Trans-vinylene Groups Research Articles

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occuring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to accoun for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation1 are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.

Correlation of the radiation chemistry of liquid hydrocarbons with the energetics of molecular-ion reactions

ABS>Exothermic reactions involving alkane parent ions are considered to precede the neutralization process in the liquid state. Thermochemical calculations show that carbon-carbon scission is energetically preferred for branched alkane ions whereas the corresponding dissociation of linear hydrocarbon ions is less favored. Hydrogenatom detachment from a parent ion is not a facile process but the removal of molecular hydrogen to produce an olefin ton is exothermic in trans-vinylene group formation. A reaction of the parent ion with a molecule is often exothermic and can lead to dimer formation. These considerations afford a rational basis for the interpretation of the radiation chemistry of hydrocarbons as a function of molecular structure; an explanation is derived for the occurrence of either cross-linking or degradation in irradiated vinyl polymers. (auth)