One-dimensional optical reflectors based on self-organization of polymeric comb-shaped supramolecules

We demonstrate that complexation of dodecylbenzenesulphonic acid, DBSA, to a diblock copolymer of polystyrene- block-poly(4-vinylpyridine), PS- block-P4VP, leads to polymeric supramolecules PS- block-P4VP(DBSA)y (y = 1.0, 1.5, and 2.0), which self-organize with a particularly large lamellar periodicity in excess of 1000 A. The structures consist of alternating PS and P4VP(DBSA)y layers, where the latter contains smaller internal structure, probably lamellar. The DBSA side chains are bonded to the pyridines by protonation and hydrogen bonding and they effectively plasticize the material. In this way relatively well-developed structures are obtained even without annealing or macroscopic alignment. Transmission and reflectance measurements show that a relatively narrow and incomplete bandgap exists for supramolecules of high molecular weight block copolymer at ca. 460 nm.     

AC field induced two-dimensional aggregation of multilamellar vesicles

The aggregation of 2D colloidal crystals can be performed by applying an AC field to a colloidal dispersion. This technique is used in this work for assembling multilamellar vesicles in suspension. The dynamics of the aggregation is followed by real-time recording of the pictures of the microsphere assembly through a phase contrast microscope. The influence of both the frequency and the amplitude of the alternating field on the dynamical evolution of the concentration of layered particles is discussed with respect to their size. A phenomenological model of double layer induced trapping of the particles is proposed and an electroconvective instability of the fluid surrounding the particles is suggested from the observation of the local dynamics of the particles, in accordance with a very recent argument of Yeh et al. [#!Yeh:97!#]. Received: 4 December 1997 / Revised: 24 March 1998 / Accepted: 4 May 1998     

A Monte Carlo ray tracing study of polarized light propagation in liquid foams

A Monte Carlo ray tracing scheme is used to investigate the propagation of an incident collimated beam of polarized light in liquid foams. Cellular structures like foam are expected to change the polarization characteristics due to multiple scattering events, where such changes can be used to monitor foam dynamics. A statistical model utilizing some of the recent developments in foam physics is coupled with a vector Monte Carlo scheme to compute the depolarization ratios via Stokes-Mueller formalism. For the simulations, the incident Stokes vector corresponding to horizontal linear polarization and right circular polarization are considered. It is observed that bubble size and the polydispersity parameter have a significant effect on the depolarization ratios. This is partially owing to the number of total internal reflection events in the Plateau borders. The results are discussed in terms of applicability of polarized light as a diagnostic tool for monitoring foams.     

A Spiderweb‐Like Metal–Organic Framework Multifunctional Foam

Processing metal–organic frameworks (MOFs) into hierarchical macroscopic materials can greatly extend their practical applications. However, current strategies suffer from severe aggregation of MOFs and limited tuning of the hierarchical porous network. Now, a strategy is presented that can simultaneously tune the MOF loading, composition, spatial distribution, and confinement within various bio‐originated macroscopic supports, as well as control the accessibility, robustness, and formability of the support itself. This method enables the good dispersion of individual MOF nanoparticles on a spiderweb‐like network within each macrovoid even at high loadings (up to 86 wt %), ensuring the foam pores are highly accessible for excellent adsorption and catalytic capacity. Additionally, this approach allows the direct pre‐incorporation of other functional components into the framework. This strategy provides precise control over the properties of both the hierarchical support and MOF.     

Elastic characterization of nanoporous gold foams using laser based ultrasonics

A resonance based laser ultrasonics technique is explored for the characterization of low density nanoporous gold foams. Laser generated zero group velocity (ZGV) lamb waves are measured in the foams using a Michelson interferometer. The amplitude spectra obtained from the processed time-domain data are analyzed using a theoretical model from which the foam Young’s modulus and Poisson’s ratio are obtained. The technique is non-contact and nondestructive, and the ZGV resonance modes are spatially localized, allowing for spatial mapping of the bulk sample properties. The technique may be suitable for process control monitoring and mechanical characterization of low density nanoporous structures.     

封装有相变材料的金属泡沫复合散热器实验研究

相变材料的固液相变具有较高的相变潜热且相变体积变化小,在间歇性工作的电子器件的温控中得到广泛的应用。本文采用将铜泡沫嵌入相变材料中的方法来强化固液相变的传热性能的方法,提出一种封装有金属泡沫和相变材料的复合式散热器结构,实验研究了该散热器的加热表面的温度与时间的变化关系,分析铜泡沫孔隙率、孔密度以及石蜡物性等各个参数对该复合式热沉散热效果的影响。     

LIGNIN IN PHENOLIC CLOSED CELL FOAMS: THERMAL STABILITY AND APPARENT DENSITY

ABSTRACT

Lignin, extracted from sugarcane bagasse, was used as a partial phenol substitute in phenolic closed cell foams. The thermal stability of phenolic and lignophenolic foams were studied using thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques, under air and nitrogen atmospheres. The results of apparent densities (Dapp) are also reported for both foams. The thermal analyses data showed that the decomposition depends on the atmosphere, that is, this process is not only a thermal one and, that it is feasible to replace part of the phenol by lignin in closed cell foams. Regarding the apparent density, that replacement was extremely advantageous because the Dapp value obtained placed the lignophenolic foam in the structural foam class.     

Polymer Foams Stabilized by Particles Adsorbed at the Air/Polymer Interface

In aqueous systems, partially hydrophobic particles are known to stabilize foams even in the absence of any added surfactant. This paper shows that the same principle can be applied to polymeric systems: particles that are partially wetted by a polymer melt can stabilize a foam of that polymer. The foam stability is attributable to the adsorption of the particles at the air/polymer interface. Remarkably, stable foams are realized even from polymers that are liquid at room temperature, and hence are otherwise unfoamable. The implications of this result to practical foaming operations are discussed.

     

A foam film propagating in a confined geometry: Analysis via the viscous froth model

A single film (typical of a film in a foam) moving in a confined geometry (i.e. confined between closely spaced top and bottom plates) is analysed via the viscous froth model. In the first instance the film is considered to be straight (as viewed from above the top plate) but is not flat. Instead it is curved (with a circular arc cross-section) in the direction across the confining plates. This curvature leads to a maximal possible steady propagation velocity for the film, which is characterised by the curved film meeting the top and bottom plates tangentially. Next the film is considered to propagate in a channel (i.e. between top and bottom plates and sidewalls, with the sidewall separation exceeding that of the top and bottom plates). The film is now curved along as well as across the top and bottom plates. Curvature along the plates arises from viscous drag forces on the channel sidewall boundaries. The maximum steady propagation velocity is unchanged, but can now also be associated with films meeting channel sidewalls tangentially, a situation which should be readily observable if the film is viewed from above the top plate. Observed from above, however, the film need not appear as an arc of a circle. Instead the film may be relatively straight along much of its length, with curvature pushed into boundary layers at the sidewalls.