A Mechanistic Study of the Interaction of Ultraviolet Laser Radiation with Low Density Polymers
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A MECHANISTIC STUDY OF THE INTERACTION OF ULTRAVIOLET LASER RADIATION WITH L0W DENSITY POLYMERS Philip J. Hargis, Jr. Sandia National Laboratories, Albuquerque, NM87185-5800 ABSTRACT Ultraviolet laser radiation at 248 nm and 266 nm was used to ablate low density (0.04 to 0.14 g/cm') microcellular polystyrene and TPX foams at 2 laser fluences between 0.1 and 1000 J/cm , The dependence of the etch rate on laser fluence and foam density is consistent with a linear absorption mechanism for the initial stages of polystyrene ablation. Studies of the morphology and chemical composition of the ablated foams as a function of laser fluence and foam density show that the degree of melting increases at the higher foam densities. Blackbody and radical specie contributions to optical emission from the ablated foam were readily resolved for correlation with laser fluence, foam density, absorption spectra, and etch depth. Temperatures in excess of 3000 0 K are calculated from emission spectra observed during the ablation of polystyrene foams. From this data we conclude that chemical and thermal changes in the ablated foam are influenced by the absorption spectrum as well as the density of the foam. INTODCION The recent development of techniques to fabricate microcellular polystyrene [1] and TPX, poly(4-methyl-l-pentene), [2,3] foams with densities between 0.01 and 0.20 g/cm3 has led to the investigation of their use in applications as diverse as artificial skin and blood vessels, inertial-confinement fusion targets, laboratory X-ray laser media, filters, catalytic substrates, and model porous media. The successful development of many of these technologically important applications requires shaped foams with features comparable to the 1 to 20 micron cell size of the foam. Since these new materials are fragile and easily deformed, they are difficult to shape using conventional machining techniques. Ultraviolet laser ablation [4] is an attractive technique for obtaining the required feature sizes in these new low density materials. This paper presents results showing that the chemical composition of the ablated foams, as determined by XPS, RaS, and laser Raman measurements, as well as the morphology, as determined by SEM measurements, is influenced by the ultraviolet absorption spectrum of the foam. The availability of chemically homogeneous foams with densities between 0.04 and 0.14 g/cm3 made it possible to correlate chemical and thermal changes with the density of the foam. The effect of density on optical emission spectra from the ablated foam was also investigated. Results at laser ablation wavelengths of 248 nm and 266 nm are reported in this paper. EXPERIMENTAL DETAILS A krypton fluoride excimer laser (Lambda-Physics Model EMG 150-ET) operating at a wavelength of 248 nm with a pulse length of 20 nsec FWHM and a frequency-quadrupled Nd:YAG laser (Quanta-Ray Model DCR-lA) operating at a wavelength of 266 nm with a pulse length of 5 nsec FWHM were used to study the ablation properties of low density polystyrene and TPX foams. Eac
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