Decay of imprinted surface waves in polymers [Elektronische Ressource] : a method to probe near-surface dynamics / Kirstin Petersen

Kirstin Petersen

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2003
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

DECAY OF IMPRINTED SURFACE WAVES IN POLYMERS:
A METHOD TO PROBE NEAR-SURFACE DYNAMICS
Dissertation
zur Erlangung des Grades
”Doktor der Naturwissenschaften”
am Fachbereich Chemie und Pharmazie
der Johannes Gutenberg-Universitat¨
in Mainz
Kirstin Petersen
geboren in Aachen
Mainz 2003”If you don’t make mistakes you are not working on hard enough problems,
and that’s a big mistake.”
Frank Wilczek
iiContents
TABLE OF CONTENTS 1
1 INTRODUCTION 2
2 LITERATURE OVERVIEW: Current Status 4
3 EXPERIMENTAL TECHNIQUES AND MODES OF MEASUREMENT 10
3.1 Experimental Techniques 10
3.1.1 Sample and Master Preparation 10
3.1.2 Experimental Setup 17
3.1.3 Hot Embossing 21
3.2 Modes of Measurement 25
3.2.1 Quasi-Static:
Temperature Ramps and an Estimate for Surface Glass Temperature - 25
3.2.2 Dynamic:
Constant Temperature, Mastering, Activation Energies and Fragilities 30
4 THEORETICAL BACKGROUND 32
4.1 Surface Waves 32
4.2 Theoretical Models for Polymer Dynamics 37
4.2.1 Bead-Spring Model 37
4.2.2 Reptation Model 41
4.3 Time-Temperature Superposition 44
4.4 Relaxation Processes and Activation Energy 47
4.5 Fragility 56
4.6 Free Volume and Expansion Coefficients 58
5 RESULTS 62
5.1 Temperature Ramps 63
5.1.1 Temperature Ramps with Diffraction 63
5.1.2 Tature with Atomic Force Microscopy (AFM) 74
5.2 Decay Experiments with Diffraction: Surface Dynamics 79
5.2.1 Mastering 79
5.2.2 Activation Energies 85
5.2.3 Determination of Fragility 92
5.2.4 Free Volume and Expansion Coefficients 106
6 DISCUSSION 109
6.1 Discussion of the PMMA Results 109
6.2 Discussion of the PS Results 110
6.3 Comparison PMMA and PS 110
7 CONCLUSIONS 115
8 ZUSAMMENFASSUNG 117
9 OUTLOOK 119
10 APPENDIX 120
10.1Ehrenfest Theorem 120
List of Figures 121
References 122
Publications 135
1

1. INTRODUCTION
Polymers are widely used in industry and science. The easy processing, as well as the
variety of unique properties, has been the focus of much research in industry and academia.
One of these unique and attractive properties is the strong dependence of mechanical and op-
tical properties on the temperature. For amorphous polymers the drastic change in polymer
properties upon heating occurs at the glass ”transition” temperature. Since this transition is not
a true transition as defined by thermodynamics, the characteristic temperature will be referred
to as the glass temperature, abbreviated as . Well below , the polymer is brittle and hard,
above it is soft, easy to form and tacky.
The temperature dependent properties have to be taken into consideration and can be very
useful in polymer processing. However, following the trend toward miniaturization, the use of
polymeric materials brings up new problems. The failure of miniaturized devices can be pre-
vented if the properties of confined polymers are known. It has to be checked if confinement,
in the form of a surface, an interface, or a thin film has an influence on the properties of the ma-
terial. This is especially true for polymers with their large intrinsic length scales, for example
the radius of gyration [Fer80]. The radius of gyration is a few nanometers and thus the range
of perturbation by a surface or interface is much larger than in metals or crystals with short
intrinsic length scales. Confined polymers could therefore have characteristics different from
the bulk. This could result in a change in , segmental mobility and relaxation time, amongst
others.
Polymers in confined geometries have attracted the interest of many scientists and some
controversial results are discussed in chapter 2. Still no single theory for these phenomena has
been agreed upon.
An all inclusive model describing the behavior of polymers under confinement would not only
be useful for material processing but it could also enlighten one’s understanding of the funda-
mental processes in polymeric materials and macromolecular physics.
2







The research presented in this work focuses on surface-induced confinement of poly(methyl
methacrylate) (PMMA) and polystyrene (PS). The work is arranged into two parts.
The first section focuses on the influence of surface perturbations on .
The effect of the polymer chain length and chain entanglement with respect to the magnitude
of this effect is investigated by a series of measurements performed with different molecular
weights below and above the entanglement molecular weight.
Investigations on the possible influence of the technique on the resulting surface glass tempera-
ture will also be presented. It may be that a reduction of the probing depth increases the surface
effect.
In the second section the dynamics at the surface are investigated. One of the favored
explanations for a decrease in near the surface is an enhanced mobility of the polymer
chains at the free surface. An enhanced mobility should affect the relaxation times and thus the
dynamics of the polymer. Measurements for PMMA and PS related to the surface dynamics
are presented. The validity of time-temperature superposition (TTS) for the surface relaxation
processes of PMMA and PS is checked for a series of molecular weights. The shift factors
extracted from the TTS are analyzed in respect to free volume, expansion, activation energies
for relaxation processes, and fragilities.
3



2. LITERATURE OVERVIEW: Current Status
This chapter is an overview on the research published in the field of polymers in confined
geometry.
The first part of this chapter gives an overview of the literature concerning glass temper-
atures, s.
In the second part literature is presented which describes dynamic effects occurring under con-
fined geometry as found at surfaces and in thin films. Publications dealing with deviations from,
and conformity with, the bulk behavior in diffusive processes, mobilities, and activation ener-
gies will be covered in this part of the overview.
Glass Temperatures
The techniques and methods to determine the for thin films and surfaces are numerous
and so are the results, which do not always agree. The techniques usually detect discontinuities
in second derivatives of the free energy. These are changes in heat capacity and expansivity or
changes in properties related to them, such as refractive index, viscosity, and diffusivity.
The technique most obvious for the determination of the in thin films is, in anal-
ogy to bulk determinations, a differential scanning calorimeter (DSC). Efremov and coworkers
[EWO 02] designed a calorimeter for ultrathin films and remedied the lack of sensitivity by us-
ing Micro-Electro-Mechanical Systems-technology (MEMS-technology). MEMS-technology
is the integration of mechanical elements, sensors, actuators, and electronics on a common
silicon substrate through the utilization of microfabrication technology. The results for polydis-
perse polystyrene (PS) revealed both an increased and an increased fictive temperature for
thin films in comparison to the bulk. The fictive temperature is the temperature of intersection
of the extrapolated equilibrium liquid and glass enthalpy versus curves - the fictive
equilibrium transition temperature.
4








Measurements performed by Grohens et al. [GBL 98], using ellipsometry also point
toward a differing from the bulk value. This was found using PMMA with different tactic-
ities on various substrates. Polymer films with thicknesses between 20 and 40 nm showed an
increased in the case of iso- and a-tactic PMMA, and a decreased in the case of syndio-
tactic PMMA on silicon and on aluminum substrates. An influence of the tacticity on might
be assumed but a deviation of the thin film caused by the interaction of the polymer with the
substrate might also be a reason for the varying .
Wallace and coworkers [WZW95] and Tsui and Zhang [TZ01] investigated the depen-
dence of on the film thickness with X-ray reflectivity and ellipsometry measurements, re-
spectively. Both groups obtained a decreasing , reducing the film thickness of monodisperse
PS films on silicon. The experiments of Tsui and Zhang cover thin film s at two different film
thicknesses for a wide range of molecular weights (13.7 to 2300 kg/mol).
Jones and coworkers tested thin supported PS films with ellipsometry for a thickness de-
pendence of . The experiments revealed an increasing reduction of with decreasing film
thickness [KJ01]. They found not only the reduction in but also that the width of the transi-
tion increases with decreasing film thickness.
The question of how much the interface influences the thin film properties, and in par-
ticular, was addressed by Fryer and coworkers [FPK 01]. Changing the interface properties
by altering the silicon substrate with a hexadimethylsilane (HDMS) coating, revealed an effect
opposite to the one on a bare silicon substrate. Comparing it with the bulk value, on silicon
the thin film of PS was higher, whereas on the HDMS treated silicon surfaces it was lower
[FNdP00]. X-ray exposures of variable doses applied to octadecyltrichlorosilane (OTS) covered
silicon substrates altered the properties of the OTS films and with it the interfacial energy at the
polymer-substrate interface. PS and PMMA samples, prepared on the modified substrates were