9 research outputs found
Adhesion and Friction Properties of Fluoropolymer Brushes: On the Tribological Inertness of Fluorine
The effects of fluorination on the
adhesion and friction properties
of covalently bound polyÂ(fluoroalkyl methacrylate) polymer brushes
(thickness ∼80 nm) were systematically investigated. Si(111)
surfaces were functionalized with a covalently bound initiator via
a thiol–yne click reaction to have a high surface coverage
for initiator immobilization. Surface-initiated atom-transfer radical
polymerization (SI-ATRP) was employed for the synthesis of four different
fluoropolymer brushes (SPF<i>x</i>, where <i>x</i> = 0, 3, 7, or 17 F atoms per monomer), based on fluoroalkyl methacrylates.
All polymer brushes were characterized with static contact angle measurements,
X-ray photoelectron spectroscopy (XPS), and infrared absorption reflection
spectroscopy (IRRAS). The polymer brushes exhibited an excellent hydrophobicity,
with static water contact angles of up to 121° depending on the
number of fluorine atoms per side chain in fluoroalkyl methacrylate.
The degree of swelling was precisely studied by using ellipsometry
in different solvents such as acetone, hexadecane, hexafluoroisopropanol,
nonafluorobutyl methyl ether, and Fluorinert FC-40. The polymer brushes
have shown nanoscale swelling behavior in all solvents except hexadecane.
The grafting density decreased upon increasing fluorine content in
polymer brushes from 0.65 chains/nm<sup>2</sup> (SPF0) to 0.10 chains/nm<sup>2</sup> (SPF17) as observed in Fluorinert FC-40 as a good solvent.
Adhesion and friction force measurements were conducted with silica
colloidal probe atomic force microscopy (CP-AFM) under ambient, dry
(argon), and lubricating fluid conditions. SPF17 showed the lowest
coefficient of friction 0.005 under ambient condition (RH = 44 ±
2%) and a further decrease with 50% under fluidic conditions. These
polymer brushes also showed adhesion forces as low as 6.9 nN under
ambient conditions, which further went down to 0.003 nN under fluidic
conditions (Fluorinert FC-40 and hexadecane) at 10 nN force
Adhesion and Friction Properties of Polymer Brushes: Fluoro versus Nonfluoro Polymer Brushes at Varying Thickness
A series
of different thicknesses of fluoro polyÂ(2,2,2-trifluoroethyl
methacrylate) and its analogous nonfluoro polyÂ(ethyl methacrylate)
polymer brushes were prepared via surface-initiated ATRP (SI-ATRP)
on Si(111) surfaces. The thiol-yne click reaction was used to immobilize
the SI-ATRP initiator with a high surface coverage, in order to achieve
denser polymer brushes (grafting density from ∼0.1 to 0.8 chains/nm<sup>2</sup>). All polymer brushes were characterized by static water
contact angle measurements, infrared absorption reflection spectroscopy,
and X-ray photoelectron spectroscopy. Adhesion and friction force
measurements were conducted with silica colloidal probe atomic force
microscopy (CP-AFM) under ambient and dry (argon) conditions. The
fluoro polyÂ(2,2,2-trifluoroethyl methacrylate) polymer showed a decrease
in adhesion and friction with increasing thickness. The analogous
nonfluoro polyÂ(ethyl methacrylate) polymer brushes showed high adhesion
and friction under ambient conditions. Friction coefficients down
to 0.0057 (ambient conditions) and 0.0031 (dry argon) were obtained
for polyÂ(2,2,2-trifluoroethyl methacrylate) polymer brushes with 140
nm thickness, which are the lowest among these types of polymer brushes
Efficient Functionalization of Oxide-Free Silicon(111) Surfaces: Thiol–yne versus Thiol–ene Click Chemistry
Thiol-yne
click (TYC) chemistry was utilized as a copper-free click
reaction for the modification of alkyne-terminated monolayers on oxide-free
Si(111) surfaces, and the results were compared with the analogous
thiol–ene click (TEC) chemistry. A wide range of thiols such
as 9-fluorenylmethoxy-carbonyl cysteine, thio-β-d-glucose
tetraacetate, thioacetic acid, thioglycerol, thioglycolic acid, and
1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-perfluorodecanethiol was immobilized using TYC under photochemical
conditions, and all modified surfaces were characterized by static
water contact angle measurements, X-ray photoelectron spectroscopy
(including a simulation thereof by density functional calculations),
and infrared absorption reflection spectroscopy. Surface-bound TYC
proceeds with an efficiency of up to 1.5 thiols per alkyne group.
This high surface coverage proceeds without oxidizing the Si surface.
TYC yielded consistently higher surface coverages than TEC, due to
double addition of thiols to alkyne-terminated monolayers. This also
allows for the sequential and highly efficient attachment of two different
thiols onto an alkyne-terminated monolayer
Hydrolytic and Thermal Stability of Organic Monolayers on Various Inorganic Substrates
A comparative study is presented
of the hydrolytic and thermal
stability of 24 different kinds of monolayers on Si(111), Si(100),
SiC, SiN, SiO<sub>2</sub>, CrN, ITO, PAO, Au, and stainless steel
surfaces. These surfaces were modified utilizing appropriate organic
compounds having a constant alkyl chain length (C<sub>18</sub>), but
with different surface-reactive groups, such as 1-octadecene, 1-octadecyne,
1-octadecyltrichlorosilane, 1-octadecanethiol, 1-octadecylamine and
1-octadecylphosphonic acid. The hydrolytic stability of obtained monolayers
was systematically investigated in triplicate in constantly flowing
aqueous media at room temperature in acidic (pH 3), basic (pH 11),
phosphate buffer saline (PBS) and deionized water (neutral conditions),
for a period of 1 day, 7 days, and 30 days, yielding 1152 data points
for the hydrolytic stability. The hydrolytic stability was monitored
by static contact angle measurements and X-ray photoelectron spectroscopy
(XPS). The covalently bound alkyne monolayers on Si(111), Si(100),
and SiC were shown to be among the most stable monolayers under acidic
and neutral conditions. Additionally, the thermal stability of 14
different monolayers was studied in vacuum using XPS at elevated temperatures
(25–600 °C). Similar to the hydrolytic stability, the
covalently bound both alkyne and alkene monolayers on Si(111), Si(100)
and SiC started to degrade from temperatures above 260 °C, whereas
on oxide surfaces (e.g., PAO) phosphonate monolayers even displayed
thermal stability up to ∼500 °C
Ambient Surface Analysis of Organic Monolayers using Direct Analysis in Real Time Orbitrap Mass Spectrometry
A better characterization of nanometer-thick
organic layers (monolayers)
as used for engineering surface properties, biosensing, nanomedicine,
and smart materials will widen their application. The aim of this
study was to develop direct analysis in real time high-resolution
mass spectrometry (DART-HRMS) into a new and complementary analytical
tool for characterizing organic monolayers. To assess the scope and
formulate general interpretation rules, DART-HRMS was used to analyze
a diverse set of monolayers having different chemistries (amides,
esters, amines, acids, alcohols, alkanes, ethers, thioethers, polymers,
sugars) on five different substrates (Si, Si<sub>3</sub>N<sub>4</sub>, glass, Al<sub>2</sub>O<sub>3</sub>, Au). The substrate did not
play a major role except in the case of gold, for which breaking of
the weak Au–S bond that tethers the monolayer to the surface,
was observed. For monolayers with stronger covalent interfacial bonds,
fragmentation around terminal groups was found. For ester and amide-terminated
monolayers, in situ hydrolysis during DART resulted in the detection
of ions characteristic of the terminal groups (alcohol, amine, carboxylic
acid). For ether and thioether-terminated layers, scission of C–O
or C–S bonds also led to the release of the terminal part of
the monolayer in a predictable manner. Only the spectra of alkane
monolayers could not be interpreted. DART-HRMS allowed for the analysis
of and distinction between monolayers containing biologically relevant
mono or disaccharides. Overall, DART-HRMS is a promising surface analysis
technique that combines detailed structural information on nanomaterials
and ultrathin films with fast analyses under ambient conditions
Ambient Surface Analysis of Organic Monolayers using Direct Analysis in Real Time Orbitrap Mass Spectrometry
A better characterization of nanometer-thick
organic layers (monolayers)
as used for engineering surface properties, biosensing, nanomedicine,
and smart materials will widen their application. The aim of this
study was to develop direct analysis in real time high-resolution
mass spectrometry (DART-HRMS) into a new and complementary analytical
tool for characterizing organic monolayers. To assess the scope and
formulate general interpretation rules, DART-HRMS was used to analyze
a diverse set of monolayers having different chemistries (amides,
esters, amines, acids, alcohols, alkanes, ethers, thioethers, polymers,
sugars) on five different substrates (Si, Si<sub>3</sub>N<sub>4</sub>, glass, Al<sub>2</sub>O<sub>3</sub>, Au). The substrate did not
play a major role except in the case of gold, for which breaking of
the weak Au–S bond that tethers the monolayer to the surface,
was observed. For monolayers with stronger covalent interfacial bonds,
fragmentation around terminal groups was found. For ester and amide-terminated
monolayers, in situ hydrolysis during DART resulted in the detection
of ions characteristic of the terminal groups (alcohol, amine, carboxylic
acid). For ether and thioether-terminated layers, scission of C–O
or C–S bonds also led to the release of the terminal part of
the monolayer in a predictable manner. Only the spectra of alkane
monolayers could not be interpreted. DART-HRMS allowed for the analysis
of and distinction between monolayers containing biologically relevant
mono or disaccharides. Overall, DART-HRMS is a promising surface analysis
technique that combines detailed structural information on nanomaterials
and ultrathin films with fast analyses under ambient conditions