Altering The Oxygen Diffusion Affected Region Biology Essay

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Polymerization that uses light (photo-polymerization) rather than heat to convert monomer to polymer have become very important, due to its use of less energy, good adhesion, compatibility, operational temperature, and lower cost of raw materials.12, 3 Free radicals have been commonly photo-polymerized because it exhibits high reaction rates, and offer a large selection of monomers and initiators.4 The photo-polymerization of interpenetrating network (acrylate and epoxides) by UV exposure in the presence of a radical and cationic photo-initiator in real-time was monitored, by the use of infrared spectroscopy.5 The free radical photo-polymerization however, have its own shortcomings- ranging from toxicity, volatility, oxygen-inhibition effect, high viscosity, unpleasant odor, and potential health hazard6- in order to minimize this negative effects, cationic photo-polymerization have been explored. The cationic photo-polymerization is insensitive to oxygen inhibition, continues to polymerize in the dark, low shrinkage, good adhesion, and excellent abrasion resistant.1, 78 In photo-polymerization, other additives that are present also affects the process in several way, such as competing with the monomers for light sources, accelerating the photo-polymerization e.t.c. as discussed in several studies. 6, 94A Hybrid resin systems consisting of monomer possessing the two different functional groups have been cross-linked and was found to be a more efficient monomer that can better reduce all the negative effect of the acrylate monomer.10

Decker, C. 1997, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 1997, 34, 4, 605, Taylor & Francis

Several monomer compositions was photopolymerized to evaluate the effect of the photo-initiator salts on the radical and cationic photo-initiation, and how each of these networks can mutually eradicate the inhibiting effects as previously discussed. The interpenetrating polymer network produced showed that the acrylate double bonds polymerizes faster than the vinyl ether double bond and reached 95% conversion in the triacrylate monomers. The sulfonium salt used as a cationic photo-initiator generates a strong protonic acid upon photolysis in the presence of a hydrogen donor molecule, which is an efficient cationic polymerization. Because the sulfonium salt also produces free radicals upon irradiation, it can serve to initiate both the cationic and radical polymerization of vinyl ether/acrylate mixtures. The penetrating network when compared to the radical-induced polymerization, the acrylate systems, cationic polymerization of vinyl ethers presents some distinct advantages. it is insensitive to oxygen inhibition because of the lack of reactivity of the propagating carbocation towards the oxygen molecules, the carbocation do no react among themselves, so that polymerization will continue to proceed in the dark in the absence of necleophilic species after the light has been switched off and the initiating specie are no longer generated. This article does not however indicate strongly the other contributing factor that can help eradicate or reduced the oxygen inhibition problems that occurs in radical polymerization, but is thorough on the cationic part of the network.5 The paper is the most important valuable source for my project, hence did not see so many negativity in it.

The vast majority of photo-polymerization monomers used in the industry are free radical polymerization, the most widely used monomer are multifunctional acrylates and methacrylates, it has been demonstrated that these monomers polymerize very rapidly and are easily modified on the ester group. However, free radical photo-polymerization has a number of problems and drawbacks that have led to the exploration of other monomers for example, they are relatively volatile, have unpleasant odor, potential health hazard, terminates once the light is off and high sensitivity to oxygen.2 The UV-initiated cationic polymerization monomer that is most widely used is the epoxides and vinyl ethers. The cationic photo-polymerization of this monomers exhibit several advantages when compared to free radical photo-polymerization, they are insensitive to oxygen and are not inhibited by oxygen, it has low shrinkage, good adhesion, an excellent abrasion resistance, ability to polymerize in the dark when the light is shut off and are very versatile. However, the cationic polymerization of epoxides does not polymerize rapidly, and can be costly because the rate of polymerization depends on other factors.1, 11 It may be necessary to ascribe the less attention paid to the cationic photo-polymerization in the past to lack of suitable UV-sensitive cationic photo-initiators. The generation of hybrid monomers consisting the free radical and cationic chain through interpenetrating networks, have been a breakthrough, these hybrid monomers behave more effectively and efficiently in addressing the individual shortcomings of the free radical and cationic monomer.

Cai,Ying; Jessop,Julie L.P. Encyclopedia of polymer science and technology. Volume 10

In order to produce free radicals that initiate polymerization, photo-initiators absorbs light of certain frequency. Upon absorption, the photo-initiator molecule at the ground state will go to a singlet or triplet excited electronic state. The excited molecules then undergo either cleavage or reaction with another molecule to produce initiating free radicals. The absorption spectrum of the initiator must overlap with the emission spectrum of the light source by good engineering; in systems with overlap such as highly pigmented systems, higher light intensities and photo-initiator concentration are often used. Initiation systems involving only one molecular species interacting with the light and producing free-radical active center ‘unimolecular photo-initiators’ will be used to evaluate the radical unit a good example will be dimethoxyphenylacetophenone. The other systems present may involve two molecular species ’bimolecular photo-initiator system’ where a photo-initiator absorbs light ad a co-initiator that serves as a hydrogen or electron donor. A photo-sensitizer is able to absorb light and use the light to enhance photo-polymerization in bimolecular photo-initiation systems, also it can improve initiation efficiency by absorbing photons from the light source that the photo-initiator cannot absorb or may not do so efficiently.2

Sipani,Vishal; Scranton,Alec B. Encyclopedia of polymer science and technology. Volume 10

There have been numerous studies on suitable photo-initiators for cationic polymerization. The first more generally applicable cationic photo-initiators that have achieved some success were diazonium salts of complex metal halide anions, but the limitation of these salts by their poor thermal stability and release of nitrogen gas as a by-product have led to further research on an alternative. The first iodonium salt photo-initiator developed GE and 3M, avoided the shortcomings of the diazonium photo-initiators.1 These iodonium salts are remarkably stable in the absence of light, thermally stable at room temperature, and do not degrade in the presence of moisture. The introduction of triarylsulfonium salts, as an efficient cationic photo-initiator that offered many of the same advantages was also a breakthrough, upon UV illumination; the onium salts produce a cationic moiety and an anion, which plays an important role in the performance of the photo-initiator. The cation of the photo-initiator plays the determining role in production of the active centers, the efficiency of the light in creating the active centers and ability to be photosensitized, but the cation have little effect on the processes that occur after the production of the active centers. The mechanism by which onium photo-initiators salts produce active center is very complex, for example upon absorption of photon from light at the ground state the iodonium salt is known to be promoted to an electronic excited state, and then undergo an irreversible photo-fragmentation in which a carbon-iodine bond is cleaved to produce arylliodium radical cation and an aryl radical. This paper did not discuss the hybrid monomer and free radical photo-polymerization neither did get to the specifics of tackling with the oxygen inhibition problems.

Cho,Jung-Dae 2004; UV-initiated free radical and cationic photo-polymerizations of acrylate/epoxide and acrylate/vinyl ether hybrid systems with and without photosensitizer

This paper repeated some of the advantages and disadvantages of the free radical and cationic photo-polymerization of acrylaye/epoxides. Interesting is the point about the product of the photolysis of free radical photo-initiation that produced a benzoyl radical and an alpha-hydroxyalkyl, both of which are capable of reacting with the acrylate double bond. The photolysis of the cationic photoinitiator in the presence of a hydrogen donor molecule produces both protonic species and free radicals, of which protonic acid initiates the cationic polymerization. The photo-sensitizer on the hybrid system of the acrylate/epoxide exhibited higher pendulum hardness, high solvent resistance, formed more highly cross-linked network films and lower break energy.

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