' INTRODUCTION Materials1À4 that can be manipulated to a "fixed" temporary shape and reverted later to the "memorized" original shape upon exposure to an external stimulus such as heat, 1À3 light, 5À7 magnetic fields, 8,9 or chemicals 10À13 are useful for a plethora of potential applications that range from mechanically adaptive materials for biomedical devices 2,14À18 to aerospace structures 18,19 to dry adhesives 20 to optical devices. 21 Polymers intrinsically show shapememory effects, on the basis of rubber elasticity, but with varied characteristics of strain recovery rate, work capability during recovery, and retracted state stability. The elasticity can be imparted by covalent or physical cross-links. The polymer is typically deformed in its rubbery state above a transition temperature (T trans ), i.e., either glass transition, crystallization, or melting temperature, is cooled to below the transition temperature (T trans ) to fix a temporary shape in which the energy used for deformation is stored. Later, the recovery to its original shape is driven by regaining the entropy lost during deformation. 22 The ability to fix temporary shapes depends mainly on the possibility to create discrete reversible phase transitions in the polymer. 23À26 Thus, a broad range of different shape-memory polymers has been or can be developed, which exploit various reversible phase transitions and are designed to respond to different external stimuli. The most widely studied behavior is a purely thermally induced shape-memory effect, which relies on heating to above and cooling to below a transition temperature; indirect heating upon exposure of appropriately modified systems to an electrical current, magnetic field, or light represent interesting variations of this approach. 27À30 A light-induced shape-memory effect was also demonstrated, which was achieved by introducing photoresponsive cinnamic acid units into acrylate hydrogels. This allows one to trigger the shape-memory effect with light under isothermal conditions. 5 Comparably few materials have been described in which the shape-memory recovery can be triggered by a chemical stimulus. One design approach for such materials relies on the reduction of the glass transition temperature (T g ) of thermoresponsive shape-memory polyurethanes by way of plasticization upon immersion in water; here, the fixation of the temporary shape involved deformation at elevated temperature and cooling below T g . 10,11,31 Recently a purely solvent-induced shape-memory effect was also reported for a chemically cross-linked polyvinyl alcohol. 12,13ABSTRACT: New biomimetic, stimuli-responsive mechanically adaptive nanocomposites, which change their mechanical properties upon exposure to water and display a water-activated shape-memory effect, were investigated. These materials were produced by introducing rigid cotton cellulose nanowhiskers (CNWs) into a rubbery polyurethane (PU) matrix. A series of materials with CNW concentrations of 2À20% v/v was produced by solution blending CNWs an...

The stiffness of 10 nm diameter cellulose nanowhiskers is reported. These whiskers are produced by acid hydrolysis. These whiskers are dispersed in epoxy resin and placed on the surface of a beam of the same material and deformed in tension and compression using a four-point bending device. By following the molecular deformation of the whiskers using Raman spectroscopy it is shown that, by theoretical models of their dispersion and matrix reinforcement, their stiffness can be derived. The effects of debonding, matrix yielding, and buckling of whiskers are also discussed using this method as a means for studying nanocomposite materials.

Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stresstransfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of μm. Polarized Raman spectroscopy of the 1095 cm -1 Raman band, associated with C-O ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers.

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The mechanically induced molecular deformation of cellulose nanowhiskers embedded in subpercolation concentration in an epoxy resin matrix was monitored through Raman spectroscopy. Cellulose nanowhiskers isolated by sulfuric acid hydrolysis from tunicates and by sulfuric acid hydrolysis and hydrochloric acid hydrolysis from cotton were used to study how the aspect ratio (ca. 76 for tunicate and 19 for cotton) and surface charges (38 and 85 mmol SO(4)(-)/kg for sulfuric acid hydrolysis of cotton and tunicate, respectively; no detectable surface charges for hydrochloric acid hydrolysis) originating from the isolation process influence stress transfer in such systems. Atomic force microscopy confirmed that uncharged cellulose nanowhiskers produced by hydrochloric acid hydrolysis have a much higher tendency to aggregate than the charged cotton or tunicate nanowhiskers. Each of these nanowhisker types was incorporated in a concentration of 0.7 vol % in a thermosetting epoxy resin matrix. Mechanically induced shifts of the Raman peak initially located at 1095 cm(-1) were used to express the level of deformation imparted to the nanowhiskers embedded in the resin. Much larger shifts of the diagnostic Raman band were observed for nanocomposites with tunicate nanowhiskers than for the corresponding samples comprising cotton nanowhiskers. In the case of nanocomposites comprising nanowhiskers produced by hydrochloric acid hydrolysis, no significant Raman band shift was observed. These results are indicative of different modes of stress transfer, which in turn appear to originate from the different sample morphologies.

Advanced energy storage technology based on phase change materials (PCMs) has received considerable attention over the last decade for used in various applications. Buildings are the major industry which needs this advanced technology to improve internal building comfort and the reduction of energy usage. However, the main barrier which affects the application of this technology in building sector is the method to incorporate the PCMs into the building materials and the method used to measure the effectiveness of the PCMs as TES in building. In this paper, a review on the TES systems based on PCMs, their thermo-physical and chemical properties, and potential application as TES for buildings have been carried out. The methodologies for the incorporation of PCMs into the building materials, and their thermal performance are discussed.

This work focuses on the preparation of AC (activated carbon) through a physical activation method using peat soil as a precursor, followed by the use of the AC as an inorganic framework for the preparation of SPCM (shape-stabilized phase change material). The SPCM, composed of n-octadecane as the core and AC pores as a framework, was fabricated by a simple impregnation method, with the mass fraction of n-octadecane varying from 10 to 90 wt.%. The AC has a specific surface area of 893 m 2 g −1 and an average pore size of 22 Å. The field emission scanning electron microscope images and nitrogen gas adsorptiondesorption isotherms shows that the n-octadecane was actually encapsulated into the AC pores. The melting and freezing temperatures of the composite PCM (phase change material) were 30.9 °C and 24.1 °C, respectively, and its corresponding latent heat values were 95.4 Jg −1 and 99.6 Jg −1 , respectively. The composite shows a good thermal reliability, even after 1000 melting/freezing cycles. The present research provided a new SPCM material for thermal energy storage as well as some new insights into the design of composite PCM by tailoring the pore structure of AC derived from peat soil, a natural resource

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