12 research outputs found
Convenient Quantification of Accessible Surface-Attached ATRP Initiators and RAFT Chain Transfer Agents on Cross-Linked Polystyrene Nanoparticles
The exact quantification of surface-attached initiators
for grafting-from
radical polymerization on micro- and nanoparticles is still a challenging
task. Here we demonstrate that, by combining UV/vis spectroscopy and
an efficient derivatization protocol, the originally nearly invisible
ATRP initiators and RAFT chain transfer agent entities can be quantified
easily and precisely. Application of an isorefractive dispersion medium
for the nanoparticles moreover provides reliable information about
the amount of chemically really âaccessibleâ surface-grafted
initiators, that is, those that are not hidden in the particleâs
shell interior. To qualify the developed procedure further, nanoparticles
of different grafting densities were generated, and the values determined
afterward for the initiator concentration were in good agreement with
expectations
Pressure Induced Structure Formation in Langmuir Monolayers of Amphiphilic Metallocene Diblock Copolymers
We present in situ structural investigations
of a metal-containing
diblock copolymer on a water surface. Monolayers of polyÂ(vinylferrocene-<i>b</i>-(2-vinylpyridine)) (PVFc-<i>b</i>-P2VP) block
copolymers are studied in a wide range of compositions by varying
molar masses of P2VP with two different molecular weights of PVFc.
We focus on the role of the respective block partners, PVFc and P2VP,
when compressing the layer on the water surface. Compression isotherms
are presented and interpreted in terms of the classical gaseous, expanded,
and condensed phases. We calculate isothermal compressibilities, which
reveal a minimum value independent of the molar masses of the respective
block partners. We find the isotherms to be dominated by P2VP while
PVFc barely contribute to the compression behavior due to its rather
compact coil structure. We consider the diblock copolymer monolayers
as a two-dimensional model system, which is reflected by two-dimensional
scaling behavior in the semi dilute and condensed regime. By X-ray
reflectometry (XR), we monitor in situ the monolayer structure change
with increasing surface pressure Î and observe the PVFc-<i>b</i>-P2VP separation at high Î
Blue-Greenish Electroluminescent Poly(<i>p</i>âphenylenevinylene) Developed for Organic Light-Emitting Diode Applications
A novel electroluminescent polyÂ(<i>p</i>-phenylenevinylene)
(PPV) derivative was synthesized via the Gilch route, which emits
in the blue-greenish region. The required monomer synthesis is a multistep
process starting from catechol and does not involve any critical step.
The polymer synthesis itself proceeds via standard Gilch conditions
and results in constitutionally homogeneous and extraordinary high-molecular-weight
PPVs. The characterization of these materials was carried out using
nuclear magnetic resonance spectroscopy and size exclusion chromatography
measurements. The highest occupied molecular orbital and lowest unoccupied
molecular orbital energy levels were estimated by combining information
provided by cyclic voltammetry and UVâvis measurements. Finally,
the electroluminescent behavior of the polymer was confirmed in an
organic light-emitting diode
Water-Soluble Poly(vinylferrocene)-<i>b</i>-Poly(ethylene oxide) Diblock and Miktoarm Star Polymers
We describe the synthesis of water-soluble diblock and
miktoarm
star polymers consisting of polyÂ(vinylferrocene) (PVFc) and polyÂ(ethylene
oxide) (PEO) blocks. First, end-functionalized polyÂ(vinylferrocene)
was generated by end-capping the living carbanionic PVFc chains with
benzyl glycidyl ether (BGE) or ethoxy ethyl glycidyl ether (EEGE).
Acidic hydrolysis of the EEGE-terminated PVFc partially oxidized the
PVFc backbone. However, the dihydroxyl end-functional PVFc was obtained
in quantitative yields by hydrogenolysis of the BGE-terminated PVFc.
A series of block copolymers and AB<sub>2</sub> miktoarm star copolymers
was obtained in a second polymerization step, utilizing the respective
end-functionalized PVFc as a macroinitiator for the ring-opening polymerization
(ROP) of ethylene oxide. All polymers were analyzed in detail, using
NMR spectroscopy and size-exclusion chromatography (SEC). Online SEC-viscosimetry
as well as MALLS was carried out, confirming the formation of miktoarm
structures. Quantitative functionalization and subsequent removal
of the acetal and benzyl protective groups, respectively were confirmed
by MALDIâToF mass spectrometry. Molecular weights of the end-functionalized
PVFcs range between 1000 and 3600 g mol<sup>â1</sup>, and block
copolymers with 10â000 to 50â000 g mol<sup>â1</sup> overall molar masses were synthesized. In addition, the water-soluble
block copolymers were investigated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). For characterization of
the morphology in aqueous solution, transmission electron microscopy
(TEM) was performed, showing micelles and multicompartment micellar
structures
Thermo-cross-linked Elastomeric Opal Films
An efficient and convenient thermal
cross-linking protocol in elastomeric opal films leading to fully
reversible and stretch-tunable optical materials is reported. In this
study, functional monodisperse coreâshell particles were arranged
in a face-centered cubic (fcc) lattice structure by a melt flow process.
A problem up to now was that un-cross-linked films could not be drawn
fully reversibly and hence lost their optical and mechanical performance.
After thermal cross-linking reaction, the obtained films can be drawn
like rubbers and the color of their Bragg reflection changes because
of controlled lattice deformation, which makes the cross-linked films
mechanochromic sensors. Different techniques were developed for the
cross-linking of the films a posteriori, after their preparation in
the melt flow process. A photo-cross-linking approach was reported
earlier. This study now deals with a very efficient thermo-cross-linking
approach based on the chemistry of hydroxyl- and isocyanate-functionalities
that form urethane bridges. The focus of the present work is the mechanism
and efficiency of this cross-linking process for elastomeric opal
films with excellent mechanical and optical properties
Redox-Responsive Polymer Brushes Grafted from Polystyrene Nanoparticles by Means of Surface Initiated Atom Transfer Radical Polymerization
Well-defined polyÂ(2-(methacryloyloxy)Âethyl ferrocenecarboxylate)
(PFcMA) brushes were synthesized by surface-initiated atom transfer
radical polymerization (SI-ATRP) on cross-linked polystyrene particles.
The ATRP of FcMA monomer was reinvestigated leading to molar masses
up to 130 kg mol<sup>â1</sup> with a good reaction control
and high monomer conversion (91%). The SI-ATRP was done with different
amounts of initiator in the PS particle shell leading to PFcMA surface
conformations from âmushroom-likeâ to dense âbrush-likeâ
polymers, which could be confirmed by dynamic light scattering (DLS)
experiments. Redox-responsive behavior of the PFcMA shell was investigated
by DLS and cyclic voltammetry (CV) measurements indicating a tremendous
increase in the hydrodynamic volume of the ferrocene-containing shell.
The transformation of PFcMA-grafted PS particles to magnetic iron
oxides after thermal treatment could be investigated by SQUID magnetization
measurements showing the typical hysteresis for ferromagnetic material
Ferrocene Polymers for Switchable Surface Wettability
The
changes in surface wettability induced by immobilized polyvinylferrocene
(PVFc) and polyÂ(2-(methacryloyloxy)Âethyl ferrocenecarboxylate) (PFcMA)
on silica wafers were studied after oxidation with two different oxidation
reagents. Surface-attached PFcMA was accessible by applying a surface-initiated
atom transfer radical polymerization (SI-ATRP) protocol, while end-functionalized
PVFc was immobilized by using a grafting onto approach. In the case
of PFcMA, a remarkable contact angle (CA) drop for water of approximately
70° after oxidation could be observed, while the effect for immobilized
PVFc after oxidation was less pronounced (CA drop of approximately
30°). In the case of PFcMA, the effect of chain length was additionally
studied, showing a more significant CA drop for PFcMA chains with
higher molar masses
Stick-Slip Mechanisms at the Nanoscale
<div><p>When two surfaces slide past each other, energy is mainly dissipated by stick-slip events. Macroscopic stick-slip is usually explained by asperities that come in and out of contact. Herein, we probe stick-slip at the nanoscale at interfaces and polymer coated interfaces by pulling single polymers covalently attached to an AFM cantilever tip laterally over solid substrates in liquid environment. We find two different stick mechanisms, namely desorption stick (DS) and cooperative stick (CS). While DS-slip resembles the velocity dependence of macroscopic stick-slip, CS-slip shows an increase in friction with velocity. For various reasons we anticipate that both stick mechanisms are necessary for a molecular understanding of stick-slip at the interface and interphase.
</p></div
Reversible Lightâ, Thermoâ, and Mechano-Responsive Elastomeric Polymer Opal Films
A new strategy to achieve easily
scalable triple stimuli-responsive
elastomeric opal films for applications as stretch-tunable photonic
band gap materials is reported. Novel monodisperse highly functional
core-interlayer-shell beads are obtained by semicontinuous emulsion
polymerization featuring a temperature-sensitive fluorescent rhodamine
dye either locally restricted in the core or the shell of prepared
beads. After extrusion and compression molding, homogeneous elastomeric
opal films with fascinating stretch-tunable and temperature-dependent
fluorescent properties can be obtained. Applying strains of only a
few percent lead to significant blue shift of the reflected colors
making these films excellent candidates for applications as deformation
sensors. Higher strains up to 90% lead to a tremendous Bragg reflection
color change caused by transition from the (111) to the (200) lattice
plane. The well-ordered opaline structure with its stop band at the
emission frequency of the incorporated fluorescent dye shows remarkable
angle-dependent fluorescence suppression. Herein described elastomeric
opal films can be valuable in a wide range of applications such as
rewritable 3D optical data storage, tunable laser action, and sensing
materials
Cohesion Mechanisms of Polystyrene-Based Thin Polymer Films
The cohesion mechanisms of end-functionalized
high molar mass polystyrene
with very low polydispersity (PS, <i>M</i><sub>n</sub> =
1.26 Ă 10<sup>6</sup> g mol<sup>â1</sup>, PDI = 1.06)
and polylysine (PLL, 150â130 Ă 10<sup>3</sup> g mol<sup>â1</sup>) on silicon (Si) supported thin PS films are investigated
by desorbing single polymers covalently bound to an atomic force microscope
(AFM) cantilever tip. The influence of film preparation conditions
and film architecture on polymer cohesion mechanisms is probed by
comparing spin-coated PS films (scPS) with a thickness range of 6â52
nm and covalently surface-attached PS films (saPS) with a thickness
of 15â83 nm. Annealed scPS prevents cohesion of further PS
polymers unless the scPS partly dewets. In all other cases, two different
cohesion mechanisms are observed: first, a previously described equilibrium
desorption similar to hydrophobic solid substrate desorption, represented
by a plateau of constant force in the forceâextension curve,
and second, a nonequilibrium mechanism with nonlinear forceâextension
behavior. The second requires a geometrical interlock between the
tip bound single molecule and the PS film. Remarkably, this mechanism
is observed below the glass transition temperature of PS films and
is promoted by good solvent conditions. These findings contrast many
bulk measurements assuming a glassy state of the complete polymer
film, but they are consistent with fluid like boundary layers having
a high mobility. Our results further underline the decisive influence
of polymer film conformation and mobility close to its solvent exposed
boundary layer for the cohesion of polymer coatings