Article

Polymer surface and thin film vibrational dynamics of poly(methyl methacrylate), polybutadiene, and polystyrene

Department of Chemistry, University of Chicago, Chicago, Illinois, United States
The Journal of Chemical Physics (Impact Factor: 2.95). 08/2008; 129(4):044906. DOI: 10.1063/1.2939018
Source: PubMed
ABSTRACT
Inelastic helium atom scattering has been used to investigate the vibrational dynamics at the polymer vacuum interface of poly(methyl methacrylate), polystyrene, and polybutadiene thin films on SiO(x)Si(100). Experiments were performed for a large range of surface temperatures below and above the glass transition of these three polymers. The broad multiphonon feature that arises in the inelastic scattering spectra at surface temperatures between 175 and 500 K is indicative of the excitation of a continuum of surface vibrational modes. Similarities exist in the line shapes of the scattering spectra, indicating that helium atoms scatter from groups of similar mass on the surface of these polymer thin films. The line shapes obtained were further analyzed using a semiclassical scattering model. This study has shown that quite different polymer thin films can have similar interfacial dynamics at the topmost molecular layer.

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Polymer surface and thin film vibrational dynamics
of polymethyl methacrylate, polybutadiene,
and polystyrene
Miriam A. Freedman,
a
James S. Becker, A. W. Rosenbaum,
b
and S. J. Sibener
c
The James Franck Institute and Department of Chemistry, The University of Chicago, 929 E. 57th St.,
Chicago, Illinois 60637, USA
Received 7 February 2008; accepted 12 May 2008; published online 28 July 2008
Inelastic helium atom scattering has been used to investigate the vibrational dynamics at the
polymer vacuum interface of polymethyl methacrylate, polystyrene, and polybutadiene thin films
on SiO
x
/ Si100. Experiments were performed for a large range of surface temperatures below and
above the glass transition of these three polymers. The broad multiphonon feature that arises in the
inelastic scattering spectra at surface temperatures between 175 and 500 K is indicative of the
excitation of a continuum of surface vibrational modes. Similarities exist in the line shapes of the
scattering spectra, indicating that helium atoms scatter from groups of similar mass on the surface
of these polymer thin films. The line shapes obtained were further analyzed using a semiclassical
scattering model. This study has shown that quite different polymer thin films can have similar
interfacial dynamics at the topmost molecular layer. © 2008 American Institute of Physics.
DOI: 10.1063/1.2939018
I. INTRODUCTION
Polymer thin films are widely used in the fabrication of
devices for technological applications such as microelectron-
ics, sensors, protective and optical coatings, and membranes
for diverse fields from engineering and material science to
medicine.
1
Significant questions remain, however, regarding
the differences between the structure and properties of these
films as compared to bulk polymer samples.
2
In particular,
nanoconfinement strongly affects the overall film properties
due to the increased free volume and difference in electro-
static potential at the free surface and interactions with the
substrate layer. Investigation into these differences began
over a decade ago with the dewetting studies of Reiter and
the ellipsometry studies of Keddie et al.
3
Many aspects of
polymer thin films have been studied using a variety of tech-
niques. For studies of the glass transition averaged over the
whole thin film system, areas of interest include but are not
limited to the effect of the substrate layer,
4
the importance
of the molecular architecture and tacticity of the polymer,
5
molecular weight,
6
relaxation,
7
and diffusion of small par-
ticles through the film.
8
Numerous studies have also been
performed to investigate increased mobility in the surface
layer. Areas of interest include
-relaxation,
9
surface mo-
lecular motion as probed by scanning probe microscopy,
10
surface relaxation studied with sum frequency generation
SFG,
11
and relaxation after surface deformation.
12
The ma-
jority of dynamics studies that have been performed probe
long length-scale motion of polymer chains or segmental
motion. One exception is neutron scattering studies in which
molecular-scale motion of the polymers is observed. Most of
the techniques used either perturb the system of interest or
probe several layers rather than just the topmost interface,
and so questions regarding the properties of the interface at
the free surface of the polymer remain largely unanswered.
To characterize the dynamics at the topmost interface,
we use the nondestructive and surface-sensitive probe of
low-energy helium atom scattering HAS. With the excep-
tion of our previous study,
13
this technique has not been used
to investigate polymer thin films. This study is both different
from traditional HAS and from most studies of polymer thin
film dynamics. As a result, a description of what we can
uniquely measure with HAS is presented. HAS has tradition-
ally been used to investigate the structure and dynamics of
single crystals and simple adsorbates on single crystals.
14,15
HAS can be viewed as a surface-sensitive analog to neutron
scattering. Because helium atoms scatter from the electron
density approximately 3 Å above the surface whereas neu-
trons scatter from atomic cores, HAS provides increased sur-
face sensitivity.
1517
HAS has been used to characterize sur-
face structure and is remarkably sensitive to defects
including point defects and steps.
18
In dynamics studies,
HAS has been used to characterize phonon dispersion curves
with inelastic HAS, diffusion through quasielastic HAS, and
multiphonon scattering.
In recent years, helium atom scattering studies have ex-
panded to include the investigation of organic thin films.
Studies have probed structure, where possible, and dynamics
of self-assembled monolayers SAMs,
19
alkanes,
20
carboxy-
lic acids,
21
liquids,
22
and fatty acids.
23
In most of these cases,
the surface is ordered as observed with helium atom diffrac-
tion. Single-phonon spectra are often found, sometimes over
a
Present address: Cooperative Institute for Research in Environmental Sci-
ences, University of Colorado, Boulder, CO 80309.
b
Present address: Intel Corporation, 2111 NE 25th Ave., Hillsboro, OR
97124.
c
Author to whom correspondence should be addressed. Electronic mail:
s-sibener@uchicago.edu.
THE JOURNAL OF CHEMICAL PHYSICS 129, 044906 2008
0021-9606/2008/1294/044906/9/$23.00 © 2008 American Institute of Physics129, 044906-1
Downloaded 26 Feb 2009 to 128.135.186.165. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
Page 1
a broad multiphonon background. In some cases, however,
only a multiphonon feature is observed,
22,23
as is the case
with polymer thin films. With the precedent of HAS studies
on organic mono- and multilayers, in our group, we have
decided to investigate polymer thin films. Helium atom scat-
tering provides a detailed picture of the low-energy surface
vibrational dynamics of the polymer at the vacuum interface.
The information we obtain is in some ways more similar to
heavy rare gas scattering studies of complex materials,
22,24
rather than traditional HAS experiments. The use of helium
atoms, however, provides a nondestructive and surface-
sensitive probe through which we can characterize vibra-
tional dynamics of the topmost interfacial layer.
Additionally, HAS studies of polymer thin films are
more similar to previous neutron scattering studies than to
other techniques commonly used in the polymer literature. In
particular, HAS is sensitive to molecular-scale vibrational
dynamics rather than segmental or long-range motion of the
polymer chains. In contrast to neutron scattering studies,
HAS is sensitive exclusively to the topmost molecular layer.
The vibrations probed are sagittally polarized modes of the
chain or substituent groups present at the polymer-vacuum
interface. Localized, short timescale motion of the topmost
molecular interface in confined polymer systems has not
been investigated in the literature. Information about surface
vibrational motion would provide new information that
would help to develop a better understanding of dynamics in
confined polymer systems.
Helium atom scattering is uniquely able to access infor-
mation on the dynamics and corrugation of the polymer-
vacuum interface without complicating contributions from
the bulk properties. In a previous paper, we have shown he-
lium atom scattering to be sensitive to the effect of nanocon-
finement on the vibrational dynamics of polymethyl meth-
acrylate兲共PMMA in the thin film limit.
13
In this paper, we
significantly expand on our prior work by examining and
comparing more extensive results for PMMA, which has a
bulk glass transition temperature of 380 K, to 1,4-trans-
polybutadiene PB, which has a bulk glass transition tem-
perature of 167 K, over the temperature range of
200500 K.
25
We also compare our PMMA results to those
from polystyrene PS兲共bulk T
g
=373 K, one of the most
widely studied polymers. Moreover, in this work, we also
present a detailed analysis of the scattering spectra for all
three systems, utilizing a semiclassical scattering model from
which we derive information about the He-polymer potential
energy surface, polymer surface structure, and the surface’s
molecular-scale vibrational dynamics.
II. EXPERIMENTAL
Experiments were conducted in a high angle- and
energy-resolution helium atom scattering apparatus, which
has been described in detail elsewhere.
26
Briefly, it consists
of a cryogenically cooled supersonic helium beam source, an
ultrahigh vacuum UHV scattering chamber equipped with
surface characterization tools, a precollision chopper chop-
per to sample distance of 0.554 m, and a rotating, long flight
path sample to ionizer distance of 1.005 m quadrupole
mass spectrometer detector. The rotating detector and inde-
pendent crystal angular drive allow for the incident and final
angles to be independently varied. The angular collimation
yields a resolution of 0.22° and a
/
of less than 1% for
most of the reported beam energies. This design allows for
precise measurements of scattering angles and energy ex-
change with the surface by acquisition of the time-of-flight
TOF of scattered helium atoms. A cross-correlation TOF
technique with a pseudorandom chopping sequence was used
to maximize the signal-to-noise ratio.
27
Spectra are deconvo
-
luted according to Ref. 28.
Three high-molecular weight, atactic polymers, PMMA
PolySciences, 1,4-trans-PB Polymer Source, and PS
PolySciences, were spin coated on the native oxide layer of
Si100. Parameters for the molecular weight, polydispersity,
and thickness of the samples can be found in Table I. Please
note that we have provided the relative thicknesses in terms
of the bulk radius of gyration, R
g
. This scaling is common
and is generally useful for comparing polymers of different
molecular weights. The scaling cannot, however, be exactly
correlated with actual film thickness because the chain con-
formation of polymers, and hence their radius of gyration,
changes in a thin film relative to the bulk. The SiO
x
/ Si100
substrates were cleaned by sequential sonication in toluene,
acetone, and methanol. The substrates used for the PB films
were additionally cleaned by 10 min immersions in 80 °C
solutions of 5:1:1 H
2
O:30% H
2
O
2
:NH
4
OH followed by
6:1:1 H
2
O:30% H
2
O
2
:40% HCl. The substrates were then
TABLE I. Parameters used for polymer experiments. M
w
is the molecular weight, M
w
/ M
n
is the polydispersity
index, T
g
is the glass transition temperature, h is the film thickness, and R
g
is the radius of gyration. Thicknesses
are given both in units of nanometers and radius of gyration.
Polymer
M
w
kg/mol M
w
/ M
n
T
g
K h nm
R
g
nm
a
h R
g
Polymethyl
methacrylate
350 1.15 380 11 15 0.67
Polymethyl
methacrylate
350 1.15 380 47 15 3.3
1,4-trans-Polybutadiene 60.9 1.05 167 16 7.1 2.2
1,4-trans-Polybutadiene 60.9 1.05 167 87 7.1 12
Polystyrene 400 1.06 373 119 17 7.0
Polystyrene 400 1.06 373 111 17 6.5
a
Values for R
g
for PB from Ref. 25.
044906-2 Freedman et al. J. Chem. Phys. 129, 044906 2008
Downloaded 26 Feb 2009 to 128.135.186.165. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
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