RESEARCH PAPER

Effects of iron oxide nanoparticles on polyvinyl alcohol:interfacial layer and bulk nanocomposites thin film

Zhanhu Guo • Di Zhang • Suying Wei • Zhe Wang • Amar B. Karki •

Yuehao Li • Paul Bernazzani • David. P. Young • J. A. Gomes •

David L. Cocke • Thomas C. Ho

Received: 9 July 2009 / Accepted: 3 November 2009 / Published online: 19 November 2009

� Springer Science+Business Media B.V. 2009

Abstract Iron oxide (a-phase) nanoparticles with

coercivity larger than 300 Oe have been fabricated at

a mild temperature by an environmentally benign

method. The economic sodium chloride has been

found to effectively serve as a solid spacer to disperse

the iron precursor and to prevent the nanoparticles

from agglomeration. Higher ratios of sodium chloride

to iron nitrate result in smaller nanoparticles (19 nm

for 20:1 and 14 nm for 50:1). The presence of

polyvinyl alcohol (PVA) limits the particle growth

(15 nm for 20:1 and 13 nm for 50:1) and favors

nanoparticle dispersion in polymer matrices. Obvious

physicochemical property changes have been

observed with PVA attached to the nanoparticle

surface. With PVA attached to the nanoparticle

surface, the nanoparticles are found not only to

increase the PVA cross-linking with an increase in

melting temperature but also to enhance the thermal

stability of the PVA. The nanoparticles are observed

to be uniformly dispersed in the polymer matrix.

Scanning electron microscopy (SEM) microstructure

also shows an intermediate phase with a strong

interaction between the nanoparticles and the poly-

mer matrices, arising from the hydrogen bonding

between the PVA and hydroxyl groups on the

nanoparticle surface. The addition of nanoparticles

favors the cross-linkage of the bulk PVA matrices,

resulting in a higher melting temperature, and an

enhanced thermal stability of the polymer matrix.

Keywords Nanoparticles �Polymer nanocomposites � Thermal properties

Introduction

Antiferromagnetic a-Fe2O3 (hematite) nanoparticles

(NPs) have attracted much interest due to their wide

potential applications in many areas, such as pigments

(Feldmann 2001), catalysts (Glisenti 1998; Ren et al.

2009; Rofer-Depoorter 1981; Xie et al. 2009; Zhang

et al. 2009; Zheng et al. 2007), photocatalysts

(Kay et al. 2006), gas sensors (Fukazawa et al.

1993; Huo et al. 2005; Neri et al. 2002; Wang et al.

Z. Guo (&) � D. Zhang � Y. Li � J. A. Gomes �D. L. Cocke � T. C. Ho

Integrated Composites Laboratory (ICL), Dan F. Smith

Department of Chemical Engineering, Lamar University,

Beaumont, TX 77710, USA

e-mail: zhanhu.guo@lamar.edu

S. Wei � P. Bernazzani

Department of Chemistry and Physics, Lamar University,

Beaumont, TX 77710, USA

Z. Wang

Mechanical and Aerospace Engineering Department,

University of California Los Angeles, Los Angeles,

CA 90095, USA

A. B. Karki � David. P. Young

Department of Physics and Astronomy, Louisiana State

University, Baton Rouge, LA 70803, USA

123

J Nanopart Res (2010) 12:2415–2426

DOI 10.1007/s11051-009-9802-z

2008), nonlinear optics (Yu et al. 2000), reinforcing

nano-fillers for multifunctional polymer nanocompos-

ites (Guo et al. 2008a, 2009a, b; Zhang et al. 2009),

and lithium ion batteries (Wu et al. 2006, 2008). In

addition, the small band gap (*2.1 eV), high-corro-

sion resistance, and low cost make iron oxide

nanoparticles suitable for serving as photoelectrodes

in solar energy conversions (Ohmori et al. 2000).

Various synthetic methods have been developed

for the preparation of hematite nanoparticles includ-

ing chemical precipitation (Glisenti 1998), hydro-

thermal reaction (Chen et al. 2002; Dhage et al. 2002;

Li et al. 1998, 2007; Wang et al. 2004), sol–gel

method (Dong and Zhu 2002; Huo et al. 2005),

solvothermal method (Zheng et al. 2007), ball milling

process (Bercoff et al. 2007; Wang and Jiang 2007),

and microemulsion technique (Nassar and Husein

2006; Santra et al. 2001). Iron oxides have strong

magnetic properties and aggregate/agglomerate spon-

taneously, resulting in nanoparticles with a wide size

distribution. In order to prevent nanoparticle agglom-

eration, various surfactants, polymers, or coupling

agents had been used. Unfortunately, organic sol-

vents, bases, and most of the surfactants are harmful

to the environment and expensive for the above

reported methods, especially, the wet chemical fab-

rications. In addition, most of the traditional methods

are conducted on a small batch scale, and are not

suitable for large-scale fabrication. Thermal decom-

position of iron nitrate has been investigated by

thermogravimetric analysis (TGA), and a-Fe2O3 was

reported (Wieczorek-Ciurowa and Kozak 1999).

However, there are no reports in the literature

regarding the nanostructural formation of a-Fe2O3

from decomposition, especially in the presence of an

inert salt. The development of a facile and economic

method for large-scale fabrication of hematite nano-

particles still remains a challenge.

One of the greatest challenges in fabricating high-

quality polymer nanocomposites is the lack of the

synergy between the polymer matrices and the

nanofillers. In principle, the polymers could serve

as a template for nanoparticle fabrication. The

nanoparticles can introduce unique physicochemical

properties into the polymer matrices, such as mag-

netism, non-linear optical properties, and improved

mechanical properties. However, the nanoparticle

materials could also have a deleterious effect on the

polymer matrix.

The effect of the nanoparticles on the polymers

attached to their surface, and on the bulk polymer

itself, has not been systematically investigated. Such

effects will play an important role in controlling the

quality and properties of the fabricated polymer

nanocomposites. In this article, a facile, economic,

and solvent-free method utilizing a mild annealing

process was developed to fabricate crystalline iron

oxide nanoparticles. Sodium chloride, which is envi-

ronmentally benign and easy to remove, was used as

the template/spacer to prevent the nanoparticles from

agglomerating. The biodegradable and water soluble

material, polyvinyl alcohol (PVA), was chosen as the

binding polymer. The ratio of the iron precursor to

sodium chloride and the presence of PVA in the

purifying solution were investigated for particle

growth. The effect of the nanoparticles on the phys-

icochemical properties of PVA was also explored, as

the PVA was chemically bound to or physically

entangled with the nanoparticle surface. The nano-

composites consisting of the PVA matrix, reinforced

with the fabricated Fe2O3 nanoparticles, were fabri-

cated by the drop casting method. The effects of

nanoparticles on the PVA, both on the nanoparticle

surface and on the bulk polymer, were investigated by

various analytical methods and discussed.

Experiment

Materials

Iron nitrite (Fe(NO3)3�9H2O, Alfa Aesar Company,

404.15 g/mol) and sodium chloride (NaCl, Alfa

Aesar Company) are used as the iron precursor and

the solid stabilizer to disperse the iron nitrate and

prevent the products from agglomerating.

Polyvinyl alcohol (PVA, MW = 88000–96800,

degree of polymerization = 2000–2200) was used

as the polymer nanocomposite matrix and the stabi-

lizer to prevent nanoparticle agglomeration. All the

chemicals were used as-received without any further

treatment.

Nanoparticle fabrication

Iron nitrate and sodium chloride were dissolved in de-

ionized water with a molar ratio of 1:50 and 1:20,

respectively. The homogeneous solid solution was

2416 J Nanopart Res (2010) 12:2415–2426

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obtained by drying at 70 �C under magnetic stirring.

The NPs were manufactured by annealing the solid

salt at 700 �C in a tube furnace for about 30 min and

cooled naturally to room temperature. The product

was washed thoroughly with de-ionized water to

remove sodium chloride and dried completely at

60 �C. In order to prevent particle agglomeration and

modify the surface chemistry of the NPs, 1 wt% PVA

aqueous solution was used to treat the NPs immedi-

ately after annealing. The particles were then washed

thoroughly with de-ionized water to remove NaCl.

Nanocomposite fabrication

The as-produced or functionalized Fe2O3 NPs with

specific amount were dispersed in 5 wt% PVA

aqueous solution (20 mL) for polymer nanocompos-

ite fabrication. The aqueous iron oxide nanoparticle

PVA solution was cast in a petri dish and dried

completely at 60 �C. It was found that the PVA-

treated iron oxide nanoparticles have a better disper-

sion quality than those washed with only de-ionized

water. Thus, only PVA-treated iron oxide nanoparti-

cles with an iron nitrate to sodium chloride ratio of

1:20 were used for PVA–Fe2O3 nanocomposite

fabrication and property characterization.

Characterization

The thermal decomposition of iron nitrate was

determined by TGA from 25 to 1000 �C with an air

flow rate of 50 cm3/min (ccpm) and a heating rate of

20 �C/min.

The phase structure of the produced iron oxide

nanoparticles was investigated by powder X-ray

diffraction. The powder X-ray diffraction analysis

of the samples was carried out with a Bruker AXS D8

Discover diffractometer with General Area Detector

Diffraction System (GADDS) operating with a Cu Karadiation source filtered with a graphite monochro-

mator (k = 1.5406 A). The detector used was a HI-

STAR two-dimensional multi-wire area detector. The

samples were loaded onto double sided scotch tape,

placed on a glass slide, and mounted on a quarter-

circle Eulerian cradle (Huber) on a XYZ stage. The

X-ray beam was generated at 40 kV and 40 mA

power, and was collimated to about 800 lm spot size

on the sample. The incident x angle was 5�. A laser/

video system was used to ensure the alignment of the

sample position on the instrument center. XRD scans

were recorded from 7 to 77� for 2h with a 0.050�step-width and a 60 s counting time for each step.

The XRD data were analyzed using the DIFFRAC-

Plus EVA program (Bruker AXS, Karlsruhe, Ger-

many), and the patterns were identified using the

ICDD PDFMaint computer reference database.

Fourier transform infrared spectroscopy (FT-IR, a

Bruker Inc. Tensor 27 FT-IR spectrometer with

hyperion 1000 ATR microscopy accessory) was used

to characterize the surface chemistry of the nanopar-

ticles, as well as to characterize their interaction with

the polymer matrix.

Scanning electron microscopy (SEM, JEOL field

emission scanning electron microscope, JSM-6700F)

was used to investigate the fabricated nanoparticles

and its dispersion in the polymer matrix. A thin gold

layer was deposited to improve the electrical con-

ductivity for better imaging.

Magnetic properties of the nanoparticles at room

temperature were investigated in a 9 T physical

properties measurement system (PPMS) by Quantum

Design.

The differential scanning calorimetry (DSC) tem-

perature and heat flow values were calibrated with an

indium standard. The heating and cooling rate was

10 �C/min, and the experimental was performed in a

continuous flow of nitrogen gas with a flow rate of

20 cm3/min (ccpm). The temperature was in the

range of 25–250 �C.

Results and discussion

Nanoparticle fabrication and property

characterization

Figure 1a and b shows the thermal decomposition

study of iron nitrate under TGA with the temperature

changing from 20 to 800 �C, and isothermal decom-

position at 700 �C, respectively. The TGA was

carried out in air with a flow rate of 40 cm3/min.

Iron nitrate was observed to decompose in both cases.

A total of 60.2% was observed to lose sharply due to

the decomposition of the iron nitrate and additional

8.0% loss at higher temperatures was due to the loss

of the physicochemical adsorption of water. The total

weight residue in both cases is about 19.4 and 19.7%,

respectively. This is almost equivalent to the

J Nanopart Res (2010) 12:2415–2426 2417

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theoretical calculation (19.80%) of iron oxide (a-

Fe2O3) from iron nitrate. The residue after each TGA

run was observed to be stable at 400 �C during the

temperature sweep. The thermal decomposition of

iron nitrate in the isothermal (700 �C) reaction was

completed within 5 min. The iron oxide nanoparticle

fabrications were carried out at 700 �C with a

reaction time of 1 h. Sodium chloride was chosen

as a spacer to disperse iron nitrate and also as a

stabilizer to prevent agglomeration of the formed

nanoparticles.

Figure 2 shows the X-ray diffraction (XRD)

patterns of iron oxide nanoparticles fabricated with

different ratios of iron nitrate to sodium chloride (20

and 50) and treated with de-ionized water or 1 wt%

PVA aqueous solution, respectively. Although, the

peak width varies, all the samples have common peak

at 24.2�, 33.4�, 35.7�, 41.1�, 49.8�, 54.4�, 62.9�,

64.5�, and 72.5�. The d-spacing is calculated using

Bragg’s law (Rudel and Zite-Ferenczy 1971):

d ¼ n� k2 sin h

ð1Þ

where n is chosen as 1 and k is 1.5406 A for the

wavelength of Cu Ka radiation.

The calculated d-spacings are 3.67, 2.68, 2.51,

2.19, 1.83, 1.68, 1.48, 1.44, and 1.30 A, respectively.

These d-spacings are in agreement with those of the

XRD PDF card #33-0664 (standard XRD pattern of

a-Fe2O3). This indicates that the formed dark reddish

brown iron oxide nanoparticles are a-Fe2O3. The

corresponding crystal planes are (0, 1, 2), (1, 0, 4),

Fig. 1 a Thermal

decomposition study of iron

nitrate and b isothermal

thermal decomposition of

iron nitrate at 700 �C under

air condition

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(1, 1, 0), (1, 1, 3), (0, 2, 4), (1, 1, 6), (2, 1, 4), (3, 0, 0),

and (1, 0, 10), respectively.

The average particle size (L) was estimated from

the Debye–Scherrer equation (Eq. 2; Talapin et al.

2001a):

L ¼ Kkb 2hð Þ cos h

ð2Þ

where b(2h) is the full width at half-maximum

(FWHM), K is a constant taken as the normal value

of 0.9, k is the wavelength of X-ray wavelength (for

copper, k = 1.5406 A), and h is the Bragg angle. The

(1, 1, 0) crystal plane at 2h = 35.7� was used to

estimate the particle size. The calculated values are

about 19, 15, 14 and 13 nm, respectively. The

particle sizes are observed to decrease with an

increase in the ratio of sodium chloride to iron

nitrite. The solution containing PVA for salt removal

was also observed to inhibit the particle growth by

serving as a stabilizer. The presence of PVA has

effectively prevented the Ostwald ripening process

(Guo et al. 2006a; Talapin et al. 2001b) and favors

smaller nanoparticle fabrication.

Figure 3 shows the FT-IR spectra of the as-

produced a-Fe2O3 nanoparticles and those function-

alized with PVA. The characteristic hydrogen-bonded

stretch bands (3,259 and 1,645 cm-1; Matveev et al.

2005; Okuno et al. 2003; Ping et al. 2001), C–H

stretch bands (1,373 and 1,319 cm-1; Carrillo et al.

2004), and C–O bands (1,142 and 1,088 cm-1;

Carrillo et al. 2004; Okuno et al. 2003) are observed

in the nanoparticles washed with PVA aqueous

solution. The absorption band at 1,142 cm-1 has

been used as an assessment tool of the PVA structure

due to the semi-crystalline nature of the synthetic

polymer (Mansur et al. 2004). Thus, the 1,142 cm-1

vibration band observed in the spectra of the nano-

particles treated with PVA aqueous solution, clearly

Fig. 2 XRD patterns of

iron oxide nanoparticles

formed (a) with a ratio of

iron nitrate to sodium

chloride (1:20) and washed

with de-ionized water; (b)

with a ratio of iron nitrate to

sodium chloride (1:20) and

treated with PVA aqueous

solution; (c) with a ratio of

iron nitrate to sodium

chloride (1:50) and washed

with de-ionized water; and

(d) with a ratio of iron

nitrate to sodium chloride

(1:50) and washed with

PVA aqueous solution

Fig. 3 FT-IR spectra of iron oxide nanoparticles formed (a)

with a ratio of iron nitrate to sodium chloride (1:20) and

washed with de-ionized water; (b) with a ratio of iron nitrate to

sodium chloride (1:20) and treated with PVA aqueous solution;

(c) with a ratio of iron nitrate to sodium chloride (1:50) and

washed with de-ionized water; and (d) with a ratio of iron

nitrate to sodium chloride (1:50) and washed with PVA

aqueous solution

J Nanopart Res (2010) 12:2415–2426 2419

123

indicates the existence of PVA. The absorption bands

at 2,949 and 2,904 cm-1 represent the sp3 bonding in

–CH3 group.(Deschenaux et al. 1999) The peaks

around 831 cm-1 are attributed to C–H bending

(Mansur et al. 2004). No C–H group peak was

observed in the spectra of the nanoparticles washed

with de-ionized water, and the obvious hydroxyl

groups indicate the presence of physically absorbed

moisture or chemically hydrolyzed water, which

make the nanoparticles compatible with PVA, and

are responsible for the subsequent uniform particle

dispersion in the PVA nanocomposite fabrication.

Figure 4 shows the TGA curves of the nanopar-

ticles treated with de-ionized water and with the PVA

aqueous solution, respectively. The small weight

percentage loss in the iron oxide nanoparticles is due

to the evaporation of the moisture or the condensation

of the hydroxyl groups. The initial decomposition

temperature (sharp weight loss) was observed to

increase from 233 to 302 �C, when the ratio of iron

nitrite to sodium chloride increased from 20 to 50,

which is due to the smaller particle size in the latter

case. The stronger interaction between the iron oxide

nanoparticles and the PVA matrix, arising from the

higher specific surface area inherent with the smaller

nanoparticles improves the thermal stability of the

entangled polymer chains.

The interaction between the nanoparticles and the

attached polymer, i.e., the effect of the nanoparticles

on the thermal properties of the PVA, was further

characterized with a differential scanning calorimeter

(DSC), which is sensitive to the thermal history of the

samples. The first heating and cooling run of the DSC

at a rate of 10 K/min and a nitrogen flow rate of

10 ccpm was used to characterize the thermal prop-

erties of all samples. The heating run was used to

obtain the melting temperature and melting enthalpy,

while the cooling run was used to characterize the

crystallization and fusion energy. The results are

presented in Fig. 5 and summarized in Table 1. The

thermograms show two endothermic peaks. The first

one at lower temperature is due to the melting of the

crystallites of the cross-linked network, and the

second peak in the range of 200–240 �C is attributed

to the melting of the PVA upon recrystallization

(Agrawal and Awadhia 2004). The melting temper-

ature arising from cross-linking decreased from

124 �C (pure PVA) to 88 �C (PVA attached to

nanoparticles fabricated with a ratio of 1:20) and

further down to 80 �C (PVA attached to nanoparticles

fabricated with a ratio of 1:50). This observation

indicates that the presence of the iron oxide nano-

particles prohibits the extent of the cross-linking

during the formation of solid PVA from the aqueous

polymer solution. The melting temperature upon

recrystallization decreased from 227.5 to 224.2 �C

after the PVA powders were processed into a thin

film structure by the aqueous drop casting. The

melting temperature of the PVA increased from 224.2

to 229 �C and 231 �C after it was attached to the iron

Fig. 4 TGA curves of (a)

pure PVA, and iron oxide

nanoparticles washed with

(a) water, and (c) 1 wt%

PVA, respectively (ratio of

iron nitrate to sodium

chloride 1:20)

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oxide nanoparticle surface fabricated with a ratio of

iron nitrate to sodium chloride of 1:20 and 1:50,

respectively. This observation is consistent with the

enhanced thermal stability with a higher initial

decomposition temperature (Fig. 5), and it is different

from the PVA in the hydroxyapatite (Hap)–PVA

composite system (Kim et al. 2008).

The melting enthalpy (DHm, based on the weight

of pure polymer rather than the total weight of

polymer nanocomposite) was observed to increase

significantly from 38.93 to 85.75 J/g and 113.3 J/g,

which is consistent with the nanocomposites com-

prised of silver nanoparticles reinforced with PVA

(Mbhele et al. 2003). This is due to the reduced

mobility of the polymer chains after being attached to

the nanoparticle surface. The dramatic difference in

melting temperature between the pure PVA and the

PVA attached to the nanoparticle surface indicates a

strong interaction between the PVA and the iron

oxide nanoparticles. The nanoparticle size was also

observed to have a significant effect on the melting

temperature and the melting enthalpy.

The degree of the relative crystallinity (Xc; Kim

et al. 2008; Peppas and Merrill 1976) of the PVA

attached on the nanoparticle surface was estimated

from the endothermic area (from cooling stage) using

Eq. 3:

Xc ¼DHf

DH0f

ð3Þ

where DHf is the measured enthalpy of fusion from

the DSC thermogram (based on the weight of the

pure polymer rather than the total weight of the

polymer nanocomposites), and DH0f is the enthalpy of

fusion of 100% crystalline PVA (DHm is 138.6 J/g;

Peppas and Merrill 1976). The crystallinity of PVA

decreased after the PVA powder was drop-casted into

the thin film structure. However, the crystallinity

Fig. 5 DSC thermograms

of (a) pure PVA thin film,

and PVA attached to iron

oxide nanoparticles

fabricated with a ratio of

iron nitrite to sodium

chloride of (b) 1:20 and

(c)1:50, respectively

Table 1 Thermal properties and calculated crystallinity of pure PVA and bounded PVA

Tm (�C) Tc (�C) DHm (J/g) DHf (J/g) Crystallinity (%)

PVA powder 227.5 198.6 52.43 43.08 37.8

PVA film 224.2 201.7 38.93 41.11 28.1

PVA-NPsa 229.0 204.6 85.75 49.69 35.8

PVA-NPsb 231.3 191.4 113.3 53.44 38.5

a PVA on the surface of the nanoparticle surface, fabricated with a ratio of 1:20 (Na:Fe)b PVA on the surface of the nanoparticle surface, fabricated with a ratio of 1:50 (Na:Fe)

J Nanopart Res (2010) 12:2415–2426 2421

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increased after PVA physicochemically bonded to the

nanoparticle surface.

Figure 6 shows the magnetic properties of the

nanoparticles fabricated with different ratios of iron

nitrate to sodium chloride. The saturation magneti-

zation (Ms) was not reached even at a high field of 9

Tesla. Ms was determined by the extrapolated satu-

ration magnetization obtained from the intercept of

magnetization versus H-1 at high field (Chen et al.

1994; Guo et al. 2006a). The calculated saturation

magnetization of the nanoparticles fabricated with the

ratio of iron nitrate to sodium chloride (1:50 and

1:20) was 3.3 and 3.0 emu/g, respectively. The

coercivity (coercive force, Hc) is 350 and 300 Oe

for the nanoparticles fabricated with the ratio of iron

nitrate to sodium chloride 1:20 and 1:50, respec-

tively. The difference is due to the particle size effect.

The coercivity is smaller than that (784.5 Oe) of the

a-Fe2O3 nanoparticles dispersed in a polystyrene

matrix (Zhang et al. 1989) and that (2,279.0 Oe) of

the cantaloupe-like Fe2O3 (Zhu et al. 2007). This

could be due to the morphological (shape and size)

difference (Zheng et al. 2007). The saturation mag-

netization (Ms) is much lower than that of c-Fe2O3

(74 emu/g; Guo et al. 2008a), and that of metallic

iron (218 emu/g; Guo et al. 2008b). However, it is

similar to the Ms observed in other nanoparticles with

different morphologies (Zheng et al. 2007; Zhu et al.

2007).

Polymer nanocomposites

Figure 7 shows the SEM microstructure of the PVA

nanocomposites reinforced with a particle loading of

10 wt%. The nanoparticles were observed to be

uniformly dispersed in the polymer matrix without

obvious agglomeration. No cracks or voids were

evident in the polymer nanocomposites. The high-

resolution SEM image (inset of Fig. 7) shows an

interface between a nanoparticle and the polymer

matrix, which is similar to that observed in nano-

composites of alumina nanoparticles reinforced vinyl

ester resin (Guo et al. 2006b). The fabricated

nanoparticles are surrounded by hydroxyl groups,

which form hydrogen bonding with the PVA, and

thus, lead to void- and crack-free structural polymer

nanocomposites. These strong hydrogen bonds are

responsible for the strong interaction between the

nanoparticles and the polymer matrix.

Figure 8 shows the TGA curves of the pure PVA

and its nanocomposites with a nanoparticle loading of

1, 6, and 10 wt%, respectively. The weight loss at

lower temperature (lower than 100 �C for pure PVA

and 116 �C for PVA reinforced with iron oxide

nanoparticles) is due to the evaporation of adsorbed

moisture. The loss at higher temperature is due to the

dehydration of the hydrogen bonding between the

PVA and the hydroxy groups in the nanoparticles.

The weight loss in the medium temperature range

Fig. 6 Magnetic hysteresis

loop (M–H) of iron oxide

nanoparticles fabricated

with ratios of iron nitrate to

sodium chloride (1:50 and

1:20); left inset shows the

enlarged hysteresis loop at

low field, the right inset

shows the magnetization

over H-1 for Ms calculation

2422 J Nanopart Res (2010) 12:2415–2426

123

(120–247 �C) is due to the condensation of the

hydroxyl groups. The sharp weight loss at tempera-

tures higher than 247 �C is due to the thermal

decomposition of the PVA. A plateau in the range of

C and D was observed in the polymer nanocompos-

ites, which is due to the degradation and carboniza-

tion of the PVA. The formed carbon further reacted

with iron oxide. In other words, the addition of the

iron oxide nanoparticles favors the thermal decom-

position of the PVA. A similar results is observed

with CuO nanoparticles (Guo et al. 2007) and Fe2O3

nanoparticles (Guo et al. 2008a), but different from

Al2O3 nanoparticles (Guo et al. 2006b) reinforced

vinyl ester resin nanocomposites.

Figure 9 shows the XRD patterns of the pure PVA

and its nanocomposites reinforced with different iron

oxide nanoparticle loadings. The peaks at 19.5� and

40.5� were observed in the pure PVA. The first peak

has a d-spacing of 0.454 nm, corresponding to the

typical doublet reflection of the (1, 0, 1) and (1, 0,

-1) planes of the semicrystalline atactic PVA. The

second peak is assigned to the (2, 2, 0) plane of the

PVA (Kim et al. 2008; Ricciardi et al. 2004). The

peak corresponding to the (1, 0, 1) plane of the PVA

becomes narrower and narrower as a function of the

particle loading. This indicates that the crystallite

sizes along the (1, 0, 1) lattice direction, as dictated

by Eq. 2, became larger with the addition of more

iron oxide nanoparticles. This suggests a higher

crystallinity of the PVA, which is in contrast to

hydroxyapatite (Hap) nanoparticles reinforced with

PVA (Kim et al. 2008). The presence of iron oxide

nanoparticles favors the recrystallization of PVA

during the polymer solution solidification process.

Concurrently, the standard Fe2O3 reflection peaks are

Fig. 7 SEM microstructures of the polymer nanocomposites

with a particle loading of 10 wt% (the inset shows the high

resolution of SEM)

Fig. 8 TGA curves of pure PVA and PVA nanocomposites

with nanoparticle loading of 1, 6, and 10 wt%, respectively

Fig. 9 XRD patterns of pure (a) PVA and PVA nanocompos-

ites with an iron oxide nanoparticle loading of (b) 1 wt%, (c) 6

wt%, and (d) 10 wt%

J Nanopart Res (2010) 12:2415–2426 2423

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observed, similar to Fig. 2. The difference in XRD

spectra between the iron oxide nanoparticles and the

polymer nanocomposites is evident in Fig. 9. The

iron oxide peak became broader, indicating a uniform

dispersion of nanoparticles, consistent with the SEM

observation (Fig. 8).

In order to investigate the effect of nanoparticle

loading on the thermal properties of the bulk polymer

matrices, such as melting and crystallization (forma-

tion of the crystalline structure), DSC measurements

were run, and the results are presented in Fig. 10 and

summarized in Table 2. The lower melting temper-

ature (123 �C) of the PVA nanocomposites with

1 wt% nanoparticle loading, as compared to the pure

PVA thin film, decreased slightly and then increased

to 161.5 �C for a particle loading of 6 wt% and

151.9 �C for a particle loading of 10 wt%. The higher

melting temperature decreased with the increase in

nanoparticle loading. Similarly, the melting temper-

ature and the melting enthalpy of the PVA matrices

decreased with the increase of the nanoparticle

loading, which is in stark contrast to the results when

the PVA is attached to the nanoparticle surface

(Fig. 5, Table 1). This behavior is similar to what

was observed in nanocomposites comprised of silver

nanoparticles embedded in PVA (Mbhele et al. 2003).

Conclusions

An environmentally benign method was developed to

fabricate iron oxide (a-phase) nanoparticles. Inexpen-

sive sodium chloride effectively serves as a solid

spacer to disperse the salt and prevent the iron oxide

nanoparticles from agglomerating. The nanoparticle

size increased with the increase of the ratio of iron

nitrate to sodium chloride. The presence of PVA in the

solution limits the particle size and favors a more

uniform particle-dispersed polymer nanocomposite

fabrication. Obvious changes of structural phase and

thermal properties, such as low-temperature cross-

linking, high-temperature recrystallization, melting

enthalpy, fusion enthalpy, and crystallinity have been

observed once the PVA was attached to the nanopar-

ticle surface. The presence of the nanoparticles

increased the cross-linking with an increase in the

lower melting temperature. The thermal stability was

enhanced when PVA was attached to the nanoparticle

surface. The nanoparticles fabricated showed that a

coercivity of 350 Oe and a lower saturation magneti-

zation. The nanoparticles were uniformly dispersed in

the polymer matrix as shown in the SEM microstruc-

ture analysis and the XRD pattern variation. SEM

images also indicated an intermediate phase, suggest-

ing a strong interaction between the nanoparticles and

the polymer matrix. In summary, the addition of

nanoparticles favored the cross-linkage of the PVA,

resulting in an increase in the lower temperature

melting point, and thus a subsequent improvement in

the thermal stability of the nanocomposite.

Fig. 10 DSC thermograms of (a) pure PVA thin film, PVA

nanocomposites with a particle loading of (b) 1 wt%, (c) 6

wt%, and (d) 10 wt% (Exo, up)

Table 2 Thermal properties

and calculated crystallinity of

pure PVA and polymer

nanocomposites

Tm (�C) Tc (�C) DHm (J/g) DHf (J/g) Crystallinity

(%)

PVA powder 227.5 198.6 52.43 43.08 37.8

Pure PVA film 224.2 201.7 38.93 41.11 28.1

1 wt% 227.3 203.0 43.16 44.72 32.3

6 wt% 223.6 201.1 31.91 44.55 32.1

10 wt% 221.8 193.8 23.53 39.27 28.3

2424 J Nanopart Res (2010) 12:2415–2426

123

Acknowledgments The project is partially supported by the

start-up grant and research enhancement grant from Lamar

University. The authors kindly acknowledge the support from

Northrop–Grumman Corporation. DPY acknowledges support

from the NSF under Grant No. DMR 04-49022. FT-IR analysis

done by Dr. Y. Mou from Department of Chemistry and

Physics at LU and financial support from the Welch

Foundation (V-1103) are kindly acknowledged.

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