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 “accessible” surface-grafted initiators, that is, those that are not hidden in the particle’s 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 poly­(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 Π and observe the PVFc-<i>b</i>-P2VP separation at high Π

Blue-Greenish Electroluminescent Poly(<i>p</i>‑phenylenevinylene) Developed for Organic Light-Emitting Diode Applications

A novel electroluminescent poly­(<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 UV–vis 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 poly­(vinylferrocene) (PVFc) and poly­(ethylene oxide) (PEO) blocks. First, end-functionalized poly­(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 AB₂ 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 MALDI–ToF mass spectrometry. Molecular weights of the end-functionalized PVFcs range between 1000 and 3600 g mol^–1, and block copolymers with 10 000 to 50 000 g mol^–1 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 core–shell 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 poly­(2-(methacryloyloxy)­ethyl 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^–1 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 “mushroom-like” to dense “brush-like” 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 poly­(2-(methacryloyloxy)­ethyl 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 Light‑, Thermo‑, 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>_n = 1.26 × 10⁶ g mol^–1, PDI = 1.06) and polylysine (PLL, 150–130 × 10³ g mol^–1) 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 6–52 nm and covalently surface-attached PS films (saPS) with a thickness of 15–83 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 force–extension curve, and second, a nonequilibrium mechanism with nonlinear force–extension 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