Infrared spectroscopy photopolymerization

Synthesis. Functionalized monomers (and oligomers) of sebacic acid (SA-Me2) and 1,6 -bis(/ -carboxyphenoxy)hexane (CPH-Me2) were synthesized and subsequently photopolymerized as illustrated in Figure 1. First, the dicarboxylic acid was converted to an anhydride by heating at reflux in methacrylic anhydride for several hours. The dimethacrylated anhydride monomer was subsequently isolated and purified by dissolving in methylene chloride and precipitation with hexane. Infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis results indicated that both acid groups were converted to the anhydride, and the double bond of the methacrylate group was clearly evident. 

Real-time infrared spectroscopy (RTIR) (21). The basic principle of this analytical technique consists of exposing the sample simultaneously to the polymerizing UV beam and to the analyzing IR beam, and monitoring on a high speed recorder the sharp decrease of the acrylic absorbance at 812 cm l. Conversion versus time curves have thus been recorded for the first time for photopolymerizations that develop extensively in a fraction of a second (211. If the reaction time drops into the millisecond range, a transient memory recorder (221 or an oscilloscope with storage function can be used to shorter the time resolution further. 

Figure 17 Performance of real-time infrared spectroscopy for kinetic analysis of high-speed photopolymerization reactions...

Thus, the applicability of real time infrared spectroscopy for monitoring high-speed photopolymerizations allows determining the rate of polymerization and the final monomer conversion. This can be used with either thin or ttiick samples exposed to intense polychromatic radiations. 

The photoinitiated polymerizations were followed by real-time infrared spectroscopy on thin films, radiation. The rates of polymerization were reported by them to increase with the light intensity according to a nearly square root law, up to an upper limit. The upper limit or the saturation effect was attributed by them to a fast consumption of flie photoinitiator under intense illumination. A strong correlation was found to exist between flie rate at which the temperature increases and fire rate of polymerization. The temperature shows the same light intensity dependence as the reaction rate, and levels off to a maximum value under intense illumination. Photopolymerization experiments carried out at a constant temperature of 25"C show that thermal runaway is not responsible for the increase of the polymerization rate observed at the beginning oftheUV exposure. 

Decker, C. and Moussa, K., A new method for monitoring ultrafast photopolymerization by real-time infrared spectroscopy, Makromol. Chem., 1988,189, 2381-2394. 

Photoinitiated Cationic Polymerization. The cationic photopolymerization of the monomers synthesized above was studied using real-time infrared spectroscopy (RTIR).i This technique involves monitoring the decrease of an IR absorption characteristic of the functional group undergoing polymerization. In these studies, 2 mol % (4-decyloxyphenyl)phenyliodonium SbF was used as the photoinitiator. Figure 4 gives individual plots of the percent conversion of the various Tg monomers as a function of time at the optimum photoinitiator concentration for each of the monomers. The rate of photopolymerization of 1-propenyl ether functional monomer IX is the fastest followed by III, V and VI. 

Lin Y, Stansbmy JW. Near-infrared spectroscopy investigation of water effects on the cationic photopolymerization of vinyl ether systems. J Polym Sci Part A Polym Chem 2004 42 1985-1998. 

Decker, C. (2005) In situ monitoring of ultrafast photopolymerizations hy real-time infrared spectroscopy. Polym. News, 30, 34. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

This is not the case of real-time infrared (RTIR) spectroscopy, " a technique that permits one to look at the chemical processes by monitoring in situ the disappearance of the monomer reactive group upon UV exposure. By this technique conversion versus time curves have been directly recorded for polymerizations occurring within a fraction of a second. RTIR spectroscopy proved also well suited to study the photopolymerization of monomer mixtures, which leads to the formation of copolymers or interpenetrating polymer networks, as it allows the disappearance of each type of monomer to be accurately followed in the course of the reaction. The performance of the three analytical techniques most commonly used to follow in real time high-speed photopolymerizations are summarized in Table 1. 

Dias et al., used, what they called, a hyphenated rapid real-time dynamic mechanical analysis (RT DMA) and time resolved near-infi ared spectroscopy to simultaneously monitor photopolymerization of acrylate coating compositions. This allowed them to determine the rate of conversion and the mechanical properties of the finished films. It is claimed that up to 374 near infrared spectra and to 50 dynamic analysis points can be accumulated within a second. They observed that modulus buildup does not linearly follow chemical conversion of acrylate bonds. The gel point is detected after passing a certain critical acrylate conversion. Their experimental data revealed a critical dependence of the mechanical property development during the later stage of acrylate conversion. 

Scherzer T, Mehnert R, Lucht H. On-line monitoring of the acrylate conversion in UV photopolymerization by near-infrared reflection spectroscopy. Macromol Symp 2004 205 151-162. 

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