Photopolymerization system

The chapters in this book are organized into three sections. A majority of the commercial photopolymerization systems are based on acrylate monomers therefore, the first several chapters focus on fundamental characterization of... 

Johnson PM, Stansbury JW, Bowman CN (2008) Kinetic modeling of a comonomer photopolymerization system using high-throughput conversion data. Macromolecules 41 230-237... 

The DnPont photopolymeric system consists of polymeric binder resins, e.g. PVA, PMMA, cellnlose acetates and styrene-acrylates, reactive acrylic monomers, e.g. aryloxy or alkoxy acrylates, a dye sensitiser and a radical or charge transfer photoinitiator, e.g. DEAW and HABI respectively (see Chapter 4, section 4.5.2), and plasticisers. The process for producing the refractive index structures is as follows ... 

Photopolymerization processes have been known since the. 1820s when a permanent photographic image was obtained using a coating of bitumen on glass (B-71MI11404). Since then photopolymerization systems have received almost as much attention as silver halide processes, but only a few have been used as substitutes for the latter in information... 

These authors interpret their results in terms of a polymerization reaction that occurs during the development step, involving the diffusion of monomer into the irradiated regions, where it had been depleted by the exposure. This mechanism is similar to that accepted for the more common photopolymerization systems. Relatively little surface modulation was observed. 

Schuster s original measurements were performed in benzene, where the cyanine borates are know to exist as tight ion pairs. In practice, the monomers used in most photopolymerization systems possess higher dielectric constants than benzene. Therefore the degree of dissociation of the cyanine borate in monomer or mixtures of monomers is expected to be higher than it is in benzene. As an example, the data... 

Recently a series of publications by Tazuke and co-workers 28.29,30) have disclosed photopolymerization systems in which ionic or charge transfer species act as the photoinitiators. It is interesting to note that the presence of oxygen in such systems causes less inhibition or retardation than in radical-type photopolymerizations. Donor-acceptor pairs such as vinylcarbazole and sodium aurochloride dihydrate typify the system ... 

The photopolymerization systems, which are used for Image recording, are photolnltlated processes. They are erroneously classified as photopolymerization processes this term should be reserved for the true chain-lengthening processes where light Is Indispensable for each propagation step and from which many examples are known (15). This true photopolymerization did not find applications In photography due to the low sensitivity of the system, which comes from the necessity to use a photon for each addition step. 

By selecting properly the irradiation sequence, a positiveworking photopolymerization system - a system wherein the irradiated parts can be washed away - could be obtained. The photochemistry of nltroso derivatives, including the nltroso dimers, has been reviewed by Heicklen and Cohen (48). 

L.B. Kong, J.P. Deng, W.T. Yang, Detailed 1D/2D NMR analyses of benzophenone-related reaction products from a photopolymerization system of vinyl aeetate and benzophenone. Macromol. Chem. Phys. 207, 2311-2320 (2006)... 

One of the most important components of the photopolymerization system is the photoinitiator, which is capable of absorbing light efficiently and generating reactive initiating species. Some photoinitiator systems require a co-initiator. 

Photoinitiators are classified according to the type of photopolymerization system they initiate (i.e., radical or ionic). The basic photochemical routes that produce radicals are photocleavage (type I), intermolecular hydrogen abstraction (type II), and electron transfer followed by proton transfer. Schematics of these photochemical routes are given in Scheme 5. The efficiency of these routes directly determines the monomer conversion, molecular weight of the polymer, and degree of polymerization, and hence the structural, physical, and mechanical properties of the final product. The general relationships of Pp, Pi, and DP were provided in the previous section. 

The EPR spectral shape undergoes marked modifications during radical decay kinetic runs in the range 363-423 K,. showing the same trend for both photopolymerized systems. An example of this behaviour is given in Figure 3.5. 

Since not all RNA molecules replicate equally well, faster mutants gradually take over. At each mutation, the front propagates faster. Evolution can be directly observed in a test tube. Propagating polymerization fronts of synthetic polymers may be useful for making new materials, and they are interesting because of the rich array of nonlinear phenomena they show, with pulsations, convection, and spinning fronts. Finally, we will consider photopolymerization systems that exhibit spatial pattern formation on the micron scale, which can be used to control the macroscopic properties. 

Photopolymerizations, which use light energy (photons) to initiate chain reactions to form polymer materials, are the basis for a growing, billion-dollar industry (1,2). Applications in which photopolymerization is used include films and coatings, inks, adhesives, fiber optics, and dentistry (2-5). Each of these industries has benefited from the high productivity and lower costs afforded by photopolymerization systems. 

Photopolymerization systems, like thermally initiated systems, contain initiator, monomer, and other additives that impart desired properties (color, strength, flexibility, etc) (6). The reaction is initiated by active centers that are produced when light is absorbed by the photoinitiator. One important class of active centers includes free-radical species, which possess an impaired electron (5,7). The highly reactive free-radical active centers attack carbon-carbon double bonds in imsaturated monomers to form pol5nner chains. Although the kinetic treatment of photopoljnner systems is similar to that in thermal systems, significant differences arise in the description of the initiation step, which in turn affect the... 

In order to produce free radicals that initiate polymerization, photoinitiators absorb light of a certain frequency. Upon absorption, the photoinitiator molecule is promoted from the ground electronic state to either a singlet or triplet excited electronic state. This excited molecule then undergoes either cleavage or reaction with another molecule to produce initiating free radicals. Numerous photoinitiators have been developed to meet the needs of a variety of photopolymerization systems, as described in a number of recent papers and reviews (3-6,8-12). 

Complex Photopolymerization Systems. Kinetic modeling of free-radical photopolymerizations becomes more complicated as comonomers are added to the reaction system and as different polymerization methods are used to tailor the pol5uner properties. Although free-radical reaction mechanisms still hold true, rates of propagation and termination must be reconsidered to account for variables such as differences in double bond reactivities, reaction diffusion, and chain transfer. 

The systems are designed in order to improve the reaction rate of the mixture and the physical properties of the photopoljmier. The flexibility of the two photoinitiation schemes in one system allows for numerons possibilities in achieving greater control of viscosity, conversion, shrinkage, adhesion, and ultimate strength. The kinetics of hybrid photopolymerization systems are more difficult because two reactive systems (free-radical and cationic) mnst be resolved from one another. Cationic photopolymerization kinetics are more difficnlt to analyze than free-radical kinetics because the pseudo-steady-state assumption is often not valid for the cationic active center concentration, and the natnre and concentration of the cationic active centers is difficult to determine (p. 376 of Ref 33, see also Photopolymerization, Cationic). 

The two-photon photopolymerization system resembles a laser scanning microscope, which doesn t need vacumn condition for operation. The system is easy to operate and maintain. 

In an efficient photopolymerization system, the excited photoinitiator L-L must be sufficiently energetic and long-lived to decompose spontaneously or to interact with a second component to produce the active free radicals. Once formed, the free radical R- (here the imidazolyl radical L- reacts with the chain transfer agent, e.g., 2-mercaptobenzoxazole, the monomer M as in thermal polymerization, undergoing propagation, chain transfer, and termination steps ... 

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