Sónia G. Pereira

Effect of curing protocol on the polymerization of dual-cured resin cements

OBJECTIVES The purpose of this study was to evaluate how curing protocol affects the extent of polymerization of dual-cured resin cements. METHODS Four commercial resin cements were used (DuoLink, Panavia F 2.0, Variolink II and Enforce). The extent of polymerization of the resin cements cured under different conditions was measured using a (1)H Stray-Field MRI method, which also enabled to probe molecular mobility in the kHz frequency range. RESULTS Resin cements show well distinct behaviours concerning chemical cure. Immediate photo-activation appears to be the best choice for higher filler loaded resin cements (Panavia F 2.0 and Variolink). A photo-activation delay (5 min) did not induce any significant difference in the extent of polymerization of all cements. SIGNIFICANCE The extent of polymerization of dual-cured resin cements considerably changed among products under various curing protocols. Clinicians should optimize the materials choice taking into account the curing characteristics of the cements.

Low viscosity dimethacrylate comonomer compositions [Bis-GMA and CH3Bis-GMA] for novel dental composites; analysis of the network by stray-field MRI, solid-state NMR and DSC & FTIR.

The effect of dilution of 2,2-bis (4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl) propane (Bis-GMA) with 2,2-bis(4-(2-methacryloxyprop-1-oxy)phenyl) propane (CH3Bis-GMA) on extent of polymerization (Ep) was investigated by FTIR and NMR. The results correlate well with Ep values of the system and its flexibility probed by Tg (obtained by DSC). Spatially resolved photopolymerization kinetics and polymerization shrinkage (PS, i.e. volumetric polymerization contraction) was also studied by stray-field imaging (STRAFI-MRI). PS was obtained in the presence of oxygen from the atmosphere as the total volumetric contraction. All the results were compared with those obtained for the Bis-GMA/TEGDMA (triethylene glycol dimethacrylate) system. Use of Bis-GMA mixed with hydrophobic low viscosity CH3Bis-GMA comonomer, as a substitute for the commonly used TEGDMA, resulted in improving significantly properties such as PS, water sorption and Ep, thereby reducing the unreacted double bond concentration.

Homo- and copolymerisation of norbornene and styrene with nickel bis(acetyl acetonate)/methylaluminoxane system

Abstract The homo and copolymerisations of norbornene and styrene with nickel bis(acetyl acetonate)/methylaluminoxane system were systematically investigated. This catalytic system shows a high activity towards homopolymerisation of both norbornene and styrene. For random copolymerisation, an increase in the initial styrene feed content leads to a gradual loss of activity relative to norbornene homopolymerisation. On the other hand, a drastic loss of activity, relative to styrene homopolymerisation, was found for very low norbornene feed contents. These results are qualitatively interpreted using the trigger coordination mechanism proposed by Ystenes. The structural characterisation of the polymers showed that norbornene polymerisation occurs via a 2,3 addition mechanism and that true copolymers are formed by random copolymerisation. At low styrene feed contents, only isolated styrene units or very short styrene sequences are present in the resulting copolymer. At higher styrene feed contents, short polystyrene sequences with more than eight styrene units may be formed. Determination of the reactivity ratios shows a much higher reactivity for norbornene (rnorbornene=17.8 and rstyrene=0.16). Size exclusion chromatography measurements have shown that polynorbornenes possess high molecular weights. Relatively low molecular weights were observed for homopolystyrene and copolymers.

Metallocene-catalysed copolymerisation of ethylene with 10-undecenoic acid : The effect of experimental conditions

The copolymerisation of ethylene with 10-undecenoic acid (UA) using the metallocene catalyst system Cp 2 -ZrCl 2 /MAO was achieved. A reaction between the comonomer and MAO, prior to the copolymerisation, was used in order to protect, to some extent, the carboxylic group. The degree of MAO protection was found to be dependent on the Al/UA ratio used in the precontact mixture. The interaction between the two compounds was investigated by means of NMR spectroscopy. The influences of comonomer concentration in feed, Al/Zr moleratio and polymerisation temperature on the copolymerisations, were investigated with respect to catalyst activity and to UA incorporation. Higher concentrations of UA in feed favour the incorporation rate but decrease the activity. The Al/Zr more ratio shows a strong influence on activity, but not on the comonomer incorporation. Both the polymerisation activity and the UA incorporation rate present an optimal polymerisation temperature of around 40°C. DSC characterisation shows a decrease in the melting temperature of the polymers for increasing UA incorporations. Bimodal melting peaks, representative of polymer inhomogeneity, were observed for the highest UA incorporations.

1H stray-field long spin-echo trains and MRI: novel studies on the photopolymerization of a commercial dental resin

Photopolymerization of a commercial dental resin has been investigated by 1H stray-field (STRAFI) magnetic resonance. The resin is a visible light-cured system, included in a new generation adhesive, which is used to bond the restorative material to enamel or dentin. Different methods were used to follow the curing reaction, which involve long and short spin-echo train acquisitions to obtain one-slice and one-dimensional data, respectively. The echo attenuation, in the limit of very short time delays, could be described as the sum of two exponentials. While the intensity of the early echoes in the train appeared mainly governed by spin-spin relaxation, the decay of the last echoes seemed to depend also on spin-lattice relaxation in the rotating frame. The relative amplitude of the long-time component was found to decrease from 84% to 10% with the photopolymerization progress, and a STRAFI degree of conversion of 74% could thus be suggested. The influence of the curing protocol was observed in STRAFI profiles.

Novel Bioadhesives for Restorative Dentistry

The objective of this study was to develop new adhesive formulations based on 2,2-bis[4-(2- methacryloxyprop-1-oxy) phenyl] propane (CH3-BisGMA) aiming to obtain high double bond conversion (DC) even in the presence of radical scavengers and monomers with acidic groups. NMR, EPR spectroscopy and spatially-resolved 1H Stray-Field MRI observations enabled to study the influence of the composition in DC, free radical concentration, reactivity of the monomers and volumetric contraction.