Cross-linking yield

The yield of cross-linking depends on the microstructure of polybutadiene and purity of the polymer as well as on whether it is irradiated in air or in vacuum. The cross-link yield, G(X), has been calculated to be lowest for trans and highest for vinyl isomer [339]. The introduction of styrene into the butadiene chain leads to a greater reduction in the yield of cross-linking, than the physical blends of polybutadiene and polystyrene [340]. This is due to the intra- and probably also intermolecular energy transfer from the butadiene to the styrene constituent and to the radiation stability of the latter unit. 

Moderate doses, between 1.0 and 20 Mrad (10 to 200 kGy) on partially or completely formed articles and polymer pellets substantially reduce the content of residual monomer98 The mechanical properties of polystyrene are changed only at high radiation doses, which is characteristic of low cross-link yield and glassy morphology.99... 

Figure 9 shows an example for application in the lower density range. The gradient system consists of mixtures of HpO and DpO with methanol. The sample under investigation is a mixture of a normal and an additionally cross-linked poly(butadiene) latex. The particle density is not uniform. The normal poly-(butadiene) has particle densities as expected around 0.89 g cm 3, but for the additionally cross-linked poly(butadiene) density values between 0.9 and 0.92 g cm-3 are obtained. Additional cross-linking yields obviously higher particle densities. In this case the experiment needs 30 min. 

Tyrosinase (polyphenol oxidase, a copper-containing monooxygenation enzyme) was used as catalyst for the modification of natural polymers. Phenol moiety-incorporated chitosan derivatives were subjected to tyrosinase-catalyzed cross-linking, yielding stable and self-sustaining gels.90 Tyrosinase also catalyzed the hybrid production between the modified chitosan and proteins. 

Neither interpretation is correct. Assumption (1) leads to the prediction that vapp should be strongly dependent on the specific surface area (S) of carbon black, for vf = SS, where d is the number of adsorption points per unit surface of black — a quantity expected to be essentially constant for all blacks of similar surface chemistry. The experimental evidence is that vapp is not strongly surface area dependent. Assumption (2) leads to the prediction that cross-link yields in vulcanization are almost invariably increased by the presence of highly reinforcing carbon blacks. Chemical evidence indicates that this is not so. There are demonstrated instances in which vapp exceeds vr by significant amounts (typically 50% or more) without confirmation by chemical means (129). 

An overall cross-link yield of ca 1-2 (100 eV)" is often found, however there is some uncertainty about the significance of this value is view of the fact that the distribution of cross-links may not be random and the theory relating the various experimental results to the number of cross-links (gel experiments, swelling, elastic moduli) may not give accurate results. 

Silicic acid gels whose formation is not due to a process of crystallisation (cf. p. 498) but to a chemical cross linking, yield an amorphous X-ray diffraction pattern A faint indication of crystallisation is only found in aged dehydrated gels, particularly after the latter have been exposed to higher temperatures. 

The gelation dose (D ) represents the minimum required dose to initiate gelation process. Pq/Qo is the parameter that allows the determination of the final results of irradiation and is equal to half of the scission yield/cross-linking yield ratio [0.5 x If Po/qo is lower, cross-linking occurs... 

The polymer is not degraded by this low-energy irradiation. The process time is short ( 1 s) and the cross-link yield is high. Of course, only the surface is cross-linked, to a depth of 50-100 nm. 

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