Thermoset, bioactive, metal–polymer composites for medical applications

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Thermoset, bioactive, metal
polymer composites for medical applications

4

Hari Madhav1, Neetika Singh2 and Gautam Jaiswar3 1

Drug Design and Synthesis Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, India 2Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, India 3Department of Chemistry, Dr. Bhimrao Ambedkar University, Agra, India

4.1 THERMOSETTING POLYMERS 4.1.1 INTRODUCTION Polymers are categorized in various ways, such as by synthesis method, their solvent properties, whether they are synthetic or natural, and their chemical properties, etc. On the basis of their response to temperature, polymers may be divided into two categories, that is, thermoplastic polymers and thermoset or thermosetting polymers. Thermoplastic polymers melt on high temperature and on cooling they again converted into solid form. These polymers can be recycled and easily converted from one form to another form, but thermoset or thermosetting polymers are just the opposite to thermoplastics. They cannot be reshaped or converted into liquid form at high temperature. Some commonly known thermoplastic polymers are PMMA, PE, PVC, PS, and ABS, etc., and the heating and cooling cycle may be repeated several times for thermoplastics without obtaining any chemical changes (Madhav et al., 2017; Rathore et al., 2017; Singh et al., 2018). The identification of thermoset polymers in chemistry is represented in Fig. 4.1. Thermosets are stable against temperature, for instance, they remain unchanged in their shape and physical form, that is, they do not melt even at high temperatures. When subjected to high temperatures they undergo direct degradation and cannot be recycled or reused. These polymers undergo a curing process during heating and shaping, which will cause crosslinking in their molecular structure. In other words, it may be stated that thermosets have a constitutional repeating unit in their structure. These polymers are also referred to as thermosetting resin. The most common known examples of thermosets include phenolic resin, urea
formaldehyde resin, unsaturated polyesters (UPEs), and epoxy resin, etc. These polymers are formed by irreversible liquid or powder to solid Materials for Biomedical Engineering: Thermoset and Thermoplastic Polymers. DOI: https://doi.org/10.1016/B978-0-12-816874-5.00004-9 © 2019 Elsevier Inc. All rights reserved.

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FIGURE 4.1 Schematic representation of the identification of thermoset polymers in chemistry.

transitions which can also be produced by other techniques, such as heating, or UV or electron beam irradiation. This process is known as cure or curing of the material. In this method the primary or mother liquid/powder is converted into polymers through chemical reactions and a highly crosslinking polymeric structure is acquired. This highly crosslinked structure is mainly creditworthy for its high mechanical strength, thermal strength, and poor elasticity or elongation. The thermosetting character of polymers could be explained by the conversion of fluid state of polymers at high temperatures. The molecules which generally allow to flow at high temperature get separated, but in thermosets the highly crosslinked structure present prevents the separation of large molecules at high temperature. Thermosets are network-forming polymers which include highly reactive functional groups, such as epoxy, phenolic, UPE, polyurethane, dicyanate, bismaleimide, acrylate, and many others. Because of these reactions, the materials first show increases in viscosity and then eventually crosslink and become set, and because of this behavior the polymer becomes unable to dissolve or flow. Curing is most often achieved through thermal activation, which gives rise to the word thermoset; this is also observed in the case of light induced network forming materials or by a crosslink dual cure mechanism, for example, thermoset adhesives (Prime, 2009). The primary liquid solution of thermosets may contain various constituents, for instance, it may be a mixture of comonomers which together can react with external actions, such as heating, UV or electron beam irradiation. The constituents of the primary solution may also include nanoparticles, initiators, pigment or

4.1 Thermosetting Polymers

dyes, catalysts, and fibers, etc. The most important and essential condition to produce or for the synthesis of thermosetting polymers is that the monomer(s) present in the primary solution should contain three or more reactive groups in each molecule. These reactive groups create a highly crosslinked three-dimensional structure through irreversible chemical reaction. To convert back into liquid state, it is essential to break the synthesized covalent bonds of crosslinking, but these processes lead to degradation rather than conversion into monomers. Schematic representation of the synthesis of thermoplastics and thermosets are represented in Figs. 4.2 and 4.3. Unlike with thermoplastic polymers, during the processing of thermosets, as shown in Fig. 4.4, as the reaction proceeds, the molecular weight is increased, which begins with the growth and branching of chains, causing an increase in viscosity and a reduction in the total number of molecules. Eventually all the chains link together to form an infinite molecular weight. An important characteristic property of thermosetting polymers is their gel point, which is defined as an irreversible transformation from a viscous liquid to an elastic gel or rubber. The gel point refers to the point at which a primary liquid converts irreversibly into solid form through a curing process. Once the primary liquid crosses gel point, it converts into solid form, which cannot be reshaped, molded, or processed. At the primary stages of the curing process the thermosets can be visualized by an increase in their viscosity η. The gel point meets with the first appearance of an equilibrium (or time-independent) modulus. The polymerization reaction continues beyond the gel point to complete the crosslinked network formation, where the physical properties, that is, modulus, tensile strength, etc., build to levels characteristic of a fully developed network. In a thermoset crosslinked system, the gelation loses its ability to flow and is no longer processable above the gel point, and therefore gelation defines the upper limit of the work life. After curing, the thermoset materials can only mill or grind in micro- or nanoparticles and then be used as a filler for other applications. This allows the thermoset polymers to make components with permanent shapes and sizes. These components

FIGURE 4.2 Schematic representation of the synthesis of thermoplastic polymers in which R represents reactive sites.

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FIGURE 4.3 Schematic representation of the synthesis of thermoset polymers in which R represent reactive sites.

FIGURE 4.4 Schematic representation of thermoset curing. Cure starts with A-stage or uncured monomers and oligomers (A) proceeds via simultaneous linear growth and branching to an increasingly more viscous B-stage material below the gel point (B) continues with the formation of a gelled but incompletely crosslinked network (C) and ends with the fully cured, C-stage thermoset (D) (Prime, 1981).

4.1 Thermosetting Polymers

can be produced by curing of the primary liquid or powder within a mold and these products can then be removed from the mold without allowing time to cool. The first manmade synthetic thermosetting polymer “Bakelite” was patented on the 7th of December 1909 by Belgian chemist Leo Hendrik Baekeland. It was a phenol
formaldehyde resin, which is chemically known as polyoxybenzylmethyleneglycolanhydride, in which the phenol monomer exhibits three reactive sites at o, o0 , and p positions, the second monomer or comonomer formaldehyde exhibits two reactive sites, and the other comonomer hexamethylenetriamine, which was added in second step, shows multifunctional activity. These polymers are used in many applications, for instance, Bakelite is used in the manufacturing of parts for electrical systems due to its high heat resistance and low electrical conductivity, urea
formaldehyde polymers are in wood agglomerates, melamine
formaldehyde polymers are used in laminates, epoxies are used in electronic applications (i.e., in capacitors, transformers, circuit boards, etc.) due to their very low electrical conductivity, UPEs are in glass fiber
reinforced plastic, and polyurethane polymers are used in insulating foams, etc. The properties of these thermosetting polymers are continuously studied by various researchers for improvement and incorporation into some nanofillers, that is, nanoclay, inorganic nanoparticles, carbon nanotubes (CNTs), nanofibers, etc. These properties are also improved by modifications in monomers or polymers through functionalization. Improvements in the properties of thermosetting polymers are focused in the electronic, mechanical, automobile, storage and production, telecommunication, medical, and biomedical fields, etc.

4.1.2 SYNTHESIS OF THERMOSET POLYMERS Plastic polymers are made through the polymerization of monomers, thereby forming macromolecular chains. Besides the monomers, many other chemical substances may be needed during the polymerization process, for example, initiators, catalysts, chain transfer agents, emulsifying agents, and solvents. The chemical processes for chain formation may be divided into chain-growth polymerizations (mainly addition polymerization) and step-growth polymerization (mainly condensation polymerization, but also addition polymerization). In addition polymerization, the monomers are reacted by opening a double bond, but with no molecules being split out. In condensation polymerization, water or other simple molecules, for example, ammonia, carbon dioxide, hydrochloric acid, ethanol, and hydrogen chloride, are split out during the reaction. Vitrification is the transformation from a liquid or rubbery state to a vitreous state. It is a completely distinct phenomenon from gelation, which may or may not occur during curing depending on the cure temperature (Tcure) relative to the glass transition temperature (Tg) for full cure. Vitrification is defined as the point where Tg 5 Tcure, and the formation of glass has been observed due to the Tg increasing from below Tcure to above Tcure as a result of the cure reaction. Vitrification can occur at any stage amid the response to form either an ungelled

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Table 4.1 Glossary of Characteristic Cure Parameters Tgel Tvit Tcure Tg TgN

Time to gelation, gel time Time to vitrification Cure temperature, a process parameter Glass transition temperature, a material property Tg for fully cured thermoset with degree of chemical conversion (α) 5 1

glass or a gelled glass. It may be evaded by restoring at or above TgN, the glass progress temperature for the completely relieved system. In the shiny state, the rate of reaction will more often than not experience a critical lessening and fall underneath the compound response rate as the response ends up being controlled by the dispersion of reactants. Usually completion of vitrification is resulted in an abatement in the rate of response by 2
3 requests of size. Not at all like gelation, vitrification is reversible by warming, and substance control of the fix might be restored by warming to devitrify the somewhat relieved thermoset. Vitrification might be distinguished by a stage increment in warm limit by tweaked temperature DSC (MTDSC) and by powerful mechanical examination (DMA) as a recurrence subordinate change bringing about a glassy modulus typically .1 GPa (Prime, 2009) (Table 4.1). Thermosetting polymers might be formed in two different ways: By polymerizing (step or chain components) monomers where one of them has a usefulness higher than 2 and by synthetically making crosslinks between already shaped direct or stretched macromolecules (crosslinking of essential chains, as vulcanization improves the situation normal elastic).

4.1.2.1 Synthesis of thermosetting polymers by polymerization Step-growth polymerization follows a step-by-step formation of elementary reactions between reactive sites, which are generally functional groups, such as alcohol, acid, and isocyanate. Each independent step forms the disappearance of two coreacting sites and creates a new linking unit between a pair of molecules. To synthesized polymers, the reactants must be at least difunctional; monofunctional reactants interrupt polymer growth. The synthesis of a thermosetting polymer through addition polymerization is represented in Fig. 4.5. Some examples are seen for the synthesis of thermoset through step-growth polymerization. For example, when monomers of amide-co-imide functional benzoxazine are heated at 200 C
215 C, then they are polymerized in a crosslinked thermoset through ring opening polymerization. In this polymerization, the presence of (
NH
CO
) linkages accelerates the polymerization in comparison to ordinary benzoxazines. The intramolecular hydrogen bond between the amide linkage and the adjacent oxazine ring acts as an internal self-complementary initiator (Zhang and Ishida, 2015).

4.1 Thermosetting Polymers

FIGURE 4.5 Linear chain formation and crosslinking via addition polymerization.

In chain-growth polymerization, propagation is followed by the direct reaction of a species bearing a suitably generated active center, such as a free radical, an anion, and a cation. The monomer itself constitutes the reactive solvent and is progressively converted into the polymer. In the polymer growth mechanism, if one of the reactants has a functionality higher than two, then branched molecules of infinite structure will be obtained. The synthesis of thermosetting polymers through condensation polymerization is represented in Fig. 4.6. If glycidyl compound and melamine are mixed together at 60 C in the presence of 4-pyro(idinopyridine) as a catalyst then they will synthesize a thermoset resin N, N-diglycidyl-4-glycidyloxyaniline with melamine. Thereafter, if the temperature is increased to 80 C, it promotes the catalytic process and completely polymerizes the resin in a crosslinked thermoset (Ricciotti et al., 2013). When homopolymer poly(2,6-dimethyl-1,4-phenylene ether) (PPE), prepared from 2,6-dimethyl phenol and a copolymer of 2,6-dimethyl phenol and 2,6-di (3-methyl-2-butenyl)phenol were polymerized by oxidative coupling polymerization using copper (I) chloride-pyridine as a catalyst at 25 C, it was converted into a thermoset polymer with a highly crosslinked structure (Matsumoto et al., 2004). A new concept has also been seen in thermoset polymer science, that is, the term biodegradable or biobased thermoset polymer is used by various researchers and scientists in their work. There are many examples of biodegradable thermoset polymers seen in the literature. For example, if glycerol and citric acid are mixed and heated to between 90 C and 150 C then they are polymerized into a crosslinked polymer through condensation polymerization with water being released as a primary byproduct (Halpern et al., 2014).

4.1.2.2 Synthesis of thermosetting polymers by crosslinking or curing A linear polymer is simply a chain in which all the carbon
carbon bonds are present in a single straight line. A network polymer is synthesized due to the reaction between linear polymer chains or to the build-up of monomeric resinous reactants of a three-dimensional fish-net configuration. This process of interaction

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FIGURE 4.6 Linear chain formation and crosslinking via condensation polymerization.

is called crosslinking and is the main distinguishing feature of a thermosetting material. The term curing is most frequently used for crosslinking or gelling phenomena, while vulcanization is the industrial term for the curing of rubber. Mostly, coreactive monomers are denoted as resin and curing agents. Resin is the resinous monomer from which the family name is derived (e.g., an epoxy plastic is an epoxy resin that has been crosslinked). The term “thermo” implies that crosslinking follows the application of heat energy; most crosslinking occurs at room temperature and below. In the past few years, some researchers have used the term chemosets, in which the reaction of a thermoset takes place at room temperature. The term “setting” references the fact that an irreversible reaction has occurred on a macroscale. The network polymer formed has an “infinite” molecular weight with chemical interconnections that restrict long-chain macromovement or slippage. Molecular functionality may be defined as a number of reactive moieties available in a molecule of reactant which indicates the potential for a crosslinking reaction. A total average functionality between reactant elements greater than two suggests the potential for crosslinking, independent of mechanism. In other words, a linear polymer is formed due to the bifunctional C 5 C bond via an addition reaction, while in a condensation reaction, thermoset structure with a polyfunctional comonomer formed via tri- or polyfunctional reactant (Dodiuk and Goodman, 2014). From Fig. 4.7, a polymerizing mixture of monomers can be tracked by observing the viscosity change versus time at a given temperature. Beginning at t0, the mixture has a viscosity of η0. The heat released from the exothermic reaction decreases the viscosity (η1). As the molecular weight of the mass increases, the resultant mixed viscosity increase outpaces and quickly surpasses any reduction caused by heat. Molecular growth continues over time until a perceptible macroscopic gel-like “lump” can be sensed. This is the gel point (tgel), or more commonly, the gel time. From this point forward, the viscosity goes to infinity and the polymeric mass becomes a macroscopic plastic solid (Dodiuk and Goodman, 2014).

4.1 Thermosetting Polymers

FIGURE 4.7 Schematic representation of the change in viscosity of thermoset prepolymer during curing.

There are many examples for the synthesis of thermoset or thermosetting polymers found in the literature, in which many hardeners or curing agents were used to produce crosslinking in thermoset prepolymer resin. For this process, firstly the thermoset monomer is prepared with various functionalities and then cured with various hardeners or curing agents with heat or else it is cured only using heat by mixing two or more appropriate thermoset monomers or compounds. For example, a phenylethynylcarbonyl terminated imide compound thermoset was prepared by curing at 200 C for 3 hours followed by 220 C for 3 hours. This curing reaction involves the disappearance of CRC and formation of CQC bonds, which was confirm by Fourier transform infrared spectroscopy (FTIR). It was seen in the reaction that polyene initially formed which participated in crosslinking (Kimura et al., 2013). If epoxy monomer diglycidyl ether of bisphenol A (DGEBA) is mixed with triethylenetetramine (TETA) as a curing or crosslinking agent at room temperature than it converts into a crosslinked thermoset (Becker et al., 2011). Another example of the synthesis of a thermosetting polymer is when 2,5-bis[(2-oxiranylmethoxy)methyl]-furan (BOF) and 1,4-bis[(2-oxiranylmethoxy)methyl]-benzene (BOB) are mixed with DGEBA using diethyl toluene diamine and 4,40 -methylene biscyclohexanamine (PACM) as amine hardeners then they are converted into thermoset polymers. When BOF reacts with PACM, the epoxy group starts reacting with the amine group and the polymerization with an epoxy ring opening reaction (Hu et al., 2014). If epoxy derivatives of vanillin monomer are melted and mixed with amine hardener isophorone diamine (IPDA) at 100 C
125 C then the monomer converts into a highly crosslinked structure through chemical reaction, as shown in Scheme 4.1 (Fache et al., 2015).

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SCHEME 4.1 Synthesis of crosslinked materials by epoxy/amine reaction (Fache et al., 2015).

Another synthesis of biobased thermoset polymers was observed when biobased monomers were cured with amine hardeners. The first biobased monomers, that is, isosorbidediferulate (IDF), butanediddiferulate (BDF), glyceroltriferulate (GTF), were synthesized with ferulic acid and various biobased diols, such as ethanol, and then functionalized with epoxy groups. When epoxy containing IDF, BDF, and GTF reacted with a diamine curing agent they were converted into having crosslinked structures (Me´nard et al., 2017). The schematic synthetic route for this synthesis is shown in Scheme 4.2. Curing of Bis-N-phenylbenzoxazine derivatives and N-propyl benzoxazine derivatives at 120 C
180 C and 140 C
200 C respectively, produces highly crosslinked thermosets. This synthesis reaction involves oxazine ring opening and gives some phenolic structures with Mannich bridges, which is represented in Scheme 4.3 (Tu¨zu¨n et al., 2016).

4.1.3 PROPERTIES OF THERMOSETTING POLYMERS Thermosetting polymers exhibit a highly crosslinked, fishnet-like chemical structure and due to this characteristic property, they show many other specific properties in comparison to other materials that make them different from others. Some of the properties of thermoset polymers are discussed here:

4.1.3.1 Formulations The formulation of thermoset polymers is highly complex. Their formulation involves curing, crosslinking, and many reactions between monomers or oligomers with other monomers, crosslinking agents, or hardeners, such as ring opening reactions, free radical reactions, and catalytic reactions. Thermosetting polymers show multistage processing, including the preparation of thermoset monomers with different functionalities, the preparation of thermoset resins with

4.1 Thermosetting Polymers

SCHEME 4.2 Syntheses of the different biobased epoxy precursors GTF3EP, BDF2EP, BDF(amide)2EP, and IDF2EP from ferulic acid and various biobased diols (Me´nard et al., 2017).

SCHEME 4.3 Thermal curing of bis-benzoxazine monomers and representative structure of the resulting networks (Tu¨zu¨n et al., 2016).

mixtures of different necessary monomers, oligomers, hardeners, and fillers, and then the curing process for long periods at different temperatures in appropriate environments.

4.1.3.2 Solvent resistant Thermosetting polymers show excellent resistance against solvents due to their chemical structures and high degree of crosslinking. For example, uncrosslinked

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polyacrylamide can be dissolved in water, but crosslinked polyacrylamide only adsorbs water.

4.1.3.3 Melt viscosity When thermoset monomers, prepolymers, or resins are heated they show high viscosity, but during the curing process after the gel point, the viscosity of resin rapidly increases to a high level or infinity and the resin enters a solid state. It has been seen from various research that when thermoset resins are heated, they do not melt and at higher temperatures they degrade through random scission.

4.1.3.4 Mechanical properties Thermoset polymers show fair to good mechanical properties at room temperature. They are much more stable then thermoplastic even at high temperatures. Most thermosets are brittle in nature while some break under external force (Li and Dingemans, 2017). The mechanical strength of thermosets can be improved using nanofillers, such as CNT, nanofibers, clay-like materials, and certain thermoplastics.

4.1.3.5 Fiber impregnation Fiber impregnation in thermosetting is easy compared to with thermoplastic (melt viscosity 100
1000 Pa) because uncured thermoset resins show low viscosity (1
10 Pa) (Karger-Kocsis, 1999).

4.1.3.6 Processing cycle The processing cycle for the synthesis of thermoset polymers is long. Mostly they take long a time for curing and hardening. After the curing process, most thermosets require a postcuring period of 2
5 hours to ensure complete crosslinking. For example, Tan et al. synthesized various phosphorus-containing polybenzoxazine derivatives as thermoset prepolymers by melting them at 160 C and 170 C to remove the air followed by a curing process at 180 C for 4 hours, then 190 C and 200 C for 2 hours each, finally postcuring them at 215 C for 1 hour to prepare a crosslinked thermoset polymer (Tan et al., 2017).

4.1.4 CHARACTERIZATION OF THERMOSET POLYMERS Thermosetting polymers are characterized using various sophisticated analytical instrumental techniques. The physical, chemical, mechanical, and thermal properties of these components, which have a broad effect on the ultimate properties of polymers, like dimensional stability, moisture and solvent resistance, and mechanical strength, were characterized using techniques such as FTIR, ultraviolet light (UV) spectroscopy, nuclear magnetic resonance spectroscopy (NMR), X-ray fluorescence spectroscopy (XRF), thermogravimetric analysis (TGA), dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC). (Forrest, 2003).

4.1 Thermosetting Polymers

4.1.4.1 Fourier transform infrared spectroscopy Infrared (IR) is an extensive identification technique, which is used for the identification of functional groups and the nature of chemical bonds, etc., in chemical compounds. Infrared spectra able to identify the presence of functional groups and chemical bonds present in cured or uncured samples. With the help of FTIR, the nature of reactions during curing and of evolved gases during degradation at high temperatures can be identified. For example, when allyl alcohol lactic acid (ALA) oligomer resin is cured in order to synthesize its thermoset, the changes in chemical structure are identified using FTIR spectra. FTIR spectra of ALA resin showed adsorption band at 1640 cm21 due to double bond of allyl alcohol, band at 3428 cm21 due to
OH stretching and 1759 cm21 due to CQO (Bakare et al., 2015). For DGEBA and epoxidized hemp oil (EHO) cured with citric acid (CA) and tartaric acid (TA) as the hardeners and triethylbenzylammonium (TEBAC) as a catalyst, the involved chemical process was studied with FTIR and is shown in Fig. 4.8. The authors explained that the presence of bands at 3409 cm21 of
OH occurs due to the opening of epoxy rings under the action of carboxyl groups. The band at 915 cm21 of the epoxy group completely disappears from DGEBA due to the same reaction. The presence of bands in cured EHO at 2800 and 3000 cm21 confirms the asymmetric and symmetric stretching of CH, CH2, and CH3 of the EHO structure. The presence of a CQC stretching band at 1609 cm21 shows the presence of an aromatic ring in cured epoxy resin of DGEBA. An absorption band at 1738 cm21 specifies the ester group of EHO in the cured

FIGURE 4.8 IR spectra of: (A) DGEBA; (B) crosslinked DGEBA/EHO/TA; (C) EHO (Mustata et al., 2016).

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thermoset (Mustata et al., 2016). So, based on the spectra of these cured and uncured resins, FTIR spectroscopy is confirmed as having an important role in the study of chemical reactions and structure of cured and uncured thermosetting resins.

4.1.4.2 Nuclear magnetic resonance spectroscopy NMR is the most sensitive and powerful tool for the determination of polymer structures and the presence of functional groups in polymer chains. This technique indicates the types of protons in cured and uncured thermoset resins. It is also used to determine the functionalization and position of functional groups, and it is possible to use solid state NMR for thermoset products. For example, when hemp oil is epoxidized with peroxyacetic acid, NMR has been proven as the best technique for the confirmation of the epoxidation of hemp oil. Proton NMR (1H NMR) spectrum of epoxidized hemp oil shows the appearance of epoxy protons, located peaks at 2.860 and 3.031 ppm, which was not observed in unepoxidized hemp oil (Mustata et al., 2016). The 1H NMR spectrum of epoxidized hemp oil is shown in Fig. 4.9. Another interesting example is the preparation of a renewable isosorbidebased monomer for the preparation of the corresponding thermoset. When the isosorbide is modified using 5-norbornene-2-yl(ethyl) chlorodimethylsilane and 5-norbornene-2,3-dicarboxylic anhydride then norbornyl functionalized isosorbide

FIGURE 4.9 1

H NMR spectrum of EHO (Mustata et al., 2016).

4.1 Thermosetting Polymers

SCHEME 4.4 Synthetic route of norbornenyl-functionalized isosorbide ISN and IN (Wang et al., 2016a,b).

is produced, known as ISN and IN respectively. The structure of functionalized ISN and IN can be determined by 1H NMR. The synthetic route for the synthesis of ISN and IN is shown in Scheme 4.4, while the 1H NMR spectra for ISN is represented in Fig. 4.10. NMR pattern of ISN shown two resonances at δ value of 6.11
6.09 ppm and 5.90
5.87 ppm of two different protons aCHQCHa of Norborne moiety, while peaks located at δ value of 4.45
3.44 ppm was correspond to eight protons of two fused rings of isosorbide segment. The NMR peaks at δ value of 2.78
2.73 and 1.92
1.78 ppm were caused by end protons (CH) and middle protons (CH2) of
CH
CH2
CH
of Norborne moiety. In addition, observed multiples at around 1.11
1.01, 0.67
0.50 and 0.15
0.05 ppm were observed for
CH2
CH2
Si(CH3)2
protons. These NMR signals confirm the proposed structure of ISN (Wang et al., 2016a,b).

4.1.4.3 Differential scanning colorimetry DSC is a useful tool for thermal analyses of thermosetting plastics using changes in heat capacity results due to exothermic or endothermic reactions. DSC can be used to identify the glass transition of thermosets. It can also be used in thermal stability studies of thermoset products and those that investigate the effectiveness of antidegradants and fire retardants. Specifically, in thermosets, DSC is used to determine the cure behavior of thermoset monomers or oligomers. The use of DSC for determining the curing behavior of maleimidobenzoxazine monomer 1-(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)maleimide (Mal-Bz) showed that exothermic transition begins from 167 C with a maximum 214 C. The total amount of exotherm was 75 cal g21. This exothermic transition corresponds to the polymerization of benzoxazine via the ring opening of oxazine rings and the addition homopolymerization of maleimide (Agag and Takeichi, 2006). DSC is

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FIGURE 4.10 1

H NMR spectrum of biobased monomer ISN (Wang et al., 2016a,b).

also used to measure the Tg of thermoset polymers. The effect of hardeners on the thermal properties of cured materials can also be examined through DSC. The effect of the epoxy/amine ratio on the Tg of cured epoxy monomer diglycidyl ether of methoxyhydroquinone is represented in Fig. 4.11, (Fache et al., 2015). The authors reported that the maximum Tg was observed for a 2:1 epoxy/ amine ratio. This can be helpful in the synthesis of appropriate flame-retardant polymers with appropriate properties. The nature of the reactions in the curing process can also be determine with the help of DSC. The DSC of DGEBA/EHO at ratio 70/30 (w/w) in presence of TEBAC as a catalyst, using CA and TA as hardeners shows single peak. This shows that the reactions during curing, that is, opening of epoxy ring, formation of ester bonds, and reaction between hydroxyl groups, epoxy groups, and carboxy groups, are completed simultaneously (Mustata et al., 2016).

4.1.4.4 Thermogravimetric analysis This technique is used for measuring mass changes as a function of temperature or time. This analysis technique is used in research and development toward determining the thermal stability of various substances and engineering materials that are both solid and liquid. This technique has been used in the quality control and assurance of raw materials and incoming goods, as well as in the failure analysis of finished parts, especially in the polymer processing industry (Forrest, 2003). The thermal stability and thermal degradation of thermoset polymers is determined using this technique. TGA cured neat DGEBA with isophorone

4.1 Thermosetting Polymers

160

140

Tg = 132ºC Tg = 125ºC

120

Tg = 113ºC

Temperature (ºC)

Tg = 102ºC Tg = 97ºC

100

80

Tg = 88ºC Tg = 78ºC

Tg = 71ºC

60

40

20

2.0/0.6

2.0/0.8

2.0/1.0

2.0/1.2 2.0/1.4 Epoxy/amine ratio

2.0/1.6

2.0/1.8

2.0/2.0

FIGURE 4.11 Tg of diglycidyl ether of methoxyhydroquinone-based materials as a function of the epoxy/ amine ratio used (Fache et al., 2015).

Table 4.2 TGA Data for DGEBA/IPDA and P3SP (Me´nard et al., 2015) Thermosets

10% Weight Loss at Temperature ( C)

% of Char Residue Remaining at 700 C

DGEBA/IPDA 1% P(P3SP) 2% P(P3SP) 3% P(P3SP)

364 352 337 325

9 13 16.1 19.3

diamine (IPDA) as a hardener showed a 10% weight loss from its initial mass at 364 C and the char residue was 9% of its initial mass at 700 C. When trithiophosphonate phloroglucinol (P3SP) was added in ratio of 1%, 2%, and 3% (w/w) to DGEBA/IPDA, the temperature for 10% weight loss of these combinations shifted to lower temperature which is represented in Table 4.2 and TGA curves are shown in Fig. 4.12 (Me´nard et al., 2015). TGA of cured ALA oligomer functionalized with methacrylic anhydride (MLA) showed that it loses 10% of its initial mass at about 290 C and 50% of its weight at about 400 C, while cured ALA oligomer pentaerythritol functionalized with methacrylic anhydride (PMLA) looses 10% of its weight at about 302 C and 50% of its weight at about 444 C. TGA curves for MLA and PMLA are represented in Fig. 4.13 (Bakare et al., 2015).

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FIGURE 4.12 Thermograms of P3SP-containing thermosets (Me´nard et al., 2015).

FIGURE 4.13 TGA curve of MLA, PMLA, GLA, and UPE resin cured (Bakare et al., 2015).

4.1.4.5 Dynamic mechanical thermal analysis DMTA or DMA is a sophisticated analytical instrumental technique which is used to record the viscoelastic properties of polymers, that is, it records the storage modulus and tan δ versus temperature for polymeric materials. The effects on the

4.1 Thermosetting Polymers

viscoelastic properties of thermoset materials over a wide temperature range 2150 C to 1200 C can be observed through DMTA. This analysis technique investigates the effect of temperature and loading on the mechanical properties of thermoset materials. For example, DMTA analysis of neat DGEBA and DGEBA/ PEI-PCLX-B formulation shows that the storage modulus and tan δ vs temperature of neat DGEBA was higher and for PEI-PCL10-B formulation both storage modulus and tan δ vs temperature were shifted to lower temperature which indicated that glass transition (Tg) was slightly decreased (Acebo et al., 2014). This shows that the tan δ curves become broader due to the heterogeneity of the structure at the molecular level. The DMTA curves for neat DGEBA and DGEBA/ PEI-PCLX-B formulations are shown in Fig. 4.14. Another example for DMTA in the literature is observed for carbazole containing a DGEBA/TTMP mixture (Korychenska et al., 2016). DMTA curves show a unimodal relaxation, typical of homogeneous materials, but as the concentration of carbazole moieties increases, the tan δ curves shift to higher temperatures and become broad, which indicates lower homogeneity. Tan δ curves of DMTA are shown in Fig. 4.15 (Korychenska et al., 2016). The loading of other polymers is affected by the viscoelastic properties of thermoset polymers, for instance, when triblock copolymer PVPy-b-PVK-b-PVPy was loaded into epoxy polymer DGEBA, it affects the storage modulus and tan δ value (Xiang et al., 2016). DMTA of neat DGEBA shows α-transition at 159 C, which was attributed to the glass
rubber transition of the thermoset. Upon loading of PVPy-b-PVK-b-PVPy at different weight percentages to DGEBA, the α-transitions shifted to a lower temperature as loadings of triblocked copolymer

FIGURE 4.14 Storage modulus and tan δ curves against temperature for neat DGEBA.

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FIGURE 4.15 Tan δ versus temperature at 1 Hz for obtained thermosets with different contents of carbazole moieties (5
30 wt.%) (Korychenska et al., 2016).

increased, but the tan δ is increased for 20 wt.% loading of triblock copolymer. This transition was responsible for the PVK nanophases (Xiang et al., 2016). DMTA curves for the neat epoxy thermoset and loaded triblock copolymers are shown in Fig. 4.16.

4.1.4.6 X-ray fluorescence spectroscopy XRF is a useful technique, which is used for obtaining semiquantitative elemental data from thermoset products and thin ashes. This analysis technique helps to identify inorganic fillers and pigments in samples. Usually, this technique is used in conjugation with IR.

4.1.5 APPLICATIONS OF THERMOSET POLYMERS Thermoset materials are those materials that are made up of polymers jointed together by chemical bonds, thereby acquiring a highly crosslinked polymer structure. These polymers cannot be melted or dissolved and possess excellent thermal stability and rigidity. Thermoset materials are directly responsible for high mechanical and physical strength as compared to thermoplastic or elastomer materials. Some thermosets have good mechanical properties and show high ionic conductivity, which can be synthesized from ionic liquid epoxy monomers. Some examples of thermoset polymers include: urea
formaldehyde resins, melamine
formaldehyde resins, polyurethanes, epoxy resins, phenol
formaldehyde resins, UPEs resins, polyelectrolytes, etc. The uses of thermosetting polymers are broad and are explained here:

4.1 Thermosetting Polymers

FIGURE 4.16 DMA curves of the control epoxy and nanostructured epoxy thermosets containing PVPy-b-PVK-b-PVPy triblock copolymer (Xiang et al., 2016).

4.1.5.1 Urea
formaldehyde resin Several examples of urea
formaldehyde uses include in textiles, paper, foundry, sand molds, wrinkle resistant fabrics, cotton blends, rayon, corduroy, etc. It is also widely used as an adhesive to glue wood together. Urea
formaldehyde is mostly used in electrical appliances, casings, and desk lamps, etc. Urea
formaldehyde is also used in the agricultural field as a source of nitrogen fertilizer, which decomposes into CO2 and NH3. This is performed by the action of microbes, which are found naturally in soils (Nuryawan et al., 2017).

4.1.5.2 Melamine
formaldehyde resins This resin is similar to urea
formaldehyde resin, but melamine resins are more moisture-resistant, harder, and stronger. Melamine
formaldehyde (MF) resins are widely used in laminate flooring, countertops, cabinetry, surface coatings, textile

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finishes, paper processing, impact resistant crockery (e.g., for hospitals and picnics), toilet seats, pan handles and knobs, stain and cut resistant decorative laminates, etc. (Doudiuk and Goodman, 2013).

4.1.5.3 Phenol
formaldehyde resin Phenol
formaldehyde thermosets are used in different industries. It is mainly used in circuit board production for making circuit boards, like PCB, etc. Phenolic resins are used in electrical equipment, for example, caps, handles, buttons, radio cabinets, furniture, knobs, vacuum cleaners, cameras, ashtrays, and engine ignition equipment. It is also used in laminated material, like laminated sheets, rods, and tubes which are made in a great variety, that is, from fabric, paper, wood veneers, etc. (Doudiuk and Goodman, 2013).

4.1.5.4 Polyelectrolytes Polyelectrolytes show many applications in fields, such as in water treatment as flocculation agents, in ceramic slurries as dispersant agents, and in concrete mixtures as super-plasticizers. Furthermore, many shampoos, soaps, and cosmetics contain polyelectrolytes. Certain polyelectrolytes are also added to food products, for example, as food coatings and release agents. Some examples of polyelectrolytes are pectin (polygalacturonic acid), alginates (alginic acid), and carboxymethyl cellulose, of which the last one is of natural origin. Polyelectrolytes are water soluble, but when crosslinking is created in polyelectrolytes they are not dissolved in water. Crosslinked polyelectrolytes swell in water and work as water absorbers and are known as hydrogels or superabsorbent polymers when slightly crosslinked. Superabsorbers can absorb water up to 500 times their weight and 30
60 times their own volume (Bolto and Gregory, 2007; Dobrynin and Rubinstein, 2005).

4.1.5.5 Polyurethane Polyurethanes have many different applications. In modern times, with advances in the different techniques for producing this polymer, manufactures are able to make a wide range of polyurethane apparel, including manmade skin and leathers, which are used for garments, sports clothes, and a variety of accessories. Polyurethanes are most commonly used in major appliances, such as in rigid foams for refrigerator and freezer thermal insulation systems. In addition to the foam that makes car seats comfortable, bumpers, interior “headline” ceiling sections, car bodies, spoilers, doors, and windows, all use polyurethanes. Polyurethane is also used as a household material which includes floors, flexible foam padding cushions. Polyurethanes play a major role in modern material science, such as in composite woods. Polyurethane-based binders are used in composite wood products to permanently glue organic materials to oriented strand board, medium-density fiberboard, long-strand lumber, laminated-veneer lumber, and even strawboard and particleboard, etc. (Guo, 2012).

4.2 Thermoset Metal
Polymer Composites

4.1.5.6 Epoxy resins Epoxy is a wonderful chemical having a wide variety of applications due to its unique chemical and physical properties. It is broadly used in industrial fields, for instance, epoxy resins are used in the paint industry, as structural or engineering adhesive, in the construction of aircrafts, automobiles, and boats, as a coating, encapsulates, casting materials, potting compounds, and binders. Some of their most interesting applications are found in the aerospace and recreational industries (Lagunas et al., 2014; Acebo et al., 2014).

4.1.5.7 Unsaturated polyester resin UPEs, which are usually strengthened by fiberglass or ground minerals, are used in the manufacturing of structural components, such as boat hulls, pipes, and countertops. The principal products include boat hulls, appliances, business machines, automobile parts, automobile-body patching compounds, bathtubs and shower stalls, flooring, translucent paneling, storage tanks, corrosion-resistant ducting, and building components. These UPEs are also used as a filler with ground limestone or other minerals and cast into kitchen countertops and bathroom vanities. Bowling balls are made from UPEs by casting into molds with no reinforcement (Mark, 1999).

4.2 THERMOSET METAL
POLYMER COMPOSITES 4.2.1 INTRODUCTION Currently, the development of polymer nanocomposites with adequate properties is one of the most active research areas in polymer science. Nanoparticles show various properties focused particularly on strengthening electrical conduction and barrier properties to temperature, mechanical properties, and the possible improvement in fire behavior on gases and liquids (Marquis et al., 2011). The polymer composites are combinations or compositions which comprise of two or more materials as separate phases at least one of which is a polymer. Combining polymers to other materials, like as glass, carbon, metal nanoparticles, ceramics, or another polymer, is generally to obtain unique combinations. Glass, carbon, or polymer fiber-reinforced thermoplastic or thermosetting resins, polymer blends, silica or mica reinforced resins, and polymer-bonded or impregnated concrete or wood, etc., are typical examples of synthetic polymeric composites. It is also often useful to consider materials such as coatings (pigment-binder combinations) and crystalline polymers (crystallites in a polymer matrix) as composites. Typical naturally occurring composites include wood (cellulosic fibers bonded with lignin) and bone (minerals bonded with collagen) (Schwartz, 2002). The addition of fillers and reinforcements has played an important role in the polymer industry. Many different types of fillers have been introduced into

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polymers, which provide a synergistic improvement to their processability and the properties of the final products, like tensile strength, heat distortion temperatures, thermal and electrical conductivity, and enhanced gas barrier properties. It has also been confirmed that the addition of high fractions of micron sized fillers results in considerable changes in rheological properties. Examples of these fillers are small solid particles of black carbon, calcium carbonate, glass-fibers, and talc, and their particle sizes usually range within the micron-level (Gupta and Bhattacharya, 2008). Nanoplate fillers may be natural or synthetic clays, as well as phosphates of transition metals. The most broadly used reinforcement is clay due to its natural abundance and its high form factor. Clay added nanocomposites exhibit an overall improvement in physical performance (Marquis et al., 2011). Thermoset polymers are mostly used in polymer nanocomposites, including phenolic resin, epoxy resins, and UPE resins, etc. UPE resins show relatively poor mechanical and thermal properties, which moderates their use in advanced composite systems (Mark, 1999). Thermoset composite materials show many important properties, such as high strength, UV resistance, light weight, nonconductance, corrosion resistance, electrical and exceptional thermal properties. There is increased interest in polymer/metal composites due to their properties, such as multifunctionality, ease in processing, potential in large-scale fabrication, and being lighter as compared to metals. In these composites, when the metal nanoparticles are embedded in the polymer matrix, they show the mechanical, electrical, and chemical properties of metal and polymers. The combination of metal or metalloid atoms and polymers, which can also exhibit newly synthesized products, properties such as mechanical, physical, thermal, electrical, and aesthetic., come under the category of polymer/metal composites (Bhattacharya, 1986). Polymer/metal nanocomposites can be used in several areas, such as catalysis, sensors, electronics, optics, medicine, and biotechnology. Many researches are on-going on novel metals, such as gold, silver, copper, and platinum. These noble metals play an important role in stable dispersion and applicability. There are two different type of metal nanoparticles used for the synthesis of polymer nanocomposites, that is, monometallic, which are formed from single metal elements and bimetallic, which are composed of an allowed or core
shell structure that differs from the two metals (Yadav and Gautam, 2017; Singh et al., 2016). Polymer composites with fillers are of great interest for many fields of engineering. As already mentioned, polymer metals show intrinsic advantages, including being low cost and easily processed among others. On the other hand, metallic materials also show useful properties and characteristics, such as high conductance, mechanical strength, and thermal conductivity. (Delmonte, 1990).

4.2.2 SYNTHESIS OF THERMOSET COMPOSITES Thermoplastic polymer nanocomposites are synthesized using various synthesis methods, but thermoset polymers have certain limitations, that is, thermosetting

4.2 Thermoset Metal
Polymer Composites

polymers cannot be melted, remolded, or reshaped after being subjected to a curing process so there are limitations associated with thermosetting plastic and the synthesis of their composites. Thermoset polymer/metal nanocomposites can be prepared using two different techniques, that is, ex situ and in situ techniques. In the ex situ method, the polymerization of monomers and the formation of metal nanoparticles occur separately and then they are mechanically mixed to form nanocomposites. In this method metal nanoparticles show wide distribution and exhibit poor dispersion in the polymer matrix. This technique cannot be applied to all types of thermoset polymers. In the in situ method, metal particles are generated inside a polymer matrix by decomposition (e.g., thermolysis, photolysis, radiolysis, etc.) or a metallic precursor is dissolved into a polymer by chemical reduction. A commonly applied in situ method is a dispersion process in which the solutions of the metal precursor and the protective polymer are combined, and the reduction is subsequently performed in solution. This method is more effective with a lower cost for the performance improvements of polymers as compared to the ex situ method. These approaches are used in many types thermoset polymers (Mittal, 2013). It has been seen from various researches and literature that thermoset polymer composites are generally prepared by mixing fillers into thermoset resins followed by a curing with hardeners or crosslinking agents and catalysis. In this method, fillers such as metal nanoparticles, glass fibers, silica particles, nanoclay, etc., are mixed through mechanical stirring in uncured resin vigorously to obtained a homogeneous mixture of fillers with resins. This mixture is cured and molded in various molds to obtain the desire shape and size, etc. An epoxy nanocomposite with Al2O3 nanoparticles and CaSiO3 microparticles was prepared through the mixing of these nanoparticles and microparticles in an uncured epoxy resin (Wetzel et al., 2003). After the incorporation of these nanofillers and microparticles into the resin, this mixture was mechanically dispersed to distribute the components homogeneously within the matrix. This mixture was cured with cycloaliphatic polyamine as a curing agent at 70 C
120 C for several hours. Organic
inorganic hybrid nanocomposites of epoxy were synthesized by blending DGEBA with nanostructured polyhedral octa aminopropyl silsesquioxane (POSS-NH2) and cured with diamino dimethyl sulfone (DSS) as a curing agent at 100 C for 25 minutes and poured into a mold (Zhang et al., 2007). Another interesting example of the synthesis of a thermoset composite is when Gd2O3 nanoparticles were mixed with DGEBA epoxy resin via mechanical mixing at 100 C for 1.5 h to disperse these nanoparticles homogeneously. This solution was cured with polyoxyalkylene amine as a curing agent at 120 C (Ma et al., 2011). It has been seen from the literature that nanoparticles also play the important role of a curing agent or hardener, for example, when DGEBA was mixed with methyl isobutyl ketone containing 30 wt.% silica nanoparticles (MIBK-ST), then MIBK was removed under reduced pressure and a viscous solution was obtained. This viscous solution was heated at 170 C to produce a thermal curing reaction. In this curing process, silica nanoparticles played the role of a curing

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agent (Liu and Li, 2005). A composite thermoset polymer was also prepared with SiC nanoparticles with EPOBOND epoxy resin through mechanical stirring and curing with polyamino as an amine hardener in a ratio of 50:25 (Nassar and Nassar, 2013). Rubber microparticles and silica nanoparticles were also used in the synthesis of thermoset polymers and fiber composites. In this method, the epoxy resin DGEBA was blended with silica nanoparticles containing 40 wt.% and carboxyl-terminated butadiene acrylonitrile (CTBN) rubber. This mixture was cured with the methylhexahydro phthalic acid anhydride as a curing agent to obtained the thermoset composite (Hsieh et al., 2010). The synthesis of a threephase polymer
ceramic
metal composite was also marked in polymer science, in which the polymer used was epoxy resin, the metal was silver, and the ceramic was Ca[(Li1/3Nb2/3)0.8Ti0.2]O32δ (CLNT) (George and Sebastian, 2009). For the synthesis of the epoxy-CLNT-Ag composite, the fine powder of sintered CLNT and Ag were mixed with uncured epoxy resin and hardener. This mixture was mechanically mixed for 30 minutes to uniformly disperse the ceramic powder and Ag in this matrix. This solution was poured in to a cylindrical mold and cured at 70 C for 2 hours. A thermoset polymer embedded fumed silica was prepared by adding fumed silica into a monomer of bisphenol E cyanate ester (BECy) through mechanical stirring to obtain the suspension solution. This solution was mixed with a catalyst at 2000 rpm for 2 minutes and poured into a silicon rubber mold followed by curing at 60 C for 1 hour (Goertzen and Kessler, 2008). Another thermoset nanocomposite of silica nanoparticles and epoxy thermoset polymer has been seen in the literature, which was prepared from the mixing of silica particles with DGEBA through mechanical stirring at 26,000 rpm. After some time, polyamine was added as a hardener and this solution was cured at 70 C for 2 hours then postcured at 120 C for 2 hours (Bondioli et al., 2005). A novolac-type phenolic/SiO2 hybrid organic
inorganic nanocomposite was also synthesized. For this, two types of materials were used and these materials were prepared by mixing two solutions. These solutions consisted of phenolic resin/THF and tetraethoxysilane/H2O/THF/HCL. Both these solutions were mixed and cured with hexamethylene tetraamine as the curing agent. The final solution was poured into aluminum dishes and aged at room temperature for 2
3 days (Chiang et al., 2003).

4.2.3 PROPERTIES OF THERMOSET POLYMER COMPOSITES When nanoscale sized nanofillers were mixed into thermosetting resins, it affected many properties of those polymers. For instance, nanoclay improved the thermal and mechanical properties, CNTs and graphene oxide (GO) improved the mechanical properties, while silver nanoparticles improved the antibacterial and conductivity properties of thermosetting polymers. Some of the properties of thermoset polymer composites are discussed here. After the loading of nanofillers into a polymer matrix, they affect the mechanical and thermal properties of those polymers. Some nanofillers decrease the

4.2 Thermoset Metal
Polymer Composites

mechanical property, while some increase the mechanical properties. For application in biomedical engineering, in this section, the effect of fillers on the mechanical properties of composites is discussed here:

4.2.3.1 Tensile strength Researchers have reported that when fillers, such as Cu, Al, and Zn were added to polyester resin, a sharp decrease in tensile strength was observed after the incorporation of 10% filler into the polyester composite. In addition, in a Cu containing composite the tensile strength was higher as compared to Al and Zn containing composites, which shows that every filler has different effects. It was reported that, the hardness of a composite increased with increasing filler content up until 20% filler loading and then it decreased with further loading. This may contribute to poor surface contact between the filler and polymer matrix (Mansour et al., 2007). Considering SiC nanoparticles, when the nanoparticles were mixed with epoxy resin at 10
20 wt.% variation, the tensile strength was decreased and further increased when the weight percentage reinforced (Nassar and Nassar, 2013). It has been seen that, tensile strength was enhanced after the incorporation of CuO nanoparticles in a vinyl
ester nanocomposite (Guo et al., 2007). Research shows that the incorporation of TiO2 nanoparticles improves the tensile strength of polymers, for example, SiC and TiO2 were used to prepare a thermoset nanocomposite and when the SiC was blended with the epoxy resin, the prepared nanocomposite showed a significantly lower impact strength as compared to the neat matrix, but after the incorporation of 7.5 vol.% TiO2 nanoparticles into the epoxy resin, the impact strength was remarkably improved, while when a 17.5 vol.% of TiO2 was mixed with epoxy resin, the material showed increases in impact strength (Wetzel et al., 2001).

4.2.3.2 Fracture surface Fillers show great impact on the fracture properties of thermosets, for example, when nanoparticles of SiC and TiO2 were added into an epoxy resin, the SiC containing epoxy composite showed the fracture surface to be much rougher than that of the epoxy/TiO2 nanocomposite. After some time, the particle agglomerates can clearly be obtained within the fracture surface (Wetzel et al., 2001). Another study was done with TiO2/polyester resin. With the incorporation of TiO2 nanoparticles into the polyester resin with a loading of 1, 2, and 3 vol.%, increases in toughness of 57%, 42%, and 41% respectively were observed as compared to virgin polyester. The result obtained for 1 vol.% of TiO2 nanoparticles was the highest value compared to that of neat polyester (Evora and Shukla, 2003).

4.2.3.3 Stress
strain behavior The influence of particulate fillers on the stress
strain behavior of polymers is well known, at least for fillers in the micrometer size range and larger. On the one hand, rigid microfillers commonly increase stiffness, but on the other hand, they may have a detrimental effect on the stress
strain behavior leading to

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breakages. The flexural strength of microparticle-filled composites may also be reduced with rising filler content, especially in cases where the load transfer between the matrix and the particles is insufficient and the interface is weak (Fridrich et al., 2005). For example, the loading of flax fibers in thermoset polyester resin modified the stress
strain properties of that polymer composite. When these fibers were loaded into the polymer matrix, it was reported that the tensile stress at break and the strain at maximum stress were increased from 31 to 304 MPa and from 0.68% to 1.73% respectively compared to neat polymer, but when the filler was replaced with E-glass fibers then the tensile stress at break increased to 695 MPa and the strain at maximum stress to 2.37 % (Hughes et al., 2007).

4.2.3.4 Dynamic mechanical properties Dynamic mechanical tests, over a wide temperature range are highly sensitive to the physical and chemical structure of polymers and composites. They allow for the study of the viscoelastic properties against the temperature of thermosetting polymers. It was observed from various researches that the interaction of some nanoparticles can increase these properties, while the interaction of other nanoparticles can decrease these properties; for example, ZnO nanoparticles loaded with epoxy resin showed a slightly improved storage modulus, while nanoparticles of barium titanate (BT) loaded with epoxy exhibited decreases in storage modulus (Medina et al., 2016; Li and Zheng, 2016).

4.2.3.5 Wear performance In general, the friction and wear properties depend on the whole tribiological system rather than a single material property. After the incorporation of fillers, composites exhibit improved wear rates. For example, TiO2 and SiC were both added into epoxy resin in a uniform distribution of particles, and it was observed that they help to enhance the wear rate (Wetzel et al., 2001). TiO2 possesses good filler characteristics, that is, when TiO2 microparticles are mixed with polyester resin, it improves the sliding wear resistance of the composite (Satapathy et al., 2010).

4.2.4 CHARACTERIZATION OF THERMOSET POLYMER COMPOSITE Many properties of thermoset composites are characterized through different sophisticated analytical techniques, for instance, morphology can be determined through scanning electron microscopy and transmission electron microscopy, while thermal properties or behavior against temperature can be characterized through TGA, different thermal analysis, DSC. Mechanical and viscoelastic properties can be determined through DMTA. The thermoset nanocomposites can be characterized against structural properties by FTIR and NMR, etc. For example, high-resolution transmission electron microscopy analysis confirmed that ZnO nanoparticles synthesized via an arc-discharge method were prismatic and rod

4.2 Thermoset Metal
Polymer Composites

shaped with average particle sizes of 56 nm (Medina et al., 2016). When these nanoparticles were loaded in epoxy resin, it was observed that the Tg of the ZnO/ epoxy composite was slightly increased, which was determined by DSC. The storage modulus and rubber zone of these composites were determined by DMTA, which showed that the ZnO nanoparticles slightly increased these parameters for the epoxy polymer. The interaction between a GO and hyper branched epoxy (HBE) system was determined through IR spectroscopy. FTIR of GO showed that the bands at 1733 cm21 for CQO, 1396 cm21 for O
H bending, 1222 cm21 for C
O
C str., 855 cm21 for C
O
C of the epoxy ring, etc., confirm the presence of functional groups, like epoxy, carbonyl, hydroxyl, etc., in GO. FTIR of pristine HBE showed bands at 3433 cm21 for O
H str., 915 cm21 for C-O str. of the epoxy ring, and 842 cm21 for C
O
C str. of the epoxy ring, but in HBE/GO composites, the shifting of the O-H band to a lower wave number 3419
3418 cm21 confirmed that GO had interacted with HBE via H-bonding (Baruah and Karak, 2016). The thermal stability of GO/HBE composites was determine through TGA, which showed that the incorporation of GO nanoparticles in the HBE matrix improved the thermal stability, while the Tg of the nanocomposites was lower than that of pristine HBE, which was determine through DSC. GO nanoparticles also improved the tensile strength, elongation at break percentage, toughness, and adhesive strength of the GO/HBE composite compared to that of pristine HBE. The incorporation of barium titanate (BT) nanoparticles improved the dielectric constant of epoxy thermosets (Li and Zheng, 2016). BT nanoparticles decreased the Tg and thermal degradation of an epoxy polymer, which was determine by DSC and TGA. DMTA of BT/epoxy nanocomposites showed increased BT nanoparticle loadings in the polymer matrix decreased the storage modulus of the composite compared to that of the neat epoxy polymer. Broadband dielectric spectroscopy showed that the dielectric properties of neat epoxy was 5.24 and 4.09 at frequencies of 103 and 106 Hz, but it increased with the loading of BT nanoparticles into the epoxy. For a 14.1 wt.% loading of nanoparticles, the dielectric constant was increased by up to 14.6 at a frequency of 103 Hz.

4.2.5 APPLICATIONS OF THERMOSET POLYMER COMPOSITES Imbedding fillers into thermoset polymers typically affect some changes to composites, such as low electrical conductivity and low fracture toughness. These composites also exhibit typically poor resistance to lightning strikes and crack growth (Ladani et al., 2015). There are many advantages of thermoset polymer matrix composites. Thermoset polymer composites show better economics properties than thermoplastic composites and these composites have high temperature properties, good wetting, and adhesion to reinforcement. They cannot be reshaped and melted as thermosets are stable against temperature and exhibit their properties even at high temperatures, that is, strength, mechanical properties, wear resistance, etc. Due to these characteristic properties, these polymers and polymer

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composites are used in various applications, such as in biomedical engineering, the electronic industry, automobile engineering, aircraft engineering, scaffold production, and in a variety of electronic devices, etc. Thermoset cynate ester (CE) composites typically exhibit high temperature resistance and these composites are found in the aerospace sector. The main applications of CE composites include redoes for fighter aircrafts, missile nose cones, skin covering phase array radar, as well as in missile fins, nozzle flaps, fairings, cowls, and inlet guide vanes in jet engines, gear cases for helicopters, etc., (Mangalgiri, 2005). Polyester composites containing glass reinforced fillers are used in several areas, including automobile-body panels, seats and panels for transit cars, boat hulls, bathroom shower and bathtub structures, chairs, architectural panels, agricultural seed and fertilizer hoppers, tanks, and housings for a variety of consumer and industrial products. Glass fiber
reinforced epoxy composites are used in filament-wound pipes and tanks, and circuit boards, while carbon fiber (CF) based epoxy composites show light weight, high strength and modulus, and also exhibit excellent fatigue properties. Due to these characteristic properties CF/epoxy composites are used in military aircraft aerospace components, fuselage panels for military aircrafts, cargo doors for space shuttles, and high-priced sports equipment, such as tennis-racquet frames, golf-club shafts, skis, and archery bows.

4.3 APPLICATIONS IN BIOMEDICAL ENGINEERING Materials used in biomedical fields should have certain special properties as well as being tunable to meet the need for selected applications, for example, they should be biocompatible, noncarcinogenic, corrosion-resistant, and should have low toxicity (Teoh et al., 2004). The selection of an appropriate material is dependent on the application of that material, for example, polymers used in scaffolds should be biodegradable so that as cells generate their own extracellular matrices and a patient’s own tissue will completely replace the polymeric material with time, but in the case of polymeric heart valves, the polymeric material should be wear-resistant and nonbiodegradable so it may remain stable and not degrade with time. These materials should not disturb or induce the opposite response from the host. These materials should be sterilizable and should not decompose or emit toxic gases during the sterilization process. These materials should meet with all the functional and mechanical requirements in order to be applied in the biomedical field, some such applications are discussed here:

4.3.1 IN DENTISTRY The development of restorative materials for dental problems for use in dentistry is challenging due to the environment of the mouth. The temperature of the

4.3 Applications in Biomedical Engineering

human mouth varies from 32 C to 37 C, moreover the intake of hot or cold food changes the temperature of mouth from anywhere between 0 C and 70 C. Another important factor that affects the environment of mouth is pH, which ranges from 4 to 8.5 and varies from 2 to 11 with the intake of various foods. The success and failure of dental material is dependent on the “selection of appropriate material for a given application” and the “ability to carry out manipulative procedures to arrive at the optimum properties” of those materials. Dental materials can be broadly categorized into ceramics, polymer composites, and metals, etc. Due to this, many researchers have tried to prepare dental material from thermoset polymers, such as Bis-GMA. A dental material was prepared from Bis-GMA and triethylene glycol dimethacrylate (TEGDMA) with a silanized glass filler (Atai and Watts, 2006). For this, 65 wt.% Bis-GMA and 35 wt.% TEGDMA was used in a matrix with 0.5 wt.% camphorquinone and 0.5 wt.% N,N0 -dimethyl aminoethyl methacryate as a light-curing initiator. Silanized glass in 3.4 μm average particle size was also mixed into this matrix to decrease the shrinkage-strain and maximum shrinkagestrain rate. Polyurethane resins are also used in dental applications, but they show high shrinkage. To reduce the shrinkage of these resins, Atai et al. modified urethane di(meth)acrylate (UDMA) with isophorone diisocynate (IPDI) to prepare a dental material, and it was observed that the shrinkage of the synthesized material was low, roughly near to that of Bis-GMA/TEGDMA resin (Atai et al., 2007). Another researcher modified Bis-GMA/TEGDMA resin with liquid rubber as a filler for dental applications to improve fracture toughness (Mante et al., 2010). Marsich et al. modified Bis-GMA/TEGDMA with silver coatings to improve its antibacterial properties for orthopedic and dental applications (Marsich et al., 2013). The coatings of Ag nanoparticles were done with polysachride-1-deoxylactil-1-yl chitosan. Another thermoset resin of Bis-GMA, methyl methacrylate (MMA), N,N-cynomethyl methylaniline (CEMA), and camphorquinone (CQ) with different weight percentages of silanized zirconia fibers was used to prepare dental material (Wang et al., 2016a,b). Polyhedral oligomeric silsesquioxane (POSS) was also used to reduce the shrinkage of Bis-GMA/TEGDMA resin for dental material. POSS decreased the shrinkage from 3.53% to 2.18% and it also increased the mechanical properties of the Bis-GMA/TEGDMA thermoset resin (Wu et al., 2010).

4.3.2 IN PROSTHETIC HEART VALVES Thermoset polymers and polymer nanocomposites have been used in the preparation of prosthetic heart valves and artificial hearts by various researchers and scientists. Thermoset polyolefins, that is, crosslinked poly(styrene-block-isobutylene-block-styrene), was used for the preparation of a polymeric heart valve (PHV) (Claiborne et al., 2013). The use of crosslinked polyolefins were also reported by Zhou et al. for making a PHV (Claiborne et al., 2012). Another

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polymer composites

thermoset, that is, poly(styrene-cobleck-4-vinylbenzocyclobutene)-polyisobutylenepoly(styrene-coblock-4-vinylbenzocyclobutene) was synthesized in order to make an efficient PHV (Sheriff et al., 2015), and it has been seen that various thermoset polyurethanes are also used for making heart valves (Rogers, 2005; Lambert et al., 2001).

4.3.3 IN BONES Bone fractures and the development of materials that meet the required properties of bone are burning issues in bone science. Researchers and scientists are continuously attempting to develop the ideal prosthetic material for bone repair. They use metals, alloys, ceramics, polymers, and polymer composites for repairing bone. Thermoset polymers and their composites along with various fillers play an important role in bone repair, for example, bone morphogenetic proteins transduced in injectable thermoset hydrogels were used to repair segmental defects in rat femurs (Rutherford et al., 2002). Epoxy nanocomposites were also used in bone applications, that is, multiwalled carbon nanotubes (MWCNTs) were loaded into epoxy-based resin to enhance the strength and modulus for making an efficient material for application in the sockets in transfemoral amputees (Arun and Kanagaraj, 2016). The use of CF/flax/epoxy hybrid thermoset composites was marked for orthopedic long bone fracture plates as an alternate to metal plates (Bagheri et al., 2013). These CF/flax/epoxy plates were closer to human cortical bone compared to clinically used metal plates. A thermoset material, CORTOSS, was used as a cortical bone void filler (Pomrink et al., 2003). It is a glass
ceramic reinforced composite that consists of Bis-GMA, 2,2-bis[4-(2-methacryloxyethoxy)]phenylpropane (Bis-EMA), TEGDMA, and 2,6-di-tert-butyl-p-cresoln (BHT). This material was primarily developed as a cortical bone void filler. E-glass fibers reinforced with a Bis-GMA/TEGDMA thermoset composite was used in the formation of intramedullary rods or intramedullary nails, which are used to treat fractures in the long bones of the body (Moritz et al., 2014).

4.3.4 IN BONE GRAFTING Many thermoplastic and thermoset polymers and their composites have been used in bone regeneration by many researchers. A biodegradable polyurethane acrylate/2-hydroxyethyl methacrylate (HEMA) grafted nanodiamond (ND) composite was synthesized for bone regeneration (Alishiri et al., 2016). It was reported that this composite showed high modulus and strength and also did not cause any negative effect on proliferation. Hybridized carbon nanofibers (CNFs) containing calcium phosphate (CaP) nanoparticles performed an important role in improving the interfacial adhesion of epoxy resin for bone repair (Gao et al., 2016). It was reported that these CNF/CaP nanoparticles also enhanced the flexural properties of epoxy composites and that these materials play an important role in bone repair. Another researcher prepared a bioactive thermoset composite from Bis-GMA reinforced with E-glass fibers to replace metallic implants for bone

4.3 Applications in Biomedical Engineering

(Kulkova et al., 2016). The fatigue resistance and mechanical properties of this composite match the properties of bone. Researchers have tried to synthesis a promising material for bone grafting from natural sources. For example, Natarajan et al. synthesized a biodegradable poly(ester amide) from soybean oil for modulated release and bone regeneration (Natarajan et al., 2016). Biodegradable star-shaped polylactide scaffolds were also synthesized for bone tissue regeneration (Timashev et al., 2016). These scaffolds provide a beneficial microenvironment for osteogenic mesenchymal stem cell differentiation in vitro and support de novo bone formation in vivo.

4.3.5 IN PROSTHETIC SOCKETS Polymeric materials were also used to develop prosthetic sockets. An MWCNT reinforced epoxy composite was developed for application in prosthetic sockets in transfemoral amputees (Arun and Kanagaraj, 2016). This synthesized thermoset material increases the comfort level by decreasing the metabolic cost of the socket in transfemoral amputees. Thermosetting polyester resin was used for lamination in prosthetics to match the skin tone of patients (Saikia et al., 2015). Natural fibers are also used to synthesize thermoset polymer composites for prosthetic sockets. A bioactive banana pseudo stem fiber reinforced epoxy composite was synthesized and used as a material to replace transtibial prosthetic sockets (Odusote et al., 2016). Another biodegradable thermoset composite was marked as a material for prosthetic sockets. Pineapple leaf fibers were loaded in different variations into thermoset polyester and epoxy resin (Odusote and Oyewo, 2016). These composites showed some efficient mechanical properties for the development of prosthetic sockets.

4.3.6 IN MEDICAL DEVICES The use of thermoset polymers and their composites have also been reported in the manufacturing of medical devices. Magnetic nickel zinc ferrite particles were loaded with ester-based thermoset polyurethane (Buckley et al., 2016). CNT fabricated polydimethylsiloxane composites were used in the manufacturing of electrodes for electrocardiography (Liu et al., 2015). Thermoset polyurethanes were also studied for application in medical devices. A polyurethane thermoset, MP5510, was investigated and reported as being particularly well suited for medical applications, especially deployment devices, that is, as stents or clot extractors (Baer et al., 2007). There are many electrical devices used in medical field. These devices are prepared using various metals, polymers, ceramics, etc. Many researches have been performed for making suitable polymer compositions for these devices, for instance, graphene/epoxy composites were synthesized for electrical applications due to their adequate mechanical and electrical properties (Wajid et al., 2013).

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Further Reading

Wu, X., Sun, Y., Xie, W., Liu, Y., Song, X., 2010. Development of novel dental nanocomposites reinforced with polyhedral oligomeric silsesquioxane (POSS). Dent. Mater. 26, 456
462. Xiang, Y., Li, L., Zheng, S., 2016. Photophysical and dielectric properties of nanostructured epoxy thermosets containing poly(N-vinylcarbazole) nanophases. Polymer (Guildf). 98, 344
352. Yadav, S., Gautam, J., 2017. Review on Undoped/doped TiO2 nanomaterials: synthesis, Photocatalytic and antimicrobial activity. J. Chin. Chem. Soc. 64, 103
116. Zhang, K., Ishida, H., 2015. Smart synthesis of high-performance thermosets based on ortho-amide
imide functional benzoxazines. Front. Mater. Available from: https:// doi.org/10.3389/fmats.2015.00005. Zhang, Z., Gu, A., Liang, G., Ren, P., Xie, J., Wang, X., 2007. Thermo-oxygen degradation mechanisms of POSS/epoxy nanocomposites. Polym. Degrad. Stab. 92, 1986
1993.

FURTHER READING Kotsilkova, R., 2007. Thermoset Nanocomposites For Engineering Applications. Smithers Rapra Technology Limited, UK.

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