Study of carbonaceous clusters in irradiated polycarbonate with UV-vis spectroscopy

Abstract

The formation of carbonaceous clusters in ion-irradiated polymer films was investigated extensively. Information about these clusters may be obtained with ultraviolet–visible (UV–vis) spectroscopy. The optical band gap (Eg), calculated from the absorption edge of the UV spectra of these polymers, can be correlated to the number of carbon atoms (N) in a cluster with the modified Tauc equation. The structure of the cluster is also related to Eg; for example, a six-membered-benzene-ring-type structure has an Eg of ≈5.3 eV, whereas a buckminsterfullerene-type structure has an Eg of ≈4.9 eV. These clusters are responsible for the electrical conductivity in these films. In this work, polycarbonate films (20 μm thick) were irradiated with 45-MeV Li ions at fluences of 1 × 1012 to 1 × 1013 cm−2 and were characterized with UV–vis spectroscopy and impedance measurements. The Eg values, calculated from the absorption edge in the 280–315-nm region with the Tauc relation, varied from 4.39 to 4.35 eV for the pristine and various irradiated samples, respectively. The cluster size showed a range of 60–62 carbon atoms per cluster. The sheet conductivity (σdc) and loss (tan δ) values of 10−16 Ω−1cm−1 and 10−3 for the pristine sample changed to 10−15 Ω−1cm−1 and 10−2, respectively, for the irradiated samples. This increase in the values of σdc and tan δ may be correlated to the increase in the size of the carbonaceous clusters. This study provides insight into the mechanism of electrical conductivity in irradiated polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1589–1594, 2000