Evaluation of the probing profile of scanning force microscopy tips

It is demonstrated that a high-temperature-treated (305) surface of a SrTiO3 crystal can be used to evaluate the probing profile of AFM tips routinely, to provide a means of selecting perfect tips and to evaluate possible image distortions. This is important in order to recognize typical AFM artifacts which are caused by tips with truncated or twinned peaks which occur rather often in the case of microfabricated AFM needles. By means of selected needles, it is shown that also defective tips can give apparently rather perfect atomic resolution from flat crystal surfaces. Scope and limitations of the resolution of structural defects are discussed as the criterion for real atomic resolution

A scanning force microscopy study on the morphology of elastomer-coagent blends

Atomic force scanning microscopy (AFM) was used to investigate the dispersion of low molecular weight compounds in ethylene-propylene copolymers (EPM). Where other microscopical techniques failed to provide morphological details of this type of blend, as a result of the restricted resolution (light microscopy) or the volatility of the low molecular weight component (SEM), the AFM technique provided surface images, which show inclusions in the matrix of the uncrosslinked polymers

Atomic force microscopy of gel-drawn ultrahigh-molecular-weight polyethylene

Gel-drawn ultrahigh molecular weight polyethylene was studied by atomic force microscopy (AFM). Three-dimensional surface profiles were recorded for tapes drawn to different extents. AFM images allowed the discrimination of different well-defined levels of the fibrillar morphology: (i) bundles of microfibrils with a diameter between 4 and 7 μm strongly depending on the elongation; (ii) microfibrils with a diameter between 0.2 and 1.2 nm which also decreased with increasing draw ratio; (iii) nanofibrils which form the elementary fibrillar building blocks; and (iv) regular chain patterns on the molecular level which correspond to the crystalline packing of polyethylene chains at the surface of the nanofibrils. The nanofibrils were formed during the initial conversion of lamellae to fibrillar crystallites and did not change considerably in diameter up to draw ratios of λ = 70

Calibration and evaluation of scanning-force-microscopy probes

It is demonstrated that a stepped (305) surface of a SrTiO3 crystal can be used routinely to evaluate the probing profile of scanning-force-microscopy probes. This provides a means to select optimal surface probes, and to evaluate possible image distortions within the range of the atomic and nanometer scale. The scope and limitations of the resolution of structural defects are discussed as a criterion for a true atomic resolution

Programming and Reprogramming the Viscoelasticity and Magnetic Response of Magnetoactive Thermoplastic Elastomers

We present a novel type of magnetorheological material that allows one to restructure the magnetic particles inside the finished composite, tuning in situ the viscoelasticity and magnetic response of the material in a wide range using temperature and an applied magnetic field. The polymer medium is an A-g-B bottlebrush graft copolymer with side chains of two types: polydimethylsiloxane and polystyrene. At room temperature, the brush-like architecture provides the tissue mimetic softness and strain stiffening of the elastomeric matrix, which is formed through the aggregation of polystyrene side chains into aggregates that play the role of physical cross-links. The aggregates partially dissociate and the matrix softens at elevated temperatures, allowing for the effective rearrangement of magnetic particles by applying a magnetic field in the desired direction. Magnetoactive thermoplastic elastomers (MATEs) based on A-g-B bottlebrush graft copolymers with different amounts of aggregating side chains filled with different amounts of carbonyl iron microparticles were prepared. The in situ restructuring of magnetic particles in MATEs was shown to significantly alter their viscoelasticity and magnetic response. In particular, the induced anisotropy led to an order-of-magnitude enhancement of the magnetorheological properties of the composites

Consommation de substances psychoactives chez les sujets suicidants (prévalence, impact pronostique et méthodes d'identification)

BORDEAUX2-BU Santé (330632101) / SudocSudocFranceF

Reversible morphological transitions of polystyrene-b-polyisoprene micelles

Reversible morphological transitions of diblock copolymer micelles in dilute solutions were monitored by light scattering and atomic force microscopy for two different polymer samples near the calculated morphological boundaries. These transitions were induced solely by temperature changes. At 25 °C, a sample of polystyrene-b-polyisoprene diblock micelles with a polystyrene block of 20.6 kDa and a polyisoprene block of 6 kDa was observed to form cylindrical micelles in heptane, a selective solvent for polyisoprene. Upon heating to 35 °C, the sample adopted a spherical micelle morphology. When the sample was cooled back to 25 °C, cylindrical micelles were once again observed. In addition, a reversible transition from vesicles to cylindrical micelles, upon heating from 25 to 40 °C, was observed for a second diblock sample with the same polystyrene block (20.6 kDa) and a shorter polyisoprene block of 4.3 kDa. The change in morphology upon heating was found to be much faster then the reverse process upon cooling. © 2006 American Chemical Society

Orthogonal self-assembly in folding block copolymers

We herein report the synthesis and characterization of ABA triblock copolymers that contain two complementary association motifs and fold into single-chain polymeric nanoparticles (SCPNs) via orthogonal self-assembly. The copolymers were prepared using atom-transfer radical polymerization (ATRP) and possess different pendant functional groups in the A and B blocks (alcohols in the A block and acetylenes in the B block). After postfunctionalization, the A block contains o-nitrobenzyl-protected 2-ureidopyrimidinone (UPy) moieties and the B block benzene-1,3,5-tricarboxamide (BTA) moieties. While the protected UPy groups dimerize after photoinduced deprotection of the o-nitrobenzyl group, the BTA moieties self-assemble into helical aggregates when temperature is reduced. In a two-step thermal/photoirradiation treatment under dilute conditions, the ABA block copolymer forms both BTA-based helical aggregates and UPy dimers intramolecularly. The sequential association of the two self-assembling motifs results in single-chain folding of the polymer, affording nanometer-sized particles with a compartmentalized interior. Variable-temperature NMR studies showed that the BTA and UPy self-assembly steps take place orthogonally (i.e., without mutual interference) in dilute solution. In addition, monitoring of the intramolecular self-assembly of BTA moieties into helical aggregates by circular dichroism spectroscopy showed that the stability of the aggregates is almost independent of UPy dimerization. Size-exclusion chromatography (SEC) and small-angle X-ray scattering analysis provided evidence of significant reductions in the hydrodynamic volume and radius of gyration, respectively, after photoinduced deprotection of the UPy groups; a 30–60% reduction in the size of the polymer chains was observed using SEC in CHCl3. Molecular imaging by atomic force microscopy (AFM) corroborated significant contraction of individual polymer chains due to intramolecular association of the BTA and UPy groups. The stepwise folding process resulting from orthogonal self-assembly-induced supramolecular interactions yields compartmentalized SCPNs comprised of distinct microdomains that mimick two secondary-structuring elements in proteins

Structure and dynamics of self-assembled poly(ethylene glycol) based coiled-coil nano-objects

Herein we describe the structure and dynamics of self-assembled nano-objects generated from poly(ethylene glycol) based (PEG-ylated) coiled-coil hybrid block copolymers. Electron paramagnetic resonance (EPR) experiments on spin-labeled samples provided a strong indication for a parallel alignment of the peptide helices in at least the dimeric coiled-coil nano-object and indicated that the PEG chains are folded rather closely around the peptide core of the nano-objects. The EPR results were supported by AFM studies, which revealed the presence of discrete nanosized objects in thin, spin cast films of the block copolymers on mica substrates. Since their size and structure may be engineered via directed mutations in the amino acid sequence, these nano-objects may be interesting building blocks for the development of supramolecular materials with various potential applications