Chains before Scission

To date two methods have been employed for the determination of the distribution N(i//) of (large) axial chain stresses  

This indirect method has a rather restricted applicability. It is limited to a temperature region T Tj where interfibrillar slip is still negligible and to molecular stresses p Furthermore the method does not permit the solution of a 

To overcome this difficulty Zhurkov and his collaborators began in 1965 to study by an independant method, the IR technique, the effect of axial stress on molecular chains [4—16]. Subsequently similar [35, 37—40] infra-red studies were carried out in the USA. All these investigations are based on the fact that frequency and intensity of skeletal or backbone vibrations of chain segments respond to a superimposed deformation of the vibrating skeleton. In polypropylene for example C—C stretch vibration modes have been assigned [16, 36,41,42] to the parallel dichroic bands (Fig. 8.1)at 1326, 1168, 1045, 975, 842, 456, and 398 cm . All of these bands have been found to be stress sensitive [36]. The stress effect manifests itself as a shift of the frequency of maximum absorption and in the occurrence of a tail at longer wavelengths (Fig. 8.2). 

The mechanisms of the distortion of IR absorption bands of stressed polymers have been discussed especially by Gubanov [7—9], Kosobukin [13], Vettegren and Novak [15] and Wool [36]. There is a general agreement that the distorted IR absorption band profile D(i ) can be related to a large number of separate oscillators with strongly overlapping absorption bands whose maxima are shifted by different amounts. The possible causes of the frequency shift of an individual stressed oscillator have been considered to be the quasielastic deformation of the harmonic oscillator (re- 

As discussed previously molecular stresses are a function of molecular strains which depend on sample morphology and chain orientation. Any real sample, therefore, contains a variety of differently stressed oscillators. The profile of the deformed band, D(p), representing such a system of oscillators can be expressed by a convolution integral  

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Fig. 20. Bond scission activation energy and lifetime (Tt) plotted as a function of applied force. The solid curve is derived from Eq. (65) based on the Morse potential, the other data are redrawn from Ref. [101]. The upper abscissa gives the overall elastic strain before failure. The numbers indicate the minimum chain lengths which will fail at a particular force...

The molecular weights of the polymers before and after irradiation were followed by GPC to determine changes in the molecular weights of the polyamides. It was found that photolysis of the polymer resulted in polymer chain scission, leading to the appearance of oligomers containing thymine bases at the end of the molecules. 

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 brittleness reported can either be a function of chain scission and/or chain branching as the AFM technique cannot distinguish between those two processes, only that the sample has become brittle as opposed to initial ductility before exposure. 

With the exception of PC, amorphous, non-oriented polymers did not produce measurable amounts of broken segments when subjected to tension. As has been shown in previous paragraphs, large axial stresses capable of chain scission in amorphous polymers can only be transmitted into the chain by friction of slipping chains requiring strong intermolecular interactions. In addition, macroscopic fracture occurs before a widespread chain overloading and scission occurs, which is opposite to the behavior of semicrystalline polymers. 

Assuming that the two carbonyl species have similar molar extinction coefficients, a simple calculation suggests that about half of the acyl radicals formed abstract hydrogen before decarbonylation. This process would thus be expected to reduce the efficiency of main-chain scission via the /3-scission mechanism. 

Clq braid around each other, but do not braid with themselves. If they did, then the ratio of different types of chains in the complex would be changed, and there would be a much smaller chance of getting the real Clq complex with six copies of three different chains. If the binding of Clq to the antibody-foreign cell did not trigger Clr s self-scission, then the cascade would be stopped in its tracks. Conversely, if Clr cut itself before Clq bound to the antibody complex, then the cascade would be prematurely triggered. And so on. ... 

Recently spin-coated PMMA thin films with a thickness of 0.45 pm on silicon wafer were irradiated with various ion beams (H+, He+, N+, Ni3+). Ion beam energy regions are from 300 keV to 4 MeV. Irradiated PMMA films were developed by isopropyl alcohol in these experiments. After the irradiation by ion beams on PMMA in a vacuum, the thickness of the films were measured both before and after development. Various radiation effects on PMMA films such as ablation (sputtering), main chain scission, and positive-negative inversion were observed as shown in Fig. 11. These phenomena are very different from those in 60 Co gamma-ray or electron beam irradiation. Large LET effects are considered to be due to high density excitation by ion beams. 

Plasma vs. Corona Treatment of Polypropylene (PP1. Corona treatments of polyolefins to modify their surfaces are very common in the polymer industry. The chemistry at such surfaces has been widely studied by XPS (4). It is generally assumed that corona treatments create abundant amounts of radicals which react with oxygen to form a hydroperoxide. This reacts further to eventually form crosslinks, oxidized products (ranging from hydroxyls to esters) with and without chain scission. The latter process is believed to lead to low-molecular weight material. There is some controversy over this material. Its role in determining the surface properties of the modified polymer is not completely understood. Its formation cannot be demonstrated directly by XPS, but only by comparing spectra before and after washing. 

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