Irradiated polymers, luminescence

A fraction of the electrons will cause an emission of light. Most electrons contributing to the luminescence of the irradiated polymer are mobilized through an erosion of cavity traps. A fraction, however, is... 

Figure 3. Luminescence of irradiated polymers (AJ, PBD (cis/trans/vinyl = 55/35/11) (B), (poly-butadiene-co-styrene) with 20% styrene (C, D), blend comprising 50 wt % of each of the two elastomers prior to and after annealing in vacuo at 423 K for 26 hr. Electron irradiation at 90 K to a dose of 3.1 lO eV g-K...

Several arylplatinum complexes of Tl+ have been reported (Table 15). The anionic /rreacts with T1N03 to produce a chain polymer 120, in which successive Pt atoms are bridged to each other by two Tl+ ions (Scheme 28).112 The compound is an acetone solvate, and there also appear to be weak Tl F interactions in the solid-state structure. Upon irradiation (A = 441 nm), the complex exhibits an intense emission with a maximum at 678 nm. The luminescence is attributed to MLCT transitions. 

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]. 

The powerful role of the exitonic migration was proved on the basis of the luminescence and photosensitivity investigations [270]. The preliminary ultraviolet illumination of PAC increases the photosensitivity and decreases the luminescence. The experimental data are given in Fig. 40. One can see the redistribution of the maxima intensity in the spectra without changing their positions. Apparently ultra violet illumination promotes the photolysis of the weak coordination bonds. This leads to the changing of the polymer homolog content. Stimulated exciton dissociation on the ruptured bonds results in an increase in the photosensitivity and a luminescence decrease. The experiments carried out at 77 K show that in the luminescence spectrum of irradiated frozen PAC a new maximum appears with a position close to the phosphorescence maximum of diphenylbutadiene. So the rupture of weak coordination bonds under ultraviolet irradiation was proved. 

Ag(I) salts and 1,8-diisocyano-p-menthane (dipm, 118) yield a 1-D polymeric structure [Ag(dipm)2(Y)]tt, Y = PFe, BF4 (119), where every tetracoordinated Ag(I) is double bridged to the two adjacent Ag atoms. All dipm bridges are oriented in the same direction. No direct Ag—Ag bonding exists (distance ca 0.5 nm) with the Ag atoms forming a zig-zag line (Ag—Ag—Ag angle 137.15°). When irradiating the polymer in EtOH solution, at 77 K, a blue luminescence appears (Amax 385 nm)210. 

Systematic studies on the nature/dynamics of the excitation processes in the UV ablation of polymers have been reported by Masuhara and co-work-ers [61-63]. To this end, they have relied on the use of dopants dispersed within polymers. Given the relevance of their approach to the topic of the present article, their work will be presented in some detail. Time-resolved absorption and luminescence spectroscopies have been used to probe the dynamics of electronic excitation and deexcitation of the dopants. In the irradiation of fluorescing dopants (biphenyl or phenanthrene), the total emis-... 

Intermediates in the radiation chemistry of high polymers include ions and trapped electrons, radicals and excited states. Free radicals trapped after irradiation have been studied mainly by electron spin resonance (ESR) and in some cases by chemical methods and by ultraviolet or infrared spectroscopy. The detection of free radicals during radiolysis has been performed by pulse radiolysis and also by ESR. Trapped ions and radical-ions were characterized by absorption spectroscopy and thermoluminescence while pulse radiolysis allows their detection during irradiation. Excited states, owing to their very short lifetime, could be observed only by pulse radiolysis or by the measurement of the luminescence spectrum and decay time during steady irradiation. 

This phenomenon was observed with polymers some years ago [177—183]. The more recent investigations are due to Buben and Nikolskii [183]. These workers measured the emission from many amorphous and crystalline polymers and observed correlations between the temperature corresponding to the glow peak maxima and the structural transition temperatures of the materials. A detailed study of polyethylene thermoluminescence was made by Charlesby and Partridge [184]. The glow curve obtained after irradiation in vacuo possesses three peaks, a, j3 and y, whose luminescence intensities are proportional to the irradiation dose for doses below 5 x 104 rads. The maxima occurs, respectively, at —110, —65 and —27°C, when the total... 

The fluorescence and phosphorescence excitation and emission spectra of commercial polypropylene and poly(4-methylpent-l-ene) are examined using a fully compensated spectrofluorimeter. The excitation spectra of the polymers are compared with the absorption spectra of model chromo-phores of those believed to be present in the polymers. The fluorescence emission is associated primarily with the presence of enone and the phosphorescence is associated with dienone impurity chromophoric units. Bromination of cold hexane extracts of the polymers reduces significantly the intensity of the fluorescence confirming the presence of ethylenic unsaturation. The behavior of the luminescent enone and dienone groups during irradiation under sunlight-simulated conditions is examined also. Possible mechanisms for the participation of these chromophoric units in the photooxidation of the polymers are discussed. 

A solution of the high-cis polymer fluoresces when excited by irradiation at 330 nm. The initial spectrum displays a weak emission at about 370 nm attributed to 2% of tra s-stilbene segments originally present in the chains. With continued irradiation (2 min) there is an increase in the emission from this band as more trans double bonds are generated, and a new emission appears at 445-500 nm. After further irradiation, this intense red-shifted luminescence becomes the predominant feature before the polymer precipitates. In a statistical copolymer of 9% of 71 and 91% of norbomene, the units of 72 are isolated between norbomene units, and the fluorescence spectrum is confined to the shorter wavelength region with a maximum around 360 nm (Miao 1994a). 

Figure 8. Luminescence of SBS block copolymer as weU as of PS and PBD homopolymers similar in microstructure and molecular weight to the component polymers of SBS. Samples were irradiated with x-rays at 90 K to 5 eV g K...

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