# Effect of Carbon Nanofiber-Matrix Adhesion on Polymeric Nanocomposite Properties—Part II

Authors: Khalid Lafdi; William Fox; Matthew Matzek; Emel Yildiz
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

A successful integration of two independent phases with good adhesion is imperative for
effective translation of superior carbon nanofiber filler properties into a physically superior
carbon nanocomposite. Carbon nanofibers were subjected to electrochemical oxidation in 0.1 M
nitric acid for varying times. The strength of adhesion between the nanofiber and an epoxy matrix
was characterized by flexural strength and modulus. The surface functional groups
formed and their concentration of nanofibers showed a dependence on the degree of oxidation.
The addition of chemical functional groups on the nanofiber surface allows them to physically
and chemically adhere to the continuous resin matrix. The chemical interaction with the
continuous epoxy matrix results in the creation of an interphase region. The ability to chemically
and physically interact with the epoxy region is beneficial to the mechanical properties of a
carbon nanocomposite. A tailored degree of surface functionalization was found to increase
adhesion to the matrix and increase flexural modulus.


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## Body

## 1. Introduction

Research on vapor-grown carbon nanofibers (VGCFs) has
been heightened in recent years by the discovery of carbon nanotubes. Such
fibers are characterized by an extraordinarily high tensile modulus, tensile
strength, and high electrical and thermal conductivity. A decrease in the
diameter of a vapor-grown carbon nanofiber causes gradual improvement in
mechanical properties. As the fiber diameter reaches the threshold value of 1μm, distinguishing the transition from a fiber
to a nanofiber, the improvement in mechanical properties becomes more
significant [1].
Vapor-grown carbon nanofibers can be prepared with diameters ranging from 15 nm
to 100 nm. These fibers are not continuous and have hollow cores. Their
morphology resembles that of carbon nanotubes. Nanofibers can have a number of
different internal structures, wherein graphene layers are arranged as
concentric cylinders, nested truncated cones, segmented structures, or stacked
coins [2]. External
morphologies include kinked and branched structures and diameter variation. The
percolation threshold for carbon nanofibers is low indicating that only a small
amount of nanofiber additive is required for conducting filler applications.
The intrinsic stiffness and strength of carbon nanofibers, combined with these
superior transport properties, present the opportunity to develop
multifunctional nanofiber composites with tailored physical and mechanical
properties.Experimental results from previously published
research efforts indicate that VGCFs are suitable reinforcing agents for
polymers. Lozano and Barrera demonstrated a 100% increase in the dynamic
mechanical properties with 2 wt% VGCF in a polypropylene matrix [3]. Other work by Kuriger  with a higher loading of VGCFs in
polypropylene produced over a 50% increase in tensile strength [4]. Patton  combined VGCFs with an epoxy and poly
(phenylene sulfide) to improve the flexural properties of the matrix materials.
They obtained 68 and 91% increases in the flexural strength in epoxy and poly
(phenylene sulfide), respectively, with a nominal fiber loading of 20% by
volume [5].Fiber functionalization is considered necessary to
improve mechanical properties in nanofiber-reinforced composites by increasing
the stress transfer between the nanofiber and the matrix of a nanocomposite
structure. Fiber-matrix adhesion is governed by the chemical and physical
interactions at the interface. Extensive literature exists on surface treatment
of conventional carbon fibers by methods such as oxidation in gas and liquid
phases and anodic etching. Poor fiber-matrix adhesion may result in composite
failure at the interface, resulting in decreased longitudinal and transverse
mechanical properties of the composite.Surface modification of carbon nanofibers changes the
graphitization extent of the fiber and increases its surface area of the fiber.
Lim  studied “stacked coin”-type
nanofibers and the impact of mechanical and chemical treatments on the
morphology of the fibers [6]. Heat treatment at 2800°C induced closed loop ends on the surface of the
nanofibers formed by folding of some planar hexagons at their edges. The heat
treatment removed C–H bonds and densely stacked hexagonal layers of graphene,
forming chemically active sites on the edges. The edges were stabilized by
bonding to each other, even though the bonding caused high tension through the
formation of a sharp curvature. Acidic oxidation of the nanofibers cuts off the
closed looped ends, resulting in improved overall alignment of graphene layers.
Both treatments generated many free edges and a high graphitization extent,
indicating the possibility of improved interfacial bonding with a polymer
matrix.Toebes et al. [7] examined
the effect of liquid phase oxidation of carbon nanofibers in nitric acid and
mixtures of nitric and sulfuric acid for times up to two hours. The graphitic
structure of the nanofibers was not altered by the treatments, but the texture
of the fibers were significantly changed through increase in the specific
surface area and pore volume due to the opening of the fiber inner tubes. The
total oxygen content and surface oxygen functional groups were affected by the
treatment time and acid type. Oxygen groups were also formed in the first 2-3  nm
of the subsurface of the fibers.Bubert et al.
[8]
investigated the influence of plasma treatment on the surface properties of
carbon nanofibers by X-ray photoelectron spectroscopy (XPS) in combination with
ion sputtering, acid-base titration, derivatization of carbonyl groups, pyrolysis,
and CH analysis. The results indicated that the fiber surface is covered by a
monomolecular oxygen-containing layer and that plasma treatment allows complete
oxygen functionalization of the uppermost surface layer. XPS provides an
average value for the content of functional groups of the first ten to fifteen
molecular layers.A number of results have been reported on the effect
of chemical modification of carbon nanotubes on nanocomposite properties.
Xu  included a nitric acid-oxidized
nanofiber/vinyl ester composite in an examination of electrical properties of
nanocomposites [9].
The resistivity of the oxidized fiber nanocomposites were much higher than that
of those produced with untreated fibers. The oxidation was reported to have
increased oxygen percentage by approximately 20% with the addition of
anhydride, quinine, ether, and ester functional groups. The oxidized layer
could reduce conductivity through percolation pathways. The functional groups
should also improve wetting by the polymer matrix. A strong bond between matrix
and fiber could encase the fiber and serve as an insulating covering.Finegan et al.
[10] examined
the mechanical properties of carbon nanofiber/polypropylene composites in an
attempt to optimize carbon nanofiber surface treatment. The fiber-matrix
adhesion was qualitatively studied by scanning electron microscopy (SEM), and
the strength and stiffness of the composites were evaluated from tensile tests.
One sample of nanofibers was oxidized in air at 450°C and a second sample was oxidized with carbon
dioxide in a tube furnace. Fiber matrix adhesion was improved by moderately
oxidizing the fibers in either air or carbon dioxide. The carbon dioxide oxidation
was more effective as it increased the external surface area and the surface
energy of the fibers. However in the preparation of the nanocomposites, the
fibers were either ball milled or force sieved, thereby possibly altering the
fiber aspect ratios resulting in decreased mechanical testing results.Cortes et al.
[11] exposed
carbon nanofibers to a series of chemical treatments in nitric acid prior to
mixing with polypropylene. The oxidized fiber nanocomposites did not improve
electrical properties of the polymer, did not produce significant changes in
the mechanical properties of the composites, and showed a decrease in tensile
strength. The nanocomposites had only produced 5 wt% VGCF compositions. Higher
fiber content may have led to increases in the mechanical properties of the
composites.In conventional carbon fiber-reinforced composites,
there have been a number of studies completed to generate strong adhesion
between the fiber surface and matrix to improve stress transfer from the matrix
to the reinforcing fibers. Continuous surface electrochemical oxidation has
been the preferred method of fiber surface treatment to enhance interfacial
bonding. Electrochemical treatments have been carried out in acid and alkaline
aqueous solutions of ammonium sulfate, ammonium bicarbonate, sodium hydroxide,
diammonium hydrogen phosphate, and nitric acid.Anodic oxidation of fibers in electrolytes can produce
a variety of chemical and physical changes in the fiber surface. Harvey et al.  [12] examined surfaces of conventional carbon fibers by
XPS after electrochemical treatment by galvanostatic and potentiostatic cell
control under varying potential, reaction time, and electrolytes. They noted
that the rise in interlaminar shear strength (ILSS) with surface treatment is
not dependent on O-1s:C-1s ratios or the amount of carboxyl functionality on
the surface, thereby supporting the view that mechanical keying of the resin to
the fiber surface plays an important role in forming the resin-fiber bond.Gulyas et al.
[13] subjected
PAN-based carbon fibers to electrochemical oxidation under a wide variety of
conditions—varying electrolyte, electrolyte concentration, and applied
voltage. The functional groups formed on the surface of the fibers were
dependent on the type of electrolyte used, and the number of functional groups
found on the fiber surface was dependent on electrolyte concentration and
voltage. A close correlation was found between surface chemistry and
fiber/matrix adhesion. The concentration of certain functional groups could be
quantitatively related to ILSS.Yue et al. [14] applied continuous
electrochemical oxidation to high-strength PAN-based carbon fibers in 1% by
weight potassium nitrate. Fiber weight loss increased with electrochemical
oxidation. A large internal micro porous surface area was generated due to the
formation of acidic functions. XPS indicated that the concentration of oxygen
within the outer 50 Å of the fibers increased on oxidation. XPS C-1s and O-1s
spectra showed an increase in primarily carboxyl or lactone groups. The
oxygen-rich surfaces in the micro porous regions chemisorbed oxygen and water.
The O-2s—C-2s peak separation increased in the valence band spectra as the
extent of oxidation increased due to carbonyl group contribution.In this study, vapor-grown carbon nanofibers were
subjected to electrochemical oxidation in 0.1 M nitric acid for varying times to
modify the interface between the nanofibers and epoxide molecules in
epoxide/nanofiber composites. X-ray photoelectron spectroscopy was employed to
characterize surfaces with regard to the content of carbon, oxygen, and
nitrogen. The strength of adhesion between the fiber and an epoxy (thermoset)
matrix was characterized by the tensile strength and modulus, and the
electrical and thermal properties of the composites were investigated from the
viewpoint of surface treatment of carbon nanofibers.


## 2. Experimental Setup and Property Measurement

The carbon nanofibers used in this study were produced
at Applied Sciences, Inc. (Detriot, MI, USA) and were from the PyrografIIITM
family of fibers. This group of nanofibers (labeled PR-24) has diameters
between 60 and 100 un and lengths ranging from 30 to 100 μm. The nanofibers were electrochemically
surface treated using nitric acid as an electrolyte in a concentration of 1 wt%.
Approximately 15 g of nanofibers for each trial were packed into covered, porous
plastic beakers and submerged in the acidic solution. Previous studies have
shown that an amount of 12% by weight added to epoxy polymer has led to maximum
mechanical performance. We chose a graphite electrode submerged into the packed
nanofibers, and a specific applied current was set at 0.1amps. The time of the
treatments was 30 seconds, 1 minute, 2 minutes, 4 minutes, 8 minutes, and 15 minutes.
Following treatment, the oxidized fibers were washed with distilled water until
attaining a neutral pH and dried in a vacuum oven at 100°C for 48 hours.The functionalized carbon nanofibers were
characterized using X-ray photoelectron spectroscopy (XPS). The samples were
oriented such that the axial direction was in the plane of the X-ray source and
the analyzer detection slit. During all XPS experiments, the pressure inside
the vacuum system was maintained at approximately 1 torr. A nonlinear least
squares curve fitting program with a Gaussian-Lorentzian mix function and
background subtraction was used to deconvolve the XPS peaks.While investigation of surface chemistry was performed
by XPS using chemical shift phenomena, in several cases, relative chemical
shifts of different groups are below the energy resolution attainable by XP
spectrometers due to the existence of significant intrinsic peak widths. In
these cases, other sample properties, like chemical reactivity, could operate
the requested discrimination. Chemical derivatization XPS (CD-XPS), based on
selective reaction labeling groups of interest, was used to improve the
selectivity and sensitivity of the XPS analysis. The methodology has been
applied mainly to organic polymers, even though some inorganic materials have
also been studied. Derivatization reactions have been established for C–OH,
COOH, C=O, and C–NH2groups. Information obtained by the simple XPS evidence of
the marker element must be complemented by careful analysis of the main
signals, to which both the organic material and the derivatizing agent
contribute. This procedure will determine whether each reaction occurs
according to the expected stoichiometry.C–OH groups can
be esterified by trifluoroacetic anhydride (TFAA). A procedure for the
derivatization of a polymer sample calls for the sample to be introduced into a
glass test tube. two mL of TFAA were injected into the test tube below the
sample, without contacting it. The test tube was sealed, and the reaction with
the TFAA vapor was allowed to proceed for 15 minutes at room temperature. The
sample was then removed from the test tube, and transferred to the XP
spectrometer for analysis.Derivatizing the sample with TFAA vapor results in the
conversion of the phenol substituent to a trifluoroester group. This leads to
the appearance of new peaks at 290.4 and 293.7 eV which are attributed to the ester
and CF3 carbon atoms, respectively. The peak area of the trifluoroacetic ester
(CF3COO) component is used for a quantitative estimate for the C–OH groups by
calculating one third of the F1s area. TFAA labels virtually all of the
hydroxyl groups within the XPS sampling depth.COOH groups can be esterified by trifluoroethanol
(TFE) vapors, making use of di-t-butyl carbodiimide (DtC) as a dehydrating
agent. A procedure for the derivatization of a polymer sample calls for the
sample to be suspended in a glass test tube; TFE (0.9 mL), pyridine (0.4 mL), and
DtC (0.3 mL) were sequentially injected below the sample, without contacting it,
at 15 minutes intervals. The test tube was sealed with a cap and the reaction
was allowed to proceed at room temperature for 12 hours. The samples were then
transferred to the XP spectrometer and analyzed.An assumption of the reaction is that the reaction
product of DtC (N,N′-di-(t-butyl)-urea) is completely removed from the
surface of the reacted organic material, though no washing stage is performed.
This behavior has been confirmed in literature. The reaction has been shown to
yield about 99% esterification of the COOH groups by TFE. Esterification by TFE
causes three fluorine atoms to be introduced into the sample for each COOH
group. An enhancement of the response factor should be obtained and the XPS
detectability of this functionality then improved. One source of error could be
due to the presence of ionized COOH groups, which are not susceptible to
esterification mediated by carbodiimides.The carbon nanofibers were added to an epoxy resin
matrix (EPON 862) forming a nanocomposite. Since bulk nanofibers are difficult
to incorporate into resin matrices, the nanofibers were first dispersed in the
epoxy resin. The resulting mixture was cured under pressure in a silicone mold.The mechanical properties of the nanocomposites were
measured using the three-point bending test according to ASTM D790-00. The
specimen size was 3.0 in (length) by 0.5 in (width), with the depth dependent
upon the composite sample. The three-point bend test had a span ratio
(span-to-depth of sample) of16:1.
This value is suggested by ASTM D790-00 for materials with a depth greater than
0.0625 in.The thermal conductivity of each nanocomposite
incorporated a xenon flash diffusivity test. The procedure is outlined in ASTM
C714-72, with the test specimens cut to 0.5 in (length) by 0.5 in (width) by
0.03 in (depth). Thermal diffusivity is calculated according to the equationα=0.139L2/t1/2,
where L is the thickness of the sample and t1/2 is the time at which the rear surface of the
test specimen reaches one half its maximum temperature. The thermal diffusivity
parameter is used in conjunction with the specific heat and density of the
nanocomposite to calculate the thermal conductivity. The tests for specific
heat and density were carried out according to ASTM E1269-89 and ASTM C693-74,
respectively. Thermal conductivity is calculated according to the equation K=Cpρα,
where Cp is the specific heat of the sample and ρ is the density of the sample.The electrical properties of the carbon nanocomposites
were studied utilizing a four-point test according to ASTM B 193-87. The two
outer leads of the tester are connected to the current source and the two inner
leads are used to measure the voltage drop through the nanocomposite. Ohm's
law, equation 6, allows for the resistance of the sample to be determined. By
coupling the calculated resistance with the known cross-sectional area and
distance between leads, the resistivityρ of the sample can be deduced


## 3. Results and Discussion

Surface characterization of the nanofibers by XPS (see
Table1) showed an increase in oxygen content from 2% in the as-received
fibers to 8.6% in the fibers treated for fifteen minutes. The N 1s peak was
negligible in all cases.Table 1
Atomic percentages of nanofibers as a function of
electrochemical treatment time.
SampleOxygen %Carbon %As-received2.098.030 s
treatment2.397.71 min
treatment2.897.22 min
treatment3.596.54 min
treatment4.295.88 min
treatment5.494.610 min
treatment7.692.412 min
treatment8.391.715 min
treatment8.691.4Deconvolution of the C 1s spectra (see Table2)
gives the potential for five peaks: graphitic carbon (284.6 eV), carbon present
in phenolic, alcohol, ether, or C=N groups (286.1–286.3 eV), carbonyl or quinone
groups (287.3–287.6 eV), carboxyl or ester groups (288.4–288.9 eV), and
carbonate (290.4–290.8 eV). Figure 2 shows the calculated percentage of
graphitic and functional carbon atoms. There is a significant decrease in the
relative content of graphitic carbon (peak I) and a rise in the relative
content of carbon bonded to oxygen-containing functions (peaks II, III, IV, and
V) with increasing amount of electrochemical treatment. The increase in
relative concentration of carbon oxygen complexes occurs because the outer
layers of the fibers become increasingly porous. The fraction of carbon atoms
in the region which exists on the pore surfaces increases. These carbon atoms
are the sights of oxidation, thereby increasing the relative amount of oxygen.Table 2
Calculated percentages of graphitic and functional
carbon atoms.
SampleIIIIIIIVV30 s treatment9064001 min
treatment87130002 min
treatment8672234 min
treatment80146008 min
treatment78964310 min
treatment791190012 min
treatment752140015 min
treatment7423200Table3 shows the deconvolution of the O 1s spectra
fitted to three component peaks: C=O groups including ketone, lactone, carbonyl
(531.2–531.6 eV), C–OH and C–O–C groups (532.2–533.4 eV), and chemisorbed and
adsorbed water (534.6–535.4 eV). The C=O contribution to the O 1s profile
increases significantly with increasing amount of electrochemical treatment.Table 3
Calculated percentages of oxygen-containing
groups.
SampleIIIIII30 s treatment574301 min
treatment673302 min
treatment702284 min
treatment752058 min
treatment7822010 min
treatment8119012 min
treatment8317015 min
treatment84160As shown in Figure1 and Table 4, a modest
degree of electrochemical treatment improved the flexural modulus of the
nanocomposite by enhancing the interfacial adhesion between the fiber and
resin. The treatment time of 30 seconds allowed for a 4.32% increase in
flexural modulus. A maximum improvement of 74% occurred with an electrochemical
treatment time of twelve minutes. Prolonged subjection to the electrochemical
treatment beyond eight minutes showed a drop in flexural modulus. This may be
due to the oxide layer actually causing failure of the fiber-resin bond or
possibly the additional treatment time may have damaged the fiber surface.Table 4
Nanocomposite
mechanical properties with relative standard deviation.
SampleSurface treatmentMassModulusSTDStrengthSTDtime (min)%(GPa)(GPa)EPON 862—01.77300.06880.09760.0018PR-24-PS0122.57830.12670.11960.0057PR-24-PS-ET
(0.5)0.5122.68980.54640.09010.0143PR-24-PS-ET
(1)1122.73990.16190.10370.0076PR-24-PS-ET
(2)2122.80140.11240.10720.0150PR-24-PS-ET
(4)4123.02610.34340.10170.0071PR-24-PS-ET
(8)8123.49150.02170.11630.0093PR-24-PS-ET
(10)10123.95350.01180.12630.0087PR-24-PS-ET
(12)12124.48560.01230.14620.0068PR-24-PS-ET
(15)15123.06900.35580.10410.0099Figure 1
Nanocomposite
mechanical properties as a function of electrochemical treatment time.Figure 2
Longitudinal
view of untreated nanofibers-based nanocomposite fracture surface.The electrochemical treatment did have a slightly
negative effect on the thermal conductivity of the manufactured nanocomposites.
As shown in Table1, there was a significant decrease in the heat capacity
of the composites with functionalized nanofibers, indicating an effect of
additional oxygen content. There was not a significant change in the thermal
diffusivity results. In addition, the electrical resistivity of the
nanocomposites decreased with increasing electrochemical treatment time. This
may be due to the increase in oxygen content causing the formation of a narrow
insulating layer along the surface of the fibers (see Table 5).Table 5
Thermal and
electrical properties of nanocomposites as a function of treatment time.
SampleSurface treatmentMassResistivityKDensitytime (min)%(Ohm-cm)(W/m-K)(g/cm3)EPON 862—03.28E
+ 100.17681.198PR-24-PS0121.58E
+ 000.37311.252PR-24-PS-ET
(0.5)0.5124.13E
+ 000.29921.237PR-24-PS-ET
(1)1125.16E
+ 010.34761.253PR-24-PS-ET
(2)2123.14E
+ 030.33031.246PR-24-PS-ET
(4)4125.57E
+ 060.30321.240PR-24-PS-ET
(8)8123.35E
+ 070.32901.235PR-24-PS-ET
(10)10125.56E
+ 100.33341.257PR-24-PS-ET
(12)12124.54E
+ 090.34931.234PR-24-PS-ET
(15)15123.69E
+ 100.33481.254The SEM examination of the fracture surface of various
samples following mechanical testing has shown that the interface between the
nanofiber and epoxy resin matrix has only minimal adhesion in the form of
covalent bonds (Figures2 and 3). An interfacial gap between the two phases is
always present and its location may vary from one sample to another (single
arrows in Figures 2 and 3).Figure 3
Cross view of
untreated nanofibers-based nanocomposite fracture surface.The inability to create an effective interface between
the carbon nanofiber and the neat resin prevents the transfer of mechanical
loads between the two regions. This hindrance of load transference results in
mechanical properties that are nominally better than those of the neat resin.
To better understand the role of surface chemistry of the nanofiber, the effect
of surface functionalization of nanofiber on the mechanical properties of
produced nanocomposites was studied.The fracture surface of the surface-functionalized
nanocomposite is markedly different from that of the PR-24 nanocomposite. Along
the length of the chemically modified nanofiber within the epoxy matrix, there
is an apparent interface devoid of the gaps present in the pyrolytically
stripped nanocomposite (see Figure4). The body of the nanofiber is actually
covered with lighter regions of “strings” and bumps arranged in the form of
“knife teeth.” These bumps are made of resin residue. This region is more
closely studied using high-resolution SEM and TEM techniques.Figure 4
SEM micrograph of surface-functionalized nanocomposite
fracture surface.In Figure5, it is shown that the surface of the
functionalized nanocomposite is covered with masses of residual epoxy resin.
This demonstrates that the nanocomposite did not fracture along the
nanofiber/resin interface. Rather, the resin remainent adhered to the nanofiber
following fracture and the mechanical failure took place within the resin phase
of the nanocomposite. TEM analysis offers a more thorough explanation for the
interaction between the modified surface of the nanofiber and the epoxy resin.Figure 5
Higher magnification SEM of surface-functionalized
nanocomposite fracture surface.The black arrows in Figures6 and 7 show the exposed
ends of nanofibers and their bodies within the resin matrix at the fracture
site. The bright-field micrograph shows a change in a grey scale to demonstrate
differences in electronic density within the resin matrix. The epoxy resin
areas do not have one consistent shade, however. The single white arrows in
Figures 6 and 7 point to areas of higher electronic density than that of areas
showing lighter contrast (double white arrows in Figures 6 and 7). This
difference in a grey-scale may indicate some structural changes within
continuous resin matrix. A longitudinal representation offers a clearer
understanding of the physical property gradient within the
surface-functionalized carbon nanocomposite.Figure 6
TEM
micrograph of surface-functionalized transverse fracture surface.Figure 7
TEM micrograph of surface-functionalized longitudinal
fracture surface.An analysis using high resolution TEM allows for a
more complete understanding of the changes that occur at the interface between
the surface-functionalized carbon nanofibers and the continuous epoxy resin
phase.At high resolution, the individual graphene planes
of a nanofiber can be seen along with the region of the epoxy region. Moving
from the upper-left towards the lower-right of the micrograph, the interface
with the nanofiber is crossed into the parallel layers within the nanofiber
representing the wall of nanofiber. Further down the micrograph, another
interface is crossed into the epoxy resin phase with very organized
turbostratic carbon (circled areas in Figure8 with a wavy, spaghetti-like
look). In this region, there is a local molecular orientation of basic
structural units that are in the form of columns and clusters with
discontinuous but preferential molecular orientation parallel to the nanofiber
axis. The polymer interphase is no longer amorphous but has gained a
two-dimensional order similar to the microstructure of carbonized PAN carbon
fiber. Epoxy generally has gravel-like appearance under HR-TEM, which indicates
an amorphous structure. This change in physical properties demonstrates the
presence of an “interphase.” Due to the chemical interaction between the
surface-functionalized nanofiber and the resin, a new material has been created
with unique physical properties.Figure 8
High resolution TEM showing an oriented interface
between nanofiber and matrix.


## 4. Conclusion

In principle, achieving high tensile strength
composites with nanofibers as the reinforcement relies on factors including
weight fraction, strength of the nanofibers, dispersion of the fibers, and the
strength of the interface. Assembly of these factors may be envisioned without
difficulty; however, the latter factor—strength of the interfacial bond
between nanofiber and matrix—is problematic due to the inert nature of the
smooth hexagonal surface commonly presented by the nanofiber class of
reinforcement.Due to the exposed graphitic edge planes of nanofiber
surfaces, it is possible to add functional groups. The addition of functional
groups will greatly increase the ability of the nanofiber to bond to polar
matrix materials such as epoxy. As a result of this surface modification, the
mechanical properties of the nanocomposites were significantly enhanced, and
the chemical properties such as heat capacity of the polymer were lowered. This
improvement might be obtained by an internal reaction between the surface
functional groups and polymer matrix. Transmission electron microscopy
characterization shows that the post-synthesis surface treatment has
contributed to the formation to a very dense and oriented interface between
nanofiber and matrix.The surface-treated nanofiber-based nanocomposites
samples exhibit better mechanical properties than any of the nonsurface-treated
nanofiber-based nanocomposites. The improvement in mechanical properties is due
to the formation of gradients at interfaces and interphases between the
nanofibers and epoxy resins. These interphases cause the polymeric
nanocomposites to behave as a continuous phase in which the mechanical
transport properties between parent individual ingredients (nanofibers and
epoxy matrix) were enhanced.There was a significant decrease in the relative
content of graphitic carbon and an increase in the relative content of carbon
bonded to oxygen-containing functions with increasing amount of electrochemical
treatment. The increase in relative concentration of carbon oxygen complexes
occurs because the outer layers of the fibers become increasingly porous
allowing for additional sights of oxidation. The increasing amounts of oxygen
with electrochemical treatment corresponded to an increase in the flexural
modulus of nanocomposites manufactured with the treated fibers. This indicates
an improvement in interfacial adhesion between the fibers and the resin. The
treated nanofibers had negative impact on the heat capacity of the
nanocomposites, but they did not affect the thermal diffusivity of the
composites. The electrical resistivity of the nanocomposites decreased with
increasing electrochemical treatment time due in part to the increased
concentration of oxygen functional groups.



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*Source: 310126-2008-01-10.xml*