Synthesis and characterization of UV curable polyvinylsilazane

Abstract

Polyvinylsilazane (PVSZ) is a well known ceramic precursor for the fabrication of high performance ceramic structures which are involved in harsh environments. The shaping process for ceramic structures usually involves thermal or photo curing techniques with the aids of polydimethylsiloxan (PDMS) molds to transform liquid preceramic polymer into well-shaped solid structures. Thermal curing of PVSZ needs more than 60 similar to 120 min at about 100 degrees C and photo curing needs about 20 min at room temperature. Both these two curing methods are very hard to get fine microstructures because of the absorption or adhesion between the surface of preceramic polymer and PDMS molds. In order to develop a preceramic photoresist based on PVSZ for the fabrication of non-oxide SiCN ceramic microstructures by a mold-free fast photocuring shaping process, UV sensitive acrylate functional groups were successfully grafted onto the backbone of PVSZ by its reaction with ethyl-4-bromocrotonate (EBC) via a high efficiency allyl bromide electrophilic substitution process. In a typical reaction, 3 g of FBC was added drop-wisely into 4 g of PVSZ dissolved 20 mL of toluene. The mixture was heated in an oil bath at 90 degrees C for 72 h with continuously magnetic bar stirring. Finally, the white HBr precipitate was removed by high speed centrifugal treatment and a filtration process; the solvent and unreacted precursor was subsequently removed by vacuum pump. All of the processes were carried out in an inert gas atmosphere to avoid exposure to moisture, utilizing standard Schlenk techniques. The as-modified polymer was characterized by H-1 nuclear magnetic resonance (H-1-NMR) and 2D-H-1-H-1-NMR (COSY) spectroscopy, and its UV sensitivity was investigated by differential photocalorimetry (DPC) and Fourier transform infrared (FT-IR spectroscopy. The samples for photosensitive checking were prepared by an addition of 5 wt% of photoinitiator IRGACURE369. The results of H-1-NMR pointed out that the hydrogens on the NH groups of PVSZ at delta = 0. 65 were obviously consumed after chemical modification, and there was a new peak at delta = 2.23 on the H-1-NMR spectra of m-PVSZ which was assigned to the hydrogen of (CH2-N) The results of 2D-H-1-H-1-NMR gave out further evidence of the newly appeared chemical bond (CH2-N). The peak at delta = 2.23 (CH2-N) was correlated to the peak at delta = 7.01 (-CH=CH-), which had confirmed that there was a group like (N-CH2-CH=CH-) on m-PVSZ. The new chemical bond of (CH2-N) strongly supported the chemical modification of grafting UV sensitive acrylate functional groups onto the backbone of PVSZ. The results of DPC pointed out that 90 % UV curing of m-PVSZ could be finished within 60 s with a peak heatflow of 1.18 w/g; however, it would take about 20 min for PVSZ for curing at the same conditions as m-PVSZ. The results of TGA pointed out that the chemical modification had some negative effects on its ceramic yield, the final ceramic yield of m-PVSZ at 1000 degrees C was 55.34 % and that was 63.13 % for PVSZ. All the above results showed that m-PVSZ is a promising candidate of negative photoresist for the fabrication of micro ceramic structures by photolithography. The dramatically increased UV curing speed would be good for the fabrication of fine ceramic microstructures, which is still under progress in our group.