Jacques Devaux

The micro-Raman spectroscopy, a useful tool to determine the degree of conversion of light-activated composite resins.

Light-activated composites are now among the most popular dental restorative materials. Nevertheless, concerns exist about the so-called depth of cure. Infrared spectroscopy (FTIR) has traditionally been used to quantify this problem by evaluating the degree of conversion of dental resins. However, Raman scattering provides an alternate method. This article describes the advantages and the limitations of micro-Raman spectroscopy, as compared to FTIR and other techniques, for calculating the local degree of conversion and the depth of cure of light-cured composites.

On the Molecular-weight Determination of a Poly(aryl-ether-ether-ketone) (peek)

Abstract Attempts have been made to determine the weight average molecular weight ( M W ), the radius of gyration ( R G ) and the second virial coefficient (A2) of five samples of poly(ether-ether-ketone) (PEEK) by light scattering (LS) in concentrated sulphuric acid. Account has been taken of the sulphonation of the polymer. Correlations between LS-molecular weights, melt viscosities and intrinsic viscosities in sulphuric acid or in a 50 50 phenol-trichlorobenzene mixture have been established. Gel permeation chromatography (g.p.c.) analyses at 115°C have been performed in this latter solvent and two calibration methods were compared.

Investigating filler morphology and mechanical properties of new low-shrinkage resin composite types

Three types of low-shrinkage composites are today commercially available: Ormocers, cationic ring-opening curing systems and highly filled methacrylate-based materials, which cure via free-radical polymerization mechanisms. The aim of this study was to characterize the inorganic fraction of materials belonging to each type and to compare their mechanical properties. Two Ormocers (Admira and an experimental Ormocer V35694), one ring-opening composite (Filtek Silorane) and five methacrylate-based composites [Filtek Supreme XT, Tetric EvoCeram, Grandio, Synergy D6 (Coltène-Whaledent, Langenau, Germany) and an experimental material, V34930] were tested. Inorganic fillers were quantified by thermogravimetric analysis and morphologically characterized by scanning electron microscopy. Dynamic modulus was determined by an impulse excitation technique, static elastic moduli and flexural strength by a three-point bending method. The results were analyzed using ANOVA tests (P < 0.05) and linear correlations. Grandio, V34930 and V35694 exhibited significantly higher filler mass fractions. Both dynamic and static moduli of Grandio and V34930 were significantly higher than the other materials (P < 0.05), although no significant difference in flexural strength was observed between material type (P > 0.05). From the present findings, it was suggested that V35694 and Filtek Silorane exhibit comparable properties to conventional methacrylate-based composites, although clinically the cavity type and location must guide material choice. Under high occlusal load, the use of Grandio and V34930 might be favoured. For small cavities, alternative technologies could be preferred as the need for mechanical resistance is lower and the potential for stress generation is greater.

Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins.

OBJECTIVES To test the influence of photoinitiator type and filler particle inclusion on the validity of exposure reciprocity law. MATERIALS AND METHODS 50/50 wt% Bis-GMA/TEGDMA resins were prepared with equimolar concentrations of camphorquinone/DMAEMA (0.20/0.80 mass%) (CQ) or Lucirin-TPO (0.42 mass%), and were used either unfilled or filled to 75 mass%. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using four different curing protocols: 400 mW/cm² for 45 s as reference protocol (18 J/cm²), 1500 mW/cm² for 12 s (18 J/cm²), 3000 mW/cm² for 6 s (18 J/cm²) and 3 s (9 J/cm²). Degree of conversion (DC) was measured in real time for 70 s by FT-NIRS and temperature rise using a thermocouple. Depth of cure was determined with a penetrometer technique. RESULTS With respect to DC and depth of cure, exposure reciprocity law did not hold for any tested material, except for the depth of cure of filled CQ-based materials. At similar radiant exposure, DC was significantly higher (p<0.05) for all unfilled and filled TPO-based materials compared with CQ-based materials. As exposure time was reduced and irradiance increased, TPO-based materials exhibited higher DC whilst an opposite trend was observed for CQ-based materials (p<0.05). For similar curing regimes, depth of cure of CQ-based materials remained significantly greater than that of TPO-based materials. Adding fillers generally reduced DC, except at higher irradiance for CQ-based materials where a positive effect was observed (p<0.05). SIGNIFICANCE The validity of exposure reciprocity law was dependent on several factors, among which photoinitiator type and filler content were important. Lucirin-TPO is a highly reactive and efficient photoinitiator, which may allow the potential for a reduction in curing time of TPO-based photoactive materials in thin sections.

The anhydride content of some commercial PP-g-MA: FTIR and titration

A set of anhydride-grafted polypropylenes was collected from various companies. They were studied in light of our recent results on polypropylene melt grafted with maleic anhydride.(1) This work confirmed that an important decrease of the anhydride content is recorded on heating or washing, due to the elimination of free, ungrafted products, respectively, by sublimation of maleic anhydride and by polymaleic anhydride solubilization. The deconvolution of the infrared spectra of washed anhydride-grafted polypropylenes (PP-g-MA) revealed two types of grafted anhydride: succinic anhydride form and polymaleic anhydride form. All in all, four forms of anhydride functions were detected: two grafted and two free, each being either on monomeric or polymeric forms. Nevertheless, one PP-g-MA (Hercoprime) can be distinguished by its very high grafting level. This polymer is therefore discussed in more detail. Finally, all the present results are discussed with regard to the principal applications of the PP-g-MA as a blend compatiblizer or as an adhesion promoter

Competitive reactions during compatibilization of blends of polybutyleneterephthalate with epoxide-containing rubber

Blends of polybutyleneterephthalate (PBT) with ethene-ethyl acrylate copolymer (E-EA) and ethene-methyl acrylate-glycidyl methacrylate terpolymer (E-MA-GMA) were investigated. E-MA-GMA terpolymers containing various concentrations of epoxide functions were synthesized in a preliminary step by melt modification of commercial E-MA-GMA with benzoic acid molecules. The blends were analyzed by several techniques including electron microscopy and separation experiments. PBT/E-EA (80/20 w/w) blends presented the general features of uncompatibilized polymer blends, such as a lack of interfacial adhesion and a relatively coarse unstabilized morphology. No evidence of transesterification reaction was found to occur according to the used blending conditions. In contrast, blends containing both virgin and modified E-MA-GMA terpolymers exhibited a very complex behavior. Fractionation experiments demonstrated that two competitive reactions take place during the melt blending viz. (1) compatibilization due to interfacial reactions between PET chains end and terpolymer epoxide groups, resulting in the formation of E-MA-GMA/PBT graft copolymer and (2) rapid crosslinking of the rubber phase due to the simultaneous presence of hydroxyl and epoxide groups on E-MA-CMA chains. This competition between compatibilization and crosslinking is clearly dependent on the type of the terpolymer, since the modified E-MA-GMA already contains hydroxyl groups before mixing but for the pure E-MA-GMA hydroxyl groups are formed as a result of the in situ compatibilization reaction with PET. Rubber phase crosslinking through double reaction with PET chain can not be excluded but is expected to occur only to a smaller extent. All these phenomena result in a very complex processing/morphology interrelationships and probably also affects the final blend properties

Influence of curing protocol on selected properties of light-curing polymers: Degree of conversion, volume contraction, elastic modulus, and glass transition temperature

OBJECTIVES The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. METHODS Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses. RESULTS ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r=0.58), ED and E (r=0.51), and ED and T(g) (r=0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r=0.54), DC and E (r=0.61), and DC and T(g) (r=0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T(g). Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous. SIGNIFICANCE A complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.

Pulpal-temperature Rise and Polymerization Efficiency of LED Curing Lights

This paper assesses the effects of light characteristics and irradiation time on the Vickers microhardness (VH) of a dual-photoinitiator commercial composite and on temperature increase in the pulp chamber (deltaT). Four recent light-emitting diodes (LEDs)--bluephaseG2 (BG2), bluephase16i (B16i), G-Light (G) and Freelight2 (F2)--and one control halogen light (XL3000-X) were tested on two shades of Tetric EvoCeram (A2 and Bleach XL), whose respective commercial formulations differed based on their concentration of camphorquinone and lucirin TPO. Three different irradiation times were applied--10, 20 and 40 seconds-and VH was measured on the upper and lower surfaces of 2-mm thick samples. The deltaT was measured by using a K-type thermocouple inserted into the pulp chamber of a molar that had been prepared to obtain a 2-mm thickness of dentin. The measurements were made either during polymerization of a 2 mm composite (Shade A2 or Bleach) or with an empty mold. The data were analyzed with the two-way ANOVA (p < 0.05) test. For shade A2, all but one irradiation condition (F2-10 seconds, lower surface) generated VH values that were statistically equal to or better than the standard chosen for this study (X-40 seconds). For Bleach shade, the VH values obtained with G and BG2-20 and 40 seconds were statistically comparable to X-40 seconds for both the upper and lower surfaces. This was not the case with either G and BG2-10 seconds or for all the procedures with other LCUs for which a VH of at least one of the surfaces was significantly lower than the reference. The results also highlight differences between the two material shades, whether the upper or lower surface is considered. Regarding temperature measurements for shade A2, B16i-20-40 seconds, BG2-40 seconds and G-40 seconds induced significantly higher deltaTs (3.98, 5.98, 5.21 and 4.95, respectively) than X-40 seconds (3.09). For Bleach shade, B16i-20 and 40 seconds, F2-20 and 40 seconds, BG2-40 seconds and G-40 seconds generated deltaTs significantly higher than the control values (2.70, 4.05, 3.03, 4.58, 2.74 and 2.44, respectively). The deltaT values obtained with uncovered tooth were generally higher than those obtained with a 2-mm layer of composite. In conclusion, this research emphasizes that a perfect correspondence between light and material spectra is of prime concern, both to insure optimal polymerization and to limit heating in the pulp chamber. Some reduction in curing time is possible, but only within certain limits.

Maleic anhydride homopolymerization during melt functionalization of isotactic polypropylene

The homopolymerization of maleic anhydride was attempted at 190 degrees C, during the melt-functionalization of polypropylene, either with or without organic peroxide using a Brabender plastograph. The free radical homopolymerization of pure maleic anhydride was also attempted either with or without organic peroxide, at 190 degrees C, in vacuum-sealed glass vials. In all cases, free low molecular weight maleic anhydride oligomers were observed by low molecular weight size exclusion chromatography (SEC). This maleic anhydride homopolymerization tends to prove that the ceiling temperature of poly(maleic anhydride) probably lies above the previously published value of 160 degrees C for these specific experimental conditions

Thermal and morphological behaviours of bisphenol A polycarbonate/poly(butylene terephthalate) blends

Crystallization of bisphenol A polycarbonate/poly(butylene terephthalate) (PC/PBT) blends at low undercoolings (less than or equal to 60 degrees C) was first studied in isothermal conditions by rapidly cooling samples from the melt. Differential scanning calorimetry (d.s.c.) half-crystallization times (t(1/2)) were obtained. At the same time, transmission electron microscopy (TEM) analyses were undertaken on microtome sections from quenched and crystallized d.s.c. samples. The d.s.c. behaviour appears to be linked to the dispersion of PBT in the blends. For blends in which PBT is finely dispersed in PC, t(1/2) lies around 10(3) s. In the reverse situation, the blends, like pure PBT, cannot be quenched, showing a t(1/2), in the same range as that of the pure polymer. In the co-continuous range, t(1/2) measurements are not very reproducible. They lie between those of dispersed PC and dispersed PBT blends. On the basis of these observations, original mechanisms of crystallization are proposed. The main feature of these mechanisms is the slowness of the PBT crystallization when it is finely dispersed. This behaviour is attributed to a low nucleation density. Crystallization of PC/PBT blends at high undercoolings was undertaken using d.s.c. cooling experiments. When some PBT is finely dispersed, an exotherm occurs on cooling, well below the T-g of the PC-rich phase. An evolution of T-ch, Delta H-ch (respectively, crystallization temperature and enthalpy on heating) and Delta H-m (melting enthalpy) as a function of annealing temperature occurs on reheating the samples. Surprisingly enough, a stepwise transition in morphology takes place around 110 degrees C, which is confirmed by the very different microstructures observed by TEM for samples annealed at 105 degrees C and 150 degrees C. A tentative phase diagram is finally proposed which shows that a spinodal-type phase decomposition should occur around 110 degrees C. Competition between a crystallization mechanism involving a homogeneous nucleation step and this spinodal phase decomposition probably takes place. The result is that the homogeneous nucleation mechanism, which is shown to occur in pure PBT when finely dispersed, is delayed until the spinodal decomposition occurs.