Frank Millich

Elements of Light-cured Epoxy-based Dental Polymer Systems

The greatest problem with current dental composite systems is their polymerization shrinkage. Extensive work is being done by many investigators to alleviate this problem. Our approach has been to examine epoxy- and spiro-orthocarbonate (SOC)-based resins. The hypothesis to be tested in this study was that the cure characteristics of experimental visible-light-cured epoxy resin systems are governed by the types and concentrations of co-reactants and activators. Resin samples containing onium salt initiators and a thiozanthone sensitizer were successfully cured by means of either an experimental visible-light irradiation system or a commercially available dental lamp. Test resins consisted of di-epoxies alone or in combination, epoxy mixtures in combination with an SOC, or an epoxy in combination with a caprolactone-derived polyol. Significant findings were as follows: (a) Resins containing the SOC had longer cure times than their counterparts; (b) the optimum ratios of epoxy to polyol for most rapid cure were 50:50 or 60:40 under conditions tested; (c) resins containing TONE 305 polyol generally were faster to cure than those containing no polyol, or TONES 201 or 310; and (d) a resin mixture was found that had a cure time of 1 to 3 min when irradiated with a commercial dental lamp. Based on this exploratory study, it should be possible for clinically relevant cure times to be achieved for visible-light-cured epoxy-based resins by careful manipulation and optimization of key elements.

Design and Development of Isocyanatoacrylates as Dental Adhesives

During the last 12 years, significant progress has been made in the development of dental adhesive systems. Some of the more promising systems are based on multifunctional structures that contain polymerizable vinyl double bonds and reactive isocyanate groups. The utility of compounds with such structures as adhesives arises in part because their isocyanate functionality is available for reaction independently, without compromising the reactivity of the vinyl groups. The hypotheses tested in this investigation were: (1) that the monomer reactivity ratios (r1, r2) for the free-radical-initiated copolymerization of ethyl a-isocyanatoacrylate (a-EIA) and 2-isocyanatoethyl methacrylate (IEM) with selected vinyl monomers can be determined; (2) that these reactivity ratios can be used to establish Q (reactivity) and e (polarity) values for a-EIA and IEM; and (3) that these reactivity parameters can be useful in designing copolymers with controlled compositions for dental adhesive applications. The free-radical copolymerization characteristics of a-EIA and IEM were studied. The isocyanate monomers were copolymerized at seven comonomer ratios with n-butyl acrylate (NBA), methyl methacrylate (MMA), and styrene (STY). Reactivity ratios, r1 and r2, were calculated for each of the copolymer systems, giving: IEM (r1) = 0.38 and STY (r2) = 0.44; IEM (r1) = 1.19 and MMA (r2) = 0.84; IEM (r1) = 2.50 and NBA (r2) = 0.40; a-EIA (r1) = 2.20 and STY (r2) = 0.06; α-EIA (r1) = 7.00 and MMA (r2) = 0.10; and a-EIA (r1) = 23.50 and NBA (r2) = 0.04. The Q (reactivity) and e (polarity) values for IEM and a-EIA were calculated from r1 and r2 with use of the Alfrey-Price equations, giving, for IEM, Q = 0.89 and e = 0.60, and, for a-EIA, Q = 7.64 and e = 0.74. These reactivity parameters are useful for tailoring copolymers with controlled compositions and properties. Based on these calculated reactivity parameters, several copolymers of IEM [for example, IEM/2-hydroxyethyl methacrylate (HEMA)] are currently being prepared and evaluated as adhesives.

Adhesion and hydrophilicity of glow‐discharge‐polymerized propylene coatings

The glow‐discharge polymerization of propylene has been studied using an electrodeless electromagnetically coupled (inductive coupling perpendicular to the reactor) tubular reactor operating at a radio frequency of 27.1 MHz. The effects of monomer flow rate (or reactor pressure at constant pump speed), polymerization time, and substrate position on the adhesion of polymeric film to glass and aluminum have been determined. Conditions have been found for excellent adhesion to a substrate. Improvement of the hydrophilicity of the glow‐discharge‐polymer surface has been achieved by oxygen etching.

Auto- and Induced Stereoregulation of Polyisocyanides

Polyisocyanides, \(\matrix{ {({\rm{ - C - }})} \cr {||{\rm{n,}}} \cr {{\rm{N - R}}} \cr } \) represent a class of polymers with unique primary structure that sets them apart from conventional polymers. Their polymeric properties in several ways are unique in kind or degree, and their exploration has already, and further promises, to broaden our experiences and understanding of macromolecular chemistry and physics. These are the subjects of some recent reviews (1–5). Poly(α-phenylethyl isocyanide), α-PPEI, is the progenitor member of the polymer class, being the first and most characterized of the few currently known soluble polyisocyanides of high molecular weight. It assumes a rigid-rod conformation, and it yields liquid crystals in solution. Most polyisocyanides are strongly aggregated solids and are insoluble in solvents other than strong acids, wherein they are protonated and show time-dependent viscosity changes. This phenomenon has led to a new viscosity concept of isoviscosity and isohydrodynamic volume (6), which has recently received some independent support (7,8). Polyisocyanides form racemic mixtures of complete stereoregular helices, and give evidence of dipole coupling of the vicinal imine groups. Hence, polyisocyanides are proposed as models of fixed dimensional coordinates for future investigation of optical, electrical and solution thermodynamic properties.