Molecular chain scission

Polyethylene displays good heat resistance in the absence of oxygen in vacuum or in an inert gas atmosphere, up to the temperature of 290°C. Higher temperature brings about the molecular-chain scission followed by a drop in the molecular-weight average. At temperatures in excess of 360°C the formation of volatile decomposition products can be observed. The main components are as follows ethane, propane, -butane, n-pentane, propylene, butenes and pentenes [7]. 

Inhibition of polyethylene thermal destruction by polypyrocatechin borate could be represented as follows. The initial molecular-chain scission of branched... 

Figure 14.15 Mechanism of PET nucleation by sodium ionomers. A particular advantage of such compounds is that they provide active nucleation sites without molecular chain scission...

U Zero stress limit of activation energy for molecular chain scission Y(X) True tensile plastic resistance of homopolymer at a tensile extension ratio of A, Y Athermal plastic resistance of homopolymer with sorbed PB Yq Athermal plastic resistance of pure homopolymer a Craze half length... 

Vq PB diluent concentration at the solubility limit under a standard state Vp Poisson s ratio of block copolymer composite Vp Atomic frequency factor in molecular chain scission Q Active craze front length per unit volume a Negative pressure (mean normal stress)... 

Chemical effects are those involved in breaking and or forming chemical bonds. In some cases further polymerization may resnlt, and in other cases molecular chain scission may result. These chanical reaction mechanisms may involve hydrolysis. Hydrolysis is the reversal of the condensation reaction in the presence of water molecules. Oxidation may result from the presence of oxidizing chemicals, and low- and high-energy radiation can also promote chanical changes. 

This might be due to a molecular chain scission of NR molecules caused by degradation during mastication taking place. According to the mastication degradation mechanism, long-chain molecular model and DSC results, it was revealed that partial NR chains were clamped by HDPE crystals and thus acted as tie molecules in the blend. 

The rate of molecular chain scission which occurred during thermal aging was obtained by the measurement of chanical stress relaxation [5]. The stress relaxation of EP07P stimulated by heat was measured at different constant temperatures ranging from 80 to 140 C in air. The thickness of the sample was about 0.5 nun, the width was 5.0 nun, and the distance of jowls was 80 nun. The stress decay was measured at a constant strain (10 %). 

Figure 8.3 shows the stress-decay curve of EP07P at 100 °C. The rate constant of chemical stress relaxation that is proportional to the molecular chain scission is expressed by Eq. (8.1) [5] ... 

The slope of curve in Fig. 8.3 increased at time tc. This phenomena means that the effect of small amount of antioxidant reagent which was added in the production process of EP07P to prevent oxidative degradation during storage of the sample was decreased by thermal aging and the rate of molecular chain scission increased from tc. 

Table 8.1 Rate constant of thermal molecular chain scission of EP07P which has various thermal and irradiation histories (mol h ml )...

In the initial stage, molecular chain scission was started, but the rate of scission was low before tc. The accumulation of carbonyl went on in this stage, but the concentration of C=0 increased logarithmically with increasing aging time. Therefore, the reactions which occurred in the initial stage didn t have an effect on the weight of the sample. 

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