Quantum yields cross-linking

Acid generation in photoresist films add photogeneration vs. dose, 3233/ acid present after irradiation, 32,34r add present before irradiation, 32 quantum yield, 3234 Acid hardening resin resists cross-linking adivation energy determination, 87,89 cross-linking chemistry, 87 determination of acid generated, 87-88 effect of postexposure bake temperature and time, 87... 

QUANTUM YIELD PHOTOCHROMISM PHOTO-CROSS-LINKING PHOTOAFFINITY LABELING PHOTODIMERIZATION PHOTOISOMERIZATION PHOTOLYSIS FLASH PHOTOLYSIS... 

Under UV-laser irradiation, photosensitive multifunctional acrylate resins become rapidly cross-linked and completely insoluble. The extent of the reaction was followed continuously by both UV and IR spectroscopy in order to evaluate the rate and quantum yield of the laser-induced polymerization of these photoresist systems. Two basic types of lasers emitting in the UV range were employed, either a continuous wave (C.W.) argon-ion laser, or a pulsed nitrogen laser. 

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. 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

In these equations, and are the initial molecular weights, 4>(s) and 4>(x) are the quantum yields for the scissioning and cross-linking reactions, respectively D is absorbed dose and is Avogadro s number. The slopes of the respective plots of I/M and I/M versus dose produce two simultaneous equations, the solution of which yields values for 4>(s) and 4>(x). We (60) have analyzed the data obtained for a number of polysilane derivatives by GPC (gel permeation chromatography) to evaluate the respective molecular weights and distributions (Table III). Polystyrene standards were used for molecular weight calibration. 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

The fate of the ally lie radicals is ultimately to recombine, to add to double bonds or to abstract hydrogen atoms from the chain. These reactions result simultaneously in the formation of crosslinks and chain scission. On exposure to 302 nm radiation in the presence of air, cross-linking has been shown to occur with a quantum yield of 4.5 x 10-3 [Ref. 50]. In the same conditions the quantum yield of chain scission is 2 x 10 3 [Ref. 51]. Moreover, oxygen is consumed with a quantum yield of 0.16 [52]. Carbonyl and hydroxyl groups are formed during the oxidation. 

Eastman Kodak. Its bifunctional chromophore absorbs strongly at 365 nm. Its cross-linking reaction has a quantum yield of 0.1. ... 

Equation (6.4) is the fundamental equation of cross-linking resists, for it relates the imaging characteristic, namely, the gel dose Eq, to molecular properties of the polymer. It expresses the fact that the resist sensitivity is proportional to the quantum yield of the cross-linking reaction, to the weight-average molecular weight Mw of the polymer, and to the radiation-gathering power em of the chromophore. ... 

In systems undergoing cross-linking polymerization, it is usually not possible to evaluate the kinetic chain length because of the formation of a polymer of infinite molecular weight One of the distinct advantages of photoinitiation is to make this quantity accessible through quantum yield measurements. The kinetic data reported in Figures 6 and 7 have been used to evaluate the polymerization quantum yield. Op, i.e. the number of epoxy or vinyl ether functions polymerized per photon absorbed by the irradiated sample. Op was calculated from the equation ... 

Therefore, the cross-linking quantum yield is significantly increased if the irradiation is performed in the presence of dioxygen. 

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