Investigation of the Interfacial Friction and Adhesion of Thin PDMS Network Lubricant Films

209 Pages·2010·3.22 MB·English

Investigation of the Interfacial Friction and Adhesion of Thin PDMS Network
Lubricant Films
by Lucas James Landherr
This thesis/dissertation document has been electronically approved by the following individuals:
Archer,Lynden A. (Chairperson)
Ober,Christopher Kemper (Minor Member)
Cohen,Claude (Minor Member)
INVESTIGATION OF THE INTERFACIAL FRICTION AND ADHESION OF
THIN PDMS NETWORK LUBRICANT FILMS

A Dissertation
Presented to the Faculty of the Graduate School
of Cornell University
in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy

by
Lucas James Landherr
August 2010
© 2010 Lucas James Landherr
INVESTIGATION OF THE INTERFACIAL FRICTION AND ADHESION OF
THIN PDMS NETWORK LUBRICANTS FILMS
Lucas James Landherr, Ph.D.
Cornell University, 2010

The lubrication properties of several variations on a polydimethylsiloxane
(PDMS) network – self-assembled monolayer (SAM) hybrid lubricant and the effect
that structural and chemical modifications can have on the friction, adhesion, and wear
are investigated. The primary lubricant structure studied in this thesis consisted of a
model cross-linked PDMS network tethered to an anchoring SAM layer.
Modifications of this structure including ultrathin hyperbranched films, micron-thick
films with pendent chains, networks swollen with free chains, and a deconstructed
polymer brush are also studied. These hybrids created high degrees of internal
tethering with a flexible surface layer exhibiting low interfacial shear and low surface
energy.
The friction and adhesion properties of these lubricants were determined from
a series of experiments conducted with atomic force microscopy, nanoindentation, and
bulk rheology and tribology. The PDMS-SAM lubricants demonstrated dramatically
low friction coefficients, as low as μ = 0.0024, which is the lowest friction coefficient
recorded for a dry polymer film. These low friction properties are a result of a
negligible contribution of adhesion to the friction force.
Structural modifications to the network allowed for the effect of the physical
properties on the friction and lubrication to be determined. Incorporating pendent
chains and free chains into the network layer increased the surface viscous dissipation.
While the elastic moduli and the network stiffness decreased, the structural
modifications also served to increase the surface shear and viscosity, resulting in
larger friction coefficients. Wear measurements conducted on the networks swollen
with free chains indicated that increasing the lubricant viscous dissipation also
dramatically reduced the wear resistance of the film.
Polymer brushes were developed at several surface coverages and with
different polymer chemistries to determine the physical and chemical effects on
friction and lubrication. High surface coverages creating stretched elastic chains
yielded a significant reduction in the friction coefficient, relative to more deformed,
mushroom conformations. PDMS brushes exhibited significantly lower friction than
brushes made from polymers with higher surface energies, despite the chemical effects
being minimized as a result of a minimal adhesion contribution to the friction force at
the ultrathin lubricant lengthscale.
BIOGRAPHICAL SKETCH

Lucas James T. Landherr was born in southeastern Connecticut on February 4, 1983.
He graduated as valedictorian from St. Bernard’s High School in Uncasville, CT, in
2001. He attended Lafayette College in Easton, PA, and received a B.S. in Chemical
Engineering in 2005, graduating Summa Cum Laude with honors. Upon completion
of the requirements for a Ph.D. in Chemical Engineering at Cornell University, Lucas
will begin a post-doctoral research investigation at the National Institute of Standards
and Technology in Gaithersburg, MD.

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for Maret and my parents
(Anyone who puts up with me deserves this as much as I do.)
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ACKNOWLEDGMENTS
Thank you to Dr. Lynden Archer and Dr. Claude Cohen for their insight,
recommendations and intellectual challenges that made me a better chemical engineer.

Thank you to Dr. Christopher Ober for serving on my thesis committee and helping at
various points along the way.

Thank you to Dr. Qing Zhang for training me during my first year at Cornell.

Thank you to Henry Lau for repeatedly serving as a sounding board over five years
and helping to refine my academic teaching skills as a teaching advisor partner for two
years. Thank you to all other members of the Archer and Cohen research groups who
have contributed with their suggestions and criticisms.

Thank you to Fluids students who showed me graduate school was the right decision.

Thank you to National Science Foundation and Department of Energy for their
funding support, and to the research facilities providing the equipment needed for the
many experiments described within.

Finally, and most importantly, thank you to my family for their support. Without the
guidance, encouragement and humor of many people over many years, I would never
have gotten to this point. Thank you to my wife, Maret, for providing a much-needed
anchor to a world outside research. Thank you to my parents, my sisters,
grandparents, pets, the rest of my extended family and a number of close friends for
helping me become someone who could take on this challenge and succeed.
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TABLE OF CONTENTS

Biographical Sketch iii
Dedication iv
Acknowledgments v
List of Figures ix
List of Tables xi

1. Introduction: Friction, Adhesion, and Lubrication Systems 1
1.1 Introduction 1
1.2 Friction and Adhesion 1
1.3 Lubrication Systems 9

2. The Interfacial Friction of Thin PDMS Network Films 17
2.1 Abstract 17
2.2 Introduction 18
2.3 Experimental Section 22
2.3.1 Preparation of SAMs 22
2.3.2 Preparation of PDMS Networks 23
2.3.3 Characterizing the Thickness of the PDMS Networks 23
2.3.4 Preparation of Monofunctional PDMS Chains 24
2.3.5 AFM Analysis 24
2.3.6 Characterizing the Elastic Modulus of PDMS Network Films 26
2.3.7 Characterizing PDMS Networks Using Solvent Swelling
Measurements 29
2.4 Results and Discussion 30
2.4.1 Ultrathin Film PDMS Network Lubricants 30
2.4.2 Thin Film PDMS Network Lubricants 40
2.4.3 Effect of the Surface Chemistry of the Bead Probe 44
2.4.4 Role of Pendent Chains 50
2.5 Conclusions 54
2.6 Acknowledgments 55

3. The Effect of Pendent Chains on the Interfacial and Bulk Properties of Thin
PDMS Networks 59
3.1 Abstract 59
3.2 Background 59
3.3 Experimental 63
3.3.1 Preparation of SAMs 63
3.3.2 Synthesis of Difunctional PDMS 64
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3.3.3 Synthesis of Monofunctional PDMS 64
3.3.4 Preparation of Cross-linked PDMS Networks 65
3.3.5 Characterizing the Thickness of PDMS Cross-linked Films 66
3.3.6 AFM Analysis 66
3.4 Results 68
3.4.1 Effect of Pendent Chains On Friction 68
3.4.2 Surface Characterization 74
3.4.3 Bulk Analysis 85
3.4.4 Surface Mobility 90
3.5 Conclusions 93

4. The Friction and Wear of PDMS Network Lubricants Swollen With Free
Chains 100
4.1 Abstract 100
4.2 Background 100
4.3 Experimental 104
4.3.1 Preparation of SAMs 104
4.3.2 Preparation of PDMS Networks 105
4.3.3 Characterizing the Thickness of the PDMS Networks 106
4.3.4 AFM Analysis 106
4.4 Results 108
4.4.1 Interfacial Friction 108
4.4.2 Surface Characterization 113
4.4.3 Wear and Tribology 127
4.5 Conclusions 131

5. Physical and Chemical Characterization of the Interfacial Friction and
Adhesion of Ultrathin Polymer Brush Films 135
5.1 Abstract 135
5.2 Introduction 136
5.3 Experimentation 138
5.3.1 Preparation of Vinyl-Terminated SAMs 138
5.3.2 Preparation of Monofunctional Vinyl-Terminated PDMS 139
5.3.3 Preparation of PDMS Brushes 139
5.3.4 Preparation of Sulfonic SAMs 139
5.3.5 Preparation of PS and PPG/PEG Brushes 140
5.3.6 Brush Thickness Measurement 140
5.4 Analysis 140
5.4.1 Surface Coverage and Bulk Properties 140
5.4.2 Friction Measurements 143
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