to >CH2 out of plane deformation. This may be explained on the basis of evolution of hydrogen gas as a result of the degradation. The intensity of IR absorption bands at 1017, 1113, 1131, 1175, 1241 cm"1 is shown to increase with increasing irradiation dose. The intensity of the bands 1069 cm 1 (C-C stretching), 1287 cm 1 (OH bending), 1377 cm"1 (symmetrical deformation of CH3) (which are initially present in IR spectra of unirradiated films) increase with absorbed dose of radiation, whereas the intensity of the sharp band at 1456 cm"1 is not affected at all. The small band at 965 cm 1 is attributed to C-H out of plane deformation of trans-vinylene groups trans RCH=CHR). 

An excitation transfer (Eq. 39) was considered in PE as a process preventing formation of an activated exciplex via Eq. (40) and, consecutively, formation of trans-vinylene groups in oxidized PE. Reactions at Eqs. 37—40 were used for explanation of the involvement of HAS in quenching of chain initiation in oxidized PE [122, 134, 168, 189]. 

The relative susceptibilities of the crystalline and amorphous portions of polyethylene to high energy radiation has been a moot question. Present evidence indicates that for linear polyethylenes without vinyl end groups the G-values for the production of hydrogen, cross-links, and trans-vinylene groups are identical for the melt at 133 °C and the crystalline state at 130°C (I). (Scission is now believed to be essentially nil at least, its occurrence is not demonstrated). 

The dosimetry utilization of polyethylene films in the 10 -10 Gy dose range is based on ultraviolet at 240 nm (Chen et al. 1980) or infrared measurement of carbonyl or trans-vinylene groups formed upon irradiation and analyzed at 5,800 nm or 10,600 nm, respectively (Charlesby et al. 1964 McLaughlin et al. 1999). 

The main products of polyethylene photo-oxidations are hydroxyl (OH), carbonyl (C=0) and vinyl groups, whereas trans-vinylene groups are formed but only in small amounts. 

Trans-vinylene groups can be formed directly from hydroperoxy groups when they are cleavaged with the formation of hydroperoxy radicals [848, 849, 855] ... 

The presence of trans-vinylene groups markedly increases the oxidizability of adjacent methylene (CHj) groups [711]. The primary products of interaction between dienes and oxygen are considered to play an important role in the initiation of the photo-oxidation reaction [339,711,2113] ... 

The increases in the concentrations of methylene and trans-vinylene groups with increasing gel fraction and the concomitant decrease in the concentration of the pendant vinyl group support this mechanism. 

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