Crosslinking and main-chain scission

The radiolysis of polymer solutions has been reviewed by Chapiro [441] and later by Henglein and Schnabel [442, 443]. Two types of reaction are added to the degradation mechanism for solid polymers. One involves the reactions between polymer radicals and solvent or between solvent radicals and polymer. The other involves energy transfer from solvent to polymer or from polymer to solvent. 

The complete reaction scheme for polymer radiolysis in solution is [443] 

Elementary reactions (65), (66), (69), (70) and (71) are specific to solutions. Steps (65) and (66), respectively, induce protection or sensitization of polymer radiolysis. Their existence is, however, often difficult to demonstrate and even more difficult to predict. Reactions (69) and (71) are responsible for inhibition of the radiolysis while step (70) leads to an increase in the free radical yield. 

Owing to the large number of possible reactions, very different crosslinking and degradation yields are obtained depending on the nature 

A rough evaluation of the ratio p 0/q o for a solution as a function of PoAfo for the solid state and the mole fraction, xp has been made by Henglein and Schnabel [443]. The fraction, f, of macroradicals which combine to form crosslinks is 

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Crosslinking and main-chain scission yields have been determined by many workers. Newtork formation is predominant. Typical values for crosslinking yields are given in Table 17. The various methods used, and the differences in purity, crystallinity and branching of the samples studied, explains the large scattering in the measured values. Most results in the literature show that main-chain scission also occurs, since a soluble fraction is always obtained even at very high doses. Early values of p0/q0 determined by Charlesby [297] and by Baskett and Miller [298] are, respectively, 0.35 and 0.18—0.20. A value of 0.3 independent of... 

Crosslinking and main-chain scission in Marlex-50 has been reinvestigated recently by Dole and co-workers [272] using the Charlesby— Pinner function modified for a Wesslau molecular weight distribution (Table 18). Evidence of increase of Gc L with dose has been obtained and related to vinylene decay. GCL and Gcs at zero dose are almost independent of temperature but at 27 Mrad, GCL increases with temperature. 

When crosslinking and main chain scission occurs simultaneously, for an initial random distribution then... 

Figure 5.8 Irradiation of poly(ethylene oxide) in vacuo with Co-y-rays. Radiation chemical yields of crosslinking and main-chain scission as a function of temperature. Adapted with permission from Ref [65] 1970, Wiley-VCH.

CHEMICAL YIELDS FOR CROSSLINKING (GCL) AND MAIN CHAIN SCISSION (Gcs) IN POLYETHYLENE IRRADIATED IN VACUO AT ROOM TEMPERATURE [287]... 

Resistance to ionizing radiation. In contrast to chemically cross-linked elastomers, which change the properties and exhibit brittleness when exposed to electron beams or gamma rays, as a consequence of crosslinking or main chain scission, TPEEs are resistant to radiation doses up to 150 kGy. 

Verbruggen and co-workers [13] prepared a model NR gumstock (i.e., cure system only, no fillers or other additives) compound and subjected a thin film (300 pm thick) of it to DPDS at 200 "C. Solvent extraction (with acetone and tetrahydrofuran), swelling experiments (in toluene) and chemical analysis by GPC and nuclear magnetic resonance (NMR) spectroscopy were used to assess the degree of devulcanisation and to characterise the samples produced. It was found that complete network breakdown was obtained with 2.4% w/w of DPDS after 2 h of heating, but that both crosslink scission and main-chain scission had occurred. 

So far we have only considered polymers that undergo main-chain scission upon exposure to radiation. PMMA is an example of such a material. If, on the other hand, one considers polymeric systems in which both scissioning and crosslinking events occur simultaneously upon exposure, the analysis depicted above will allow determination only of the net scission-... 

The radiation chemical yields are expressed in terms of G-values. G(scission), G(s), equals the number of main chain scissions produced per 100 eV of energy absorbed and G (cross-linking), G(x), the number of crosslinks formed per 100 eV absorbed. The G-value is a structure dependent constant similar to quantum efficiency in photochemistry. 

Elastomeric materials undergo both thermal and oxidation degradation over time. Main chain scission and loss of sulfur crosslinks can occur with either factor or by both factors by a thermo-oxidative mechanism. 

Vehicle tire scrap containing polyisoprene rubber, SBR, and butadiene rubber was devulcanized by low-temperature phase-transfer catalyst. Both the devulcanizing agent composition and the process were patented. The novelty of this process lies in the use of low-temperature phase-transfer catalyst and a process temperature lower than 150" C. The devulcanized rubber of this invention is distinguishable from conventional reclaimed rubber in that the devulcanized rubber is substantially free from polysulfide crosslinks, which are selectively broken during the process with negligible main chain scission. 

Polyimides, chlorinated poly(methylstyrene) and novolac resins are all crosslinking polymers under ordinary UV light irradiation, whereas PMMA and PMGI are main-chain scission-type polymers under deep UV irradiation (23). Even for such scission-type polymers, photo-thermal effects cannot be neglected. For crosslinking-type polymers, photo-thermal effects become more important and are the predominant mechanism for photo-ablation. 

The ion track radius is also an important parameter in such reactions, reflecting the local spatial distribution of energy deposited by an incident ion and influencing the character of subsequent chemical reactions [8-12]. We recently reported on main-chain scission and crosslinking reactions in a variety of polymer systems and proposed chemical core sizes in ion tracks based on discussion of the non-homogeneous spatial distribution of reactions [9-15]. Intratrack crosslinking reactions are also of interest with respect to the potential for the direct formation of nano-structured materials, and materials exhibiting these reactions have been successfully visualized in recent years [11,13]. However, despite the extensive experimental and theoretical study undertaken to date, many factors in the relationship between the ion track structure and the chemical core radius remain unclear. This paper proposes a new formulation that determines the chemical core radius in an ion track based on the initial... 

The main chemical changes induced by ionizing radiation are (a) main-chain scission, (b) crosslinking formation, (c) volatile products formation, (d) formation and decay of unsaturation, and (e) cyclization. The yields strongly depend on the chemical structure of the polymer. 

Main-chain scission has been shown to occur in polyphenylvinylketone under UV irradiation (366 nm). Norrish type-2 scission due to reaction of the first n-II triplet state of the ketone has been shown to occur [422]. Under 7-irradiation, this polymer, which was expected to crosslink according to the Miller rule, was shown to undergo main-chain fracture with a G value of 0.35. Inhibition of the degradation in the presence of napthalene and diphenyldisulphide demonstrates the participation of radicals and excited n-II triplet states in the radiolysis [423]. 

Two processes that can take place during the photodegradation of polymers are of prime importance because even a small extent of a reaction can alter profoundly the mechanical properties of the sample. These are main chain scission and crosslinking. 

Actually, with most polymers crosslinking is accompanied by main chain scission to a small extent. In these conditions the determination of quantum yields requires the measure of the soluble fraction as a function of dose. The following equation is useful, provided the initial distribution of molecular weights is random and crosslinking also occurs at random [7] namely,... 

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