The significance of both good dispersionand interfacing nanodiamonds in the matrix has been recently acknowledged as acritical factor in improving nanodiamonds –polymer composites and should befurther emphasized. Many novel multifunctional nanodiamonds–polymer compositesare still to be developed. A unique combination of properties has been provideby nanodiamonds should be more broadly explored in composites for electronics,packaging, membranes, biomedical applications, etc10,147,167–169. It can involve for example,tissue engineering scaffolds and biomedical polymer devices incorporating nanodiamondsto recount mechanical strength, bioimaging modality (due to luminescent nanodiamonds),drug delivery modality (by adsorption/desorption or chemical linking/release ofthe drugs to/from nanodiamonds), improve biomineralization, etc.
Finally,synergism between nanodiamonds and other fillers should be explored. There is ahandful of results in this area and it should be studied further. For example,incorporation of nanodiamonds in traditional epoxy–carbon fiber composites may reinforce the matrixbetween the fibers and thus increase themechanical properties and failure tolerance of the composite170. At the same time, the use of nanodiamondsfor this purpose is clearly more advantageous than the use of CNTs or graphenenanoplatelets, because nanodiamonds can be introduced in higher concentrationswithout increasing the viscosity of the resin beyond processability limits, andsmall spherical nanodiamonds particles won’t be filtered by carbon fibers,allowing uniform infiltration and dispersion171. In first attempts, to strengthen thethermoplastic polymers, non-modified as-produced nanodiamonds were simply mixedwith the polymers.
It is shown that even in low concentrations and withoutsurface functionalization, nanodiamonds sometimes can strengthen somethermoplastics. Although this “trial and error” approach aquire in earlystudies is not approved because the chances to attain the advancement are slim,it is still in use due to its simplicity172. For example, improved elasticstrain at uniaxial extension was noticed due to the addition of glass beads in combinationwith only 0.1 wt.% of as-received nanodiamonds to polydimethylsiloxane.
According to the viscosity measurements of the melts, nanodiamonds alter theconformation of polydimethylsiloxane macromolecules, resulting in bettermechanical performance of the composite173 . This is an example ofinterphase-mediated property change. In another study, increased Young’smodulus and glass transition temperature (Tg) were aquired due to the additionof 0.25 wt.
% of as-received ND to polyurethane-2-hydroxyethylmethacrylate (PU-PHEMA). These development, in this case is explained by a reaction betweennanodiamonds – COOH groups and the isocyanate groups formed during thepolymerization of PU-PHEMA, give an example of interface-mediated propertychange. In most situations, nevertheless, refinement of nanodiamonds isrequired to achieve the enhanced performance in composites174.
To increase the affinity of nanodiamonds to polyethylene (a polymer consisting of a –CH2–backbone with no side chains) alkyl groups of inconsistant length were graftedto nanodiamonds. DSC measurements carry out on these polyethylene– nanodiamonds composites indicate the increasedcrystallinity, as well as higher crystallization and melting temperatures ofthe nanocomposites with increasing nanodiamonds content and alkyl chain length. In parallel, 2.5 times higher Young’smodulus and 4.
5 times higher hardness were also evaluate for these compositesby AFM, with a larger increase correlate with longer grafted alkyl chains.These enhancement were elucidate by a significant increase in crystallinity ofthe polymer — an instance of how a proper nanodiamonds surfacefunctionalization can be used to design an interphase175. Electrospun PAN and PA11nanofibers with high contents of nanodiamonds show dramatically modifiedmechanical properties.
In this study, nanodiamonds was purified by airoxidation, followed by HCl treatment to minimize the content of metals and tohydrolyze the anhydrides and lactones formed on nanodiamonds surface duringoxidation in air and in order to maximize the number of carboxylic groups onthe surface, which was confirmed by FTIR134. The electrospun fiber mats were then melted, yielding uniform nanodiamonds–polymer films on the substrate. A 400% increase in Young’s modulus and a 200%increase in hardness were measured by nanoindentation for these films.Importantly, even at nanodiamonds concentrations upto 20 wt.
% the films remainoptically transparent while strongly absorb the UV radiation. This study has signifythat nanodiamonds –polymer, nanofibers with very high ND content (up to 80wt.%) can be formed by electrospinning. At high concentration of nanodiamonds thecomposite fibers act as ceramic fibers, in particular, indicating the brittlefailure typical for ceramic materials, with no necking or crazing, that aretypical for pure PAN or CNT-containing PAN nanofibers176.
A pronounced agglomeration of nanodiamondsobserved at higher concentrations shows that characteristics of these compositescan be even higher when dispersion of nanodiamonds is enhance. These nanodiamonds–polymer films are favourable candidates for optically transparent UV absorbingscratch- and wear resistant coatings andpaints, and indicate that nanodiamonds could be used to squash the photoaging of polymers177. For coatings specifically, astrong adhesion between the substrate and the coating composite film is needed.
Nanodiamonds can improve the adhesion through creating nano-roughness at thecoating-substrate interface, as well as through other mechanisms. A remarkableupgrade adhesion of the Nylon-11 coating to steel due to the addition of 7 wt.%purified nanodiamonds has been reported, further underlined the potential of nanodiamondsin polymer-based coatings and paints178. In various studies nanodiamondshave shown to be non-toxic and biocompatible, which give it an excellentmaterial for biomedical applications.
Most polymer synthesized for suchapplications, in particular biodegradable polymers, suffer from poor mechanicalproperties, sharply limiting their potential for the replacement of metals inapplications such as bone fracture fixation179. Various studies have explore the use ofnanodiamonds to reinforce polyvinyl alcohol (PVA), which is used in soft tissuereplacing, artificial cartilage, skin, pancreas, as well as catheters andhemodialysis membranes, and a number of other biomedical applications. A 40%enhanced tensile modulus and a 70% larger fracture energy were measured due tothe addition of 1 vol.% of nanodiamonds -carrying detonation soot purified bytreatment with o-xylene to remove adsorbed organics180.
The noticed enhancement weredescribed by favorable interactions between oxygen-containinggroups (including COOH, C-O and alike), reveal on the surface of nanodiamonds-containingsoot and OH groups of PVA. It is likely that removal of adsorbed hydrocarbonsand other weakly bonded species from the surface of nanodiamonds containing soot by boiling it in o-xylene accelerateweak interactions (mainly hydrogen bonding) between these groups and thefunctional groups of polymer matrix, which collectively result in a quitestrong nanofiller-matrix interface181. Acid-purified nanodiamonds wasexhibit to improve mechanical properties of another common biodegradablepolymer, poly-(methyl methacrylate) (PMMA) which is used, for example in dentalimplants. The introduction of the acid-purified nanodiamonds in concentrationsas small as 0.1 wt.% resulted in increased strength and fracture toughness ofPMMA. At 0.8 wt.
% nanodiamonds content, Young’s modulus was 80% higher and theTg increased by 20 °C182. Similar to PVA, the developedproperties are described by hydrogen bonding between the polymer carbonylgroups and ND OH groups. TEM micrographs of the ND–PMMA nanocomposites indicatesignificant agglomeration of nanodiamonds, suggesting that further advancementare possible when well dispersed single ND particles become available183. Polycarbonate (PC) and PMMAwere reinforced with as-received and different functionalized nanodiamonds.Hardness and Young’s modulus of these nanocomposites were increased by up to100%, when the appropriate surface functionality was selected. The largestimprovements in mechanical properties were obtained by using amidefunctionalized nanodiamonds for PMMA and amino-functionalizednanodiamonds for PC emphasize the importance of nanodiamonds surface chemistryin designing the interface to different matrices184. It is worth mentioning that inmany cases it is difficult to disentangle the contributions of nanodiamonds surfacechemistry into improved nanodiamonds –matrix interface and better nanodiamondsdispersion in the matrix, which normally interpret into a larger volume of theinterphase. Judiciously chosen surface chemistry is often advantageous forboth, resulting in a stronger interface, while at the same time maximizing theinterphase185.
Poly-Lactic Acid (PLA) is awell-known biodegradable polymer used in tissue engineering. Storage modulus,tensile modulus, and the tensile strength of PLA were enhanced by the additionof 5 wt.% nanodiamonds. The reasons for the improvements might be due to theincreased crystallinity of the matrix (measured by DSC) as well as theattractive molecular interactions between nanodiamonds and PLA molecules assuggested by the increased thermal stability of the composite measured in TGA.An enantiomer of PLA, poly-L-(Lactic-Acid) (PLLA), that are used in producingfixation devices for bone fracture surgery, was reinforced by octadecylamine (ODA)-modified nanodiamonds.Long hydrophobic chains of ODA were transplant to nanodiamonds surface in orderto increase the affinity of nanodiamonds towards the polymer matrix and improvethe dispersion of the nanofiller, while at the same time decreased unpleasant nanoparticle–nanoparticleattractive interactions by exchange nanodiamonds polar functional groups proneto hydrogen bonding and other types of stronger interactions with alkyl chains,which can only interact via weak van der Waals forces186. Successful covalent binding ofODA to nanodiamond was established by detection of newly produced amide bondsin FTIR and NMR and indirectly by AFM. In contrast to pristine ND, the resultingnanodiamond–ODA material is hydrophobic, resulting in good miscibility withhydrophobic polymers and solvents.
TEM images of PLLA– nanodiamond –ODA filmsshow single nanodiamonds – ODA particles and loosely bonded nanodiamonds –ODAagglomerates dispersed in the matrix. Good dispersion translates into highmechanical properties: PLLA–ND–ODA composites carrying upto 10 wt.% ND–ODAshows upto 2 times higher Young’s modulus and up to 8 times higher hardnesswhen compared to neat PLLA, measured using nanoindentation187. As expected, unmodified nanodiamondsreinforced PLLA to a lesser extent. The bulk compression modulus of a PLLA– nanodiamonds–ODA composite consist 10 wt.% nanodiamonds was increased the ODA by 22%, and a316% increase in fracture energy was observed at the same time.
It is theorizethat ND–ODA-induced crazing, as confirmedby light and TEM microscopy, which is responsible for the large increase instrain to failure and fracture energy. It is essential to mention that ND canbehave as multifunctional nanofiller. Beyond mechanical reinforcement, nanodiamonds–ODA have blue fluorescent when illuminated with UV radiation, and assistbiomineralization, giving additional advantages in bone surgery and tissueengineering188. In a similar biodegradablesystem, poly-L-(lactide-co-?-caprolactone) or poly(LLA-co-CL), a copolymer, inwhich ?-caprolactone lower the glass transition temperature and increases theelongation at break of L-lactic acid, the reinforcing effects of three differentNDs involving milled acid-purified oxygen terminated nanodiamonds, nanodiamondswith grafted polylactide (ND– PLA), and benzoquinone functionalized nanodiamonds(ND–BQ) were investigated. As expected acid purified hydrophilicoxygen-terminated nanodiamonds did not diffuse well in hydrophobicpoly(LLA-co-CL) and resulted in degradation of mechanical properties atconcentrations of 5 wt.% or higher. Nanodiamonds –BQ had necessarly noinfluence on Young’s modulus, stress, and ultimate strain186.
In contrast, nanodiamonds –PLAindicate a clear trend towards improved Young’s modulus with increasing amountof nanodiamonds –PLA. At 10 wt.% nanodiamonds –PLA in poly(LLA-co-CL) theYoung’s modulus was ~6 times higher, while keeping improved elasticity at breakprovided by poly(LLA-co-CL) in comparison to PLLA. Nanodiamonds –BQ and nanodiamonds–PLA signify good dispersion in THF and in poly(LLA-co-CL) matrix, which incase of nanodiamonds –PLA transformed into improved mechanical properties asdescribed by favorable interactions and involvement between nanodiamonds insertedPLA chains and the matrix, an effect which is lack in case of nanodiamonds –BQ.The alter in mechanical properties on addition of nanodiamonds –PLA resemblewith the increase of the composite glass transition temperature from 7 to 18 °C, represent the increased crystallinity of the matrix, i.e. an interphaseeffect of nanodiamonds –PLA188,189. Nanodiamonds with covalentlylinked polymer chains (nanodiamonds –polymer brushes) were produced by usingthe atom transfer radical polymerization of poly (iso-butyl methacrylate) at the nanodiamonds surface.
The brusheswas identify by TGA, FTIR, NMR and AFM. Single ND–polymer brushes envision by usingAFM can have diameters upto 300 nm, i.e. ~100 times larger than the averagediameter of single ND particles. Using a similar approach, polyimides was developon nanodiamonds particles190. It was established that the XRDdiffraction peak of polyimide at 4.9° vanish in nanodiamonds composites, givingverification that long-range interactionsbetween polyimide chains are disrupted due to the introduction of nanodiamonds.
The produced nanocomposite had a 25% higher Young’s modulus and a 15% higherhardness at a nanodiamonds content of 5 wt.% . Long chains on nanodiamondssurface anchor the particle in the matrix, increase affinity between thenanoparticle and the matrix and, by mixing with its molecules, effect thestructure of the host near the ND, resulting the changes in the interphase andthe interface191. A common thermosetting polymer isepoxy, broadly used as matrix material for carbon-fibrereinforced composites in aerospace, shipbuilding,and sports industries. Many nanofillers have been analyse to reinforce epoxysystems. On comparing between nanodiamond–epoxy and CNT– epoxy composites at relatednanofiller loadings in the low concentration range exhibit a significantincrease in glass transition temperature, 37 °C and 17 °C for nanodiamonds andcarbon nanotubes, respectively192. The fracture surfaces of bothcomposites indicate a superior resistance to crack the propagation as comparedto neat epoxy.
Tensile characteristics of nanodiamonds – and carbon nanotubes–epoxycomposites indicate the improvement of 6.4% and 2.9%, respectively.
Thenanocomposites also show 41% increase inmicrohardness for nanodiamonds and 12% for carbon nanotubes. The authorsconclude that at identical nanofiller concentrations in the range 0.1–0.5 wt.% nanodiamondsshow superior improvement as compared to the carbon nanotubes in epoxy matrixcomposites. For further enhancement the mechanical properties of the epoxy, nanodiamonds– epoxy composites with nanodiamonds content upto 35 vol.% were produced193.
Hardness and Young’s modulus ofthese composites calculated by nanoindentation which was higher by 300% and700% respectively, reaching modulus values upto 20 GPa and resulting the increasedscratch resistance. The traditional view, where a polymer is considered as thematrix and nanoparticles are considered as the filler, need to be reversed insituations where the nanofiller is present in high concentrations. At such high nanodiamonds loadings asindicated above, the composite material should be rather considered as a nanodiamondsnetwork infiltrated by a polymer acting as a binder. Direct contacts betweenthe nanodiamonds particles in such composites result the modified thermalconductivity194. It is essential to emphasizethat due to their small sizes, shorter interparticle distances and directcontacts between nanodiamonds are attain at lower nanodiamonds contentscompared to larger nanofillers, e.
g., 10 nm diameter silica. Tribologicalstudies indicate that an alumina counter bodycan harm by the nanodiamonds –epoxy agglomerates hold within these composites,suggesting very high hardness of the agglomerates, which can eventually replacemicron-sized diamond particles in drilling and cutting tools. Averagemacroscale friction coefficients of epoxy composites carrying 7.
5 vol.% nanodiamondswere reduced 4 times approaching 0.1.
While high loadings of nanodiamondsresult in remarkably high hardness and Young’s moduli of the epoxy– nanodiamondscomposites, lower concentrations of nanodiamonds can be used to improve themechanical properties177. The bulk Young’s modulusmeasured in tensile tests was 25% higher upon addition of 0.5 wt.% as-received nanodiamonds,which also increased the decomposition temperature. However, due to poor nanodiamondsdispersion, the storage modulus of the epoxy composite was significantlyreduced, emphasizing the importance of a good dispersion to optimize themechanical properties of nanodiamonds –polymer composites.
A study on the modeI and II fracture toughness of nanodiamonds –epoxy composites has shown that,beyond an improved Young’s modulus and hardness, the mode II fracture toughnessof epoxy– nanodiamonds composites with 0.1 wt.% nanodiamonds is increased.
Thisis because nanodiamonds is thought to inhibit shear deformation, improvingfracture toughness195. For covalent binding to epoxy,pointed to form the most strong ND– polymer interface, nanodiamonds terminatedwith reactive amino groups was formed by combining ethylenediamine to nanodiamonds–COOH surface via amide bond (yielding nanodiamonds –CONH(CH2)2NH2, here afternamed nanodiamonds –NH2). The rational behind using amino terminated ND is similar to molecular curing agents, reactionof nanodiamonds –NH2 with epoxy resin is predicted to result in a covalentlybonded network of nanodiamonds and epoxy molecules. However, to get fulladvantage of covalent nanodiamonds –polymer interface, it is unfavourablyessential to have covalent bonds all the way from nanodiamonds surface to themacromolecules of the matrix. Therefore, firstcovalent bonding between the diamine molecules and nanodiamonds particles wasconfirmed by FTIR, TG and DSC.
When nanodiamonds –NH2 reacted with the epoxyresin, a strong covalent nanodiamonds –epoxy interface was formed as proof byDSC, which was used to monitor the reaction. As a result, Young’s modulus of acomposite containing 3.5 vol.% nanodiamonds –NH2 was enhance by 60%196.
Also, it was found that inorder to manufacture ND–NH2–epoxy composites with uniformly dispersed nanodiamondsit is important to keep nanodiamonds –NH2 dispersed in a compatible and inertsolvent without drying. Tetrahydrofuran(THF) was chosen for this purpose as it provides a good dispersing medium for nanodiamonds–NH2, dissolves epoxy resin and, according to a previous report, it does notreact with components of the epoxy system . Moreover, it can be vaporize fromthe system by moderate heating over few days197.
This study also discusses theinterference of amino groups of nanodiamonds –NH2 with the amino groups of thecuring agent (PACM) used for this epoxy system and emphasizes that it isnecessary to adjust the amount of the curing agent to compensate for the aminogroups introduced by nanodiamonds –NH2 in order to maintain the rightstoichiometry in the system and to maximize the Young’s modulus of thecomposite. Elastomers are cross-linked, amorphouspolymers above their Tg, possessing high elastic deformation and resuming theiroriginal shape after deforming force is removed. They are heavily used inmodern life, especially in the automobile industry (tires, braking systems,chassis, interior parts, etc.). In manycases, improvements in mechanical properties of elastomers are needed to extendtheir lifetime and further broaden their applications.
Improved mechanicalproperties, such as cohesive strength, rupture, and wear resistance were describedfor a variety of nanodiamonds filled elastomers involving fluorinatedelastomers and rubbers198. The effect of surfacefunctionalization of nanodiamonds on the mechanical properties of polysiloxanefilms was studied as well, where mechanical properties such as engineeringstress and tensile strength were increased due to the addition of silanized ND.The enhancement in mechanical properties was assigned to a reduction in nanodiamondsagglomerate size during a silylation reaction, which in addition removesadsorbed water from the nanodiamonds surface, rendering the materialhydrophobic166.