The role of nanoconfinement on the glass transition temperature (Tg) of polymers has been extensively studied in the thin film geometry. This work looks at the Tg of nanoconfined polystyrene in spherical nanoparticles (27- 200 nm), individually dispersed in crosslinked polybuthylmethacrylate matrices. A reduction of Tg compared to the bulk was observed in PS nanospheres via neutron scattering-mechanical tests and was found to decrease with decreasing diameter for D< 50nm. The general trend of Tg depression in nanospheres was found to be similar to the results on thin film geometry

RHARBI, Yahya

YOUSFI M.

I-Revues

18ème Congrès Français de Mécanique Grenoble, 27-31 août 2007 
Nanomechanical Properties of NanoConfined Polystyrene Nanoparticles in 
Nanoblends 
 
Yahya Rharbi*, M. Yousfi 
 
Laboratoire de Rheologie, UMR 5520 
PB 53 Domaine universitaire 38041 Grenoble cedex  9 France 
rharbi@ujf-grenoble.fr 
 
 
 
Abtract  : 
 
The role of nanoconfinement on the glass transition temperature (Tg) of polymers has been extensively 
studied in the thin film geometry. This work looks at the Tg of nanoconfined polystyrene in spherical 
nanoparticles (27- 200 nm), individually dispersed in crosslinked polybuthylmethacrylate matrices. A 
reduction of Tg compared to the bulk was observed in PS nanospheres via neutron scattering-mechanical 
tests and was found to decrease with decreasing diameter for D< 50nm. The general trend of Tg 
depression in nanospheres was found to be similar to the results on thin film geometry.    
 
 
Key-words :  
 
glass transition ; nanomechanics ; polymer nanoblends ; confinement 
 
1 Introduction 
 
The glass transition (Tg) of nanoconfined polymers has been extensively studied 
throughout the last decade (Rieter (1993) - Riggleman and Al (1997)). The majority of this 
research has focused on thin film geometry and was carried out on supported and free-standing 
films using various experimental techniques; Brillouin light scattering, (Forrest and Al (2002)) 
ellipsometry (Keddie and Al (1994), Kawana and al (2001)), dielectric relaxation (Fukao and Al 
(2000)), x-ray reflectivity (van Zanten and Al (1996)), fluorescence (Ellison and Al. (2003)). 
Mechanical measurements on nano-bubble inflation have also been used to probe Tg in ultrathin 
films (O’Connell and Al (2005)). These reports substantially agreed that the Tg of polystyrene 
(PS) decreases with decreasing film thickness for h < 50 nm. They demonstrate the relevance of 
polymer structure and polymer/substrate interactions in confined Tg. Some reports also raised 
the question of the influence of thin film sample preparation (spin coating) on Tg reduction 
(Alcoutlabi and Al (2005)). This issue could be settled by studying samples prepared by other 
means.  
Although polymers are often found nanoconfined in geometries such as spheres, little 
effort has been dedicated to the study of their glass transition. This information could be useful 
in many industrial and environmental applications; blends, copolymers, nanocomposites, 
colloids, coatings… One example is zero-VOC coating, which uses polymer nanoparticles in 
the film-making process (Steward and Al (2000), Tirumkudulu and Al (2005)). VOCs are used 
in coatings to lower the particle Tg, which permits the fabrication of crack-free films at room 
temperature. If the Tg is reduced by decreasing the nanoparticle size, the use of VOCs could be 
avoided, which would have a positive impact on the environment. Another example is in nano-
blends, where hard polymer nanoparticles are used to reinforce soft matrices. Reducing 
nanoparticle size would be counterproductive in this application if it causes the reduction of 
their Tg. It is therefore of utmost importance to tune in to the study of nano-confined glass 
transition of polymers in spherical geometries.  
1 
18ème Congrès Français de Mécanique Grenoble, 27-31 août 2007 
This paper describes the influence of the size and the environment of spherical PS 
nanoparticles, on their glass transition and their nanomechanical properties. It looks at the 
properties of polystyrene confined in nanospheres (27 nm to 120 nm) within 
Polybutylmethacrylate (PBMA) matrices. We use a method which combines Nanomechanics 
with Small Angle Neutron Scattering (SANS) to measure the reduction of the glass transition of 
PS nanoparticles and we compare these results to those found on thin film geometry. This study 
also focuses on the effect of proximity of two polymers on their glass transition. 
 
2 Experimental 
 
Polybutylmathacrylate (PBMA) particles (56 nm) and deuterated polystyrene (dPS) 
particles were prepared using emulsion polymerization, at 70°C for the PBMA and 80°C for the 
dPS. The polymer concentrations were 10 wt % for the PBMA and 2 wt % for dPS. The PBMA 
particles were crosslinked at 10% using ethylene glycol dimethylacrylate (EGDMA) during 
polymerization. The dPS particle size was controlled between 20 and 200 nm by the amount of 
surfactant SDS in the reaction. The molecular weight measured using GPC was found to be 
around 400 kg/mol – 600 kg/mol for all the particles investigated here. Particle diameters (D) 
were measured using dynamic light scattering (Malvern 5000). The bulk glass transition 
measured by DSC (Perkin-Elmer, DSC7) yielded the following Tg values:  
 
D,nm 27.8 29.8 39.4 52.4 91.2 132.6 
Tg,°C 102.2 99.95 99.2 101.9 99.09 99.44 
 
The surfactant and free ions were removed from the dispersions using a mixture of anionic 
and cationic exchange resins (Dowex, Aldrich). The nano-blends were prepared by mixing dPS 
and PBMA dispersions to make dPS concentrations of 0.5, 1 and 2 wt % of the solid PBMA. 
Solid films were obtained after water evaporation at 45 °C. 
The small angle neutron scattering (SANS) experiments were carried out on the PAXY 
instrument at Orphée, Saclay. The scattered neutrons collected on an XY bi-dimensional multi-
detector were regrouped according to circular rings to yield spectra of intensities (I) versus the 
magnitude of the scattering wave vector (q). The range of accessible q values was between 2.3 
10-3 and 3.4 Ả -1. 
 
3 Results and discussions 
 
The SANS spectra of films containing up to 2 wt % dPS infer that the dPS nanoparticles 
are indubitably  individually dispersed within the PBMA matrices (FIG.1). The scattering 
intensity I(q) is the product of the form factor P(q) and the structure facture S(q) of the dPS 
particles. Up to a volume fraction of 2 %, the structure factor equals 1 and I(q) = P(q). The I(q) 
intensities were compared to the P(q) of poly-dispersed hard spheres and the diameter and poly-
dispersity of the particles were calculated from the best fit.  These diameters are similar to those 
from quasi elastic light scattering (FIG. 1). 
 
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18ème Congrès Français de Mécanique Grenoble, 27-31 août 2007 
0.1
1
10
100
0.005 0.01
Q, A-1
Diameter, nm
29.8
39.4 
52.4
91.2 
 
FIG. 1 – Scattering intensities vs. wave vector q for PS nanoparticles dispersed in PBMA 
matrices. 
 
  Films measuring 5×5mm in size and 0.5-1mm thick were compressed in a homemade 
apparatus in a temperature controlled silicon oil  bath by applying a stress of  ~4 MPa on the 
samples for 20s. The samples were then quenched (for about 20s) to room temperature in the 
deformed state. The total experiment lasted around 60s. During the experiment, the film 
thickness was reduced to 1/4 of its original size. Because the compressed films were observed 
with the deformation axis parallel to the neutron beam, the SANS spectra were isotropic. The 
anisotropy would be seen if the films are visualized with the deformation axis ⊥ to the neutron 
beam. The scattering intensity of the deformed film was radially averaged and compared to the 
spectra prior to deformation. At a temperature above Tg of PS, the spectra of deformed films are 
shifted toward the smaller q values (FIG. 2) and fit the form factor of spherical particles with 
larger diameters than before deformation. Since the spectra of deformed films below dPS Tg are 
identical to the non-deformed films (FIG 2), the idea that cavities form around hard dPS 
particles is discarded.  
By assuming dPS nanoparticles deform at a constant volume, one can imagine that they 
flatten with λ in the compression direction and expand with λ1/2 in the two other axes. The 
spectra of the deformed film describe the particle expansion in the two directions perpendicular 
to the neutron beam. The mathematical form factor of deformed spherical particles, with a 
deformation axis parallel to the neutron beam was calculated and was found  to be exactly equal 
to the P(q) of a spherical particle with diameter D =  D0 λ1/2. The deformation rate of individual 
particles is calculated as λ = (D/D0)2. Figure 3 shows λ vs. [temperature – Tgbulk] for 91.2 nm 
and 27.8 nm dPS nanoparticles within 10% cross-linked PBMA matrices. The deformation rate 
(λ) for 91.2 nm dPS particles remains constant and equal to 1 for temperatures below Tonset = 
104 °C, and increases rapidly above this temperature (Fig. 3). The general tendency of λ vs. 
[temperature – Tgbulk] is the same for all particle sizes, however, the whole nanomechanical 
curve shifts toward low temperatures as the particle diameter decreases. The 27.8 nm dPS 
particles begin to deform at a lower temperature (Tonset = 91 °C) than the 91.2 nm particles.  This 
shift is a clear indication that mechanical behavior of small particles is different from that of 
large ones. Since the mechanical tests were exactly the same for all particle sizes, the shift of the 
mechanical curve proves, beyond error, that the glass transition of dPS nanoparticles decreases 
below bulk value as the particle size decreases. 
  
3 
18ème Congrès Français de Mécanique Grenoble, 27-31 août 2007 
0.1
1
10
100
0.004 0.01
q, A-1
0.005 0.01
q, A-1
In
te
ns
ity
a b
Before 
deformation
Before 
deformation
Before 
deformation
Before 
deformationIn
te
ns
ity
In
te
ns
ity
 
FIG. 2 –  SANS spectra before and after deformation of 91 nm dPS particles in PBMA matrics . 
a) deformed at 65°C and b) deformed at 120°C 
 
The mechanical curves are super-imposable for particle diameters higher than 90nm, 
which infers that these large particles exhibit a bulk behavior. The temperature shift of the 
mechanical curve from that of large particles is most likely equal to the deviation of the 
nanoconfined glass transition (∆Tg) from bulk. Therefore Tg deviation (∆Tg) from bulk is 
calculated as the temperature shift of the nanomechanical curve of each sample from that of the 
130 nm particle. Although the onset temperature of the transition from glassy to rubbery 
regimes (Tonset = 104 °C) is close to Tgbulk = 99.4 °C, for large particles (D > 90 nm), the Tonset is 
not used here as the nanoconfined glass transition.   
The contribution of Laplace pressure (∆P = 4 γPS-PBMA/D) to particle Tg is negligible; dTg
ST 
= + 0,03 °K for the smallest diameter investigated here. dTgST is calculated using γPS-PBMA = 
0.72 mN/m  and dTgST/dP = 0,30°K/MPa. The Laplace pressure (dP = 0.1 MPa for 27 nm and 
0.02 for 130 nm) could act as a barrier for the deformation of small particles, which could 
reduce the Tg depression as measured from mechanical tests. However, this is not likely to be 
the case because the applied stress is larger than the Laplace pressure.  
  
91 nm27.8 nm
0.9
1
1.1
1.2
1.3
1.4
1.5
-40 -20 0 20 40
λ
T -Tgbulk , °K
Tgbulk
 
FIG. 3 – Deformation rate (λ) of dPS nanoparticles vs. the deformation [temperature – Tgbulk] for 
27.8 nm and 91.2 nm. 
 
 
 
 
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18ème Congrès Français de Mécanique Grenoble, 27-31 août 2007 
4 Conclusions 
 
This report shows the first direct comparison between confinement in thin film geometry 
and spherical nanoparticles. A shift from bulk Tg (∆Tg) in spherical nanoparticles dispersed in 
nano-blends was measured. The ∆Tg decreases with decreasing particle diameters. 
 
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Nanomechanical properties of nanoconfined polystyrene nanoparticles in nanoblends

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