Structural changes in photopolymers based on the acrylate oligomers studied by positron annihilation lifetime and X-ray diffraction methods

Abstract

This letter presents the results of studies of photopolymers based on acrylate oligomers. Their polymerization occurs under the in uence of ultraviolet (UV) exposure. The measurements were made on photopolymer specimens with different UV exposure times. In the investigated photopolymers there are free volumes, which trap positronium before its decay. Ortho-positronium lifetimes and the free volume radius were calculated as a result of positron lifetime measurements. From the X-ray diffraction (XRD) intensity data the radial distribution function and reduced radial distribution function were calculated. The nearest neighbour distance and the average coordination number for investigated samples were determined from these functions. The processes of physical ageing, fatigue of polymers, and formation and decay of free radicals are closely connected with changes of free volumes. The in uence of UV exposure time on free volume size and on the nearest neighbour distances was investigated. In recent years, there has been increasing interest in photopolymers based on acrylate oligomers due to the possibility of applications in different branches, such as optoelectronics, holography and polygraphical material engineering [1]. Three photocompositional mixtures were chosen for the current investigations. The compositions of the mixtures are presented in Table I, where i-BEB is the photo-initiator. The chemical formulae of the represented chemical substances are shown in Fig. 1. In the case of non-conductive molecular materials such as polymers, besides the annihilation of the positron, the formation and annihilation of positronium (Ps) also take place. Positronium is a bound state of e‡ and ey with an atomic radius comparable to that of the hydrogen atom. Ps exists in two spin states. One is called para-positronium (p-Ps), in which positron and electron spins are antiparallel, and this