Polymers in the recording layer

POLYMERS IN THE RECORDING LAYER 9.3.1 Photochemical and thermal efficiencies... 

First, the recording layer which contains photochromic spirobenzothiopyran in liquid-crystal polymer or polymer such as vinyl chloride-vinylidene chloride copolymer, is made colored by UV irradiation. In the recording (writing) process, a colorless recording dot in the recording layer is formed by semiconductor laser beams (789 nm, 15-20 mW).100 This process is essentially thermal decoloration of the photomerocyanine form by laser beam. 

A number of these dyes were applied, mixed with a polymer for the control of the aggregation state, to CD-R and DVD-R recording systems. The aggregation state in the recording layer was controlled by choosing the set of axial substituents (Scheme 8 R1, R2, and R3). The interaction between the phthalocyanine dyes and the polymers was dependent on the length of the axial substituents or, more carbon atoms in the alkyl group were found to be necessary for the axial substituents to mix with the polymers.218... 

The simplest recording medium is a bilayer structure. It is constructed by first evaporating a highly reflective aluminum layer onto a suitable disk substrate. Next, a thin film (15-50 nm thick) of a metal, such as tellurium, is vacuum deposited on top of the aluminum layer. The laser power required to form the mark is dependent on the thermal characteristics of the metal film. Tellurium, for example, has a low thermal diffusivity and a melting point of 452 °C which make it an attractive recording material. The thermal diffusivity of the substrate material should also be as low as possible, since a significant fraction of the heat generated in the metal layer can be conducted to the substrate. For this reason, low cost polymer substrates such as poly (methylmethacrylate) or poly (vinyl chloride) are ideal. 

In addition, the surface should be free of contaminating particles and occlusions that would interfere with the information retrieval process. A typical ablative-mode optical disk has the structure shown in Figme 16.14. The substrate is an optically transparent material such as polycarbonate, PMMA, polyfethylene terephthalate), or polyfvinyl chloride) topped by a subbing layer to provide an optically smooth surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as poly(a-methyl styrene) and, finally, the absorbing layer where the information pits are created is added. The latter can be a metal-polymer composite (silver particles in a gel) or a dye molecule dispersed in a polymer matrix such as squaryllium dyes, which act as infrared absorbers for GaAs lasers, typically... 

Polymers play three different roles in optical media as the recording layer, as the protective coating on this recording layer, and as the transparent substrate. Since the last role is by far the most important, the emphasis in this review will be placed on substrate materials. In particular the results of recent work done at our laboratories on the problem of birefringence in optical disc substrates and how this is related to processing and to polymer chain structure will be discussed. 

In the previous section it was shown that detailed information about electrochemical processes and the kinetics of follow-up chemical reaction steps can be investigated by UV/Vis/NIR spectroelectrochemical experiments in transmission mode in the diffusion layer at optically transparent or microstructured non-transparent electrodes. Many metal electrodes show a high reflectivity and therefore optical spectra may also be recorded under in situ conditions in reflection mode [70, 71]. This approach is essential for the study of adsorbed species, the formation of solid layers at the electrode surface, reactions of solids [72-74], and the redox behaviour of conducting polymer layers [75-77]. Furthermore, in reflection mode, the angle of incidence may be modified and polarised light maybe used in ellipsometry studies [78]. 

Fig. 2.3.4 Film formation of a photoinitiated the lower surface (left) after a 90 min induction cross-linking latex coating as measured by period due to oxygen absorption. The profiles CARField. (a) The coating is exposed to air shown were recorded 10, 90, 100 and 110 min (evaporation) and light from above, (b) A sam- and 2, 3, 4, 5, 6 and 17 h after casting the layer, pie comprising a combination of only polymer (d)The full formulation film forms in the central and water dries from the upper surface (right) layers first. In this final time series, the profiles as shown by a time series of profiles, recorded shown were recorded after 10 min (dotted at 10, 20, 30, 40, 50, 60, 70, 100 and 120 min trace, T) attenuated) and then, from the top after casting the layer, (c) A combination of down, 30, 60 and 90 min and 2, 3, 6 and 17 h polymer and photoinitiator only cures from after casting the layer.

Fig. 2.3.6 The two dimensional Pe-xD space, which characterizes the drying of semi-crystal-line polymer solutions, in this case PVOH. In the lower right quadrant (Pe = 0.18, xD = 5.8), the drying is uniform but, because it is slow (the profiles shown were recorded at approximately 12 h intervals), a crystalline layer forms that traps residual water. In the lower left...

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