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DOI 10.1140/epje/i2011-11099-2
Regular Article
Eur. Phys. J. E (2011) 34:99 THE EUROPEAN
PHYSICAL JOURNAL E
Effect of nanoparticle dispersion on glass transition in thin films
of polymer nanocomposites
S. Chandran and J.K. Basua
Department of Physics, Indian Institute of Science, Bangalore, 560012, India
Received 30 April 2011 and Received in final form 15 July 2011
Published online: 23 September 2011 – c
EDP Sciences / Societ`a Italiana di Fisica / Springer-Verlag 2011
Abstract. We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites
consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness
variation is used to estimate the glass transition temperature, Tg. In these thin films we find a significant
dependence of Tgon the nature of dispersion of the embedded nanoparticles. Our work thus highlights
the crucial role played by the particle polymer interface morphology in determining the glass transition in
particular and thermo-mechanical properties of such nanocomposite films.
1 Introduction
Polymer nanocomposites (PNC) are a novel class of ma-
terials usually consisting of inorganic nanoparticles with
tunable electrical, optical and magnetic properties embed-
ded in a polymer matrix which can be readily processed
using solvent or thermal treatments [1–3]. The enormous
technological potential of such materials has also led to
rapid growth in fundamental investigation on the disper-
sion and in correlation with the ultimate physical prop-
erties attainable in such materials [1–10]. Since most of
the polymer used play the role of the passive matrix
and are glassy in the bulk, a large amount of work has
been devoted to understand how the glass transition and
glassy dynamics is affected by incorporation of nanopar-
ticles [4,5,7,10]. This is not only helpful in understanding
and and hence controlling the thermo-mechanical prop-
erties of these PNCs but also obtaining a general under-
standing of the much studied but controversial field of
finite-size and interface effects on glass transition of poly-
mers [5, 8, 10–18]. Although the equivalence of finite-size
effects in bulk PNCs and polymer thin films [5,10] or poly-
mers confined in nanopores [14] has been shown, the role
of dispersion of nanoparticles in PNC films has not been
adequately studied or understood [15, 19]. Here, we show
how the dispersion of 1-octadecanethiol (ODT)-capped
gold nanoparticles (AuNP) at various densities inside thin
poly(styrene) (PS) films of thickness 75 ±3 nm is related
to the glass transition temperature, Tg, of the thin films.
The Tg’s have been estimated from spectroscopic ellip-
sometry on these thin films prepared with various volume
fractions, φp, of ODT-capped AuNP in PS matrix and
ae-mail: basu@physics.iisc.ernet.in
under different conditions of thermal annealing to control
their dispersion.
2 Experimental details
2.1 Synthesis
The thiol functionalized gold nanoparticles (AuNPs) were
synthesized by reduction of hydrogen tetrachloroaurate
trihydrate (HAuCl4.3H2O) with super hydride (lithium
triehtylborohydride) in Tetrahydrofuran (THF) after stir-
ring for 20 minutes in the presence of ODT as described
earlier [20–23]. All chemicals, unless stated otherwise, were
purchased from Sigma-Aldrich. After stirring the growth
solution for a couple of hours, the growth is stopped by
adding ethanol (Merck). The particles were cleaned for
excess ODT by selective precipitation using the mixture
of THF and ethanol and then centrifuging the mixture.
Finally the ethanol-THF mixture was decanted and the
residue was dried for 12 hours.
2.2 Characterization ODT-capped AuNPs
The size of the synthesised particles were estimated from
Transmission Electron Microscopy (TEM) images, ob-
tained using Technai G20 High Resolution TEM. The
TEM micrograph in fig. 1 shows that reasonably monodis-
perse gold particles were obtained. Thermo gravimetric
analysis (TA Instruments) is used to find the relative frac-
tion of ODT and gold in ODT-capped AuNPs. The ob-
tained fraction along with the size were used to estimate
the grafting density σ(= 5.5 chains/nm2) of ODT on the
nanoparticle surface.
Page 2 of 5 The European Physical Journal E
Fig. 1. Transmission electron microscopic images of ODT-
capped AuNPs. Inset: histogram of the particle diameters and
a Gaussian fit to show the mean particle size 3.79 nm.
Table 1. Sample identification.
Sample Volume Mean interparticle h/Rg
Fraction percentage distance (a)
φph(nm)
A1 0.1 50.49 6.33
A2 0.3 33.07 4.14
A3 0.5 26.89 3.37
A4 0.55 26.84 3.24
A5 0.75 22.67 2.84
A6 1.22 18.30 2.29
(a)The mean interparticle distance given here is applicable only for
the annealed samples, where the particles are well dispersed in the vol-
ume [5].
2.3 Preparation of nanocomposite thin films
ODT-grafted gold nanoparticles and Poly-(styrene)
(Mw= 97400 g/mol with a calorimetric Tgof 106 ◦C; ra-
dius of gyration, Rg= 8nm) were dissolved separately in
toluene for 12 hours. Once both were dissolved completely,
they were mixed and then stirred for ∼12 hours for en-
suring a homogeneous dispersion. Highly polished silicon
wafers (Vin Karola, USA) with a native silicon dioxide
layer (∼2nm) served as substrates for ellipsometric mea-
surements. The silicon surface is thoroughly cleaned with
acidic piranha, to make the surface hydrophilic before spin
coating. The films as identified in table 1 were then spin
coated at a rate of 3000 rpm (for 100 s). The films were
annealed at two different conditions: (a)∼150 ◦C for 12
hours, (b) ∼70 ◦C for 4 hours in vacuum better than
∼5×10−3mbar. Annealing is done at different temper-
atures one below and another above the Tgof the matrix
to control different dispersions of the particles and still
ensuring the removal of trapped solvent.
2.4 Atomic force ficroscopy
Atomic force microscopy (AFM, NT-MDT, Russia) is used
in contact mode to get the surface morphology of the sam-
ples. Cantilevers (force constant = 0.03–0.2 N/m) with a
radius of curvature 10nm were used. The images were col-
lected at a scan frequency of 1 Hz and at a minimum set
point as determined from force-distance curves, to ensure
that the tip does not drag on the sample.
2.5 Ellipsometry
Temperature-dependent ellipsometry measurements were
carried out at a fixed angle of incidence of 70 ◦using
a spectroscopic ellipsometer SE850 (Sentech, Germany)
connected to a home-made vacuum (3 ×10−2mbar) tem-
perature cell. The measurements were performed in cool-
ing at a rate of 1 ◦C/min starting from 150 ◦C(Tgof
bulk PS from differential scanning calorimetry is 105 ◦C
and that measured in thin film without nanoparticles is
106 ◦C). The Ψand Δvalues were collected from the high-
est to room temperature continuously in the cooling cycle.
An effective medium layer with Maxwell-Garnett type of
dispersion [24] is assumed for the sample (PNC thin film)
layer for finding the complex dielectric function fand
thereby the thickness dof the film. The Maxwell-Garnett
type dispersion assumes a mixture of two distinct mate-
rials, each possessing the optical properties of the bulk
material, and requires that the particles dispersed in the
host material do not interact with one another. This can
be satisfied by keeping the volume fraction of the dispersed
particles low. The complex dielectric function, f,ofthe
film can be written as
f−m
f+2m
=Fp−m
p+2m
,(1)
where m,pare the dielectric constants of the matrix
and the particulate fillers, respectively, and Fis the fill
factor of the particles. The thickness dof the films was
obtained by fitting Ψand Δwith a model as specified. The
obtained thickness is plotted as a function of temperature,
and the respective Tgis obtained from the change in slope,
as shown in fig. 3.
3 Results and discussion
The AFM images in fig. 2 shows the dramatic role of
high temperature (150 ◦C) thermal annealing on disper-
sion of ODT-capped gold nanoparticles in PS thin films.
The changes in morphology of low temperature (70 ◦C
for 4 hours) annealed films (fig. 3) is less significant, in
comparison, but is also dependent on volume fraction. Al-
though it thus seems to indicate the possibility of the pres-
ence of some surface mobility even at this low temperature
(as compared to the bulk Tg) along the lines of [17, 18].
The dispersion of the nanoparticles is complete when an-
nealed above bulk Tg. The height of the nanoparticle do-
mains visible in unannealed and 70 ◦C annealed films are
S. Chandran and J.K. Basu: Effect of nanoparticle dispersion on glass transition . . . Page 3 of 5
Fig. 2. AFM image showing the surface morphology of un-
annealed (a,b,c,d) and annealed at 150 ◦C (e,f,g,h) for samples
A1, A4, A5, A6, respectively.
Fig. 3. AFM image showing the surface morphology of un-
annealed (a,d), annealed at 70 ◦C (for 4 hours) (b,e) and at
150 ◦C (for 12 hours) (c,f) for the samples A1 and A6, respec-
tively.
∼3 nm, which seems to indicate that it is a monolayer of
the nanoparticles. The dispersion of nanoparticles in poly-
mers is a great challenge [25] and the nature of dispersion
depends, to a large extent, on the interaction between the
nanoparticles and the host polymer. We (not presented
here) and others [15, 26] have found that gold nanopar-
ticles capped with appropriate polymers can form stable
dispersions in suitable host polymer matrix even without
annealing. We will discuss the role of thermal treatment
and especially of the consequent degree of dispersion of
nanoparticles in the polymer matrices on the glass transi-
tion behaviour of PNC films.
Several authors have reported their investigations of
thermo-mechanical properties in general and glass transi-
tion in particular on polymers confined in pores [12–14]
or in thin films as well as bulk and thin film poly-
mer nanocomposites [8–11, 16, 17]. Widely varying ther-
mal treatment have been used for thin polymer films and
this has resulted in a great controversy about the finite-
size effect on glass transition of such films [11, 16, 18].
The available literature on bulk [5, 7–10] and thin film
PNCs [15, 19, 27, 28] is also controversial in terms of the
conclusions drawn on the role of confinement and sur-
face effects in such systems. For example, some authors
Fig. 4. (a) Δas a function of temperature, T,atafixedwave-
length of 350 nm for samples A2 and A4 as indicated in the
panel. (b) Thickness as a function of T. The continuous lines
in (a) and (b) are the linear fits in the respective regions. The
Tgof the respective samples is indicated in the panel.
have observed large changes in Tgof thin film polymer
nanocomposites as a function of φpof nanoparticles in
polymers. However, in most of these cases, it turns out
that the films have either not been annealed or are an-
nealed at temperatures below the Tgof the host poly-
mer [15]. In other cases, some authors have found small
changes or no change with increasing volume fraction
of nanoparticles when the films have been annealed at
temperatures above the host polymer Tg[27, 28]. We
have, hence used a thermal-expansivity–based study of
glass transition of, otherwise identical, thin film polymer
nanocomposites which have been annealed at tempera-
tures below and above the host polymer Tg. In fig. 3, a typ-
ical variation of the spectroscopic ellipsometry parameter,
Δ(at a wavelength of 350 nm) as a function of measured
temperature, T, is shown for two such films which have
been annealed at 150 ◦C. The Tgcan be clearly identified.
We have also analyzed the full spectroscopic ellipsometry
data to extract the thickness at each temperature. Within
errors, the Tgobtained from the two analyses are identi-
cal; hence for all other films we have used the variation of
Δwith Tto extract the respective, Tg.
The results are summarised in fig. 4 for both the low
temperature and high-temperature annealed films. From
fig. 4(a) we observe that for the low-temperature annealed
films,atthelowestφp,Tgincreases with respect to the PS
Page 4 of 5 The European Physical Journal E
Fig. 5. Variation of Tgas a function of volume fraction per-
centage, φp, of the ODT-capped AuNPs after annealing at
70 ◦C (a) and 150 ◦C (b). The dot-dashed line in both pan-
els gives the calorimetric Tgof PS.
film of identical thickness. However, with increasing φp,
we observe significant depression of Tg. This is something
which is quite unusual and, to our knowledge, has not
been observed earlier for similar systems. It seems clear
from the AFM images that there is considerable surface
segregation of the nanoparticles for all the unannealed and
low-temperature annealed films. Such an effect was found
to lead to an enhancement of Tgin an earlier work [15].
However, the depression in Tgis not expected for our films
with higher φpsince the nature of the dispersion does not
seem to change significantly with increasing φp. The de-
crease of Tgwith increasing φpis due to the possible de-
pletion of polymer chains around the large nanoparticle
domains at the film surface. The variation of Tgchanges
quite dramatically with high-temperature annealing of the
films. From fig. 4(b) we find that the Tgvalues are either
equal to neat PS or depressed compared to it, irrespec-
tive of φp. However, the effect is not as strong as that
observed for the films annealed at lower temperature. For
well-dispersed ODT-capped Au NP particles in PS this
would be expected, since the PS chains are expected to be
depleted from the ODT interface due to unfavorable in-
teractions leading to enhancement in segmental mobility
at the nanoparticle polymer interface and hence a reduc-
tion in Tg. It might be noted here that an increase in φp
corresponds to either a reduction in film thickness or pore
size for bulk polymers confined in pores. At the volume
fractions investigated, the mean interparticle separation
is ∼50–100 nm. At such confinement dimensions the re-
duction in Tghas been found to be quite small compared
to the bulk polymers [12–14, 17]. Thus the observed de-
viations in Tgfor the PNC thin films at the studied φp
is not unusual. However, our primary finding is that the
extent of such variations as well as the anomalous nature
of such variations, that has been observed earlier [15], is
very sensitive to the nature of dispersion of nanoparticles
and hence only equilibrated films which have been well
annealed give the appropriate nature of variation of Tgin
thin films of PNCs.
4 Conclusion
In conclusion we have shown how thermal treatment of
nanoparticle-embedded polymer films can lead to signif-
icant variation in the morphology and dispersion of the
nanoparticles in the host polymer. We have shown that
as a consequence, the nature of variation of glass transi-
tion in such films changes dramatically due to sensitive
surface and interface effects at the polymer-nanoparticle
interface. Further wok involving the interplay of confine-
ment and surface effects is underway to better understand
the physics of glass transition in such systems.
The authors would like to acknowledge IISc Nanoscience Ini-
tiative for providing access to TEM facility.
References
1. R.B. Thompson, V.V. Ginzburg, M.W. Matson, C. Balazs,
Science 292, 2469 (2001).
2. S.C. Warren, L.C. Messina, L.S. Slaughter, M. Kamper-
man, Q. Zhou, S.M. Gruner, F.J. DiSalvo, U. Wiesner,
Science 320, 1748 (2008).
3. M.R. Bockstaller, E.L. Thomas, Phys. Rev. Lett. 93,
166106 (2004).
4. M.K. Corbierre, N.S. Cameron, M. Sutton, K. Laaziri,
R.B. Lennox, Langmuir 21, 6063 (2005).
5. S. Srivastava, J.K. Basu, Phys. Rev. Lett. 98, 165701
(2007).
6. F.W. Starr, T.B. Schroder, S.C. Glotzer, Phys. Rev. E 64,
021802 (2001).
7. A. Bansal, H. Yang, C. Li, B.C. Benicewicz, S.K. Kumar,
L.S. Schadler, J. Polym. Sci. Part B-Polym. Phys. 44, 2944
(2006).
8. V. Pryamitsyn, V. Ganesan, Macromolecules 43, 5851
(2010).
9. J.M. Kropka, V. Pryamitsyn, V. Ganesan, Phys. Rev. Lett.
101, 075702 (2008).
10. A. Bansal, H. Yang, C. Li, K. Cho, B.C. Benicewicz, S.K.
Kumar, L.S. Schadler, Nat. Mater. 4, 693 (2005).
11. S. Napolitano, M. Wubbenhorst, Nat. Commun. 260,2
(2011).
12. C.L. Jackson, G.B. McKenna, J. Non-Cryst. Solids 131-
133, 221 (1991).
S. Chandran and J.K. Basu: Effect of nanoparticle dispersion on glass transition . . . Page 5 of 5
13. J. Zhang, G. Liu, J. Jonas, J. Phys. Chem 96, 3478 (1992).
14. Y.P. Koh, Q. Li, S.L. Simon, Thermochim. Acta 492,45
(2009).
15. A. Arceo, L. Meli, P.F. Green, Nano Lett. 8, 2271 (2008).
16. M. Tress, M. Erber, E.U. Mapesa, H. Huth, J. Muller, A.
Serghei, C. Schick, K.-J. Eichhorn, B. Voit, F. Kremer,
Macromolecules 43, 9937 (2010).
17. Z. Fakhraai, J.A. Forrest, Science 319, 600 (2008).
18. Z. Yang, Y. Fujii, F.K. Lee, C.-H. Lam, O.K.C. Tsui, Sci-
ence 328, 1676 (2010).
19. J.Q. Pham et al., J. Polym. Sci. Part B-Polym. Phys. 41,
3339 (2003).
20. M.K. Corbierre, N.S. Cameron, M. Sutton, S.G. Mochrie,
L.B. Lurio, A. Ruhm, R.B. Lennox, J. Am. Chem. Soc.
123, 10411 (2001).
21. M.K. Corbierre, N.S. Cameron, R.B. Lennox, Langmuir
20, 2867 (2004).
22. S. Srivastava, S. Chandran, A.K. Kandar, C.K. Sarika,
J.K. Basu, S. Narayanan, A. Sandy, J. Chem. Phys. 133,
151105 (2010).
23. A.K. Kandar, S. Srivastava, J.K. Basu, M.K. Mukhopad-
hyay, S. Seifert, S. Narayanan, J. Chem. Phys. 130, 121102
(2009).
24. J.C. Maxwell-Garnett, Philos. Trans. R. Soc. London, Ser.
A203, 385 (1904).
25. M.E. Mackay, A. Tuteja, P.M. Duxbury, C.J. Hawker, B.V.
Horn, Z. Guan, G. Chen, R.S. Krishnan, Science 311, 1740
(2006).
26. X.C. Chen, P.F. Green, Langmuir 26, 3659 (2010).
27. J.H. Xavier, S. Sharma, Y.S. Seo, R. Isseroff, T. Koga, H.
White, A. Ulman, K. Shin, S.K. Satija, J. Sokolov, M.H.
Rafailovich, Macromolecules 39, 2972 (2006).
28. T. Koga, C. Li, M.K. Endoh, J. Koo, M. Rafailovich, S.
Narayanan, D.R. Lee, L.B. Lurio, S.K. Sinha, Phys. Rev.
Lett. 104, 066101 (2010).
