Radiation, ultraviolet

Irradiation of dioxane solutions of MA gives dimer 2, accompanied by oligomer containing about four units derived from monomer and one dioxane unit per polymer molecule. Mass spectroscopy, NMR, IR, elemental, and other analytical studies support a complex reaction mechanism (equations 1-8), consisting of monomer excimer, ionic intermediates, dioxanyl radical, and other intermediates formation during photoexcitation. 

Ionic intermediates in the above reactions are believed to be consistent with other work on electrical conductivity of MA-dioxane solutions where conductivity increases when the mixture is exposed to UV radiation/ Termination by disproportionation is neglected, since NMR spectra of various oligomers failed to exhibit a vinyl proton for a terminal MA residue/ An alternate mechanism for dimer and oligomer formation could consist of direct coupling of excited dimer species 3 and 

Extended exposure of acetic anhydride solutions of monomer to UV light, in the presence of 1% biacetyl, gives polymeric material in about 15% yields/MA oligomers, containing aromatic residues, have also been obtained from the benzophenone-sensitized photopolymerization of MA in a variety of aromatic solvents/ These sunlight-induced polymerizations were not inhibited by hydroquinone or /-butylhydroquinone. 

Maleic anhydride and hexamethylbenzene form a 1 1 charge-transfer complex in methylcyclohexane solvent with a UV maximum at 340 tx. Ultraviolet irradiation of these solutions brings about the formation of carbon dioxide, pentamethylbenzylsuccinic anhydride, and resinous products (see Chapter 6). Analytical data suggest the polymeric material has a decarboxylated MA building block. Charge-transfer complexes were not detected in solid-state mixtures of MA and pentamethyl-benzene. Ultraviolet irradiation of these solid mixtures gives cyclic dimer, 1,2,3,4-cyclobutanetetracarboxylic anhydride 2, and resinous products. Pentamethylbenzylsuccinic anhydride, due to the possible absence of charge-transfer complexes, was not isolated from the solid-state photoreaction. 

For a short time it was believed that UV irradiation of MA in tetrahy- 

Some limitations are associated with UV radiation for disinfection. These include (1) The process performance is highly dependent on the efficacy of upstream devices that remove suspended solids (2) Another key factor is that the UV lamps must be kept clean in order to maintain their peak radiation output (3) A further drawback is associated with the fact that a thin layer of water ( 0.5 cm) must pass within 5 cm of the lamps. 

One way of implementing the UV disinfection process at existing activated sludge plants involves suspending the UV lights (in the form of low-pressure mercury arc UV lamps with associated reflectors) above the secondary clarifiers. The effluent is exposed to the UV radiation as it rises over the wire in a thin film. 

UV light can cause burns to skin and cataracts to the eyes. UV classifications can include near, medium, and far UV energy. Consider near UV radiation as nonionizing. UV light produces free radicals that induce cellular damage, which can be carcinogenic. UV light also induces melanin 

5 Functionalization of Polyester Electrospun Scaffolds with Bioactive Molecules 

A variety of methods have been employed to functionalize polyesters with bioactive molecules. One approach is to graft the biomolecule in polyester before it is subject to the electrospinning process. Grafting methods permit control of the extent of functionalization in all stages. However, using this method after electrospinning, a part of the bioactive molecules is located in the core of the fibers, inaccessible for the cells. Therefore, an additional step for polymer purification can be necessary. This step can increase the cost of scaffold production. 

Another alternative is the functionalization of polyester after electrospinning. 

In covalent immobilization of biomolecules onto the fibers, chemical modifications are made in electrospun polyester in order to produce reactive functional groups in its chain. Primary amine and carboxylate are chemical groups frequently used as intermediates of reaction. Through this strategy, the amino or carboxyl groups present on biomolecules are cross-linked to free carboxyl or amino groups on activated electrospun polyesters. The l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and M-hydroxysuccinimide (NHS) are the most used intermediary reagents in activation reactions of polyesters. 

EDC is a zero-length cross-linking agent which reacts with carboxyl groups 

TNFa is thought to be an important mediator in the suppression of CHS. This hypothesis is supported by the finding that injection of antibodies to TNFa reverses the suppression of CHS that usually occurs following UVR exposure. Conversely, intracutaneous injection of recombinant TNFa at the site of sensitization suppresses the development of a contact sensitivity response. Also, the genetic difference between UVR-resistant and UVR-susceptible strains of mice appears to involve a polymorphism in a regulatory region of the TNFa gene. 

Three epidermal photoreceptors have been identified that convert UVR energy into biologic signals that mediate immune suppression DNA, urocanic acid, and 

UVR is frequently referred to as a complete carcinogen because it has both initiation and promotion properties. Immune suppression may be responsible for promotion. It should be noted that many chemical carcinogens are also immunosuppressive. The observation that DNA damage induces immune suppression has ramifications that extend beyond UVR-induced immune suppression. Furthermore, similar mechanisms account for immune suppression observed after dermal exposure to jet fuels, particularly JP-8, suggesting that mechanisms described here are not unique to UVR. 

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Ozone, known for its beneficial role as a protective screen against ultraviolet radiation in the stratosphere, is a major pollutant at low altitudes (from 0 to 2000 m) affecting plants, animals and human beings. Ozone can be formed by a succession of photochemical reactions that preferentially involve hydrocarbons and nitrogen oxides emitted by the different combustion systems such as engines and furnaces. 

Hepburn J W 1995 Generation of coherent vacuum ultraviolet radiation applications to high-resolution photoionization and photoelectron spectroscopy Laser Techniques in Chemistry vol 23, ed A B Myers and T R Rizzo (New York Wley) pp 149-83... 

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

Using QMOBAS, TMOBAS, or Mathcad and the method from Computer Project 6-2, calculate the energy separation between the HOMO and LUMO in units of p for all compounds in Table 6-1 and enter the results in Table 6-2. Enter the observed energy of ultraviolet radiation absorbed for each compound in units of cm . The reciprocal wavelength is often used as a spectroscopic unit of energy. 

The substance is examined in a dilute solution in a solvent. A wide choice of solvents, transparent to ultraviolet radiation, is available. The paraffin hydrocarbons are all suitable, as are the ahphatic alcohols and the chlorinated hydrocarbons, such as chloroform and carbon tetrachloride. The most useful solvents are re-hexane, cycZohexane, chloro-... 

Lowest Wave Length (mp) at which Solvents Transmit Ultraviolet Radiation... 

UV-VIS Simple ethers have their absorption maximum at about 185 nm and are trans parent to ultraviolet radiation above about 220 nm... 

Hydroxybenzophenones represent the largest and most versatile class of ulbaviolet stabilizers that are used to protect materials from the degradative effects of ulbaviolet radiation. They function by absorbing ultraviolet radiation and by quenching elecbonically excited states. 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Another factor in oxidative degradation is ultraviolet radiation, of which sunlight is a rich source. The oxidation of parylene appears to be enhanced by ultraviolet radiation. 02one may play a mechanistic role in the ambient temperature exposure of parylenes to ultraviolet radiation in the presence of oxygen. For the best physical endurance, exposure of the parylenes to ultraviolet light must be minimised. 

Sinks, chemical species, or method OH, reaction with OH radical S, sedimentation P, precipitation scavenging NO, reaction with NO radical uv, photolysis by ultraviolet radiation Sr, destmction at surfaces O, adsorption or destmction at oceanic surface. 

Ozone, which occurs in the stratosphere (15—50 km) in concentrations of 1—10 ppm, is formed by the action of solar radiation on molecular oxygen. It absorbs biologically damaging ultraviolet radiation (200—300 nm), prevents the radiation from reaching the surface of the earth, and contributes to thermal equiHbrium on earth. 

Although inert in the lower atmosphere (troposphere), the hilly halogenated CFCs and Halons diffuse into the upper stratosphere where they are photodissociated, ie, photolyzed, by the intense ultraviolet radiation. 

A more energy-efficient variation of photohalogenation, which has been used since the 1940s to produce chlorinated solvents, is the Kharasch process (45). Ultraviolet radiation is used to photocleave ben2oyl peroxide (see Peroxides and peroxide compounds). The radical products react with sulfuryl chloride (from SO2 and CI2) to Hberate atomic chlorine and initiate a radical chain process in which hydrocarbons become halogenated. Thus, for Ar = aryl,... 

Ultraviolet radiation causes cleavage of the aryl ether linkage (23). DMPPO undergoes oxidation when exposed to ultraviolet light and oxygen by direct attack on the aromatic ring to produce a variety of ring-cleaved and quinoidal stmctures (24). 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

The Kestner-Johnson dissolver is widely used for the preparation of silver nitrate (11). In this process, silver bars are dissolved in 45% nitric acid in a pure oxygen atmosphere. Any nitric oxide, NO, produced is oxidized to nitrogen dioxide, NO2, which in turn reacts with water to form more nitric acid and nitric oxide. The nitric acid is then passed over a bed of granulated silver in the presence of oxygen. Most of the acid reacts. The resulting solution contains silver at ca 840 g/L (12). This solution can be further purified using charcoal (13), alumina (14), and ultraviolet radiation (15). 

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