IONIC AND EXCITED INTERMEDIATES IN PULSE-IRRADIATED POLYPROPYLENE DOPED WITH AROMATICS

A pulse radiolysis study of isotactic polypropylene (PP) film has been carried out with the main aims of investigating charge trapping in an undoped system and solute radical ion generation in an pyrene (Py) do ped matrix. In PP, pulse radiolysis gives electron-positive hole pairs . The electron can be stabilized in the undoped system as a trapped el ectron, e(t)(-). The transient absorption spectrum of e(t)(-) in the n ear-IR (up to 1800 nm) was observed in the temperature range 30-100 K. This IR absorption was not detected in the case of oxidized PP. In su ch a matrix electrons can be scavenged by oxidation products generatin g respective radical-anions (absorption in the UV range, lambda < 350 nm). In a doped matrix transient absorption bands centered at 450 and 500 nm were observed which can be assigned to the Py radical cation an d anion, respectively. The recombination of these ionic species leads to monomer excited-state formation observed during and after the 17 ns pulse. Contrary to the Py-doped polyethylene no excimer emission was detected at room temperature even if Py content in PP was close to 0.0 2 mol dm(-3). The rate of Py radical-ion decay was found to be tempera ture dependent. Two linear parts of the Arrhenius plot were observed w hich intersected at ca. 240 K, the glass transition temperature, T-g, for PP. The activation energies calculated for two parts of Arrhenius plot were equal to 111 and ca. 0.78 kJ mol(-1) for T > T-g and T < T-g , respectively. Some preliminary results concerning the ionic processe s in PP containing two solutes (Py, 3,3'-dimethyldiphenyl) were presen ted. The mechanism of ionic recombination in PP will be proposed and d iscussed. (C) 1998 John Wiley & Sons, Inc.