Rate of molecular chain scission

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 %). 

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. 

Sol-Gel Measurements. The relative rates of crosslinking and scission may be estimated from soluble-fraction measurements. For an initial random molecular weight distribution and random chain scission, extrapolating a curve of S + S vs. 1/D, where S = sol fraction and D —... 

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] ... 

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. 

Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

In flow-induced degradation, K is strongly dependent on the chain length and on the fluid strain-rate (e). According to the rate theory of molecular fracture (Eqs. 70 and 73), the scission rate constant K can be described by the following equation [155]... 

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