University of Groningen
Synthesis and Application of Thermally Reversible Polymeric Networks from Vegetable Oils
Yuliati, Frita
DOI:
10.33612/diss.127911343
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Publication date: 2020
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Yuliati, F. (2020). Synthesis and Application of Thermally Reversible Polymeric Networks from Vegetable Oils. University of Groningen. https://doi.org/10.33612/diss.127911343
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1
INTRODUCTION
Thermosets have a vast variety of applications in modern societies. However, repair and reprocessing of these materials is often difficult. Furthermore, most are produced from fossil resources. The use of fossil resources is under pressure due to environmental concerns, fluctuating prices and uncertainties regarding future availability. This thesis aims to contribute to solving these challenges, by providing synthetic methodology for reprocessible thermosets from renewable resources. This chapter provides a literature overview relevant to the work reported in this thesis. The first part discusses various methods employed for self-healing polymeric systems. The emphasis will be on methodology using the Diels-Alder reaction. Moreover, the synthesis of biobased polymers based on vegetable oils is described.
2 CHAPTER 1
1.1 Self-healing polymers
Thermosetting polymers and composites are used in many applications, for example in transportation, construction, electronics and sporting goods1. Unfortunately, the materials have some major drawbacks, for example mechanical properties degradation over time as a result of stresses and strains build up. Damage usually starts with micro crack formation that is visually difficult to detect. Larger cracks can be repaired by using adhesives; however, this may not restore the original properties of the material1,2. In this context, the design and preparation of self-healing thermosets constitute a popular research area among academic and industrial researchers.
In general, two self-healing methods can be distinguished. The first is known as autonomous self-healing. Here, the polymers do not require external stimuli to heal. This method is inspired by self-healing mechanisms operative within living organisms3. Examples in the thermosetting field are self-healing epoxy resins made by embedding microcapsules with dicyclopentadiene monomer and separate capsules containing a Grubb catalyst. In the case of crack formation, the capsules rupture, the monomer and catalyst are released and polymerization occurs, leading to repair of the crack4–6. The major drawback of this system is that it is limited to a single healing action at a given specific spot in the sample. To answer this challenge, new systems were developed by embedding a 3-dimensional microvascular network containing dicyclopentadiene monomer in an epoxy coating, and dispersing a Grubb catalyst into the substrate. When cracks are formed, the monomer flows towards the fracture as a result of capillary forces, polymerization occurs and the cracks are healed. This new system was found to be able to perform 7 healing cycles. However, the healing ability is limited by the availability of active catalyst, and as such the catalyst intake determines to a large extent the maximum number of healing cycles7.
The second self-healing strategy involves the use of reversible polymerization strategies, where the polymers can return to the monomeric, oligomeric, or non-cross-linked state under certain external stimuli. Examples of this strategy
1
involving covalent bond formation/breakage are the disulfide/thiol system, thethermally reversible NO-C bonds, photo-induced reversibility, and the use of reversible Diels-Alder chemistry2. The disulfide/thiol system was used for the synthesis of reversible gels. Here, the disulfide was synthesized under oxidizing conditions, and cleaved into two thiol groups via a reduction reaction8–10. The NO-C bonds in alkoxyamines have low bond dissociation energy, and were exploited in synthesizing reversible grafts and cross-links of polymers via reversible radical crossover of alkoxyamine units. The reaction was reversed by heating in anisole solvent containing excess alkoxyamines11–13. The proof of concept for the third strategy, the photo-induced reversibility, was provided for the dimerization of anthracene under photoirradiation (O> 350 nm), and cleaving of the dimer back into anthracene during photoirradiation atO < 300 nm14.
Heat is a stimulus for Diels-Alder chemistry, which is easy to perform compared to other options15. Another advantage of this chemistry is that the reaction may be carried out without a catalyst and yet with relatively fast kinetics16. As a consequence, the use of the Diels-Alder (DA) chemistry in thermosetting polymers has attracted high attention in the last decades. The DA reaction is known to take place at relatively low temperatures (from 20 oC to 90 oC)17,18, and can be reversed via the retro-Diels-Alder (rDA) reaction by increasing the temperature up to 200
o
C18,19, the actual temperature depending on the chemical structure of the molecules involved. The decomposition of the DA adduct enables the polymers to be reprocessed due to the absence of cross-linking points. Therefore, the use of DA chemistry allows the synthesis of self-healing thermosetting polymers and facilitates the recycling of this product 16,20,21.
Besides the need for self-healing and recyclable polymers, the replacement of fossil based feeds for renewable ones is also high on the research agenda. Among the potentially attractive renewable feedstock for the chemical industry, oils and fats from vegetable and animal origin are considered notably important. These resources are available worldwide at relatively low cost and have multiple applications22,23. Synthetic methodology for polymers from vegetable oils have been developed, including polymers based on DA reactions. The objective of this
4 CHAPTER 1
chapter is to provide an overview on the application of DA chemistry for the synthesis and applications of reversible materials, with an emphasis on the use of renewable feeds, particularly vegetable oils.
1.2
Thermally reversible polymers based on the DA reaction
The DA reaction is a cycloaddition between a conjugated diene and a dienophile (a component with at least one bond), to form a six-membered ring (Figure 1.1).
Diene Dienophile
New bond
New bond
Figure 1.1. The Diels-Alder reaction24 a
The dienes should be in the cis configuration, and it is well established that cyclic dienes display a higher reactivity than open forms. The electronic properties of the substituents on both the diene and dienophiles have a major impact on the reaction rates. In general, substituted dienes with electron-donating properties leading to higher reaction rates for reactions with dienophiles with electron-withdrawing substituents24,25. Relevant examples of dienes and dienophiles are given in Table 1.124,25.
a
Figures describing chemical structures in this thesis were not prepared with stereochemistry consideration for brevity reasons.
1
Table 1.1. Representative dienes and dienophiles24 for the DA reaction.
The use of the DA reaction has been studied for various polymeric systems. Well known combinations of dienes and dienophiles are furans-maleimides, cyclopentadiene-cyclopentadiene, anthracene-maleimides, fulvene-fulvene, fullerene-dienes and cyclopentadiene-dithioesters2,26. Among these systems, the furan-maleimide combination is the most studied, mainly due to the high availability of reagents and relatively fast kinetics27. Different strategies have been investigated: (i) reacting multifunctional monomers in a direct DA cycloaddition; (ii) utilizing DA linkages in cross-linkers or initiators to perform polymerization and (iii) DA cycloaddition of pendant functional groups on linear polymers16, see Figure 1.2 for details.
Dienes
Open Chain Outer Ring Inner-outer Ring Across Ring Inner Ring
OSiMe3 OMe O O O CH2 N O O O O O O O O O O Dienophiles Acyclic Cyclic CHO COMe CN O O O O O N O O H2C C CHMe HC CO2Me OEt O N N N O O Ph Me2C S Ph N O ArN NCN O O O O
6 CHAPTER 1
Figure 1.2. Common strategies in implementing the DA chemistry in polymers: (i) polymerization of multifunctional monomers via the DA reaction, (ii) utilizing cross-linkers containing DA adducts, and (iii) cross-linking of linear polymers by using pendant functional groups16,28–30.
An early example of a cross-linked polymer synthesized with the first strategy (i in Figure 1.2) involves the DA cycloaddition between a diene comprising 4 furan moieties per molecule (4F) and a dienophile with 3 maleimide moieties per molecule (3M) (Figure 1.3). The reaction formed a hard and fully transparent polymeric material. The polymerization was performed in a solvent (dichloromethane) at 24 – 95 oC. The mechanical properties of the polymer were in the range of commercial epoxy and unsaturated polyesters. To understand the healing capability of the polymer, some samples were scratched and subjected to loads at directions perpendicular to the cracks during a scratch toughness measurement. After failure, the samples were healed by heating them at about 150 oC. The healed samples were measured again and showed healing efficiencies (defined as the ratio between the scratch toughness after and before healing) of about 50%31.
'
'
'
Thermally reversible bond Irreversible bond
(i)
(ii)
1
Figure 1.3. A diene comprising 4 furans (4F); and dienophiles comprising 2 (2M and 2MP) and 3 maleimides (3M)32.
To avoid the use of solvents, the original dienophile was replaced by molecules containing two maleimide moieties (2M and 2MP). The mechanical properties of the resulting polymers (2M-4F and 2MP-4F) were determined and shown to have lower Young’s modulus than that of 3M-4F. In addition, the polymeric materials showed an average of 80% healing efficiency32.
Besides the synthesis of cross-linked polymers, the first strategy is also applicable to obtain linear polymers. An example involves the use of a molecule with 2 furan units combined with a bismaleimide. A biobased monomer with two furan groups was synthesized by reacting furfural and trehalose. The product was reacted with a bismaleimide to produce a linear polymer. The optimum temperature for the DA reaction was 70 oC, higher temperatures were shown to lead to lower yields due to the occurrence of the retro-DA reaction33.
Another interesting example of a linear polymer synthesized via the DA reaction involves the use of cyclopentadiene as both diene and dienophile. Cyclopentadiene is thermally labile and has to be stored at -20 oC to prevent dimerization to dicyclopentadiene34 This property can be used to prepare new reversible polymers (Figure 1.4). The reaction successfully generated a hard and transparent polymeric material35.
C O O O O 4 N O O N O O O O N N O O 3 N N O O O O 4F 3M 2M 2MP
8 CHAPTER 1 Figure 1.4. Polymerization of a dicyclopentadiene-based monomer35.
The second polymerization strategy (ii in Figure 1.2) comprises the use of DA linkages in cross-linkers or initiators to perform an irreversible polymerization reaction. The incorporation of such DA adducts in the cross-linker enables the cross-linked polymer to be reprocessed upon heating. This strategy was applied in a diamine linker containing DA adducts at both ends (Figure 1.5). The cross-linker was able to cure commercially available epoxy resins such as diglycidyl ether bisphenol A (DGEBA) at 60 oC28. The cross-linker was also used to cure TGAP (trifunctional triglycidyl p-amino phenol). The best curing condition was at 80 oC for 24 h, giving a conversion as high as 85%. However, higher curing temperature lead to the retro-DA reaction and possibly also to Michael additions between the maleimide groups and unreacted amines, which is known to occur at 70 oC. The optimum condition for the rDA reaction was 150 oC for 10 min29. Both epoxy polymers were found to possess self-healing capabilities. Scratches of about 40 micrometer wide on the polymer surface completely disappeared after heating at 140 oC for 30 min and subsequent annealing at 75 oC for 5 h for the DGEBA-based epoxy, and at 130 oC for 5.4 min for the TGAP-based polymer28,29.
The third strategy (iii in Figure 1.2) uses DA active moieties as pendant groups on monomers or linear polymers and further cross-linking of the polymers by making use of the DA reaction. Examples include the use of furan groups attached to epoxy resins, polymethacrylate, alternating polyketone, and EPM rubber; and
O O O O O O O O O O O O O O O O O O 120 oC
1
N N O O NH2 NH2 O O O OFigure 1.5. A cross-linker containing amine groups (red) for irreversible bond formation and the reversible DA adduct (blue)28,29.
cross-linking them with the commercially available 1,1’-(methylene-di-1,4-phenylene)bismaleimide. The resulting cross-linked polymers were found to show reversible behavior upon a thermal treatment30,36–38.
Polymers synthesized with the third strategy can be also be cross-linked by a combination of the DA chemistry and another approach. A thermosetting polymer was cross-linked with a conventional epoxy ring-opening reaction and the DA reaction. The resin contained both epoxide groups at both ends and a pendant furan group (Figure 1.6). It was cross-linked with a mixture of a traditional curing agent (methylhexahydrophtalic anhydride) and a bismaleimide. The end-product contained two types of bonds, namely thermally stable bonds from the reaction product of epoxide and anhydride groups; and thermally reversible bonds from the DA reaction between the furan and maleimide moieties. The Young’s and flexural modulus of this polymer were found to be higher than those of conventional epoxy polymers (2.5 GPa and 4.6 GPa, compared to 1.8 GPa and 2.6 GPa), although the tensile strength was somewhat lower (53 compared to 65 MPa)36. Other polymers were synthesized, for example, by using modified polymethacrylate38 and a Novolac epoxy resin37. The latter produced a polymer with a storage modulus of 2.37 GPa at 30 oC37, lower than the Novolac resin cross-linked conventionally with a methyl etherified amino resin39. The high functionality of the starting materials enabled high amounts of pendant furans groups grafted along the backbone. The rDA reaction occurred upon heating, allowing reprocessing37,38. The Novolac-based polymer also showed self-healing capabilities, as shown by healing scratches on polymer films by heating the samples at 130 oC for 10 min and subsequent annealing at 60 oC for 24 h37.
10 CHAPTER 1
N
O O
O
Figure 1.6. An epoxy resin containing epoxy groups (red) and pendant furans (blue)36.
Earlier research in our group showed that pendant furan groups can be attached to polyketones by the Paal-Knorr reaction of such polyketones with furfurylamine (Figure 1.7). Reaction of the resulting polymer with bismaleimides by a DA approach leads to cross-linked material with thermally reversible properties, even after six heating and cooling cycles30. Higher levels of furan groups attached to the polyketones and the use of higher maleimide concentration resulted in higher cross-link densities. Even though molecular reversibility was shown to be less than quantitative, the mechanical properties were found to be completely reversible26. The Tg of the product can be increased by making use of free primary amines
which form hydrogen bonds with the carbonyl groups present in the backbone40. In addition, electric conductivity of the polymer can be enhanced by adding carbon nanotubes as a filler to the formulation41.
O NH2 O R R N O R N R R O Paal-Knorr, 110 oC O R R N O R N R R O N N R R R O N O O O N O O O R O R R N R N R R Diels-Alder, 50 oC Retro-Diels-Alder, 150 oC N N O O O O O R R N O R N R R O
1
A similar strategy is applicable to EPM rubbers, for instance by the introduction ofpendant furan groups to the rubber, followed by cross-linking with a commercial bismaleimide42. The mechanical properties of the products were shown to be tunable by using different types of bismaleimides43. These resulting rubbers were found to be fully thermally reversible, indicating that the Diels-Alder chemistry is also applicable in this system.
The three strategies utilizing the DA and rDA reactions to prepare thermally reversible and self-healing polymers have specific features that affect their applicability. The first strategy uses relatively small molecules as monomers, that will be detached from the polymeric structure during the rDA reaction, potentially resulting in a dramatic decrease of the mechanical properties. This condition is acceptable in the case of recycling, but potentially more cumbersome in the case of healing processes. Similar issues occur with the second strategy, though to a lower extent, because the thermally stable bonds will not be cleaved during heating. The changes in mechanical properties during recycling or healing of a polymer can be tailored by changing the composition of the thermally reversible and irreversible bonds in the polymer. Furthermore, attention needs to be paid to the temperature at which the irreversible bonds are formed. If it is in the range where the rDA reaction occurs, the detachment of the DA adducts should be taken into account in handling the polymer. On the other hand, the third strategy only uses the DA and rDA reactions for network formation and disruption, therefore the behavior of the thermosets is more predictable. In case of the rDA reaction where the adducts are separated into the original DA pairs, the products still consist of large molecules. Consequently, the decrease in mechanical strength during heating is less dramatic than those in the previous strategies. These consequences should be considered when selecting the best strategy for a given application.
12 CHAPTER 1
1.3 Applications of thermally reversible polymers
Thermally reversible polymers offer new possibilities and advantages in many applications. Here we describe some potential applications of the materials in adhesives, coatings, and composites.
1.3.1 Removable adhesives from thermally reversible polymers
Thermosetting adhesives are potentially applicable in the aerospace, automotive, electronics and medical industry. Removable adhesives are required when disassembling of joined parts is an important aspect in the design, e.g. for recycling, repair or upgrading of an expensive component. The use of removable adhesives, for instance, a thermally reversible one, enables easy disassembling without causing collateral damage to the components or the entire unit44,45.
A number of strategies can be envisaged for the use of thermally reversible epoxy adhesives, for example by incorporating a DA adduct in the epoxy resin or in the cross-linker. A number of removable adhesives were designed using conventional curing agents for cross linking in combination with modified resins, e.g. with thermally labile DA adducts. The DA adducts can be inserted in the resin structure44, or utilized as a bridging structure between a linear polymer and pendant oxiranes46. When a non-modified commercial epoxy resin is preferred, the DA adduct can also be incorporated in the cross-linker, e.g. in the diamine used45. These strategies gave adhesives with similar lap shear strengths as a commercial adhesive (about 4 MPa and 2 MPa for the two strategies respectively44,45. The reversibility of these adhesives were examined by repeatedly detaching and re-attaching the substrates bonded by the adhesives. It was found that the bond strength was similar or even increased after several detaching-re-attaching cycles44–46.
A major concern for thermally reversible adhesives described above is the limited temperature range of application. As such, alternatives with improved properties using a different approach are required. A “lock” and “unlock” mechanism was proposed to enable a removable adhesive to operate at higher temperature under
1
“locked” condition. When it was “unlocked”, the adhesive could be removed byheating. Such an adhesive was prepared from a DA adduct where the diene component was integrated into a photoresponsive hexatriene (Figure 1.8). After the DA reaction, the hexatriene is attached to the cyclohexene of the DA adduct. Exposure of the polymer to ultraviolet light was shown to trigger ring closure of the hexatriene while converting the cyclohexene part of the DA adduct into cyclohexanes, and consequently prevented the retro DA reaction to happen. When the system was exposed to visible light, the photoreaction occurred by returning the hexatriene and also the cyclohexene of the DA adduct, and therefore “unlocked” the adduct (enabling the rDA reaction to take place)47.
Figure 1.8. A removable adhesive using a lock/unlock mechanism triggered by light. Ultraviolet light promotes the ring closing of the hexatriene (blue), removing the cyclohexene of the DA adduct (red) and prevents the rDA reaction. Visible light exposure opens the hexatriene ring and enables the rDA reaction to occur47.
N O C H 3 S S CH3 O O C H3 Cl O O O O N O C H3 S S C H3 O O CH3 Cl N O C H3 S S O O C H3 Cl O O O O N O C H3 S S O O Cl CH3 CH3 C H 3 312 nm >435 nm
14 CHAPTER 1
1.3.2 Thermally reversible polymers for self-healing coatings
Coatings are used to protect their substrates from damage caused by the environment, for example to prevent corrosion of metals. Since coatings usually have a high surface to volume ratio, they are very susceptible to damage, which will ultimately also lead to the damage of the substrate. Thermally reversible polymers offer advantages for coating applications by allowing easy repair within certain limits of damage48,49.
Methacrylate-based polymers are widely used as coatings due to their fast curing kinetics and good weather resistance. Augmenting self-healing properties into the polymeric structure can be achieved by incorporation of a Diels-Alder adduct in the polymer structure. A number of methods have been proposed for this purpose. For example, furans were incorporated as pendant groups to a methacrylate-based block copolymer and the resulting copolymer was subsequently cross-linked with an aliphatic bismaleimide via a DA reaction50. To avoid the complications of using two components, incorporation of both pendant furans and maleimides into a linear methacrylate backbone was also reported49. Both methods were shown to lead to self-healing polymers with interesting properties49,50.
Other polymers used extensively in the coating industry are epoxy resins and polyurethanes. Epoxy coatings are applicable with minimum preparation and are used for a wide range of applications, from primer to top coatings and applications where chemical resistance is a key product property. Polyurethane coatings are known for their high UV resistance, gloss retention, and color stability. There is a high interest to incorporate self-healing properties into both classes of coatings. An example is the use of strategy (ii) in Figure 1.2 using an epoxy resin and a polyol. A thermally-responsive epoxy resin was synthesized by reacting furfuryl glycidyl ether with aromatic or aliphatic bismaleimides, yielding a diepoxy resin with two DA adducts between the oxirane groups50. For polyurethanes, the synthesis of a soybean oil-based polyol containing DA adducts connecting the triglyceride structure with the alcohol groups is worth mentioning51. In both cases, the monomers were cross-linked using conventional
1
amines and isocyanates, respectively50,51, producing polymers with irreversibleand reversible bonds. Both epoxy and polyurethane coatings showed self-healing properties without notable changes in other product properties, such as chemical resistance against MEK for the epoxy, and gloss retention for the polyurethane.50,51
Fouling caused by marine organisms is a major challenge for the development of coatings for marine applications. Hyperbranched fluorinated polymers (HBFP) cross-linked with poly(ethylene glycol) (PEG), possess good anti-biofouling capabilities and have shown to be suitable for marine coatings. However, durability is limited and this is considered a major disadvantage. The introduction of DA adducts into the material in the form of furan groups into HBFP and maleimides into PEG was shown to have a very positive effect on service life. The coating prepared from furan-functionalized HBFP and maleimide-functionalized PEG was cured at 60 oC for 24 h. It was shown to reheal after thermal reprocessing and to have similar resistance to protein adsorption as a conventional HBFP-PEG coating, confirming its anti-biofouling property52.
1.3.3 Self-healing composites from thermally reversible polymers Cross-linked polymers are important components in polymer composites. Defects in such composites usually arise at the interface between the polymer matrix and the reinforcement. At this interface, mechanical stress is transferred from the matrix to the reinforcement. However, the difference in mechanical properties between the materials may lead to stress concentration and eventually to crack formation. Using thermally reversible polymers in the composite systems enables easy repair of the cracks resulting in prolonged service life of the composite53,54.
An effort to create rehealable interphases was made by introducing DA linkages between the epoxy matrix and a typical reinforcement material (glass fiber). For this purpose, furan groups were introduced into the epoxy matrix by mixing DEGBA and furfuryl glycidyl ether (FGE), while the glass fibers were functionalized with maleimides. The interfacial bond strength between the matrix and reinforcement was measured by a micro-droplet single fiber pull-out testing. A
16 CHAPTER 1
single fiber was embedded into a droplet of the resin as a model of a composite, then the fiber was pulled out, followed by healing of the composite. Debonding between the matrix and the fiber was found to be possible at higher temperatures and the interfacial bonds were shown to remain recoverable after five healing cycles55. A similar strategy was applied when using carbon instead of glass fibers, however, the healing efficiency was found to decrease with the number of debonding/healing cycles. During debonding, some furan groups were attached to the fiber in the form of DA adducts. This resulted in a decrease in furan content in the matrix, thus reduced the number of DA linkages at the interphase53.
A novel nanocomposite with thermally reversible properties was prepared by incorporating DA adducts into polyhedral oligomeric silesquioxanes (POSS). POSS are organic-inorganic hybrid materials with an empirical formula of Rn(SiO1.5)n
(n=8, 10, 12), with R comprising organic groups (e.g. glycidyl, phenyl, cyclohexyl) or organic-inorganic hybrids such as -OSiMe2OPh. DA adducts were incorporated
in the material by first reacting POSS with furfurylamine, and a subsequent reaction with a bismaleimide. The thus obtained material was cracked and subsequently heated at 135 oC and 150 oC for 30 min. The results suggest that thermal healing was successful, provided that the damaged parts are in close contact during heating56.
1.3.4 Shape-memory polymers
The DA reaction is also found to be useful for the synthesis of recyclable shape-memory polymers. A successful example involves incorporation of furan groups into shape memory bisphenol-A type resins and subsequent cross-linking with a maleimide linker. The thus produced epoxy polymers have shape memory properties and can be recycled at temperatures above 120 oC57. In another study, modified poly(lactic acid) was functionalized with furan groups and cross-linked with a flexible maleimide linker, producing a recyclable shape memory polymer58. The polymers were shown to be self-healing without the need for external forces to close cracks59.
1
1.4 Vegetable oils as raw materials for thermally reversible
polymers
Vegetable oils are considered attractive resources for the synthesis of chemicals and polymers. The feeds are available in high amounts (an average of 202 million tons per year worldwide in 2016-201860) at relatively low cost (US$ 400 – 800 per ton in 201961). Several studies on the utilization of vegetable oils for the synthesis of thermally reversible polymers have been performed, for example using castor, tung, soybean, linseed, and jatropha oils. Here we briefly describe some of these studies.
Early in the 1970s, the first reports on the synthesis of DA adducts from safflower oil were published. The oil contains up to 70% of linoleic acid, which may serve as the diene in DA reactions after double bond isomerization. Maleic anhydride was used as the dienophile. It was found that the dienophile is preferably present in the trans,trans configuration, in contrast to common believe that the reaction takes place when the dienophile is in the s-cis configuration62. In subsequent studies, the isomerized linoleic ester was allowed to react with styrene (1-2 molecules of the latter per molecule of linoleic ester). The reaction was favored at higher temperature, higher styrene input, and higher pressure, although a considerable amount of side products were also formed63. Beside maleic anhydride and styrene, acrylic, methacrylic, crotonic and cinnamic acids and their esters may also serve as dienophiles. When performing the reaction between 160 to 220 oC, mixtures containing more than 45% of DA adducts were obtained, with a relatively low content of the side products64. These reports suggest that DA reactions with linoleic acid and its ester are well possible, and that polymers can also be obtained via the DA reaction with proper dienophile selection.
Soybean oil is one of the most produced vegetable oils in the world. Its relatively high content of unsaturated fatty acids23 enables various chemical modifications, thus make it an attractive source for chemicals. Commercially available acrylated epoxidized soybean oil (AESO) can be modified to obtain DA-active chemicals. Furan groups were attached to AESO via a Michael addition with furfurylamine
18 CHAPTER 1
(Figure 1.9), and the product (AESO-FA) was subsequently reacted with a bismaleimide and n-phenylmaleimide at 65 oC in different solvents (acetonitrile, dimethylformamide, chloroform, and tetrachloroethane). It was found that solvents influenced the Tg of the polymers obtained, and the temperature at
which the rDA occurred. The product with the highest Tg (74 oC) was synthesized with an AESO-FA to bismaleimide ratio of 1 to 1 in tetrachloroethane65.
Figure 1.9. Commercial AESO modified with furfurylamine via a Michael addition65.
Oils obtained from the seed of the tung tree contain D-eleostearic acid, an interesting molecule with a conjugated triene. This makes the oil suitable for the preparation of paints and varnishes, not at least because of its outstanding drying property66. Tung oil is also a potential precursor for synthesis of DA polymers, for example, using the synthesis routes given in Figure 1.1067. The first route involves a direct polymerization with three types of bismaleimides, with and without solvent (1,1,2,2-tetrachloroethane). The second route started with ester aminolysis of the oil using an excess of furfurylamine, to attach furans at the ester sites of the oil. The furan-functionalized triene was then cross-linked with three different bismaleimides. It was found that the properties of the cross-linked materials can be tailored by modifying the structure of the bismaleimides. Partial
O O H O O O O O O O H OH NH O O O NH O O O NH O O O O H O O O O O O O O H OH O O O O NH2 O DMSO 30 o C
1
O O O O O O O O O O O O N O O N O O N O O R O O O O O O N O O N O O N O O NH O O NH O N O O N O O O R N O O O NH O N O O T u n g o il ( 8 5 % -e le o st e a ri c a ci d ) Direct DA polymerisation with bismaleimide Ester aminolysis (excess furfurylamine) DA polymerization with bismaleimide O R =Figure 1.10. Two approaches of tung oil polymerization via DA reaction67.
depolymerization by the retro-DA reaction was only possible for the materials produced by the second route67.
Epoxidized linseed oil is one of the few commercially available epoxidized vegetable oils. It was shown to be a suitable starting materials for polymers with DA linkages (Figure 1.11). The synthetic approach involves the introduction of furan groups to the epoxidized oil by reaction with furfurylamine at temperatures between 80 and 95 oC. Both ester aminolysis and epoxide ring opening reactions occur, resulting in furan units attached at the epoxy and glyceride ester sites. Subsequently, the materials are cross-linked with a bismaleimide, leading to a linear DA polycondensation product with a glass transition temperature of 80-100
o
C. TGA analysis showed that the polymer was thermally stable up to 250 oC. Retro-DA reactions occurred at about 110 oC. It was proposed that the polymer is suitable to be used for thermally reversible coatings and adhesives68
20 CHAPTER 1 O O O O O O O O O O O O NH2 (excess) 95 C NH O O HO NH O NH O O HO NH O O NH O O HO NH O O H NH O
Figure 1.11. Reaction of epoxidized linseed oil with an excess of furfurylamine68.
10-Undecenoic acid, the pyrolysis product of Castor oil, is an interesting renewable chemical due to the presence of a terminal C-C double bond. Furan groups may be introduced at both ends of the molecule whereas the combination of one furan and one maleimide unit at each end is also possible (Figure 1.12). The introduction of two furan groups was performed by the esterification of 10-undecenoic acid with furfuryl alcohol and an ene reaction with furanmethanethiol at the terminal double bond, resulting in monomer AA (a). An alternative approach involves esterification of 10-undecenoic acid with an allyl alcohol followed by an ene reaction at both alkenyl ends, producing monomer AA’ (b). The synthesis of monomers with one furan and one maleimide unit at each end was done by the esterification of 10-undecen-1-ol by a masked 4-maleimidobutyric acid, followed by an ene reaction with furanmethanethiol at the terminal double bond, producing monomer AB (c). Monomers AA and AA’were polymerized with 1,6-bismaleimidohexane. Repeated heating and cooling cycles revealed that all polymers show thermally reversible properties69,70.
1
C H2 OH O O OH C H2 O O O O SH O O O S O C H2 OH O C H2 O O CH2 O SH O O S O S O C H2 OH N O H O O O O C H2 OH H2C O N O O O O O N O O O O S O AA AA' AB O SH (a) (b) (c)Figure 1.12. Synthesis of monomers based on the pyrolysis products of castor oil, to be used for the synthesis of thermally reversible polymers69,70.
Jatropha curcas oil, a non-edible oil, contains high amounts of unsaturated fatty acids, and as such it is an interesting vegetable oil for further modifications. Recent research in our group indeed showed that thermally reversible polymers may be prepared from this oil (Figure 1.13). It involves epoxidation of the C-C double bonds in the oil by performic acid, followed by a reaction with furfurylamine to introduce furan groups. However, ester aminolysis also occurred to a significant extent, leading to di- and mono-glycerides. The product was cross-linked with a bismaleimide, resulting in a brittle solid product. Thermoreversibility of the product was determined by a DSC analysis comprising a number of heating and cooling cycles between 20 and 190 oC. Analysis suggests that the retro-DA reaction occurs at about 125 oC in every cycle71.
22 CHAPTER 1 NH O O OH N H O NH O O O O O O O O O N H O OH OH N H O O H NH O N N O O O O Diels - Alder retro - Diels - Alder
R1 O OH NH N O O O R2 O O O N H R1 O OH R2 =
Figure 1.13. Cross-linking of a furan-functionalized Jatropha curcas oil with a bismaleimide71.
1.5 Aim and scope of this thesis
The aim of this research is to prepare environmentally benign polymeric materials based on vegetable oils and to investigate their possible applications. The emphasis is on the synthesis of polymers with mechanical properties of that of a cross-linked structure combined with the ability to be reprocessed to enable straightforward repair and recycle. The selected strategy involves the introduction of furan groups to vegetable oils, followed by cross-linking by the thermally reversible Diels-Alder approach using bismaleimides.
The synthesis of suitable monomers with furan units from non-edible Jatropha oil is described in Chapter 2. It entails a novel two-step approach involving epoxidation and furan-functionalization, in contrast to a three-steps route proposed in the literature (involving epoxidation, acrylation, and furan-functionalization)65. The major challenge is the limited chemoselectivity of the reaction, leading to product formation. Experiments aimed to reduce by-product formation, e.g. by tuning process conditions, are described.
1
The furan-bearing monomers described in chapter 2 are used for the synthesis ofpolymers by cross linking reactions with commercially available bismaleimides (Chapter 3). Mechanical properties of the polymers could be tuned by varying the chemical structure of the bismaleimide, and/or by using mixtures of aliphatic and aromatic bismaleimide in various ratios. The influence of the bismaleimides composition on the thermal reversibility of the obtained polymers is also investigated. To determine whether the synthetic methodology is applicable for a range of plant oils and not restricted to Jatropha oil only, additional experiments with sunflower oil were performed.
Vegetable oils are also used as starting materials for the synthesis of epoxy resins72–76. Such polymers are typically more easy to synthesize compared to the thermally reversible ones described in the previous chapter, though are not easy to reprocess. In Chapter 4, the properties of thermally reversible and epoxy polymers with similar structures are compared.
In the final chapter of this thesis, attempts are described to identify applications and to further modify the polymers to extend the application range. The thermally reversible polymers are tested as adhesives and their bond strengths are compared with a commercially available adhesive. In addition, the use of fillers is investigated to improve the mechanical properties. Finally the use of these polymers in formulations with a thermosetting polyketone is investigated to improve the self-healing properties.
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