Comment on ‘Absorption of CO 2 and subsequent viscosity reduction of an acrylonitrile copolymer’ by Michael J. Bortner and Donald G. Baird

Abstract

Dimethacrylate monomers of various types are commonly used to form the matrix phase of dental composite materials. Traditional dental composites are reinforced with a filler phase consisting of silanized glass particles of various sizes and shapes. Polymerization of these acrylic resin composites is usually effected by free radical photopolymerization via visible light irradiation. These resin based composites are subject to two significant problems that arise from the rapid photopolymerization of the resin matrix phase: polymerization shrinkage (PS) and stress development (SD) in the polymeric composite, which are related to the chemical structure of matrix resin and its degree of conversion (DC) on photopolymerization. Recent research in our laboratories has focused on similar problems in bioactive resin-based composites that utilize a unique calcium phosphate, amorphous calcium phosphate, ACP, as the filler phase rather than the conventional silanized glass fillers (1). This type of composite is capable of providing a sustained release of calcium and phosphate ions in aqueous milieus such as exists in the oral cavity (2). Significantly, this type of composite has the potential for repairing and preserving contiguous mineralized tissues such as enamel and dentin (3). While there have been some promising improvements recently (3–6) with regard to mechanical properties, the less than optimal PS and SD, along with DC are still considered shortcomings of these as well as conventional glass-filled composites. This study was designed to determine the effects of a new high molecular mass oligomeric urethane dimethacrylate co-monomer (PEG-U) on PS, SD and DC. In the past, urethane dimethacrylate (UDMA), a commonly used dental monomer, has been blended with 2-hydroxyethyl methacrylate (HEMA) in formulating ACP composites, see Fig. 1 for structures of UDMA, HEMA and PEG-U. Specifically, the aim of this study was to assess the effects of the UDMA resin matrix composition on DC, PS, SD and the biaxial flexure strength (BFS) of ACP composites when PEG-U was used in place of HEMA. Fig. 1 Chemical structure of monomers and photo-initiator system. Experimental Formulation of the resins Four types of experimental resins, photoactivated with camphorquinone and ethyl-4-N, N-dimethylaminobenzoate were formulated from the commercially available UDMA, PEG-U and HEMA (Table 1). Chemical structures of the monomers and the components of the photoinitiatior system are shown in Fig. 1. Table 1 Composition of resins (% mass fraction; * based on the equivalent molar concentration of UDMA comonomer). Synthesis and characterization of ACP filler Zirconia-hybridized ACP (Zr-ACP) was synthesized as previously described (3). Its amorphous state was verified by powder X-ray diffraction (XRD: Rigaku X-ray diffractometer, Rigaku/USA Inc., Danvers, MA, USA) and Fourier-transform spectroscopy (FTIR: Nicolet Magna-IR FTIR System 550 spectrophotometer, Nicolet Instrument Corporation, Madison, WI, USA) before the ACP was utilized to make the composite specimens. Morphological/topological features of Zr-ACP were examined by scanning electron microscopy (SEM; JEOL 35C instrument, JEOL Inc., Peabody, MA, USA). Particle size distribution (PSD) of the filler (dispersed in isopropanol and ultrasonicated for 10 min) was determined by gravitational and centrifugal sedimentation analyzer (SA-CP3 model; Shimadzu Scientific Instruments, Inc., Columbia, MD, USA).