Investigation of Molecular Chain Orientation Change of Polymer Crystals in Phase Transitions by Friction Anisotropy Measurement

Direct observation of the molecular orientation change in polymer crystals provides us visible information for understanding their structural phase-transition mechanisms. In this letter, we successfully identified the main-chain orientation of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) crystals over all directions using friction anisotropy measured by lateral-modulation friction force microscopy (LM-FFM). This technique made possible our investigation of molecular orientation changes caused by a ferroelectric phase transition and also a fabrication process for artificial nanometer-scale structures. These results give us visual information that is directly connected to the transition mechanisms

Electrospray Deposition, Model, and Experiment: Toward General Control of Film Morphology

Poly(vinylidene fluoride) film formation with electrospray deposition has been studied with support of a droplet evaporation model. The input parameters of the model consist basically of the solvent, the solute concentration, the flow rate, and the solution conductivity. The model provides the droplet size, the solute concentration, the droplet velocity, and the shear stress of the droplet at impact as a function of the distance between the nozzle and the substrate. With some additional experimental information such as the size change of the film with spray distance and the viscosity of the solution, the growth rate of the film and the shear rate of the droplet at impact can be determined. Growth rate is shown to define distinct regimes of film formation. In those regimes, only a single factor or a limited number of factors controls the film morphology. The most important factors include the shear rate and the surface energy. It is found that at a specific range of growth rates only the shear rate determines the morphology of the polymer film. Growth rate, as the defining quantity of film morphology, is not limited to polymer solutions. Therefore, the growth rate, in combination with the control factors mentioned above, functions as a general framework through which understanding and control of film formation with electrospray deposition can be improved

Structured Water Molecules on Membrane Proteins Resolved by Atomic Force Microscopy

Water structuring on the outer surface of protein molecules called the hydration shell is essential as well as the internal water structures for higher-order structuring of protein molecules and their biological activities in vivo. We now show the molecular-scale hydration structure measurements of native purple membrane patches composed of proton pump proteins by a noninvasive three-dimensional force mapping technique based on frequency modulation atomic force microscopy. We successfully resolved the ordered water molecules localized near the proton uptake channels on the cytoplasmic side of the individual bacteriorhodopsin proteins in the purple membrane. We demonstrate that the three-dimensional force mapping can be widely applicable for molecular-scale investigations of the solid–liquid interfaces of various soft nanomaterials

Alkyl and Alkoxyl Monolayers Directly Attached to Silicon: Chemical Durability in Aqueous Solutions

For practical application of self-assembled monolayers (SAMs), knowledge of their chemical durability in acidic or basic solutions is important. In the present work, a series of SAMs directly immobilized on a silicon (111) surface through Si−C or Si−O−C covalent bonds without a native oxide layer were prepared by thermally activated chemical reactions of a hydrogen-terminated Si(111) substrate with linear molecules, i.e., 1-hexadecene, 1-hexadecanol, 1-dodecanol, and n-dodecanal, to investigate the durability of the SAMs to HF and Na2CO3 solutions. While grazing incidence X-ray reflectivity measurements showed that all the as-prepared SAMs had almost the same film density and molecular coverage, keeping the original step and terrace structure of Si(111) as is observed by atomic force microscopy, they gave different degradation behaviors, i.e., pitting and concomitant surface roughening in both solutions. 1-Hexadecene SAM was stable against immersion in both solutions, while the other SAMs were damaged within 60 min, most likely due to the difference in chemical bonding modes at the SAM/Si interface, i.e., Si−C and Si−O−C

Beyond the Helix Pitch: Direct Visualization of Native DNA in Aqueous Solution

The DNA double helix was first elucidated by J.D. Watson and F.H.C. Crick over a half century ago. However, no one could actually “see” the well-known structure ever. Among all real-space observation methods, only atomic force microscopy (AFM) enables us to visualize the biologically active structure of natural DNA in water. However, conventional AFM measurements often caused the structural deformation of DNA because of the strong interaction forces acting on DNA. Moreover, large contact area between the AFM probe and DNA hindered us from imaging sub-molecular-scale features smaller than helical periodicity of DNA. Here, we show the direct observation of native plasmid DNA in water using an ultra-low-noise AFM with the highly sensitive force detection method (frequency modulation AFM: FM-AFM). Our micrographs of DNA vividly exhibited not only overall structure of the B-form double helix in water but also local structures which deviate from the crystallographic structures of DNA without any damage. Moreover, the interaction force area in the FM-AFM was small enough to clearly discern individual functional groups within DNA. The technique was also applied to explore the synthesized DNA nanostructures toward the current nanobiotechnology. This work will be essential for considering the structure–function relationship of biomolecular systems <i>in vivo</i> and for <i>in situ</i> analysis of DNA-based nanodevices

Thermal Conversion of Precursor Polymer to Low Bandgap Conjugated Polymer Containing Isothianaphthene Dimer Subunits

Thermal conversion strategy has been utilized in the synthesis of a novel low bandgap polymer containing isothianaphthene (ITN) dimer structure and benzodithiophene (BDT) unit in the backbone (PBIBDT). First, a highly soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]octadiene-fused thiophene dimer and BDT (PPBIBDT) was synthesized by a palladium(0)-catalyzed Stille coupling reaction. Then, heating of the yellow PPBIBDT film spin-coated on a glass plate yielded a dark blue film of PBIBDT that was insoluble in any organic solvents. Thermogravimetric analysis of PPBIBDT showed 14% weight loss with an onset at 230 °C, corroborating the occurrence of the thermally induced retro-Diels–Alder reaction. The PBIBDT film showed red-shifted, broad absorption in the visible and near-infrared regions with a maximum at 706 nm compared to the precursor polymer PPBIBDT with an absorption peak at 445 nm. The introduction of an ITN dimer unit in the backbone lowered the bandgap owing to the stabilized quinoid resonance structure. The field-effect hole mobility of PBIBDT was determined to be 1.1 × 10^–4 cm² V^–1 s^–1 with an on–off ratio of 2.5 × 10², while the PPBIBDT-based device revealed no p- and n-type responses. Organic photovoltaic devices were fabricated based on the planar heterojunction structure of PBIBDT and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and showed a power conversion efficiency of 0.07% under standard AM1.5 sunlight (100 mW cm^–2). These results obtained here will provide fundamental information on the design of thermally induced low bandgap polymers for device applications

Donor–Acceptor Alternating Copolymer Based on Thermally Converted Isothianaphthene Dimer and Thiazolothiazole Subunits

A novel donor–acceptor-conjugated polymer PBITT consisting of isothianaphthene (ITN) dimer donor unit and thiazolothiazole acceptor unit was synthesized by thermal conversion method. First, a soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]­octadiene (BCOD)-fused thiophene dimer and benzodithiophene (PPBITT) was synthesized by palladium(0)-catalyzed Stille coupling reaction. The BCOD moiety underwent the retro-Diels–Alder reaction by the thermal treatment of a red PPBITT film to afford a dark blue film of PBITT that was insoluble in any organic solvents. The optical bandgap of PBITT (1.3 eV) became significantly narrow compared with that of PPBITT (2.1 eV) due to the stabilized quinoid resonance structure of the PBITT main chain. The field-effect hole mobility (μ_h) of PBITT was determined to be 2.2 × 10^–4 cm² V^–1 s^–1 with on–off ratio (<i>I</i>_on/<i>I</i>_off) of 2.5 × 10², whereas the corresponding PPBITT-based device did not show any p- and n-type response. Organic photovoltaic (OPV) devices were fabricated based on the bulk heterojunction film of the polymers and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). The device with the PBITT:PCBM film exhibited higher short-circuit current and lower open-circuit voltage than those of the PPBITT:PCBM-based device, resulting in the comparable power conversion efficiency (∼0.3%). These results obtained here will provide fundamental information on the design of thermally induced donor–acceptor alternating polymers for organic electronics

A Photoconductive, Thiophene–Fullerene Double-Cable Polymer, Nanorod Device

Gold/double-cable copolymer/gold multisegmented nanorods were prepared electrochemically via a template-based method. These “bulk heterojunction” nanorods showed photoconductivity providing us with a platform to study photoinduced charge separation/transport at the nanointerface and begin to think about the rational design of nanoscale solar cells based on such structures