Ingoglia, Bryan

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University of Kentucky

KaleidoscopeVolume 11 Article 74July 2014Microencapsulation of Diels-Alder Monomers inSelf-Healing MaterialsBryan IngogliaFollow this and additional works at: https://uknowledge.uky.edu/kaleidoscopeRight click to open a feedback form in a new tab to let us know how this document benefitsyou.This Summer Research and Creativity Grants is brought to you for free and open access by the The Office of Undergraduate Research at UKnowledge.It has been accepted for inclusion in Kaleidoscope by an authorized editor of UKnowledge. For more information, please contactUKnowledge@lsv.uky.edu.Recommended CitationIngoglia, Bryan (2013) "Microencapsulation of Diels-Alder Monomers in Self-Healing Materials," Kaleidoscope: Vol. 11, Article 74.Available at: https://uknowledge.uky.edu/kaleidoscope/vol11/iss1/74Microencapsulation of Diels-Alder Monomers in Self-Healing Materials Student: Bryan Ingoglia Faculty Mentor: Susan Odom  Project Background and Goals Developing  a   polymer   in  which mechanical   damage   is   autonomically restored  has  widespread  application  in structural  materials, coatings,    and electronics.[1] In  particular, I  have committed   to  designing  a  system utilizing core-shell microcapsules and a particular organic reaction – the Diels- Alder   reaction,   specifically   one   that occurs at   room  temperature.   These microcapsules  were   incorporated  into polymer  films,  which were   then                 Figure 1. A schematic of self-healing polymers scratched to release the core components of the microcapsules in order to repair the mechanical damage. The benefits to my approach include inexpensive and non-toxic components, no need to apply heat or use expensive catalysts, and can be considered a “green” reaction (i.e. organic solvents are not used and reaction yields are high).  In the Diels-Alder reaction, an electron-rich diene reacts with an electron-poor dienophile to form new carbon-carbon bonds. In my system, each reactant was bis-functionalized: the reactive portions of the molecules are the same on each side. I thus selected a bis(furan) as the diene and a bis(maleimide) as the dienophile (Figure 2). The monomers, when combined upon rupturing the microcapsules, form a repeating polymer unit under ambient conditions (Figure 3).                 Figure 2. Bis(furan) and bis(maleimide) synthesis. Figure 3. Polymerization of bis(furan) and bis(maleimide). Methods In order to create the Diels-Alder system, each monomer had to be custom synthesized. The bis(furan) monomer was created by reacting furan with n-butyl lithium followed by 1,6- dibromohexane (Figure 2). The bis(maleimide) was synthesized by reacting maleic anhydride with 1,3-diaminopropane, followed by a ring- closing reaction with sodium acetate and acetic anhydride (Figure 2). The poly(urea- formaldehyde) core-shell microcapsules were made by emulsification polymerization around a chlorobenzene core. Capsules were isolated by size range using a RoTap® (Figures 4 and 5).                  Figure 4.  Capsules containing 100% bis(furan). (“Neat” microcapsules).  Several diagnostic techniques were used to characterize the organic compounds synthesized throughout this project. Gas chromatography/mass spectrometry (GCMS) combined with  1H Nuclear  Magnetic  Resonance  (NMR)  spectroscopy were  used  in  order  to  confirm  that  the synthesis and encapsulation of each monomer. In addition, polymerization reactions were analyzed by Matrix-Assisted Laser   Desorption/Ionization   (MALDI),   a type of mass spectrometry.             Figure 5.  Capsules containing bis(maleimide) in chlorobenzene. Once microcapsules were prepared on a multi-gram scale and were separated by size, they were incorporated into several kinds of polymer films: epoxy-amine, poly-acrylic acid, and a polyurethane elastomer—which were doctored on to epoxy-coated steel plates. Each film was scratched and immersed in a salt water bath in order to see if rusting occurred.    If healing occurs efficiently, rusting will be prevented.  Results The synthesis of each Diels-Alder monomer was successful and supported by NMR data. The bis(furan) reaction had a 96% yield which was distilled to improve purity. The bis(maleimide) reaction, while successful, only gave a 18% yield. Since the bis(maleimide) synthesis was not successful on a multi- gram scale, I searched for alternative dienophiles and identified  a  similar  maleimide,  1-1-(methylenedi-4,1- phenylene)bismaleimide,  shown  in  Figure  6.  Both dienophiles  do  indeed  undergo  a  room-temperature Diels-Alder   reaction,   as   evidenced   by   the   NMR  Figure 6—new bis(maleimide) spectrum of the products and qualitatively by the brown solid formed when the two monomers are mixed, shown in Figure 7.  Since beginning my research I have made progress and also met challenges to overcome. I first incorporated chlorobenzene, an organic solvent, into poly(urea- formaldehyde) microcapsules. However, within a few days, the core had diffused out of the microcapsule wall, depleting their function. In order to resolve this issue, I added Desmodur L75, a polyisocyanate, to my microcapsule recipe. The role of Desmodur L75 is to form a second polyurethane shell in addition to the urea-formaldehyde wall; a diagram is shown in Figure 8 to indicate the difference in microcapsule integrity. Microcapsules made using the Desmodur L75 recipe keep their cores for at least 8 weeks without drying out. I isolated each microcapsule core individually and analyzed them by 1H NMR spectroscopy. To date, I have  Figure 7— bis(furan) and bis(maleimide) polymerization. successfully made four distinct capsule types: chlorobenzene, bis(maleimide) in chlorobenzene, bis(furan) in chlorobenzene and more recently, bis(furan) with no solvent (called a “neat” capsule as the bis(furan) is a liquid and does not require a solvent to be put into a microcapsule).  The next step was to incorporate monomer microcapsules into a polymer film for scratch testing. However, the bis(maleimide) does not dissolve well in chlorobenzene, thus those microcapsules contained a low weight percent of bis(maleimide). To overcome this obstacle, solid bis(maleimide) was incorporated into the polymer film itself, along with bis(furan)- containing microcapsules  in  a  system  similar  to  the  right-most  diagram  in  Figure  1.  Opting  for  this particular system film excluded the epoxy-amine film because the amine reacted with the bis(maleimide),      effectively destroying the monomer. Alternate  film  choices included poly-acrylic acid and the polyurethane elastomer; however, I discovered that poly-acrylic acid dissolves in water, rendering it a non-ideal film. Thus I identified the polyurethane elastomer as the best option for creating films containing       bis(maleimide) and microcapsules with a bis(furan) core. Figure 8—Diagram of core-shell microcapsules with (right) and without (left) Desmodur L75 polyurethane shell wall.  After incorporating bis(maleimide) and bis(furan) microcapsules into a viable polyurethane film, the films were scratched and left alone for two days before immersion in the salt water bath. After a day in the salt water, evidence of rusting was observed in plates with and without microcapsules (Figure 10). Possible future goals for the Fall semester include alternate forms of scratch testing and the investigation of internal phase separation in order to get a higher weight percentage of bis(maleimide) into microcapsules in order to see a significant amount of self- healing.                 Figure 9—Polyurethane films; without microcapsules (left) and with microcapsules (right). Figure 10—Polyurethane films after salt water bath; without microcapsules (left) and with microcapsules (right).   Discussion Analysis of each 1H NMR spectrum shows that the monomers have been synthesized. Furthermore, each monomer was successfully encapsulated into a microcapsule. The difficulty of encapsulating a high weight percent of the bis(maleimide), or alternatively incorporating it into the polymer film, appear to indicate that this Diels-Alder reaction may not be ideal for microencapsulation. Investigating internal phase separation could prove a solution to this problem,[2] or perhaps identifying another dienophile that is more soluble in organic solvents.  Since scratched films rusted regardless of the presence of microcapsules, the polymerization reaction that presumably occurs likely did not form a long enough chain to protect the underlying steel substrate from corrosion. Possible issues could arise from any non-homogeneity regarding the location of the solid bis(maleimide), especially if a minute amount if present in the scratch gap.  References [1] Caruso, M.M.; Davis, D.A.; Shen, Q.; Odom, S.A.; Sottos, N.R.; White, S.R.; Moore, J.S. “Mechanically-Induced  Chemical  Changes  in  Polymeric  Materials.”  Chem. Rev.  2009,  109, 5755–5798. DOI: 10.1021/cr9001353 [2] Dowding, P.J.; Atkin, R.; Vincent, B.; Bouillot, P. Langmuir 2004, 20, 11374-11379. 

Microencapsulation of Diels-Alder Monomers in Self-Healing Materials

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Microencapsulation of Diels-Alder Monomers in Self-Healing Materials

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