Lucirin TPO Research Articles

Light-curing effectiveness using led lamps: a review

LED lamps have a new light-curing technology which can be monowave or polywave, which allows it to reach more initiators such as camphorquinone, Lucirin TPO and Propanodione, which have a wide variety of advantages and disadvantages. These lamps have evolved over time, as have different ergonomics, longevity, systems and quality standards. The objective of this literature review is to improve the clinician on the proper use of different LED lamps and how they influence the efficiency of resin photopolymerization. Extensive research has been carried out in the existing literature on this topic. From the beginning of this information until April 18, 2022, the bibliographic search carried out includes 86 articles published in the Medline database through PubMed, LILACS, Science Direct and SciELO, and there is no language restriction. The photopolymerization effects of Polywave and Monowave LED lamps present significant differences between the compressive strength of the light-cured resin, with single-wave and polyvalent LED lamps where the types of light and lamp directly influence the compressive strength of the resin. composite resins. The type of light and lamp directly affects the efficiency of the photopolymerization of the composite resin, so it is concluded that LED lamps with single wave technology (Monowave) produce a greater depth of photopolymerization than those with multiple wave technology (Polywave).

The Influence of Various Photoinitiators on the Properties of Commercial Dental Composites.

The aim of this article was to compare the biomechanical properties of commercial composites containing different photoinitiators: Filtek Ultimate (3M ESPE) containing camphorquinone (CQ); Estelite Σ Quick (Tokuyama Dental) with CQ in RAP Technology®; Tetric EvoCeram Bleach BLXL (Ivoclar Vivadent AG) with CQ and Lucirin TPO; and Tetric Evoceram Powerfill IVB (Ivoclar Vivadent AG) with CQ and Ivocerin TPO. All samples were cured with a polywave Valo Lamp (Ultradent Products Inc.) with 1450 mW/cm2. The microhardness, hardness by Vicker’s method, diametral tensile strength, flexural strength and contraction stress with photoelastic analysis were tested. The highest hardness and microhardness were observed for Filtek Ultimate (93.82 ± 17.44 HV), but other composites also displayed sufficient values (from 52 ± 3.92 to 58,82 ± 7.33 HV). Filtek Ultimate not only demonstrated the highest DTS (48.03 ± 5.97 MPa) and FS (87.32 ± 19.03 MPa) but also the highest contraction stress (13.7 ± 0.4 MPa) during polymerization. The TetricEvoCeram Powerfill has optimal microhardness (54.27 ± 4.1 HV), DTS (32.5 ± 5.29 MPa) and FS (79.3 ± 14.37 MPa) and the lowest contraction stress (7.4 ± 1 MPa) during photopolymerization. To summarize, Filtek Ultimate demonstrated the highest microhardness, FS and DTS values; however, composites with additional photoinitiators such as Lucirin TPO and Ivocerin have the lowest polymerization shrinkage. These composites also have higher FS and DTS and microhardness than material containing CQ in Rap Technology.

Photo-polymerisation variables influence the structure and subsequent thermal response of dental resin matrices

ObjectiveThe structure of the polymer phase of dental resin-based-composites is highly sensitive to photo-polymerisation variables. The objective of this study was to understand how different polymer structures, generated with different photo-polymerisation protocols, respond to thermal perturbation. MethodsExperimental resins were prepared from a series of Bis-GMA/TEGDMA blends (40/60, 50/50 and 60/40 wt.%), with either Camphorquinone/DMAEMA or Lucirin TPO as the photo-initiator system. Resins were photo-polymerised, in a disc geometry, at either relatively ‘high’ (3000 mW cm−2 for 6 s) or ‘low’ (300 mW cm−2 for 60 s) irradiances ensuring matched radiant exposures (18 J cm−2). Specimens were heated, from 20−160 °C at a rate of 5 °C min−1, whilst simultaneous synchrotron X-ray scattering measurements were taken at 5 °C increments to determine changes in polymer chain segment extension and medium-range order as a function of temperature. For each unique resin composition (n = 3), differential scanning calorimetry was used to measure glass transition temperatures using the same heating protocol. A paired t-test was used to determine significant differences in the glass transition temperature between irradiance protocols and photo-initiator chemistry at ɑ = 0.05. ResultsResins pre-polymerised through the use of TPO and or high irradiances demonstrated a reduced rate of chain extension indicative of lower thermal expansion and a larger decrease in relative order when heated below the glass transition temperature. Above the transition temperature, differences in the rate of chain extension were negligible, but slower converted systems showed greater relative order. There was no significant difference in the glass transition temperature between different photo-initiator systems or irradiance protocols. SignificanceThe evolution of chain extension and medium-range order during heating is dependent on the initial polymer structure which is influenced by photo-polymerisation variables. Less ordered systems, generated at faster rates of reactive group conversion displayed reduced chain extension below the glass transition temperature and maintained lower order throughout heating.

Solubility and mixing thermodynamic properties of (2,4,6-trimethylbenzoyl) diphenylphosphine oxide in pure and binary solvents

The solubility of (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (Lucirin TPO) in ten pure solvents and two binary solvent mixtures was measured from 273.15 K to 308.15 K by gravimetric method. The solubility of Lucirin TPO increased non-linearly with rising temperature in all the studied solvents. Furthermore, the solubility in mixed solvents presented a maximum-solubility effect. The modified Apelblat model, λh model, CNIBS/R-K model, NRTL model and Jouyban-Acree model were employed to correlate the experimental solubility data. The modified Apelblat model provided the best agreement in pure solvents, while the modified Apelblat model and CNIBS/R-K model gave better correlation results in binary solvent mixtures. Moreover, the thermodynamic properties of the mixing process, including the mixing Gibbs free energy, mixing enthalpy and mixing entropy were calculated from the solubility data using the NRTL model.

Effect of blue and violet light on polymerization shrinkage vectors of a CQ/TPO-containing composite

ObjectiveTo evaluate the effect of light-curing wavelengths on composite filler particle displacement, and thus to visualize localized polymerization shrinkage in a resin-based composite (RBC) containing camphorquinone (CQ) and Lucirin TPO (TPO). MethodsThree light-curing units (LCUs) were used to light-cure a RBC containing CQ and TPO: a violet-only, a blue-only, and a dual-wavelength, conventional (Polywave®, emitting violet and blue wavelengths simultaneously). Zirconia fillers were added to the RBC to act as filler particle displacement tracers. LCUs were characterized for total emitted power (mW) and spectral irradiant output (mW/cm2/nm). 2-mm high, 7-mm diameter silanized glass cylindrical specimens were filled in a single increment with the RBC, and micro-computed tomography (μ-CT) scans were obtained before and after light-curing, according to each LCU (n=6). Filler particle movement identified polymerization shrinkage vectors, traced using software, at five depths (from 0 up to 2mm): top, top-middle, middle, middle-bottom and bottom. ResultsConsidering different RBC depths within the same LCU, use of violet-only and conventional LCUs showed filler particle movement decreased with increased depth. Blue-only LCU showed homogeneous filler particle movement along the depths. Considering the effect of different LCUs within the same depth, filler particle movement within LCUs was not statistically different until the middle of the samples (P>.05). However, at the middle-bottom and bottom depths (1.5 and 2mm, respectively), blue-only LCU compared to violet-only LCU showed higher magnitude of displacement vector values (P<.05). Use of the conventional LCU showed filler displacement magnitudes that were not significantly different than blue-only and violet-only LCUs at any depth (P>.05). With respect to the direction of particle movement vectors, use of violet-only LCU showed a greater displacement when close to the incident violet LED; blue-only LCU showed equally distributed particle displacement values within entire depth among the samples; and the conventional LCU showed greater filler displacement closer to the blue LED locations. SignificanceFiller particle displacement in a RBC as a result of light-curing is related to localized application of light wavelength and total emitted power of the light emitted on the top surface of the RBC. When the violet LED is present (violet-only and conventional LCUs), filler particle displacement magnitude decreased with increased depth, while results using the blue-only LED show a more consistent pattern of displacement. Clinically, these results correlate to production of different characteristics of curing within a RBC restoration mass, depending on localized wavelengths applied to the irradiated surface.

Cytotoxicity and genotoxicity of a low-shrinkage monomer and monoacylphosphine oxide photoinitiator: Comparative analyses of individual toxicity and combination effects in mixtures

ObjectiveTo compare cytotoxicity and genotoxicity of novel urethane-based monomer FIT-852 and monoacylphosphine oxide photoinitiator (Lucirin TPO) with conventional Bisphenol A-glycidyl-methacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) monomers and camphorquinone (CQ)/amine photoinitiator system, respectively. Moreover, we quantified and analyzed the combinatorial effects of individual substances in resin-based mixtures concerning the nature of the combinatorial effects. MethodsCytotoxic and genotoxic effects of BisGMA, FIT, TEGDMA, CQ, DMAEMA and TPO and their combined toxicity in four clinically relevant mixtures (FIT/TPO, FIT/CQ, BisGMA/TPO, BisGMA/CQ) were tested on human fetal lung fibroblasts MRC-5 using MTT and Comet assays. We assessed combination effects of monomers and photoinitiators on overall toxicity from the measured concentration-effect relationships. Combination index (CI) was calculated on the basis of the median-effect equation derived from the mass-action law principle. ResultsIndividual substances showed decreasing cytotoxic effects in the following order: BisGMA>TPO>FIT>CQ>DMAEMA>TEGDMA. Experimental mixtures showed decreasing cytotoxic effects in the order BisGMA/TPO>BisGMA/CQ>FIT/CQ>FIT/TPO. FIT-based mixtures exhibited antagonistic cytotoxic effects between components while BisGMA-based mixtures demonstrated synergistic effects at ED50. TPO amplified both antagonistic and synergistic cytotoxic effects in mixtures. Pure substances showed genotoxicity in the following order: TPO>BisGMA>FIT>CQ>TEGDMA. We did not detect the genotoxic potential of DMAEMA. The rank of genotoxic concentrations of the mixtures was: BisGMA/TPO>BisGMA/CQ>FIT/CQ>FIT/TPO. SignificanceLower cytotoxicity and genotoxicity of FIT than BisGMA suggests its greater biocompatibility. Conversely, photoinitiator TPO was significantly more cytotoxic and genotoxic than both CQ and DMAEMA. CI values showed that components of FIT-based mixtures exhibit an antagonistic cytotoxic effect, while compontents of BisGMA-based mixtures show synergism.

Photochemical Deposition of Silver Nanoparticles on Clays and Exploring Their Antibacterial Activity.

Photochemical method was used to synthesize silver nanoparticles (AgNPs) in the presence of citrate or clay (SWy-1, SYn-1, and Laponite B) as stabilizers and Lucirin TPO as photoinitiator. During the photochemical synthesis, an appearance of the plasmon absorption band was seen around 400 nm, indicating the formation of AgNPs. X-ray diffraction results suggested that AgNPs prepared in SWy-1 were adsorbed into interlamellar space, and moreover, showed some clay exfoliation. In the case of SYn-1, AgNPs was not intercalated. For the AgNP/Lap B sample, the formation of an exfoliated structure occurred. Transmission electron microscopy revealed the spherical shape of AgNPs for all samples. The particle sizes obtained for AgNP/SWy-1, AgNP/SYn-1, and AgNP/Lap B were 2.6, 5.1, and 3.8 nm, respectively. AgNPs adsorbed on SYn-1 reveal nonuniform size and aggregation of some particles. However, AgNP/SWy-1 and AgNP/Lap B samples are more uniform and have diameters smaller than those prepared with SYn-1. This behavior is due to the ability to exfoliate these clays. The antibacterial activities of pure clays, AgNP/citrate, and AgNP/clays were investigated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). AgNPs in the presence of clays (AgNPs/SYn-1 and AgNPs/SWy-1) showed a lower survival index percentage compared to those obtained for pure clays and AgNPs. The AgNP/SWy-1 sample showed good antibacterial activity against both tested species and the lowest survival index of 3.9 and 4.3 against E. coli and S. aureus, respectively. AgNPs are located in the interlayer region of the SWy-1, which has acid sites. These acidic sites may contribute to the release of Ag(+) ions from the surface of AgNPs. On the other hand, Laponite B and AgNP/Lap B samples did not demonstrate any bactericidal activity.

Effect of resin and photoinitiator on color, translucency and color stability of conventional and low-shrinkage model composites

ObjectiveTo study the effect of a low-shrinkage methacrylate monomer and monoacylphosphine oxide photoinitiator on color, translucency, and color stability of model resin-based composites (RBCs). MethodsFour micro-hybrid RBCs were prepared containing barium-glass fillers in bisphenol A-glycidyl-methacrylate (BisGMA) and triethyleneglycol-dimethacrylate (TEGDMA) or urethane-based low-shrinkage monomer FIT-852 (FIT; Esstech Inc.) and TEGDMA matrix. Camphorquinone (CQ)/amine or Lucirin TPO were used as photoinitiators. Commercial low-shrinkage RBCs (Charisma Diamond, Heraeus Kulzer and N’Durance, Septodont) and conventional RBCs (Tetric EvoCeram, Ivoclar Vivadent and Filtek Z250, 3M ESPE) were used as controls. Color and translucency were measured using Thermo Scientific Evolution (Thermo Fisher Scientific) and SpectroShade™ Micro (MHT Optic Research) spectrophotometers. Color stability was evaluated after immersion in black tea (pure, with milk or lemon) and distilled water. Data were analyzed using analyses of variance with Tukey's post-test (α=0.05). ResultsPhotoinitiators had no significant effect on baseline color. Initially whiter FIT-based RBCs showed greater staining in all staining solutions than BisGMA-based RBCs. TPO-containing RBCs showed better color stability than CQ-containing RBCs irrespective of the base monomer. Tea and tea with lemon induced greatest color changes. Adding milk to tea significantly reduced material staining. SignificanceUrethane-based low-shrinkage monomer FIT and conventional BisGMA affected color, translucency and color stability of their respective RBCs. Despite being used in posterior teeth, low-shrinkage RBCs are expected to have favorable optical and esthetic properties. Manufacturers are urged to provide information on optical properties of monomers and monomer mixtures in their low-shrinkage RBCs to allow understanding of interaction with fillers and photoinitiators.

Degree of conversion of resin‐based materials cured with dual‐peak or single‐peak LED light‐curing units

There is a lack of data on polymerization of resin-based materials (RBMs) used in paediatric dentistry, using dual-peak light-emitting diode (LED) light-curing units (LCUs). To evaluate the degree of conversion (DC) of RBMs cured with dual-peak or single-peak LED LCUs. Samples of Vit-l-escence (Ultradent) and Herculite XRV Ultra (Kerr) and fissure sealants Delton Clear and Delton Opaque (Dentsply) were prepared (n = 3 per group) and cured with either one of two dual-peak LCUs (bluephase(®) G2; Ivoclar Vivadent or Valo; Ultradent) or a single-peak (bluephase(®) ; Ivoclar Vivadent). High-performance liquid chromatography and nuclear magnetic resonance spectroscopy were used to confirm the presence or absence of initiators other than camphorquinone. The DC was determined using micro-Raman spectroscopy. Data were analysed using general linear model anova; α = 0.05. With Herculite XRV Ultra, the single-peak LCU gave higher DC values than either of the two dual-peak LCUs (P < 0.05). Both fissure sealants showed higher DC compared with the two RBMs (P < 0.05); the DC at the bottom of the clear sealant was greater than the opaque sealant, (P < 0.05). 2,4,6-trimethylbenzoyldiphenylphosphine oxide (Lucirin(®) TPO) was found only in Vit-l-escence. Dual-peak LED LCUs may not be best suited for curing non-Lucirin(®) TPO-containing materials. A clear sealant showed a better cure throughout the material and may be more appropriate than opaque versions in deep fissures.

Phase separation kinetics and morphology induced by photopolymerization of 2-hydroxyehyl methacrylate (HEMA) in poly(ethyl acrylate)/HEMA mixtures

Morphology and phase separation kinetics induced by polymerization of 2-hydroxyethyl methacrylate (HEMA) in a HEMA/poly(ethyl acrylate) (PEA) mixture were observed by laser scanning confocal microscope in the presence of lucirin TPO used as an initiator. The results were analyzed by 2D-Fourier transform (2D-FFT). The photopolymerization is driven by irradiation with visible light λ = 405 nm. The mixture exhibits the Trommsdorff–Norrish effect which is responsible for a drastic increase in the reaction rate during the irradiation process. The concentration fluctuations and the increase in the viscosity of the medium play an important role in promoting the reaction yield. PHEMA droplets were found to develop in the rhodamine-B-labeled poly(ethyl acrylate) (PEA-Rh) continuous matrix. The characteristic length of the morphology increases with increasing irradiation intensity, revealing the tool to control the morphology by varying the light intensity.

Transparent magnesium hydroxide/acrylate nanocomposite films

AbstractNanocrystalline Mg(OH)2 was synthesized using four different methods and the crystallite size was calculated from X‐ray powder diffraction patterns. For the stabilization of the particles in polymer dispersions, two different surface modifiers were used [poly(N‐vinyl‐2‐pyrrolidone) and 3‐methacryloxypropyltrimethoxysilane]. IR spectroscopy measurements show that the surface of the particles is covered with the surface modifiers rendering the particles dispersable in monomer solutions such as butanediol monoacrylate. The dispersions were polymerized into films of 100 μm thickness by addition of a UV initiator (2,4,6‐trimethylbenzoyldiphenylphosphine oxide, Lucirin TPO) and a crosslinking agent (trimethylolpropanetriacrylate, Laromer TMPTA) between two glass plates by UV‐polymerization. The resulting films were characterized by UV–Vis spectroscopy, thermogravimetry, and scratch resistance tests. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Surface functionalization of ZrO 2 nanocrystallites for the integration into acrylate nanocomposite films

Surface functionalized zirconia nanoparticles were prepared by covalent grafting of a methacrylate functionalized silane (methacryloxypropyltrimethoxysilane, MPTS) onto the surface of the zirconia nanoparticles (tetragonal and mixed monoclinic/tetragonal phase) obtained by hydrothermal treatment of zirconyl chloride octahydrate. The particles are 70 nm aggregates of nanometric primary grains (4–12 nm) with inter particle porosity. BET measurements show that the specific surface area of the particles after activation at 100 °C is between 85 and 204 m 2/g depending on the mineralizer used (Sr 2+, Ca 2+, Mg 2+). IR-measurements show that the surface of the particles can be covered with functional groups bound through a variable number of Zr O Si bonds to render them organocompatible. The surface modified particles were dispersed in monomer solution (butanediol monoacrylate, BDMA) and polymerized to form films by adding a cross linking agent (trimethylolpropane triacrylate, Laromer TMPTA) and an UV initiator (2,4,6-trimethylbenzoyldiphenylphosphine oxide, Lucirin TPO). The received films were characterized with thermogravimetry and UV–vis spectroscopy.

Can CQ Be Completely Replaced by Alternative Initiators in Dental Adhesives?

Despite good clinical acceptance, photoinitiating systems based on camphorquinone and amines raise concerns in terms of yellowing, aging, toxicity, or degradation in low pH conditions. This study aimed to prove whether CQ could be successfully replaced by alternative initiators in adhesive systems. Further, the efficiency of a prototype dual-wavelength LED (= Light Emitting Diode) curing unit was analyzed. In two commercial adhesive systems, CQ was completely replaced by Lucirin TPO. The commercial adhesives and their experimental counterparts were evaluated after curing for 10 seconds and 20 seconds with two dual-wavelength LED units and one regular LED unit, by applying the curing unit on the adhesive surface at two distances of 0 mm and 5 mm. Degree of cure and mechanical properties (Vickers hardness and modulus of elasticity) were assessed after 24-hour storage in distilled water at 37 degrees C. Experimental data showed that the CQ-amine system could be completely replaced by Lucirin TPO when dual-wavelength LED unit was used for photoactivation.

Preparation of protected photoinitiator nanodepots by the miniemulsion process

The solid photoinitiator Lucirin TPO was encapsulated within a polymer shell by using the miniemulsion process. A solution of Lucirin TPO in methyl methacrylate (MMA) or butyl acrylate (BA)/MMA mixture was miniemulsified in water followed by a polymerization process in which phase separation of the Lucirin TPO and the formed polymer led to amorphously solidified Lucirin TPO nanoparticles encapsulated by polymer. These nanocapsules were freeze-dried and could be redispersed in acidic monomers, which are applied in polymeric dental adhesives. It is shown by 1H nuclear magnetic resonance spectroscopy that the shell separates the Lucirin TPO, which is sensitive to degradation in acidic media, from an ambient acidic monomer phase and protects it from fast decomposition. Investigations of the release kinetics of Lucirin TPO from the nanocapsules reveal that the kinetics are strongly dependent on the composition of the surrounding continuous phase.

The initiating radical yields and the efficiency of polymerization for various dental photoinitiators excited by different light curing units

To evaluate the efficiency of the photopolymerization of dental resins it is necessary to know to what extent the light emitted by the light curing units is absorbed by the photoinitiators. On the other hand, the efficiency of the absorbed photons to produce species that launch the polymerization process is also of paramount importance. Therefore, the previously determined PAE (photon absorption efficiency) is used in conjunction with the polymerization quantum yields for the photoinitiators, in order to be able to compare the total process on an equivalent basis. This parameter can be used to identify the best performance for the photochemical process with specific photoinitiators. The efficiency of LED (Ultrablue IS) and QTH (Optilux 401) lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). The extent of photopolymerization per absorbed photon was determined from the polymerization quantum yields obtained by using the photoinitiators to polymerize methyl methacrylate, and afterwards combined with the previously determined PAEs. Although CQ presents a rather low polymerization quantum yield, its photopolymerization efficiency is practically the highest when irradiated with the Ultrablue LED. On the other hand, Lucirin is much more efficient than the other photoinitiators when irradiated with a QTH lamp, due to its high quantum yield and the overlap between its absorption spectrum and the output of the visible lamp light. Difference in photopolymerization efficiencies arise when combinations of photoinitiators are used, and when LED sources are used in preference to QTH. Mechanistic understanding is essential to optimal initiator formulation.

Photopolymerizable Systems Containing Organophosphorus Compounds as Photoinitiators

p-Methoxybenzoyldialkylphosphonates, terephtaloylbis(dialkylphosphonates), (alkyl=methyl, ethyl), p-methoxybenzoyldiphenylphosphine oxide and terephtaloylbis(diphenylphosphine oxide) were synthesized by Michaelis–Arbuzov reaction. These compounds were used as photoinitiators for radical polymerization of acrylic monomer 1,6-hexandiol diacrylate. The photoinitiating activity was determined by differential photocalorimetry (DPC), in isothermal conditions, using 1,6-hexandiol diacrylate as monomer, in the absence of amines and in the presence of amines as synergist additive. The influence of amine on photoreactivity parameters was studied. The kinetic study of polymerization of 1,6-hexandiol diacrylate sensitized by these radical photoinitiators have been studied by DPC. These compounds performed good photoinitiating activity in comparison with commercial product Lucirin TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide).

Molar extinction coefficients and the photon absorption efficiency of dental photoinitiators and light curing units

The light absorption of dental photoinitiators should correlate with the spectral emission profiles of dental light curing units compared on an equivalent basis. Spectral data of dental photoinitiators and light curing units can be used to define the photon absorption efficiency (PAE) obtained by integrating the product of the absorption and emission spectra in terms of photons. This parameter can be used to identify the best performance for photochemical process with specific photoinitiators. The efficiency of two LED and one QTH lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). Absorption and emission spectra of the photoinitiators and the LED (Ultrablue I and Ultrablue IS) and QTH (Optilux 401) LCUs were determined in the 360-550nm range. CQ exhibited an absorption centered in the blue region and, although the maxima of PPD, MAPO, and BAPO were in the UV-A region, their absorption extended to the visible region. Power output maxima of the LCUs were at 467 (Ultrablue I), 454 (Ultrablue IS) and 493nm (Optilux 401), and the total power densities were 170+/-1, 470+/-4 and 444+/-4mW/cm(2), respectively. The use of the PAE allows a prediction of the most efficient photoinitiator/LCU systems. For similar photoinitiator concentrations, Lucirin and CQ are most efficiently photoinitiated by the QTH unit, whereas the high-power LED device is more efficient for Irgacure. PPD is photoactivated similarly by both LCUs.

Tetrasubstituted bi(phosphine sulfides) as free radical photoinitiators

Photolysis of tetrasubstituted bi(phosphine sulfides) at 254 nm yields disubstituted thiophosphinyl radicals that initiate the polymerization of methyl methacrylate. The phosphorus-containing chain ends of the polymers produced in these polymerizations were easily measured and characterized using 31P-NMR spectroscopy. These measurements showed that thiophosphinyl radical additions to methyl methacrylate are regiospecific. The yields obtained in bi(phosphine sulfide)-initiated MMA polymerizations compare favorably with yields obtained using a commercially available photoinitiator, Lucirin TPO, but the reaction times required are considerably longer. The molecular weights of PMMA samples obtained from bi(phosphine sulfide)-initiated polymerizations are lower than those of TPO-initiated samples, due to the ability of bi(phosphine sulfides) to also act as chain transfer agents.

Electro-Optical, Spectroscopical, and Calorimetric Investigations of Electron Beamand Ultraviolet- Cured Polymer Dispersed Liquid Crystal Materials

The must convenient method used for the preparation of polymer dispersed liquid crystal (PDLC) films is the polymerization induced phase separation (PIPS) of mixtures composed of polymer precursors and low molecular weight liquid crystals (LMWLC). Ultraviolct (UV) and electron beam (EB) radiation was employed to initiate the PIPS process. A mixture of the nematic LMWLC material E7, an aromatic polyester acrylatc, and additional monomeric acrylates was exposed to the EB radiation. Lucirin TPO as a photoinitiator has been included in the initial mixture in the case of UV exposure. The clectro-optical properties of the obtained PDLC films strongly depend on the curing conditions, and arc highly reproducible. In general it was found that the EB-cured PDLC films show a better electro-optical performance as compared to the UV-cured samples. The extent of cure has been determined by infrared spectroscopy measurements of the carbon-carbon double bond of the acrylate groups. Differential scanning calorimetry was employed to obtain the glass transition temperature of the polymer and the LMWLC solubility limit in the polymer matrix.