Free Volume Determination of Azobenzene−PMMA Copolymer by a Pulsed Low-Energy Positron Lifetime Beam with in-Situ UV Illumination

Abstract

The free volume properties of a poly(methyl methacrylate)−azobenzene copolymer were for the first time directly probed by use of a low-energy positron lifetime beam with in-situ excitation capabilities, showing that the free volume cavity size was not appreciably influenced by photoisomerization and thermal isomerization in the temperature range 34−180 °C. Isomerization is therefore suggested to occur without any molecular rearrangement of the glassy polymer matrix, which would also account for the lack of any shift to shorter wavelengths for the photoisomerization occurring in a glassy polymer in comparison to a chloroform solution. A decrease in the thermal isomerization rate at room temperature caused by the glassy polymer is explained in terms of a model in which only a fraction of the azobenzene is free to isomerize. The cis-azobenzene was found to be an efficient inhibitor of positronium formation, which enabled measurements of thermal isomerization rates and changes in the steady-state concentration of cis-azobenzene for an illuminated sample as a function of temperature.

Free volume change of elongated polyethylene films studied using a positron probe microanalyzer

“…1-3 Positron annihilation lifetime spectroscopy (PALS) has been used in a variety of polymer researches because it can be used to quantify nanometer-sized free volume and pores not only in bulk polymers 4-6 but also in thin films such as reverse osmosis membranes, ion exchange membranes, low k interlayer dielectrics for semiconductors. [7][8][9][10][11][12] However, only a few researches have been performed on the free volume in deformed polymers using PALS. 13,14 The typical positron lifetime spectrum of a polymer contains three exponentially decaying components, which are due to the intrinsic annihilation of the spin-antiparallel electron-positron bound state, para-positronium (p-Ps), non-Ps positron annihilation, and spin-parallel ortho-positronium (o-Ps) annihilation.…”

Gamma irradiation effects on polymethyl methacrylate

“…Many studies using positron annihilation and other material characterization techniques have been performed [5,6]. The free volume properties of a poly(methyl methacrylate)-azobenzene copolymer were directly probed by use of a low-energy positron lifetime beam [7]. Rubiolo and co-workers assessed the effect of gamma irradiation on the structural state and the corresponding inelastic deformation behavior of polymethyl methacrylate (PMMA) [8].…”

Azobenzene Polymers for Photonic Applications

“…This photomechanical response presumably occurs because of the shortening of the polymer chains upon trans-cis conversion. However, photoexpansion would seem to be contradicted by positron lifetime experiments that suggest no change in microscopic free volume cavity size during irradiation (Algers et al, 2004). More conclusive experiments are in order.…”