urrent Research & Results
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Highly localized probing
of surface nanomechanical properties with a
submicron resolution can be accomplished with scanning probe microscopy (SPM)
technique. The SPM ability to probe local surface topography in conjunction
with mechanical, adhesive, friction, thermal, magnetic, and electric
properties is unique. We focus on
organized molecular films from amphiphilic
molecules, molecules with reactive ends and functional oligomers.
These films are fabricated by means of Langmuir-Blodgett
technique and physical/chemical self-assembly. The interfacial properties of molecular
coatings critical for their lubrication or adhesive performance at a nanoscale are friction forces, shear strength, surface
stiffness, compressive elastic behavior, viscoelastic
response and adhesive forces.
Recent highlights:
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Four generations of amphiphilic
dendrons containing a benzyl-15-crown-5 polar focal
point, photochromic spacer, and alkyl tails as
peripheral groups have been investigated for their ability to form Langmuir monolayers at the
air-water interface and on solid surfaces.
The cross-sectional area was found to be proportional to number of
alkyl chains for low generations but higher dendrimer
generations demonstrate loose packing because of steric
hindrances among the branches the outer shells of the dendrons. Photomechanical behavior of Langmuir monolayers under
UV-illumination was observed for intermediate generations with optimal
balance between bulky polar heads and dendrimer
shells. We found significant
acceleration of photomechanical response during monolayer compression to
solid state due to increasing cooperative interactions among photochromic groups.
A.
Sidorenko, C. Houphouet-Boigny,
O. Villavicencio, M. Hashemzadeh,
D. V. McGrath, V. V. Tsukruk, PHOTORESPONSIVE
LANGMUIR MONOLAYERS FROM AZOBENZENE-CONTAINING DENDRONS, Langmuir,
2000, 16, 10569.
p-A isotherms for Langmuir monolayers from dendrimers AD0,
AD1, AD2, AD3, reference azobenzene compound AH
with three tails, and stearic acid (StA)
Rate
and amplitude of photomechanical transformations during the illumination of
the AD1 monolayer at various surface pressure
Molecular
structures of isomers of AD0 monodendron. Tilt of mesogenic group increases when trans-cis isomerization occurs.
SPM topographical and phase images of the monolayers of AH and AD1 dendrons.
Vertical scales are 20 nm for AH topography and 5 nm for AD1, 90° for AH
phase image and 10° for AD1 phase image. Lateral scale is 5´5 mm.
Bright parts correspond to higher elevations
We present the results
of microthermal studies of ultrathin
polymeric films deposited on a silicon substrate. We propose the procedure for microthermal analysis of ultrathin
polymer films, elucidate limits of applicability of this technique to study
their microthermal properties, and observe thermomechanical response during local heating for films
with thickness above 10 nm. The glass
transition temperature of ultrathin PS films
deduced from experiments decreases for film thickness below 200 nm. For the thinnest PS film with clearly
detectable response (25 nm), glass transition temperature is 20oC
below its bulk value. Our estimation
of heat dissipation in the tip-surface contact area supports conclusion that
observed micro-thermal-analysis (μTA) response
for polymer films is associated mainly with temperature induced elastic
variation of the contact area.
V. V. Gorbunov, N. Fuchigami,
I. Luzinov, V. V. Tsukruk,
Microthermal Probing Of Ultrathin
Polymer Films, High Performance Polymers, 12, 603, 2000.
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The
variation of its glass transition temperature as probed with µTMA of the PS
films with different thickness on a silicon substrate
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We fabricated robust ultrathin
film of a tri-block copolymer, poly[styrene – b - (ethylene-co-butylene) - b- styrene] (SEBS), functionalized with 2% of
maleic anhydride by melt/solution grafting to a
chemically reactive silicon surface.
The thickness of grafted block-polymer, t, was varied from 1.35
nm to 9.1 nm to test and the ratio t/d
was changed from 0.05 to 0.33, where d is the interdomain spacing.
The contact angle measurements demonstrated that the surface of the
complete block copolymer films was totally occupied by poly[ethylene-co-butene] (PEB) chains.
When the SEBS film thickness reached 8.4 nm, the film possessed the
well-defined microphase structure of the typical
thermoplastic elastomer material, where PS phase
formed the microdomain network that reinforced the elastomeric matrix.
The microphase separation was completely
suppressed only for ultrathin films with t/d
< 0.08. We found that tethered monolayers were extremely stable and preserved their microdomain structure at elevated temperatures unlike spincoated films that dewetted
under similar conditions. |
Microstructure
of grafted SEBS layer Luzinov, D. Julthongpiput, V.
V. Tsukruk, Thermoplastic Elastomer
Monolayers Grafted to a Silicon Substrate, Macromolecules,
33, 7629, 2000 I. Luzinov, D. Julthongpiput, V. V. Tsukruk,
Stability Of Microdomain Morphology In Tethered
Block-Polymer Monolayers, Polymer, 42,
2267, 2001. |
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Wear resistance test for
grafted (left) and spin-coated (right) polymer films |
Nanodomain structure of 8 nm thick grafted SEBS layer |
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Scanning thermal microscopy (SThM)
is used for the probing microthermal surface
properties of a wide range of materials ranging from low thermally conductive
polymers to highly conductive metals.
We demonstrate the unique capability of SThM
technique in measuring microthermal surface
conductivity. We discuss the detail
experimental procedure for the microthermal
measurement including the selection of the probing parameters for different
surfaces, data collection, and data processing. We propose and test the relationships for
the evaluation of the thermal probe heat dissipation and the surface temperature variation during approaching-retracing microprobing. For
the direct thermal probe-surface contact, a linear relationship is
demonstrated between the probe heat dissipation and the initial temperature
gradient that opens the way for the direct estimation of the absolute value
of the surface thermal conductivity. We proposed the procedure for the evaluation of
the surface thermal conductivity from the microthermal
probing with scanning thermal microscopy (SThM). This approach is based on the theory of a steady
heat transfer between the thermal tip and a surface. The variation of the surface microthermal
conductivity as measured by SThM correlates fairly
well with known bulk materials values.
We used
the model of the quasi-steady heat transfer between the thermal tip and the
surface to interpret our experimental results obtained in part I (Gorbunov et al, 2000).
This model describes the observed increase of the heat dissipation of
the thermal probe approaching the surface due to the temperature gradient
presented on a surface before the probe-surface physical contact. The model predicts that there is a lower limit for
the thermal probe velocity for obtaining meaningful thermal data. Higher
sensitivity of the microthermal probing can be
achieved by increasing probe velocity and by using positional/temperature
modulation. V. V. Gorbunov, N. Fuchigami,
J. L. Hazel, V. V. Tsukruk, Microprobing
Surface Thermal Properties By Scanning Thermal Microscopy, Langmuir, 1999, V15, N24, p.8340-8343 V. V. Gorbunov, N. Fuchigami, V. V. Tsukruk, Microthermal Analysis With Scanning Thermal Microscopy.
I. Methodology and Experimental, Probe Microscopy, 2, 53,
2000.
V. V. Gorbunov, N. Fuchigami, V. V. Tsukruk, Microthermal Analysis With Scanning Thermal Microcopy. II: Calibration, Modeling, and Interpretation. Probe Microscopy, 2, 65, 2000. |
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A sketch of the
SThM detection scheme and tip-surface approaching
and retracting data for gold and polystyrene surfaces showing difference in
heat dissipation jump at the point of physical contact. |
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Thermal image (right) and
topography (left) for 15 x 15 µm silicon oxide-silicon grid. |
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Dense, homogeneous, and complete self-assembled monolayers (SAMs) with epoxy
surface groups were fabricated from epoxysilanes to
serve as a template for chemical anchoring of polymer layers. Epoxysilane SAMs were truly monomolecular films with a
virtually normal molecular orientation of densely packed molecules, which
were firmly attached to the substrate.
Carboxylic
acid and anhydride-terminated polystyrenes of different molecular weights
from 4500 to 672000 were grafted from
melt onto silicon substrates modified with epoxysilane
monolayer. The grafted chains are
densely packed with a density close to the known value for the bulk
material. The tethered polymer layers
are very smooth, uniform, mechanically stable, and cover homogeneously the
modified silicon. At the degree of
polymerization (N) close to the
critical molecular weight, the grafting process is the most effective
resulting in the grafted unperturbed macromolecules. We suggest that the grafting from the reactive
polymer melt is controlled by steric constrains
through the minimum available free
volume between the grafted macromolecules, which is reachable by another
reactive end. V. V. Tsukruk,
I. Luzinov, D. Julthongpiput,
Sticky Molecular Surfaces: Epoxysilane
Self-Assembled Monolayers, Langmuir, 1999, 15, 3029. I. Luzinov, D. Julthongpiput,
H. Malz, J. Pionteck, V.
V. Tsukruk, Polystyrene Layers Grafted To
Epoxy-Modified Silicon Surfaces, Macromolecules,
33, 1043, 2000. |
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We fabricated and studied
several sets of composite molecular films such as self-assembled monolayers and fullerene molecules, complexes of fatty
acids and rigid polymers (ladder polymers, polyimides).
Their structural ordering and microtribological
properties were studied in comparison with classic boundary lubricants by
AFM/FFM. We suggested the characterization of the organic layers with a
figure of merit parameter introduced through the friction coefficient
normalized to the maximum limits of dissipated energy. Correlation between
this parameter and a packing density of the monolayers
was found. V. N.
Bliznyuk, et al. "Nanotribological properties of organic
boundary lubricants: Langmuir films versus
self-assembled monolayers" J. Tribology, October, 120, 489, 1998 |
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Self-assembly of polymer (grafted polymer, polyelectrolyte, and dendrimer) layers was studied. Nano-
and micro-phase separated structures were observed for mixed monolayers. Organized multilayer films from globular dendritic macromolecules were fabricated and
characterized. V. V.Tsukruk, et al. "Miscibility and segregated structures in mixed
polymer monolayers", Supramolecular
Sciences, 2, 219, 1995; V. V. Tsukruk "Dendritic Macromolecules at Interfaces", Advanced
Matl., 10, 253, 1998 |
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SPM was used for probing nanomechanical
properties of compliant polymeric materials. Sneddon’s,
Hertzian, and Johnson-Kendall-Roberts theories of
elastic contacts were tested for a variety of polymeric materials, ranging
from very compliant rubbers (e.g., natural rubber) to hard glassy polymers
(e.g., polystyrene). All models of elastic contact allowed microprobing depth profile and gave consistent values of
elastic moduli. S. A. Chizhik, Z. Huang, V. V. Gorbunov,
N. K. Myshkin, V. V. Tsukruk,
Micromechanical Properties Of Elastic Polymeric Materials As Probed By
Scanning Force Microscopy, Langmuir, 14, 2606,
1998. V. V. Tsukruk, V. V. Gorbunov,
Z. Huang, S. A. Chizhik, Dynamic Microprobing Of Viscoelastic
Polymer Properties, Polymer Intern. 49,
441, 2000. |
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