UV-polymerization

Photopolymerization reactions of monolayers have become of interest (note Chapter XV). Lando and co-workers have studied the UV polymerization of 16-heptadecenoic acid [311] and vinyl stearate [312] monolayers. Particularly interesting is the UV polymerization of long-chain diacetylenes. As illustrated in Fig. IV-30, a zipperlike process can occur if the molecular orientation in the film is just right (e.g., polymerization does not occur readily in the neat liquid) (see Refs. 313-315). 

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

The chemistry involved in LfV-curable resin systems has been extensively investigated and thoroughly surveyed [88-94]. LfV-radiation polymerization, is in principle, completely analogous to the conventional addition polymerization. A photoinitiator is used in UV polymerization. Its function is the same as the free-radical initiator. A conventional initiator possesses a thermally labile bond which is cleaved to form free-radical species, but the photoinitiator has a bond which breaks upon absorption of radiant energy. Benzoin ethers, benzyldialkyl ketals, benzophenone, and acetophenone derivatives are the important LfV-photoinitiators [95-99]. 

Unlike the approach in which LB multilayers are formed from polymerizable fluoromonomers followed by UV-polymerization,69 we started to use functional fluoropolymers for the preparation of LB films. These films seem to be better, because in the case of monomeric LB film the polymerization process causes contraction and hence possible defects in the final cured film. Two different kinds of functional amphiphilic fluoropolymers were used for LB-film preparation. One type (referred to in Section 7.4.2), which has large fluoroalkyl groups as side... 

The dried polymer network 1 was then swollen in monomer Il/methanol solution (NlPAAm/CH30H) to equilibrium. The solution contained 1 mol% of TEGDMA and 1 wt% of DMPA. The swollen gel was UV polymerized for 10 min to form the IPN. The IPN was then washed in pH = 7.4 buffer solution to remove the unreacted monomers. 

The use of UV-polymerized, vacuum-deposifed film has enabled fhe development of mulhlayer barrier coatings for thin-film phofovolfaics and flexible electronics. The patented system uses thin polymer layers that are deposited alternatively with thin barrier layers. The mulhlayer structure is composed from organic (polymer) and inorganic (oxide) layers. 

In this report, vacuum evaporated PDA(12-8) film is used as an optically nonlinear layer with a grating coupler for nonlinear coupling for all optical bistability. Grating coupler on a substrate was prepared at the same periodicity and depth as the SHG devices. Vacuum evaporation of PDA on a substrate with previously rf-sputtered Corning 7059 buffer layer film were carried out at 5 x 10 5 torr with tungsten boat heater. Rapid evaporation can avoid thermal polymerization of the undesirable red phase PDA during the process. UV polymerization of the film for the useful blue phase PDA was carried out by Xe lamp 500 w for 20 min. at a... 

A liquid preparation with solid polystyrene (0.6 g) dissolved in liquid styrene monomer (1.5 mL) was cast against a mold. Polymerization was accomplished with UV irradiation (21°C, 18 h). Solid PS was included to reduce the degree of shrinkage that occurred when monomeric styrene was photopolymerized [85]. In a similar manner, PMMA dissolved in MMA was cast against a Si master. Upon UV polymerization (with BME as the photoinitiator), a PMMA chip is formed. Nearly 100 PMMA chips can be replicated using a single Si master [223]. 

FIGURE 3.30 Photopolymerized piston with cross-sectional area change. The widths of the channels are 150 and 50 pm, and the channels are 25 pm deep. The piston was first UV-polymerized in the outlined region and then displaced slightly to the left to show the molded sealing surface. The inset shows the schematic of the piston [456]. Reprinted with permission from the American Chemical Society. 

Photoinitiated polymerization of the same mixtures at 20°C generally yields monoliths with larger pores compared to those initiated thermally. Thus, reduced contents of dodecanol in the polymerization mixture has to be used for UV initiated polymerizations in order to obtain pore sizes comparable to those of their thermally polymerized analogs. For example, a polymerization mixture containing only 30% dodecanol can be used to produce a 2-hydroxyethyl methacrylate monolith with 1,000 nm pores by UV polymerization at 20°C. These shifts can readily be explained by the effect of the polymerization temperature, since the creation of larger pores is favored at lower temperatures [59],... 

Absorption, precipitation, air oxidation polymerization, UV polymerization, evaporation,... 

A dispersion of nanoparticles of Au or other metals in a polymer matrix may also be obtained by a one-pot process of microemulsion polymerization. For instance, the UV-polymerization of a microemulsion of 35 wt% MMA, 35 wt% AUDMAA and 30 wt% of 0.1 M HAUCI4 aqueous solution would produce a Au-polymer nanocomposite, as shown in Fig. 12 [104]. This TEM micrograph shows a microtoned thin film of the sample. It is clearly apparent that Au particles of about 10-15 nm are well dispersed in the polymer matrix. 

Oriented Polymers Obtained by UV Polymerization of Oriented Low Molecular Weight Species... 

There is an early report in the literature claiming absence of the autocatalytic reaction enhancement in TS if the reaction is induced by UV-excitation of the monomer crystal. The implication would be that thermal and UV-polymerization involve different mechanisms. Later on, however. Chance and Patel found this to be an artifact caused by the neglect of spatially inhomogeneous absorption by polymer molecules which effectively competes with monomer excitation at increasing conversion and prematurely terminates the reaction. Although it is difficult to correct X(t)-curves obtained under UV-excitation for polymer absorption quantitatively, particularly if irradiation is done with unpolarized non-monochromatic light, it turns out that there is a qualitative agreement between X(t)-curves obtained under y-and UV-irradiation. Application of this correction, however, does not solve the puzzle why in case of y- or UV-polymerization of TS, the reaction rate increases less dramatically with conversion, than observed upon thermal conversion. 

Fig. 14. Time dependent absorption of the reaction-intermediate B (trimer) observed upon UV-polymerization of TS-6 under hydrostatic pressure (a 1 bar, b 0.5 kbar, c 1.0 kbar, d 2,2 kbar). Excitation was by a single flash of excimer laser operated at 308 nm (from Ref...

Fig. 22. Summary of recent results concerning the determination of activation energies. (I) Thermal polymerization (2) 7-polymerization (3) to (5) UV-polymerization...

Mellott, M.B. Searcy, K. Pishko, V. Release of protein from highly cross-linked hydrogels of poly(ethylene glycol) diacrylate fabricated by UV polymerization. Biomaterials 2001, 22, 929-941. 

Though the quantum yield for photopolymerization with UV light is high at low polymer conversions, it rapidly decreases as the polymer concentration increases [2). The limiting yield for UV polymerization in 4BCMU is about 35%. It is believed that this effect has its origin in quenching of the monomer excited states by the polymer chains, i.e., the monomer excited states created on irradi-... 

To reduce or eliminate debonding, a UV-polymerized, hydro-phobic sealant was painted around the dentin-methacrylate interface. Thus, it was hoped that hydrolytic degradation of the nd would be minimized. The rationale for this procedure is that the coating would seal off the adhesive from contact with water, or at least the water permeation through the protective film to the adhesive surface would be reduced. Suitable protective formulations should (1) cure within 1 minute, (2) be hydrophobic,... 

Fig. 33 s-Polarized PWR curves acquired with only buffer in the PWR flow cell (1), after bis-SorbPC PSLB formation (2), after Rho incorporation (3), after UV polymerization (4), and after yellow light activation of Rho (5). Reflectance was measured at 632.8 nm. Reprinted with permission from [140]. Copyright 2005, American Chemical Society... 

Ward, J.H. and Peppas, N.A. (2001) Preparation of controlled release systems by free-radical UV polymerizations in the presence of a drug, J. Contr. Rel., 71, 183-192. 

Fig. 9 Field-enhanced water dissociation increases ionic currents in (a) a reverse-biased 20 nm thick bipolar-junction nanolluidic channel containing positive and negative surface charges (IVnil > 0.6 V), and (b) a UV-polymerized bipolar membrane V < —10 V). (c) Hydroxide ions and protons are produced at the bipolar membrane Junction and transport to opposite sides of the membrane. The pH change of the solution in the microchannels can be observed with a mixture of universal pH indicator. Left half of the bipolar membrane is positively charged whereas the right half is negatively charged...

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