Fluorescence Lifetime Distributions of Labeled Amorphous Polymers in Bulk

Abstract

ABSTRACT:Polystyrene, poly(methylmethacrylate), poly(cyclohexyl methacrylate) and poly(1-butylmethacrylate) were labeled with anthryl groups by copolymerization. Films of labeled polymers as well as blends of them with unlabeled polystyrene samples of different polydispersity and molecular weight, were prepared by solvent casting. The time resolved emission of anthryl groups in those films was measured by Single Photon Counting with front face excitation at the standard 30• incident angle and with much lower incident angles to photoselect chromophores on the polymer-air surface. Fluorescence decays were fitted with bimodal fluorescence lifetime distributions, which were analyzed taking into consideration some polymer characteristics and the compatibility of polymer blends. It was thus concluded that solid-like and liquid-like environments are both in the bulk and in the surface of the film although liquid-like domains are more frequent on the surface than in bulk. Polymer T g determines the position of the two modes and polydispersity justify the broadness of the modes in the fluorescence lifetime distribution. KEY WORDS Fluorescence Lifetime Distribution / Fluorescence Decay / Poly(1-butylmethacrylate) / Poly(methylmethacrylate) / Poly(cyclohexylmethacrylate) / Polystyrene / Steady state or time-resolved fluorescence measurements of chromophores added to polymers as probes or labels can yield valuable information on the structural or dynamic properties of the system. 1 Anthryl groups have been broadly employed as polymer labels due to its well known photophysical behavior as well as to its relatively small size and high fluorescence quantum yield. 2, 3 Nevertheless, it was found 4 that the chain mobility of polystyrene (PS) is slightly reduced by the steric hindrance of the anthryl group with phenyl rings and therefore, minimum proportions of labeling groups must be introduced in the polymer to preserve unmodified the macroscopic polymer characteristics. Fortunately, due to the very high sensitivity of this technique, only very low chromophore concentrations are needed, which does not disturb the system properties. But even such low concentrations still report on its environment and on processes occurring in the nanosecond time range.It was shown 2 for anthracene dissolved in PS that rigidity and dimension of the matrix free volume are related with the vibronic structure of the fluorescence spectrum. The non-radiative deactivation processes, which involve out of plane vibrational modes and intermolecular interactions, are inhibited in polymer matrices. 2 As a consequence of the probe-polymer matrix interaction the fluorescence intensity changes with temperature following and revealing polymer relaxation processes. 5 Matrix effects on the radiationless deactivation of substituted anthracenes were also observed in films of poly(methylmethacrylate) (PMMA) and they were attributed to competing intersystem crossing and very fast internal conversion. 6 For 9-methylanthracene, 7 the radiationless deactivation rate coe...

Photoluminescence properties of polystyrene-hosted fluorophore thin films

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