Julian R. G. Evans

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Organization: University College London , England
Department: Department of Chemistry
Title: (PhD)

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Co-reporter:Theodore Hayes;Yingxue Hu;Sra A. Sanchez-Vazquez;Helen C. Hailes;Abil E. Aliev
Journal of Polymer Science Part A: Polymer Chemistry 2016 Volume 54( Issue 19) pp:3159-3170
Publication Date(Web):
DOI:10.1002/pola.28203

ABSTRACT

The use of biomass-sourced chemical feedstocks creates a conflict over land use between food and fuel/chemical production. Such conflict could be reduced by making use of the annual 1.3 Pg food waste resource. Oleic acid is available from seed oils such as pumpkin, grape, avocado and mango. Its esterification with diols 1,3-propanediol, resorcinol and orcinol was used to form diesters and the naturally occurring norspermidine was used to prepare a diamide, all under ambient conditions. These compounds were then epoxidized and polymerized. When esterification was followed by epoxidation and subsequent curing at elevated temperature with p-phenylenediamine or diethylenetriamine, hard insoluble resins were formed. When the sequence was changed such that the epoxidized oleic acid was first reacted with cis-1,2-cyclohexanedicarboxylic anhydride and then esterified with orcinol and resorcinol, insoluble crosslinked polymers were also obtained. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3159–3170

Co-reporter:T. Rowland, H. C. Hailes and J. R. G. Evans  
RSC Advances 2015 vol. 5(Issue 49) pp:38926-38930
Publication Date(Web):28 Apr 2015
DOI:10.1039/C5RA03469H
One way to increase planetary albedo in order to ameliorate global warming is to use ocean foams. The energy needed to maintain such foams, even if drawn from wave or wind is high and so they must be persistent, ideally surviving up to a month. While studying combinations of surfactants and gelling agents to make long-lived foams in salt water, it was apparent that some foams developed progressively higher reflectance as the upper surface dried. Scanning electron microscopy of the upper surface revealed 0.5 μm, near-monodisperse salt crystals. Thus with a refractive index of ∼1.5, sea salt, being in somewhat prodigious supply, now enters the arena as a potential mirror material. Its reflectance depends on the microstructure, specifically on the inhibition of Ostwald ripening. Studies were therefore carried out to investigate particle size effects in a membrane such as a foamed gel in which seawater dries on the upper surface. It was demonstrated that the organic additives used to create foam persistence also mediated sea salt crystallisation and in this way drastically modified the reflectance of the upper layer. These experiments suggest a new direction of research in planetary surface albedo modification.
Co-reporter:H.M.T.U. Herath, L. Di Silvio, J.R.G. Evans
Materials Science and Engineering: C 2015 Volume 57() pp:363-370
Publication Date(Web):1 December 2015
DOI:10.1016/j.msec.2015.07.052
•Biocompatibility of chemically identical, topologically different ZrO2 was tested.•ZrO2 promoted cell adhesion, proliferation, differentiation and nodule formation.•Proliferation was high on polished ZrO2 but initial recruitment was high on abraded ZrO2.•With open porosity, proliferation was low; cells cannot establish a microenvironment.Zirconia-3 mol% yttria ceramics were prepared with as-sintered, abraded, polished, and porous surfaces in order to explore the attachment, proliferation and differentiation of osteoblast-like cells. After modification, all surfaces were heated to 600 °C to extinguish traces of organic contamination. All surfaces supported cell attachment, proliferation and differentiation but the surfaces with grain boundary grooves or abraded grooves provided conditions for enhanced initial cell attachment. Nevertheless, overall cell proliferation and total DNA were highest on the polished surface. Zirconia sintered at a lower temperature (1300 °C vs. 1450 °C) had open porosity and presented reduced proliferation as assessed by alamarBlue™ assay, possibly because the openness of the pores prevented cells developing a local microenvironment. All cells retained the typical polygonal morphology of osteoblast-like cells with variations attributable to the underlying surface notably alignment along the grooves of the abraded surface.
Co-reporter:Alex Aziz, Helen C. Hailes, John M. Ward and Julian R. G. Evans  
RSC Advances 2014 vol. 4(Issue 95) pp:53028-53036
Publication Date(Web):28 Oct 2014
DOI:10.1039/C4RA08714C
This work explores the challenge of making persistent foams in salt water to provide high reflectance. While stable foam is essential in the food industry and in fire fighting, this is the first work aimed at combining foam persistence with reflectance. One application is the use of oceanic foams to increase planetary albedo: extending foam lifetime moderates the energy required to maintain large areas of ‘ocean mirror’. Two compositions to produce such foams in seawater are described. The first is based on high methyl ester pectin-type A gelatin complexes which produced foams with a reflectance of ∼0.5. The second produces stable foams using cellulose ethers and iota carrageenan gelling agents. These foams gelled in the presence of sea water to give measured reflectance of 0.65–0.75. Both had lifetimes, without wave action, beyond three months at which point the experiment ended. In contrast, single protein species such as gelatin B, whey protein isolate and albumin produced short-lived foams. Foam stability was measured by recording liquid drainage and foam height as a function of time. In the event that climate interventions are needed, such additives would be appropriate for nutrient-deficient ocean regions that support low levels of marine life.
Co-reporter:Xuesong Lu, Julian R.G. Evans, Steve N. Heavens
Journal of the European Ceramic Society 2012 Volume 32(Issue 3) pp:681-687
Publication Date(Web):March 2012
DOI:10.1016/j.jeurceramsoc.2011.10.014
Tape casting is a low cost fabrication process for flat ceramic sheet and this paper demonstrates how it can be combined with vacuum forming to make α-alumina domes of 250–300 μm sintered thickness. The suspension formulation makes use of systems of double plasticizers and double solvents in a poly(methyl methacrylate) binder to provide plastic ceramic films by tape casting from which preforms for vacuum forming were cut. In order to retain the shape of vacuum formed domes during debinding and sintering, an initial low temperature anneal is needed. The study indicates that the ratio of plasticizers to binder and the particle size are two dominant parameters in influencing the elongational flow. The whole process provides a novel method to make thin ceramic domes.
Co-reporter:Xuesong Lu, Julian R.G. Evans, Stephen N. Heavens
Journal of the European Ceramic Society 2012 Volume 32(Issue 16) pp:4219-4228
Publication Date(Web):December 2012
DOI:10.1016/j.jeurceramsoc.2012.07.013
This work concerns the manufacture of planar cell configurations for molten sodium batteries in energy storage devices such as vehicle batteries and stationary storage cells. Tape casting of beta-alumina electrolyte could provide a low-cost mass production route but intriguingly there are only a few reports of tape casting using beta-alumina directly as the raw powder. We first compared tape casting of α-alumina and beta-alumina using polyvinyl butyral (PVB) in conventional formulations. While it is relatively easy to obtain homogeneous α-alumina tape cast sheet, beta-alumina resulted in adhesion to the substrate and cracking. These problems were shown to be attributable to particle characteristics. When the binder was changed to polymethylmethacrylate (PMMA), tape casting of beta-alumina and three different α-alumina powders was facilitated, producing a formulation that was more tolerant to different powder types. Screening of several commercial dispersants provided two which were effective with PMMA and a conventional MEK/ethanol dual solvent.
Co-reporter:Xuesong Lu;Lifeng Chen;Negar Amini;Shoufeng Yang
Journal of Porous Materials 2012 Volume 19( Issue 5) pp:529-536
Publication Date(Web):2012 October
DOI:10.1007/s10934-011-9501-x
New methods for fabrication of 3D macroporous carbon scaffolds and synthesis of mesopores on carbon surfaces are proposed. Ordered macroporous filamentary carbon structures were made by rapid prototyping using solvent-based extrusion freeforming which allows the scaffold to be designed on computer and downloaded directly to a building platform. The surface of extruded filaments was decorated with 20–25 nm open mesopores by coating with nano-silica as a hard template followed by pyrolysis and dissolution of the silica. This left an open mesoporous surface to serve as a host for catalysts or enzymes while retaining integrity in the core for electrical and mechanical performance. The combination of these two methods could be used to make different hierarchical, multi-functional carbon structures which could be applied in fuel cells as the catalyst carrier and biofuel cell electrode.
Co-reporter:Biqiong Chen
Journal of Polymer Science Part B: Polymer Physics 2011 Volume 49( Issue 6) pp:443-454
Publication Date(Web):
DOI:10.1002/polb.22202

Abstract

Thirty-three polystyrene (PS)/acrylonitrile-butadiene-styrene (ABS) and high impact PS/ABS polymer blends with organoclay and copolymer additives were prepared by melt processing using different mixing sequences in order to test the putative capability of clay to perform a compatibilizing role in polymer blends. In general, the addition of clay increased the tensile modulus and had little effect on tensile strength. For the blends studied in this work, the addition of organoclays caused a catastrophic reduction in impact strength, a critical property for commercial viability. The polymer-blend nanocomposites adopted a structure similar to that for ABS/clay nanocomposites as determined by X-ray diffraction and transmission electron microscopy. It is suggested that clay reinforcement inhibits energy absorption by craze formation and shear yielding at high strain rates. Simultaneous mixing of the three components provided nanocomposites with superior elongation and energy to failure compared to sequential mixing. The clay pre-treated with a benzyl-containing surfactant gave the best overall properties among the various organoclays tested and of the two clay contents studied 4 wt % was preferred over 8 wt % addition. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Co-reporter:F.A. Saeed, J.R.G. Evans
Analytica Chimica Acta 2010 Volume 677(Issue 1) pp:79-89
Publication Date(Web):10 September 2010
DOI:10.1016/j.aca.2010.07.007
Using a luminescence spectrometer as a platform, a system of fibre-optic probes was created that allows full colour characterisation, fluorescence and phosphorescence spectra to be recorded in diffuse reflectance and in transmission from thick or thin film arrays of combinatorial samples of diameter down to 2 mm and from liquids. An integrating sphere is not required and the method is more versatile than the instrument's fibre-optic plate reader which has conjoined fibre bundles set at a fixed angle. Incident and detected light is routed via separate optical fibre bundles which remain stationary above or below a two-axis table. The validation and calibration are described. A library of 25 members was scanned for both diffuse reflectance (colour) and fluorescence in less than an hour. The method thus combines techniques that conventionally rely on different instruments and makes them amenable for high throughput libraries.
Co-reporter:Lifeng Chen, Julian R.G. Evans
Journal of Colloid and Interface Science 2010 Volume 351(Issue 1) pp:283-287
Publication Date(Web):1 November 2010
DOI:10.1016/j.jcis.2010.07.037
Recent research on the flow patterns during the drying of droplets of solutions or suspensions has revealed a characteristic flow of dissolved or suspended material to the droplet periphery to produce the ‘coffee ring’ phenomenon. This effect was used to make ceramic well-plates by spontaneous manufacturing. Here we demonstrate that when a colloidal droplet dries on a superhydrophobic surface, the effect is rather different. Evaporation from the region adjacent to the three phase line becomes so restricted that the interior flows, and hence the final destination of particles, changes and the characteristic bowl-shape becomes inverted.Graphical abstractThe upside-down relic of a droplet of alumina suspension that has dried on a superhydrophobic surface: instead of an upward facing bowl produced on a wetting surface, a downward facing bowl is produced.Research highlights► We explore the morphology of colloidal drops when they dry. ► When the contact angle is low, an upturned bowl forms. ► When the contact angle is high (e.g., 150–160°) the bowl is inverted.
Co-reporter:H. M. T. U. Herath;L. Di Silvio;J. R. G. Evans
Journal of Biomedical Materials Research Part A 2010 Volume 94A( Issue 1) pp:130-136
Publication Date(Web):
DOI:10.1002/jbm.a.32667

Abstract

The biocompatibility of natural samarium (III) oxide, which has previously been used for treatment in bone-related diseases was determined as a first step in its evaluation as a bone implant material. Assessment for 28 days using osteoblast-like cells revealed no indications of cytotoxicity. The cells adhered and proliferated on the surface. Furthermore, the differentiation and mineralization were observed, indicating a normal biological response of the cells on the samarium (III) oxide surface. The in vitro, short term biocompatibility assessment of this oxide has indicated its biosafety with no damaging toxic effects on the cells and biofunctionality; with an appropriate cell response for a bone-contacting material. Hence, samarium (III) oxide deserves recognition in the field of biomaterials for its excellent in vitro performance and demonstrates that the class of potential bioceramics may be larger than previously thought. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010

Co-reporter:H. Y. Yang;X. P. Chi;S. Yang
Journal of Materials Science: Materials in Medicine 2010 Volume 21( Issue 5) pp:1503-1510
Publication Date(Web):2010 May
DOI:10.1007/s10856-010-4009-5
Hydroxyapatite–tricalcium phosphate mixtures of various compositions were extruded by a solid freeforming process to form lattice structures to serve as hard tissue scaffolds. The unwelded filaments, sintered at temperatures from 1100 to 1300°C, had radii from 115 to 135 μm and were tested in three point flexural loading using a purpose-built fixture. Flexural strength ranged from 20 to 100 MPa depending on composition and sintering temperature. Weibull moduli up to 13 were obtained. Compositions with 50% or more tri-calcium phosphate did not develop strengths much above 40 MPa and the strength of most compositions fell when the sintering temperature exceeded 1250°C. Multiple layer lattice structures were created and tested in compression.
Co-reporter:Biqiong Chen and Julian R. G. Evans  
Soft Matter 2009 vol. 5(Issue 19) pp:3572-3584
Publication Date(Web):21 Jul 2009
DOI:10.1039/B902073J
The first reports of polymer-clay nanocomposites sprang out of industrial research from the motor industry in 1987. Since then, academic research has flourished and far outstrips industrial implementation of these new materials; approximately 700 papers were published in 2007. Much of this work, although it emphasises mechanical properties, neglects the characterisation parameter that industry regards as critical; impact strength. The expression ‘toughness’ has become ambiguous and inferences from the area under the tensile stress-strain curve (conducted at low strain rates) can directly contradict impact testing results. The mechanical, barrier and thermal stability properties of polymers are usually enhanced by incorporation of nano-clays, often to a remarkable extent and these improvements are well-documented. It seems to be important now to take a closer look at progress in retaining toughness when a polymer is converted to a nanocomposite and what matters is sensitivity to defects and energy absorption under high strain rate conditions. This is a lacuna that might be holding back the deployment of nanocomposites. This Review shows how difficult it is to identify the factors contributing to retention of toughness. Most of the polyamide studies report slight reductions in impact strength or very modest increases, explicable in terms of micro-cavitation as observed in the electron microscope. There is a growing view that clay reinforcement is only effective in retaining toughness above Tg. In other polymers, it is difficult to predict how nano-reinforcements will affect impact strength because of the confounding effects of crystallinty, spherulite size, preferred orientation or processing variables.
Co-reporter:Xuesong Lu, Yoonjae Lee, Shoufeng Yang, Yang Hao, Julian R.G. Evans, Clive G. Parini
Journal of Materials Processing Technology 2009 Volume 209(Issue 10) pp:4654-4661
Publication Date(Web):1 June 2009
DOI:10.1016/j.jmatprotec.2008.11.039
This paper describes the main factors affecting the rapid prototyping of fine lattices by extrusion freeforming of powder, notably equipment accuracy, paste preparation, extrusion and post-processing and their effects on filament deposition and the defects that might be caused. Effective methods were devised in order to reduce the incidence of these defects. The results provide guidance for fabrication of very fine lattices from powder extrusion (comprising <100 μm diameter filaments) and improvement of sample quality.
Co-reporter:Lifeng Chen and Julian R. G. Evans
Langmuir 2009 Volume 25(Issue 19) pp:11299-11301
Publication Date(Web):September 2, 2009
DOI:10.1021/la902918m
When a sessile drop containing colloidal particles or solute dries, the well-known “coffee ring” forms at the periphery because of radial flows. Here we demonstrate for the first time that if such drops are in close proximity to each other they form arches as they dry. The interaction between adjacent drops causes spatial variations in humidity so that drying from neighboring regions slows down and particles pile up in places where drying is most rapid. This discovery shows that the deceptively simple process of droplet drying still has more to reveal, and the effect probably influences all droplet technologies in an environment of locally nonuniform humidity.
Co-reporter:Biqiong Chen, Julian R. G. Evans, H. Christopher Greenwell, Pascal Boulet, Peter V. Coveney, Allen A. Bowden and Andrew Whiting  
Chemical Society Reviews 2008 vol. 37(Issue 3) pp:568-594
Publication Date(Web):13 Dec 2007
DOI:10.1039/B702653F
The surge of interest in and scientific publications on the structure and properties of nanocomposites has made it rather difficult for the novice to comprehend the physical structure of these new materials and the relationship between their properties and those of the conventional range of composite materials. Some of the questions that arise are: How should the reinforcement volume fraction be calculated? How can the clay gallery contents be assessed? How can the ratio of intercalate to exfoliate be found? Does polymerization occur in the clay galleries? How is the crystallinity of semi-crystalline polymers affected by intercalation? What role do the mobilities of adsorbed molecules and clay platelets have? How much information can conventional X-ray diffraction offer? What is the thermodynamic driving force for intercalation and exfoliation? What is the elastic modulus of clay platelets? The growth of computer simulation techniques applied to clay materials has been rapid, with insight gained into the structure, dynamics and reactivity of polymer–clay systems. However these techniques operate on the basis of approximations, which may not be clear to the non-specialist. This critical review attempts to assess these issues from the viewpoint of traditional composites thereby embedding these new materials in a wider context to which conventional composite theory can be applied. (210 references)
Co-reporter:Biqiong Chen;Shuo Liu
Journal of Applied Polymer Science 2008 Volume 109( Issue 3) pp:1480-1483
Publication Date(Web):
DOI:10.1002/app.28224

Abstract

Polymer and polymer–clay nanocomposite laminates were prepared as transparent thermal actuators with possible uses in automatic aeration and ventilation or as thermal switches. Low levels of smectite clay addition reduced the thermal expansion coefficient of poly(methyl methacrylate) (PMMA) but retained optical clarity and reduced water absorption. X-ray diffraction and transmission electron microscopy were used to confirm the formation of PMMA–clay nanocomposites, and dynamic mechanical analysis was used to measure the coefficients of thermal expansion. The experimental values of the radius of curvature of the laminates from cantilever bending tests were in good agreement with the theoretical predictions for composite bars with only 4 wt % (nominally 1.3 vol %) mineral reinforcement. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Co-reporter:Xuesong Lu, Yoonjae Lee, Shoufeng Yang, Yang Hao, Rick Ubic, Julian R.G. Evans, Clive G. Parini
Metamaterials 2008 Volume 2(Issue 1) pp:36-44
Publication Date(Web):May 2008
DOI:10.1016/j.metmat.2007.12.001
Electromagnetic crystals have potential applications from microwave to optical frequencies. At present, several methods can be applied to fabricate these structures, for example, stereolithography and wafer etching. In this work, extrusion freeforming, one of the linear solid freeforming methods, was used to fabricate electromagnetic crystals and the forming, drying, debinding and sintering stages are described. This method allows structures to be designed on a computer file and rendered directly as physical objects so that prototypes can be created quickly. A wide range of ceramic powders can be used so that dielectric constant and loss factor can be tailored. Structures with deliberate defects such as missing filaments or circular structures can be made.
Co-reporter:H. Y. Yang;I. Thompson;S. F. Yang
Journal of Materials Science: Materials in Medicine 2008 Volume 19( Issue 11) pp:
Publication Date(Web):2008 November
DOI:10.1007/s10856-008-3473-7
The dissolution behaviour of calcium phosphate filaments made by extrusion freeforming for hard tissue scaffolds was measured. The solubility of filaments with different HA/β-TCP ratios sintered at temperatures from 1,100 to 1,300°C was measured under simulated physiological conditions (tris buffer solution: tris(hydroxyl) methyl–aminomethane–HCl), pH 7.4, 37°C). Calcium and phosphate concentrations were measured separately by inductively coupled plasma (ICP) atomic emission spectroscopy. Surface morphologies and composition before and after immersion were analyzed by SEM and EDS. The results clearly show that as the β-TCP content increased, the dissolution increased. Higher sintering temperatures, with consequent closure of surface pores, resulted in lower dissolution. Examination of the surface suggested dissolution on preferred sites by pitting.
Co-reporter:Julian RG Evans, Shoufeng Yang
Tsinghua Science & Technology (June 2009) Volume 14(Supplement 1) pp:94-99
Publication Date(Web):1 June 2009
DOI:10.1016/S1007-0214(09)70074-4
A view of manufacturing processes is presented in which five distinct categories are defined as casting, deformation, machining, joining, and solid freeforming. Solid freeforming is essentially biomimetic and shares problems of morphogenesis with natural processes. Our team in University of London has been exploring three mechanisms of solid freeforming. In dry powder deposition and direct ink-jet printing, the emphasis has turned to the problem of delivering a complex shape in which the three dimensional spatial arrangement of composition is delivered from the design file. In extrusion freeforming, the aim is to control microstructure at hierarchical levels also from the design file. The quest for 3-D functional gradients is satisfied by acoustic and ultrasonic dispensing and mixing of powders so that each layer can be patterned. These methods could be extended to deliver the complex patterns demanded by left-handed microwave metamaterials. Dry powder deposition and direct ink-jet printing are turning towards combinatorial methods in which multiple sample libraries are used to accelerate discovery. In turn, this paves the way for ‘autonomous research machines’ which steer their own search refinements in response to our requests for new materials. In this way, solid freeforming used for sample preparation can give an ‘arm’ to an intelligent machine so that it can conduct its own experimentation and learning; an idea that originated with Alan Turing in the late 1940s.
Co-reporter:Xuesong Lu, Yoonjae Lee, Shoufeng Yang, Yang Hao, ... Clive G Parini
Tsinghua Science & Technology (June 2009) Volume 14(Supplement 1) pp:168-174
Publication Date(Web):1 June 2009
DOI:10.1016/S1007-0214(09)70087-2
Extrusion freeforming can be used for the rapid prototyping of millimeter-wave electromagnetic bandgap (EBG) structures. In this work, an alumina-polymer paste with a relatively high volatility solvent (propanol) was used and the characteristics of the ceramic paste, particularly the rheological features are described. The advantage of high volatility solvent is that the viscosity and elastic modulus of the paste are increased sharply as the solvent evaporates. Thus, the rigidity of the extruded filament is quickly increased as a small amount of solvent evaporates. Finally, by employing this procedure, different EBG structures such as 2-D, 3-D woodpile and aperiodic structures were fabricated and their bandgaps were measured. The experimental results show that extrusion freeforming is a relatively simple and easy method to fabricate these woodpile structures with a bandgap in the 90–110 GHz region.
Co-reporter:Biqiong Chen, Julian R. G. Evans, H. Christopher Greenwell, Pascal Boulet, Peter V. Coveney, Allen A. Bowden and Andrew Whiting
Chemical Society Reviews 2008 - vol. 37(Issue 3) pp:NaN594-594
Publication Date(Web):2007/12/13
DOI:10.1039/B702653F
The surge of interest in and scientific publications on the structure and properties of nanocomposites has made it rather difficult for the novice to comprehend the physical structure of these new materials and the relationship between their properties and those of the conventional range of composite materials. Some of the questions that arise are: How should the reinforcement volume fraction be calculated? How can the clay gallery contents be assessed? How can the ratio of intercalate to exfoliate be found? Does polymerization occur in the clay galleries? How is the crystallinity of semi-crystalline polymers affected by intercalation? What role do the mobilities of adsorbed molecules and clay platelets have? How much information can conventional X-ray diffraction offer? What is the thermodynamic driving force for intercalation and exfoliation? What is the elastic modulus of clay platelets? The growth of computer simulation techniques applied to clay materials has been rapid, with insight gained into the structure, dynamics and reactivity of polymer–clay systems. However these techniques operate on the basis of approximations, which may not be clear to the non-specialist. This critical review attempts to assess these issues from the viewpoint of traditional composites thereby embedding these new materials in a wider context to which conventional composite theory can be applied. (210 references)
Silane, triethoxy(heptadecafluorooctyl)-
1,3-PROPANEDIYL DIOLEATE
Nickelous Nitrate
Carbonate (8CI,9CI)
Cerium(IV) ammonium nitrate
O-[(2,3,4,5,6-PENTAFLUOROPHENYL)METHYL]HYDROXYLAMINE
(3AR,4R,5R,6AS)-4-FORMYL-2-OXOHEXAHYDRO-2H-CYCLOPENTA[B]FURAN-5-YL 4-BIPHENYLCARBOXYLATE
Polyethylene glycol 200 diacrylate
Aluminum cobalt hydroxide