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4.5 Surface Glass Transition and Transitions in Thin Films
Index
4.5 Surface Glass Transition and Transitions in Thin Films .............................................. 1
4.5.1 Background ............................................................................................................ 1
4.5.2 Interfacial confinement effects and film preparation history ................................. 3
4.5.3 Liquid-like surface models that address Tg depletion in thin films ....................... 5
Capillary wave induced surface melting ..................................................................... 5
Near surface polymer chain sliding (sliding model) ................................................... 5
4.5.4 Shear modulation scanning force microscopy (SM-SFM) .................................... 6
4.5.5
Mobile surface layer theories and preliminary SM-SFM results .................... 7
4.5.6 SM-SFM transition measurements of ultrathin supported films........................... 8
References ........................................................................................................................... 9
4.5.1 Background
Extensive literature deals with the determination and the interpretation of the glass
transition temperature, Tg, of homopolymer systems. Within bulk systems, many debates
have meanwhile been settled. At free surfaces and in thin films however, because of
constraints and size effects, the relaxation dynamics is still poorly understood.
It has been recognized theoretically and experimentally that in thin homopolymer
films the proximity of a free surface, substrate interactions, and stress induced anisotropy
within the film, are responsible in an intricate way for shifts of the glass transition
temperature with respect to the bulk.1-20 Glass transition temperatures have been reported
to be depressed up to tens of degrees Celsius for films thinner than a few hundred
angstroms.11,14,20 It has also be recognized that substrate effects, can cause an opposite
shift in Tg from the bulk to higher values.11,17
Models that have been developed over the last few years favor the idea of a liquidlike surface layer that is responsible for a Tg depletion in none-substrate-confined thin
films.5,20,21 One of them, inspired by dewetting studies of thin films,22,23 in which
morphological changes below the bulk Tg value were reported, proposes coupling of
capillary waves with flow properties in thin films responsible for surface melting.21 In
another liquid-like surface model (referred to it as the sliding model), the scenario of the
glass transition is split into two; i.e., a standard bulk transition that is based on freezingunfreezing of certain local degrees of freedom), and a near-surface transition that
originates from polymer chain sliding motions. Depending on the temperature and
thickness of the film, the model suggest either a sandwich structure of bulk and surface
phases, or a single semifluid phase.5,24
The two liquid surface models are applied to different regimes of molecular weights:
The capillary wave model21 that is based on capillary wave-induced dewetting deals with
low molecular weight polymers melts. The sliding model 5,20 that is considering in
parallel standard bulk transition and near-surface chain sliding motions is proposed to
apply to polymers above 100k. Both models do conceptually not depend on the film
thickness, i.e., they apply for ultrathin films (< 100 nm) and at surfaces of bulk films.
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Liquid-like surface theories have been partially supported by macroscopically
averaging techniques, such as, for instance, ellipsometry11,25,26, Brillouin light
scattering20,27, and thermal expansion spectroscopy28. The macroscopically determined
Tg depletion is often related to the film thickness. However, more local probes used in
dielectric measurements 29 and scanning force microscopy, SFM,12,17 led to results,
where the glass transition temperature is much less affected by the film thickness.
Contrary to the last statement about local probes are some recently published
interpretations of SFM friction force measurements30. Possible misinterpretations are
addressed below.
After the development of a nanorheological technique that has shown to be quite
successful in determining Tg values of polymer films over a wide range of molecular
weights, one has recently investigated (a) the possibility of surface melting below the
glass transition temperature, and (b) interfacial substrate confinement effects on apparent
Tg values.
Glass transition values obtained of monodisperse, homopolymer bulk material or at
surfaces of >200 nm thick films by macroscopic techniques (i.e., differential scanning
calorimetry (DSC) and electron spin resonance (ESR)), and by local techniques (i.e.,
shear modulation SFM), respectively, are found to correspond over a wide range of
molecular weights, Figure 1. The finding that surface and bulk transition properties
correspond agrees well with the low surface energy of these systems, and the van der
Waals liquid-like description of polymer melts.
Figure 1. Glass transition of atactic bulk polystyrene
as function of the molecular weight determined
independently by three different methods:
- Differential scanning calorimetry (DSC),3
- Electron spin resonance (ESR)31 and
- Shear modulation scanning force microscopy
(SM-SFM)1
The solid line represents a Fox-Flory fit.
400
380
Tg [K]
360
340
DSC (Claudy)
ESR (Kumler)
shear modulation SFM
Fox Flory Fit
320
300
280
3
1x10
4
1x10
5
1x10
6
1x10
7
1x10
MOLECULAR WEIGHT [Mn]
However, the more complex the polymer system, i.e., the more anisotropic it is, the
less correspondence is found between bulk, thin film and surface Tg measurements.
Origins for deviations have to be carefully analyzed. Common assumptions have to be
revisited and challenged. A common hypothesis is that thermal annealing of a spin coated
homopolymer film, with a thickness exceeding the pinning regime of a few nanometers,
is sufficient to relax the film back to the bulk phase. It is interesting to note that near
surface Tg values determined with shear modulation SFM method correspond well to
bulk Tg measurements as illustrated above in Figure 1. Hence, the annealing presumption
of ultrathin films seems to be justified.
Over the last few years however, various experimental findings have challenged
common hypotheses such as the annealing presumption. It has, for instance, been found
that the polymer rheological properties are modified over tens to hundreds of
nanometers.1,17,32-34
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4.5.2 Interfacial confinement effects and film preparation history
While materials such as ceramics, metals, oxides, exhibit size limitations ('quantum
well effects') only noticeable below 10 nm, it was found that in polymer systems,
interfacial effects could be noticeable over distances of tens to hundreds of nanometers.
Over the last few years, various groups reported bulk-deviating structural and dynamic
properties for polymers at interfaces.15,34-39 For instance, increased molecular mobility
was observed at the free surface for thick films.15 Reduced molecular mobility at the film
surface of ultrathin films was reported based on forward recoil spectroscopy
measurements.35 In secondary ion mass spectrometry (SIMS) and scanning force
microscopy (SFM) studies of graft-copolymers, it was found that the degree of molecular
ordering significantly affects dynamic processes at interfaces.34 Self-organization of graft
and block-copolymers at surfaces and interfaces were found with transmission electron
microscopy (TEM) and neutron reflectivity (NR).36-39
Application of mean-field theories to interfacially constrained and size-limited
polymer systems failed to describe the rather unexpected mesoscale behavior observed
experimentally. The extension of the interfacial boundary far into the bulk is unexpected
because many amorphous polymer systems are theoretically well treated as van der
Waals liquids with an interaction length on the order of the radius of gyration, i.e., the
effective molecular size. At solid interfaces the radius of gyration is further compressed,
like a pancake, and thus, any memory effects of the solid are expected to be even more
reduced to a pinning regime of only 0.5 to 2 nm.40 Within the pinning regime, it is
commonly accepted that the material is structurally altered and exotic properties (for
instance, quantum-well effects) are expected. Outside the pinning regime, the polymer is
expected to behave bulk-like. Experiments show however, that such scaling theories, i.e.,
mean-field theories, fail in describing the observed unique mesoscale properties because
they do not consider effects that occur during the film coating process, e.g., rapid solvent
evaporation. For instance, recent SFM experiments revealed that the spin coating process
altered the structural properties of polyethylene-copropylene (PEP) at silicon interfaces
due to anisotropic molecular diffusion that is caused by process-induced structural
anisotropy.41 The polymer structure at the interface affects properties such as the shear
mechanical properties, the entanglement strength, and dewetting instabilities and
velocities, as illustrated in Figure 2.
An extensive SFM analysis involving also density measurements by X-ray diffraction
and self-diffusion measurements by neutron reflectivity (NR) revealed a three component
system after spin coating: (i) An adjacent to the surface immobilized and fully
disentangled sublayer (20 nm thick, Fig. 2), (ii) a partially disentangled intermediate
layer (100-200 nm thick, Fig. 2), and (iii) beyond the intermediate layer the bulk polymer
phase. he strained interfacial sublayer can be pictured as highly disentangled and
laterally anisotropic system with a thickness on the order of the polymer's radius of
gyration. NR reveals that the polymers adjacent to the surface immobilized sublayer can
diffuse through the sublayer's pores.39 X-ray diffraction measurements exhibit a
shrinkage of the pores if annealed which immobilizes the diffused polymers
'permanently'.32 Hence, a boundary layer that exceeds by orders of magnitudes the
pinning regime is formed between the interfacial sublayer and the polymer bulk phase.
Naturally one could expect that the glass transition temperature be affected within the
constrained boundary regime. Indeed, it has been observed that substrate supported spin
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coated ultrathin films of polystyrene exhibit an increase in Tg (by 5 oC at a thickness of
30 nm) compared to the bulk.11,17 Much larger changes in Tg by tens of degrees Celsius
and in the opposite direction have been observed for substrate unsupported ultrathin
homopolymer monodisperse PS films.11,14,20 These findings have let to theoretical
models that go beyond the free volume theory and consider either coupling of capillary
waves with flow properties, or near surface polymer chain sliding motions responsible for
the observed depletion in Tg in ultrathin films. 5,20,21
D
1.0
c
0.8
Frictiondecreasesbecauseof
pinningforces!
0.6
Normalized Lateral Force
0.4
< 200 nm
Thin Film
Regime
0.2
0.0
Liquid
Bulk Regime
Pinning Regime
0
50 100 150 200 250 300 350 400
PEP Film Thickness D[nm]
Si
Figure 2(a): (Left) SFM rheological lateral force
measurements on thermally annealed PEP reveal a
comparable qualitative film thickness behavior-as the
dewetting velocities in Fig. 4(b). A decrease in the
measured lateral force is interpreted as an increase in
the shear mechanical properties (moduli) of the
interfacially confined polymer film.
(Right) The range over which the confinement is
recognizable (Thin Film Regime, 100-200 nm,)
exceeds by orders of magnitude the surface pinning
regime (< 5 nm). Above the thin film regime, the
polymer behaves bulk-like.
Figure 2(b): Lateral (friction) force vs. load SFM
experiments provide fundamental insight into the
origin for changes of the rheological properties
in spin coated thin PEP films. The transition
point Px (x = thickness of polymer film), which
corresponds to the kink in the friction vs. loading
curve is a measure of the entanglement strength
of the polymer (PEP). The bulk value is reached
for films thicker than 230 nm. Films thinner than
230 nm are partially disentangled due to the spin
coating process, and thus, the transition point
occurs earlier. Within a 20 nm boundary regime
the film is entirely disentangled (gel-like).
0.8
0.7
0.6
0.5
nm/s
Plot Title
0.4
0.3
0.2
0.1
0.0
0
100
200
300
400
500
Thickness of PEP [nm]
Figure 2(c): (left a+b) Illustrative sketch of the
dewetting process measured by SFM. (right) 5050
m recorded dewetting pattern (topography (left) and
friction (right)) of PS/PEP. (top) Large and deep
dewetting holes for thick (400 nm) PEP films.
(bottom) Small and interfacially bound dewetting
holes for thin (4 nm) PEP films.33,42
Figure 2(d): PS/PEP dewetting velocities
measured by optical microscopy reveals a
decrease in velocity for PEP films thinner than
100-200 nm. The triangle indicates that the
history of the sample preparation is very
important ( PEP spin coated on silicon wafer,
PEP floated on polyvinylpyridine (PVP)). 34
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4.5.3 Liquid-like surface models that address Tg depletion in thin films
Capillary wave induced surface melting
In this model, capillary wave coupling on the free surface to the bulk flow of the polymer
is responsible for the formation of a melt in the vicinity of the surface. Herminghaus'
capillary model21 involves coupling between interfacial eigenmodes of the viscoelastic
polymer melt with capillary waves. This model is considering only low molecular weight
polymers. (Note: The polymer-chain-sliding model (discussed below) applies for high
molecular weights exceeding 100k). The capillary model is not restricted in polymer film
thickness, i.e., applies to the free surface. It suggests a continuum scenario responsible
for the glass transition, which is largely independent of the molecular weight. The theory
considers strain relaxation fluctuations (density fluctuations are neglected) and its
viscoelastic eigenmode spectrum. The central assumption is that the physical cause for
the melting or freezing of the film (or surface layer) are memory effects in the polymer
material (i.e. using as a theoretical treatment: a convolution integral with memory
kernel). It is found that only high frequency modes contribute appreciably to the
reduction of the glass transition temperature. The model predicts that the highest
eigenvalue modes are those of the molten layer alone, which depend inversely on the
thickness of this layer. The mean square amplitude of the strain fluctuation of these high
modes, a measurable quantity that can be obtained from rheological spectrum analysis,
should be appreciable only down to a critical thickness of the melt. The model predicted
thickness of the molten layer, d, scales as d  (Tgb-T)- with a critical exponent =1.
Recently developed other multilayer models predict =0.5.20 Herminghaus' model
consequently proposes for ultrathin films that the transition temperature value measured
over the entire film is continuously changing from the time surface-melting occurs until
the thickness of the molten layer corresponds to the thickness of the film. Note however,
in thick films, i.e., in films in which the thickness of the film exceeds any molten surface
layer thickness below the bulk Tgb value, two transition values should be obtainable with
a surface sensitive tool.
Near surface polymer chain sliding (sliding model)
Experimentally observed extraordinary depleted Tg values for ultrathin freely suspended
polystyrene films10,43 have recently been interpreted by de Gennes via a two "meltingscenario" 5: (a) a standard bulk transition, related to the freezing-unfreezing of certain
local degrees of freedom, and (b) sliding motions of each polymer chain along its own
path. In the bulk it is assumed that chain sliding is hindered by the end points. At or near
the free surface, however, a thin fluid skin allows the chains to slide. The skin is
estimated to be less than a nanometer thick. The two transition processes are illustrated in
Figure 3.4
B
A
C
Figure 3: Model of polymer chain in a thin film. Two
contributions arise from a segment formed by loop AB and a
bridge BC. For films thinner than the coil size, the dominant
process maybe the collective motion of a loop which does not
involve the chain ends.4
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The bulk Tg motions are based on short-range rearrangements. These types of
motions are contributions of bridge formations shown in the polymer chain touching
points A and B in Figure 3. The other type of motion is based on polymer sliding, where
a chain advances along a path via mobile kinks and the free volume required for sliding is
less than the bulk cooperative motion. These types of motions are contributions of “loop”
formations shown in the polymer chain touching points B and C in Figure 3. A loop at the
surface should slide easily, however, it is believed that sliding is hindered in the bulk
because chain ends would have to invade new territory, thus requiring more free volume.
The two transition model lead to a Tg that depends on the distance from the free surface
(h-h*), described by
M 
(1)
Tg  Tg*  b ln  *w  h  h*
M
 W
where Tg, Mw, and h refer to the glass transition temperature, molecular weight, and
film thickness respectively. The parameters b, Tg*, Mw*, and h* are fitting parameters
from Tg vs. h plots characteristic of the observed deviation from bulk Tg values. The
model predicts bulk behavior at the surface for very high molecular weights (> several
1,000k), which is experimentally confirmed in ultrathin films.5,11
Although the mobile surface model has been motivated by ellipsometric ultrathin film
studies, which lead to discussions about experimentally observed apparent transition
values and theoretically predicted two scenario transitions, it is not restricted to thin films
only.44 It could also be applied to free polymer surfaces of thick films. An experimental
test of the model demands a surface sensitive tool such as the shear modulation SFM.




4.5.4 Shear modulation scanning force microscopy (SM-SFM)
The working principle of the shear modulation scanning force microscopy, SM-SFM,
method is sketched in Figure 4.17 The technique is well suited for any surface rheological
study involving thermally activated transitions or relaxations. Over the last two years, the
method has shown to be a highly accurate method for determining near surface glass
transition temperatures of thin polymer films. The method involves a nanometer sharp
SFM cantilever tip that is brought into contact with the sample surface as sketched in
Figure 4. A constant load of a few nanonewton is applied, and the probing tip is laterally
modulated (with a nanometer amplitude that guarantees no relative probe-sample
slippage) while the temperature is stepwise increased by 0.1oC. At each temperature step
the system is idle until thermal equilibrium is obtained before any viscoelastic responses
are recorded. The recorded response amplitude, which is a measure of the contact
stiffness,17 is then plotted versus the temperature. The Tg value is determined from the
"kink" in the response curve as documented in Figure 4(a). The figure also illustrates the
high accuracy of the method. It allows Tg studies of various parameters such as, for
instance, the molecular weight dependence as shown in Figure 4(b), (see also Fig. 1
above).
It is important to note that the SM-SFM method is a non-scanning method. The
reason is briefly describe here: To obtain high accuracy in Tg measurements, it is
essential, not to induce by other means than temperature, changes in the contact area.
This is to avoid system-driven artifacts in the contact stiffness, kc. To be precise, kc(AL,
G*), i.e. the resistance of the contact to deform, is dependent on (a) the laterally projected
contact area, AL, (e.g., the side wall of an indentation dip), and (b) the relative shear
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properties of the two materials, G*. Thus, any local plastic deformation, for instance, the
generation of a deformation wave (Schallamach wave)45 that travels ahead of a scanning
SFM tip can change kc. Any plastic deformation is intrinsically rate and load dependent.
Thus, it is not astonishing that scanning methods such as the lateral (friction) force
microscopy revealed scanning velocity dependent apparent transition values for Tg, Fig.
5.8
Shear Response
 xL
380
Tg / K
(a)
Cantilever
Tip
(1) 3 k
(2) 7 k
(3) 9.5 k
(4) 65 k
(5) 6,500 k
 xs
Shear Displacement
 xmod
Figure 4: Schematic representation of the SFM based SMSFM method. The sample is sinusoidally modulated (xmod)
relative to the probing cantilever tip. In response the contact
and the cantilever are deformed by xs and xL, respectively.
(a) Tg values correspond to dominating kinks in shear
response amplitude vs. temperature curves, as here illustrated
on polystyrene for a wide range of molecular weights.
Amplitude / a.u.
Sample
370
360
(b)
Surface
Bulk
350
340
330 3
10
4
5
6
10
10
10
Molecular weight
7
10
342 K
(1)
355 K
(2)
360 K
(3)
(4)
(5)
300
320
374 K
374 K
340
360
380
Temperature / K
400
By placing the SFM tip stationary at constant load onto the polymer surface, contact
area changes occur only due to temperature induced changes in the rheological properties
of the material. Consequently the experimental observable in the SM-SFM method, kc, is
changing only due to changes in the polymer material properties.
1 m/s
5 m/s
20 m/s
LATERAL FORCE [nN]
120
100
80
372K
60
378K
40
360
365
370
375
380
385
Figure 5: The absolute temperature for friction
measurements is ill-defined due to sliding
velocity dependent changes in the contact area,
i.e., the contact stiffness. The scan distance was
5 m and the load 15 nN. At high speed, no
transition is observed. An apparent transition
corresponding to Tg can be observed at very low
speed. At intermediate scan speeds, the apparent
transition is higher than Tg.8
TEMPERATURE [K]
Hence, the "kinks", observed in Figure 4(a), are true measures of the transition property
4.5.5 Mobile surface layer theories and preliminary SM-SFM results
In SM-SFM experiments on high molecular weight polystyrene films (230 nm thick, Mw
= 6.5M) a subtle change in the SM-SFM curve was found at about 25 oC below the bulk
Tg value, Fig. 6(a). A more careful analysis using adhesion force SFM, Fig. 6(b),
confirmed that at Ta = 350 K the PS surface changed, i.e., softened, forming a larger
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contact. with the SFM tip. It is important to note that the SM-SFM measurements were
contacted at lower loads (i.e., smaller penetration depth) than the prior experiments.
Considering DeGennes model, the here discussed finding is not unexpected, as the model
predicts Ta to be distance dependent from the surface.
0.030
30
Tgb = 374 K
ADHESION (nN)
28
Ta(h)  350 K
0.025
0.020
26
T = 354K
24
22
20
18
340
360
380
400
TEMPERATURE (K)
Fig. 6(a): Shear rheological response measurements
(SM-SFM, applied load:10 nN) on PS, 6.5M (Mw),
230 nm thickness. The graph shows a load
dependent (i.e., initial penetration depth, h,
dependent) apparent surface transition T a(h) of 350
K. The bulk glass transition temperature, T gb, is
374 K.
300
320
340
360
380
400
420
TEMPERATURE [K]
Fig. 6(b): SFM adhesion pull-off force vs.
temperature measurements (maximum applied
load 10 nN). The transition value of 354 K
corresponds to the apparent surface transition
value determined by SM-SFM.
4.5.6 SM-SFM transition measurements of ultrathin supported films
In thin film studies one has to pay particular attention to the film preparation
technique employed. As discussed above, spin coated films in the vicinity to the substrate
can exhibit quite complex strain structures that can impact the glass transition properties.
This is illustrated in Figure 7. The plot in Figure 7 compiles distance-dependent Tg values
in a single plot for spin coated polystyrene films. As expected from previous rheological
studies on thin supported films (as discussed above), the glass transition for ultrathin
homopolymer films deviates from the bulk Tg value for films that are thinner than about
100-200 nm. Astonishingly, two regimes were obtained: On one hand, the apparent T g
value increased by an average of 4 oC within 30-100 nm. On the other hand, the apparent
Tg value dropped by about 8 oC compared to the bulk at a film thickness of about 15 nm.
While an increase in Tg could be expected due to interfacial confinement effects, the
finding of a reduction in Tg in the boundary regime to the substrate is rather unexpected.
The Tg-thickness dependence can be interpreted with the multilayer layer model
(sublayer and transition layer) in spin coated films. The fully disentangled layer that is
directly adjacent to the silicon substrate exhibits a morphology in which Tg occurs earlier,
while the intermediate, partially disentangled layer (sandwiched between the sublayer
and the bulk) is constrained, and hence exhibits higher Tg values.
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Figure 7: Glass transition of PS vs. film
thickness determined from SM-SFM plots, for
various molecular weight PS (MW). The films
had been spin coated directly on silicon and
thermally annealed over 4 hours at 130 oC.
The near surface measured Tg value is bulk-like
for t > 200 nm. Within a "thin film transition
region" of about 20 to 200 nm (compare with
Fig. 2), the Tg value is increased by up to 5oC
from the bulk value. A significant decrease in
Tg is observed for 15 nm thick films.
Note: No bulk deviating Tg values were found
for low interaction substrate surfaces, such as
PVP or OTS coated silicon surfaces, after
temperature annealing.17
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