Memory materials

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

Actually, some fluids and solids have both elastic (solid) properties and viscous (fluid) properties. These are said to be viscoelastic and are most notably materials composed of high polymers. The complete description of the rheological properties of these materials may involve a function relating the stress and strain as well as derivatives or integrals of these with respect to time. Because the elastic properties of these materials (both fluids and solids) impart memory to the material (as described previously), which results in a tendency to recover to a preferred state upon the removal of the force (stress), they are often termed memory materials and exhibit time-dependent properties. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

A second type of behavior existing in the PLZT s is the linear (Pockels) effect which is generally found in high coercive field, tetragonal materials (composition 3), This effect is so named because of the linear relationship between An and electric field. The truly linear, nonhysteretic character of this effect has been found to be intrinsic to the material and not due to domain reorientation processes which occur in the quadratic and memory materials. The linear materials possess permanent remanent polarization however, in this case the material is switched to its saturation remanence, and it remains in that state. Optical information is extracted from the ceramic by the action of an electric field which causes linear changes in the birefringence, but in no case is there polarization reversal in the material. 

The photodimer (Scheme 27) of acridizinium ethylhexanesulfonate has been used as a non-destructively read-reversible optical memory material to control the intensity and wavelength of emission from a laser (72MI21000). 

Otsuka. K. and C.M. Wayman Shape Memory Materials, Cambridge University Press, New York. NY, 1998. 

The purple membrane is harvested semiindustrially from halobacteria mutants which are bred in fermenters. The BR is then embedded into a polymeric matrix of poly(vinyl alcohol) or polyacrylamide. The BR films manufactured in this way are used for different applications, preferably in holography, for example, as a reversible transient data storage system for optical information processing (159). Another example is real-time interferometry by using the property of BR films to integrate over time (160). BR has been proposed also as a two-photon memory material because of its unusually large two-photon cross section. 

Shape-memory materials are those materials that return to a specific shape after being exposed to specific temperatures. In other words, these materials are able to remember their initial shape. This process of changing the shape of the material can be repeated several times. The shape-memory effect has been observed in different materials, such as metallic alloys, ceramics, glasses, polymers and gels. 

Stress-strain curve for a shape memory material. The lower curve is for deformation when the material is entirely martensitic. The deformation occurs by movement of variant boundaries. After all of the material is of one variant, the stress rises rapidly. The upper curve is for the material above its Af temperature. Adapted from a sketch by D. Grummon. 

The above analysis takes the synthesis methods, the performance affected by the dispersion of CNTs, enhanced physical properties and the latest applications of carbon nanotube/polyurethane composites described in literature reports as the reference point. In the interest of brevity, this is not a comprehensive review, however, it goes through numerous research reports and applications which have been learned and described in the recent years. Despite that, there are still many opportunities to synthesize new carbon nano-tube/polyurethane systems and to modify carbon nanotubes with new functional groups. The possibility of producing modern biomedical and shape memory materials in that way makes the challenge of the near future. 

C. T. Liu, H. Kunsmann, K. Otsuka, andM. Wuttigeds, Shape Memory Materials and Phenomena-Frmdamental Aspects and Applications , MRS Symposium Proceedings, Vol. 246, Materials Research Society, Pittsburgh, PA, 1992. 

K. Otsuka and C. M. Wayman, eds, Shape Memory Materials , Cambridge University Press, Cambridge, 1999. 

Cuilum, B. M., Mobley, J., Bogard, J. S., Moskovitch, M., Phillips, G. W., and Vo-Dihn, T. Three-dimensional optical random access memory materials for use as radiation dosimeters. And/. Chem. 2000, 72, 5612-5617. 

Chromium(lV) oxide utilized in the manufacture of magnetic memory materials. 

Temperature-sensitive materials shape-memory materials Temperature-sensitive gels... 

James R. D. and Hane K. R, Martensitic Transformations and Shape Memory Materials, Acta Mater., 4S, 197 (2000). 

Shell-cross-linked micelles have also been aligned and patterned on a flat silicon substrate by a microfluidic technique (see Figure 3.8). Ordered magnetic nanoceramic arrays derived from a pyrolysis process may be of interest as magnetic memory materials or as catalysts for the growth of carbon nanotubes.49... 

R. Kainuma, H. Nakano, K. Oikawa, K. Ishida, T. Nishizawa High Temperature Shape Memory Alloys of Ni-Al Base Systems. In C.T. Liu, M. Wuttig, K. Otsuka et al. Shape-Memory Materials and Phenomena - Fundamental Aspects and Applications. MRS, Pittsburgh (1992) 403-408. 

The idea of teaching materials to remember their past or their modified physical state so that they can get back to it when an external stimuli is applied is fascinating and one that has manifested itself in shape memory materials. 

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