Coarse‐grained models for frontal photopolymerization with evolving conversion profile

We introduce a series of ‘minimal’ models to describe a common light‐driven directional solidification process, known as frontal photopolymerization (FPP ), focusing on experimental observables: the solidification kinetics, light attenuation and spatiotemporal monomer‐to‐polymer conversion. Specifically, we focus on FPP propagation that yields conversion profiles that are not invariant with time, and which cannot be simply described by the presence of mass or heat diffusion. The models are assessed against experimental data for the photopolymerization of a model trimethacrylate system. We find that the simplest model, comprising a single equation of motion for the conversion fraction ? and a generalized Beer–Lambert law, can only describe the experimental data by assuming an unphysical variation in optical absorption. Introducing a ? ‐dependent reaction constant K _eff is found to require a time dependence, regardless of the functional form in ? . We conclude by introducing a ‘minimal’ chemical model, which is based on a simple three‐step reaction scheme involving the spatiotemporal evolution of the photoinitiator fraction, relative fraction of radicals and monomer conversion fraction, that is able to capture the experimental data with a small number of parameters and under reasonable FPP assumptions. Our framework provides important predictive ability for ubiquitous solidification and patterning processes, including three‐dimensional printing, via photopolymerization. © 2017 Society of Chemical Industry     

Propagating waves of network formation induced by light

The exposure of a photopolymerizable liquid (e.g., multifunctional thiol-ene) to ultraviolet radiation often leads to a propagating wavefront of network formation that invades the unpolymerized material from the illuminated surface of the photosensitive material. We theoretically describe this light-driven frontal photo-polymerization (FPP) process, which is the basis of many commercially important fabrication methods, in terms of an order parameter ?(x,t) characterizing the extent of monomer-to-polymer conversion, the temporally and spatially evolving optical attenuation μ(x,t) of the medium, and the height h(t) of the resulting solidified material. The non-trivial aspects of this frontal polymerization process derive from the coupling of μ(x,t) and the growing non-uniform network ?(x,t) and we consider limiting situations in which the optical attenuation increases (‘photodarkening’) or decreases (‘photobleaching’) in time to illustrate the general nature of FPP front propagation and the essential variables on which it depends. Since FPP fabrication of complex three-dimensional structures containing components having different material characteristics would greatly extend the practical utility of this method, we explore the influence of nanoparticle (silica, titania, and multi-wall carbon nanotube) additives on FPP front propagation. We also characterize the influence of temperature on the kinetics of FPP since this factor can often be controlled in practice. The experiments elucidate basic physical aspects of FPP and are well described by our model with a sensible variation of relevant physical parameters (optical attenuation and chemical rate constant) governing frontal growth. Our results are of interest both from the standpoints of complex structure fabrication and for understanding the fundamental nature of the FPP process.     

Improvement of Chromatic Aberration of the Plastic Rod‐Lens Array, 2

Summary: We propose in this paper a new plastic rod‐lens with low chromatic aberration. Since a plastic rod‐lens has a parabolic refractive index distribution, it usually also shows a distribution of Abbe number. This Abbe number distribution is thought to cause the chromatic aberration of the plastic rod‐lens array. We have studied novel materials for a new plastic rod‐lens with low chromatic aberration and have provided new transparent polymer blends consisting of two polymers with different refractive indices and with almost equal Abbe numbers by using a fabrication process based on photopolymerization of methacrylate monomer(s) in the presence of other methacrylic polymers. The process can give new transparent polymer blends which cannot be formed using conventional techniques for polymer blend formation. In this work, tricyclo[5,2,1,0^2,6]deca‐8‐yl methacrylate is used as a high refractive index monomer and tert‐butyl methacrylate is used as a low refractive index one. By using polymer blends including these monomer units, we have created a plastic rod‐lens with lower chromatic aberration.

Molecular structures of comonomers in the study.     

Polymerization of styrene using N-(p-tolyl)-N′,N′-diethyldithiocarbamoylacetamide as photoiniferter

N-(p-Tolyl)-N′,N′-diethyldithiocarbamoylacetamide was prepared and used as a new photoiniferter for polymerization of styrene in bulk under UV light. This photopolymerization showed some specific character, i.e., molecular weights of resulting polymers increase with reaction time and monomer conversion, respectively. The reversible reaction between growing radical and dormant species was confirmed by electron spin resonance using 2-methyl-2-nitrosopropane as a spin trapping agent. Thus, the end functionalized polymer, N,N-diethyldithiocarbamoyl-terminated polystyrene, reinitiated photopolymerization of methyl methacrylate and vinyl acetate to form block copolymers. The molecular weight distribution about 2.7 indicated that the polymerization is not a strictly living radical polymerization. © 1996 John Wiley & Sons, Inc.     

Photoinitiated polymerization in ionic liquids and its application

Ionic liquids (ILs) are low‐melting organic salts often liquid at room temperature, whose unique properties are the reason of increasing interest for their applications as solvents, reaction media and functional additives. The exceptional properties of ILs have proved to be particularly useful in polymer science giving the potential to produce polymeric materials with improved properties or to immobilize ILs in polymer matrices while keeping their special characteristics. One of the possibilities is polymerization in ILs which can also affect positively polymerization reactions. An especially attractive technique is photopolymerization due to the ease of process control, short reaction time and ambient working temperature. This review gives a literature survey of developments in photopolymerization processes carried out in ILs as well as applications of these processes. It covers both the photopolymerization in ILs as well as photopolymerization of IL monomers. The first part presents a short overview of physicochemical and photochemical properties of ILs; it includes also photochemical reactions and photoinitiation of polymerization in ILs. The second part covers both the basic research (kinetics of photopolymerization including polymerization rate coefficients and polymerization of IL monomers) as well as applications of UV‐induced polymerization in ILs. © 2016 Society of Chemical Industry     

The influence of N-vinyl-2-pyrrolidinone in polymerization of holographic polymer dispersed liquid crystals (HPDLCs)

Holographic polymer dispersed liquid crystals (HPDLCs) are targeted for application in a wide range of devices as dynamically switchable transmission or reflection diffraction gratings tunable through the visible spectra. The inclusion of N-vinyl pyrrolidinone (NVP) has been shown to reduce liquid crystal (LC) droplet size in HPDLC gratings and subsequently improve HPDLC performance. In this work, the influence of NVP on HPDLC polymer/LC morphology is examined and correlated to the influence of NVP on HPDLC photopolymerization kinetics and LC phase separation. As in other photopolymer systems, NVP significantly increases the rate of polymerization in HPDLC photopolymerization. In all the HPDLC formulations studied, NVP is completely incorporated into the polymer network while less than half of acrylate double bonds react. Furthermore, as the highly cross-linked polymer network forms, the small mono-vinyl NVP appears to react preferentially with acrylate double bonds, facilitating additional conversion of pendant double bonds otherwise trapped in the polymer network. NVP also induces a delay in the onset of reaction diffusion termination and extends the range of conversions for which reaction diffusion is observed. Interestingly, NVP also impacts polymer/LC morphology by delaying LC phase separation to higher double bond conversions. Together, the influence of NVP on the polymerization kinetics and LC phase separation alters HPDLC morphology by limiting LC droplet size, consequently resulting in improved HPDLC performance.     

Rate enhancement of amines in the photopolymerization of methyl methacrylate under oxygen

The photopolymerization of methyl methacrylate was accelerated by amines such as N,N-dimethylbenzylamine, triethylamine, and diethylamine under nitrogen, in which additional initiating radicals were generated by the reaction of the amines with excited MMA. Enhanced rates were observed under oxygen for this photopolymerization in the presence of amines. Because UV spectra of the amines under oxygen indicated that the amines form charge-transfer complexes with oxygen, the rate enhancement was ascribed to photodecomposition of complexes that yield radical species. Molecular weights of polymers obtained in the photopolymerization in the presence of amines decreased under oxygen, supporting the assumed mechanism. © 1998 Society of Chemical Industry     

Novel carbazole phenoxy-based methacrylates to produce high-refractive index polymers

A novel, high-refractive index homopolymer was produced by incorporating carbazole and phenol into the methacrylate monomer structure. The reaction of phenol with 9-(2,3-epoxypropyl)-carbazole, followed by the reaction of the carbazole phenoxy-based intermediate with methacryloyl chloride or methacrylic anhydride, and recrystallization from methanol, produced a good yield of highly pure carbazole phenoxy functionalized methacrylate monomer. Subsequent free radical polymerization or UV photopolymerization of the functionalized methacrylate monomer, in addition to copolymerizations with methyl methacrylate, provided for high-refractive index materials well suited for lightweight optical applications. Unlike N-vinyl carbazole, the novel carbazole phenoxy-based methacrylate readily copolymerized with methyl methacrylate. Statistical copolymers of carbazole based methacrylates with methyl methacrylate were produced by free radical solution polymerization in DMAC or by photopolymerization in DMF. The carbazole phenoxy-based methacrylate monomer was characterized for molecular weight using gel permeation chromatography (GPC), for melting point and glass transition temperature using differential scanning calorimetry (DSC), for decomposition using thermal gravimetric analysis (TGA), and for chemical composition by one- and two-dimensional ¹H nuclear magnetic resonance (NMR) spectroscopy and by elemental analysis. The AIBN initiated carbazole phenoxy-based methacrylate polymerization was followed using in situ FTIR, which showed the reaction to be complete within 40 min in DMAC at 90 °C. Refractive indices of the carbazole based methacrylate homopolymers and copolymers ranged from 1.52 to 1.63. PhotoDSC was used to determine the heat of polymerization (ΔH_p) for the carbazole phenoxy-based methacrylate (ΔH_p=−39.4 kJ/mol). The carbazole phenoxy-based methacrylate homopolymer had a surprisingly high onset of decomposition temperature (T_onset=316 °C). ¹³C NMR spectroscopy experiments and molecular modeling were used to explore the configuration of the polymerized carbazole phenoxy-based methacrylate. The lack of head-to-head linkages due to steric considerations reasonably explains the high thermal stability observed for the carbazole phenoxy-based methacrylate polymer.     

Novel visible photoinitiators systems for free-radical/cationic hybrid photopolymerization

Free radical/cationic hybrid photopolymerization of acrylates and epoxides was induced using an initiator system comprised of the dye derivative 5,12-dihydroquinoxalino[2,3-b]quinoxaline or 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine as the sensitizers and hexafluoroantimonate triarylsulfonium salt as the initiators. The curing experiments were carried out in the presence of air and the consumption of each monomer upon VIS-radiation was monitored in situ by real-time infrared spectroscopy. Hardness (H), elastic modulus (E) and the H/E ratio of the coatings obtained by visible-initiated acrylate/epoxide hybrid photopolymerization were determined using nanoindentation. DSC measurements show that the initiator system presented here may produce an interpenetrating polymer network. The pencil hardness of the obtained coatings indicates that the dye/hexafluoroantimonate triarylsulfonium salts systems studied here may have practical applications as visible-light hybrid initiators.     

Novel visible photoinitiators systems for free-radical/cationic hybrid photopolymerization

Free radical/cationic hybrid photopolymerization of acrylates and epoxides was induced using an initiator system comprised of the dye derivative 5,12-dihydroquinoxalino[2,3-b]quinoxaline or 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine as the sensitizers and hexafluoroantimonate triarylsulfonium salt as the initiators. The curing experiments were carried out in the presence of air and the consumption of each monomer upon VIS-radiation was monitored in situ by real-time infrared spectroscopy. Hardness (H), elastic modulus (E) and the H/E ratio of the coatings obtained by visible-initiated acrylate/epoxide hybrid photopolymerization were determined using nanoindentation. DSC measurements show that the initiator system presented here may produce an interpenetrating polymer network. The pencil hardness of the obtained coatings indicates that the dye/hexafluoroantimonate triarylsulfonium salts systems studied here may have practical applications as visible-light hybrid initiators.     

Preparation and characterization of UV-cured epoxy nanocomposites based on o-montmorillonite modified with maleinized liquid polybutadienes

Polymeric nanocomposites based on an epoxy cycloaliphatic resin and o-montmorillonite (Cloisite 30B^®) modified by reaction with maleinized liquid polybutadienes were prepared through photopolymerization. The conditions for the o-montmorillonite modification were settled. After the reaction, a strong increase of the basal spacing of the nanoclays was evidenced through XRD analysis. The modified clays were dispersed in the epoxy resin (5 wt.%) and subjected to the UV-curing process. The kinetics of the photopolymerization process was evaluated by means of Real-Time FT-IR spectroscopy. The obtained nanocomposites were studied by means of XRD and TEM analyses; the formation of either intercalated or quasi-exfoliated structure was assessed. The thermal and mechanical properties of the cured films were evaluated and correlated to the morphology of the obtained nanocomposites.     

Highly branched polyurethane acrylates and their waterborne UV curing coating

A series of UV curable highly branched waterborne polyurethane acrylates (BWPUAs) were synthesized using an “oligomeric A₂ + B₃” approach. The thiol-endcapped difunctional oligomeric A₂ was synthesized first by the addition reaction of isophorone diisocyanate, α,α-dimethylol propionic acid and 2-hydroxyethyl acrylate, then further underwent thiol-Michael reaction with 1,6-hexamethylene bis(thioglycolic acetate). Trimethylolpropane triacrylate was used as a B₃ monomer. The molecular structures were characterized with FT-IR and ¹H NMR spectroscopy. 1,6-Hexanediol diacrylate (HDDA) was incorporated into the polymeric chain for preparing the HDDA-modified BWPUAs (BWPUA-Hs). For the comparison, the linear waterborne polyurethane acrylate (LWPUA) was synthesized. The UV curing kinetics study results by using the photo-DSC approach showed that the BWPUAs possessed higher photopolymerization rate and final unsaturation conversion in the UV cured films compared with the LWPUA, which increased with the increase of unsaturation concentration in BWPUA. Moreover, the photopolymerization performance, and water and solvent resistance properties were greatly enhanced by the incorporation of HDDA segment into the BWPUA chain. The dynamic mechanical thermal analysis results showed that the elastic modulus in the rubbery plateau, and the glass transition temperature of UV cured film increased with increasing unsaturation concentration in BWPUA, whereas decreased with the introduction of HDDA flexible segment. The thermogravimetric analysis confirmed the high thermal stability of UV cured BWPUA films. All UV cured BWPUA and BWPUA-H films showed better flexibility and middle refractive indices due to the thioether linkage in the polymer network.     

Crosslinked microparticle preparation in supercritical carbon dioxide with photopolymerization

An antisolvent processing technique by simultaneous compressed antisolvent precipitation and photopolymerization for cross-linked polymer microparticles formation was presented in this paper. In this process, photopolymerization of the homogeneous solution composed of methylene chloride, poly(ethylene glycol)600 diacrylate (PEG600DA) as monomer and diphenyl-(2,4,6-trimethylbenzoyl)-phosphine oxide (TPO) or 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator resulted to microparticle when it was sprayed into supercritical carbon dioxide (scCO₂) and simultaneously exposed to initiating light. High miscibility of the solvent in scCO₂ made methylene chloride quickly extracted from the dispersion phase, leaving very high concentrations of monomer (PEG600DA) dispersed in scCO₂. The high monomer concentration combined with photo initiating polymerization facilitates rapid reaction rates and ultimately lead to polymer precipitation. Particle size and morphology were adjustable by changing the processing conditions, such as temperature and pressure. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Cryochemical Synthesis of Mn-Containing Poly(p-Xylylene)

A Mn-containing polymer was produced by solid-state photopolymerization of a cocondensated mixture of p-xylylene with Mn at 80 K. It was shown that during simultaneous deposition p-xylylene and Mn formed both σ- and π-complexes. Irradiation of the system by UV light resulted in polymerization of p-xylylene with the destruction of the unstable π-complexes, while the σ-bonded compounds were incorporated into the polymer chains. Complexes of Mn with benzyl-type radicals of the polymerized system were also observed. Prolonged storage of the polymerized material at ambient temperature under vacuum led to the gradual decomposition of the organomanganese compounds and complexes.     

Sesquiterpene Synthases Cop4 and Cop6 from Coprinus cinereus: Catalytic Promiscuity and Cyclization of Farnesyl Pyrophosphate Geometric Isomers

Sesquiterpene synthases catalyze with different catalytic fidelity the cyclization of farnesyl pyrophosphate (FPP) into hundreds of known compounds with diverse structures and stereochemistries. Two sesquiterpene synthases, Cop4 and Cop6, were previously isolated from Coprinus cinereus as part of a fungal genome survey. This study investigates the reaction mechanism and catalytic fidelity of the two enzymes. Cyclization of all‐trans‐FPP ((E,E)‐FPP) was compared to the cyclization of the cis–trans isomer of FPP ((Z,E)‐FPP) as a surrogate for the secondary cisoid neryl cation intermediate generated by sesquiterpene synthases, which are capable of isomerizing the C2? C3 π bond of all‐trans‐FPP. Cop6 is a “high‐fidelity” α‐cuprenene synthase that retains its fidelity under various conditions tested. Cop4 is a catalytically promiscuous enzyme that cyclizes (E,E)‐FPP into multiple products, including (?)‐germacrene D and cubebol. Changing the pH of the reaction drastically alters the fidelity of Cop4 and makes it a highly selective enzyme. Cyclization of (Z,E)‐FPP by Cop4 and Cop6 yields products that are very different from those obtained with (E,E)‐FPP. Conversion of (E,E)‐FPP proceeds via a (6R)‐β‐bisabolyl carbocation in the case of Cop6 and an (E,E)‐germacradienyl carbocation in the case of Cop4. However, (Z,E)‐FPP is cyclized via a (6S)‐β‐bisabolene carbocation by both enzymes. Structural modeling suggests that differences in the active site and the loop that covers the active site of the two enzymes might explain their different catalytic fidelities.     

Synthesis and photoinitiating behavior of benzophenone-based polymeric photoinitiators used for UV curing coatings

Three benzophenone-based type II polymeric photoinitiators, poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane maleate)) (PBM), poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane succinate)) (PBS), and poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane-co-2-(phenyl-methylene-ether)-1,3-dihydroxypropane maleate)) (PBPM) used for free radical UV curing systems, were prepared through the step-growth polymerization of 4-(2,3-epoxypropyloxy) benzophenone (EBP) with maleic anhydride and succinic anhydride, as well co-polymerization with phenyl glycidyl ether and maleic anhydride, respectively. The molecular structures were characterized with ¹H NMR and FT-IR spectroscopy, and GPC analysis. For equimolar EBP and MA reaction system, the M_n led to the maximum of 6868 g/mol with the PDI of 1.22. The UV spectroscopy analysis showed that the synthesized polymeric photoinitiators possess higher UV absorption intensity in the wavelength range of 300–400 nm compared with benzophenone (BP). The photoinitiating activity was examined based on the photopolymerization of tripropyleneglycol diacrylate (TPGDA) in the presence of triethylamine as a coinitiator by using Photo-DSC method. It was found that PBM and PBS showed higher photoinitiating efficiency than BP in the photopolymerization of TPGDA. Moreover, the side BP moiety incorporated into the polymeric chain possessed higher initiating activity than end-capped BP moiety. Moreover, PBM with higher molecular weight was more efficient to photoinitiate TPGDA UV-cured. The side chromophore group distribution in the molecular chain also affected the photoinitiating activity. The highest photopolymerization rate at the peak maximum was obtained by the photoinitiation with PBM prepared with the molar feed ratio of 1.0 of EBP to MA.     

连续微流体在光聚合制备聚合物颗粒中的应用进展

光聚合以其对环境友好、反应条件温和、适应性广、成本低等优点备受关注。连续微流体在光聚合中的应用有助于提高反应效率并实现聚合的高度控制。连续微流体在光聚合制备聚合物颗粒中的优点包括可重复性高、合成精度高、能控制聚合物的粒径、多分散性和内部结构。在这篇综述中，总结了连续微流体在光聚合合成纳米颗粒中的应用，其中包括单相微流体光聚合和多相微流体光聚合，具有各种形态的聚合物颗粒的形成机理以及典型实例。     

Non-fickian diffusion with reaction in glassy polymers with swelling induced by the penetrant—effects of consecutive and parallel reactions

A mathematical model is presented for non-Fickian diffusion of a penetrant A into a granular glassy polymer containing a reactive group B, resulting in the desired product P. Further, both a consecutive reaction between A and P (producing X) and a parallel reaction between A and C (producing Y) are incorporated, with C initially present in the particle. The swelling of the polymer, induced by the penetrant, is described by power-law kinetics for the velocity of the swelling front. Kinetics are considered to be first order in each of the two reactants. Concentration profiles in the particle and selectivity to desired product are calculated as function of the swelling behavior of the polymer grain. In case of a consecutive reaction the local concentration of P reaches a maximum value independent of the swelling rate. However, the position of the maximal concentration of P moves towards the center of the grain with a rate depending on the kinetics of swelling. For Case II diffusion this velocity equals the velocity of the advancing front between glassy and rubbery polymer. The selectivity of the desired reaction decreases with decreasing swelling rate. A low swelling rate also results in an inhomogeneous product distribution within the granule. A criterion is derived predicting under what conditions the consecutive reaction can be neglected and a pure product is obtained. The analysis further reveals that both a more homogeneous product and a higher selectivity toward a desired product can be obtained by realizing preswelling of the polymer with an inert swelling agent. For Case II diffusion the concentration profiles of the side product of the parallel reaction, Y, are flat in the rubbery part of the polymer. This is caused by the relatively low swelling rate allowing Y to redistribute in the swollen polymer. If additional C is continuously supplied from the gas phase, then the selectivity decreases continuously with increasing conversion of B.     

Photodegradation of chloromethyl vinyl ketone polymer and copolymers with styrene and α-methylstyrene

The photodegradation behaviour of chloromethyl vinyl ketone (CMVK) polymer has been compared with that of polymers of methyl and tertiary butyl vinyl ketones, MVK and t-BVK, respectively. The results obtained for poly(CMVK) showed that the quantum yields of main chain scission in dioxane and of the Norrish type I reaction (ø_I) in film were 0.18 and 0.61 times, respectively, greater than those of poly(MVK), while ø_I for poly(t-BVK) was about 19 times that of poly(MVK). It was also found that the photolysis of poly(CMVK) produced a considerable amount of hydrogen chloride and its quantum yield ø_HCl was higher than ø_I, i.e.$\text{ø}{}_{\mathrm{<mtext>HCl</mtext>}}\text{ø}{}_{\mathrm{<mtext>I</mtext>}}\text{= 1.7}$. This was attributed to the intramolecular transfer of photoexcited energy from the ketone chromophore to the adjacent chloromethyl group. The existence of such a reaction was supported from the remarkable acceleration of the photopolymerization of methyl methacrylate. The photodegradation mechanism of poly(CMVK) is discussed, in the light of the results obtained above and from the studies on quenching and on the photodegradation of CMVK copolymer.     

CFD study on the radial distribution of coolants in the inlet section of rod-baffle-multi-tubular reactor

CFD method was employed to investigate the radial distribution of coolants in the inlet section of rod baffle multi-tubular reactor. It was found that the reactor had poor distribution of coolants. In view of this problem, the detailed structures of incident channel, shape of baffle ring and distance between tube plate and the first rod baffle were discussed to study their effect on the radial distribution. The simulation results showed that incident channels 0^#, 1^#, 3^#, 5^# and 6^# could improve radial distribution of coolants significantly, and the effect of incident channels 2^# and 4^# was relatively limited. The zigzag baffle ring was proved to prevent short pass near the shell wall. In addition, the radial distribution of coolants was also improved with distance between tube plate and the first rod baffle increasing from 85 mm to 242mm. The simulation results can provide guidance for designing multi-tubular reactor with rod baffles.