/pdf/chemical-strategies-to-design-textured-materials-from-cb2un3cizv.pdf

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

Engineering applications of synthetic polymers are widespread due to their availability, processability, low density, and diversity of mechanical properties (Figure 1a). Despite their ubiquitous nature, modern polymers are evolving into multifunctional systems with highly sophisticated behavior. These emergent functions are commonly described as “smart” characteristics whereby “intelligence” is rooted in a specific response elicited from a particular stimulus. Materials that exhibit stimuli-responsive functions thus achieve a desired output (O, the response) upon being subjected to a specific input (I, the stimulus). Given that mechanical loading is inevitable, coupled with the wide range of mechanical properties for synthetic polymers, it is not surprising that mechanoresponsive polymers are an especially attractive class of smart materials. To design materials with stimuli-responsive functions, it is helpful to consider the I-O relationship as an energy transduction process. Achieving the desired I-O linkage thus becomes a problem in finding how to transform energy from the stimulus into energy that executes the desired response. The underlying mechanism that forms this I-O coupling need not be a direct, one-step transduction event; rather, the overall process may proceed through a sequence of energy transduction steps. In this regard, the network of energy transduction pathways is a useful roadmap for designing stimuli-responsive materials (Figure 1b). It is the purpose of this review to broadly survey the mechanical to chemical * To whom correspondence should be addressed. Phone: 217-244-4024. Fax: 217-244-8024. E-mail: jsmoore@illinois.edu. † Department of Chemistry and Beckman Institute. ‡ Department of Materials Science and Engineering and Beckman Institute. § Department of Aerospace Engineering and Beckman Institute. Chem. Rev. XXXX, xxx, 000–000 A

Mechanically-induced chemical changes in polymeric materials

The advantages in using nanoscale materials for electrochemical energy storage are generally attributed to short diffusion path lengths for both electronic and lithium ion transport. Here, we consider another contribution, namely the charge storage from faradaic processes occurring at the surface, referred to as pseudocapacitive effect. This paper describes the synthesis and pseudocapacitive characteristics of block copolymer templated anatase TiO2 thin films synthesized using either sol−gel reagents or preformed nanocrystals as building blocks. Both materials are highly crystalline and have large surface areas; however, the structure of the porosity is not identical. The different titania systems are characterized by a combination of small- and wide-angle X-ray diffraction/scattering, combined with SEM imaging and physisorption measurements. Following our previously reported approach, we are able to use the voltammetric sweep rate dependence to determine quantitatively the capacitive contribution to the ...

Templated Nanocrystal-Based Porous TiO2 Films for Next-Generation Electrochemical Capacitors

In this paper, we report the complete synthesis and characterization procedures to generate highly organized and oriented mesoporous titania thin films, using poly(ethylene oxide) (PEO)-based templates. Controlled conditions in the deposition, postsynthesis, and thermal treatment steps allow one to tailor the final mesostructure (2D hexagonal, p6m, or 3D cubic, Im3m). Various techniques were used to determine the time evolution of the mesostructure. Spectroscopic techniques (UV/vis, (17)O NMR) and EXAFS/XANES have been used to follow the chemical changes in the Ti(IV) environment. Crossing these techniques spanning all ranges permits a complete description of the chemistry all the way from solution to the mesostructured metal oxide. A critical discussion on all important chemical and processing parameters is provided; the understanding of these features is essential for a rational design and the reproducible construction of mesoporous materials.

Controlled formation of highly organized mesoporous titania thin films: from mesostructured hybrids to mesoporous nanoanatase TiO2.

TOPICAL REVIEW: Supermagnetism

A simple sol−gel route has been developed for the preparation of mesoporous and nanocrystalline anatase thin layers. An anatase hydrosol was first synthesized at room temperature from acidic hydrolysis of titanium isopropoxide. The optimization of the synthesis parameters, including titanium concentration, HCl/Ti and H2O/Ti ratios, temperature, and aging time, enabled us to produce a clear sol with a very low HCl/Ti ratio (equal to 1). As a function of the further thermal treatment conditions, it was then possible to control the size of the anatase crystallites (from 5 to >10 nm) and the O/Ti stoichiometry (from 1.9 to 2.0). Ordered mesoporosity was obtained by using a triblock copolymer as the templating agent. Preliminary experiments evidenced the photocatalytic activity of the prepared layers.

A Simple Route for Low-Temperature Synthesis of Mesoporous and Nanocrystalline Anatase Thin Films

A previous paper described the low-temperature synthesis of mesoporous nanocrystalline anatase thin layers obtained by mesophase templating (Chem. Mater. 2003, 15, 2463). This work deals with the structural characterization of these anatase thin layers and with the extension of the synthesis method to films with a different mesostructure. Thus, two triblock copolymers are used as structuring agents and the effect of the volume fraction of surfactant in the dried layers is also considered. The determination of the mesostructures is mainly based on 2D X-ray diffraction experiments, completed with observations by transmission electron microscopy and atomic force microscopy. Nitrogen adsorption−desorption measurements are performed to investigate the porous texture of the thin layers.

Mesostructure of Anatase Thin Films Prepared by Mesophase Templating

It has been shown that by annealing three dimensional (3D) disordered and fcc supracrystal assemblies of 7.5 nm Co nanocrystals, a structural transition of the nanocrystals from a poorly crystallized fcc structure to a pure hcp phase occurs. The annealing process does not result in either oxidation or coalescence of the nanocrystals. Not only is the system highly stable, but also the thermal treatment actually improves the mesoscopic structural order in the fcc supracrystals. In the native state, an effect of the mesoscopic order on the magnetic properties of the 3D assemblies is observed which we attribute to the difference in distribution of dipolar interaction energies. In this paper, we extend this study to the same systems after annealing in order to investigate the effect of increased anisotropy and crystallinity on the magnetic behavior. We find a significant increase in both blocking temperature (to a near room-temperature value) and saturation magnetization. Surprisingly, the effects of mesoscopi...

Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals: I, Structural Investigation

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by 2H-NMR measurements. Various materials have been swollen with small amount ( λE, at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to λA, whereas the crystallinity increases linearly with the macroscopic draw ratio λ. The hardening σ ∼ (λ - λA)2 is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.

Chain orientation in natural rubber, Part II: 2H-NMR study

We report quantitative experimental study correlating the structure and mechanical properties of fibers made from single-walled carbon nanotubes (SWNTs) and polyvinyl alcohol (PVA). A post-synthesis solvent drawing treatment is used to vary nanotube alignment, whose detailed understanding is a prerequisite for fiber development. Quantitative analysis of nanotube alignment within the fibers with different draw ratios is performed using x-ray scattering. The method is described in detail, and we also show that the improvement of nanotube alignment with draw ratio can be understood within a model of induced orientation at constant volume. Young's modulus and tensile strength increase with nanotube alignment. This is modeled using continuum mechanics in qualitative agreement with experiment, however quantitative differences show that nanotube alignment is not the only parameter controlling the fiber mechanical properties. We suggest that interaction between the SWNTs and PVA chains should also play a significant role.

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Pierre-Antoine Albouy

Papers

A Simple Route for Low-Temperature Synthesis of Mesoporous and Nanocrystalline Anatase Thin Films

Mesostructure of Anatase Thin Films Prepared by Mesophase Templating

Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals: I, Structural Investigation

Chain orientation in natural rubber, Part II: 2H-NMR study

Structural and mechanical properties of single-wall carbon nanotube fibers