Real-time IR Spectroscopy Research Articles

Holographic polymer dispersed liquid crystals (HPDLCs) containing triallyl isocyanurate monomer

The morphology and corresponding performance of holographic polymer dispersed liquid crystals (HPDLCs) based on thiol–ene polymer are dependent on a number of factors including the gel point conversion of the polymer, polymerization kinetics, and extent of liquid crystal (LC) phase separation. Previous research of HPDLC reflection gratings made from thiol–allyl ether polymer indicates that increasing polymerization rate in systems with moderate gel point conversion can improve diffraction efficiency (DE). This work examines HPDLC reflection gratings that contain the ene monomer triallyl isocyanurate (TATATO). In HPDLCs, thiol–TATATO polymerization is two times faster than the thiol–ene polymerization of triallyl ether. By substituting TATATO for triallyl ether, the LC droplet size within HPDLC reflection gratings decreases from 100nm to 25nm. The dramatic reduction in LC droplet size for thiol–TATATO HPDLCs increases baseline transmission from 55% in thiol–triallyl ether HPDLCs to 90% at 450nm. Unfortunately, the DE of thiol–TATATO HPDLCs is only approximately 10% due to poorly defined lamellae in the grating morphology. As determined with real-time IR (RTIR) spectroscopy, thiol–TATATO HPDLCs have significantly faster LC demixing kinetics in comparison to thiol–allyl ether HPDLCs. During holographic photopolymerization, the increased rate of LC demixing causes formation of LC droplets throughout the grating. The low DE of thiol–TATATO HPDLCs can be improved by mixing TATATO and allyl ether monomer. The morphology of ternary thiol–ene HPDLC formulations containing TATATO and allyl ether has a well-defined grating structure due to increased LC solubility in the system, an average LC droplet size of 50nm, and baseline transmission of nearly 85% at 450nm.

Effect of Spacer Unit on Photo-Initiated Thermal Crosslinking of Polymers Having Carbamoyloxyimino Groups in Side-Chains

Carbamoyloxyimino (COI) groups are known to generate amino groups photochemically and isocyanato groups thermally, and these generated groups have been used for highly efficient crosslinking systems. In this article, a series of polymers which were connected to COI groups with or without spacer units were prepared, and their crosslinking behavior on irradiation followed by heating was investigated to clarify the effect of the length of side-chain. There was no effect of spacer units on photoreactivity of COI groups. On the other hand, thermal reactivity of COI groups in polymers with spacer unit was faster than that in polymers without spacer unit. Thermogravimetric analyses and real-time IR spectroscopy revealed lower thermal decomposition temperature, lower glass transition temperature, and higher thermal decomposition rate of COI groups in copolymers with spacer unit compared to corresponding polymers without spacer unit. These behaviors were reflected on the crosslinking on irradiation followed by heating, where films of polymers with spacer unit showed more effective insolubilization than those without spacer unit. Other factor such as comonomer and the presence of α-methyl group on main-chain were also investigated.

Depth-resolved characterization of UV cured coatings by confocal Raman and two-photon microscopy

UV cured coatings can be characterized very effectively by confocal Raman microscopy, real-time IR spectroscopy and two-photon laser scan microscopy. The first two vibrational spectroscopic methods for chemical imaging and kinetics of curing conversion, respectively, can help to optimize UV formulations and curing conditions, e.g. UV dosage or temperature. In contrast, two-photon laser scan confocal microscopy can be used to monitor changes in refractive indices and intrinsic or tracer fluorescence with three-dimensional spatial and fast temporal resolution. This technique is well suited to analyze buried interfaces to substrate and other coating layers and to visualize distribution of fillers. In this regard, aluminum flakes in metallic coatings can be investigated without additional sample preparation as a function of their individual orientation and size. With this information, one can optimize the process of coating formation yielding improved coloristic properties. Illustrative examples will be presented to elucidate the perspective of these techniques in coatings characterization.

Real-time kinetic study of the polymerization activity of the photoinitiator system p-nitroaniline/N,N-dimethylaniline by real time IR spectroscopy

The polymeriztion process of different acrylic monomers photoinduced by the bimolecular system p-nitroaniline/N,N-dimethylaniline (pNA/DMA) under UV radiation was analyzed in situ by real-time IR spectroscopy. To this purpose an inexpensive, automatized and simple apparatus based on commercially available items easily assembled was developed. The conversion versus time curves recorded allow the polymerization rate and quantum yield to be determined at any stage of the reaction. The effect of various factors such as incident light-intensity, initiatior and coinitiator concentration, functionality of the monomer on the kinetic of the polymerization process was analyzed. This technique allowed the evaluation of the dark polymerization developed just after a dose of UV radiation by being able to measure and control the polymerization degree. A comparison with alternative methods as Photo-DSC technique allows to evaluate the effect of the shample thickness on the kinetics of a photoinitiated acrylate polymerization.

Polymerisation kinetics photoinduced by derivatives of 4-nitroaniline in the presence of N,N-dimethylaniline studied by Real-Time IR spectroscopy

The behaviour of nitro-aromatic amines 4-nitro-1-naphthylamine (NNA), N-acethyl-4-nitro-1-naphthylamine and N,N-dimethyl-4-nitroaniline, in the presence of a tertiary amine such as N,N-dimethylaniline as photoinitiators of the dodecyl acrylate polymerisation has been studied by Real-Time IR spectroscopy upon UV exposure and the results compared with those obtained with 4-nitroaniline (pNA). The rate and quantum yield of polymerisation as well as the residual unsaturation in the cured polymer result are strongly dependent on the photoinitiator nature. The formulation based on the NNA compound improves the polymerisation activity of pNA exhibiting a reaction rate up to 2.1Ms−1 and reaching a conversion of nearly 85%.

Photoinitiated cationic polymerization of epoxy alcohol monomers

A series of epoxy alcohols were prepared by simple, straightforward methods. These compounds were very reactive monomers that polymerized rapidly on UV irradiation in the presence of cationic photoinitiators. The kinetics of the cationic photopolymerization of these monomers were studied with diaryliodonium salt photoinitiators and real-time IR spectroscopy. The rate of epoxide ring-opening polymerization was enhanced markedly by the presence of the hydroxy group. Using model compounds, the monomers were shown to polymerize via an activated monomer mechanism. Simple epoxy alcohols polymerized to give polymers with a hyperbranched structure. The novel monomers also were observed to accelerate the rate of the photopolymerization of mono- and multifunctional epoxides. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 389–401, 2000

Light-stabilization of polymeric materials by grafted UV-cured coatings

The weathering resistance of organic materials has been substantially increased by protecting their surface with a photocured coating containing both a UV absorber (UVA) and a radical scavenger (HALS). A kinetic study by real-time IR spectroscopy has shown that HALS have no effect on the cure rate, whereas UV absorbers slow down the cure process, due to their radiation inner filter effect. 3D analysis of the depth of cure profile revealed an incomplete cure at the coating/substrate interface, leading to adhesion failure. To prevent this detrimental effect, the UV-cured coating was photochemically grafted onto the substrate. The polymer material was first coated with a thin layer of a benzophenone solution in a diacrylate monomer. Polymer radicals, generated by hydrogen abstraction from the substrate by the excited benzophenone molecules, effectively initiate the polymerization of the acrylate functions, thus ensuring a chemical bonding between the coating and the substrate. The grafting reaction was characterized by ATR spectroscopy analysis and by surface energy measurements. Excellent adhesion was achieved by applying to the treated substrate a photocurable acrylate coating, containing the light stabilizers, because of the copolymerization reaction taking place with the unreacted acrylate double bonds of the base coat, upon UV exposure. The efficiency of this on-line stabilization process has been demonstrated on poly(vinyl chloride) that was made eight times more resistant to accelerated weathering. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2571–2580, 1998

Spectroscopy of Hydrothermal Reactions. 9. IR and Raman Spectroscopy of Hydrolysis and Self-Reaction of Cyanamide and Dicyandiamide at 130−270 °C and 275 bar

The hydrothermolysis reaction scheme of cyanamide, NH2CN, and dicyandiamide, (NH2)2CNCN, applies to the role of these compounds in chemical evolution and to their destruction in aqueous waste streams by hydrothermal methods. Real-time IR spectroscopy with an optically accessible flow cell and Raman spectroscopy with a stopped-flow cell were used at set temperatures of 130−270 °C and a pressure of 275 bar to specify the details of the pathway. Rate constants and Arrhenius parameters were determined for the major steps, i.e., hydrolysis of cyanamide, conversion of cyanamide into dicyandiamide, hydrolysis of dicyandiamide, and hydrolysis of the guanylurea intermediate. Previously reported hydrolysis kinetics for guanidine and urea at hydrothermal conditions were used to complete the kinetic scheme. In addition, the apparent equilibrium constants for deprotonation of cyanamide and the protonation of dicyandiamide and its monoanion were determined. The conversion of dicyandiamide to ammeline and the hydrolysis...

Photocrosslinking of Silicones. Part XV. Cationic Photocrosslinking of α, ω, 4-Terminated Disiloxanes

The photoinduced cationic crosslinking of α, ω-terminated disiloxanes (epoxy, vinyl ether, propenyl ether) has been investigated by means of Real-Time IR spectroscopy. A lipophilic iodonium salt and three lipophilic sulfonium salts were used as photoinitiator. The crosslinking rate is influenced by the type of α, ω-terminated disiloxane used and differed by a factor of more than 100 from the aliphatic epoxy to the vinyl ether derivatives. Moreover, the sulfonium salts were found to have a lower initiation efficiency than the lipophilic iodonium salt in the various systems studied. These results are in good agreement with the quantum yield of proton formation in a hexamethyldisiloxane/dimethoxyethane mixture. The final degree of conversion is larger with the ene derivatives than with the epoxy derivatives. The application of a kinetic method allows us to estimate the rate constant of the termination step (kt and for the propenyl derivative the rate constant of the propagation step kp. The termination step can be described by means of a first order reaction. kt was found to depend on the light intensity and the type of initiators used, whereas kp is independent of the initiator used.

Pigment polymer layers as sensitizers for the photopolymerization of trimethylolpropane triacrylate

Layers of organic dye pigments dispersed in poly(vinyl butyral) can be used as heterogeneous sensitizers for the photopolymerization of trimethylolpropane triacrylate. Reactions of the bulk monomer with different 3, 4, 9, 10-perylenetetracarboxylic diimides and copper phthalocyanine were studied by means of real-time IR spectroscopy. Compared to all polymerizations sensitized with perylene diimide layers, the relative rate of polymerization r p of reactions sensitized with the phthalocyanine layer is higher. In all cases, an increase of the pigment concentration leads to higher r p values. No simple relationship exists between incident light intensity and r p values. The initiating activity of the pigments correlates with the maximum voltage U max of their photo-emf.

A new series of type II (benzophenone) polymeric photoinitiators

Polyimides derived by the reaction of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) with amino-terminated polyethers have been used as photoinitiators in conjunction with tertiary amine synergists for the polymerization of acrylates. The performance of the new, fully characterized materials was compared with that of benzophenone and BTDA. The performance of the novel photoinitiators was assessed by their ability to cure thin films of multifunctional acrylates. Real time IR spectroscopy and photo-differential scanning calorimetry were used to assess the cure rate and extent of photopolymerization. Cured films were subjected to solvent extraction to determine the level and nature of the extractable components and these were shown not to contain any of the photoinitiator. The polyimide photoinitiators were found to be absent in the extracts. The polymeric photoinitiators were shown to have lower reactivities than benzophenone and BTDA. The reactivity of the new photoinitiators is affected by such factors as the composition of the polyimide and molecular weight (which markedly changes the viscosities of the formulations).