Effect of surfactant architecture on the properties of polystyrene- montmorillonite nanocomposites

Article abstract of DOI:10.1021/la904827d

A series of surfactants were designed and synthesized for use as clay modification reagents to investigate the impact of their chemical structure on the nanocomposites morphology obtained following polymerization. The behavior of the surfactant-modified clays at three different stages were investigated: after ion exchange, following dispersion in styrene monomer, and once polymerization was complete. The propensity of the styrene monomer to swell the surfactant-modified clay was observed to be a useful indicator of compatibility and predictor of the resultant polystyrene nanocomposite morphology which was directly observed using small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (TEM). It was found that the key components of surfactant design driving exfoliated morphologies were (1) the position of the ammonium group, (2) the inclusion of a polymerizable group, (3) the solubility of the surfactant in the monomer, (4) the length of the alkyl chain, and (5) sufficient concentration of surfactant used to exchange the clay. This understanding should lead to better design of clay modifications for use in polymer nanocomposites.

Full text of DOI:10.1021/la904827d

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Effect of Surfactant Architecture on the Properties of
Polystyrene-Montmorillonite Nanocomposites
Ranya Simons,†,‡ Greg G. Qiao,*,† Clem E. Powell,† and Stuart A. Bateman*,‡
†Polymer Science Group, Department of Chemical and Bimolecular Engineering, University of Melbourne,
Parkville, VIC 3010, Australia, and ‡Materials Science and Engineering, CSIRO, Graham Road,
Highett, VIC 3190, Australia
Received December 22, 2009. Revised Manuscript Received January 27, 2010
A series of surfactants were designed and synthesized for use as clay modification reagents to investigate the impact of
their chemical structure on the nanocomposites morphology obtained following polymerization. The behavior of the
surfactant-modified clays at three different stages were investigated: after ion exchange, following dispersion in styrene
monomer, and oncepolymerizationwas complete. The propensity ofthe styrene monomer to swell the surfactant-modified
clay was observed to be a useful indicator of compatibility and predictor of the resultant polystyrene nanocomposite
morphology which was directly observed using small-angle X-ray scattering (SAXS) and cryogenic transmission electron
microscopy (TEM). It was found that the key components of surfactant design driving exfoliated morphologies were
(1) the position of the ammonium group, (2) the inclusion of a polymerizable group, (3) the solubility of the surfactant in
the monomer, (4) the length of the alkyl chain, and (5) sufficient concentration of surfactant used to exchange the clay.
This understanding should lead to better design of clay modifications for use in polymer nanocomposites.
1. Introduction
investigated.5-13 Several groups have probed the effect of differ-
ent clay modifiers on platelet dispersion, specifically different
alkyl- or arylammonium4,10,12 or alkylphosphonium surfac-
tants6,7 cation exchanged with the clay. More recently, surfactants
which are capable of copolymerizing with monomers like styrene
have been investigated.4-6,8,11,14,15 Nonpolymerizable surfactant-
modified clays resulted in intercalated polystyrene-clay nano-
composite morphologies, while the polymerizable surfactants
provided predominately exfoliated morphologies. The latter ob-
servations were attributed to the surfactant participating in the
polymerization reaction. Taking a different tact, Lee et al.16
prepared styrene nanocomposites using montmorillonite that
had been intercalated with a cationic radical initiator. Exfoliated
structures were formed due to the predominant intragallery poly-
merization over the extra-gallery polymerization using the an-
chored radical initiator. Zang et al.17 ion-exchanged polystyrene-
based quaternary ammonium ions onto montmorillonite to form
polystyrene montmorillonite composites and formed mixed inter-
calated/exfoliated structures, without calculating the amount of
polystyrene which was exchanged onto the clay.
Polymer-clay nanocomposites represent an attractive alter-
native to conventional polymer composites considering that
significant physical property improvements can be achieved at
very low clay concentration.1-3 Such advantages and the resul-
tant flow on benefits in rheology and density have been captured
commercially through the use of polymer-clay nanocomposites
in automobile components, packaging, and construction mate-
rials.1,2 Dispersion or “exfoliation” of montmorillonite (MMT)
clay into its individual clay platelets within a polymer matrix is
known to depend on the polarity and structure of the polymer and
the interaction between the clay and the polymer.4 Previous
investigations have focused on either modifying the clay to
become more organophilic or the polymer to become more
hydrophilic in order to improve the compatibility of the clay
and polymer.
Polystyrene-clay nanocomposites prepared though both
in situ polymerization and melt processing techniques have been
*Corresponding authors. E-mail: Stuart.Bateman@csiro.au (S.B.); gregghq@
unimelb.edu.au (G.Q.).
(1) Chen, B. Br. Ceram. Trans. 2004, 103(6), 241–249.
(2) Usuki; Kojima; Kawasumi; Okada; Fujushima; Kurauchi; Kamigaito
J. Mater. Res. 1993, 8(5), 1179–84.
(3) Fischer, H. Mater. Sci. Eng., C 2003, 23(6-8), 763–772.
(4) LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Appl. Clay Sci. 1999, 15(1-2),
11–29.
(5) Chigwada, G.; Wang, D.; Wilkie, C. A. Polym. Degrad. Stab. 2006, 91(4),
848–855.
Despite previous efforts, the intercalation mechanisms are not
well understood.1 Furthermore, the essential surfactant design
(structural) features required to realize exfoliated platelet mor-
phologies remains unclear, including the position of the ammo-
nium group within the surfactant, the length of the alkyl chain,
and the requirement for polymerizable groups, and have not been
explicitly investigated in the same study.
(6) Zhu, J.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem. Mater. 2001, 13
(10), 3774–3780.
(7) Zhu, J.; Wilkie, C. A. Polym. Int. 2000, 49(10), 1158–1163.
(8) Fu, X.; Qutubuddin, S. Mater. Lett. 2000, 42(1-2), 12–15.
(9) Liu, G.; Zhang, L.; Zhao, D.; Qu, X. J. Appl. Polym. Sci. 2005, 96(4), 1146–
1152.
In this work, a series of surfactant structures were designed
and synthesized for use as clay modifiers in order to study
the relationship between surfactant structure and the resultant
polystyrene/clay nanocomposite morphology. The objective of
(10) Zhang, W. A.; Chen, D. Z.; Xu, H. Y.; Shen, X. F.; Fang, Y. E. Eur. Polym.
J. 2003, 39(12), 2323–2328.
(11) Samakande, A.; Hartmann, P. C.; Cloete, V.; Sanderson, R. D. Polymer
2007, 48(6), 1490–1499.
(12) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694–1696.
(13) Vaia, R. A.; Price, G.; Ruth, P. N.; Nguyen, H. T.; Lichtenhan, J. Appl.
Clay Sci. 1999, 15(1-2), 67–92.
(14) Zeng, C.; Lee, L. J. Macromolecules 2001, 34(12), 4098–4103.
(15) Fu, X.; Qutubuddin, S. Polymer 2001, 42(2), 807–813.
(16) Uthirakumar, P.; Song, M.-K.; Nah, C.; Lee, Y.-S. Eur. Polym. J. 2005,
41(2), 211–217.
(17) Zang, Y.; Xu, W.; Qiu, D.; Chen, D.; Chen, R.; Su, S. Thermochim. Acta
2008, 474(1-2), 1–7.
Langmuir 2010, 26(11), 9023–9031
Published on Web 02/16/2010
DOI: 10.1021/la904827d 9023

Article
Simons et al.
the study was to gain insights into the mechanisms behind the
driving force for exfoliation to enable more rational clay modi-
fication. Montmorillonite was modified with newly designed
organic surfactants through ion-exchange methodology and dis-
persed in polystyrene systems through in situ free-radical poly-
merization. The newly designed surfactants varied in structure in
several ways in order to study the effect of (a) the position of the
ammonium group (head or tail), (b) the inclusion of a polymeri-
zable group, and (c) the length of the alkyl chain. The behaviors of
these modified clays at three different stages (after ion-exchange
onto montmorillonite, the modified clay dispersed in styrene
monomer, and in polystyrene composites) were investigated.
Small-angle X-ray scattering (SAXS) and cryogenic TEM were
used to study the modified clay/monomer interactions. The
structure and properties of the resulting composites have been
determined using wide-angle X-ray scattering (WAXS), transmis-
sion electron microscopy (TEM), dynamical mechanical thermal
analysis (DMTA), and thermal gravimetric analysis (TGA).
Small-angle X-ray scattering (SAXS) was undertaken on the
SAXS/WAXS beamline at the Australian Synchrotron, Victoria,
Australia, at an energy of 12 keV for q values between 0.05
and 0.65 A-1. Styrene/clay solutions were contained in 1.5 mm
diameter quartz capillaries. Scattering data were calibrated for
background scattering and normalized to the primary beam
intensity.
Wide-angle X-ray scattering (WAXS) was performed on a
Phillips PW 1729, Cu KR1 source λ = 0.154 nm. For WAXS of
the nanoclays 3 g of clay was pressed into a sample holder and
placed in the diffractometer. For the nanocomposites themselves
a 4 mm thick polymer sample was prepared via injection molding,
fitted to the sample holder, and placed in the diffractometer for
scanning. The 2θ angles were varied between 1.5° and 30° in order
to measure the d001 spacing of the montmorillonite.
Thermal gravimetric analysis (TGA) was performed on both the
clays and the resulting nanocomposites using a Perkin-Elmer-7
TGA. 5-8 mg of sample was heated from 50 to 800 °C using
a scan speed of 15°/min.
To image the composites, 70-90 nm sections of the composite
samples were microtomed at room temperature using an Ultracut
E microtome at a cutting speed of 0.05 mm/s. A Jeol 100S TEM
was used at 100 keV to study the dispersion of clay particles in
polystyrene.
2. Experimental Section
2.1. Materials. Styrene (99%) was purchased from Sigma-
Aldrich and purifiedbypassing througha basic alumina (Aldrich)
column. Methanol, chloroform, diethyl ether, and ethyl acetate
were purchased from British Drug Houses (BDH) Ltd. and used
without further purification. Tetrahydrofuran (THF) was dis-
tilled from sodium benzophenone ketyl and sodium metal under
argon and stored over 4 A molecular sieves. Dodecyltrimethyl-
ammonium chloride (98%), 4-vinylbenzyl chloride (90%),
N,N-dimethyldodecylamine (97%), N,N-dimethylhexylamine,
2,6-di-tert-butyl-p-cresol (BHT), 1,5-dibromopentane (97%),
1,11-dibromoundecane (98%), ethylbenzyl chloride (70% 4-ethyl-
benzyl chloride, 30% 2-ethylbenzyl chloride), magnesium flakes,
lithium chloride (LiCl), copper(I) chloride (CuCl2), 1,1-diphenyl-
picrylhydrazyl (95%), and trimethylamine hydrochloride (98%)
were purchased from Sigma-Aldrich and used without further
purification. The initiator 2,2-azobis(2-methylpropionitrile) (AIBN)
(DuPont Australia Vazow 64) was recrystallized from ethanol
and stored below 4 °C prior to use. Sodium montmorillonite
(Na-MMT) was obtained from Southern Clay Products and had
a cation exchange capacity (CEC) of 92 mequiv/100 g clay.
2.2. Characterization of Nanoclays and Nanocompo-
sites. Fourier transform infrared (FTIR) was used to observe the
chemical modification of the clay via a Bruker FTIR/NIR with a
resolution of 4 cm-1. Clay was ground into KBR powder and
pressed into disks prior to being placed in the FTIR for scanning.
1H NMR spectra were collected in deuterated chloroform
(CDCl3) using a Varian Unity Plus 400 MHz spectrometer using
tetramethylsilane (TMS) and the deuterated solvent as lock and
residual solvent.
Critical micelle concentration (cmc) determination was per-
formed using the conductivity method.18 Solutions with a range
of concentrationswere madeand the conductivity measured using
a TPS WP-81 conductivity probe. The point at which the gradient
of the curves changed corresponded to the cmc.
Cryogenic transmission electron microscopy (cryo-TEM) was
performed on a Tecnai TF30 300 kV transmission electron
microscope on styrene/clay solutions that had been frozen cryo-
genically onto a coated copper grid by pipetting a drop of the
liquid onto a grid and quickly blotting and freezing in liquid
ethane to form a frozen film.
The dynamic mechanical behavior of the cured samples were
measured on a Pyris Diamond dynamical mechanical analyzer
(DMA) using rectangular samples in tension. The glass transition
temperature (Tg) was determined at the maximum tan δ in the
dynamic mechanical thermal analysis spectrum at 1 Hz.
2.3. Synthesis of Surfactants. The structures of the surfac-
tants used in this study are shown in Figure 1. The structures of
the N,N-dimethyl-N-(4-vinylbenzyl)hexan-1-aminium (6H) and
N,N-dimethyl-N-(4-vinylbenzyl)dodecan-1-aminium (12H) were
synthesized based on the method of Morimoto et al.,19 and the
structures were confirmed by 1H NMR. N,N-Trimethyl-6-
(4-vinylphenyl)hexan-1-aminium (6T) was synthesized based on
the method of Wu et al.20
N,N-Trimethyl-12-(4-vinylphenyl)dodecan-1-aminium (12T)
was a new surfactant, and the synthesis was based on the method
of Wu et al.20 with modifications to account for the longer alkyl
chain. 4-Vinylbenzyl chloride (0.045 mol, 6.93 g) in 20 mL of dry
diethyl ether was added to a stirred suspension of magnesium
flakes (0.09 mol, 2.19 g) in 20 mL of dry diethyl ether at room
temperature, under argon for 30 min using 0.1 g of iodine as a
catalyst. The Grignard reagent was transferred via a canular over
a period of 30 min to a stirred solution of 1,11-dibromoundecane
(0.045 mol, 14.04 g) in dry THF (40 mL) at room temperature,
with LiCl and CuCl2 as catalyst (200 and 100 ppm, respectively).
The mixture was stirred overnight at room temperature. The
reaction mixture was then filtered, and the filtrate was concen-
tratedinvacuum. The concentrate was diluted inether, and 20mL
of 0.2 N HCl was added to the mixture to stop the reaction.
The organic layer was subsequently extracted with saturated
hydrogen carbonate solution, followed by saturated brine solu-
tion. The ether layer was dried over MgSO4 and concentrated
under vacuum. The concentrate was distilled under vacuum in
the presence of 1,1-diphenylpicrylhydrazyl to remove unreacted
dibromo compound. Trimethylamine was made by mixing
trimethylamine hydrochloride (0.065 mol, 6.35 g) with 32.5 mL
of 0.1 M KOH for 3 days at room temperature. The distilled
product was dissolved in acetone and quaternarized with
trimethylamine at room temperature for 5 days. The solvent
was evaporated, and the crude product was dissolved in
chloroform and filtered. The solution was then precipitated
in diethyl ether, washed several times in ether and hexane, and
dried under reduced pressure. 3.9 g (23%) of a white crystalline
compound was obtained. The structure was confirmed by 1H
NMR (400 MHz, CDCl3, TMS): δ 1.25 (m, 20H, -(CH2)10-),
2.9 (s, 2H, benz-(CH2)-), 3.53 (s, 9H, -Nþ-(CH3)3),
3.57 (m,2H, -CH2-Nþ-), 5.2 (m, 1H, -CHdCH2a), 5.7
(19) Morimoto, H.; Hashidzume, A.; Morishima, Y. Polymer 2003, 44(4), 943–
952.
(20) Wu, H.; Kawaguchi, S.; Ito, K. Colloid Polym. Sci. 2004, 282(12), 1365–
1373.
(18) Cochin, D.; Zana, R.; Candau, F. Polym. Int. 1993, 30(4), 491–498.
9024 DOI: 10.1021/la904827d
Langmuir 2010, 26(11), 9023–9031

Simons et al.
Article
Figure 1. Structures of surfactants used in this study to modify montmorillonite.
b
(m, 1H, -CHdCH2 ), 6.7 (m, 1H, =CH-benz), 7.25 (m, 4H,
benzene). The compound was also verified with an accurate mass
spectroscopy measurement (calculated: 330.32; actual: 330.3154).
N-(4-Ethylbenzyl)-N,N-dimethyldodecan-1-aminium (EB) was
also a new surfactant and was synthesized using a modification of
the synthesis for N,N-dimethyl-N-(4-vinylbenzyl)dodecan-1-ami-
nium (12H).19 In a 200 mL flask, ethylbenzyl chloride (70%
4-ethylbenzyl chloride, 30% 2-ethylbenzyl chloride) (0.0647 mol,
10 g) was mixed with N,N-dimethyldodecylamine (0.0647 mol,
13.8 g), 60 mL of ethyl acetate, and 200 ppm 2,6-di-tert-butyl-
p-cresol (BHT) as inhibitor. This reaction was carried out for 72 h
at 40 °C. A colorless precipitate was formed. The product was
washed with diethyl ether and dried under vacuum at room
temperature. The product was purified by recrystallization from
acetone/diethyl ether (1/10, v/v). 18.6 g (86.3%) of a white sticky
compound was obtained (containing 30% of the 2-isomer).The
structure was confirmed by 1H NMR (400 MHz, CDCl3, TMS):
δ 0.88 (m, 3H, -CH3), 1.25 (m, 18H, -(CH2)9-), 1.32 (m, 3H,
CH3-CH2-benz), 1.79 (m, 2H, Nþ-CH2-CH2-), 2.7 (m,
2H, benz-CH2-CH3 3.29 (s, 6H, -Nþ-(CH3)2), 3.5 (m, 2H,
-CH2-Nþ-), 4.97 (s, 2H, benz-CH-Nþ), 7.3, 7.6 (m, 4H,
benzene).
2.5. Synthesis of Polystyrene-Montmorillonite Compo-
sites. The bulk polymerization method of styrene was performed
based on prior work.8,15 The required amount of styrene mono-
mer was charged into a round-bottom flask with the required
amount of clay. The mixture was stirred by vortex for 1 h and
sonificated for 4 h to ensure proper dispersion of the clay. The
mixture was degassed with argon for 20 min prior to addition of
0.5 wt % AIBN initiator. The mixture was then polymerized in an
oil bath at 60 °C for 48 h to obtain polystyrene nanocomposites.
The solid polystyrene was dissolved in the minimum amount of
chloroform, precipitated in methanol, and dried under vacuum to
affordthepolystyrene nanocomposites. Toformbarsformechan-
ical and chemical testing, the nanocomposites were put through a
DSM—small scale twin screw extruder with injection molder—to
form bars with a width of 1 cm and length of 8 cm. The molding
was performed on a 15 cm3 microcompounding extruder and
injection molder (DSM, The Netherlands) at a melt temperature
of 185 °C, a residence time of 5 min, and a screw speed of 65 rpm
The injection molder had the injector barrel set at 195 °C, a mold
temperature at 40 °C, and a pressure at 60 psi.
3. Results and Discussion
The cmc’s of the surfactants in water were determined to be A:
14.5 mmol/L, 6H: 230 mmol/L, 6T: 6.3 mmol/L, 12H: 4 mmol/L,
12T: 0.09 mmol/L, and EB: 4.25 mmol/L. These values agree well
with previous calculations of similar surfactants.20
Quaternary ammonium ions have been widely used to modify
clays for use as polymer fillers. They improve the compatibility of
hydrophilic clay with organic polymers by changing the surface
polarity of the clay to better suit the polarity of the polymer.23 For
polystyrene-clay composites, well-dispersed nanocomposites are
most likely to occur when a reactive functionality is incorporated
into the modifying surfactant.24
In this study, a series of surfactants were designed and then
synthesized for use as clay modifiers in order to investigate the
effect of the architecture of the surfactant on the resultant
nanocomposites. Figure 1 shows the structures (and abbre-
viations) of the different organic modifications used to change
the surface properties of montmorillonite. The surfactants were
varied to show the effect of changing (a) the position of the
ammonium group within the surfactant, (b) the presence of
a polymerizable group, and (c) the length of the alkyl chain.
The 6H, 12H, 6T, and 12T surfactants contain a styrene group,
which can react with styrene during the in-situ polymerization and
2.4. Montmorillonite Modification. The clay modifica-
tion was based on ion-exchange methodolgies.1,4,21,22 Sodium
montmorillonite (Na-MMT) (cation exchange capacity (CEC)=
92 mequiv/100 g) was suspended in 70 °C distilled water (20 g
Na-MMT/3 L water) and stirred for 1 h. It was then allowed to cool
slightly (around 40 °C). A 10% excess of the modifying ion
dissolved in 250 mL of water and 1% BHT as inhibitor (dissolved
in minimal ethanol) were added to the solution, and the resultant
suspension was stirred without heat for a further 3 h. The resulting
suspension was filtered and washed repeatedly with warm distilled
water until no chloride ions were detected using 0.1 M AgNO3
solution. The resulting modified clay was preliminary dried (75 °C)
for 12 h, ground to a particle size of less than 45 μm, and further
dried at 75 °C prior to processing or analysis. The resultant modified
clays are termed as A-MMT, 6H-MMT, 12H-MMT, 6T-MMT,
12T-MMT, and EB-MMT modified with surfactants A, 6H, 12H,
6T, 12T, and EB, respectively.
(23) de Paiva, L. B.; Morales, A. R.; Valenzuela Dıaz, F. R. Appl. Clay Sci. 2008,
´
(21) Suh, D. J.; Lim, Y. T.; Park, O. Polymer 2000, 41(24), 8557–8563.
(22) Xu, L.; Lee, L. J. Polym. Eng. Sci. 2005, 45(4), 496–509.
42(1-2), 8–24.
(24) Carastan, D. J.; Demarquette, N. R. Int. Mater. Rev. 2007, 52(6), 345–380.
Langmuir 2010, 26(11), 9023–9031
DOI: 10.1021/la904827d 9025

Article
Simons et al.
hence increase the movement of the clay platelets to increase
disorder. The styrenic group is attached to the alkyl chain in either
the head position adjacent to the ammonium group (H) or the tail
position at the end of the alkyl chain (T). The alkyl chain length
varies from either 6 or 12 units in order to investigate what effect
the size of the surfactant has. Surfactant EB is a new molecule and
was selected to implicitly investigate the need for a polymerizable
group; it has an identical structure to surfactant 12H apart from
an ethyl group instead of a vinyl group. An alkylammonium
surfactant (A) with the chain length of 12 was also used as a
comparison to determine the importance of including the styrene
group.
While head-type surfactants such as 6H and 12H have been
investigated in the past in styrene-montmorillonite systems,8 the
effect of tail-type surfactants such as 6T and 12T have not been
investigated, and the comparison of both can provide insights into
their effects. It was initially postulated that for the tail-type isomer
the position of the styrenic functionality on the opposite end of
the surfactant to the ammonium group (and hence the ion
exchange site) could mean that the styrene group would be more
accessible to styrene monomer during the polymerization than in
the case of the head-type surfactants and hence lead to better
dispersed systems. It is known that isomers of surfactants of this
structure result in polymers with very different properties;20
however, the impact of isomer on clay modification and resultant
composites has not been investigated in the past.
The cmc’s of the surfactants were calculated and are reported in
section 2.3. The cmc’s of the tail-type surfactants (6T and 12T)
were 2 orders of magnitude smaller than the values for the head-
type surfactants (6H and 12H), which has also been observed in
the past.20 This indicates that the tail-type surfactants form
micelles more easily and at lower concentrations than the head-
type surfactants. The head-type surfactants are less likely to form
micelles because of the structure of the surfactants containing the
hydrophilic ammonium ion between two hydrophobic regions
and hence have higher cmc values. The molecules would need to
twist or bend to form micelles. The EB surfactant has a similar
cmc to the 12H surfactant because the structures of the surfac-
tants are very similar. For the 12H, 12T, and EB surfactants, the
ion exchange onto the clay (at 5.66 mmol/L) was performed at a
concentration above the cmc. For the 6T, 6H, and A surfactants
the ion exchange was performed below the cmc. Because of the
large amount of agitation during the ion-exchange process,
however, the surfactants are most likely not in micelles during
the ion exchange.
3.1. Modification of Montmorillonite with Organic Sur-
factants. The various organo-modified clays studied in this work
were characterized using WAXS, FTIR, and TGA to measure the
exchange level of the organo groups to the intergallery spacing
both qualitatively and quantitatively. FTIR was performed onthe
modified clays to confirm the attachment of the reactive surfac-
tant onto the clay for 6H-MMT, 12H-MMT, 6T-MMT, and
12T-MMT, and the relevant portion of the spectra is shown in
Figure 2. The peaks at 1514, 1487, 1470, and 1413 cm-1 show the
styrene stretches and confirm the attachment of the styrene onto
the clay.25 The peak at 1413 cm-1 corresponds to the double bond
stretch from the styrene group, and the observation of this stretch
provides evidence that the reactive group is preserved during the
ion-exchange reaction.25,26 The peak at 1514 cm-1 corresponds
to the phenyl ring.27 The other characteristic styrene stretches at
Figure 2. Fourier transform infrared (FTIR) spectra of Na-
MMT, 6H-MMT, 12H-MMT, 6T-MMT, and 12T-MMT, con-
firming the presence of reactive surfactant on the clay surface.
Table 1. Basal Spacings via WAXS and Organic Content via TGA for
Modified and Unmodified Montmorillonite
basal spacing
(d001, A)
organic
content (%)
% exchanged
of cation
of modified clay of modified clay exchange capacity
clay
(WAXS)
(TGA)
(CEC)
Na-MMT
A-MMT
11.7
15.7
14.5
18.8
14.5
14.3
18.3
15
19
25
20
21
24
71
84
82
88
69
79
6H-MMT
12H-MMT
6T-MMT
12T-MMT
EB-MMT
around 1631, 1000, and 890 cm-1 are hidden by the strong peaks
from the MMT.
The basal spacings of the clays were calculated from WAXS
spectra for the modified and unmodified montmorillonite, and
they are shown in Table 1.
All modified clays exhibited an increase in basal spacing. The
A-MMT, 6H-MMT, 6T-MMT, and 12T-MMT exhibited only
modest increases in the basal spacing (14-15.7 A). The 12H-
MMT has the largest increase at 18.8 A, followed by the EB-
MMT with a spacing of 18.3 A. The orientation of alkylammo-
nium ions within montmorillonite has been investigated in the
past,28 and it has been found that alkylammonium ions may lie
parallel to the clay surface as a monolayer (corresponding
to a d001 of 1.4 nm), a lateral bilayer (corresponding to a d001 of
1.8 nm), a pseudotrimolecular layer (corresponding to a d001
of 2.3 nm), or an inclined “paraffin” structure (corresponding to a
d001 of 4.6 nm). If these reactive surfactants behave similarly to
alkylammoniums ions in clay, the observation of larger basal
spacings for 12H-MMT and EB-MMT suggests that these
surfactants were orientated in a bilayer arrangement between
the clay layers. Similarly, it is most likely that the 6H-MMT,
6T-MMT, 12T-MMT, and A-MMT surfactants were arranged in
a monolayer. This indicates that the presence of the bulky styrene
group adjacent to the ammonium ion in the head-type arrange-
ment influences the orientation of the surfactants within the clay
layers, and the position of the ammonium group in combination
with the higher alkyl chain length is responsible for the increase in
the basal spacing for the 12H-MMT and EB-MMT. This is
surprising as it is opposite to the initial expectations of the
surfactant structure design that the 12T-MMT would lead to a
modified clay with more accessible functional groups.
TGA analysis of the modified clays was used to determine the
extent of cation exchange. Direct quantitative analysis of the organic
(25) Khalil, H.; Mahajan, D.; Rafailovich, M. Polym. Int. 2005, 54(2), 423–427.
(26) Bachiller-Baeza, B.; Anderson, J. A. J. Catal. 2002, 212(2), 231–239.
(27) Bellamy, L. J. The Infra-red Spectra of Complex Molecules; Chapman and
Hall: London, 1975.
(28) Lagaly, G. Solid State Ionics 1986, 22, 43–51.
9026 DOI: 10.1021/la904827d
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Simons et al.
Article
Figure 3. Possible orientations of the surfactants within the clay layers for (a) 12H-MMT, (b) 12T-MMT, (c) EB-MMT, and (d) 6H-MMT.
The outer arrows indicatethe basal spacing (which includesthe thickness ofone silicate layer at9.6 A plusthe distancebetween silicatelayers),
and the inner arrows indicate the distance between galleries only.
layer was calculated by subtracting the mass loss due to physisorbed
water (occurs around 170 °C) and to dehydroxylation of the mineral
(occurs around 550-680 °C) from the total mass loss.29
3.2. Swelling of Organo-Clay in Styrene Monomer Solu-
tion. To understand the behavior of modified montmorillonite in
the styrene monomer solution, and relate this to final nano-
composite morphologies, solution SAXS and cryogenic-TEM were
performed on the styrene monomer solutions containing organo-
clays after vortex mixing and sonification. It has been observed that
an important indicator of the chemical compatibility between
organo-clay and a solvent or monomer is the swelling behavior
of the clay in the solvent or monomer, with higher swelling degrees
leading to better dispersion in the final composite.31,32
The observed visual swelling of the organo-clays in styrene
monomer is detailed in Table 2. While the unmodified clay
exhibited no swelling, A-, 6H-, 6T-, and 12T-MMT exhibited
partial swelling. 12H-MMT swelled completely and caused gel-
ling of the solution upon sonification, while EB-MMT appeared
to swell completely but only partially gelled. This gel effect of
MMT in styrene has been observed previously by Fu et al.15 The
viscosity increase and gelation by organophilic clay dispersed in
organic media are most likely due to the expansion and delamina-
tion of clay layers and structure formation between the layers due
to strong hydrogen bond or ionic interaction at the edge-to-edge
and edge-to-face contacts.33 This gelation is a good indication
that the clay and styrene are chemically compatible for these
organoclays.
While it has been observed that the swelling of clay in monomer
or solution is a good indication of the compatibility of the clay
with the monomer,31 the effect of clay swelling in monomer on the
basal spacing, to our knowledge, has not been investigated in the
past. In this work, the styrene monomer-clay solution after
vortex and sonication was measured using small-angle X-ray
spectroscopy (SAXS) and the basal spacing data presented in
Table 2. All clays exhibit an increase of the basal spacing upon
addition of the styrene. This increase indicates that styrene is
intercalating between the clay layers. The largest increases were
observed for 12H-MMT and EB-MMT, corresponding to the
best visual swelling and gelation and highest initial basal spacing
of clay. Many of the clay-styrene solutions exhibited two d001
Table 1 lists the calculated organic content of the modified clays
from the TGA data as well as the organic content as a percentage
of the cation exchange capacity (CEC) of montmorillonite. The
surfactants used in the study were exchanged to 69-88% of the
available sites, which is comparable to similar surfactants used to
modify clay in the past.15,29 For comparison, clays exchanged for a
longer period of 24 h were also investigated, and no further
increase in the exchanged amount of surfactant was observed.
It has been suggested that for molecules with unsaturated
aromatic compounds such as styrene, the molecules tend to prefer
the parallel orientation when intercalated between clay layers, in
order to present the dipole parallel to the surface.30 There has also
been evidence of concentration dependence—lower concentra-
tions favor the parallel orientation, while higher concentrations
tend to favor the perpendicular structure. Combining this know-
ledge with observed basal spacing from WAXS, the possible
orientation of the surfactants is postulated in Figure 3.
The models suggest that for 12H-MMT and EB-MMT the
alkyl chain of the surfactant is freer to move in a bilayer, whereas
in 12T-MMT the molecule lies flat along the clay surface due to
interaction with the silicate surface at both ends of the molecule
(the ammonium ion and the styrene group), forming a monolayer.
This orientation of the 12T-MMT could also mean that surfac-
tants lying flat along the clay surface are obstructing ion exchange
sites and thus limit complete ion exchange of the clay as observed
via TGA (Table 1) which showed that the 12T-MMT clay had the
lowest amount of cation exchanged onto the clay (69%). For the 6H-
MMT, the alkyl chain is not long enough to form a bilayer structure
as it does with the 12H-MMT, despite the styrene group being in the
head position of the molecule and so also forms a monolayer. These
models of the surfactant orientations likely account for the differ-
ences in basal spacing observed. Another possible intergallery
structure is that the surfactants are inclined at an angle rather than
forming a mono- or bilayer. Because of the likelihood of interaction
between the styrene and the silicate surface, the monolayer/bilayer
structure is more likely, as postulated in Figure 3.
ꢀ
(31) Arioli, R.; Gonc-alves, O. H.; Castellares, L. G.; da Costa, J. M.; Araujo,
P. H.; Machado, R.; Bolzan, A. Macromol. Symp. 2006, 245-246(1), 337–342.
ꢀ
ꢀ
(29) Osman, M. A.; Ploetze, M.; Suter, U. W. J. Mater. Chem. 2003, 13, 2359–
(32) Burgentzle, D.; Duchet, J.; Gerard, J. F.; Jupin, A.; Fillon, B. J. Colloid
2366.
Interface Sci. 2004, 278(1), 26–39.
(33) Granquist, W. T. M.; L, J. J. Colloid Sci. 1963, 18, 409–420.
(30) Theng, B. K. G. Clays Clay Miner. 1971, 19, 383–390.
Langmuir 2010, 26(11), 9023–9031
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Table 2. Solubility of Surfactants in Styrene and Observed
Simons et al.
Swelling and Basal Spacing of 5% Modified Clay in Styrene
Solution via SAXS
solubility of
swelling of
organo-clay
in styrene
(visual)
basal spacing
(d001, A) of 5%
clay in styrene
surfactant
(not attached
to MMT) in
styrene (g/mL)
clay
monomer (SAXS)
Na-MMT none
A-MMT
10.8
15.7, 35 (broad, very weak)
partial
0
6H-MMT partial
12H-MMT complete
6T-MMT partial
12T-MMT partial
EB-MMT complete
22.6, 14.3
16.7, 35
15.5
14.3
16.7, 33.8
0
0.8
0
0
0.8
peaks in the SAXS, which corresponds to two main populations
of intercalated clay in the solutions. The small increase in basal
spacing observed for the A-, 6H-, 6T-, and 12T-MMT-styrene
solutions may be due to the monolayer arrangement of surfactant
within the clay spacing which does not allow as much styrene to be
intercalated within the layers. The solubility of the surfactants
(when not attached to clay) in styrene was also tested in correla-
tion with the swelling behavior (Table 2). The 12H and EB
surfactants were the only surfactants soluble in styrene monomer,
and the modified clays arising from these surfactants show the
best compatibility in styrene monomer as observed visually and in
the SAXS data. This suggests that during the styrene swelling
phase the surfactants 12H and EB assume more of a vertical
position within the clay layers because they are soluble in styrene,
adopting these orientations to minimize energy and thus increas-
ing in basal spacing.
SAXS and WAXS can only measure the clay structure that
is still ordered within the styrene; any disordered clay that
arises from exfoliation is not observed from these techniques, and
so only partial information is gleaned from these techniques.
To directly observe the clay dispersions in styrene monomer,
cryogenic-TEM images of 2.5% Na-MMT-styrene, 2.5% 12H-
MMT-styrene, and 2.5% 12T-MMT-styrene solutions were
obtained and are shown in Figure 4.
To our best knowledge, this is the first time the clay dispersion
states have been directly observed in monomer solutions by
cryogenic TEM. In the Na-MMT-styrene solution (Figure 4A),
there is indication of large tactoids (groupings of clay platelets
associated closely together) present with occasional individual
platelets and some smaller tactoids. This ordered platelet structure
was also observed by SAXS. The TEM of the 12T-MMT-styrene
solution does not show any evidence of large tactoids, but smaller
tactoids and single platelets are common (Figure 4B). The 12H-
MMT-styrene shows mostly single platelets or tactoids of only a
few platelets together (Figure 4D). Figure 4C shows a region
where the platelets are ordered in an overlapping pattern, which
may indicate possible edge-to-face34 interactions. These TEM
observations correspond to the swelling observations and the
solution SAXS results. Overall, the swelling observations indicate
that the 12H-MMT exhibits the best dispersion of clay platelets in
the styrene solution, through both visual (TEM, and visual
swelling) and analytical (SAXS) techniques.
Figure 4. Cryogenic TEM images of (A) 2.5% Na-MMT,
(B) 2.5% 12T-MMT, and (C, D) 2.5% 12H-MMT in styrene in
different regions and at different magnifications.
polymerization was chosen as the method to form the nanocom-
posites based on work by Wang et al.,35 who found that solution
polymerization always formed intercalated structures whereas
bulk polymerizations were more likely to form exfoliated struc-
tures. The morphology of the polystyrene-montmorillonite
(referred to herein as surfactant-MMT-PS) composites was
determined using TEM and WAXS, which in combination
provide an indication of the dispersion of the clay within the
polystyrene.36 The TEM images of the 5% clay-polystyrene
nanocomposites are shown in Figure 5. The basal spacing’s of
the 1%, 2.5%, and 5% composites as determined via WAXS are
reported in Table 3, and the spectra for the 5% composites
are presented in Figure 6.
TEM of 5% Na-MMT-PS (Figure 5a) indicates a poorly
dispersed system with large tactoids. Minimal increase in the
basal spacing was shown via WAXS (Table 3) for all concentra-
tions, indicating that the composite was a microcomposite with
poor dispersion. The TEM of the 5% A-MMT-PS nanocompo-
site (Figure 5b) shows a better dispersed composite of smaller clay
tactoids, corresponding to a composite with clay particles still
exhibiting order, and this corresponds to a very large peak as
observed in the WAXS spectra (Figure 6). This structure is
considered as an intercalated structure. TEM of the 5% 6H-
MMT-PS, 6T-MMT-PS, and 12T-MMT-PS (Figure 5, parts c, e,
and f, respectively) show platelets and tactoids of various sizes—
ranging from individual platelets, smallertactoidsthat are reason-
ably well dispersed, and tactoids up to 2 μm in diameter,
indicating intercalated structures. The WAXS spectra (Figure 6)
indicate only a very slight increase in the basal spacing, which
indicates that the clay is still in an ordered state (intercalated),
with only a small amount of polystyrene between the clay layers.
Previous work in the area has shown that polymerizable groups on
the clay can sometimes lead to “pinning” of the clay layers.37-39
In these cases, the one growing styrene chain will react with several
styrene surfactants on the clay surface, in effect pinning the layered
structure and leading to a smaller basal spacing as the regular
3.3. Polystyrene-Montmorillonite Nanocomposites. Poly-
styrene composites with unmodified and modified montmorillo-
nite were formed using bulk polymerization methodology. Bulk
(36) Sinha Ray, S.; Okamoto, M. Prog. Polym. Sci. 2003, 28(11), 1539–1641.
(37) Zhang, J.; Jiang, D. D.; Wilkie, C. A. Polym. Degrad. Stab. 2006, 91(4),
641–648.
(34) Luckham, P. F.; Rossi, S. Adv. Colloid Interface Sci. 1999, 82(1-3), 43–92.
(35) Wang, D.; Zhu, J.; Yao, Q.; Wilkie, C. A. Chem. Mater. 2002, 14(9), 3837–
3843.
(38) Zhang, J.; Wilkie, C. A. Polymer 2006, 47(16), 5736–5743.
(39) Zhang, J.; Jiang, D. D.; Wang, D.; Wilkie, C. A. Polym. Degrad. Stab. 2006,
91(11), 2665–2674.
9028 DOI: 10.1021/la904827d
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Article
Table 3. Basal Spacings of Clay-Polystyrene Composites at 1, 2.5,
and 5 wt % As Determined via WAXS
1%
composite
(A)
2.5%
composite
(A)
clay
5% composite (A)
12.3
13.9
14.8
49, 19 (broad, weak)
Naþ-MMT
A-MMT
6H-MMT
12H-MMT
6T-MMT
12T-MMT
EB-MMT
11.8
13.8
15.2
10.5
13.8
14.7
14.7
14.3
18.3
14.4
14.1
17
14.8
14.2
18 (small), 33 (broad)
Figure 6. Wide-angle X-ray scattering spectra of 5% unmodified
and modified montmorillonite-polystyrene composites.
structure of the clay is maintained.37-39 It is possible that this is
happening for these modified clays which have not exhibited vastly
improved intercalation.
TEM images of the 5% 12H-MMT-PS composites show a
predominantly exfoliated structure with some occasional smaller
tactoids visible. From the WAXS (Table 3), at 1 and 2.5%
loading, there is no evidence of a peak in the spectra, indicating
no order to the clay platelets. At 5% there is a low intensity, broad
peakat48.5 A, and a smaller peak at 19.5 A, whichcorresponds to
the occasional small tactoid observed in the TEM (Figure 5d).
This is in agreement with earlier observations by Fu et al.,15 with
the same surfactant used to modify clay for polystyrene compo-
sites, and with Akelah et al.,40 who used a longer chained version
of this surfactant to modify montmorillonite. TEM of 5% EB-
MMT-PS (Figure 5e) indicates better dispersed clay layers than
in the Na-MMT-PS, A-MMT-PS, 6H-MMT, 6T-MMT, and
12T-MMT-PS systems, with some individual clay platelets visible;
however, there are still small tactoids evident (corresponding to
the small peak in the WAXS trace in Figure 6). It is not as
well dispersed as the 12H-MMT-PS composites as observed via
TEM, even at lower clay loadings, which is also confirmed by the
presence of a peak at 1 and 2.5 wt % in the WAXS.
The TEM and WAXS observations indicate that the 12H-
MMT-PS composites are the best dispersed systems. This can be
attributed to the presence of the double bond, as the correspond-
ing surfactant EB does not lead to as well-dispersed systems
despite being identical in structure apart from the double bond.
The position of the ammonium ion in the 12H-MMT in the
head position is also vital, as the 12T-MMT (which varies from
12H-MMT only in the position of the ammonium group) does
not lead to exfoliated structures. The length of the alkyl chain
also played an important role, as the 6H-MMT-PS did not lead
to well-dispersed systems. These observations are expected to
apply for similarly designed surfactants that contain a reactive
Figure 5. TEM images of 5% (a) Na-MMT, (b) A-MMT, (c) 6H-
MMT, (d) 12H-MMT, (e) 6T-MMT, (f) 12T-MMT, and (g) EB-
MMT in polystyrene composites.
(40) Akelah, A.; Rehab, A.; Agag, T.; Betiha, M. J. Appl. Polym. Sci. 2007,
103(6), 3739–3750.
Langmuir 2010, 26(11), 9023–9031
DOI: 10.1021/la904827d 9029

Article
Simons et al.
group, such as for methacrylate- or acrylate-based alkylam-
monium ions.
Table 4. Basal Spacings of Clays Modified at 50% Cation Exchange
Capacity (CEC) and Corresponding Composites As Determined
via WAXS
Physical properties of the surfactants in water were investigated
including the cmc (section 2.3). The ability of the surfactants to
form micelles was not related to the final morphology of the
polymer-clay nanocomposites; the 12T surfactant had the lowest
cmc but only led to intercalated nanocomposites, whereas the
12H surfactant had a higher cmc but led to well-exfoliated
structures. The chemical properties and structure of the surfac-
tants have been observed to be the most important aspects of the
surfactant for designing structures that can lead to well-dispersed
composites.
As observed earlier, the solubility of the surfactants in styrene
(Table 2) showed that only the 12H and EB surfactants were
soluble in styrene. These surfactants led to the highest dispersed
clay not only in styrene monomer but also in the final polystyrene
composite; however, only the 12H-MMT led to completely exfoli-
ated nanocomposites. This work demonstrates that the solubility
of the surfactants in the monomer is a necessary condition for
achievingexfoliation of the clay in the polymer; however, it is not a
sufficient condition. Solubility testing may be an easy and quick
way to screen potential surfactants for use in clay modification.
These insights into clay modifications may provide insight for
other cationic clays exchanged with organic surfactants and for
other free radical polymerized systems.
The work presented so far investigated clay that had been
synthesized with 100% of the CEC (cation exchange capacity)
with respect to surfactant loading. To study the effect of surfac-
tant exchange level, the 6H, 12H, 6T, and 12T surfactants were
also exchanged onto montmorillonite at 50% the CEC of the clay,
and the resultant basal spacings are shown in Table 4. The basal
spacings of these clays as determined by WAXS were less than the
clays modified by 100% the CEC amount (Table 1) in all cases
and were of similar value to each other. These results clearly show
that the surfactants did not expand the intergallery space to the
same level as in the case when there was sufficient surfactant
for maximum ion exchange. For the 12H-MMT, at 50% ion
exchange the surfactant is most likely forming a monolayer
arrangement (with a basal spacing of 14.7 A) in the clay spacing
rather than the bilayer arrangement as hypothesized in Figure 3
for the 100% CEC exchanged clays. The corresponding 5%
composites of the 50% CEC clays also exhibited lower basal
spacings, including the 12H-MMT composites, which in this
case did not lead to exfoliated structures (although it did when
exchanged to 100% onto the clay). This demonstrated that the
clay needs to be modified with 100% surfactant in order to
sufficiently improve the compatibility and increase the basal
spacing sufficiently for styrene to enter the spacing and polymerize
within the clay layers to form exfoliated structures. These results
contrast the report by Suh et al.,41 who used stearylamine as a clay
modifier, in which case 50% exchanged clays led to exfoliated
nanocomposites, whereas 100% exchanged clays did not. They
hypothesized at 50% surfactant loading there was more room
within the clay layers for the polymer to intercalate. The differ-
ences in results between the reported work and the current work
may be due to the different organic modifications used, where the
presence of the styrene group in the surfactant in the current work
is the driving force for styrene monomer to intercalate within
the clay layers. Clays exchanged to 150% of the CEC were also
synthesized and used as fillers in polystyrene nanocomposites (not
shown). However, these composites showed similar behaviors to
those at 100% CEC. This indicates excess surfactant was washed
basal spacing of
polystyrene composites
(5% clay loading) (A)
basal spacing of
clay (A)
clay
6H-MMT 50% CEC
12H-MMT 50% CEC
6T-MMT 50% CEC
12T-MMT 50% CEC
14.3
14.7
14.5
14.0
14.4
14.7
14.5
14.3
away after ion exchange was complete, leaving the equivalent
surfactant concentration on the clay to the 100% CEC clays.
3.4. Physical Properties of Polystyrene-Montmorillonite
Nanocomposites. The physical properties of the polystyrene-
montmorillonite composites were investigated to determine what
effect the morphology of the composites had on the properties
and are reported in Table 5 for nanocomposites containing 5%
modified and unmodified MMT. The thermal mechanical beha-
vior of the polystyrene composites was measured from -100 to
140 °C on a DMTA, and the resulting storage modulus and Tg
were calculated. The storage modulus E0 at 25 °C increased for all
modified clay composites by up to 119% for the 5% 12H-MMT-PS
and 58% for the 5% EB-MMT-PS, whereas the unmodified clay
(Na-MMT) composite decreased by 35% over unreinforced poly-
styrene. This is further proof that the modified clays have improved
the interaction of the clay with the polystyrene, allowing the
reinforcement of the clay platelets to improve the physical proper-
ties, particularly for the best dispersed systems of 5%12H-MMT-PS
and 5% EB-MMT-PS. This enhancement in storage modulus is due
to the high aspect ratio of the dispersed clay, which improves the
interaction between the clay layers and polymer chains, resulting in
a decrease in the polymer chains mobility near the polymer-clay
interface. Modulus values above the Tg (> 1 1 0 °C) in the rubbery
region did not exhibit a plateau value as is commonly observed as
the composites melted in the sample holders, making it difficult to
measure modulus values in this region.
One phase transition was observed via DMTA, corresponding
to a glass transition (Tg) of around 110 °C. Addition of Na-MMT
decreased the Tg by 2 deg compared to unfilled polystyrene, which
had a Tg of 111 °C. The 5% A-MMT-PS, 5% 6H-MMT-PS, 5%
6T-MMT-PS, and 5% 12T-MMT-PS had an increased Tg by
2-4 °C. The best dispersed systems of 5% 12H-MMT-PS and 5%
EB-MMT-PS actually showed a slight decrease in the Tg, although
these differences in Tg are not highly significant with respect to the
error associated with the technique. Decreases in Tg have been
observed previously for well-dispersed systems and have been
attributed to the high viscosity of the styrene-modified mont-
morillonite solutions which affects the dispersion of initiator
molecules in the bulk free radical polymerization.8
The addition of clay increases the peak differential temperature
observed using TGA for the major thermal decomposition phase in
the polystyrene by up to 23 °C. There is little difference between the
unmodified Na-MMT composite and the modified clay composites
which indicates that the ion exchange of a reactive styrenic
functionality onto the clay does not make the resultant nanocom-
posite more susceptible to thermal degradation. The improvement
in the thermal stability of the clay nanocomposites has been
attributed to the restriction of polymer chains within and close to
the silicate layers, leading to greater resistance to thermal degrada-
tion and has been noted for different clay-polymer systems.42,43
(42) Blumstein, A. J. Polym. Sci., Part A 1965, 3, 2665.
(43) Dean, K. M.; Bateman, S. A.; Simons, R. Polymer 2007, 48(8), 2231–2240.
(41) Suh, D. J.; Park, O. O. J. Appl. Polym. Sci. 2002, 83(10), 2143–2147.
9030 DOI: 10.1021/la904827d
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Simons et al.
Article
Table 5. Thermal and Mechanical Properties of Polystyrene-Montmorillonite Composites
storage modulus E0
at 25 °C (Pa)a
glass transition
Tg (°C)b
peak degradation
temperature (°C)c
char (%)d
expt (calcd)
sample
polystyrene
1.67 ꢀ 109
111
109
112
114
110
114
114
110
427
446
441
447
439
450
450
442
0.8
5% Na-MMT-PS
5% A-MMT-PS
5% 6H-MMT-PS
5% 12H-MMT-PS
5% 6T-MMT-PS
5% 12T-MMT-PS
5% EB-MMT-PS
1.1 ꢀ 109
5.2 (5.15)
5.2 (4.95)
4.9 (4.95)
6.1 (4.35)
5.2 (4.95)
5.0 (4.75)
5.3 (4.55)
2.33 ꢀ 109
2.04 ꢀ 109
3.65 ꢀ 109
2.14 ꢀ 109
2.15 ꢀ 109
2.65 ꢀ 109
a Storage modulus E0 (Pa) of polystyrene and 5 wt % montmorillonite based composites from dynamic mechanical thermal analysis (DMTA). b Glass
transition temperature Tg (°C) of polystyrene and 5 wt % montmorillonite based composites from dynamic mechanical thermal analysis (DMTA). c Peak
differential temperature (°C). d Char % (experimental and calculated) obtained from thermal gravimetric analysis (TGA).
The peak differential temperature of the 12H-MMT-PS is lower
than for the intercalated and microstructured composites but
higher than pristine polystyrene. The lower thermal stability of
exfoliated nanocomposites over intercalated was observed pre-
viously11 and attributed to the ion-exchanged modifying surfactant
having poorer thermal properties which decreases the overall
stability of exfoliated samples.
The char of the clay composites is also presented in Table 5 and
is calculatedas the remainingmass at 700°C as a percentageof the
startingmass. The calculatedchar in parentheses was based onthe
known organic content of the modified clay as determined by
TGA (section 3.1) plus the char remaining from pristine poly-
styrene as determined byTGA. It can beseen that the 12H-MMT-
PS has the highest char and is of a higher value than the calculated
char. This is an indication that the 12H-MMT-PS has improved
fire-retardancy properties, as the buildup of this high-perfor-
mance carbonaceous char improves retardancy due to its thermal
insulating and low permeability to further degradation.44,45
This increase in char has been attributed to the barrier effect,46,47
which assumes the formation of a carbonaceous silicate char that
builds up on the surface of the polymer melt and acts as a mass
and heat transfer barrier.
the final nanocomposite morphology. For in situ free radical
polymerized polystyrene-montmorillonite composites, it was
determined that the important aspects of surfactant design to
achieve well-dispersed and -exfoliated nanocomposites include
the following:
1. The position of the ammonium ion in the surfactant.
Locating the ammonium group in the head position (adjacent
to the styrene group) led to exfoliated structures whereas locating
the ammonium ion in the tail (at the end of the alkyl chain) only
led to intercalated structures.
2. The presence of a polymerizable group which can react with
the styrene monomer during polymerization.
3. The solubility of the surfactant in the styrene monomer.
Soluble surfactants in styrene led to the best dispersed systems,
whereas partially soluble or insoluble surfactants only led to
intercalated morphologies.
4. The concentration of the surfactant exchanged on the clay.
Exchanging surfactant to 100% of the clays cation exchange
capacity (CEC) is necessary to lead to well-dispersed systems.
5. The length of the alkyl chain; for the head-modified clays,
homologues of alkyl chain 6 did not lead to well-dispersed
samples whereas an alkyl chain of 12 did.
The behavior of the modified clays in styrene monomer is also
an important indicator of the ability of the organic modification
to lead to well-dispersed composites. The physical properties were
improved for the modified clay/polystyrene systems, particularly
for the best dispersed clay composites. This understanding of the
key criteria for surfactant design for use as clay modification
agents can lead to better designed nanocomposite systems for free
radical polymerized systems.
4. Conclusion
A series of polystyrene montmorillonite composites have
been developed with differently modified clays in order to
probe the effect of the structure of the organic modification on
(44) Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R.; Manias, E.;
Giannelis, E. P.; Wuthenow, M.; Hilton, D.; Phillips, S. H. Chem. Mater. 2000,
12(7), 1866–1873.
(45) Wang, S.; Hu, Y.; Song, L.; Wang, Z.; Chen, Z.; Fan, W. Polym. Degrad.
Stab. 2002, 77(3), 423–426.
(46) Gilman, J. W. Appl. Clay Sci. 1999, 15(1-2), 31–49.
(47) Bourbigot, S.; VanderHart, D. L.; Gilman, J. W.; Awad, W. H.; Davis,
R. D.; Morgan, A. B.; Wilkie, C. A. J. Polym. Sci., Part B: Polym. Phys. 2003, 41
(24), 3188–3213.
Acknowledgment. The authors thank Ken Goldie at the Bio21
Institute, Victoria, Australia for the cryogenic TEM images. The
authors also thank Nigel Kirby and the SAXS/WAXS beamline
at the Australian Synchrotron, Victoria, Australia.
Langmuir 2010, 26(11), 9023–9031
DOI: 10.1021/la904827d 9031