Yu. V. Chechet

General conditions and experimental design of sustained frontal photopolymerization in photopolymerizable liquid compositions

It has been shown that, in order to accomplish frontal photopolymerization in the mode of a self-sustained travelling wave, it is necessary to use compositions that are optically transparent behind the front and experience the onset of gelling at a minimal degree of polymerization, e.g., systems based on oligoether (meth)acrylates. With the use of a composition containing oligocarbonate methacrylate and the o-benzoquinone-amine photoinitiator system, frontal photopolymerization in a layer of more than 100 mm in thickness was experimentally revealed. It was shown that the photoinitiator and monomer conversion fronts synchronously propagate into the layer. It was found that the coordinate of the leading edge of the monomer conversion front h is related to the irradiated time τ by the equations h = A1 log τ − B and h = A2τ − B at the first and the second step of frontal polymerization, respectively. It was found that the thickness of the area of the formulation layer in which the condition h = A1 logτ − B is fulfilled is equal to the width of the photoinitiator and monomer conversion front. A general equation of motion of the coordinate of the photoinitiator conversion label in the layer of a liquid photopolymerizable composition during sustained frontal photopolymerization was proposed for describing experimental data.

Influence of o-benzoquinone nature on initiation of radical polymerization by the o-benzoquinone—tert-amine system

Abstracto-Benzoquinones initiate radical polymerization of methacrylates under visible light irradiation in the presence of tertiary amines. Spectral sensitivity of the initiating system coincides with absorption bands of o-benzoquinone attributed to the S(π→π*) (λmax ≈ 400 nm) and S(n→π*) (λmax ≈ 600 nm) transitions. The amine radicals (Am·) initiating polymerization are generated by the photoreduction of Q in the presence of AmH from the triplet radical pair 3(QH·, Am·). The yield of Am· depends on the difference between the volumes of substituents in the 3 and 6 positions of the quinoid ring and is maximal for symmetrically substituted o-benzoquinones. For a series of derivatives of symmetrical 3,6-di-tert-butyl-o-benzoquinone, the rate of photopolymerization of α,ω-bis(methacryloyloxyethyleneoxycarbonyloxy)ethyleneoxyethylene (OCM-2) in the presence of N,N-dimethylaniline is determined by the free energy (ΔGe) of electron transfer from the amine to photoexcited o-benzoquinone. The ΔGe value includes the energies of oxidation of the amines and reduction of the o-quinones and the energy of the 0→0 transition of the triplet excited state of o-benzoquinones, which are equal to their redox potentials. The photopolymerization rate is maximal for ΔGe ≈ 0.

Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λmax ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (kH) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔGe.t). The ΔGe.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔGe.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex.

Photoinitiation of methacrylate polymerization with an o-benzoquinone-amine system

The kinetic features of the photopolymerization of mono- and dimethacrylates in the presence of the binary initiating system comprising a substituted o-benzoquinone and a tertiary amine under the action of visible light are studied. In the case of methyl methacrylate, the limiting conversion of the monomer does not exceed 10%. The photopolymerization of oligo(ester methacrylates) yields a polymer glass with a monomer conversion of 60–90%. As is shown for a series of seven o-benzoquinones, the rate of photopolymerization increases with an increase in the volume of substituents in positions 3 and 6 of the quinoid ring of o-benzoquinone. It found that trialkylamines (dimethylethanolamine and dimethylcyclohexylamine) are more efficient as coinitiators of photopolymerization than N,N-dimethylaniline. For compositions based on 3,6-di-tert-butylbenzoquinone-1,2, the spectral-sensitivity range in the visible region is 400 to 650 nm with a maximum at 600 nm. This value coincides with the maximum of the absorption band of quinone, which corresponds to the S(n → π*) electronic transition of carbonyl groups.

Molecular and crystalline structure of pyrocatechol and hydroquinone dimethacrylates and their reactivity in melts

X-ray diffraction analysis of pyrocatechol and hydroquinone dimethacrylates (Tm = 18 and 86–88°C, respectively) shows that the oligomer molecules within crystals are packed in stacks where the methacrylate fragments of neighboring molecules are parallel to each other. The minimum distances between the centers of double bonds of adjacent methacrylate fragments in crystals of pyrocatechol and hydroquinone dimethacrylates are 4.621(3) and 4.269(4) A. The curves showing the reduced rate of photopolymerization of oligomer melts versus conversion (9,10-phenanthrenequinone used as the initiator) display a maximum at conversions of 1.5–3.0%. The limiting conversion in photopolymerization of molten pyrocatechol dimethacrylate at 25 and 40°C is 20%; for hydroquinone dimethacrylate at 95°C, it is approximately 10%. As the temperature rises from 25 to 40°C, the maximum reduced rate of photopolymerization of pyrocatechol dimethacrylate increases by a factor of 1.4.

Molecular and crystalline structure of 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate: Reactivity in melts

The X-ray diffraction analysis of 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate has shown that oligomer molecules within crystals are packed in stacks, where (meth)acrylate fragments of neighboring molecules are parallel to each other. The minimum distances between the centers of double bonds C=C of (meth)acrylate fragments in 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate are 4.208, 4.012, 3.621, and 3.739 describing the reduced rate of photopolymerization of molten monomers (with 9,10-phenanthrenequinone used as a photoinitiator) versus conversion show maxima at degrees of polymerization of 8, 16, 22, and 38%; the limiting conversions are 29, 36, 44, and 86%, respectively. The maximum reduced rates of photopolymerization of 2,2-di(phenyl-4-ol)propane dimethacrylate and diacrylate are nearly the same, whereas the rates of photopolymerization of hydroquinone diacrylate and pyrocatechol diacrylate are higher by a factor of 4 than those of the corresponding dimethacrylates.

Crystal packing and reactivity of di(meth)acrylates of some derivatives of hydroquinone and pyrocatechol in melts

The X-ray diffraction study of 2,2′-(1,2-phenylene-bis(oxy)diethanol and 2,2′-(1,4-phenylenebis(oxy)diethanol dimethacrylates and 2,2′-(1,4-phenylenebis(oxy)diethanol diacrylate (T m = 40–42, 68–70, and 62–64°C, respectively) indicates that oligomer molecules are packed in crystals as stacks in which methacrylate fragments of adjacent molecules are parallel to each other. The minimum distances between the centers of C=C double bonds of adjacent methacrylate fragments in crystals of di(meth)acrylates are 4.373, 4.215, and 3.996 respectively. The conversion dependences of the reduced rates of photopolymerization of melted oligomers (9,10-phenanthrenequinone as a photoinitiator) pass through maxima at conversions of 40, 11, and 2%, while the ultimate conversions are 85, 33, and 73%, respectively. The addition of ionic liquids based on phosphonium and imidazolium cations to dimethacrylates of 2,2′-(1,2-phenylenebis(oxy)diethanol and triethylene glycol increases the maximum reduced rate of photopolymerization.

Effect of Viscosity of Dimethacrylate Ester-Based Compositions on the Kinetics of Their Photopolymerization in Presence of o-Quinone Photoinitiators

The kinetics of photopolymerization of compositions based on dimethacrylate oligomers with viscosities ranging from 1.5 to 376.7 cSt under the action of visible light (9,10-phenanthrenquinone and a mixture of 3,6-di-tert-butyl-1,2-benzoquinone with N,N-dimethylcyclohexylamine as initiators) is studied. Dimethacrylates of poly(ethylene glycols) with the number of ethoxy fragments of n = 1–4 and 8, dimethacrylates of OKM-2 and MDF-2 trademarks, and solvents (benzene, acetonitrile, and dinonyl ester of phthalic acid) are used. At the initial stages of the reaction, the dependence of the reduced rate of photopolymerization of such compositions on their initial viscosity is described by a curve attaining a plateau at a viscosity of 100 cSt or above. The dependences of viscosity of all dimethacrylates on temperature ranging from –10 to +80°С are determined, the effective activation energies of monomer viscous flow are calculated, and the temperature dependences of the number of molecules in associates for each of the oligomers are ascertained. At Т = 20°С, the number of molecules in the associates of poly(ethylene glycol) dimethacrylates with n = 1–4 does not exceed 10, for n = 8 the number of molecules in the associates is ∼102, and for dimethacrylates of OKM-2 and MDF-2 trademarks this value is above 104.

Experimental determination of microphase separation conversion during photopolymerization of cross-linking monomers to control the evolution of optical inhomogeneities in the volume of the forming polymer

The method of small-angle scattering of optical radiation has been used to study the dynamics of the concentration inhomogeneities evolution in a photopolymerizable composition based on cross-linking monomer, which is restrictedly compatible with its own polymer. The example of oligocarbonate dimethacrylate OCM-2, shows that after the composition photoinitiation is stopped the inhomogeneities evolve depending essentially on the phase state of the polymerizable medium. In the homophase state, these inhomogeneities relax due to diffusion, and in the heterophase state their amplitude grows due to microphase separation. Basing on this regularity an optical technique is proposed to determine the values of conversion at which the polymerizable medium passes from the homophase state to the heterophase state combining the studies on the kinetics of polymerization by IR spectroscopy. The method to optically control the process of concentration inhomogeneities evolution in the volume of the polymerizing layer by switching the polymerization initiation modes during the homo- and heterophase states of the composition is proposed and experimentally realized.

New antireflection coatings on silicate glass, deposited from a silicon dioxide sol containing a nonionogenic surfactant and an oligoether based on ethylene oxide

Transparent nanoporous thin films with low refractive index (1.23–1.25) were produced on glass substrates by application of a formulation based on a silicon dioxide sol into which two organic compounds, an oligoether based on ethylene oxide and a nonionogenic surfactant, are introduced. It is shown that the antireflection capacity of the nanoporous coating can be substantially raised at comparatively low concentrations of silicon dioxide and organic additives in the sol by making higher the rate at which the formulation is applied to the substrate.