J. L. Mateo

Photopolymerization of hydroxyethylmethacrylate in the formation of organic–inorganic hybrid sol–gel matrices

Hydroxyethylmethacrylate (HEMA) monomer in different contents was used in the preparation of transparent organic-inorganic hybrid sol-gel matrices. The matrices were studied by analytical techniques before and after irradiation. An increase in the homogeneity of the composite material after the irradiation of HEMA, and a chemical linkage of the monomer with the surface has been observed. Photopolymerization processes were studied by differential scanning calorimetry (DSC). Details of the irradiation times, polymerization rates, and DSC related parameters of HEMA in the sol-gel matrices are reported.

Sorption and diffusion of organic solvents through photo-crosslinked SBS block copolymers

Abstract Styrene-butadiene-styrene (SBS) block copolymer exposed to UV radiation in the presence of unimolecular and bimolecular photoinitiators yields a crosslinked polymer. When 2,2-dimethoxy-2-phenylacetophenone is used as photoinitiator the photocrosslinking takes place fundamentally through the addition of the benzoyl and/or dimethoxyphenyl radicals to the double bonds (cis, trans and vinyl) of the SBS block copolymer. The reactivity of the radicals towards the three double bonds is as follows: vinyl > cis > trans. Diffusion and sorption of chloroform and toluene through SBS block copolymer have been studied in the temperature interval of 25–60°C by conventional weight gain experiments. The diffusion results have been analyzed in terms of the simple Fickian model. Diffusion of toluene and chloroform in crosslinked SBS block copolymer was found to deviate slightly from the normal Fickian character which was attributed to the diphasic morphology of the SBS copolymer. The approach to a normal Fickian behaviour as the temperature and crosslinking increase has been explained considering the decrease in the degree of completion of the microphase separation with the increase of both the crosslinking and temperature. Temperature was found to activate the diffusion process, which was supported by higher intrinsic diffusion coefficient values and slopes values at higher temperatures. Entropy and enthalpy of sorption were estimated by using the Van’t Hoff relationship. Crosslinked SBS block copolymer–solvent interaction parameter χ were computed using the Flory–Rehner equation. The χ parameter (0.40 and 0.41 for chloroform and toluene, respectively) indicated strong polymer–solvent interaction. The crosslinking densities of the SBS block copolymer irradiated were relatively low ( M c values go from 23,000 for an irradiation time of 30 min until 5000 for an irradiation time of 120 min).

Photochemistry and photopolymerization study on 2-acetoxy and methyl-2-acetoxy derivatives of thioxanthone as photoinitiators

Abstract The initiation efficiencies of 2-acetoxy thioxanthone and its methyl derivatives (in the 1, 3 and 4 positions) were studied by photocalorimetry when they were used as photoinitiators in the polymerization of butyl acrylate in bulk. The behaviour of the excited states of these new thioxanthones has been studied by spectroscopic measurements. As initiation results from the interaction of the thioxanthone molecule with tertiary amines, their photoreduction in the presence of a tertiary amine 2-N,N-diethylamino ethanol has also been investigated. Generally, most of the compounds exhibit high phosphorescence and triplet lifetimes at 77 K in addition to high photopolymerization efficiencies except 1-substituted derivatives where intramolecular hydrogen bonding is involved which enhances the rate of radiationless conversions.

Photoinitiated bulk polymerization of lauryl acrylate by N-acetyl-4-nitro-1-naphthylamine in the presence of N,N-dimethylaniline

Abstract Photoinitiated polymerization at 365 nm of lauryl acrylate in bulk was studied using a combination of N -acetyl-4-nitro-1-naphthylamine (ANNA) and N,N -dimethylaniline (DMA) as the initiator. The initiation involves exciplex formation between the excited ANNA and the amine DMA. A detailed kinetic study of the polymerization showed that the rate of polymerization is proportional to the square root of both the incident light intensity and the ANNA concentration. The influence of amine concentration and oxygen were also examined. The efficiency of this photoinitiated system was studied in comparison with other conventional aromatic ketone photoinitiators.

Thioxanthone photopolymers I: A kinetic study of methyl methacrylate polymerization using 2-benzyloxy thioxanthone, free and polymer bound, as photoinitiator

The general photochemistry and photopolymerization activity of free and polymer-bound 2-benzyloxy thioxanthone were studied. The polymeric photoinitiator was prepared by chemical modification of an atactic polystyrene (Mn = 97 500). The spectroscopic properties of free and polymer-bound 2-benzyloxy thioxanthone were examined in various solvents and the data were compared with their behaviour as photoinitiators of polymerization. Quantitative steady state and microsecond flash photolysis studies in the absence and presence of a tertiary amine were also related to their photopolymerization activity. The dependence of the photopolymerization rates and quantum yield of polymerization of methyl methacrylate on the concentration of amine, photoinitiator and monomer was studied by photodilatometry. Furthermore, all the polymers obtained were characterized by gel permeation chromatography and the ratio of the propagation constant to the square root of the termination rate constant kp/kt12 was obtained.

Synthesis, characterization and photopolymerization activity of a novel thioxanthone monomer and photopolymers

Abstract The synthesis of 2-acryloylthioxanthone monomer is described together with its copolymerization with acrylic monomers such as methyl methacrylate. The preparation of novel polymers containing pendant thioxanthone groups is also described, by reaction with substituted chloromethyl groups. The efficiencies and potential advantages of the new monomer and polymers in initiating the photopolymerization of a commercial triacrylate resin are described.

New fluorescent probes for monitoring polymerisation reactions: 1. Synthesis, solvatochromism and emission properties

Abstract The synthesis of three new highly fluorescent compounds is reported and the basic spectroscopic properties of them described, discussed and compared with those of some structurally related fluorescent compounds usually used as fluorescent probes. The effect of the solvent on ambient temperature absorption and fluorescence spectra, and the solvatochromic properties observed for both ground and first excited states of all the probes were used to evaluate their first excited dipole moments using the solvatochromic shift method (Bakhshiev’s correlation).

Photochemistry of novel water-soluble parasubstituted benzophenone photoinitiators: A photocalorimetric and photoreduction study

Abstract The photopolymerization quantum yields of five novel water-soluble para-substituted benzophenone derivatives were determined in a water-soluble monoacrylate resin using N , N ′-dimethylaniline as a co-synergist and these are compared with their corresponding photoreduction quantum yields in water in the presence of 2- N , N -diethylethanolamine. A linear correlation is found between the two sets of data, confirming that the photopolymerization activity of these photoinitiators is associated with photoreduction of the excited carbonyl triplet state with the amine. Cationic structures were found to be less active than the anionic structures and this is reflected by a corresponding marked difference in their triplet lifetimes. One of the photoinitiators, 4-methylsulphonium benzophenone sodium salt, exhibited a photoreduction quantum yield of 1.5 under the experimental conditions used. This is associated with secondary processes due to the reaction of alkylamino radicals with a ground state ketone molecule.

New Fluorescent Probes for Monitoring Polymerization Reactions: Photocuring of Acrylic Adhesives, 2

The results of following the fluorescence of selected probes during the entire range of curing of acrylic-based adhesives are presented in this work. The change of fluorescence parameters has been continuously measured during the photocrosslinking reactions of acrylic adhesive systems under UV-irradiation. Real-time FT-IR (RT-FT-IR) has been used to measure the conversion reached at different irradiation times. Intensity ratio, maximum emission wavelength and the first moment of fluorescence have been used as parameters to correlate with the double bond conversion degree. Two-slope plots have been obtained during UV-curing, which correspond to the different stages of the crosslinking reaction. Also, the size of fluorescent probe appears to be a parameter that influences the sensing time in which the probe can detect the changes in viscosity/polarity occurring along the UV-curing process. The results are compared with those of commercial probes such as dansylamide and pro- dan. The use of the fluorescence technique allowed us to understand the mechanism taking place and the role of the binder matrix, depending on the photoinitiator used. This information is not obtained when the reaction is followed by the conventional RT-FT-IR technique.

New fluorescent probes for monitoring the polymerization reaction: p-vinyliden derivatives of N,N-dimethylaminoaryl compounds

The fluorescence emission of two new fluorescent probes have been studied in environments of different polarity and viscosity. The probes have been shown to be sensitive to changes in both microviscosity and micropolarity of the surroundings, and their dipolar moments in the excited state have been estimated. They have been used as fluorescent sensors to monitor the photopolymerization reactions of some mono- and di-functional (meth)acrylate monomers, and two commercial photocurable acrylic adhesives. The fluorescence emission band of the probes showed an increase in intensity as the degree of conversion increases, throughout the entire polymerization range in the different systems checked. Their sensitivity have been compared with that of their parent compound N,N-dimethylamino-bencilidenenmalononitrile.