Light-absorbing polymer INDEX

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

Perfectly transparent, light-fast color filters and UV absorbers can be obtained by combining metal clusters of coin metals (silver, gold, etc.) with optical polymers [i.e., amorphous polymers with a visible refractive index close to 1.5, such as polystyrene, poly(methyl methacrylate), or polycarbonate]. The high extinction coefficients that characterize the surface plasmon absorption of these metals allows intensive coloration at very low filling factors, and the nanoscopic filler size makes possible the realization of ultrathin color filters [Carotenuto, 2001 Zheng et al., 2001]. 

The equipment widely used for the detection of carbohydrates in the HPLC method is the differential refractive index (RI) detector. The principle involved in this detection depends on the continuous measurement of the variation of the RIs of the mobile phase containing the samples with little or no chromophores such as carbohydrates, lipids, and other polymer compounds that do not absorb UV light. RI detection method presents high degree of reproducibility and is very convenient for the analysis of polysaccharides. However, other detectors such as evaporative light scattering detector and pulsed amperometric detector have been used for the detection of polysaccharides [100]. 

Transparency, in general drops with crystallinity (e.g. polyethylene), and with an increase of crystallite size which causes light scattering. Most fillers, colorants and auxiliary additives lead to opacity. Transmittance depends on the refractive index, so that some fillers may preserve full or partial transparency (translucent). There are also dyes that dissolve in the polymer, so that a colored transparent polymer is produced. It is also possible to find stabilizers (including antioxidants or UV absorbers) that do not affect the polymer transparency. Any chemical change in the polymer like degradation or oxidation, or diffusion of some components, may reduce light transmission. 

In order to increase the contrast in light microscopy, staining with selective dye molecules that absorb light or have a dilferent refractive index is often used. This process is not widely used in the synthetic polymer field, but there are three specific instances that are worth noting ... 

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