World Journal Of Engineering
Glass Transition Temperature and Structural Relaxation of Polymer
Nanoparticles Under Soft and Hard Confinement
Yunlong Guo, Chuan Zhang, Christine Lai and Rodney D. Priestley
Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544, USA
Hybrid polymer/silica core-shell nanoparticles were
prepared by coating a ~30 nm thick silica shell around
PS nanoparticles using a procedure adapted from the
Stober method. Bulk polymer samples were created
for each PS nanoparticle sample by drying the
nanoparticles under vacuum and subsequently
annealing at 423 K for 20 h.
Introduction
Understanding the dynamics of nanoscopicallyconfined polymer remains an intriguing scientific
endeavor and one with significant technical
motivation. Confined polymers are featured
prominently in emerging applications ranging from
plastic solar cells to nanocomposites, to smart
coatings, and to membranes for energy efficient
separations. When a polymer film is confined to the
nanoscale, its glass transition temperature (Tg) and
associated dynamics can differ substantially from the
bulk [1]. Irrespective of deviations in Tg within
confinement, confined glasses undergo structural
relaxation (i.e., physical aging). Structural relaxation is
the spontaneous relaxation of glasses toward
equilibrium, which results in a time dependence of end
use properties. Aside from investigations on submicron thick polymeric membranes, few studies have
characterized the structural relaxation of confined
polymer glasses [2].
Here, we advance the current understanding of how
confinement impacts the glass transition temperature
and aging by investigating how different confinement
conditions, i.e., soft versus hard confinement, alter Tg
and rate of structural relaxation. This is achieved via
studies on aqueous suspended polystyrene (PS)
nanoparticles (the case of soft confinement) and silicacapped PS nanoparticles (the case of hard
confinement) by calorimetry. We show that the type of
confinement has a significant effect on the deviation in
Tg and structural relaxation of confined glasses
compared to the bulk.
Characterization
Sizes of PS nanoparticles suspended in water were
determined from dynamic light scattering (DLS)
(Malvern Instruments Zetasizer Nano-ZS ZEN 3600).
The Tgs of PS nanoparticles suspended in water (and
dried PS-silica core-shell particles) were determined
using MDSC (TA Instruments Q2000, second heat
with a modulation rate of (0.2 K/20 s, heating rate of 5
K/min) in hermetically sealed aluminum pans. All
reported Tgs are the midpoint value between the
tangents of the glass and liquid line from the total heat
flow.
Physical aging measurements were performed
using DSC in both standard mode (for PS-silica coreshell nanoparticles and bulk PS) and modulated mode
(MDSC) (for PS nanoparticles suspended in water). A
nitrogen environment was employed in this study. The
DSC was calibrated on heating at 20 °C /min and
isothermally calibrated at 75 °C. The MDSC was
calibrated on heating at 5 °C /min. Calibrations were
performed using the modules in the Thermal
Advantage software provided by TA Instruments.
Results and Discussion
Figure 1 shows the diameter dependence of TgTg,bulk (squares) for PS spheres and PS-silica core-shell
nanoparticles. Decreasing the diameter of PS
nanoparticles led to a reduced Tg with respect to Tg,bulk.
The onset diameter for Tg reductions was ~700 nm.
Furthermore, the magnitude of the deviation in Tg with
decreasing diameter was significant; e.g., for PS
nanoparticles with a diameter of ~90 nm, Tg - Tg,bulk = 58 K.
Experimental
Materials
PS nanoparticles were synthesized from surfactantfree emulsion polymerization. Desired nanoparticle
sizes were achieved by changing the monomer
concentration, initiator concentration, and/or initiator
type.
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World Journal Of Engineering
We further explored the free surface as the cause of
the reduced Tg by comparing the size dependence of Tg
for bare PS nanoparticles and the corresponding
PS/silica core-shell nanoparticles. Capping of the PS
nanoparticles with a silica shell should remove the free
surface by confining the polymer within a hard shell,
i.e., the case of hard confinement. The thickness
dependence of Tg for PS and PS/silica core-shell
nanoparticles is shown in Figure 1 (circles). We
observed that PS nanoparticles confined within a silica
shell did not exhibit a size dependent Tg as observed
for bare PS nanoparticles. The absence of the Tgconfinement
effect
for
PS/silica
core-shell
nanoparticles can be partially understood if the free
surface is considered a major reason for the size
dependence of Tg for polymer nanoparticles. The inset
shows SEM images of up-capped and capped PS
nanoparticles [3].
shorter for PS nanoparticles confined within a silica
shell than aqueous suspended PS nanoparticles [4].
This implies that PS and silica do not possess strong
attractive interactions, a premise consistent with direct
interfacial glass transition temperature measurements
of PS supported on a silica substrate [5].
a
10
200 nm diameter
PS nanoparticles
Tf - Ta (K)
8
6
4
Tg - 4 K
Tg - 4 K
Tg - 6 K
Tg - 6 K
Tg - 8 K
Tg - 8 K
Tg - 10 K
Tg - 10 K
2
0
1
10
2
10
3
10
10
4
5
10
6
10
Time (s)
b
12
Tg - 4 K
Tg - 4 K
Tg - 6 K
Tg - 6 K
Tg - 8 K
Tg - 8 K
Tg - 10 K
Tg - 10 K
Tg - 6 K MDSC
200 nm diameter
PS-silica core-shell
nanoparticles
10
Tf - Ta (K)
8
6
4
2
0
1
10
2
10
3
10
10
4
5
10
6
10
Time (s)
Fig. 2. Aging isotherms of 200 nm diameter (a) bare
PS particles and (b) PS-silica core-shell nanoparticles
aged at different quench depths (Tg – Ta).
Conclusions
Fig. 1. Change in Tg versus particle diameter for PS
(■) and PS/silica core-shell nanoparticles (●). Insert:
SEM images of bare (left) and capped (right) particles.
We found a striking similarity of size-dependent
effects on the Tg for polymer nanoparticles. The
deviation of Tg can be suppressed by capping the freesurface with a hard layer. The structural relaxation
behavior of 3D-confined polymers greatly depends on
the conditions of confinement.
The structural relaxation of PS nanoparticles under
soft and hard confinement was investigated by DSC
and MDSC, respectively. Figure 2a illustrates aging
isotherms for aqueous suspended PS nanoparticles. All
aging isotherms asymptotically approached Tf – Ta = 0
which indicated the attainment of (or approach to)
equilibrium. For all systems, increasing the quench
depth (Tg – Ta) resulted in longer aging times required
to reach equilibrium. At the same value of Tg – Ta, the
time required to reach equilibrium, teq, was
significantly shorter for bulk PS than aqueous
suspended PS nanoparticles.
Figure 2b shows aging isotherms of PS
nanoparticles confined within a silica shell. Similar to
aqueous suspended PS nanoparticles, all aging
isotherms asymptotically relaxed to or towards a value
of Tf – Ta = 0. At the same value of Tg – Ta, teq was
References
[1]. Keddie, J.L.; Jones, R.A.L.; Cory, R.A. Europhys. Lett.
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[2]. Priestley, R.D.; Ellison, C.J.; Broadbelt, L.J.; Torkelson,
J.M. Science 2005, 309, 456-459.
[3]. Zhang, C.; Guo, Y.; Priestley, R.D. Macromolecules
2011, 44, 4001-4006.
[4]. Guo, Y.; Zhang, C.; Lai, C.; Priestley, R.D. et al. ACS
Nano, accepted.
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World Journal Of Engineering
[5]. Ellison, C.J.; Torkelson, J.M. Nat. Mater. 2003, 2, 695700.
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