Molecular methods chain scission

We also note data from atomic force microscopy (AFM) versus depth, carried out by using a diamond tip for scratching patterns into the surface [12], Because of the 2° microtoming method reported, these authors were able to examine the depth profile of brittle behavior in weathered samples with excellent resolution. The data showed a very rapid decrease in the brittleness with depth into the sample which, of course, was a strong function of exposure time. The brittleness was more in line with the IR data (see above) versus depth than the molecular weight data, hence suggesting that some chain scission and branching can be tolerated in the system before it manifests brittle behavior. 

The methods of increasing molecular weights of polyesters and poly-amids below the melting point by transesterification and transamidation are worth mentioning as a still only little studied tool to improve crystal perfection by tempering through chain scission. Although this t5q>e of reaction is restricted to few polymer classes it should by proper use allow considerable increase in crystal perfection. 

Flow-Induced Chain Scission. If the chains are overstretched, that is, e increases along the plateau in Figure 2, they will eventually break 12, 13). The molecular weight of the fracture products can then be determined by the method just outlined. Thus, through our technique we can both break chains in a controlled manner and analyze the resulting fracture products within the same apparatus, this double capacity enabling a systematic study of the flow-induced chain fracture. The principal results, to be of consequence later, are as follows ... 

It can be therefore concluded that the above results of the mechanochemical experiments are directly related to the slowing down of chain mobility upon deterioration of solvent quality. Consequently, these experiments are uniquely useful to study chain dynamics in semiconcentrated polymer solution as a function of the thermodynamic conditions at high shear rates. Hence, these experiments can give information about molecular parameters not being accessible by any other method. For example, one can quote the probability of chain scission along the backbones obtained by the full kinetic analysis of the data (cf. Sect. 3). This distribution of rupture sites is obviously connected with the distribution of strain along the chains, which may be probed by degradation experiments. 

The standard measurement of the yield for an event resulting from the irradiation process is expressed as the G factor. This factor is universally accepted [16] and is defined as the event yield per 100 eV of energy deposited in the material. The SI unit for G is xmol For the purposes of this review events per 100 eV will be used throughout. The most commonly quoted G factors are for crosslinking, chain scission, and gas evolution, G(X), G(S), and G(Gas), respectively. There may be several different values for the G-factor in the literature since several different methods of measurement may be used to determine the event yield. If different methods or standards are used to determine molecular weights, they can lead to widely different G-factors. For example, in the case of a low density polyethylene values for G(X) of 0.9 or 1.7 are obtained using the hydrogenated polybutadiene or polystyrene calibrations, respectively [17]. 

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