Cross photopolymerization

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

Chain Cross-Linking Photopolymerization of Tetraethyleneglycol Diacrylate... 

Fig. 7 Topographic images, SFM scan lines, and schematic cross-sectional representation of glass slides with chromium patterns and the formation of the polymer and copolymer layer A cleaned sample, B sample modified with SBDC, C after photopolymerization of styrene for 5h, D after photopolymerization of styrene for 15 h, and E after photopolymerization of styrene for 15 h and subsequently methyl methacrylate for 10 h. (Reproduced with permission from [45] American Chemical Society)...

Because the cast films are relatively thin, the optical density of the light absorbing species can he low and can vary with time of exposure. Additionally, the depth penetration of the absorbed light can be inhomogeneous in some systems. However, thin films can be mounted directly in UV/visible or infrared spectrometers, and so the course of the photopolymerization (and the rate) can be monitored directly in some systems. The most common observation made is the disappearance of monomer (e.g., loss of double bond absorption in the IR) as a function of irradiation time. It must be emphasized that in most thin film compositions important industrially, the monomers used are multifunctional. The polymer which results is then highly cross-linked and simple kinetic arguments are usually not valid. 

MIP films, applied to a QCM transducer, have been employed for chiral recognition of the R- and 5-propranolol enantiomers [107]. MIP films were prepared for that purpose by surface grafted photo-radical polymerization. First, a monolayer of 11-mercaptoundecanoic acid was self-assembled on a gold electrode of the quartz resonator. Then, a 2,2 -azobis(2-amidinopropane) hydrochloride initiator (AAPH), was attached to this monolayer. Subsequently, this surface-modified resonator was immersed in an ACN solution containing the MAA functional monomer, enantiomer template and trimethylolpropane trimethacrylate (TRIM) cross-linker. Next, the solution was irradiated with UV light for photopolymerization. The resulting MIP-coated resonator was used for enantioselective determination of the propranolol enantiomers under the batch [107] conditions and the FIA [107] conditions with an aqueous-ACN mixed solvent solution as the carrier. The MIP-QCM chemosensor was enantioselective to 5-propranolol at concentrations exceeding 0.38 mM [107]. 

Another photopolymerization strategy towards electrically switchable gratings is the crossed-beam irradiation of a mixture of a non-LC monomer (typically with... 

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. 

Herr, A.E., Singh, A.K., Photopolymerized cross-linked polyacrylamide gels for on-chip protein sizing. Anal. Chem. 2004, 76, 4727-4733. 

Fig. 6.35. SEM micrograph of a cross section of a photopolymerized 1 mm long inlet frit with embedded 1.5 pm ODS beads within a 75 pm i.d. capillary. (Reprinted with permission from [86], Copyright 2000 American Chemical Society).

The temperature effect on the polymerization of p-PDA Et is described in the range from —50 to 15 °C in Figs. 4 and 5. p-PDA Et, with a melting point of 100 °C (96 °C by the capillary method) and a crystal transition point of 56 °C (DSC), photopolymerizes quantitatively to a crystalline high polymer at a temperature below ca. 0°C as is obvious in Figs. 4 and 5. However, above ca. 25 °C, a partially cross-linked amorphous polymer is obtained in poor yield. 

Fig, 24. Energy level diagram of the photophysical and photochemical primary processes of the low temperature photopolymerization reaction in diacetylene crystals. IC Internal Conversion ISC Intersystem Crossing... 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

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