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Chemical strategies to design textured materials: from microporous and mesoporous oxides to nanonetworks and hierarchical structures.

Supramolecular chemistry has a very large impact on chemistry of current interest and the use of non-covalent but directional forces is appealing for the construction of 'supramolecular architectures'. The invention of scanning probe microscopy techniques has opened new doorways to study these concepts on surfaces. This review deals with recent progress in the study of two-dimensional supramolecular self-assembly on surfaces probed by scanning tunneling microscopy, with a special emphasis on structure, dynamics and reactivity of hydrogen bonded systems.

Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy

A lubricating oil composition formulated with a viscosity index (VI) improver composition including a combination of an ethylene α-olefin copolymer having no greater than 66 mass % of units derived from ethylene, and a linear diblock copolymer including at least one block derived primarily from a vinyl aromatic hydrocarbon monomer, and at least one block derived primarily from diene monomer.

Lubricating oil compositions.

In light of the considerable impact synthetic 2D polymers are expected to have on many fundamental and applied aspects of the natural and engineering sciences, it is surprising that little research has been carried out on these intriguing macromolecules. Although numerous approaches have been reported over the last several decades, the synthesis of a one monomer unit thick, covalently bonded molecular sheet with a long-range ordered (periodic) internal structure has yet to be achieved. This Review provides an overview of these approaches and an analysis of how to synthesize 2D polymers. This analysis compares polymerizations in (initially) a homogeneous phase with those at interfaces and considers structural aspects of monomers as well as possibly preferred connection modes. It also addresses issues such as shrinkage as well as domain and crack formation, and briefly touches upon how the chances for a successful structural analysis of the final product can possibly be increased.

Two‐Dimensional Polymers: Just a Dream of Synthetic Chemists?

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

Self-Organized Porphyrinic Materials

Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.

https://science.sciencemag.org/content/sci/348/6230/102.full.pdf

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

We report on a technique that simultaneously quantifies the contact stiffness and dissipation of an AFM cantilever in contact with a surface, which can ultimately be used for quantitative nanomechanical characterization of surfaces The method is based on measuring the contact resonance frequency using dual AC resonance tracking (DART), where the amplitude and phase of the cantilever response are monitored at two frequencies on either side of the contact resonance By modelling the tip?sample contact as a driven damped harmonic oscillator, the four measured quantities (two amplitudes and two phases) allow the four model parameters, namely, drive amplitude, drive phase, resonance frequency and quality factor, to be calculated These mechanical parameters can in turn be used to make quantitative statements about localized sample properties We apply the method to study the electromechanical coupling coefficients in ferroelectric materials and the storage and loss moduli in viscoelastic materials

https://iopscience.iop.org/article/10.1088/0957-4484/22/35/355705/pdf

Mapping nanoscale elasticity and dissipation using dual frequency contact resonance AFM.

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene–polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time–temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

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Viscoelastic Property Mapping with Contact Resonance Force Microscopy

Tessellation of nine free-base porphyrins into a 3 × 3 array is accomplished by the self-assembly of 21 molecular entities of four different kinds, one central, four corner, and four side porphyrins with 12 trans Pd(II) complexes, by specifically designed and targeted intermolecular interactions. Strikingly, the self-assembly of 30 components into a metalloporphyrin nonamer results from the addition of nine equivalents of a first-row transition metal to the above milieu. In this case each porphyrin in the nonameric array coordinates the same metal such as Mn(II), Ni(II), Co(II), or Zn(II). This feat is accomplished by taking advantage of the highly selective porphyrin complexation kinetics and thermodynamics for different metals. In a second, hierarchical self-assembly process, nonspecific intermolecular interactions can be exploited to form nanoscaled three-dimensional aggregates of the supramolecular porphyrin arrays. In solution, the size of the nanoscaled aggregate can be directed by fine-tuning the properties of the component macrocycles, by choice of metalloporphyrin, and the kinetics of the secondary self-assembly process. As precursors to device formation, nanoscale structures of the porphyrin arrays and aggregates of controlled size may be deposited on surfaces. Atomic force microscopy and scanning tunneling microscopy of these materials show that the choice of surface (gold, mica, glass, etc.) may be used to modulate the aggregate size and thus its photophysical properties. Once on the surface the materials are extremely robust.

Dalia G. Yablon

Papers

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

Mapping nanoscale elasticity and dissipation using dual frequency contact resonance AFM.

Viscoelastic Property Mapping with Contact Resonance Force Microscopy

Designing supramolecular porphyrin arrays that self-organize into nanoscale optical and magnetic materials.

Controlled Hierarchical Self-Assembly and Deposition of Nanoscale Photonic Materials