Effect of Low Surface Energy Chain Ends on the Glass Transition Temperature of Polymer Thin Films

Abstract

Many polymer thin film applications
are intimately related to the chemical functionality of
the chain end groups. For example, use of hydrophilic
end groups is a common strategy to end-graft hydrophobic
polymer chains onto high-energy substrate surfaces; introduction of polar end groups to the polyfluoroalkyl
ether improves its ability to protect a computer
hard disk against wear; using high molecular weight
end-functionalized chains as additives can improve the
durability of polymer thin films against deweting;
polymer surfaces that are modified by thermodynamically
segregated low-energy chain ends can be exploited
to make “self-healing” surfaces.4 Previous experiments
showed that polymer surfaces enriched with segregated
chain ends exhibit higher molecular mobility,5-7 which
are consistent with theoretical predictions8 assuming
the generally larger free volume associated with chain
ends compared to chain segments. A local interfacial
layer with higher molecular mobility has been attributed
to be the cause of a global reduction in the Tg of
polymer thin films observed recently. In this study,
we investigate the Tg of thin films of PS with low surface
energy fluoroalkylsilyl (Rf) termination on both ends
(alpha,omega-PS-(Rf)2). Results show that the amount of reduction
in the Tg found in thin films of alpha,omega-PS-(Rf)2 with
decreasing film thickness is about twice of that found
in thin films of sec-butyl-initiated-proton-terminated PS.

Full text

Effect of Low Surface Energy Chain Ends on
the Glass Transition Temperature of Polymer
Thin Films
Fengchao Xie, H. F. Zhang, Fuk Kay Lee,
Binyang Du, and Ophelia K. C. Tsui*
Department of Physics, Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong
Y. Yokoe, K. Tanaka, A. Takahara, and T. Kajiyama
Department of Applied Chemistry, Faculty of Engineering,
Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan
Tianbai He
State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun, Jilin 130022, China
Received September 27, 2001
Introduction. Many polymer thin film applications
are intimately related to the chemical functionality of
the chain end groups. For example, use of hydrophilic
end groups is a common strategy to end-graft hydro-
phobic polymer chains onto high-energy substrate sur-
faces;1introduction of polar end groups to the polyfluoro-
alkyl ether improves its ability to protect a computer
hard disk against wear;2using high molecular weight
end-functionalized chains as additives can improve the
durability of polymer thin films against deweting;3
polymer surfaces that are modified by thermodynami-
cally segregated low-energy chain ends can be exploited
to make “self-healing” surfaces.4Previous experiments
showed that polymer surfaces enriched with segregated
chain ends exhibit higher molecular mobility,5-7which
are consistent with theoretical predictions8assuming
the generally larger free volume associated with chain
ends compared to chain segments. A local interfacial
layer with higher molecular mobility has been attrib-
uted to be the cause of a global reduction in the Tgof
polymer thin films observed recently.9-14 In this study,
we investigate the Tgof thin films of PS with low surface
energy fluoroalkylsilyl (Rf) termination on both ends
(R,ω-PS-(Rf)2). Results show that the amount of reduc-
tion in the Tgfound in thin films of R,ω-PS-(Rf)2with
decreasing film thickness is about twice of that found
in thin films of sec-butyl-initiated-proton-terminated PS
(H-PS).
Experimental Section. The R,ω-PS-(Rf)2(Mn)
47.8K Da, Mw/Mn)1.18, TgDSC )376 K) was synthe-
sized by living anionic polymerization using potassium
naphthalene as a bifunctional anionic initiator and
(tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchloro-
silane as a terminator. The H-PS, synthesized by anionic
polymerization using sec-butyllithium as the initiator,
had been purchased from Scientific Polymer Products
(abbreviated com. H-PS,15 Mn)530K Da, Mw/Mn)1.04,
TgDSC )382 K) and also self-made in our own laboratory
(abbreviated home H-PS, Mn)52K Da, Mw/Mn)1.05,
TgDSC )372 K). Thin films of the polymers with different
thicknesses (∼8-200 nm) were prepared by spin-coating
solutions of the polymers in toluene (0.3-4 wt %) onto
cleaned silicon substrates16 covered with a ∼1.2 nm
thick native oxide layer. Upon spin-coating, the R,ω-
PS-(Rf)2thin films were kept under vacuum at ambient
temperature for 3 days to remove the residual solvent.
Before measurement, all samples were annealed at
or above 120 °C for 5-9 h inside a vacuum oven to
relax the polymer, whereupon they were allowed to
cool to room temperature under vacuum. Spectroscopic
ellipsometric measurements over wavelengths 350-1300
nm were carried out in a J.A. Woollam (Lincoln, NE)
variable-angle spectroscopic ellipsometer (VASE). A
home-built hot-stage with temperature controllable to
within (1 °C was incorporated to heat the sample in
situ. To obtain the temperature scans, both the wave-
length and incident angle of incoming light were fixed
at settings empirically found to optimize the VASE
signal;9the ellipsometric angles, δand ξ, which are
directly related to the film thickness and sample refrac-
tive index,9were recorded as a function of temperature,
T, at a constant heating rate of 2 K/min. The Tgof a
polymer film was determined as the temperature at
which slope of its the temperature dependence data
displays a discontinuity.
Results and Discussion. Shown in Figure 1 are the
measured results plotted as Tg(t)-Tg(∞)vst, where t
is the film thickness and Tg(∞) is the asymptotic Tgin
the limit of large t. The data exhibit a monotonic
reduction in Tg(t) with decreased t, in accordance with
previous measurements.9-14 A number of models have
been proposed to explain this behavior.9,11-14 While they
may differ by the physical process involved, all these
models are based on the idea that a nanometer scale
mobile (or lower Tg) region exists, whose influence on
the Tgof the film increases as the film thickness is
decreased, and end in the same phenomenological
expression: Tg(t))Tg
∞[1 -(ξ0/t)ν], where ξ0is the
length scale of the mobile region and νa constant. We
have model-fitted our data to this expression and found
excellent agreement (smooth lines). From Figure 1, one
may also notice that data points of the two H-PS fall
within each other’s experimental error bars (or 1σ
confidence interval) and hence are indistinguishable. On
the other hand, the Tgof the R,ω-PS-(Rf)2thin films are
noticeably more reduced (by ∼2 times, which amounts
to a sizable difference of ∼3.5σon average). According
to Table 1, the fitted value of νand ξ0are within the
range of values obtained previously.9,13 Comparing
between polymers, the fitted values of νvary only by
Figure 1. Tg(t)-Tg(∞) vs thickness, t, for thin films of PS
with different Mnand chain ends. Smooth lines through the
data are fits to the phenomenological expression discussed in
the text.
1491Macromolecules 2002, 35, 1491-1492
10.1021/ma011689a CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/24/2002

∼10%. However, the fitted value of ξ0for the R,ω-PS-
(Rf)2films is about 2 times larger than those of the H-PS
films, suggesting a doubling in the length scale of the
mobile region in the former. In the following, we will
try to understand these different results in terms of the
degree of surface segregation of chain ends and surface
molecular motion in these two kinds of PS.
In H-PS, numerous measurements showed that the
sec-butyl end groups preferentially segregated to the
polymer surface.5,17 The preferential segregation of
surface chain ends is attributable to the enhanced
surface molecular motion found in H-PS by scanning
viscoelasticity microscopy (SVM)5and lateral force
microscopy (LFM),6as a later finding7showed that no
enhanced surface molecular mobility was present in PS
with higher surface energy chain end groups, namely
R,ω-PS-(COOH)2and R,ω-PS-(NH2)2.
In the fluoroalkylsilyl-terminated PS, angle-depend-
ent X-ray photoelectron spectroscopy (ADXPS) and
neutron reflectivity revealed that the polymer surface
was ∼5 times more concentrated with the fluoroalkyl-
silyl end groups than the bulk,18 which is about 2.5
times bigger than that found in H-PS.4In PS terminated
with both kinds of end groups, i.e., R-H-PS-ω-(Rf), the
fluoroalkylsilyl end group had been found to dominate
at the free surface.17 LFM study also revealed that
surface relaxations began to be discernible at a much
lower temperature (∼140 K) in R,ω-PS-(Rf)2than in
H-PS (∼300 K). It is thus justifiable to presume that
the degree of segregation of chain ends to the free
surface and the surface molecular mobility are substan-
tially larger with the fluoroalkylsilyl termination than
with the sec-butyl-initiated proton termination.
These previous findings enable useful insights to the
understanding of data displayed in Figure 1. The
notably lower Tgfound in R,ω-PS-(Rf)2than in H-PS thin
films reveals that when the surface of a polymer is
sufficiently plasticized, the reduction in the Tgof the
polymer confined in thin films may be promoted. To
inspect whether this conclusion can be generalized to
any kind of chain end that causes a lowering in the Tg
of the surface, we examine data of the two H-PS
displayed in Figure 1. Clearly, the same degree of
depression in Tgprevails in thin films of both polymers
despite their notably different Mn. This is consistent
with previous experiments on the Tgof similar system,
namely uncapped H-PS supported on Si substrates,
where no Mndependence was discernible for Mn)12.4-
2900 kDa.9,19,20 Correlating these findings with results
of ref 6 showing that the surface molecular motions of
H-PS with Mn<∼30K Da are enhanced, one may
conclude that existence of a plasticized surface layer
does not always warrant a promotion in the depression
of Tgin supported polymer films.
In conclusion, we have measured the Tgof thin films
of PS with sec-butyl-initiated proton terminations and
those with the lower energy fluoroalkylsilyl termination.
In comparison, the latter exhibit a drastically higher
reduction in the Tgwith decreasing film thickness. The
present result evidences the first time that a surface
layer sufficiently plasticized by segregated chain ends
may have a global effect on the Tgof a polymer film.
Acknowledgment. This work was supported by
HKUST through the University Grants Council of Hong
Kong under Project DAG 98/99.SC24.
References and Notes
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Table 1. Comparison between Fitted Parameters for
Tg(t)-Tg(∞)vstData of Figure 1 by Using the
Phenomenological Expression Discussed in the Text
sample ξ0(nm) νTg(∞) (K)
R,ω-PS-(Rf)21.46 (0.23 1.37 (0.1 375 (0.9
com. H-PS (Mn)530K) 0.86 (0.12 1.35 (0.06 373.5 (0.2
home H-PS (Mn)52K) 0.72 (0.06 1.22 (0.03 372.2 (0.3
1492 Communications to the Editor Macromolecules, Vol. 35, No. 5, 2002