Characterization of Cross-linking Structures in UV-cured Acrylic Ester Resin by MALDI-MS Combined with Supercritical Methanolysis

Abstract

IntroductionUltraviolet (UV) curable acrylic esters are applied to many industrial products such as printing inks, coatings for optical fibers and disks, adhesives, dental materials, photoresist, and three-dimensional stereolithography. [1][2][3][4][5] In order to utilize these in advanced technology, it is often requested to elucidate the cross-linking network structure of the resulting cured resins by UV-irradiation. However, it is not an easy task to analyze cured resins even using advanced spectroscopic methods mainly because of their insoluble nature. Recently, the authors characterized the chain length distribution of network junctions in UV-cured poly(ethylene glycol) diacrylate (PEDA) using pyrolysis-gas chromatography in the presence of tetramethylammonium hydroxide. 6 In this case, methyl acrylate (MA) oligomers formed through thermally assisted hydrolysis and methylation selectively occurred at ester linkages in UVcured PEDA were attributed to the sequences of the network junctions in the cross-linking structure. The chain length distribution of the network junctions composed of up to six acryloyl units in the UV-cured PEDA was estimated from the peak intensities of the corresponding MA oligomers observed in the pyrogram. However, the network structure containing longer sequences comprised of more than seven acryloyl units was not elucidated because the corresponding higher MA oligomers were not observed in the pyrogram, mainly due to their lower volatility.Meanwhile, Burkoth et al. evaluated the kinetic chain length of the network junctions of UV-cured dimethacrylated sebacic anhydride through matrix-assisted laser desorption/ionizationmass spectrometric (MALDI-MS) measurements of its decomposition products. 7 This polymer was readily hydrolyzed in phosphate buffered saline at pH 7.4 and 37˚C during several hours into sebacic acid and poly(methacrylic acid). They observed that poly(methacrylic acid)s reflected the sequences of network junctions in the range between pentamer and 24-mer in the MALDI mass spectra. This approach, however, was limited for the loosely cross-linking polymer composed of the hydrophilic monomers and oligomers suitable for biodegradable applications. Such materials should be much easier to be hydrolyzed than practical UV-cured resins in the industrial fields with rigidly cross-linking structure essential for high chemical resistance and heat resistance.On the other hand, supercritical fluids such as supercritical methanol have been utilized to depolymerize intractable polymers such as poly(ethylene terephthalate) (PET) 8 The cross-linking structure of the ultra violet (UV)-cured resin prepared from dipentaerithritol hexacrylate (DPHA) was characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) combined with supercritical methanolysis. The MALDI-mass spectrum of the decomposition products obtained by supercritical methanolysis contained a series of peaks of sodium-cationized methyl acrylate (MA) oligomers up to around m/z = 4000 formed through...

Characterization of network structure in radiation-cured resins of polyfunctional acrylic ester and N-vinylpyrrolidone by MALDI-MS combined with supercritical methanolysis

“…The procedure for preparing UV-cured resin samples is basically the same as that described in our previous papers. 11,14 Prepolymer was compounded by dissolving the photoinitiators into the…”

“…The apparatus and procedure for supercritical methanolysis are almost the same as those in our previous paper. 14 The autoclave was made of a stainless steel (SUS 316) tube (1/4 inch F, ca. 7 cm long) with the inner volume of 1 ml, of which both ends were sealed with Swagelok caps.…”

“…The reaction temperature and time were empirically optimized by using size exclusion chromatographic measurements to attain higher decomposition efficiency with minimizing pyrolytic cleavage of C-C bonds in the polymer chains. 14 Under these conditions, methanol in the reaction tube was in supercritical state over the critical point.…”

“…In the case of supercritical methanolysis of UV-cured DPHA, methyl acrylate (MA) oligomers and/or polymers were mainly formed through selective cleavage and methylation at ester linkages in the cured resin. 14 Provided that supercritical methanolysis of the radiation-cured PETA/VP resins proceeds in the same manner as that of UV-cured DPHA, selective cleavage and methylation should occurr at ester linkages in the resins. Scheme 2 illustrates the most probable decomposition pathway of the radiation-cured PETA/VP resins in supercritical methanol.…”

“…Recently, in order to overcome this limitation, the authors evaluated the kinetic chain length of the network junctions of UV-cured acrylic ester resins prepared from dipentaerythritol hexacrylate (DPHA) by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) combined with supercritical methanolysis. 14 In this case, UV-cured DPHA was selectively decomposed at the ester linkages to give poly(methyl acrylate)s (PMAs) reflecting cross-linking sequences, which were observed in the MALDI-mass spectrum of the decomposition products as their sodium-cationized molecules. Furthermore, collecting a size exclusion chromatographic fraction of the products followed by MALDI-MS analysis enabled us to evaluate almost the whole range of molar mass distribution of PMA reflecting the widely distributed sequences of the network junctions.…”