Scission polymer

The quantum yields for scissioning ( >g) and crosslinking ( ) were determined for some representative polysilane derivatives both in solution and in the solid state (8). In all cases polymer scission is the predominant process and the values ranged from 0.2 to 1.0. For two cases, poly (methyl phenylsilane) and poly(cyclohexyl methylsilane) which were also examined in the solid state, the quantum yields were reduced by at least an order of magnitude from the solution values. 

Reference 1 contains the most extensive mathematical treatment of polymer scission and crosslinking by light. Two text errors in that publication require correction. 

Oxidative degradation occurs at much lower temperatures than conventional pyrolysis temperatures, and it is characterised by the complete random scission in the polymer chain backbone, compared to thermal degradation where polymer scission occurs randomly and at the chain end. 

TEM (refer to Figure 1) is used whenever a more in-depth study (when domain sizes are less than 1 micron or so) is required on polymer phase morphologies such as dynamically vulcanized alloys and Nylon/EP filler location as in carbon black in rubber compounds and also in the morphology of block copolymers. Thin sections are required and take anywhere from one hour to one day per sample depending on the nature of the sample. They must be 100 nm in thickness and are prepared usually by microtoming with a diamond knife at near liquid nitrogen temperatures (-150° C). The same contrasting media for SEM apply to TEM. In addition, PIB backbone polymers scission and evaporate in the TEM which helps... 

Highly controlled ultrasound-induced polymer scission of 0.1 wt% PMMA in MMA in the presence of the radical scavenger i,i-diphenyl-2-piciylhydrazyl (DPPH) to prevent further polymerization of the formed macro-radicals. 

Ultrasound Frequency. The frequency of ultrasoimd has a significant effect on the cavitation process. At very high frequencies (>1 MHz), the cavitation effect is reduced as the inertia of a cavitation bubble becomes too high to react to fast changing pressures. Most ultrasoimd-induced reactions are therefore carried out at frequencies between 20 and 900 kHz. The optimum ultrasoimd effect as a function of frequency depends on the reaction system eg, water dissociation has an optimum frequency at approximately 500 kHz (21). For bulk pol5mierizations the maximum radical formation rate is obtained at 20 kHz. At this frequency the highest strain rates are produced, which results in a high radical formation rate by polymer scission (22). 

Generally, free-radical polymerization consists of four elementary steps initiation, propagation, chain transfer, and termination (see Radical Polymerization). When ultrasound is used to initiate polymerization, radicals can be formed both from monomer and from polymer molecules. This implies that because of radical formation by polymer scission, an additional elementary step is involved in ultrasound-induced polymerization, as indicated in Figure 4. 

In terms of product properties, the molecular weight distribution is an important characteristic of polymers. In the polymer industry often a post-processing step is applied to alter the molecular weight of the polymers eg, the peroxide-induced degradation of polypropylene (53). In this process, fracture of the polymer chain occurs at a random site. An alternative method is ultrasound-induced polymer scission, which involves a much better controlled, nonrandom process (7). This enables a relatively straightforward production of the desired molecular weight as well as the formation of block copolymers, as described in the previous section. 

In semicrystalline polymers, scission occurs by a mechanism typical of the amorphous phase which results in crystal destruction. The crystallinity index is therefore reduced, an effect actually recorded with all semicrystalline polymers subjected to grinding. 

Fig. 21.15. Molecular weight distributions of an ultrasound-induced polymer scission of PMMA into MMAwith an initial M of 18.0 X 10 g moM [151].

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