Teresa G. Nunes

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.

Synthesis of novel polyurethanes from sugars and 1,6-hexamethylene diisocyanate

Abstract Synthesis of two new polyurethanes was accomplished by reaction of 1,6-hexamethylene diisocyanate with the sugars methyl 2,6-di-O-pivaloyl-α- d -glucopyranoside (1) and methyl 4,6-O-benzylidene-α- d -glucopyranoside (2) catalysed by 1,4-diazabicyclo[2.2.2]octane. The polymers 4 and 5 obtained were characterised by physical and spectroscopic methods.

The hardening of Portland cement studied by NMR stray-field imaging

Hydration and hardening processes of Portland cement (type I) were studied by analysis of the one-dimensional projections (profiles) obtained periodically with the stray-field imaging technique over two days. The influence of additives, such as gypsum, in Portland cement (type IA) was also investigated. The decay of the signal intensity as a function of time was found to be bi-exponential for type I and mono-exponential for type IA.

\23Na,27Al and31P NMR and X-ray powderdiffraction study of Na/Ca/Al phosphate glasses and ceramics

The techniques of X-ray powder diffraction and solid-state magic angle spinning (MAS) 23 Na, 27 Al, and 31 P nuclear magnetic resonance (NMR) spectroscopy have been combined to investigate the speciation in a series of glasses and glass ceramics of general formula (P 2 O 5 ) 0.45 (CaO) 0.24 (Na 2 O) 0.31-x (Al 2 O 3 ) x ,x=0.0–0.05. The principal phosphate species are shown to be various P 2 O 7 4- containing phases, and cylic trimetaphosphates bridged by Ca, i.e. Na 4 Ca(PO 3 ) 6 (instead of open-chain metaphosphates). Higher concentrations of Al 2 O 3 result in glass ceramics which are phosphate-depolymerised (Q 2 →Q 1 ) with respect to the parent glasses. At a lower level of Al 2 O 3 (2%) the aluminium is present in octahedral coordination, while the higher level (5%) results in the formation of tetrahedrally coordinated aluminium. X-Ray powder diffraction of the ceramic with the higher aluminium content indicated the presence of Na 5 Ca 2 Al(PO 4 ) 4 , and the 31 P NMR spectrum provides evidence for Q 1 2 species similar to phosphorus in aluminium metaphosphate. The more detailed structural information available from the aluminium-free glass ceramic, and the similarity in the Q 1 /Q 2 ratio between the glass and its derived ceramic leads to the thesis that the ceramic structure may, in favourable cases, be used to model phosphate speciation in the glass.

Stray-field imaging of quadrupolar nuclei of half integer spin in solids.

A report is presented on the observation of Hahn echoes from the following quadrupolar nuclei of half integer spin (I) in polycrystalline solids in the large static magnetic field gradient (37.5 T/m) which exists in the fringe field of a superconducting solenoid: 7Li, 23Na, 11B, 65Cu (I = 3/2); 27Al (I = 5/2); 51V, 59Co (I = 7/2); and 115In (I = 9/2). 23Na echo-trains from NaCl (with non-selective excitation) and from Na2SO4 (with selective excitation) are compared quantitatively for two different RF pulse sequences: 90x-(tau-90y-tau-echo-)n and 90x-(tau-90x-tau-echo-)n. The signals obtained from RF pulses corresponding to non-selective 90 degrees pulses were shown to be quantitative, whereas in the selective case smaller signals were obtained since only the central transition contributed. The loss of signal from this cause can be distinguished from small signals resulting from low density of nuclei by use of the second sequence. A 7Li image obtained from LiF in a cylindrical glass-vial is shown.

The study of a commercial dental resin by 1H stray-field magnetic resonance imaging

Abstract A dentine/enamel resin containing methacrylate monomers, included in a new generation adhesive system, was used to evaluate the potentialities of recent nuclear magnetic resonance imaging (MRI) techniques to obtain spatially-resolved information on photo-polymerization reaction and subsequent polymerization shrinkage. 1 H stray-field (STRAFI)-MRI one-dimensional images (1D profiles) of visible-light cured resins were obtained in the presence of oxygen from the atmosphere, and the variation of magnetization with irradiation time was recorded for each resin slice. The polymerization shrinkage was obtained from 1D profiles. The spatial distribution of the unreacted methyl methacrylate groups was obtained from 3D STRAFI experiments. In particular, the thickness of the surface remaining unpolymerised was measured.

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.

The Structure of Hybrid Gels by Drift and NMR Spectroscopies

Hybrid inorganic-organic gels have been prepared by the sol-gel process using tetraethoxysilane (TEOS) as precursor, mixed with a low concentration of polytetrahydrofuran (PTHF), under acid catalysis. The hybrid xerogels were characterized by DRIFTS and Solid State 1H, 13C and 29Si NMR. The DRIFT spectra indicate that the polymer is responsible for decreasing the number of free silanol groups in comparison to pure silica. Solid-state NMR spectra reveal the types of silicate structures formed and the conditions for establishing chemical bonds between the two phases, which are responsible for the silica network flexibility. We have concluded that it is possible to design a hybrid gel with tailored properties, even at very low polymer concentration, by selecting the appropriate preparation route.