Theoretical and Experimental Study of Thymine Based Photoresists Polymers

J VICH BERLANGA; C LUCIANI; D. MARTINO; D. ESTENOZ; G. MEIRA

Viña del Mar, Chile

Simposio; ARCHIPOL 2007 IV Simpósio Binacional de Polímeros Argentino-Chileno; 2007

ARCHIPOL

Styrene-based biopolymers containing ionic and thymine groups chemically attached to the polystyrene structure were experimental- and theoretically investigated. The ionic groups make the polymer watersoluble, eliminating the environmental and health concerns associated with the use of volatile organic solvents as in conventional photoresist processes. When biopolymers are illuminated with UV light, a photocyclization reaction between adjacent thymines takes place, rendering it water-insoluble. The material obtained is called a photoresist. Vinylbenzyl thymine(VBT) was copolymerized in a batch solution process with vinylbenzyl triethylammonium chloride (VBA) or vinylphenylsulfonic acid sodium salt (SSA), using different comonomers ratios and initiator concentrations. The reactions were carried out at 65 C in a batch-stirred tank reactor using 2,2’-azobisisobutyronitrile (AIBN) as initiator and isopropanol as solvent. The synthesis recipes are shown in Table1. Samples were taken along the reactions to determine: (i) monomer conversion by gravimetry, (ii) chemical composition by elemental analysis, and (iii) molecular weight distribution by chromatography (SEC). Fig. 1 shows the evolution of conversion andaverage molecular weights for [VBA]/[VBT]= 4. As expected, the molecular weights decrease during the polymerization. The copolymer composition remains almost unchanged along the reaction and it´s approximately equal to the feed comonomer ratio, indicating similar copolymer reactivities. The obtained copolymers were cured by UV irradiation at room temperature. The addition of sensitizer molecules to irradiate at wavelengths in the visible range (520nm), and the effects of irradiation dose, molecular structure and chemical composition of the polymer on the curing process were investigated. The cured material was characterized by AFM, FTIR, and UV-Vis Spectroscopy. Fig. 2 shows some spectra crosslinked at a given irradiation dose. Polymers with lower SSA content have more thymine groups and as a result undergo more crosslinking. A mathematical model that simulates the polymers synthesis was developed [Casiset al.]. The model enables the calculation of the evolution of the reagent and product concentrations and the molecular weight distribution of the copolymer.Theoretical predictions are in good agreement with measurements (see Fig. 1). Also, for the curing process, a recursive statistical model was presented to compute the evolution of the weight-average molecular weight (mw) before gelification, the gel point, and the evolutionof the weight fraction (ws) of soluble material after gelification (Fig. 3).