Open Journal of Synthesis Theory and Applications, 2012, 1, 36-43
http://dx.doi.org/10.4236/ojsta.2012.13007 Published Online October 2012 (http://www.SciRP.org/journal/ojsta)
Mechanical and Thermal Properties of Chemically
Modified Epoxy Resin
Sumeera Ikram, Arshad Munir*
Centre of Excellence for Science and Advanced Technology, Islamabad, Pakistan
Received September 12, 2012; revised October 6, 2012; accepted October 19, 2012
ABSTRACT
Diglycidyl ether of bisphenol-A (DGEBA), having number average molecular weight (Mn) 375, was modified by in-
corporating the hydroxyl terminated polybutadiene (HTPB) based prepolymer using isophorone diisocyanate as a cou-
pling agent. To increase the compatibility between the epoxy resin and HTPB part, polar groups were introduced in the
later to achieve physical and chemical interactions between the two phases. The finally modified DGEBA system was
cured with amine based hardener. FTIR and 1H-NMR were used to monitor the whole modification procedure. The
rubber particles size and distribution was monitored as a function of HTPB contents in the resin system using scanning
electron microscopy (SEM). The mechanical, thermal and thermo-mechanical properties have shown that the tensile
strength, toughness, ductility and impact strength of the modified cured system have been successfully increased at
some optimum HTPB contents without affecting the inherent thermal and thermo-mechanical stability associated with
DGEBA resin system. Some of the mechanical properties like flexural modulus, tensile modulus and compressive
strength decreased with increasing rubber contents.
Keywords: DGEBA; HTPB; Impact Strength; Thermo-Mechanical; Mechanics Properties
1. Introduction
Thermo set resins form a rigid network which is dimen-
sionally stable under many conditions, having good che-
mical and thermal resistance. As a result these polymers
are very important engineering materials, accounting for
nearly 20% of the polymer market, including use in com-
posites, coatings and adhesive applications [1-3]. Epoxy
resins are thermo sets with extraordinary adhesion ac-
companied by simple cures, good strength, creep resis-
tance, heat tolerance, chemical durability and relatively
low shrinkage. Adversely, these polymers are also brittle
and have a high degree of cross linking. Toughening ep-
oxy resins would allow them to be used in many addi-
tional applications including structural applications [3].
Toughening of thermosetting polymers can also be
accomplished by adding a flexibilizer to the thermo set.
Adding a flexibilizer will indeed increase the toughness,
but it results in adverse effect such as lowering of the
glass transition temperature (Tg) and modulus. Another
method of toughness is inclusion of rubber particles into
the primary phase. The secondary rubber can improve the
impact strength without loss in polymer other properties
[4].
Epoxy resins are frequently modified by dissolving in
a small proportion (10% - 20%) of a liquid rubber con
taining reactive end groups such as carboxyl-terminated
acrylonitrile copolymer (CTBN) [5] and amine-termi-
nated butadiene acrylonitrile copolymer (ATBN) [6].
Although CTBN and ATBN oligomers are very efficient
in improving the fracture properties of epoxy resins but
make the resultant material thermally unstable.
To avoid deterioration in the inherent stiffness and
strength and a reduction in the glass-transition tempera-
ture of the resin, some engineering plastics, such as poly
ether sulphone (PES) [7] and poly ether imide [8,9] have
been used to produce systems containing rigid, ductile
particles dispersed in epoxy resin. Particles of crystalline
polymers, such as poly vinylidene fluoride (PVDF), poly
butylenes terephthalate (PBT) and nylon 6 has been used
for the toughening of an aromatic amine cured epoxy
resin. It has been reported that nylon 6 and PVDF tough-
ened epoxy resins to an extent similar to that of CTBN.
The fracture toughness, however, was increased two fold
by the inclusion of PBT over that achieved with nylon 6
and PVDF [10].
It has been reported that both poly ether ether ketone
(PEEK) and PBT could form crystals in the form of tri-
block copolymers with amorphous PES. The most effect-
tive modifier for increasing toughness appeared to be the
triblock PEEK-PES-PEEK system, which gave an almost
*Corresponding author.
C
opyright © 2012 SciRes. OJSTA

S. IKRAM, A. MUNIR 37
single-phase structure [11]. Various carboxyl-terminated
elastomeric acrylate oligomers have been used to toughen
a DGEBA resin. Acrylate oligomers exhibited extremely
good miscibility with conventional epoxy resins and
cured castings exhibited enhancements in impact strength
comparable to those of traditionally toughened epoxy
systems. A different approach has been taken [12] to
synthesize two polymers, one of which formed a network
by simultaneous independent reactions in the same con-
tainer. Intercross linking reactions were eliminated by the
combination of free-radical (acrylate) and condensation
(epoxy) polymerization. The mechanic cal properties of
the resultant system improved when the extent of mo-
lecular mixing was minimized and hetero phase semi-
interpenetrating polymer networks (semi-IPNs) were
produced. Similar observations were reported in the work
concerning the production of heterogeneous interpene-
trating polymer networks (IPNs) of poly methyl metha-
crylate within a poly dimethyl siloxane elastomer [13].
Recently, epoxy resin acrylated polyurethane semi-
IPNs [14] were synthesized that were compatible and
sufficiently flexible. A great deal of literature has been
devoted to the toughening of epoxy resins via polyure
thane incorporation as a secondary phase in a specific
matrix to form a grafted or un-grafted IPN [15-17]. Si-
loxane elastomers [18] have also been used as attractive
alternatives to traditional toughening systems. For exam-
ple, the addition of room temperature vulcanized (RTV)
silicon rubber in amount up to two times that of the un-
modified resin has been reported. Although some of these
oligomers are quite expensive, these oligomers are im-
miscible in the resin systems and tend to migrate to the
outside. To resolve this problem, epoxy resin has been
chemically modified with functionally terminated poly
dimethyl siloxane and poly dimethyl-co-diphenyl silox-
ane oligomers [19].
Rubber modification of bifunctional and tetrafunctional
epoxy matrices by means of block co-polymer of poly
dimethyl siloxane and poly oxyethylene elastomer or an
anhydride-grafted polybutene [20] improved the impact
strength for bifunctional epoxy systems but failed for
tetrafunctional epoxy resin systems.
Within this framework, the main objective of the pre-
sent work is to toughen epoxy resins by chemically in-
corporating hydroxyl terminated poly butadiene (HTPB)
into the backbone of epoxy resin and its characterization.
Different ratio by weight of epoxy terminated HTPB based
prepolymer was used as a reactive modifier and its effect on
the particle morphology, impact strength, toughness, flexi
bility and other mechanical, thermal and thermo-mechanical
properties of the final system was studied.
2. Experimental
2.1. Materials and Preparation of Epoxy
Terminated Htpb Based Prepolymer
The epoxy resin synthesized in our lab was a diglycidyl
ether of bisphenol-A (DGEBA) with an epoxy equivalent
weight of 194g/eq. and Mn of 375 g/mol. Hydroxyl ter
minated polybutadiene (HTPB, supplied by Aldrich) with
Mn 2800 (Mn stands for number average molecular
weight of the polymer and does not have units) and OH
content of 0.72 m eq./g was used as a rubber modifier.
Isophorone diisicyanate (IPDI, with 99% purity, supplied
by Sigma) was used as a coupling agent between epoxy
resin and HTPB.
N-(3-dimethylaminopropyl)-1,3-propylene diamine (Ar-
aldite 2011B, supplied by Ciba-Giegy) was used a hard-
ener. Tetrabutyl ammonium chloride (TBAC, sup plied
by BDH, 98%) was used as a catalyst for the reaction
between epoxide and isocyanate groups.
In the first step of reaction, IPDI and HTPB were
charged in 1L reaction kettle. The mixture was continu-
ously stirred while the temperature was raised to 130˚C
that was kept constant for two hr under nitrogen atmos
phere. The hydroxyl group of HTPB reacted with the
-NCO group of IPDI to produce a urethane linkage (see
Scheme 1). Unreacted -NCO groups were titrimetrically
determined. After a series of experiments using different
molar ratios of HTPB with IPDI, it was found that 53%
of -NCO groups remain unreacted when 16 g (0.072
moles) of IPDI was mixed with 84 g (0.04 moles) of
HTPB. In the second step of reaction, a small amount (5
g) of tetra butyl ammonium iodide was added into 750 g
of DGEBA epoxy resin as a catalyst, which was mixed at
145˚C ± 5˚C. Then the temperature was raised to 200˚C
while vigorously stirring the mixture for 30 min. This
results in the reaction between the epoxide group of
DGEBA and the NCO group of IPDI-HTPB based tel
echelic polymer to form epoxy-terminated HTPB-based
prepolymer (ETHTPB) with number average molecular
weight (Mn) of 3553 as determined by vapour pressure
osmometry (see Scheme 2). ETHTPB prepolymer was
further diluted with different amount of DGEBA epoxy
resin to get various concentrations on the basis of HTPB
part of the polymer. For 0.1%, 0.3%, 0.7%, 1.0%, 1.5%
and 2.0% the amount of ETHTPB prepolymer added per
100 g of resin was 100 mg, 300 mg, 700 mg, 10 mg, 15
mg and 20 mg respectively. Finally curing agent, N-(3-
dimethylaminopropyl)-1,3-propylene diamine, was added
into the mixture at a weight ratio of 100:100, followed by
curing at 120˚C for 10 hrs.
2.2. Measurements
FTIR and H1NMR (300 MHz) were used in the whole
process to monitor the reaction product while free NCO

S. IKRAM, A. MUNIR
38
NCO
CH3
CH3CH3
NCO CH2
CH
CH
CH2 OH
OH
y
y
N
O
O
CH 2
CH
CH
CH 2 O
O
N
CH3
CH 3
CH3
NCO
H
CH3
CH3
CH3
NCO
H
+
130 C
o
2 hrs
Scheme 1. Synthesis of IPDI-HTPB-IPDI telechelic poly-
mer.
Scheme 2. Reaction between telechelic polymer and DGEBA
to synthesize prepolymer.
groups were determined by the usual chemical titration
method. FTIR spectra were obtained using Thermo Elec-
tron Corporation IR 200 FT-IR spectrometer. All FTIR
analysis were carried out in the range of 400 - 4000 cm–1
using KBr pellets and CDCl3 was used as a solvent dur-
ing 1H-NMR analysis. The microstructure of diamond-
poxy nanocomposites was observed under scanning elec-
tron microscope (SEM, JED 2300). SEM was performed
on the fractured surfaces after dipping the sample into
liquid N2 to make the material more brittle. Fractured
surfaces were sputtered with gold and all the measure-
ments were done using 10 kV voltage electron beam.
Many important mechanical properties were also inves-
tigated including The Izod impact strength was measured
using 64 mm long, 12.7 mm wide and 6.4 mm thick
specimens on a pendulum impact testing machine ac-
cording to the specification, ASTM D 256. The depth
under the noch of a specimen is 10.2 mm. Drop height of
pendulum was kept 1.46 meter with a striking velocity of
5.35 m/s. Tensile, flexural and compressive properties
were measured using a universal testing machine, (Tes
tometric, UK), at room temperature at a cross head speed
of 5 mm/min, according to the specifications, ASTM 638 -
52, D-790, D-695, respectively. The Vickers hard ness
number (VHN) was calculated using micro-hardness
testing instrument manufactured by M/s. Frank, UK. For
hardness measurement at ambient conditions, films of
uniform thickness (0.8 - 1.0 mm) were used. The hard
ness of a material was measured by forcing an indenter
into the surface of the film with a constant speed. A force
of 2.94N was applied slowly by pressing the indenter at
90o into the material surface being tested. The indenter
material, in the form of square based pyramid made up of
diamond was used for indentation. The indentation time
was kept fixed (i.e. 30 seconds) for all samples. An em-
pirical hardness number was then calculated by measur-
ing the length of the diagonal through microscope at-
tached with the instrument. To evaluate the effects of
HTPB contents on the degradation behaviour of modified
epoxies at various temperatures, thermogravimetric analy-
sis was conducted on a Perkin Elmer Diamond TGA/
DTA machine at a heating rate of 10˚C/min. The dy-
namic mechanical thermal analysis measurements were
performed in a temperature range of 50˚C - 300˚C using
Perkin Elmer Diamond DTMA at a heating rate of 2˚C/
min and at a frequency of 1Hz.
3. Results and Discussion
3.1. IPDI-HTPB-IPDI Telechelic Polymer
In the first step of modification process, i.e. the IPDI was
always taken in excess of the equimolar ratio between
NCO groups of IPDI and OH groups of HTPB. The con-
centration of IPDI studied was 16%, 20%, 24% and 32%
w/w with respect to HTPB which correspond to 2:1;
2.5:1; 3.0:1 and 4.0:1 moles of NCO groups per mole of
OH groups in IPDI and HTPB, respectively. The molar
ratio 2:1 (16% w/w of IPDI with respect to HTPB) was

S. IKRAM, A. MUNIR 39
considered ideal as it could leave one NCO group free to
react with one epoxide group of epoxy resin, in the sec
ond step of reaction. The peak in the FTIR spectrum ap
peared at 1720 cm–1 indicates the formation of urethane
linkage between HTPB and IPDI, Figure 1. The peak
due to unreacted NCO’s is also present at 2260 cm-1,
while in 1H-NMR the NH proton of urethane linkage
appeared at δ 7.5 ppm and a doublet appeared at δ 2.9
ppm due to four protons in 2 CH2 attached to nitrogen.
3.2. Epoxy Terminated HTPB Based Prepolymer
In the second steps i.e. the reaction of unreacted isocy-
anate groups of IPDI-HTPB-IPDI telechelic polymer
with the epoxide groups of DGEBA results in the forma-
tion of oxazolidone linkage. The characteristic peak of
carbonyl group appeared at 1749 cm–1 and 1723 cm–1 in
FTIR spectrum due to oxazolidone and urethane linkage,
respectively as shown in Figure 1(b). The band appear-
ing at 1456 cm–1 is attributed to the C-N linkage in oxa-
zolidone. No peak appeared in FTIR spectrum at 2260
cm–1 corresponding to NCO’s antisymmetric stretching
indicating the completion of the reaction. While in
1H-NMR the doublet at δ 4.19 ppm and multiplet at δ
5.70 ppm appeared due to four protons of 2CH2 and two
protons of 2CH in oxazolidone ring, respectively. While
the formation of two tertiary nitrogen were confirmed by
the appearance of multiplet at δ 3.58 ppm by the 2CH
proton attached to tertiary nitrogen The peaks observed
at 967 cm–1 and 913 cm–1 are attributed to the stretching
and bending vibration of cyclo-oxirane in FTIR spec-
trum.
SEM micrographs of the fractured surface of modified
epoxy-amine system having different proportion of
HTPB rubber i.e. 0.3 wt%, 0.7 wt%, 1.0 wt% and 2.0
wt% are shown in Figures 2(A)-(D). The elastomer ap-
peared as distinct particles in epoxy-amine network. These
rubber particles were clearly observed to be homogene-
ously dispersed in the epoxy-amine matrix. The diameter
of the particles was in the range of 0.2 μm to 2.6 μm.
Although boundaries were not very sharp but still the
particles appeared as distinct constituents with separate
boundaries, which confirmed that there was less chemi-
cal interaction between the two constituents. The dif-
fused nature of the boundaries observed in the micro-
graph may be due to the physical interaction (hydrogen
bonding etc.) between the two phases. It means that the
polar groups that had been inserted in the rubber chains
i.e., epoxide groups, -OH-groups and NCO-groups make
the prepolymer compatible to large extent with the ma-
trix creating physical interaction that assist the homoge-
neous distribution of rubber particles. Due to the pres-
ence of terminal epoxide group most of the prepolymer
could become the part of the main polymer chains de-
creasing the number of distinct particles that could be
expected, especially at lower concentration of prepoly-
mer. According to the SEM results, the particle size in-
creased with the increase of rubber contents in the matrix
as shown in Figure 3. Initially the effect of elastomer
concentration on the particle size was very small but after
0.7 wt% there was a steep incline suggesting that the
rubber particles agglomerate when its concentration ex-
ceeded certain value and had pronounced effect on parti-
cle diameter.
3.3. Mechanical Properties
One of the reasons behind modifying epoxy-amine mate-
rial was to improve its impact strength. Impact strength
of neat epoxy-amine polymer was 2.3 kJ/m which was
doubled by modifying the system with HTPB contents as
shown in Figure 4 (rectangular symbols). Impact strength
was observed to increase linearly with increasing HTPB
concentration up to 1.0%. At 1.0% HTPB contents the
value of impact strength was found to be 4.23 kJ/m. Be-
yond 1.0% of HTPB the impact strength remained almost
constant with a very little deviation and reached to 4.5
kJ/m when 2.0% HTPB contents were incorporated to the
system. Same effect has been observed in the material
prepared with an epoxy resin modified with hydroxyl
terminated poly(butadiene-acrylonitrile) by Sankaran and
Chandas [21].
To evaluate the hardness property, microhardness meas-
urement were performed on both unmodified and modi-
fied samples at 25˚C, using Vickers microhardness in-
dentation. Hardness of modified system was also found
Figure 1. FTIR spectra of (a) IPDI-HTPB-IPDI telechelic
polymer in transmission mode, (b) epoxy terminated HTPB
based prepolymer.

S. IKRAM, A. MUNIR
40
Figure 2. SEM micrograph of (A) 0.3% (B) 0.7% (C) 1.0%
(D) 2.0% HTPB modified cured epoxy-amine system.
Figure 3. Mean particle size as a function of HTPB concen-
tration in the epoxy-amine system.
to be decreased due to the incorporation of soft prepoly-
mer contents. Vicker’s hardness number decreased from
13.9 to 2.7 from neat system to 2.0% modified system as
shown in Figure 4 with circular symbols.
The tensile properties like fracture strength, Young’s
modulus, and elongation at break were calculated as
shown in Figure 5. Fracture strength of unmodified sys
tem observed in case of neat system is 31.72 MPa (Fig-
ure 5(a) shown with colour square symbol). This low
strength value of epoxy-amine system is due to the in-
Figure 4. Vicker’s hardness and Impact strength as a func-
tion of HTPB concentration in the modified epoxy-amine
system.
herent brittleness of the system. The fracture strength of
the system was increased by the addition of prepolymer.
Maximumvalues i.e., 42 - 44 MPa were achieved when
0.3% - 1.0% HTPB was incorporated into the epoxy-
amine system. It can be attributed to the increase in frac-
ture energy and resistance to the crack propagation when
flexible HTPB parts are introduced to the brittle system.
Beyond 1.0% addition of HTPB adversely affects the
fracture strength of the resulting system, be cause HTPB
itself is low strength material. As the percentage of rub-
bery segment increases the resulting material acquires
intermediate properties of both the components resulting
in the decrease in ultimate strength of the material.
Young’s modulus also showed a decrease with increase
in HTPB contents ((Figure 5(a) shown with blank square
symbol). However initially the addition of only 0.1% of
HTPB in the system lowered its tensile modulus by about
700 MPa. Further addition of HTPB has very insignifi-
cant effect on the Young’s modulus of the system. The
ductility of the material was found to be increased by in-
creasing rubber contents in the resulting material. The
tensile elongation of unmodified epoxy-amine system
was 1.7% at break (Figure 5(b)). Up to 0.3% HTPB
concentration there was a steep incline in the ductility
where the ductility reached to 5.5% with 0.3% HTPB
incorporation. Beyond 0.3% to 2.0% HTPB contents the
increase in ductility of material was comparatively slow
and the final ductility of material reached to 8.7% before
the failure of material by the applied pulling force. HTPB
based flexible pre polymer in the main chain are proba-
bly responsible for increase in the ductility of the result-
ing material by reducing the cross-linking density and
facilitating the chains to slide-pass each other. Rubber
contents removed the inherent brittleness thus resisting
the crack initiation with the applied external force, while

S. IKRAM, A. MUNIR 41
initial strength of the material was decreased. Rubber
particles that were weekly bonded to the epoxy-amine sys-
tem started agglomerating at high concentration of pre-
polymer, as supported by SEM results, which increased
the ductility but decreased the modulus and tensile strength
of the material.
Flexural properties of the material were determined by
applying force in 3-point bending mode. The flexural
strength and flexural modulus of the system was found to
be decreased as rubber content in the modified system
was gradually incorporated, as shown in Figure 5(c).
Similar results have been obtained by C. Kaynak and
coworkers [22]. The value of flexural modulus in elastic
region was always smaller than tensile modulus for any
concentration of HTPB in epoxy-amine system. It sug
gests that the deformation produced in modified or un-
modified material is greater in 3-point bending mode
than tensile mode.
Compressive strength measure the ability of material
to resist the crushing force. When a compressive force
was applied on the sample the material bulged out from
sides and adopted a barrel shape with cracks on sides as
shown in Figure 6(b). Compressive strength also de
creased linearly with increase in HTPB contents as shown
in Figure 6(a). Compressive strength of unmodified ep-
oxy-amine system was observed to be 132.3 MPa which
decreased by 54 MPa by incorporating 2.0 % HTPB con-
tents in the system. The decrease in compressive strength
can be attributed to the decrease in crosslink density and
increase in flexible alipahatic character in resulting modi-
fied epoxy-amine system.
3.4. Thermo-Mechanical Properties
The general trend of pyrolysis for all the samples includ-
ing unmodified system are similar showing one step de-
gradation starting with the weight loss from about 300oC
upto about 500oC (Figure 7(a)). It was indicated that the
mechanical performance has been improved by adding
elastomer or rubber, but it has no adverse effect on the
inherent thermal stability of the epoxy-amine thermo set
resin. This can be attributed to the fact that the thermally
less stable rubber contents are very small in quantity as
compared to the matrix system to affect its thermal sta-
bility and secondly the rubber con tents might have been
well masked by the network of thermally stable ep-
oxy-amine network. There were very small changes in
percent weight loss at various temperatures with increas-
ing rubber contents. The variation in thermal stability at
higher temperatures can not be correlated with increasing
rubber contents. The maximum rate of decomposition
(Tmax) calculated from the peak of the differential of the
TGA curve and char content for each formulation of un-
modified and modified system was also calculated as
shown in Table 1.
Glass transition temperature (Tg) was calculated from
the peak of tanδ curve (Figure 7b) during DTMA analysis
Figure 5. Variation in (a) ultimate stress and Young’s
modulus, (b) elongation at break, and (c) Flexural strength
and Flexural modulus with increase in HTPB content in
epoxy-amine system during tensile test.

S. IKRAM, A. MUNIR
42
Figure 6. (a) Compressive strength as a function of HTPB in
the epoxy-amine system; (b) Samples after compressive
testing become barrel shaped with visible large cracks on
sides.
Table 1. Maximum decomposition temperature, Tmax, and
char portion at 700˚C for HTPB modified and unmodified
systems.
Sample ID Tmax (˚C) Char Portion (%)
Neat 375.0 4.31
0.1% 369.2 4.10
0.3% 370.0 4.01
0.7% 370.1 3.59
1.0% 370.0 4.05
1.5% 370.0 4.01
2.0% 368.5 2.15
as shown in Figure 7(c). Tg of 2% HTPB modified sys-
tem is about 6˚C lower as compared to the unmodified
system and Tg was observed to be inversely related with
HTPB concentration in the modified epoxy-amine sys-
tem, Figure 7(c). This could be explained by taking into
account the free volume concept. However there is a very
small change in the glass transition temperature at vari-
ous rubber concentrations in the modified. From thermal
and thermo-mechanical data it is inferred that improved
mechanical properties can be achieved by modifying the
system with HTPB based elastomer without having sig-
nificant effect on the thermal and thermo-mechanical pro-
perties of epoxy-amine system.
Figure 7. (a) Thermogravimetric curves showing the thermal sta-
bility of the modified epoxy-amine systems containing different
HTPB concentrations; (b) Tanδ curves of the modified epoxy-
amine systems containing different HTPB concentrations; (c) Glass
transition temperatures (Tg) from tanδ maxima as a function of
HTPB conc. as determined by DMTA at frequency 1Hz.

S. IKRAM, A. MUNIR
43
4. Conclusion
The toughening of epoxy resin, DGEBA, with hydroxyl-
terminated polybutadiene (HTPB) was success fully
conducted. A chemical linkage between the elastomer
and resin was developed by employing isophorone diiso-
cyanate (IPDI) as a coupling agent to synthesize epoxy
terminated HTPB based prepolymer. Test results showed
increases in some of the important mechanical properties
like toughness and impact strength by 100% with only
1% - 2% HTPB concentration. Other mechanical proper-
ties like flexural strength, flexural modulus, stiffness, and
compressive strength decrease linearly with increasing
rubber concentration. The toughening of epoxy resin,
DGEBA, with hydroxyl-terminated polybutadiene (HTPB)
was success fully conducted. A chemical linkage between
the elastomer and resin was developed by employing iso-
phorone diisocynate (IPDI) as a coupling agent to syn-
thesize epoxy terminated HTPB based prepolymer.
5. Acknowledgements
The authors are grateful to the Centre of Excellence for
Science and Advanced Technology, Islamabad, Pakistan
for funding the project and University of Peshawar for
providing SEM facility.
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