Changwoon Nah

Three-dimensional actuators transformed from the programmed two-dimensional structures via bending, twisting and folding mechanisms

Combining the physical principle of actuators with the basic concept of photonic crystals, colour-tunable three-dimensional (3D) photonic actuators were successfully fabricated. By controlling the d-spacings and the refractive index contrasts of the self-assembled 3D colloidal photonic crystals, colours of the photonic actuators were tuned. Various shapes of these 3D actuating objects were constructed by transforming the programmed 2D structures via bending, twisting and folding mechanisms. These 2D structures were first programmed by breaking the symmetry. The selective swellings were then applied as driving forces to control the shapes and colours of the photonic actuators. Scroll photonic actuators had been first demonstrated by bending the traditional 2D cantilever structure (K.-U. Jeong, et al., J.Mater.Chem., 2009, 19, 1956). By breaking the symmetry of a cantilever structure perpendicular to its long axis, polypeptide-/DNA-like 3D helical photonic actuators were obtained from the programmed 2D structure via twisting processes. Both left- and right-handed scrolls and helices with various colours can be achieved by changing the polarity of solvents. Different types of 3D actuators, such as cube, pyramid and phlat ball, were also demonstrated via the folding mechanism. The reversible 3D photonic actuators transformed from the programmed 2D structures via the bending, twisting and folding mechanisms may be applied in the field of mechanical actuators, and optoelectronic and bio-mimetic devices.

Preparing PP/clay nanocomposites using a swelling agent

Abstract Polypropylene (PP)/clay nanocomposites (PPCN) were prepared using a swollen organoclay, which had a larger interlayer spacing than pristine organoclay. The organoclay was first treated with a swelling agent (maleic anhydride, MA) and a co-swelling agent in solution. Then, it was melt blended with PP-g-MA to generate a pre-intercalated composite (PIC). Finally, the PIC was blended with PP to obtain a PPCN, which contained small amounts of PP-g-MA. The materials were characterized using X-ray diffraction (XRD), Scanning electron microscopy, Transmission electron microscopy (TEM), and TGA. The XRD graphs showed that the basal spacing of the pristine clay treated with MA was 1.6 nm, which was larger than that of the original clay, but smaller than that of the organoclay. The XRD graphs of the organoclay treated with MA showed double peaks at 3 and 5.5°. As the ratio of MA to the organoclay increased, the peak at 5.5° decreased gradually. TEM micrographs indicated that the clay layers in the pre-intercalated blends were still stacked in an orderly manner. However, partial exfoliation of the clay layers was observed in the PPCN. The nanocomposites prepared with the aid of swelling agents contained some PP-g-MA. Good dispersion of the clay layers gave the PPCN greater thermal stability and an enhanced storage modulus, which indicated a reinforcing effect of the clay in the PP matrix. The increased Tg (derived from Dynamic mechanical analysis) of PPCN implied that the PP macromolecules were intercalated between interlayers of the silicate.

Effects of trans-Polyoctylene Rubber (TOR) on the Properties of NR/EPDM Blends

trans-Polyoctylene rubber (TOR) was melt blended with an incompatible NR/EPDM (70/30) blend. Mixing torque and temperature were reduced as TOR was added to NR/EPDM blend. The curing characteristics of the blend were affected as TOR participated in vulcanization and became a part of network. A scanning electron micrograph demonstrated that addition of TOR improved the compatibility of the blend and thereby led to a finer phase morphology. The ozone resistance of the blends was determined in terms of a critical stress-strain parameter. The critical stored energy density for ozone cracking was significantly enhanced for the TOR containing rubber blend. It was believed that the improvement in ozone resistance arised from finely dispersed ozone-resistant EPDM particles in the blend. TOR caused an improvement in dynamic properties and an increase in tensile modulus, but a decrease in tensile stress and elongation at break of the rubber blend.

Colour-tunable spiral photonic actuators

Combining the multi-faceted environmental responsiveness of polymers with photonically active structures, we developed spiral photonic actuators which can reversibly change both shape and colour in response to the chemical environment.

Some Physical Characteristics of Double-Networked Natural Rubber

ABSTRACT: A double-networked natural rubber (DNNR) was prepared by a ‘‘two-stepcrosslinking’’ method, in which the crosslinking was achieved while the natural rubberwas in a stretched condition. The swelling behavior, tensile properties, creep, recovery,and permanent set were investigated. Generally, the observed mechanical propertiesof DNNR, such as the Young’s modulus, tensile strength, toughness, creep, recovery,and permanent set, were considerably improved as the residual extension was in-creased. They were, however, rather inferior to those of a single-networked naturalrubber and showed a minimum at a lower residual extension of about 1.55. The degreeof crosslinking and elongation at break were not greatly affected by the residual exten-sion. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 917–924, 1997 Key words: natural rubber; double networks; swelling; tensile property; creep andrecovery INTRODUCTION having different strain conditions. Based on thetheory, the degree of permanent set can be de-When a rubbery material is crosslinked in a state scribed quantitatively by using only two factors:of macroscopically zero strain, i.e., in the typical (1) the relative ratio of the number of the twomanner of the rubber industries,

Effects of Blend Composition and Mixing Method on Mechanical and Morphological Properties of Zinc Dimethacrylate-Reinforced Acrylonitrile-Butadiene Copolymer Nanocomposites

Abstract Two kinds of rubber composites were prepared by melt mixing of acrylonitrile-butadiene rubber (NBR) with zinc dimethacrylate (ZMA). In one system ZMA was added in the form of powder; and, in another system, the ZMA was prepared by in-situ reaction of ZnO and methacrylic acid (MA) during melt mixing. Both systems showed similar tensile strength, however the elongation at break, tear strength, and fatigue resistance were higher for the in-situ prepared system. The composite prepared by direct addition of ZMA powder showed a corresponding peak for ZMA particles in the wide angle x-ray diffraction (WAXD) spectra, indicating the existence of two phases, while there was no such peak for the in-situ prepared blend. The comparison of cross-link densities of these two blends showed a higher proportion of total cross-link density (calculated from covalent and ionic cross-linking) for the blend prepared from the direct addition method. The morphology of torn surfaces based on field-emission scanning electron microscopy (FE-SEM), showed a different nature of fracture behavior in these blends. The mean particle size and distribution of ZMA in NBR matrix as measured by atomic force microscopy (AFM) and small angle x-ray scattering (SAXS) indicated the formation of nanostructured domains of ZMA of particle size in the range of 10–40 nm with more uniform distribution in the in-situ blends.

Carbon nanotube-reinforced elastomeric nanocomposites: a review

This review is focused on carbon nanotube (CNT)-elastomeric polymer nanocomposites, which have attracted industrial and academic interest over the years due to their enhanced properties. Major factors notably CNT type, surface modification, dispersion of CNT, and processing techniques that affect the physical properties of CNT-elastomeric polymer nanocomposites are reviewed, and several key physical properties, including tensile, electrical, and thermal properties, were also included in this review. Some of the key challenges that undermine the effectiveness of CNTs and their composites with elastomeric polymers, and the potential applications of CNT-elastomeric composites are also captured.

Liquid crystal micro-lenticular array assembled by a fringing field

Based on dielectrophoretic effect, the phase separation morphology of liquid crystal (LC) in a liquid monomer can be manipulated by a fringing field. Applying the fringing field generated from interdigitated electrode upon the LC/monomer mixture, the randomly dispersed LC droplets can be assembled to grating-like stripes. When the field is removed, the LC stripes break into tiny droplets again. This process is reversible and stable. Our results show that the surface profile of each LC stripe exhibits a lenticular shape. The response time of the LC morphology converting from droplets to stripes and from stripes to droplets is ~0.96 s. Potential applications of this controllable morphology can be found in optical communications, beam steering, imaging, and displays.

Free-Standing and Circular-Polarizing Chirophotonic Crystal Reflectors: Photopolymerization of Helical Nanostructures

The preparation of materials exhibiting structural colors has been intensively studied in biomimetic science and technology. Utilizing a newly synthesized cholesteric liquid-crystal (CLC) monomer (abbreviated as BP1CRM), we have prepared CLC films. Photoinitiated copolymerization of this monomer with a common achiral liquid-crystalline monomer produced free-standing films with homogeneous and nanoscale pitch distributions. Employing the thermal sensitivity of the CLC monomer, chirophotonic crystal reflectors were prepared exhibiting a range of colors. The free-standing and circular-polarizing chirophotonic crystal films maintain excellent thermal, mechanical, and chemical stabilities, and the composition can readily be applied as polarized optical films and smart paints.

A macroscopically oriented lyotropic chromonic liquid crystalline nanofiber mat embedding self-assembled Sunset-Yellow FCF nanocolumns

A macroscopically oriented anisotropic nanofiber mat embedding self-assembled Sunset-Yellow FCF nanocolumns was fabricated by the electrospinning technique. From the 2D WAXD and polarized FTIR results, it was realized that the nanocolumns were aligned parallel to the long axis of the nanofiber.