nanocrystalline fe-ni alloy/polyamide 6 composites of high mechanical performance made by...

Nanocrystalline Fe-Ni Alloy/Polyamide 6 Compositesof High Mechanical Performance Made byUltrasound-Assisted Master Batch Technique

Marwa A. A. Mohamed,1,2 Azza El-Maghraby,2 Mona Abd El-Latif,2 Hassan Farag,3 Kyriaki Kalaitzidou1,4

1The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology,Atlanta, Georgia 30332-0405

2Fabrication Technology Department, Institute of Advanced Technology and New Materials, City of ScientificResearch and Technological Applications, New Borg El-Arab, Alexandria 21934, Egypt

3Department of Chemical Engineering, Faculty of Engineering, Alexandria University,Alexandria 21544, Egypt

4School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405

The focus of this study is to improve the dispersionstate of nanocrystalline (nc) Fe-Ni particles in polyamide6 (PA6) matrix and the filler-matrix interfacial interac-tions to provide Fe-Ni alloy/PA6 nanocomposites ofremarkable mechanical performance for engineeringapplications. nc Fe40Ni60 particles were chemically syn-thesized. Then Fe40Ni60/PA6 nanocomposites of variousnanofiller loading were prepared by compounding via anewly modified master batch technique called ultra-sound assisted master batch (UMB), followed by injec-tion molding (IM). Their mechanical properties,morphology and structural parameters were character-ized and compared with the corresponding propertiesof Fe40Ni60/PA6 nanocomposites made by solution mix-ing (SM) and IM. The study reveals that the UMB pro-cess is more cost effective and time efficient, simplerand easier to scale up compared with the SM process.In addition, UMB nanocomposites exhibit superiormechanical properties and distinctive morphology com-pared with the corresponding SM ones. Moreover, struc-tural analyses indicate that physical structural changesoccurred in PA6 due to presence of alloy particles areaffected differently by the different compounding meth-ods, profound understanding of such phenomenon isfocused throughout the article. These distinctive advan-tages recommend that UMB technique can be of greatpotential in commercial production of polymer nano-composites (PNCs). It is concluded that the sonication

of nc Fe40Ni60 particles in dilute polymer solution duringUMB compounding, a new step that is incorporated forthe first time in the master batch process, is mainlyresponsible for the good wetting between nanoparticlesand polymer chains, strong filler-matrix interactions andconsequently the remarkable mechanical performanceof UMB PNCs. POLYM. COMPOS., 00:000–000, 2014. VC 2014Society of Plastics Engineers

INTRODUCTION

Polymer nanocomposites (PNCs) have a great potential

for a wide variety of engineering applications including

automotive, aerospace, construction, food packing and

electrical applications. They represent a radical alternative

to conventional polymer composites where the smallest

dimension of the reinforcement is on the order of at least

few microns [1]. The value of PNCs is mainly due to the

unique properties [2] and the extremely high surface area

of nano-size fillers which enables strong filler-polymer

interactions, leading to significant performance improve-

ments even at very low nanofiller loading (1–10 wt%).

Conversely, in conventional composites similar perform-

ance improvement is only possible at high-filler loadings

(10–40 wt%). The low content of nano-size reinforce-

ments enables greater retention of the inherent processi-

bility and toughness of the neat polymer and avoids

adding excessive weight to the final product [1, 3].

Nanocrystalline (nc) Fe-Ni alloy is considered of

great importance for various technological applications

including high-density magnetic storage devices, bimetal-

lic temperature sensors, electromagnetic shielding and

Correspondence to: Marwa Mohamed; e-mail:

[email protected]

Contract grant sponsor: Egyptian Ministry for High Education and Sci-

entific Research; contract grant number: PhD scholarship 51218.

DOI 10.1002/pc.22901

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2014 Society of Plastics Engineers

POLYMER COMPOSITES—2014

microelectromechanical systems (MEMS) [4], due to its

unique mechanical and magnetic properties, low thermal

expansion and good electrical conductivity [5, 6]. The potential

of using nc Fe-Ni alloy, as a nanofiller in polymers has been

explored for the first time in our previous study [7] where the

mechanical and thermomechanical properties of Fe-Ni alloy/

polyamide 6 (PA6) nanocomposites were investigated and the

potential of using this novel nanocomposite as structural mate-

rial in engineering applications was explored. The study

revealed that the Fe-Ni alloy/PA6 PNCs produced by melt

mixing (MM) have deteriorated mechanical performance,

mainly because the weak mixing forces, applied in MM pro-

cess, could not overcome the strong agglomeration tendency

of the supermagnetic Fe-Ni alloy, leading to poor filler-matrix

interfacial interactions. In contrast, the Fe-Ni alloy/PA6 PNCs

produced by solution mixing (SM) are promising, compared

with the currently PNCs used in engineering applications, in

which PA6 is reinforced with other nanofillers including clay,

silica and carbon nanotubes. However, the alloy agglomerates

within PA6 matrix, appeared in SEM micrographs of SM

PNCs, implies that the performance of SM composites is still

lower than the ideal one expected based on micromechanical

models and the rule of mixtures.

This study aims at further improving the overall perform-

ance of Fe-Ni alloy/PA6 PNCs by minimizing particle

agglomeration and improving the interfacial interactions

between metallic nanofiller and PA6 matrix. Effective com-

pounding of nanocomposite constituents using a newly modi-

fied master batch technique called ultrasound assisted master

batch (UMB), is the technical route employed for improving

the dispersion state of Fe-Ni nanoparticles in PA6 matrix and

the filler-matrix interfacial interactions. Thus, the powerful

mixing forces applied during compounding by UMB counter-

act the strong agglomeration tendency of the ferromagnetic

Fe-Ni nanoparticles and thereby facilitate breakup of nano-

particle agglomerates, which is essential for the nano-scale

dispersion of the filler within the polymer matrix.

The study has been proceeded as follows. The nc

Fe40Ni60 fine particles were synthesized by chemical

reduction of the corresponding metals’ salts in aqueous

solution. Then the Fe40Ni60/PA6 nanocomposites were

prepared by compounding via UMB followed by injection

molding and their mechanical properties, morphology and

structural parameters were determined. Furthermore, the

performance of the composites made by UMB process is

compared with the performance of the corresponding

composites made in our previous study [7] by solution

mixing (SM), which has great potential but is limited to

lab or medium scale production of PNCs [8].

EXPERIMENTAL

Materials

The chemicals used for the synthesis of nc Fe40Ni60

alloy were nickel chloride hexahydrate (NiCl2.6H2O,

98%) and ferrous chloride (FeCl2, 99%) as sources of

metal ions; hydrazine hydrate (N2H4.H2O, 99%) as a

reducing agent, sodium hydroxide (NaOH, 99%) as a cat-

alyst and distilled water as a solvent. NiCl2.6H2O and

N2H4.H2O were purchased from Panreac Quimica, Spain.

FeCl2 and NaOH were obtained from Spectrum Chemical

MFG. CORP, USA and El Nasr Pharmaceutical, Egypt,

respectively. The polymer used was PA6 pellets

(BX3WQ662[X] (ISO)) kindly supplied in sealed pack-

ages by Custom Resins, Inc. Formic acid (HCO2H, 98%)

and ethanol (C2H5OH, 96%) used as a solvent and a pre-

cipitant for PA6, respectively, were purchased from

VWR.

Synthesis of nc Fe40Ni60 Particles

The synthesis of the nc Fe40Ni60 particles was done

according to the protocol reported in our previous work

[9]. Briefly, FeCl2 and NiCl2.6H2O were dissolved in dis-

tilled water to form aqueous solution of 0.6 M with a

[Fe21]/[Ni21] ratio of 4:6. The solution was vigorously

stirred and heated to 80�C. Then, a second solution of

aqueous hydrazine, N2H4.H2O, (50 wt%), and aqueous

NaOH (0.1 M) was added to it. Fine black particles were

precipitated as a result of the reduction reaction. The

resulting particles were separated magnetically, washed

repeatedly with distilled water until neutral pH and dried

in vacuum at 35�C for two days.

Preparation of Fe40Ni60/PA6 PNCs

The PNCs samples were made in a two-step process:

first the nc Fe40Ni60 particles were compounded with

PA6 pellets followed by injection molding of standard

Izod bars (ASTM D256). A DSM Micro 15 ml extruder,

(vertical, co-rotating twin-screw micro extruder) and 10

ml injection molding machine were used for the fabrica-

tion of PNCs with nc Fe40Ni60 content of 0, 1, 3, and 5

wt%. The conditions used were: 240�C as Tbarrel in the

compounder and injection molding machine, screw speed

of 150 rpm, Tmold of 70� and injection pressure of 0.76

MPa. Before compounding, the PA6 pellets were dried in

a vacuum oven at 80�C for 48 h. After molding, the

specimens were sealed and placed in a desiccator for a

minimum of 24 h before testing. The detailed sample

codes are listed in Table 1.

Compounding by Solution Mixing (SM). PA6 pellets

were dissolved in formic acid at ambient temperature

using magnetic stirring. The polymer concentration in the

solution was 10 wt%. The nc Fe40Ni60 particles were then

added to the polymer solution and the mixture was vigo-

rously stirred for 30 min. Addition of ethanol, with the

ethanol/formic acid ratio being 2:1 vol/vol, initiated the

precipitation of PA6 after 30 min and led to formation of

the composite powder which was completed within 20

min. The composite powder was separated from mixed

2 POLYMER COMPOSITES—2014 DOI 10.1002/pc

solvent through filtration and dried in vacuum oven at

80�C for 48 h.

Compounding by Ultrasound-Assisted Master Batch

(UMB). nc Fe40Ni60 particles were compounded with

half of the PA6 amount via ultrasound-assisted solution

mixing (USM) to prepare concentrated PNCs containing

2, 6, and 10 wt% nc Fe40Ni60. The composite powders

were then compounded (diluted) with the rest of PA6 pel-

lets via melt mixing using the DSM Micro 15cc extruder

to prepare PNCs with 1, 3, and 5 wt% nc Fe40Ni60,

respectively.

Concentrated USM PNCs were prepared as follows.

First, a PA6-formic acid solution of 15 wt% PA6 concen-

tration was prepared. Second, a very dilute solution of

PA6 in formic acid (polymer concentration �3 wt%) was

prepared and nc Fe40Ni60 particles were dispersed and

sonicated into it for 30 min, using ultrasonic bath. The

alloy particles/PA6 ratio was 1:1 wt/wt. The particles sus-

pension was then added to the first polymer solution, so

that the concentration of polymer in the mixture was 10

wt%. After that, the process was continued in the same

manner as in solution mixing to produce concentrated

USM PNCs.

At this point, it is worth mentioning that in the con-

ventional master batch technique, the concentrated com-

posites are usually prepared via melt mixing using a twin

screw extruder [10–12]. This is the first time the solution

mixing route has been employed for the preparation of

the concentrated PNCs, as indicated in the above detailed

UMB process. This enabled incorporating an additional

step in the UMB process as compared with the conven-

tional master batch, helping in improving the interfacial

interactions between polymer and nanofiller. Such step is

the sonication of nc Fe40Ni60 particles in a very dilute

solution of PA6 in formic acid.

Characterization of nc Fe40Ni60 Particles

X-ray diffraction (XRD) was employed to characterize

the crystallographic texture and crystal size of the synthe-

sized Fe-Ni alloy. XRD measurements were carried out

on a Shimadzu XRD-7000 diffractometer (30 kV, 30 mA;

Cu-Ka 1 Ni-filtered radiation, k 5 0.15406 nm). The 2hrange was 30 to 110�, at a scanning rate of 4�/min and a

scanning step of 0.018�. Chemical composition was esti-

mated by an area analysis using energy dispersive X-ray

spectroscopy (EDS) equipped with the scanning electron

microscope (SEM, JEOL, Model JSM 6360 LA). The

morphology and particle size were analyzed by a field

emission scanning electron microscope (Zeiss Ultra-60

FE-SEM), operating at 5 kV. The SEM sample was pre-

pared by dropping a diluted dispersion of Fe-Ni particles

onto a silicon wafer and letting the solvent to evaporate.

The image was captured at a magnification of 340,000.

Characterization of Fe40Ni60/PA6 PNCs

Flexural test was performed using a MTS 810 testing

machine (MTS Systems Corporation), according to

ASTM-D790. Notched Izod impact strength was deter-

mined using a CS-137-083 impact tester (Custom Scien-

tific Instruments, Inc.), according to ASTM-D256. All the

tests were conducted at ambient temperature and the

reported results reflect an average of three measurements.

The degree of agglomeration and distribution of nc

Fe40Ni60 particles in PA6 matrix were assessed through

SEM examination of the impact fracture surface using

Zeiss Ultra-60 FE-SEM operating at accelerating voltage

of 5 kV. The specimens were gold coated using a Desk

IV Sputter/Etch Unit (DENTON Vacuum, LLC). The

images were captured at a magnification of 320,000.

Glass transition temperatures (Tg) of neat PA6 and

PA6 phase in the nanocomposites, were characterized

using dynamic-mechanical analyzer (DMA Q800, TA

instruments). Samples with dimensions of about 35 mm

3 13 mm 3 3 mm were cut from the injection molded

bars and then heated from ambient to 180�C at a heating

rate of 3�C/min. The test was conducted using a single

cantilever mode applying constant strain. The amplitude

of strain was 0.15% and the frequency (ƒ) was 1 Hz.

Modulated differential scanning calorimeter (MDSC

Q200, TA instruments) was used to determine the degree

of crystallinity and analyze the melting and crystallization

behaviors of neat PA6 and PA6 phase in the nanocompo-

sites. Slices had a net weight of about 7 mg were cut

from the injection molded bars. The sliced samples were

heated from ambient temperature to 270�C, held at this

temperature for 3 min to erase the thermal history and

then cooled to 25�C. All MDSC runs were conducted

under nitrogen atmosphere with heating/cooling rates of

5�C/min. The degree of crystallinity, XC, was calculated

from the enthalpy evolved during melting based on the

heating scans, using the following formula [8]:

XC5DHm

ð121ÞDH0m

3100 (1)

where DHm is the apparent enthalpy of melting of sample,

DH0m is the extrapolated value of the enthalpy

TABLE 1. Sample codes.

Sample Code

Solution mixed nanocomposites (SM)

Control PA6 SM0

1 wt% p-Fe40Ni60/PA6 SM1



Ultrasound-assisted master batch nanocomposites (UMB)

Control PA6 UMB0

1 wt% p-Fe40Ni60/PA6 UMB1



DOI 10.1002/pc POLYMER COMPOSITES—2014 3

corresponding to the melting of 100% crystalline pure

PA6, which is taken as 190 J/g [13], and � is weight frac-

tion of nc Fe40Ni60 in the composites.

XRD was employed to determine the relative fraction

of a and g crystalline phases in neat PA6 and PA6 phase

within the PNCs. This is important in interpreting the

mechanical performance of Nylon compounds, because a-

phase is much stronger and less ductile than the g-phase

[14, 15]. XRD patterns were obtained using X’Pert PRO

MPD diffractometer in reflection mode (45 kV, 40 mA;

Cu-Ka 1 Ni-filtered radiation, k 5 0.154 nm). The analy-

sis was performed on the injection molded Izod bars, at

ambient temperature with a 2h range between 8 and 80�,at a scanning rate of 4�/min and a scanning step of

0.016�. XRD traces were de-convolved to diffraction

peaks corresponding to amorphous phase and crystalline

phase which is composed of a and g crystal forms, using

MDI Jade 9 software. The percentage of g-phase with

respect to the total crystalline phase was calculated by:

c %ð Þ5 Ac

Aa1Ac3100 (2)

where Aa is the area under a diffraction peaks and Ac is

the area under c diffraction peaks [16]. In order to check

the influence of high temperature processing on the nano-

structure of nc Fe40Ni60 alloy, crystallite sizes of nc

Fe40Ni60 particles within the PNCs were also determined

using the Scherrer formula [17].

RESULTS AND DISCUSSION

Characteristics of nc Fe40Ni60 Particles

Figure 1 shows the XRD pattern of the chemically

synthesized particles. The four characteristic peaks for the

disordered FCC g-Fe-Ni taenite phase 2h 5 44.05�, 51.3�,75.6�, and 91.5�, marked by the Miller indices (111),

(200), (220), and (311), respectively, are observed [9, 18].

This reveals that the synthesized particles are pure Fe-Ni

alloy. The average crystallite size of the synthesized

alloy, based on the full width at half maximum of the

(111) peak using the Scherrer formula [17], is 11 nm.

According to EDS quantitative analysis the alloy compo-

sition is 36 at.% Fe and 64 at.% Ni, excluding the other

elements like carbon in the supporting film and oxygen in

the oxide layer formed on the particle surface. Addition-

ally, the oxygen content is only 6 at.%, indicating that the

oxide layer formed on particle surface is very thin. The

SEM image, presented in Fig. 2, shows that the produced

Fe40Ni60 alloy is composed of spherical particles with a

mean diameter of about 150 nm. This suggests that each

spherical particle is nanostructured, i.e. it is composed of

several nanocrystallites, as previously reported [9, 18].

Based on XRD, EDS, and FE-SEM results, it is con-

cluded that the chemically synthesized black particles are

nc Fe40Ni60 submicron-sized particles.

Mechanical Properties of Fe40Ni60/PA6 PNCs

Figure 3a and b show the relative flexural modulus

and strength respectively of SM and UMB processed

PNCs to those of the corresponding neat PA6, as a func-

tion of nc Fe40Ni60 content. Regardless of the nanofiller

content, SM and UMB PNCs acquire much higher modu-

lus and strength than the corresponding neat PA6. This

indicates that the intensive mixing applied during these

processes can sufficiently break up the particle agglomer-

ates and provide an adequate distribution of the alloy par-

ticles within the polymer matrix, resulting in good filler-

matrix interactions and thus efficient stress transfer

between the two phases. UMB processing results in larger

property enhancement than SM processing probably

because of better wetting between the nanoparticles and

the PA6 molecules that leads to stronger filler-matrix

FIG. 1. XRD pattern of the chemically synthesized black particles.

FIG. 2. SEM image of the nc Fe40Ni60 alloy particles.


interactions. The ultrasonic waves during sonication of the

nc Fe40Ni60 particles in dilute PA6 solution disperse the

alloy agglomerates into small nanoparticles which are then

wrapped by PA6 molecules and thereby can’t be re-

agglomerated during the remaining UMB processing steps.

It is also noted that feeding a mixture of PA6 pellets

and concentrated nanocomposite powder into the extruder

during the second stage of the UMB process, was much

easier than feeding just the nanocomposite powder result-

ing from the SM process. Therefore efficient mixing and

homogenization of nanocomposite components was

achieved in shorter processing time, �4 min as compared

with �8 min in SM processing. Accordingly, degradation

of PA6 molecules and oxidation of nc Fe40Ni60 particles

during thermal processing, which were noticed in case of

SM processing and have negative effect on the nanocom-

posite performance, were avoided or minimized.

Figure 3 also indicates that the modulus and strength

are enhanced with the increase of nc Fe40Ni60 content

within SM and UMB nanocomposites from 0 to 3 wt%.

However, beyond this content the enhancement of

mechanical properties levels off, probably due to the

increased particle agglomeration and agglomerate size at

higher filler content, as confirmed by SEM micrographs

presented in the next section. Thus the maximum

improvements in flexural properties, calculated according

to Eq. 3, are 35% (modulus) and 26% (strength) for SM

nanocomposites and 65% (modulus) and 50% (strength)

for UMB ones, at 3 wt% nc Fe40Ni60. The results clearly

reveal that the enhancement of flexural properties for

UMB processed PNCs is almost twice that for SM proc-

essed ones, as compared with neat PA6. This proves the

great potential of UMB compounding in intensifying the

filler-matrix interfacial interactions, leading to enhance-

ment of mechanical performance of PNCs.

Property improvement ð%Þ

5Pr of nanocomposite 2Pr of PA 6

Pr of PA 63100

(3)

where Pr is referred to property

The relative impact strength of Fe40Ni60/PA6 PNCs to

that of the corresponding neat PA6, as a function of the

nanofiller loading and compounding method is presented in

Fig. 4. Unlike flexural properties, all SM and UMB proc-

essed PNCs exhibit lower impact strength compared with

that of the corresponding neat PA6. The decrease in impact

strength becomes more pronounced at higher particle load-

ing for SM PNCs. This may be because (i) addition of nc

Fe40Ni60 particles to PA6 boosts its hardness and stiffness

which, in turn, reduce toughness and/or (ii) the increased

particle agglomeration and agglomerate size at higher filler

content, act as crack nucleation sites [19]. In case of UMB

processed PNCs, however, the decline in impact strength is

comparable at all nanofiller loadings, probably due to

improved distribution of particles and presence of fewer

and smaller particle agglomerates as shown by SEM.

Morphology of Fe40Ni60/PA6 PNCs

SEM micrographs of SM and UMB processed PNCs

are presented in Fig. 5a–c and d–f respectively, with the

FIG. 3. Flexural properties of Fe40Ni60/PA6 PNCs as a function of

nanofiller content. (a) Flexural modulus, (b) Flexural strength.

FIG. 4. Impact strength of Fe40Ni60/PA6 PNCs as a function of nano-

filler content.


nc Fe40Ni60 particles indicated by arrows for reasons of

clarity. The number average particle sizes (PS) of nc

Fe40Ni60 particles within the PNCs are indicated, to pro-

vide quantitative assessment of the degree of agglomera-

tion of the alloy particles. The data of PS have been

calculated by individual measurement of about 50

particles from SEM micrographs of each PNC, using

SmartTiff software.

For SM processed PNC, large agglomerates and small

nanoparticles of nc Fe40Ni60 alloy can be clearly observed

in the SEM micrographs. In SM1 and SM3, the number

of agglomerates is few and the particles appear to be

homogeneously distributed within the PA6 matrix. With

the increase of nanofiller content, the degree of agglomer-

ation (number and size of agglomerates) is increased and

the particle distribution within the polymer matrix is

getting worse, as shown in the micrograph of SM5.

Obviously, the particle size of nc Fe40Ni60 particles

within all SM processed PNCs is significantly smaller

than that estimated from the SEM image of the chemi-

cally synthesized alloy powder, shown in Fig. 2. These

observations confirms that the intensive mixing applied

during solution mixing is effective in breaking up the

submicron supermagnetic agglomerates into smaller nano-

particles, improving the nanoparticle distribution within

PA6 matrix and the filler-matrix interactions, leading to

good mechanical properties of SM PNCs. Larger particle

size and worse distribution of particles are the major rea-

sons behind the less pronounced enhancement of the flex-

ural properties and the decline of impact strength at

larger filler content in SM processed PNCs.

In UMB processed PNCs, nc Fe40Ni60 particles are

much smaller than those in the corresponding SM proc-

essed PNCs and appear more uniformly distributed within

FIG. 5. SEM micrographs of Fe40Ni60/PA6 PNCs.


the polymer matrix. The PS of UMB processed PNCs are

approximately one half of those reported for the corre-

sponding SM processed PNCs. Moreover, the particle size

and distribution are not significantly changed with the fil-

ler content; only few agglomerates can be seen in the

micrograph of UMB5. These observations confirm that

the first step in the UMB process, which is the sonication

of nc Fe40Ni60 particles in dilute polymer solution, has a

profound role in controlling the morphology and hence

the mechanical performance of UMB processed PNCs.

Thus, although half of the corresponding PA6 amount is

added via melt compounding in UMB process, the SEM

study reveals better nanoparticle wetting by the polymer

molecules and stronger filler-matrix interactions as com-

pared with the SM processed PNCs. Relatively increased

PS in UMB5, shown in Fig. 5f, contributes to the

decreased flexural properties as compared with UMB

PNCs with lower filler content. This however may not be

the main reason for the observed decreased flexural per-

formance of UMB5.

Structural Analyses of Fe40Ni60/PA6 PNCs

The variation of the loss tangent (tan d) with tempera-

ture, resulted from DMA analysis, as a function of the

nanofiller loading and compounding method is shown in

Fig. 6. The Tg of neat PA6 controls and PA6 matrix in

SM and UMB processed PNCs can be determined from

the peak of these curves. As shown the Tg increases with

nanofiller content regardless of the process used. The

enhancement of Tg is more significant in UMB processed

PNCs; addition of 5 wt% filler results in Tg increase of

13% and 8% in UMB and SM composites compared with

the corresponding PA6 controls. As reported in Ref. 20,

an increase in the Tg of polymer upon addition of a rein-

forcing nanofiller is evidence for good distribution of the

filler within the polymer matrix which causes severe con-

straint on the polymer segmental motion, so that, the tran-

sition from glassy state to rubbery state in the

nanocomposite occurs at higher temperature than in neat

polymer. Accordingly, the Tg larger enhancement in

UMB PNCs is in agreement with the SEM results and

represents another evidence for the more uniform distribu-

tion of nc Fe40Ni60 particles in PA6 matrix when UMB

process is used. The observed restriction of the polymer

chain mobility in the PNCs contributes to the enhance-

ment of flexural properties and the decreased impact

strength compared with the properties of corresponding

neat PA6 controls. However, unlike the SM processed

PNCs, the impact strength of the UMB processed PNCs

remains the same or slightly reduces compared with that

of the corresponding PA6 control indicating the presence

of other counteracting phenomena i.e., crystallization

behavior of PA6 may be affected differently by the differ-

ent processing methods.

The DSC melting and crystallization thermograms

have been used to characterize the degree of crystallinity

(Xc), melting (Tm), and crystallization temperatures (Tc)

of neat PA6 controls and PA6 matrix in SM and UMB

processed PNCs. The data are listed in Table 2. Upon

analyzing the DSC data, it is noticed that all SM and

UMB processed PNCs have significantly variant degree

of crystallinity and higher crystallization temperature, Tc,

of PA6 phase, compared with the corresponding PA6 con-

trol. The increase in Tc indicates that the nc Fe40Ni60 par-

ticles act as effective nucleating agent, thus allowing the

PA6 crystallization to start earlier (at higher temperature

upon cooling from the melt) and inducing the rate of

crystallization leading eventually to increased degree of

crystallinity. It is noted that the nucleating action of the

nanoparticles is competing with the increased restriction

of PA6 chain mobility especially at higher filler content,

as revealed from Tg values, which hinders the PA6 crys-

tallization and tends to decrease the degree of crystallin-

ity. This is because polymer molecules of limited

mobility need long time to assume their lowest energy

conformation and then pack together to form crystals.

These counteracting effects of the nc-Fe40Ni60 particles

are the reason behind the observed trend of the degree of

crystallinity of PA6 phase in the PNCs, i.e. exhibits maxi-

mum at 1 wt% nc-Fe40Ni60 and then decreases as the

nanofiller content increases for SM processed PNCs. The

same behavior is observed in case of UMB processed

PNCs but as the restriction of the PA6 chain mobility is

FIG. 6. Tan d of Fe40Ni60/PA6 PNCs versus temperature.

TABLE 2. DSC data for SM and UMB processed PNCs.

Sample Xc (%) Tm (�C) Tc (�C)

SM0 34 223 195.5

SM1 39.5 223 197.5

SM3 38 223 197

SM5 36 223 197

UMB0 38 221.5 195

UMB1 40 222.5 197

UMB3 39 222.5 197

UMB5 35 222.5 196.5


much more pronounced than in SM ones, the enhance-

ment of crystallinity is lower at low nanofiller contents

and UMB5 exhibits lower crystallinity than the corre-

sponding control PA6 (UMB0). Unlike the degree of

crystallinity and crystallization temperature, no significant

change in the melting temperature, Tm, with the nanofiller

content is observed, revealing that the size and perfection

of PA6 crystals appear unaffected by the increase of

nanofiller content regardless of the processing method

used.

XRD patterns for SM and UMB processed PNCs as a

function of the filler content are presented in Fig. 7. The

peaks around 2h � 20� and 23� are assigned to a1 and a2

crystal planes of PA6, respectively while the peaks

around 2h � 10�, that appears only in the XRD scan of

SM5 and UMB processed PNCs, and 21� are assigned to

g1 and g2 crystal planes of PA6, respectively [21]. The

observed peaks around (2h 5 44�, 51�, 75�) are attributed

to the disordered FCC g-Fe-Ni taenite phase, as men-

tioned before in Characteristics of nc Fe40Ni60 Particles

section. The g-phase (%) and crystallite sizes of nc

Fe40Ni60 in SM and UMB PNCs are listed in Table 3.

The a-crystal form is predominant in both PA6 controls.

For SM PNCs, the relative fraction of g-form to a-form

crystals significantly increases as the nanofiller content

increases and g-form becomes predominant in SM3 and

SM5. Based on previous studies on the crystallization of

clay/PA6 [16] and MWNTs/PA6 nanocomposites [22], it

is speculated that the nc Fe40Ni60 particles constrain the

mobility of PA6 molecules, and as a result, chain packing

into well ordered crystals becomes more difficult than in

neat PA6. Therefore, the less ordered g-crystal form is

favored in the presence of nc Fe40Ni60 particles. In addi-

tion, the larger but still homogeneously distributed nc

Fe40Ni60 particles disturb the PA6 crystals while being

formed during the crystallization process, leading to the

profusion of less ordered g-phase crystals in the PNCs as

compared with neat PA6. In UMB processed PNCs, the

PA6 g-phase (%) increases also with the nanofiller

content but at a lesser extent than in the corresponding

SM processed PNCs. This is probably because the aver-

age size of nc Fe40Ni60 particles in UMB PNCs is much

smaller (41–54 nm) than in SM ones (81–114 nm). So,

the disturbance of crystalline order by nanofiller particles

and consequently crystal transformation from a-phase to

g-phase crystals is lower than in case of SM processed

PNCs. The only exception is UMB3 in which a-phase

crystals are much more favorable and is the dominant

crystal form although the reason behind this is not clear.

Table 3 also shows that the crystallite sizes of nc

Fe40Ni60 particles in both types of PNCs are slightly

larger than that of as synthesized alloy powder indicating

that the nc Fe40Ni60 particles maintain their unique stiff-

ness and strength, as their crystal sizes are within the crit-

ical range (�10–20 nm) [23, 24] even after fabrication of

PNCs. The low Fe40Ni60 crystal growth is due to the ther-

mal energy applied during the processing. The phenom-

enon is more obvious in case of SM process where

feeding of the materials into extruder was very difficult

and slow, as reported earlier, leading to more crystal

growth of nanocrystalline filler. This highlights another

advantage for the UMB process over the SM one. The

UMB process is fluent and smooth and results in PNCs

with remarkable mechanical properties and distinctive

morphology.

Based on DSC and XRD analyses, it is revealed that

the increased degree of crystallinity of PA6 in SM and

UMB processed PNCs except UMB5, is a possible rein-

forcing mechanism that contributes to the increased flex-

ural modulus and strength but lower impact strength of

such PNCs as compared with neat PA6. Damping in the

increased degree of crystallinity and increase of the rela-

tive fraction of g-form to a-form crystals, however, lead

to the reduced rate at which the flexural modulus and

strength are enhanced with the increase of the nanofiller

content in the PNCs regardless of the process used. The

favorable formation of a-phase crystals in UMB3, might

be one reason for the significant enhancement of flexural

properties of this PNC compared with the flexural proper-

ties of the other PNCs studied. The dominant mechanism

responsible for the reduction in the flexural properties in

case of UMB5 as compared with other UMB PNCs,

FIG. 7. XRD patterns of Fe40Ni60/PA6 PNCs.

TABLE 3. XRD data for SM and UMB processed PNCs.

Sample c (%)

Crystallite size of

Fe40Ni60 (nm)

Fe40Ni60 nanoparticles 11

SM0 35.5

SM1 48 15

SM3 63 15

SM5 66 15.5

UMB0 33

UMB1 38 13.5

UMB3 29 13.5

UMB5 55 11.5


seems to be the lower degree of crystallinity, compared

with that of the corresponding PA6 control, and the sig-

nificantly increased g-phase (%).

Finally, the relative mechanical properties of UMB

processed PNCs are compared with those of previously

investigated clay, carbon nanotubes (CNT) and silica rein-

forced PA6 nanocomposites, for evaluating the novel Fe-

Ni alloy/PA6 PNCs made by UMB technique among the

currently existing PA6 PNCs. The data are illustrated in

Table 4. The preparation method of PNCs and whether

the reinforcing nanofiller is pristine (p-) or functionalized

with polar molecules, are indicated. This is because; these

two parameters significantly affect the morphology, crys-

talline structure and therefore the overall performance of

PNCs. Clearly, that the performance of p-Fe40Ni60/PA6

PNCs made by UMB is remarkable compared with those

of other PA6 PNCs, indicating that novel nc Fe-Ni alloy

based polymer nanocomposites are promising and can be

good candidate materials for engineering applications.

CONCLUSIONS

Fe40Ni60/PA6 PNCs were fabricated via a newly modi-

fied master batch technique called ultrasound assisted

master batch (UMB). Their mechanical properties, mor-

phology and structural parameters have been then charac-

terized as a function of the nanofiller content and

compared with the corresponding properties of Fe40Ni60/

PA6 PNCs made in a previous study by solution mixing

(SM).

It was determined that the UMB technique has some

advantages over the SM one. The UMB process can be

scale-up and is compatible with commonly used polymer/

composite processing techniques like melt mixing. It is

time efficient and cost effective, as only half of the

required polymer amount for each PNC composition has

to be compounded with nc Fe40Ni60 via USM, which

saves time and decreases by 50% the amount of solvent

needed compared with the SM process. Feeding of the

materials into the extruder when the UMB compounding

method was used was easier and faster, reducing the cycle

time and limiting the thermal degradation of PA6 and the

oxidation of the nanofiller. In addition, the UMB process

leads to PNCs with enhanced flexural properties com-

pared with both the PA6 control and the PNCs made by

SM process. The impact strength remains the same as

that of the PA6 control in case of UMB process whereas

it reduces significantly when SM is used.

TABLE 4. Relative mechanical properties of Fe40Ni60/PA6 PNCs made by UMB and previously studied PA6 nanocomposites.

Nanofiller Ref. no Nanofiller

Filler

content

(wt%)

Composite

preparation

method

Relative

flexural

modulus

Relative

flexural

strength

Relative

impact

strength

Clay/PA6

nanocomposites

[25] Commercial fluoromica/

PA6 nanocomposite

from Unitika of Japan

(M1030D)

5 Injection molding – 1.45 0.98

[26] Organophilic montmoril-

lonite; Cloisite 30B;

modifier concentration

of 90 meq/100 g clay

1; 3; 5 Direct melt mixing

followed by injection

molding

1.08; 1.11;

1.21

1.17; 1.21;

1.27

0.98; 0.99;

1.01

[27] Sondium Saponite; cation

exchange capacity of

71.2 meq/100 g clay

2.5; 5 In situ polymerization

followed by compres-

sion molding

1.13; 1.24 – 0.98; 0.95

[28] 12-Aminododecanoic

acid-treated

montmorillonite

5 In situ polymerization


molding

2.24 1.6 0.35

CNTs/PA6

nanocomposites

[29] Multiwalled CNTs

(p-MWNTs)

1; 2; 4 Master batch technique 1.08; 1.12;

1.19

1.08; 1.09;

1.16

–

[30] Single-walled CNTs

(p-SWNTs)

1 Direct melt mixing


molding

– 1.03 1.03

Silica/PA6

nanocomposite

[31] p-Nanosilica 4.3 In situ polymerization


molding

– – 0.5

g-glycidoxypropyltrime-

thoxysilane treated

nanosilica

4.3 – – 1.26

[32] Pristine nanosilica 1; 3; 5 Ultrasound assisted in-

situ polymerization


molding

– – 1.05; 1.16;

0.9

p-nc Fe40Ni60

particles

p-nc Fe40Ni60 particles 1; 3; 5 Ultrasound assisted mas-

ter batch followed by

injection molding

1.43; 1.65;

1.41

1.35; 1.50;

1.33

0.91; 0.95;

0.95


The most important new finding of this study is that

the sonication of nanofiller aggregates in dilute polymer

solution during compounding of nanocomposite constitu-

ents, the first step in UMB process, has a great potential

in improving the overall performance of the PNCs.

According to the SEM study, the applied power ultrasonic

waves created good distributed tiny filler nanoparticles

within nanocomposites stabilized by polymer molecules

against re-agglomeration, leading to good nanoparticle

wetting with polymer molecules and strong filler-matrix

interactions. In addition, deep interpretations for the alter-

ation of PA6 structural parameters, Tg, degree of crystal-

linity and ratio of g-form to a-form crystals, with the

alloy content within PNCs and the compounding process,

which have not been previously reported in a single

study, are provided throughout the article. Finally, the

study revealed that there are four competing factors deter-

mining the overall performance of PNCs regardless of the

compounding process used: (i) degree of agglomeration

and particle distribution within PA6 matrix and physical

structural changes that occurred in PA6 due to presence

of alloy particles including (ii) increased Tg; (iii) variant

degree of crystallinity; and (iv) ratio of g-form to a-form

crystals of PA6.

It is concluded that the newly modified master batch,

UMB, technique is promising for commercial production

of nc Fe40Ni60/PA6 PNCs. In addition, the 3 wt% UMB

processed PNC, UMB3, was found to be the composite

with the best overall mechanical performance. Thus, com-

pared with Fe40Ni60/PA6 nanocomposites made by SM

and currently utilized PA6 nanocomposites in engineering

applications, in which PA6 is reinforced with other nano-

fillers, UMB3 can be a good candidate material for engi-

neering applications.

ACKNOWLEDGMENTS

Special thanks to Custom Resins, Inc, USA for supplying

PA6, and Prof. Satish Kumar, School of Materials Science

and Engineering at Georgia Institute of Technology, for

facilitating DSC analysis. The authors thank Dr. Mehdi Kar-

van and Dr. Md. Bhuiyan from Georgia Institute of Technol-

ogy, for their kind assistance throughout the experimental

work.

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