Quantum crosslinking

The quantum crosslinking yield ( c) is defined following a similar manner in Eq. (4) ... 

The low quantum yield of the photografting process (0 = 2 X 10 ) provides a good opportunity to control the network formation (curing time control), and accordingly, the desirable properties of the crosslinked or grafted copolymer might be obtained. 

Table 2. Quantum yields for oolymer scission and crosslinking for some representative polysilanes (SiR R )n (a) Molecular weights were measured by GPC using polystyrene calibration standards (b) Ban, H Sukegawa, K. J. Polym. Sci., Polym. Chem. Ed., 1988, 26, 521...

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

Skaff H, Lin Y, Tangirala R, Breitenkamp K, Boker A, Russell TP, Emrick T. Crosslinked capsules of quantum dots by interfacial assembly and ligand crossUnking. Adv Mater... 

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. 

Site-selective double-quantum build-up curves and correlation with crosslink density... 

The strongest residual dipolar couplings were also edited in a non-rotating crosslinked poly(isoprene) series by exciting DQ and triple-quantum coherences in the short time regime [62,63]. From this, the dynamic order parameters of the methylene and methyl groups were estimated and correlated with the crosslink density. Essentially the same behaviour was found as for SBR. 

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. 

Static H Multiple Quantum (MQ) NMR spectroscopy, on the other hand, has shown the ability to more reliably quantitatively characterize elastomer network structure and heterogeneities (14-19). H MQ NMR methods allow for the measurement of absolute residual dipolar couplings (cooperative dynamics without interference from magnetic susceptibility and field gradients which complicate relaxation measurements (13, 14, 20,21). It has previously been shown that the residual dipolar couplings are directly related to the dynamic order parameter, Sb, and the crosslink density (1/N)(P) ... 

One of the most interesting problems was evaluation of quantum yield for light induced crosslinking of poly(vinyl butyral) in the presence of diphenyl sulphoxide. Mathematical treatment of the molecular changes produced in polymers by light is based on that by ionizing radiation. In our case, as the changes above the gel point were examined, the only suitable approach... 

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