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      The significance of both good dispersion
and interfacing nanodiamonds in the matrix has been recently acknowledged as a
critical factor in improving nanodiamonds –polymer composites and should be
further emphasized. Many novel multifunctional nanodiamonds–polymer composites
are still to be developed. A unique combination of properties has been provide
by 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 nanodiamonds
to recount mechanical strength, bioimaging modality (due to luminescent nanodiamonds),
drug delivery modality (by adsorption/desorption or chemical linking/release of
the drugs to/from nanodiamonds), improve biomineralization, etc. Finally,
synergism between nanodiamonds and other fillers should be explored. There is a
handful 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 matrix
between the fibers and thus increase the
mechanical properties and failure tolerance of the composite170. At the same time, the use of nanodiamonds
for this purpose is clearly more advantageous than the use of CNTs or graphene
nanoplatelets, because nanodiamonds can be introduced in higher concentrations
without increasing the viscosity of the resin beyond processability limits, and
small spherical nanodiamonds particles won’t be filtered by carbon fibers,
allowing uniform infiltration and dispersion171.

         In first attempts, to strengthen the
thermoplastic polymers, non-modified as-produced nanodiamonds were simply mixed
with the polymers. It is shown that even in low concentrations and without
surface functionalization, nanodiamonds sometimes can strengthen some
thermoplastics. Although this “trial and error” approach aquire in early
studies is not approved because the chances to attain the advancement are slim,
it is still in use due to its simplicity172. For example, improved elastic
strain at uniaxial extension was noticed due to the addition of glass beads in combination
with only 0.1 wt.% of as-received nanodiamonds to polydimethylsiloxane.
According to the viscosity measurements of the melts, nanodiamonds alter the
conformation of polydimethylsiloxane macromolecules, resulting in better
mechanical performance of the composite173 . This is an example of
interphase-mediated property change. In another study, increased Young’s
modulus and glass transition temperature (Tg) were aquired due to the addition
of 0.25 wt.% of as-received ND to polyurethane-2-hydroxyethylmethacrylate (PU-PHEMA). These development, in this case is explained by a reaction between
nanodiamonds – COOH groups and the isocyanate groups formed during the
polymerization of PU-PHEMA, give an example of interface-mediated property
change. In most situations, nevertheless, refinement of nanodiamonds is
required 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 grafted
to nanodiamonds. DSC measurements carry out on these polyethylene– nanodiamonds composites indicate the increased
crystallinity, as well as higher crystallization and melting temperatures of
the nanocomposites with increasing nanodiamonds content and alkyl chain length.
 In parallel, 2.5 times higher Young’s
modulus and 4.5 times higher hardness were also evaluate for these composites
by AFM, with a larger increase correlate with longer grafted alkyl chains.
These enhancement were elucidate by a significant increase in crystallinity of
the polymer — an instance of how a proper nanodiamonds surface
functionalization can be used to design an interphase175. Electrospun PAN and PA11
nanofibers with high contents of nanodiamonds show dramatically modified
mechanical properties. In this study, nanodiamonds was purified by air
oxidation, followed by HCl treatment to minimize the content of metals and to
hydrolyze the anhydrides and lactones formed on nanodiamonds surface during
oxidation in air and in order to maximize the number of carboxylic groups on
the surface, which was confirmed by FTIR134.

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         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 remain
optically transparent while strongly absorb the UV radiation. This study has signify
that nanodiamonds –polymer, nanofibers with very high ND content (up to 80
wt.%) can be formed by electrospinning. At high concentration of nanodiamonds the
composite fibers act as ceramic fibers, in particular, indicating the brittle
failure typical for ceramic materials, with no necking or crazing, that are
typical for pure PAN or CNT-containing PAN nanofibers176. A pronounced agglomeration of nanodiamonds
observed at higher concentrations shows that characteristics of these composites
can be even higher when dispersion of nanodiamonds is enhance. These nanodiamonds
–polymer films are favourable candidates for optically transparent UV absorbing
scratch- and wear resistant coatings and
paints, and indicate that nanodiamonds could be used to squash the photoaging of polymers177. For coatings specifically, a
strong adhesion between the substrate and the coating composite film is needed.
Nanodiamonds can improve the adhesion through creating nano-roughness at the
coating-substrate interface, as well as through other mechanisms. A remarkable
upgrade 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 nanodiamonds
in polymer-based coatings and paints178. In various studies nanodiamonds
have shown to be non-toxic and biocompatible, which give it an excellent
material for biomedical applications. Most polymer synthesized for such
applications, in particular biodegradable polymers, suffer from poor mechanical
properties, sharply limiting their potential for the replacement of metals in
applications such as bone fracture fixation179.

        Various studies have explore the use of
nanodiamonds to reinforce polyvinyl alcohol (PVA), which is used in soft tissue
replacing, artificial cartilage, skin, pancreas, as well as catheters and
hemodialysis membranes, and a number of other biomedical applications. A 40%
enhanced tensile modulus and a 70% larger fracture energy were measured due to
the addition of 1 vol.% of nanodiamonds -carrying detonation soot purified by
treatment with o-xylene to remove adsorbed organics180. The noticed enhancement were
described by favorable interactions between oxygen-containing
groups (including COOH, C-O and alike), reveal on the surface of nanodiamonds-containing
soot and OH groups of PVA. It is likely that removal of adsorbed hydrocarbons
and other weakly bonded species from the surface of nanodiamonds containing soot by boiling it in o-xylene accelerate
weak interactions (mainly hydrogen bonding) between these groups and the
functional groups of polymer matrix, which collectively result in a quite
strong nanofiller-matrix interface181. Acid-purified nanodiamonds was
exhibit to improve mechanical properties of another common biodegradable
polymer, poly-(methyl methacrylate) (PMMA) which is used, for example in dental
implants. The introduction of the acid-purified nanodiamonds in concentrations
as small as 0.1 wt.% resulted in increased strength and fracture toughness of
PMMA. At 0.8 wt.% nanodiamonds content, Young’s modulus was 80% higher and the
Tg increased by 20 °C182. Similar to PVA, the developed
properties are described by hydrogen bonding between the polymer carbonyl
groups and ND OH groups. TEM micrographs of the ND–PMMA nanocomposites indicate
significant agglomeration of nanodiamonds, suggesting that further advancement
are possible when well dispersed single ND particles become available183. Polycarbonate (PC) and PMMA
were reinforced with as-received and different functionalized nanodiamonds.
Hardness and Young’s modulus of these nanocomposites were increased by up to
100%, when the appropriate surface functionality was selected. The largest
improvements in mechanical properties were obtained by using amide
functionalized nanodiamonds for PMMA and amino-functionalized
nanodiamonds for PC emphasize the importance of nanodiamonds surface chemistry
in designing the interface to different matrices184. It is worth mentioning that in
many cases it is difficult to disentangle the contributions of nanodiamonds surface
chemistry into improved nanodiamonds –matrix interface and better nanodiamonds
dispersion in the matrix, which normally interpret into a larger volume of the
interphase. Judiciously chosen surface chemistry is often advantageous for
both, resulting in a stronger interface, while at the same time maximizing the
interphase185. Poly-Lactic Acid (PLA) is a
well-known biodegradable polymer used in tissue engineering. Storage modulus,
tensile modulus, and the tensile strength of PLA were enhanced by the addition
of 5 wt.% nanodiamonds. The reasons for the improvements might be due to the
increased crystallinity of the matrix (measured by DSC) as well as the
attractive molecular interactions between nanodiamonds and PLA molecules as
suggested by the increased thermal stability of the composite measured in TGA.
An enantiomer of PLA, poly-L-(Lactic-Acid) (PLLA), that are used in producing
fixation devices for bone fracture surgery, was reinforced by octadecylamine (ODA)-modified nanodiamonds.
Long hydrophobic chains of ODA were transplant to nanodiamonds surface in order
to increase the affinity of nanodiamonds towards the polymer matrix and improve
the dispersion of the nanofiller, while at the same time decreased unpleasant nanoparticle–nanoparticle
attractive interactions by exchange nanodiamonds polar functional groups prone
to hydrogen bonding and other types of stronger interactions with alkyl chains,
which can only interact via weak van der Waals forces186. Successful covalent binding of
ODA to nanodiamond was established by detection of newly produced amide bonds
in FTIR and NMR and indirectly by AFM.       In contrast to pristine ND, the resulting
nanodiamond–ODA material is hydrophobic, resulting in good miscibility with
hydrophobic polymers and solvents. TEM images of PLLA– nanodiamond –ODA films
show single nanodiamonds – ODA particles and loosely bonded nanodiamonds –ODA
agglomerates dispersed in the matrix. Good dispersion translates into high
mechanical properties: PLLA–ND–ODA composites carrying upto 10 wt.% ND–ODA
shows upto 2 times higher Young’s modulus and up to 8 times higher hardness
when compared to neat PLLA, measured using nanoindentation187. As expected, unmodified nanodiamonds
reinforced 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 a
316% increase in fracture energy was observed at the same time. It is theorize
that ND–ODA-induced crazing, as confirmed
by light and TEM microscopy, which is responsible for the large increase in
strain to failure and fracture energy. It is essential to mention that ND can
behave as multifunctional nanofiller.

          Beyond mechanical reinforcement, nanodiamonds
–ODA have blue fluorescent when illuminated with UV radiation, and assist
biomineralization, giving additional advantages in bone surgery and tissue
engineering188. In a similar biodegradable
system, poly-L-(lactide-co-?-caprolactone) or poly(LLA-co-CL), a copolymer, in
which ?-caprolactone lower the glass transition temperature and increases the
elongation at break of L-lactic acid, the reinforcing effects of three different
NDs involving milled acid-purified oxygen terminated nanodiamonds, nanodiamonds
with grafted polylactide (ND– PLA), and benzoquinone functionalized nanodiamonds
(ND–BQ) were investigated. As expected acid purified hydrophilic
oxygen-terminated nanodiamonds did not diffuse well in hydrophobic
poly(LLA-co-CL) and resulted in degradation of mechanical properties at
concentrations of 5 wt.% or higher. Nanodiamonds –BQ had necessarly no
influence on Young’s modulus, stress, and ultimate strain186. In contrast, nanodiamonds –PLA
indicate a clear trend towards improved Young’s modulus with increasing amount
of nanodiamonds –PLA. At 10 wt.% nanodiamonds –PLA in poly(LLA-co-CL) the
Young’s modulus was ~6 times higher, while keeping improved elasticity at break
provided 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 in
case of nanodiamonds –PLA transformed into improved mechanical properties as
described by favorable interactions and involvement between nanodiamonds inserted
PLA chains and the matrix, an effect which is lack in case of nanodiamonds –BQ.
The alter in mechanical properties on addition of nanodiamonds –PLA resemble
with the increase of the composite glass transition temperature from 7 to 18 °C
, represent the increased crystallinity of the matrix, i.e. an interphase
effect of nanodiamonds –PLA188,189. Nanodiamonds with covalently
linked polymer chains (nanodiamonds –polymer brushes) were produced by using
the atom transfer radical polymerization of poly (iso-butyl methacrylate) at the nanodiamonds surface. The brushes
was identify by TGA, FTIR, NMR and AFM. Single ND–polymer brushes envision by using
AFM can have diameters upto 300 nm, i.e. ~100 times larger than the average
diameter of single ND particles. Using a similar approach, polyimides was develop
on nanodiamonds particles190. It was established that the XRD
diffraction peak of polyimide at 4.9° vanish in nanodiamonds composites, giving
verification that long-range interactions
between polyimide chains are disrupted due to the introduction of nanodiamonds.
The produced nanocomposite had a 25% higher Young’s modulus and a 15% higher
hardness at a nanodiamonds content of 5 wt.% . Long chains on nanodiamonds
surface anchor the particle in the matrix, increase affinity between the
nanoparticle and the matrix and, by mixing with its molecules, effect the
structure of the host near the ND, resulting the changes in the interphase and
the interface191.

        A common thermosetting polymer is
epoxy, broadly used as matrix material for carbon-fibre
reinforced composites in aerospace, shipbuilding,
and sports industries. Many nanofillers have been analyse to reinforce epoxy
systems. On comparing between nanodiamond–epoxy and CNT– epoxy composites at related
nanofiller loadings in the low concentration range exhibit a significant
increase in glass transition temperature, 37 °C and 17 °C for nanodiamonds and
carbon nanotubes, respectively192. The fracture surfaces of both
composites indicate a superior resistance to crack the propagation as compared
to neat epoxy. Tensile characteristics of nanodiamonds – and carbon nanotubes–epoxy
composites indicate the improvement of 6.4% and 2.9%, respectively. The
nanocomposites also show 41%  increase in
microhardness for nanodiamonds and 12% for carbon nanotubes. The authors
conclude that at identical nanofiller concentrations in the range 0.1–0.5 wt.% nanodiamonds
show superior improvement as compared to the carbon nanotubes in epoxy matrix
composites. 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 of
these composites calculated by nanoindentation which was higher by 300% and
700% respectively, reaching modulus values upto 20 GPa and resulting the increased
scratch resistance. The traditional view, where a polymer is considered as the
matrix and nanoparticles are considered as the filler, need to be reversed in
situations where the nanofiller is present in high concentrations.

       At such high nanodiamonds loadings as
indicated above, the composite material should be rather considered as a nanodiamonds
network infiltrated by a polymer acting as a binder. Direct contacts between
the nanodiamonds particles in such composites result the modified thermal
conductivity194. It is essential to emphasize
that due to their small sizes, shorter interparticle distances and direct
contacts between nanodiamonds are attain at lower nanodiamonds contents
compared to larger nanofillers, e.g., 10 nm diameter silica. Tribological
studies indicate that an alumina counter body
can harm by the nanodiamonds –epoxy agglomerates hold within these composites,
suggesting very high hardness of the agglomerates, which can eventually replace
micron-sized diamond particles in drilling and cutting tools. Average
macroscale friction coefficients of epoxy composites carrying 7.5 vol.% nanodiamonds
were reduced 4 times approaching 0.1. While high loadings of nanodiamonds
result in remarkably high hardness and Young’s moduli of the epoxy– nanodiamonds
composites, lower concentrations of nanodiamonds can be used to improve the
mechanical properties177. The bulk Young’s modulus
measured 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 nanodiamonds
dispersion, the storage modulus of the epoxy composite was significantly
reduced, emphasizing the importance of a good dispersion to optimize the
mechanical properties of nanodiamonds –polymer composites. A study on the mode
I and II fracture toughness of nanodiamonds –epoxy composites has shown that,
beyond an improved Young’s modulus and hardness, the mode II fracture toughness
of epoxy– nanodiamonds composites with 0.1 wt.% nanodiamonds is increased. This
is because nanodiamonds is thought to inhibit shear deformation, improving
fracture toughness195. For covalent binding to epoxy,
pointed to form the most strong ND– polymer interface, nanodiamonds terminated
with reactive amino groups was formed by combining ethylenediamine to nanodiamonds
–COOH surface via amide bond (yielding nanodiamonds –CONH(CH2)2NH2, here after
named nanodiamonds –NH2). The rational behind using amino terminated ND is  similar to molecular curing agents, reaction
of nanodiamonds –NH2 with epoxy resin is predicted to result in a covalently
bonded network of nanodiamonds and epoxy molecules. However, to get full
advantage of covalent nanodiamonds –polymer interface, it is unfavourably
essential to have covalent bonds all the way from nanodiamonds surface to the
macromolecules of the matrix. Therefore, first
covalent bonding between the diamine molecules and nanodiamonds particles was
confirmed by FTIR, TG and DSC. When nanodiamonds –NH2 reacted with the epoxy
resin, a strong covalent nanodiamonds –epoxy interface was formed as proof by
DSC, which was used to monitor the reaction. As a result, Young’s modulus of a
composite containing 3.5 vol.% nanodiamonds –NH2 was enhance by 60%196. Also, it was found that in
order to manufacture ND–NH2–epoxy composites with uniformly dispersed nanodiamonds
it is important to keep nanodiamonds –NH2 dispersed in a compatible and inert
solvent 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 not
react with components of the epoxy system . Moreover, it can be vaporize from
the system by moderate heating over few days197. This study also discusses the
interference of amino groups of nanodiamonds –NH2 with the amino groups of the
curing agent (PACM) used for this epoxy system and emphasizes that it is
necessary to adjust the amount of the curing agent to compensate for the amino
groups introduced by nanodiamonds –NH2 in order to maintain the right
stoichiometry in the system and to maximize the Young’s modulus of the
composite.

        Elastomers are cross-linked, amorphous
polymers above their Tg, possessing high elastic deformation and resuming their
original shape after deforming force is removed. They are heavily used in
modern life, especially in the automobile industry (tires, braking systems,
chassis, interior parts, etc.).  In many
cases, improvements in mechanical properties of elastomers are needed to extend
their lifetime and further broaden their applications. Improved mechanical
properties, such as cohesive strength, rupture, and wear resistance were described
for a variety of nanodiamonds filled elastomers involving fluorinated
elastomers and rubbers198. The effect of surface
functionalization of nanodiamonds on the mechanical properties of polysiloxane
films was studied as well, where mechanical properties such as engineering
stress and tensile strength were increased due to the addition of silanized ND.
The enhancement in mechanical properties was assigned to a reduction in nanodiamonds
agglomerate size during a silylation reaction, which in addition removes
adsorbed water from the nanodiamonds surface, rendering the material
hydrophobic166. 

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