Light-induced crosslinking polymerization of a novel N-substituted bis-maleimide monomer

The photoinduced polymerization of a N-alkyl substituted bis-maleimide (MI) has been studied by real-time infrared spectroscopy. The polymerization of the neat monomer proceeds rapidly and extensively because of both its great absorbance in the UV-range, and the presence on the alkyl chain of easily abstractable hydrogen atoms, which are needed to produce the initiating radicals. Such photoinitiator-free resin was found to be less sensitive to oxygen inhibition than typical UV-curable acrylate resins, the peroxyl radicals formed being capable to propagate the chain reaction. This N-substituted bis-maleimide was also successfully used as monomeric photoinitiator to induce the crosslinking polymerization of acrylate monomers through the free radicals formed upon its photolysis.     

The kinetics of vinyl acrylate photopolymerization

The photopolymerization of vinyl acrylate has been investigated using real-time FTIR. The acrylate and vinyl groups exhibit different polymerization kinetics with the acrylate functionality polymerizing at a much faster rate. Vinyl acrylate not only self-initiates its own free-radical polymerization, but also photoinitiates the polymerization of mono- and difunctional acrylates. Kinetic studies using a model monomer system indicate that it is essential for the acrylate and vinyl group to be part of the same molecule in order to function as an effective initiator for acrylate polymerization.     

In situ monitoring of mechanical properties during photopolymerization with particle tracking microrheology

Microrheology was employed to perform in situ monitoring of the liquid-to-gel transition during free-radical photopolymerization. Photosensitive acrylate resins were exposed to ultraviolet light, while the Brownian motion of micrometer sized, inert fluorescent tracer particles was tracked via optical videomicroscopy. Statistical analysis of particle motion yielded the rheological properties of the embedding medium as a function of time and location, thereby relating UV exposure to the progress of polymerization and gelation. Microscopy enabled a detailed study of three-dimensional gelation profiles; other experimental parameters that were varied include photoinitiator concentration, monomer composition, the presence of oxygen and light intensity. Significant changes in gelation time were observed with increasing distance from the illuminated surface into the sample under all conditions. The results were used to test the accuracy of the standard energy threshold model, which is often used to empirically predict the outcome of photopolymerization reactions.     

The effect of monomer structure on oxygen inhibition of (meth)acrylates photopolymerization

Oxygen inhibition during the free-radical photopolymerization of various monomers containing ether groups was investigated using photo-DSC and real-time FTIR (RTIR). Methacrylates and acrylates containing different number and types of ether groups were selected to determine the effects of ether group concentration and structure on oxygen inhibition. Also, the oxygen sensitivity of the free-radical polymerization of methacrylate and acrylate was compared. Compared to acrylates, methacrylates are much less sensitive to oxygen. Model poly(tetramethylene oxide) and poly(propylene glycol) system were evaluated to determine the effect of ether groups on polymerization kinetics in air. The polymerizations of the (meth)acrylates containing ether groups were found to be relatively insensitive to oxygen inhibition. The reduction of oxygen inhibition occurs by a series of chain transfer/oxygen scavenging reactions.     

Synthesis and photopolymerization kinetics of new flexible diacrylate and dimethacrylate crosslinkers based on C18 diacid

A series of new di(meth)acrylate monomers was synthesized from the reactions of methyl α-hydroxymethylacrylate (MHMA), ethyl α-hydroxymethylacrylate (EHMA), hydroxyethyl acrylate (HEA) and hydroxyethylmethacrylate (HEMA) with α,ω-C18 diacid chloride. The photopolymerization behavior and reaction kinetics of the synthesized monomers were investigated using photoinitiation with differential scanning calorimetry. The polymerization rates, conversions and kinetic constants for propagation and termination were determined for each of the monomers. The maximum rate of polymerizations of the diacrylate monomers was higher than that of the dimethacrylate monomers and followed the order: HDDA (1,6-hexanediol diacrylate)>HEA-C18>EHMA-C18∼HEMA-C18>MHMA-C18. The total conversions obtained were 78, 75, 72, 64 and 69% for MHMA-C18, EHMA-C18, HEMA-C18, HEA-C18 and HDDA, respectively, indicating comparable or higher conversions for methacrylates despite their lower rates of polymerization. Propagation and termination mechanisms of the monomers were investigated by plotting propagation and termination rate constants as a function of conversion.     

Radiation chemistry aspects of polymerization and crosslinking. A review and future environmental trends in ‘non-acrylate’ chemistry

Radiation induced polymerization of acrylate based materials has been in industrial use for almost twenty years. This field is still growing rapidly and the advantages of the technology, e.g. being solvent free and rapid ‘cure’, are very attractive properties from an industrial and environmental point of view. Mixtures of mono- and multifunctional acrylates are today the ‘heart’ of radiation curable systems. However, serious concerns about the health hazards connected with the handling of the liquid coating is an important issue. Different approaches to overcome this problem have been addressed and the obvious thing to start with was the investigation of a great number of acrylates regarding their potential health effects. Acrylates do exhibit some specific unattractive properties, regarding their toxicology profile. Sensitization and skin irritation are considered to be limiting factors for a continuing rapid expansion of this successful technology. A further development and improvement of the photoinitiator chemistry together with a more efficient use of high powered irradiators with a narrow bandwidth distribution, will improve still the acrylate technology. The importance of degree of conversion of carbon-carbon double bonds as a function of dose rates, i.e. residual unpolymerized acrylic monomers, will be discussed. An additional approach, since the acrylate technology is well established, is to further optimize the ‘reactivity’ and conversion on ‘new’ acrylates in order to obtain maximum conversion, thus minimizing the residual amount of monomers that can migrate out of the coating. The increasing functionality of the acrylate/oligomer will of course result in an increasing probability of monomer/oligomer being attached to the crosslinked network. Furthermore, this paper emphasizes potential acrylate replacements, and ‘new’ or alternative chemistry for acrylates will be introduced. The state of the art and the associated problems for cationically induced polymerization, free radical alternating copolymerization, hybrid systems and direct photolysis of donor/acceptor pairs are discussed.     

Unique morphology and properties study of polyacrylate obtained via frontal photopolymerization

Remarkable radial temperature distribution at the advancing reaction front was determined in frontal photopolymerization (FPP). Through SEM observation of the cross-fracture section and vertical fracture section for the heat-insulated FPP polymer rod, a cylindrical multi-layer structure was first observed in the FPP product. A sleeve-like model was proposed to describe the unusual morphology on the basis of polymer self-drawing and convection around reaction front. Vertical gradient distribution of molecular weight and its polydispersity in the poly(isobornyl acrylate) rod obtained via FPP was found through GPC determination, which might be developed as an in situ self-fractionation technique for polymer. Thermal analysis of the polymers showed that the FPP tended to produce purer polymer than the traditional thick film photopolymerization.     

Investigation of PSt-MWCNT concentration on epoxyacrylate photopolymerization and conductivity of polymer films

Photopolymerization kinetics and conductivity changes of epoxyacrylate composites for various loading modified PSt-MWCNT weight fractions changing from 0.0025 to 0.2 wt.% were evaluated by performing photo differential scanning calorimetry (photo-DSC) and four point conductivity measurements. 0.2% PSt-MWCNT additive polymeric films had their electrical conductivity boosted by 6% more than non-additive polymeric films.     

Synthesis and photopolymerization of novel,highly reactive phosphonated-urea-methacrylates for dental materials

An urea methacrylate (1) and two phosphonated methacrylates (2–3) were synthesized from 2-isocyanatoethyl methacrylate (IEM) and benzyl amine (1), diethyl aminomethylphosphonate (2) and diethyl amino(phenyl)methylphosphonate (3). Their photopolymerization rates are notably higher than commercial monomers, despite the presence of only one double bond. Their polymerization rates follow the order 1 ～ 2 > 3 ～ triethylene glycol dimethacrylate (TEGDMA) > 2-hydroxyethyl methacrylate (HEMA). A tendency toward high crosslinking density during thermal bulk polymerizations, low oxygen sensitivity and high conversions with benzophenone during photopolymerization indicated the importance of hydrogen abstraction/chain transfer reactions. It was found that the addition of the monomers to HEMA significantly increased its polymerization rate, proving their utility as replacements for TEGDMA as reactive diluents for 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propyloxy) phenyl] propane (Bis-GMA). Copolymer systems containing 2 and 3 showed improved T_g values compared to Bis-GMA/TEGDMA systems.     

Silsesquioxane functionalized with methacrylate and amine groups as a crosslinker/co-initiator for the synthesis of hydrogels by visible-light photopolymerization

A silsesquioxane bearing methacrylate and amine groups was synthesized by reacting half of the NH groups of N-[3-(trimethoxysilyl)-propyl]ethylene diamine with glycidyl methacrylate, followed by the hydrolytic condensation of methoxysilane groups. This led to a water-soluble silsesquioxane functionalized with methacrylate and amine groups (SFMA) that was characterized by UV-MALDI TOF MS. A formulation containing safranine-O as sensitizer and SFMA as co-initiator (electron donor)/crosslinker was used for the visible-light photopolymerization of water solutions of 2-hydroxyethylmethacrylate (HEMA) and acrylamide (AAm). Without SFMA addition no reaction was observed while in the presence of 0.8 wt% SFMA with respect to monomers, crosslinked polymers (hydrogels) were obtained. Swelling of the resulting hydrogels in a broad pH range was investigated.     

Morphological studies of LC polymer networks prepared by photopolymerization of (LC monomer/LC) blends

The morphology of LC polymer networks prepared by photopolymerization of (LC monomer/LC) blends containing photoinitiator and crosslinker was investigated. For the blends of 4-acryloyloxyhexyloxy-4′-cyanobiphenyl and 4-hexyloxy-4′-cyanobiphenyl, which had the same mesogen, orientation order of LC textures was memorized by photopolymerization, while any structure of LC polymer networks was not observed under an optical microscope because the networks did not phase separate from the low molecular weight LC. However, specific anisotropic phase-separated structures of LC polymer networks were observed for the blends of 4-acryloxloxyhexyloxy-4′-cyanobiphenyl and 4-hexyloxybenzoic acid, which had dissimilar mesogens, on condition that photopolymerization was carried out under the LC phase. If photopolymerization was performed under the isotropic phase conditions, polygonal or continuous phase-separated structures of LC polymer networks were observed for the dissimilar mesogenic blend. These morphologies were strongly dependent on the phase diagrams of (LC monomer/LC) blends and (the corresponding LC polymer/LC) blends. It has been found that the ordering field of LC molecules can give LC polymer networks anisotropic morphologies, which have the long range as the same length scale of LC textures.     

A visible light photoinitiator system to produce acrylamide based smart hydrogels: Ru(bpy)3 as photopolymerization initiator and molecular probe of hydrogel microenvironments

A novel visible light bimolecular photoinitiator system (tris(2,2′-bipyridine)ruthenium(II)/N,N-dimethylaniline) is shown to be able to polymerize N-isopropylacrylamide (NIPAM) and 2-Acrylamido-2-Methylpropanesulfonic Acid (AMPS), in aqueous solution, to render high molecular weight polymers and crosslinked gels. The photoinitiator is especially useful to synthesize thermosensitive polymers and gels, because it could be used at temperatures below the phase transition, allowing the polymer chain to grow in its uncoiled state. The polymerization and conversion rates are affected by the structure of the monomer, decreasing in the order NIPAM > AAm > AMPS. The properties of the gels agree with literature data, suggesting that the method is able to produce conventional and smart hydrogels. The microenvironments present near linear polymers and inside crosslinked gels were investigated by measuring fluorescence lifetimes and steady state anisotropy of the metallic complex (Ru(bpy)₃⁺², present in the solution. Clear effects of the polymer presence on the photophysical properties of the complex are observed. Therefore, the same metallic complex could be used as photoinitiator of vinyl polymerization and as molecular probe to sense the hydrogel microenvironments.     

新型紫外光产酸剂的合成及其光聚合动力学研究

以2-硝基噻吩,甲基磺酰氯及苯乙腈为原料合成了一种紫外光产酸剂(5-甲磺酸酯亚胺-5H-噻吩-2-亚基)-苯基-乙腈(MTPA),通过傅里叶红外光谱仪、核磁共振仪和紫外吸收光谱仪及热重分析对所合成的产物的结构和性能进行了表征,并利用实时红外(RT-IR)对该引发剂进行了光聚合反应动力学研究。考察了单体、产酸剂浓度对引发速率及单体转化率的影响。结果表明,在一定浓度范围内,随着引发剂浓度增大,聚合速率加快,最终的双键转化率也增高。     

Development of a comprehensive free radical photopolymerization model incorporating heat and mass transfer effects in thick films

A comprehensive kinetic model describing photopolymerization is developed which allows variation of temperature, species concentrations, and light intensity through the thickness of a photopolymerized film. Heat and mass transfer effects are included, as is the generation of heat by both reaction and light absorption. In addition to initiation, propagation, and termination mechanisms, both primary radical termination and inhibition are incorporated into the model. The possible presence and diffusion of an inert solvent are also accounted for. Thus, the model is useful for examining complex polymerization kinetics and behavior in industrially and commercially important thick film photopolymerizations, such as the curing of contact lenses, dental restorative materials, photolithographic resists, and optoelectronic coatings. The comprehensive model is used to predict polymerization rate, temperature, and conversion profiles in a variety of systems. The effects of heat generation and the thermal boundary conditions are explored, with the result that heat generation in thick samples leads to greatly increased conversions approaching 100 percent. Increased temperature in these samples also may lead to the appearance of two rate maxima, with the first due to the temperature increase and the second caused by the autoacceleration process. The magnitude of the temperature increase, along with the resultant effects, is more pronounced in insulated systems.     

巯基／乙烯基硅氮烷体系的紫外光聚合机理及动力学研究

乙烯基硅氮烷(VL20)是一种液态陶瓷前驱体,在光引发剂存在下与巯基化合物快速发生光聚合反应。文中利用先进的紫外光差示扫描量热(UV-DSC)实时跟踪和博里叶变换红外光谱(FTIR)原位跟踪等方法,研究了巯基-乙烯基硅氮烷VL20体系的光聚合反应机理和动力学特征。     

Molecular modeling of cellulose in amorphous state part II: effects of rigid and flexible crosslinks on cellulose

It is essential to understand the molecular level response of crosslinked cellulose chain segments upon deformation, in order to develop new agents which convey high durable-press (DP) rating to cellulose fibers with minimal strength loss. In this work models of amorphous cellulose crosslinked with both rigid and flexible crosslinks were constructed computationally for this purpose. Rigid crosslinks bound cellulose molecular segments together and blocked the chain slippage, providing cellulose models with a higher initial modulus and better elastic response. However, the loss of the chain slippage led to stress being distributed unevenly among cellulose chains. Chains in some regions were subjected higher stress and these regions were opened up much more than the rest of the cellulose, which presumably caused models to fail. When conformationally flexible crosslinks were used, breaking strain of cellulose was not significantly reduced but deformation recovery was not improved either, in comparison with the models of untreated cellulose. Conformational transitions were observed in the flexible crosslinks during extension. These results help to explain how and why rigid crosslinks work to provide wrinkle resistant properties and why they also lower tensile strength, and that just using a conformationally flexible crosslinking will not provide any recovery.     

A mechanistic and laser flash photolysis investigation of acrylamide polymerization photoinitiated by the three component system safranine-T/triethanolamine/diphenyliodonium chloride

The photopolymerization of acrylamide (AA) initiated by the synthetic dye safranine-T in the presence of triethanolamine (TEOHA) as co-initiator has been investigated in aqueous solution. It was found that the addition of diphenyliodonium chloride (DPIC) to the system has a marked accelerating effect on the polymerization rate. In the absence of DPIC an inhibiting effect of TEOHA is observed at high concentrations of the amine. This effect is suppressed by the presence of the onium salt. At the same time the photobleaching of the dye caused by the amine is suppressed by the presence of AA or DPIC. Laser flash photolysis experiments show that the presence of the onium salt increases the yield of triplet state of the dye. Also the yield of radicals in the quenching of the dye triplet by TEOHA increases by the presence of DPIC. The semi-reduced form of the dye decays faster in the presence of DPIC, suggesting a possible way of generating extra active radicals at the same time that the dye is regenerated. It is concluded that are several factors that contribute to the accelerating effect of DPIC on the polymerization rate.     

Removal of carbon disulfide: Experimental and modeling results

A mathematical model describing the rate of carbon disulfide (CS₂) removal has been developed. Kinetic studies were carried out in a fixed-bed reactor under atmospheric pressure and a range of temperatures (30-70 °C). The effects of flow rate, CS₂ inlet concentration, temperature and relative humidity were analyzed. A kinetic model based on axial dispersion, external and internal mass-transfer resistances, as well as effects of S deposition on the inner-face of the catalyst was in agreement with the CS₂ experimental breakthrough curves. The mathematical model can be used for process design and scale-up of similar systems.     

Crosslinking kinetics of methyl and ethyl (α-hydroxymethyl)acrylates: effect of crosslinker type and functionality

Photopolymerizations were carried out using monofunctional monomers methyl or ethyl-(α-hydroxymethyl)acrylate (MHMA and EHMA, respectively) with multifunctional crosslinkers. The effect of crosslinker type (acrylate versus methacrylate) and functionality (di-, tri-, tetra- and penta/hexaacrylates) on polymerization rates and conversions were investigated. RHMA systems (monomer plus crosslinker) reached very high conversions (70-83%) for all systems with the exception of dipentaerythritol penta/hexaacrylate systems. Network T_g played an important role in final conversions with dipentaerythritol penta/hexaacrylate and pentaerythritol tetraacrylate, while EHMA (with a lower T_g) went to higher conversions than MHMA.