Photopolymerization Of Monomers Research Articles

A Study on the Photopolymerization of Acrylic Monomers Having Fluorene Moiety for the Application of Holographic Data Storage System

We have prepared photopolymer compositions for holographic data storage system from aromatic diacrylate having fluorine moiety, a hybrid sol-gel, and photoinitiator. The fluorenyl diacrylate monomer was synthesized via nucleophilic substitution reaction of fluorene diphenol with an excess of 2-chloroethoxyethanol, followed by the reaction with acryloyl chloride. The physical and holographic properties of photopolymer were controlled by the ratio of precursor triethoxysilylpropyl polyethyleneglycol carbamate (TSPEG) in a hybrid sol-gel binder and the content of monomer. Also, we measured the photopolymerization rate and conversion of monomer by photo-differential scanning calorimeter (photo-DSC). Holographic recording was attempted by photopolymerization of the monomers in the photopolymer film using a 532 nm laser light in the presence of Irgacure 784 and holographic gratings were recorded into the photopolymer films by interfering two collimated plane wave beams. Contents of monomer and TSPEG were changed in the range of 0.25-1.0 equiv. and the composition were optimized in terms of diffraction efficiency. Finally, we obtained that efficient photopolymer films showed diffraction efficiency of 78% when 0.5 equiv. of TESPEG in sol-gel binder and 40% of compound 4 were added.

THE ROLES OF REACTION INHOMOGENEITY IN PHASE SEPARATION KINETICS AND MORPHOLOGY OF REACTIVE POLYMER BLENDS

The roles of reaction inhomogeneity in phase separation of polymer mixtures were described and summarized via two examples: photocross-link of polymer mixtures in the bulk state and photopolymerization of monomer in the liquid state. The reaction kinetics, the reaction-induced elastic strain and the phase separation kinetics were monitored respectively by UV-Vis spectroscopy, Mach-Zehnder interferometry and laser-scanning confocal microscopy. It was found that phase separation in the bulk state was strongly influenced by the elastic strain associated with the intrinsic inhomogeneity of the reaction, whereas the autocatalytic behavior of the polymerization plays an important role in the resulting morphology in the liquid state. These experimental results are discussed in conjunction with the morphology control of polymer mixtures by using chemical reactions.

Photo-controlled conformation-assisted permanent optical storage device employing a polymer network liquid crystal

A new type of optical storage device is described employing a material consisting of a host nematic liquid crystal, a photoisomerisable azobenzene component and a photopolymerizable monomer. The principle of image storing involves selectively controlling the birefringence of the medium immediately prior to photopolymerization of the monomer. We show that photoisomerisation driven nematic to isotropic transition can be employed to achieve this through proper timing of the reverse isomerization of the azobenzene compound before the nematic director fluctuations get quenched. It is also suggested that grey-shades can be created in this device using the recently discovered phenomenon of electric-field acceleration of reverse isomerisation.

Epoxy silsesquioxane resists for UV nanoimprint lithography

Photopolymerizable epoxy polyhedral oligomeric silsesquioxane monomers with variable aliphatic spacer have been prepared and cured under UV radiation. Due to their organic/inorganic composition, these materials are promising candidates for microelectronic applications which requires high thermal and mechanical stabilities as well as a low dielectric constant, for example, for the fabrication of electrical interconnects. The authors pointed out by ellipsometric measurements that the use of a sensitizer in addition to the photoinitiator enhances the kinetic of the cationic polymerization of epoxy groups. The polymerization performed either in an EVG770® nanoimprint lithography stepper or under normal atmospheric conditions has been investigated by infrared spectroscopy, delivering information concerning polymerization mechanisms and allowing the optimization of the resin composition. Thermogravimetric analyses have shown that an annealing step is necessary after the UV curing of the resin to eliminate inert residues of the photoinitiator dissociation and thus improving the thermal resistance of the polymer. Finally they have demonstrated that the obtained polymers form interpenetrated networks and that their mechanical properties are retained up to a temperature of 375°C.

Studies on electro‐optical properties of polymer matrix/LC/SiO2 nanoparticles composites

AbstractPolymer‐dispersed liquid crystal (PDLC) films were prepared by ultraviolet light‐induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers/SiO2 nanoparticles composites, and the effect of SiO2 nanoparticles on the electro‐optical properties of PDLC films was studied. The observed effect showed that by the adjustment of the SiO2 nanoparticles content, the refractive index ratio of the LC and polymer could be modulated, and the electro‐optical properties of the polymer matrix/LC/SiO2 nanoparticles composites could be optimized. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Numerical Modeling of Tunable Liquid-Crystal-Polymer-Network Lens

A gradient polymer stabilized liquid crystal (G-PSLC) structure has recently been proposed for a tunable-focus lens application. A homogeneously-oriented nematic liquid crystal (NLC) cell, doped by a few percent of photopolymerizable monomer, is initially illuminated by a laser beam with a Gaussian spatial intensity distribution. This induces a spatially inhomogeneous polymer network in the cell. The electro-optical response of this system to a uniform electric field exhibits an inhomogeneous circularly symmetric pattern. The radial distribution of the effective refractive index possesses a maximum in the center of the beam. This cell acts as a positive focal length lens on the extraordinary polarized light component passing through it. The profile of the refractive index in the plane of the cell can be changed by varying the voltage across the cell. Thus the focal length of the lens changes with voltage. Here we present a numerical approach to our earlier theoretical model. The model describes the dependence of the focal length of the G-PSLC lens on applied voltage. The new feature of the model is that we have used several trial functions for the form of the polymer profile. The director profile in the cell was determined as a function of voltage. The model qualitatively agrees with the experimental data. The results can be applied to develop G-PSLC lenses with no moving parts and permit electro-optical zooming.

Control of the microstructure of polymer network and effects of the microstructures on light scattering properties of UV‐cured polymer‐dispersed liquid crystal films

AbstractPolymer‐dispersed liquid crystals (PDLC) films were prepared from ultraviolet (UV) irradiation‐induced polymerization of the photopolymerizable monomers in photopolymerizable monomers/nematic liquid crystal (LC) mixtures. The effects of the composition of the mixtures, the curing temperature, and the UV light intensity on the microstructure of the polymer network in the PDLC films were investigated. Furthermore, the effects of the microstructures on the light scattering properties of the PDLC films in the wavelength region of 300–2500 nm were studied experimentally and theoretically based on the combination of three kinds of classical light scattering theories: the Rayleigh‐Gans (RG) approach, the anomalous diffraction (AD) approach, and the geometrical optics (GO) approach. It was found that the sizes of LC domain in PDLC films increased with the increase of the LC content as well as the decrease of the UV curing intensity, while increased at first and then decreased with the increase of the curing temperature. Moreover, smaller LC domain sizes could exhibit strong scattering properties in a smaller VIS wavelength region and the transmittance in NIR region (especially in the wavelength range of 1300–2500 nm) obviously decreased with the increasing sizes of LC domain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2090–2099, 2008

Effect of a Photopolymerizable Monomer Containing a Hydrogen Bond on Near-Infrared Radiation Transmittance of Nematic Liquid Crystal/Monomers Composites

Polymer dispersed liquid crystal (PDLC) films were prepared by ultraviolet (UV) radiation induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers composites. The effect of a photopolymerizable monomer 2-hydroxyethylmethacrylate (HEMA) containing a hydrogen bond on the transmittance of the PDLC films in the wavelength region of 300−2500 nm was investigated by UV−visible (VIS)−near-infrared (NIR) spectrophotometers, scanning electron microscopy, and an infrared spectrometer. It was surprisingly found that the addition of a small amount of HEMA monomer containing a hydrogen bond could dramatically decrease the off-state transmittance in the NIR region partially due to the formation of a compact and thick-border polymer network. Then the conceivable mechanism concerning the effect of HEMA monomers containing a hydrogen bond on the microstructure of the polymer network was investigated experimentally and theoretically. This provided a potential approach for the electrically controlled modulation of NIR radiation.

Effects of the mixture composition on the microstructure of polymer matrix and light scattering properties of liquid crystal/photo-polymerizable monomers composites

The microstructure and electro-optical effects based on light scattering have been investigated for nematic liquid crystal (LC)/photo-polymerizable monomers composites. It was demonstrated that the optimum electro-optical properties of the composites were obtained when the LC content was between 60 and 70 wt%. Furthermore, the variety of the light scattering intensity for the composites in the wavelength region of 300–2500 nm was studied experimentally and theoretically with the combination of three kinds of classical light scattering theories: the Rayleigh–Gans (RG) approach, the anomalous diffraction (AD) approach, and the geometrical optics (GO) approach. Then a simple mathematic simulation was established to describe the relationship between the transmittance and the incident wavelength in this wavelength region.

Microfluidic large-scale integration on a chip for mass production of monodisperse droplets and particles

In this study, we report the mass production of monodisperse emulsion droplets and particles using microfluidic large-scale integration on a chip. The production module comprises a glass microfluidic chip with planar microfabricated 16-256 droplet-formation units (DFUs) and a palm-sized stainless steel holder having several layers for supplying liquids into the inlets of the mounted chip. By using a module having 128 cross-junctions (i.e., 256 DFUs) arranged circularly on a 4 cm x 4 cm chip, we could produce droplets of photopolymerizable acrylate monomer at a throughput of 320.0 mL h(-1). The product was monodisperse, having a mean diameter of 96.4 microm, with a coefficient of variation (CV) of 1.3%. Subsequent UV polymerization off the module yielded monodisperse acrylic microspheres at a throughput of approximately 0.3 kg h(-1). Another module having 128 co-flow geometries could produce biphasic Janus droplets of black and white segments at 128.0 mL h(-1). The product had a mean diameter of 142.3 microm, with a CV of 3.3%. This co-flow module could also be applied in the mass production of homogeneous monomer droplets.

UV tuning of the electro‐optical and morphology properties in polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro‐optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro‐optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro‐optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro‐optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro‐optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.

Two-Photon Lithography of Platinum-Porphyrin Oxygen Sensors

Oxygen sensing structures were generated by two-photon microfabrication. By copolymerizing metalloporphyrins with a two-photon (2P) photo-initiated polymer, oxygen sensors were patterned into complex 3-D shapes. The sensors were generated on the interior walls of small bore capillaries to allow for controlled concentrations of oxygenated water and cell-rich media to be pumped through their local environment. Phosphorescence lifetime of the patterns were acquired at known levels of O2 as a standard for measuring the respiration rate of a tiny population of bacterial cells. In addition, we report that the inclusion of the Pt-Porphyrin significantly reduces the 2P polymerization threshold. Fabricating near the inferred polymerization threshold, 3-D structures as small as 50 nm were observed in both the Pt-Porphyrin enhanced and the pure photopolymerizable monomers

Wide‐band reflective polarizers from cholesteric liquid crystals with stable optical properties

AbstractA wide‐band reflective polarizer from glassy siloxane cyclic side‐chain liquid crystals with stable optical properties was developed. The pitch gradient that formed was first stored by rapid cooling, which, by preventing the crystallization of glassy materials, was a very easy way of keeping the molecular arrangement. Then, the polymer network that formed through UV‐initiated chain crosslinking of the photopolymerizable monomers in the reflective polarizer further stabilized the cholesteric structure stored by the former rapid cooling. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Characterization of poly(isobornyl acrylate) as a construction material for microfluidic applications

AbstractIsobornyl acrylate (IBA) is a photopolymerizable monomer that is employed in microfluidic devices because of desirable properties, such as inertness, transparency, and resolution. However, some of the mechanical properties of poly(isobornyl acrylate) are greatly affected by subtle changes in the manufacturing techniques. In this study, the parameters of exposure time, UV intensity, and aging are varied to study their effect on the material properties of thin samples of isobornyl acrylate construction material (<0.25 mm). Mechanical testing was used to obtain properties, such as elasticity, maximum strength, and maximum strain. It was observed that when using high levels of both exposure time and intensity, the polymers strength was increased. Lowering one of these two parameters immediately reduced the construction materials strength. It was also noted that aging weakens the material in as little as 1 day. In addition, an anisotropic response that produces curling in samples has been studied. It showed to have a negligible effect on the mechanical properties of the material; however it may have a major effect on device quality and shelf‐life. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Effect of a chiral dopant on the electro‐optical properties of polymer‐dispersed liquid‐crystal films

AbstractPolymer‐dispersed liquid crystal (PDLC) films were prepared by the ultraviolet‐light‐induced polymerization of photopolymerizable monomers in nematic liquid crystal/monomer/chiral dopant composites, and the effect of the chiral dopant on the electro‐optical properties of the PDLC films was studied. It was demonstrated that the addition of a small amount of the chiral dopant increased the driving voltage somewhat but decreased the turn‐off time significantly. Furthermore, the transmittance of ultraviolet, visible, and near‐infrared light of the off state of PDLC films showing light scattering increased with increasing content of the chiral dopant, and the optimum electro‐optical properties of the PDLC films were obtained when the content of the chiral dopant was not more than 2 wt %. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Cholesteric liquid crystalline materials reflecting more than 50% of unpolarized incident light intensity

Cholesteric liquid crystals (CLCs) selectively reflect light when the wavelength matches the helical pitch. The reflectance is limited to 50% of ambient, unpolarized light because only circularly polarized light of the same handedness as the helix is reflected. Here the elaboration procedure and the properties of a CLC gel whose optical characteristics go beyond the 50% reflectance limit are reported. Photopolymerizable monomers are introduced into the volume of a CLC exhibiting a thermally induced helicity inversion and the blend is then cured with UV light when the helix is right‐handed. The reflectance exceeds 50% when measured at the temperature assigned at a cholesteric helix with the same pitch but a left‐handed sense before reaction. The reflection properties are investigated in the infrared region. From scanning electron microscopy investigations, it is shown that the organization of the mesophase is transferred onto the structure of the network. The gel structure is discussed as consisting of a polymer network with a helical structure containing two populations of low molar mass LC molecules. Each of them is characterized by a band of circularly polarized light which is selectively reflected. The monitoring of the optical response with temperature offers the opportunity to discriminate the respective contributions of the bound and free fractions of LC molecules to the reflectance, and to give evidence of the progressive increase of the reflected flux when the temperature decreases from the curing temperature. Novel opportunities to modulate the reflection over the whole light flux range are offered. Potential applications are related to the light management for smart windows or reflective polarizer‐free displays with a larger scale of reflectivity levels. **Now at Philips Research, Display Applications and Technologies, Postbox MS41, Building 34 (WB), High Tech Campus 34, S656 AE Eindhoven, The Netherlands.

Going beyond the reflectance limit of cholesteric liquid crystals

Cholesteric liquid-crystalline states of matter are abundant in nature: atherosclerosis, arthropod cuticles, condensed phases of DNA, plant cell walls, human compact bone osteon, and chiral biopolymers. The self-organized helical structure produces unique optical properties. Light is reflected when the wavelength matches the pitch (twice periodicity); cholesteric liquid crystals are not only coloured filters, but also reflectors and polarizers. But, in theory, the reflectance is limited to 50% of the ambient (unpolarized) light because circularly polarized light of the same handedness as the helix is reflected. Here we give details of a cholesteric medium for which the reflectance limit is exceeded. Photopolymerizable monomers are introduced into a cholesteric medium exhibiting a thermally induced helicity inversion, and the blend is then cured with ultraviolet light when the helix is right-handed. Because of memory effects attributable to the polymer network, the reflectance exceeds 50% when measured at the temperature assigned for a cholesteric helix with the same pitch but a left-handed sense before the reaction. As cholesteric materials are used as tunable bandpass filters, reflectors or polarizers and temperature or pressure sensors, novel opportunities to modulate the reflection over the whole light flux range, instead of only 50%, are offered.

Analysis of Optical Properties with Photopolymers for Holographic Application

Optical transparency and high diffraction efficiency are two essential factors for high performance of the photopolymer. Optical transparency mainly depends on the miscibility between polymer binder and photopolymerized polymer, while diffraction efficiency depends on the refractive index modulation between polymer binder and photopolymerized polymer. For most of organic materials, the large refractive index difference between two polymers accompanies large structural difference that leads to the poor miscibility and thus poor optical quality via light scattering. Therefore, it is difficult to design a high-performance photopolymer satisfying both requirements. In this work, first, we prepared a new phase-stable photopolymer (PMMA) with large refractive index modulation and investigated the optical properties. Our photopolymer is based on modified poly (methyl methacrylate) as a polymer binder, acryl amide as a photopolymerizable monomer, triethanolamine as initiator, and yellow eosin as a photosensitizer at 532 nm. Diffraction efficiency over 85% and optical transmittance over 90% were obtained for the photopolymer. Second, Organic-inorganic nanocomposite films were prepared by dispersing an aromatic methacrylic monomer and a photo- initiator in organic-inorganic hybrid sol-gel matrices. The film properties could be controlled by optimizing the content of an organically modified silica precursor (TSPEG) in the sol-gel matrices. The photopolymer film modified with the organic chain (TSPEG) showed high diffraction efficiency (> 90%) under an optimized condition. High diffraction efficiency could be ascribed to the fast diffusion and efficient polymerization of monomers under interference light to generate refractive index modulation. The TSPEG modified photopolymer film could be successfully used for holographic memory.

Free-flow zone electrophoresis and isoelectric focusing using a microfabricated glass device with ion permeable membranes

This paper describes a microfabricated free-flow electrophoresis device with integrated ion permeable membranes. In order to obtain continuous lanes of separated components an electrical field is applied perpendicular to the sample flow direction. This sample stream is sandwiched between two sheath flow streams, by hydrodynamic focusing. The separation chamber has two open side beds with inserted electrodes to allow ventilation of gas generated during electrolysis. To hydrodynamically isolate the separation compartment from the side electrodes, a photo-polymerizable monomer solution is exposed to UV light through a slit mask for in situ membrane formation. These so-called salt-bridges resist the pressure driven fluid, but allow ion transport to enable electrical connection. In earlier devices the same was achieved by using open side channel arrays. However, only a small fraction of the applied voltage was effectively utilized across the separation chamber during free-flow electrophoresis and free-flow isoelectric focusing. Furthermore, the spreading of the carrier ampholytes into the side channels resulted in a very restricted pH gradient inside the separation chamber. The chip presented here allows at least 10 times more efficient use of the applied potential and a nearly linear pH gradient from pH 3 to 10 during free-flow isoelectric focusing could be established. Furthermore, the application of hydrodynamic focusing in combination with free-flow electrophoresis can be used for guiding the separated components to specific chip outlets. As a demonstration, several standard fluorescent markers were separated and focused by free-flow zone electrophoresis and by free-flow isoelectric focusing employing a transversal voltage of up to 150 V across the separation chamber.

Synthesis of novel highly reactive silicone-epoxy monomers for cationic photopolymerizations

A series of novel cationically photopolymerizable silicone-epoxide monomers bearing substituted benzyl ether groups were prepared. Monomers with methoxy groups on the aromatic ring of the benzyl ether group were synthesized by sequential hydrosylation reactions of 1,1,3,3-tetramethyl disiloxane. Kinetic studies using real-time infrared spectroscopy of the cationic photopolymerizations of these novel monomers showed that those monomers with methoxy groups exhibited elevated rates of photopolymerization. The acceleration effect was explained on the basis of synergistic effects of two mechanisms, namely, the normal ring opening polymerization catalyzed by a Brönstead acid, and the cationic photopolymerization induced by the redox interaction between the free radicals formed in the backbone of the monomer and the onium salt used as photoinitiator. The methoxy groups on the aromatic ring of the benzyl ether enhance the stabilization of the carbocation formed in the structure of the monomer. Silicone-epoxy monomer with 2,5-dimethoxybenzyl ether group exhibited the higher photopolymerization rate. The latter compound can also be used as accelerator of the cationic photopolymerization of commercial silicone-epoxy monomer when used in low concentrations.