Photostability of polymers

The direct and indirect effects of fillers oti the photostability of polymer compositions are well known. With the recent interest in nanopowdered fillers reports of their effectiveness as photostabilizers is beginning to be reported in the literature. For instance ZnO and Ti02 nanoparticles (primary particle size 25-70 nm) were studied in acrylic coatings. A layer that carried at least 5 weight percent of the nanoparticles was needed to shield the underlying layers from UV exposure and hence degradation [162]. Nanoscale titania was also evaluated as a stabilizer in epoxy coatings [163]. 

Bussiere, P.-O., Peyroux, J., Chadeyron, G., Therias, S. Influence of functional nanoparticles on the photostability of polymer materials Recent progress and further applications. Polym. Degrad. Stab. 98, 2411-2418 (2013)... 

Besides carbonyl groups, hydroperoxides formed during the polymer processing play an important role in the increasing polymer sensibility on the UV radiation [20]. Other impurities such some traces of catalyst (titanium compounds) decompose the hydroperoxides and affect the thermal and photostability of polymer over time [21]. 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

As will be shown throughout the chapter, the parent PPV 1 is extensively used as a holetransporting layer (HTL) in combination with other EL polymers. Recently, improved photostability of organic inorganic hybrid EL material, prepared by incorporating PPV 1 into zeolite capsules, was described [54]. However, the material showed only weak EL (at driving voltage of 2.5 V). 

Similarly, fluorescent silver clusters could be prepared in so called molecular hydrogels, formed by polyglycerol-b/oc -poly(acrylic acid) (PG-b-PAA), using a ratio COOH Ag of 2 1 with UV irradiation (365 nm). The emission band centered at 590 nm reached a maximum after 200 min of irradiation. The authors claim improved photostability of the clusters since they are still luminescent even after 9 h of irradiation, but it has to be mentioned that the irradiation source was weak, only 0.5 mW/cm2. They claim that it is the number of arms in the star polymer rather than the length of the arms (thus the density of COOH) that plays a crucial role in the formation of silver clusters [30]. 

Quantum clusters are highly photostable when compared with organic fluorophores. A study was conducted to check the photostability of clusters in comparison to organic fluorophores and semiconductor quantum dots [12]. Photostability of a gold cluster capped with dihydrolipoic acid (AuNC DHLA) was compared with polymer coated CdSe/ZnS semiconductor quantum dots and two different organic fluorophores namely fluorescein and rhodamine 6G (Fig. 5). For the study, 20 pi of fluorescent AuNC DHLA was dissolved in sodium borate buffer of pH 9. The sample was loaded into a quartz cuvette and was exposed to blue-light (480 nm)... 

Fig. 5 Photostability of fluorescent Au nanoclusters (AuNC DHLA) compared with semiconductor quantum dots (polymer-coated QD 520 from Invitrogen) and organic fluorophores (fluorescein, rhodamine 6G) [12]...

Fig. 6. Measurement of photostability of two polymeric electro-optic materials as carried out by researchers at IPITEK (TACAN) Corporation. The data represented by the solid line correspond to the LRD-3 DEC material of Dalton and co-workers [138] while the data represented by open circles correspond to a diaminonitrostilbene chromophore/poly(methyl methacrylate) guest/host material produced by IBM Almaden Laboratories. The dramatic improvement observed for the DEC material can be associated with increased lattice hardness from the chromophore coupling to adjacent polymer chains...

RNA and DNA, and to study the effects of phosphorylation state and charge on the results. " In the polymer field. Berry et al. used BDE calculations for model alkyl urethanes to identify vulnerable sites to radical attack, and to examine the effect of branching on the photostability of urethane coatings. ... 

As shown in Fig. 10.3, the fiuorescence quantum yield, monodispersity and photostability of these polymer-coated quantum dots are strongly dependent on the molar capping ratio (MCR), which is calculated by dividing the sum of basic groups (amine or thiol) on the polymer by the sum of cadmium and tellurium atoms on the quantum dot surface. When the MCR values are below 1.0, the amount of polymer is insufficient to completely coat 2.5-nm... 

Fig. 10.3. Effects of polymer capping ratios on quantum dot properties, (a) Fluorescence quantum yield blue curve) and polydispersity index red curve) of 2.5 nm CdTe quantum dots as a function of molar capping ratio. Polydispersity indices were calculated from gel filtration chromatograms, (b) Photostability data at various capping ratios (MCR = 1.5, 1.0, or 0.5) and in the absence of polymer (MCR = 0)...

It has been demonstrated in this short review that the understanding of the photosensitization mechanism in polymers may be applied for the improvement of their photostability and for the development of polymers with controlled lifetime. 

We have found that different polymeric matrices provide diverse response patterns of polymer-incorporated CdSe nanocrystals to vapors of different polarity. Such work promises to complement existing solvatochromic organic dye sensors with more photostable and reliable sensor materials. The stability of the sensor response pattern from 2.8 and 5.6-nm nanocrystals in PMMA film over more than 10 h of continuous exposure of the film to laser radiation is very attractive for diverse applications where continuous monitoring is needed. 

The photooxidation and photodegradation of polymers continues to attract some interest but is not as widespread as in previous years. Review articles have appeared dealing with poly(2,6-dimethyl-l,4-phenylene oxides) , photocatalyst fibres, polymers with azo links and accelerated weathering specifications. Other articles of interest include the design of an integrating sphere for repeatability in polymer ageing and the use of FTIR for monitoring the photostability of clearcoats . 

The photophysical properties and photostability of oxazine dye molecules for use in luminescent solar concentrators have been reported. Tests have shown that the optical efficiencies of luminescent solar concentrators prepared using a single dye in a liquid methyl methacrylate polymer are as efficient as those fabricated using the same single dye dissolved in methyl methacrylate, and which is then thermally polymerised. ... 

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation . Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer . Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air . These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments... 

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