Polymer matrices, photochromism

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. 

One recent trend has been away from using a photochromic dye itself merely as an individual component of a solution, polymer film or bulk polymer matrix. Instead, the photochromic is chemically linked to a polymer, which may be a natural polymer such as a cellulose derivative, an enzyme, a protein, or synthetic polymers from acrylates, urethanes, and vinyl compounds. The properties of the polymer can then be modified by external irradiation, and conversely, the properties of the photochromic are modified by the polymer. A recent biochemical example is the photocontrolled binding of monosaccharides to concanavalin A (Con A) modified with spiropyran units.208... 

Y. Tateoka, T. Sagawa, and M. Kawasaki, Photochromic materials containing photochromic substance and plasticizer in polymer matrix for quick response, Jap. Pat. Appl. JP 6327,837A to Nissan Motor and Unitika (1988). 

For practical applications, fulgides must retain their excellent photochromic properties in solvent and in a polymer matrix, because the ultimate photomemoiy media will certainly be provided as a plastic layer containing photochromic... 

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

A comparative study of the photochromic transformations for 6-phenoxy-5,12-naphthacenequinone in toluene and polymer matrix showed that the value of the quantum yields was unchanged with a polymer binder in place of toluene. 61 Quantum yields for direct and back photoreactions were equal to cpB= 0.3 and (Pa = 0.005 under UV light and q>A = 0.005 under visible irradiation. 

As can be seen from Table 9.12, photochromism is also strongly affected by humidity. It is well known that thiazine and its derivatives undergo photoreduction smoothly in the presence of such activated surfaces as silica gel and alumina with water molecules,37 and that methylene blue is also photochemically reduced in acid solution, even with only available water as the reducing agent38 Therefore the water present in polymer films must produce a thionine-water hydration system, which accelerates the rate of the photoreduction of thionine, as well as promoting the contact of reductants by a plasticizer effect. PVA, used as the matrix, can also play the role of reductant, but its extent may be minor as compared with the added reductant. The effect of water is supported by the fact that a less hydrophilic polymer matrix such as poly(methyl methacrylate) does not exhibit photochromism even though the system contains an appreciable amount of reductants. 

Anew experimental method based on the polarization-selective photochromic reactions is proposed to monitor extremely slow reorientation dynamics of molecular tracers in glassy polymer matrix. The correlations between the local relaxation processes of polymers and the reorientation dynamics of the tracers with different sizes are found from the experimental results obtained by this method. 

On the other hand, stretching the polymer matrix causes molecular orientation of the photochrome as shown by dichroism measurements it wfll also restrict their isomerization possibilities Thus, stretching of a film of poIy(bisphenol-A-pimelate) (M 32000)... 

Fulgides and related diarylethenes have been investigated extensively because of the long-term thermal stability of their photocyclized colored forms, which could lead to this application in erasable optical recording materials and photoswitchable optical elements. The entrapment of these photochromic molecules in polymer films is necessary for these practical applications. Picosecond laser photolysis was employed to study the electrocyclic reaction of a furylfulgide (18, Figure 8) in polymer solids and revealed that the colored structure was formed with a time constant of ca. lOps irrespective of the nature of the polymer matrix.49... 

H. Kamogawa, T. Masui, and S. Amemiya, Organic solid photochromism by photoieduction mechanism Viologen embedded in solid polar aprotic polymer matrix, J. Polym. Sci., Polym., Chem. 22, 383-390 (1984). 

It is not necessary to carry out synthesis, if the triggering photochromic compound has good affinity to the polymer matrix. A mixture of the polyacrylate with BMAB which exhibits an excellent function as trigger is equally photoresponsive. While the monomer model compound (i.e. the acrylate before polymerization) does not provide a liquid crystalline phase, the polymer shows a clear nematic - isotropic transition at ca. 61 °C and the glass transition temperature at 24 °C as shown in Fig. 4. Tj j depends very much on the length of the alkyl spacer. In comparison with the... 

Polymer types and polymer matrix effects will obviously affect stability of both states. Recently, spiropyran has been attached to polystyrene Wang resin, which serves as a solid support, to minimize thermal back reaction and improve bistability (see Fig. 6.3). The resultant photochromic microbeads suspended in toluene greatly... 

As with all photochromic polymers, the polymer matrix effect is very important in fulgides and fulgimides as the polymer free volume affects rate of photocoloration and photobleaching. For example, Bahajaj and Asiri recently reported that the rate of photochemical reactions of fulgide doped in a variety of polymer matrices decreased in the order polystyrene > poly(methyl methacrylate) > epoxy resin [37]. Polystyrene possesses a larger free volume than poly(methyl methacrylate) and epoxy resin... 

Ramachandran, D., Urban, M.W., 2012. Sensing polymer matrix rearrangments in composites by photochromic crosslinkers. Abstracts of Papers of the American Chemical Society, 242. 

Photochromism is the phenomenon where the absorption speetrum of a molecule or crystal changes reversibly when the sample is irradiated by light of a eertain wavelength. When such a photoisomerization reaction is carried out in a polymer matrix instead of in solution, a strong decrease in the reaction rate (coloration = decoloration) is observed." The poly-... 

The maximum wavelength absorbance was measured for all photochromic pigments. Samples were irradiated with a specific UV wavelength of 325 nm for 30 s, which activated the photochromic compound in the polymer matrix. The material developed a characteristic color upon irradiation. Immediately after irradiation, the samples were scanned from 380 nm to 900 nm at 20°C. The visible spectrum was recorded using a UV/VIS Spectrophotometer Phillips Scientific PU8730. 

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