FTIR Analysis of Expanding Monomer and Dioxirane Photopolymerizations

A major problem with current dental composites is shrinkage upon polymerization with subsequent internal stress development. New expanding monomer/oxirane-based systems are being developed. Objectives: In order to begin to understand their polymerizate structure, the purpose of this study was to identify characteristic FTIR spectral changes occurring during visible light polymerization of candidate materials, and to estimate the relative rates of oxirane and spiral ring-opening. Methods: Monomers:diglycidyl ether of bisphenol A (EPON^TM 825);  bis(methyl oxiranylcyclo hexylmethyl) tetraoxaspiro[5.5]undecane BMOCHM; Solvent: methylene chloride; Photoinitiatior system: diaryl iodonium hexafluoroantimonate salt photoacid (PI); camphorquinone (CQ) photosensitizer; ethyl (4-dimethylamino)benzoate (EDMAB) electron donor. Homopolymerizations were carried out at 20 wt % in methylene chloride using visible light irradiation (3M XL 2500 dental curing light; 8 min @ 940 mW/cm²) at 26 ^oC. Samples taken at selected intervals up to the gel point were transferred to disposable FTIR cards. Spectra were compared to preirradiation spectra. For EPON^TM 825, an absorbance band at 1608 cm^-1 (aromatic ring) was used to ratio the oxirane band at 915 cm^-1.  For BMOCHM changes in the OH region (3522-3418 cm^-1), C=O region (1754-1747 cm^-1), and oxirane region (905 cm^-1), were ratioed against a C-H region band (2842 cm^-1). Results: EPON^TM 825 (PI/CQ/EDMAB=1.5/0.5/0.1 wt %) demonstrated 3-5 % oxirane conversion after 8 min irradiation and 35 % oxirane conversion @ 108 min post irradiation (gel point). BMOCHM (PI/CQ/EDMAB=1.5/0.5/0.1 wt %) showed no reaction; at (PI/CQ/EDMAB=3/1/0.1 wt %) there was 65 % oxirane conversion after 8 min irradiation with concomitant increases in OH, and C=O absorptions consistant with spirocyclic and oxirane ring opening. Conclusions: This first FTIR study of visible light solution homopolymerization of these monomers provides a means for identifying conversion rates so that oligomeric units can be prepared and identified in future studies. eickj@umkc.edu