Polymerization laser-induced

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. 

Both the benzoyl and the methyl radicals react with the double bond of the monomer and thus initiate the polymerization. Other types of photoinitiators, like 1-benzoylcyclohexanol or 2,2-dimethyl-2-hydroxyacetophenone, were shown to be as efficient as DMPA in initiating the polymerization of acrylate monomers (15,16) however, their absorption in the near UV is less pronounced so that the overall rate of the laser-induced polymerization is substantially... 

The kinetics of the laser-induced polymerization was followed either by measuring the thickness of the insoluble polymer film formed on the quartz plate after laser exposure and solvent development, using UV spectroscopy or by monitoring the decrease of the IR absorption of the coating at 810 cm-1 which corresponds to the twisting vibration of the acrylate CH2=CH double bond. This last method permits accurate evaluation of the rate of polymerization (Rp) by observing the variation of the 810 cm-1 band, and using Equation 2 ... 

Under UV-laser irradiation, photosensitive multifunctional acrylate resins become rapidly cross-linked and completely insoluble. The extent of the reaction was followed continuously by both UV and IR spectroscopy in order to evaluate the rate and quantum yield of the laser-induced polymerization of these photoresist systems. Two basic types of lasers emitting in the UV range were employed, either a continuous wave (C.W.) argon-ion laser, or a pulsed nitrogen laser. 

Figure 6. Dependence of the polymerization quantum yield ( ) on the light-intensity (Iq) in the laser-induced polymerization of epoxy-acrylate photoresists (— expected variation of on I0 for a half-order kinetic law).

A number of nonsteady polymerization rate techniques can be used to measure ftp [11]. The most widely used method involves pulsed-laser-induced polymerization in the low monomer conversion regime. Briefly, a mixture of monomer and photoinitiator (Section 6.5.3) is illuminated by short laser pulses of about 10 ns (10 sec) duration. The radicals that are created by this burst of ligh propagate for about 1 sec before a second laser pulse produces another crop of radicals. Many of the initially formed radicals will be terminated by the short, mobile radicals created in the second illumination. Analysis of the number molecular weight distribution of the polymer produced permits the estimation of ftp from the relation... 

UV-Radiation- and Laser-Induced Polymerization of Aciylic Monomers... 

Table n. Performance analysis of various methods of kinetic investigation of UV radiation or laser-induced polymerization... 

Keywords conversion dependence laser-induced polymerization methacryiic acid ... 

Experimental details of pulsed-laser induced polymerizations of methacrylic acid (MAA) in aqueous solution with poly(MAA) being added. The overall concentration of MAA + poly(MAA) is 20 wt.-% in all experiments. Pulsed-laser polymerizations were performed at 25 °C and ambient pressure using a photolnitlator (DMPA) concentration of Cdmpa = 2 mmol L and a laser pulse repetition rate of 40 Hz. Listed are the virtual degree of monomer conversion, X in uai. the MAA concentration, c aa. the number of applied laser pulses, the molecular weight (MW) at the first point of inflection (POI), M and the ratio of MWs at the first and second POI. The final column contains the resulting propagation rate coefficients, kp. 

Nakamoto, T, Yamaguchi, K., Abraha, P.A., and Mishima, K. (1996) Manufacturing of three-dimensional micro-parts by UV laser induced polymerization. J. Micromech. Microeng., 6 (2), 240-253. 

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