Nanoclays reinforced glass ionomer cements: dispersionand interaction of polymer grade (PG) montmorillonitewith poly(acrylic acid)

Muhammad A. Fareed • Artemis Stamboulis

Received: 11 March 2013 / Accepted: 19 September 2013 / Published online: 29 September 2013

� Springer Science+Business Media New York 2013

Abstract Montmorillonite nanoclays (PGV and PGN)

were dispersed in poly(acrylic acid) (PAA) for utilization

as reinforcing filler in glass ionomer cements (GICs).

Chemical and physical interaction of PAA and nanoclay

(PGV and PGN) was studied. PAA–PGV and PAA–PGN

solutions were prepared in different weight percent load-

ings of PGV and PGN nanoclay (0.5–8.0 wt%) via exfo-

liation-adsorption method. Characterization was carried out

by X-ray diffraction (XRD), X-ray photoelectron spec-

troscopy (XPS) and fourier transform infrared (FTIR)

spectroscopy. XRD results of PAA–PGN demonstrated that

the interlayer space expanded from 12.83 to 16.03 A

indicating intercalation whereas the absence of the peak at

d001 in PAA–PGV indicated exfoliation. XPS scans of PGV

and PGN nanoclays depicted the main peak of O 1s pho-

toelectron due to Si–O–M (M = Mg, Al, Fe) whereas, Si–

O–Al linkages were identified by Si 2p or Si 2s and Al 2p

or Al 2s peaks. The disappearance of the Na peak con-

firmed that PAA molecules exchanged sodium ions present

on surface of silicate layers and significantly reduced the

electrostatic van-der-Waals forces between silicate plates

resulting in intercalation or exfoliation. FTIR spectra of

PAA–nanoclay suspensions demonstrated the presence of a

new peak at 1,019 cm-1 associated with Si–O– stretching

vibrations which increased with increasing nanoclays

concentration. Information concerning the dispersion of

nanoclay in PAA aqueous solutions, chemical reaction and

increase interlayer space in montmorillonite nanoclay is

particularly useful regarding dispersion and reinforcement

of nanoclay in PAA.

1 Introduction

Polymer nanocomposites, especially polymer-layered-sili-

cate nanocomposites with fully-exfoliated platelet structure

of organo-modified montmorillonite (MMT) (commonly

known as nanoclay) when dispersed in a polymer matrix

greatly improves its mechanical properties [1]. The incor-

poration of low concentrations of nanoclay (1–5 %) has

been a popular strategy to improve polymer materials in

recent years [1–13]. The high aspect ratio of layered-sili-

cates (Mica, Talc, MMT and Hectorite) is ideal for polymer

reinforcement but they are generally not easily dispersed in

polymers due to the preferred face-to-face stacking [1–3].

The reinforcement effect of nanoclay dispersions has lar-

gely been reported in polymers such as epoxy resins [2],

nylon [3], polystyrene [4], polymethyl methacrylate [5],

polycaprolactone [6], polyethylene [7], polyurethane [8],

polyamide [9], chitosan [10, 11] and poly(acrylic acid)

(PAA) based composites [12, 13]. Polymer grade (PG)

montmorillonites (PGV and PGN) are alumina–silicate

minerals of high purity and are frequently used as additives

in hydrophilic polymers such as polyvinyl-alcohols, poly-

saccharides and PAA. In the structure of MMT, single

layer nanoclay has a central alumina octahedral sheet

sandwiched between two silica tetrahedral sheets by shar-

ing their apex oxygen to form a single clay sheet of

0.96 nm thickness (Fig. 1). The face-to-face stacking of

clay layers leads to van-der-Waals forces between the

layers and the gap is called the interlayer space or clay

M. A. Fareed (&) � A. Stamboulis

Biomaterials Group, School of Metallurgy and Materials,

University of Birmingham, Edgbaston, Birmingham B15 2TT,

UK

e-mail: docamberfareed@hotmail.com

M. A. Fareed

FMH College of Medicine and Dentistry, University of Health

Sciences, Lahore, Pakistan

123

J Mater Sci: Mater Med (2014) 25:91–99

DOI 10.1007/s10856-013-5058-3

gallery which can trap polymer molecules during the pro-

cess of nanocomposite formation [14].

Conventional glass ionomer cement (GIC) is a popular

dental restorative material since its discovery by Wilson

and Kent [15]. It is used in restorative dentistry extensively

for luting, lining, as a base material and for dental fillings.

The liquid component of GIC is an aqueous solution of

(PAA) stabilized with 5 % tartaric acid copolymers. The

powder component of GIC is a fluoro-aluminosilicate

glass. GIC is formed by acid–base reaction between PAA

solution and ion-leachable basic glass powder in the pre-

sence of water. Dentists prefer GIC due to chemical bond

to the tooth structure, anti cariogenic properties and

excellent biocompatibility [15]. However, GIC exhibits

some limitations such as low abrasion and wear resistance,

poor mechanical strength and desiccation in dry conditions.

The mechanical properties of GICs are influenced by the

composition of polyacid and glass powder [16]. The

strength of GIC can be further improved by dispersion of

nanoclay particles in the polymeric matrix. To achieve

maximum polymer–nanoclay interaction, the choice of

suitable nanoclay for PAA is imperative in the develop-

ment of nanoclay-reinforced glass ionomer dental cement.

A uniform dispersion and controlled association of nano-

particles with PAA matrix is required to improve the

properties of the set material. The incorporation of

nanoclays in aqueous solutions of PAA has been reported

to increase the interlayer d-spacing of the nanoclays [17,

18]. Use of nanoclay reinforced GIC as posterior dental

filling material has also been reported previously [19]. The

present paper characterizes the nanoclay interaction with

PAA matrix and investigates the reinforcement effect of

nanoclays as a function of their concentration and

dispersion.

2 Materials and methods

Purified nanomer/polymer-grade nanoclay (PGV and

PGN) were generously supplied by Nanocor Inc. (Chicago

IL, USA) through NRC Nordmann, Rassmann GmbH

(Hamburg, Germany). The chemical analysis of PGV

nanoclay and PGN nanoclay reported by Nanocor is given

in Table 1. The nanoclay was purified to a level greater

than 98 % montmorillonite and the degree of cations

exchange capacity for PGV was 145 meq/100 mg and for

PGN was 120 meq/100 mg. Different weight percentages

of PGV nanoclay and PGN nanoclay (0.5, 1.0, 2.0, 4.0,

6.0 and 8 wt%) were dispersed in aqueous solution of

PAA. Sokalan PA-110 (PAA solution) Mw * 110,000,

water content 65 wt% and pH * 2 was supplied by

BASF plc, Cheshire, UK. The suspensions of PAA solu-

tion–PGV nanoclay (PAA–PGV) and PAA solution–PGN

nanoclay (PAA–PGN) were formed by mixing and heat-

ing procedure similar to the exfoliation-adsorption

method. However, no solvent was used for dispersion of

nanoclay in PAA. PGV nanoclay and PGN nanoclay

powders weighing 0.25 g (0.5 wt%), 0.50 g (1.0 wt%),

1.0 g (2.0 wt%), 2.0 g (4.0 wt%), 3.0 g (6.0 wt%) and

4.0 g (8.0 wt%) on a balance accurate to 0.001 g

(TS4000, Ohaus, Pine Brook, NJ, USA) were added to

50 g of 65 % solution of PAA (PA-110). After hand

Fig. 1 The crystalline layered

nanoclay has a central alumina

octahedral sheet sandwiched

between two silica tetrahedral

sheets in the structure of MMT

[14]

92 J Mater Sci: Mater Med (2014) 25:91–99

123

mixing, PAA–PGV samples (D0.5VP, D1.0VP, D2.0VP,

D4.0VP, D6.0VP and D8.0VP) and PAA–PGN samples

(D0.5NP, D1.0NP, D2.0NP, D4.0NP, D6.0NP and

D8.0NP) were mixed by a mechanical-stirrer (IKA Lab,

Sweden). A 100 ml two necked round-bottom glass flask

with a condenser fitted to cooling tubes was used as a

reactor. PAA–PGV and PAA–PGN suspensions were

heated on an isomantle maintained at *75 �C in a fume

cupboard for 24 h. The schematic presentation of the

polymer–nanoclay solutions prepared is shown in Table 2.

The solid residue was collected from PAA–nanoclays

suspensions via a centrifuge (J2-21M/E centrifuge Beck-

man, UK at 18,000 rpm and 15 �C). The residue was then

washed with water and dried in air prior to X-ray dif-

fraction (XRD) and X-ray photoelectron spectroscopy

(XPS). The PAA solution was freeze dried for XRD and

XPS analysis. XRD analysis was conducted using a Phi-

lips analytical X-Pert XRD at 40 kV and 40 mA using Cu

Ka radiation with a wavelength of k = 1.5418 A between

2h (Theta) angles of 2�–30� at a step size of 0.014� and a

count time of 1 s/step. XPS was carried out using an

Auger VG ESCALLAB 200i XL with a photoelectron

spectrometer and a monochromated AlKa source

(hv = 1486.6 eV). The spectra were acquired with the

photoelectron take-off axis perpendicular to the sample

surface. Fourier transform infrared (FTIR) spectra of

nanoclays, PAA solution and PAA–nanoclay suspensions

were obtained on a Nicolet FTIR spectrometer (FT-

Raman Module, MGNA-IR 860) equipped with a mid-

infra red source using a DTGS detector and a XT-KBr

beam-spliter with a Golden Gate Single Reflection Dia-

mond ATR attachment. For each sample 100 scans were

recorded with a resolution of 4 cm-1 in the range of

700–4,000 cm-1and FTIR spectra of at least three sam-

ples of each group were collected.

3 Results

Figure 2 shows the XRD patterns of PAA–PGV and PAA–

PGN at different weight percentages of PGV nanoclay and

PGN nanoclay. The main peak in the diffraction pattern

of PGN nanoclay (2h * 7.13�) and PGV nanoclay

(2h * 6.95�) was attributed to the formation of the inter-

layer space by a regular stacking of the silicate layers along

the [001] direction. The interlayer distance of PGV nanoclay

at 2h * 6.95� was 12.83 A and the absence of the main peak

at 2h * 6.95� in PAA–PGV samples (D0.5VP, D1.0VP,

D2.0VP, D4.0VP, D6.0VP and D8.0VP). An additional peak

at 2h * 19.74� identified in PGV nanoclay can be attributed

to the in-plane structures formed by stacking along the (a,b)

direction. The peak at 2h * 19.74� associated with the [101]

plane becomes very broad and insignificant in PAA–PGV

samples after dispersion of nanoclays (Fig. 2a). The inter-

layer distance of PGN nanoclay at 2h * 7.13� was 12.42 A

and the d001 space between the silicate layers of PAA–PGN

(D0.5NP, D1.0NP, D2.0NP, D4.0NP, D6.0NP and D8.0NP)

increased to 18.83 A. Two more peaks observed at

2h * 19.74� and *26.51� in the X-ray pattern of PGN

attributed to the in-plane structures formed by stacking along

the (a,b) direction. The peak at 2h * 19.74� associated with

the [101] plane broadened and became insignificant in PAA–

PGN after the dispersion of nanoclays (Fig. 2b).

Figure 3a shows the wide-angle XPS spectra of PAA,

PGV nanoclay and PAA–PGV samples (D4.0VP and

Table 1 Chemical analysis PGV and PGN nanoclay as reported by

Nanocor Inc

Element PGV PGN

O 48.43 48.7

Na 4.01 3.21

Mg 2.97 1.65

Al 11.06 12.35

Si 29.82 29.65

P 0.03 0.03

S 0.12 0.06

K 0.15 0.04

Ca 0.19 0.20

Ti 0.20 0.10

Mn 0.02 0.02

Fe 3.00 3.99

Total 100.00 100.00

Table 2 Dispersion of PGV nanoclay and PGN nanoclay in PAA

solutions

Specimen ID Clay (wt%) Clay type Liquid

PAA–PGV

[D0.5VP] 0.5 PGV PA-110

[D1.0VP] 1.0 PGV PA-110

[D2.0VP] 2.0 PGV PA-110

[D4.0VP] 4.0 PGV PA-110

[D6.0VP] 6.0 PGV PA-110

[D8.0VP] 8.0 PGV PA-110

PAA–PGN

[D0.5NP] 0.5 PGN PA-110

[D1.0NP] 1.0 PGN PA-110

[D2.0NP] 2.0 PGN PA-110

[D4.0NP] 4.0 PGN PA-110

[D6.0NP] 6.0 PGN PA-110

[D8.0NP] 8.0 PGN PA-110

J Mater Sci: Mater Med (2014) 25:91–99 93

123

D6.0VP) and Fig. 3b presents the XPS spectra of PAA,

PGN nanoclay and PAA–PGV samples (D4.0NP and

D6.0NP). In the wide scan of PGV and PGN nanoclays, the

main peak of O 1s photoelectron was attributed to various

oxygen containing species such as Si–O–M (M: Mg, Al,

Fe, etc.) within the two layers of the silicate plate, whereas

Si–O–Al linkages can be identified by the Si 2p or Si 2s

and Al 2p or Al 2s peaks. The minor peaks of Mg KLL and

Fe 2p were attributed to the presence of a small amount of

Si–O–Mg and Si–O–Fe species. The Auger Na KLL (Na

1s) photoelectron peak in nanoclays spectra was due to the

Na ions on the surface of silicate plates. The main O 1s and

C 1s peak in the wide scan of PAA was attributed to the

presence of oxygen and carbon in –COOH and C–C

groups. In the wide scan XPS of PAA–PGV (D4.0VP and

D6.0VP) and PAA–PGN (D4.0NP and D6.0NP) C 1s peak

was due to the presence of PAA whereas other peaks e.g. Si

2p, Al 2p and Fe 2p were due to the presence of nanoclays.

The Na 1s and the Na KLL peaks were not detected in the

wide scan of D4.0VP, D6.0VP, D4.0NP and D6.0NP

(Fig. 3). The high resolution narrow-angle XPS spectra of

C 1s and O 1s are shown in Fig. 4. In Fig. 4a narrow-angle

C 1s XPS spectra of PAA, the peak at 287 eV was attrib-

uted to C–C and the peak at 293 eV was attributed to

C–COOH of PAA backbone. In addition to the broadening

and shifting of the C–C peak in the C 1s spectrum of PAA–

PGV there was shifting of –COOH peak in the C 1s spectra

for both D4.0VP and D6.0NP. Specifically for the latter,

the peak is so small that it could be considered absent. The

C 1s narrow-angle scans of PAA–PGN depicted C–C and

–COOH peaks shifted towards higher binding energies.

The presence of C 1s peak at the binding energy 290 eV in

the spectrum of nanoclays was unexpected and can be

attributed to the presence of surface adventitious carbon

Fig. 2 XRD graphs of PAA–PGV and PGV nanoclay (a), and PAA–PGN and PGN nanoclay (b)

Fig. 3 Wide-angle XPS spectra of PAA, PGV nanoclays and PAA–

PGV (a), and PAA, PGN nanoclays and PAA–PGV (b)

94 J Mater Sci: Mater Med (2014) 25:91–99

123

ions. In the O 1s scan of PAA the peak at 537 eV was

attributed to the presence of carbonyl and hydroxyl groups

of the carboxylic group as shown in Fig. 4b. The O 1s peak

at 538 eV in the scans of PGV and PGN nanoclays was due

to the presence of metallic oxides O–M (M: Si, Al, Fe,

Mg). The O 1s scan at 536 eV of D4.0VP exhibits a strong

peak which moved slightly towards a lower binding energy

compared to the peak present in PAA spectrum indicating

changes in the environment around the chemical species

containing oxygen. The O 1s peak in the PAA–PGN sus-

pension was similar to the reference peak of PAA but

increased in intensity.

FTIR spectra with major peak position assignments of

PGV and PGN nanoclay are presented in Fig. 5. The

characteristic bands due to Si–O stretching vibrations in the

tetra-hydrate layer of PGV and PGN nanoclays can be

observed. The description of the main peaks is given in

Table 3. FTIR spectra of PAA–PGV shows a diagnostic

peak of nanoclay at 1,019 cm-1 which increased in

intensity with increasing nanoclays content from 0.5 to

8.0 wt%. This in-turn resulted in an increase in the

absorbance intensity of Si–O group.

4 Discussion

Nanoclays are the focus of research due to remarkable

improvements in the various properties of polymer com-

posites. Traditionally, nanoclays were combined with the

host polymer either by a neat or volatile solvent followed

Fig. 4 Narrow-angle C 1s XPS

spectra (a), and O 1s XPS

spectra (b)

Fig. 5 FTIR spectra of PGV nanoclay and PGN nanoclay

J Mater Sci: Mater Med (2014) 25:91–99 95

123

by extensive mechanical mixing of nanoclay-solvent and a

polymer employing high-shear mixer or 3-roll-mill for

liquid resin, as well as the Brabender mixer for viscous

resin or twin extruder for solid resin [20]. PGV and PGN

nanoclay were shear-mixed in PAA solution at 75 �C for

24 h. The use of high-speed mixing may cause the break-

age of nanoclay sheets and reduced the aspect ratio for

better polymer–nanoclay interactions. Tran et al. reported a

similar technique and suggested that mixing of nanoclays

with polymer resulted in the formation of silicate (Si–O–

M) or silica (Si–O–Si) nano-plates [21]. Previous studies

on PAA–nanoclay interactions [12, 18, 22] showed that

increasing the temperature results in better exfoliation of

nanoclays within polymer matrix. When mixing of poly-

mer–nanoclays takes place at 75 �C, the polymer chains

possibly extend the silicate layers until a thermodynamic

equilibrium is reached between the polymer and the higher

surface energy of silicate-plates of nanoclay. Ngo et al.

studied the effects of temperature, speed, and duration of

mixing on the dispersion of nanoclays (e.g., cloisites) in

epoxy resin and reported that the effect of mixing speed

was more evident than the mixing temperature in terms of

nanoclays dispersion [23]. However, the charge density of

silicate layers, nature of the interlayer exchanged ions;

curing conditions as well as the chemistry of the polymer

determine the intercalation or the exfoliation.

The PAA–nanoclay interaction was identified by a shift

of the XRD diffraction peaks towards lower angle values in

comparison with the original interlayer spacing of PGV

and PGN. It showed that PAA–PGV were exfoliated while

PAA–PGN were intercalated (Fig. 2). The increase in the

interlayer distance was due to the adsorption and segre-

gation of PAA chains into the interlayer space. The lack of

d001 peak in the XRD pattern of PAA–PGV indicates ex-

foliation of PGV in PAA though there is a possibility that

the [001] diffraction peak may have moved to a lower

angle which was not in the detectable range at 2h * 3.5�.

In PAA–PGN samples, the shifting of 2h values to a lower

angle indicates intercalation due to the small increase in the

interlayer space. In PAA–PGN samples it is clear that by

increasing PGN nanoclay content from 0.5 to 8.0 wt% did

not influenced the interlayer space between the silicate

plates of PGN and the d001 peak was present at the similar

2h values for all samples. In intercalated nanoclays, the

interlayer space increased due to the adsorbed PAA chains

on the surface of PGN nanoclay. The XRD graph showed a

broad peak shifted to lower angles associated with the out-

of plane direction of the silicate plates [17]. Similarly, the

XRD pattern of fully exfoliated nanoclays did not exhibit

the d001 peak at low angle values. On the basis of XRD

results, it is evident that in PAA–PGN one or more polymer

chains might have penetrated in between the silicate plates

of PGN and the lamellar structure of nanoclays remained

intact resulting in intercalation. On the other hand, the

separation and random orientation of the silicate plates in

PAA–PGV suggested exfoliation of nanoclays. Further-

more, during the interaction of the hydrophilic nanoclay

platelet with PAA, their natural interstitial metal cations

(K?, Na?, Li?, Mg2?, Ca2?, etc.) were exchanged with

protons and/or released within the organic PAA solution

[22]. A possible exchange of Na? ions by H? ions can form

surface hydroxyl groups leading to hydrogen bonding with

Fig. 6 FTIR spectra of PAA and PAA–PGV after dispersion in PAA

solutions

Table 3 Description of FTIR peaks present in spectra shown in

Figs. 5 and 6

Wavenumber (cm-1) Assignment

Nanoclay

3,626 Si–OH stretching vibrations

1,741 H–O–H bending vibration

1,642 H–O–H bending vibration

973 Si–O stretching vibration

904 Al–OH stretching vibration

814 (Al, Mg)–OH vibration mode

797 Si–O–Al vibration

527 Al(Mg)–O–Si

480 Si–O stretching vibration

PAA–nanoclay

3,354 H bonded O–H stretching vibration

1,705 C=O stretching vibration

1,642 C–O vibration

1,642 H–O–H bending vibration

1,453 C–H bending vibration

1,266 C–O stretching vibration

1,019 Si–O stretching vibration

96 J Mater Sci: Mater Med (2014) 25:91–99

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the carboxyl groups in PAA. This cations exchange created

more organophilic galleries within nanoclays thereby

increasing the d-spacing and simultaneously reducing the

attractive forces between the platelets [17]. Therefore, at

the nanometre scale PAA intercalated more readily and

further expanded the d-spacing in nanoclays for efficient

nanoclay dispersion into the polymer solution.

XPS provided the detailed information about structure

of the materials and specifically the chemical interactions.

Intercalation of clay galleries, in the case of PAA–PGN

may accommodate a few PAA chains. In the case of PAA–

PGV, exfoliation of silicate plates may result in a larger

number of PAA chains between the interlayer spaces is

shown in Fig. 7. Si–O–Al linkages can be identified in

PAA–PGV and PAA–PGN at the lower binding energy of

the wide-scan XPS (78–160 eV) which were associated

with the Si 2p or Si 2s and Al 2p or Al 2s photoelectrons

from the silicate layers. The presence of one additional

small peak assigned to Mg KLL at 281 eV in PGV and

PGN nanoclays indicated the presence of a small amount of

Si–O–Mg in the silicate layers. The intensity of Si 2p, Si

2s, Al 2p, Al 2s and Mg KLL peaks associated with the

presence of nanoclay was slightly higher in PAA–PGN and

PAA–PGV compared to the respective nanoclays (Fig. 3).

However, the significance of this observation is not clear.

Wide-scan XPS showed that Fe 1p and Fe 2p peaks were

present at 773 eV associated with the presence of Si–O–Fe

species in the silicate layers (Fig. 3). It is in good agree-

ment with PAA–MMT interactions reported in literature

which mentioned that PAA caused oxidation of Fe2? ions

resulting in enhanced dispersion of MMT [12]. In wide-

scan XPS, Na ion was not detected in PAA–PGV and

PAA–PGN indicating the removal of Na ions from the

surface of silicate plates during the interaction of PAA with

nanoclays at 75 �C and probable replacement by protons

from the PAA solution. The presence of O 1s peak is

clearly associated with nanoclays and PAA whereas the

presence of C 1s is associated with PAA. In the narrow-

angle XPS scans of C 1s and O 1s (Fig. 4), a small C 1s

peak at 536 eV present in PGV and PGN nanoclay narrow-

scans was due to adventitious carbon [24] as Nanocor Inc.

certified that nanoclays were not treated. The C 1s narrow-

scan of PAA–PGV shows broadening and slight shifting

of the C–C peaks however, the peak associated with

C–COOH at 292 eV became very weak and PAA–PGN

shows that the peaks associated with both C–C and C–

COOH moved to higher binding energies indicating PAA

and nanoclay interaction [20]. In the narrow-angle O 1s

scans of PAA–PGV, main peak associated with C=O in

PAA–PGV appeared broader with higher intensity and

shifted towards lower binding energies at 536.65 eV

compared to PGV nanoclay and PAA indicating changes in

the environment around the chemical species containing

oxygen and consequently some interactions with PAA. The

main peak in the narrow-angle O 1s scans in the case of

PAA–PGN is present at 537.84 eV close to the reference

peak of PAA. Comparing the changes in Figs. 3 and 4 it is

clear that stronger interactions were observed in PGV than

PGN. This is in good agreement with XRD analysis that

showed that PGV nanoclay fully exfoliated in PAA

whereas PGN nanoclay intercalated. XPS can also be

implicated by physisorption and chemisorption of PAA

molecules on the silicate surface of nanoclays. In terms of

physisorption, the increase interlayer distance in the

nanoclays may easily accommodate the PAA molecules

(Fig. 7). The extensive reorganisation of PAA at the sur-

face of MMT resulted in the formation of linear PAA

chains instead of random coils. The interlayer space can

only accommodate a maximum of two layers of fully

extended PAA chains that are bound to the silicate surface

by a number of carboxylic groups or hydrogen bonds [17,

25]. In terms of chemisorption, the organic molecules can

penetrate the interlayer spaces by interacting with nanoc-

lays surface in one of the following ways; (a) cationic

bonding, in which protonated molecules replace the sodium

ions in the interlayer spaces of nanoclays, (b) ion–dipole

interaction, in which polar organic molecules are related to

sodium ions in the nanoclay plates, or (c) dipole–dipole

interactions, which include the hydrogen bonding that

associate the polar organic molecules with the hydroxyl

groups or oxygen in the layers of nanoclays [26]. The

formation of siloxanes (C–O–Si) at the surface of the

nanoclay plates and the re-arrangement of coordination

complexes between the metal ions within the silicate plates

may also be one of the possible chemisorption mechanisms

that could describe the PAA–nanoclay interactions [21]. In

the light of the aforementioned explanation, it can be stated

that the removal of the cations from the nanoclays inter-

layer galleries by the PAA chains can significantly reduce

the electrostatic van-der-Waals forces between the silicateFig. 7 Illustration of the interactions a exfoliated b intercalated,

between the polymer-grade nanoclay and PAA

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123

plates. Additionally, the steric effect created by the PAA

chains on the silicon plates may also prevent the face-to-

face interaction of nanoclay plates, which resulted in

intercalation or exfoliation. The detailed analysis of the

peaks at lower binding energy using narrow-angle XPS

would be interesting in order to study the influence of

cations in the chemisorption of nanoclay and PAA.

In the FTIR studies of nanoclays, –OH and Si–O groups

play an important role in the differentiation of nanoclays

from each other and from the host polymer. FTIR spectra

of PGV and PGN nanoclay showed characteristic bands at

3,626 cm-1 due to O–H stretching vibrations, at

1,642 cm-1 due to H–O–H bending vibrations and at

973 cm-1 due to Si–O stretching vibrations. The sharp

peak centred at 973 cm -1 in PGV and PGN nanoclay was

attributed to the distinctive nanoplates of the tetra-hydrate

layers of silicon. The absorbance at 3,620 cm-1 in the

spectrum of PGV and PGN is typical for montmorillonite

with high Al content in the octahedral layer of nanoclay. In

2:1 family of smectite montmorillonite, the main peak at

3,620 cm-1 is associated with the high Al content in the

octahedral layer, whereas any peak at around 3,567 cm-1

is associated with the Fe2OH groups in the octahedral

sheets in Fe rich nontronites [27]. However, in our case,

this peak was not present in any of the nanoclays spectra

and therefore, it is well confirmed that the clays belong to a

typical high aluminium content type of montmorillonites.

Despite the fact that PGN contains a higher amount of Fe

than PGV (Table 1), the peak that corresponds to the

Fe2OH group was missing from both nanoclays spectra. A

broad band near 3,430 cm-1 observed in both nanoclays

was due to the H–O–H vibration of adsorbed water but this

band was less prominent in PGN nanoclays. FTIR spectra

PAA–PGV shows the absorbance intensity of the peak

observed at 1,019 cm-1 associated with Si–O– stretching

vibrations increased with increasing the clay loading,

irrespective of the type of the clay used (Fig. 6). It is

noticeable that the Si–O vibrations band present in PGV

nanoclays at 973 cm-1 moved to higher wavenumbers at

1,019 cm-1 in PAA–PGV. The shift of the peak at

973 cm-1 associated with Si–O– stretching vibrations can

be due to the change of the chemical environment indi-

cating a strong interaction between the silicate plates of

PGV and PAA. Madejova et al. reported that the acid

attack (HCl) on nanoclay lead to the successive release of

the central atoms from the octahedral layer and the release

of Al from the Si tetrahedral sheets from dioctahedral

smectites and hectorite clay [28]. In the case of PAA which

is a weak acid compared to HCl, it can be suggested that

the protons from the –COOH groups may enter the nano-

clay layers and attack the structural –OH groups resulting

in de-hydroxylation of nanoplates is connected with the

successive release of the central atoms. This can be readily

followed by observing the changes in the characteristic

absorption bands attributed to the vibration of –OH groups

(3,626, 1,642 and 904 cm-1) and octahedral cations

(814 cm-1). In addition, a gradual transformation of the

tetrahedral sheets to protonated amorphous silica can also

be observed in the region of the stretching vibrations of the

Si–O groups at 973 cm-1 which shifted to 1,019 cm-1

with a simultaneous increase in absorbance indicating

interaction of silicate nanoplates in PGV. A diagnostic

peak of nanoclay at 1,019 cm-1 increased in intensity with

increasing the nanoclays content in the polymer–nanoclay

solutions. It is clear that increasing the of nanoclay con-

tents from 0.5 to 8.0 wt% resulted in an increase in the

absorbance intensity of Si–O group at 1,019 cm-1 (Fig. 6).

The shift of the Si–O stretching vibrations from 973 cm-1

in nanoclays to 1,019 cm-1 in nanoclay–PAA solutions

could be an indication of the PAA interaction with the

silicate nanoplates and specifically the formation of an

amorphous silica gel layer on the surface of nanoclays after

PAA attack [29]. A smaller shoulder at 1,086 cm-1 in the

spectra of PAA–PGV especially at contents higher than

2.0 wt% PGV unfortunately could not be identified but it

could be connected with Si–O stretching vibrations. The

peaks associated with the carboxylic group were present at

1,705 cm-1 and at 1,629 cm-1 attributed to the C=O

stretching vibrations and –OH bending vibrations in the

carboxylic group, respectively. The changes in the intensity

of the peaks at 1,629 and 1,705 cm-1 associated with the

carboxylic groups in PAA–PGV may suggest the interac-

tion of protons from PAA with the PGV nanoclay [29].

Furthermore, a significant broadening and decrease in

intensity of the peak associated with C–O stretching

vibrations in PAA at 1,266 cm-1 was observed in the

spectra of D4.0VP, D6.0VP and D8.0VP with the increase

in the nanoclays content. It is not clear, however why an

increase in nanoclays content would have an effect on C–O

stretching . The FTIR spectra of nanoclays and PAA–

nanoclay support the observations of XRD and XPS on the

interactions observed between the nanoclays and PAA.

The mixing of nanoclay with PAA resulted in formation

of exfoliated/intercalated nanoplates of silicates (Si–O–M,

M = Al, Mg etc.) and/or silica (Si–O–Si) when dispersed

in the liquid phase. The increase in the interlayer distance

was due to the adsorption and segregation of PAA chains

into nanoclay gallery or interlayer space. According to the

FTIR and XPS results a chemical bonding at the interface

between PAA matrix and nanoclay is present because

silicon in nanoclay reacted with PAA by carbon and

oxygen, also depletion of sodium ions at the silicate

surface of nanoclay. Therefore, it is revealed that carbon,

silicon and oxygen peak shift are principally due to

chemical (ionic or hydrogen) bonding between PAA and

nanoclays.

98 J Mater Sci: Mater Med (2014) 25:91–99

123

5 Conclusions

The reinforcing capability of nanoclays is due to their high

modulus, high strength and high aspect ratio. PAA–nanoclay

interactions were observed when PGV–nanoclay and PGN–

nanoclay dispersed in the PAA following a suitable pro-

cessing method. It is suggested, that during the PAA–PGN

interactions, one or more polymer chains penetrated in

between the silicate plates of PGN whereas the lamellar

structures of nanoclays remained intact resulting in interca-

lation. On the other hand, the PAA–PGV interaction resulted

in the separation and random orientation of the silicate plates

in nanoclays indicating exfoliation of nanoclays. The dis-

persion of nanoclays and the reinforcement effect of the

dispersed nanoclays were possibly associated with the

chemisorption and physisorption of PAA on the silicate

nanoplates resulted in increasing the interlayer space due to

the segregation and adsorption of PAA molecules in the

interlayer space. The interaction of PAA molecules onto

silicate plates of PGV may be due to possible exchange of

sodium ions by hydrogen ions that leads to the formation of

hydroxyl groups which can form hydrogen bonds with the

carbonyl groups of PAA. The present approach may be

useful to study the potential of the reinforcement of nanoclay.

Acknowledgments The authors gratefully acknowledge the PhD

studentship from School of Metallurgy and Materials, University of

Birmingham and financial assistance from Higher Education Com-

mission Pakistan toward this research work. We also thank Nanocor

and BASF for munificent supply of experimental materials.

References

1. Alexandre M, Dubois P. Polymer-layered silicate nanocompos-

ites: preparation, properties and uses of a new class of materials.

Mater Sci Eng R Rep. 2000;28:1–63.

2. Yoonessi M, Toghiani H, Kingery WL, Pittman CU. Preparation,

characterization, and properties of exfoliated/delaminated

organically modified clay/dicyclopentadiene resin nanocompos-

ites. Macromolecules. 2004;37:2511–8.

3. Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y,

Kurauchi T, et al. Synthesis of nylon 6-clay hybrid. J Mater Res.

1993;8:1179–84.

4. Vaia RA, Jandt KD, Kramer EJ, Giannelis EP. Kinetics of

polymer melt intercalation. Macromolecules. 1995;28:8080–5.

5. Biasci L, Aglietto M, Ruggeri G, Ciardelli F. Functionalization of

montmorillonite by methyl methacrylate polymers containing

side-chain ammonium cations. Polymer. 1994;35:3296–304.

6. Kojima Y, Usuki A, Kawasumi M, Okada A, Kurauchi T, Ka-

migaito O. Synthesis of nylon 6-clay hybrid by montmorillonite

intercalated with ?A-caprolactam. J Polym Sci. 1993;31:983–6.

7. Jeon HG, Jung H-T, Lee SW, Hudson SD. Morphology of

polymer/silicate nanocomposites: high density polyethylene and a

nitrile copolymer. Polym Bull. 1998;41:107–13.

8. Wang Z, Pinnavaia TJ. Nanolayer reinforcement of elastomeric

polyurethane. Chem Mater. 1998;10:3769–71.

9. Fedullo N, Sorlier E, Sclavons M, Bailly C, Lefebvre JM, Devaux

J. Polymer-based nanocomposites: overview, applications and

perspectives. Prog Org Coat. 2007;58:87–95.

10. Gunister E, Pestreli D, Unlu CH, Atici O, Gungor N. Synthesis

and characterization of chitosan–MMT biocomposite systems.

Carbohydr Polym. 2007;67:358–65.

11. Depan D, Kumar AP, Singh RP. Preparation and characterization

of novel hybrid of chitosan-g-lactic acid and montmorillonite.

J Biomed Mater Res. 2006;78:372–82.

12. Tran NH, Wilson MA, Milev AS, Dennis GR, Kannangara GSK,

Lamb RN. Dispersion of silicate nano-plates within poly(acrylic

acid) and their interfacial interactions. Sci Technol Adv Mater.

2006;7:786–91.

13. Billingham J, Breen C, Yarwood J. Adsorption of polyamine,

polyacrylic acid and polyethylene glycol on montmorillonite: an

in situ study using ATR-FTIR. Vib Spectrosc. 1997;14:19–34.

14. G-105 Polymer grade montmorillonite, Product Sheet, Nanocor,

Inc., Arlington Heights, IL 6004 USA. http://www.nanocor.com/

tech_sheets/G105.pdf (2006). Accessed 05 May 2006.

15. Wilson AD, McLean JW. Glass-ionomer cement. Chicago:

Quintessence; 1988.

16. Moshaverinia A, Rohpoor N, Darr JA, Rehman IU. Synthesis and

characterization of a novel N-vinylcaprolactam-containing

acrylic acid terpolymer for applications in glass-ionomer cement.

Acta Biomater. 2009;5:2101–8.

17. Tran NH, Dennis GR, Milev AS, Kannangara GSK, Wilson MA,

Lamb RN. Interactions of sodium montmorillonite with poly

(acrylic acid). J Colloid Interface Sci. 2005;290:392–6.

18. Solhi L, Atai M, Nodehi A, Imani M, Ghaemi A, Khosravi K.

Poly (acrylic acid) grafted montmorillonte as novel fillers for

dental adhesives: synthesis, characterization and properties. Dent

Mater. 2012;28:369–77.

19. Dowling AH, Stamboulis A, Fleming GJP. The influence of

montmorillonite clay reinforcement on the performance of a glass

ionomer restorative. J Dent. 2006;34:802–10.

20. Koo JH. Polymer nanocomposites processing, characterization

and application. USA: McGraw-Hill Companies Inc; 2006.

21. Tran NH, Wilson MA, Milev AS, Dennis GR, McCutcheon AL,

Kannangara GSK, et al. Structural–chemical evolution within

exfoliated clays. Langmuir. 2006;22:6696–700.

22. Lin J, Wu J, Yang Z, Pu M. Synthesis and properties of poly

(acrylic acid)/mica superabsorbent nanocomposite. Macromol

Rapid Commun. 2001;22:422–4.

23. Ngo TD, Ton-That MT, Hoa SV, Cole KC. Effect of temperature,

duration and speed of pre-mixing on the dispersion of clay/epoxy

nanocomposites. Compos Sci Technol. 2009;69:1831–40.

24. Miller ML, Linton RW. X-ray photoelectron spectroscopy of

thermally treated SiO2 surfaces. Anal Chem. 1985;57:2314–9.

25. Taylor TJ, Stivala SS. Small-angle X-ray scattering study

of a weak polyelectrolyte in water. J Polym Sci. 2003;41:

1263–72.

26. Choi YS, Ham HT, Chung IJ. Effect of monomers on the basal

spacing of sodium montmorillonite and the structures of polymer-

clay nanocomposites. Chem Mater. 2004;16:2522–9.

27. Madejova J. FTIR techniques in clay mineral studies. Vib

Spectrosc. 2003;31:1–10.

28. Madejova J, Bujdak J, Janek M, Komadel P. Comparative FT-IR

study of structural modifications during acid treatment of dioc-

tahedral smectites and hectorite. Spectrochim Acta. 1998;54:

1397–406.

29. Feng B, Hong RY, Wu YJ, Liu GH, Zhong LH, Zheng Y, et al.

Synthesis of monodisperse magnetite nanoparticles via chitosan-

poly(acrylic acid) template and their application in MRI. J Alloy

Compd. 2009;473:356–62.

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