Dielectric Characteristics of Polyimides Modified by Additions of C 60 -Fullerene

Transcription

1 Solid State Phenomena Vols (2004) pp nline available since 2004/Jul/31 at (2004) Trans Tech Publications, Switzerland doi: / Dielectric haracteristics of Polyimides Modified by Additions of 60 -Fullerene J. Subocz 1, A. Valozhyn 1, P. Berczyński 2, E. Herko 4 1 Technical University of Szczecin, Institute of Electrical Engineering, Poland, Szczecin, ul. Sikorskiego 37 2 Technical University of Szczecin, Institute of Physics, Poland, Szczecin, ul. Sikorskiego 37 3 Technical University of Szczecin, Institute of Electronics, Telecommunication and Informatics Keywords: optoelectronic, modified polyimides, fullerene, Kapton, Upilex, dielectric characteristics Abstract. The paper presents the results of research into the dielectric characteristics of modified Kapton and Upilex polyimides with 60 fullerene contents of % wt. The influence of temperature and frequency on the time constants of β-relaxation of modified polyimides is shown. Introduction The polyimides (PI) belong to a group of thermally stable polymers possessing good mechanical and chemical properties. The strong chemical bonds in the macro-particles of PI and the chain benzene ring symmetry give polyimide high resistance to all the solvents and the β -, γ - and x - radiation. Polyimides are used as electro-insulating materials in such industries as electrical engineering, electronics, aviation or space engineering, where extremely severe exposure occurs, in particular the high temperatures of up to 400. Moreover, polyimide s good electrical characteristics, i.e. high resistivity ρ v Ωm and high electric strength E p 300 MV/m together with relatively low permitivity ε r 3,5, make the materials potentially applicable for other purposes, e.g. as electret materials [1]. However, if this is to be the case then it is necessary to fully comprehend the processes of space charge conductivity, polarization and accumulation. The analysis of dielectric relaxation and electrical conductivity of Kapton can be found, with others, in [1,2] whist an extensive description of the characteristics of polyimide is given in [3,4]. urrently, numerous research centres focus on polyimide modifications that should lead to an increase in potential areas of applications of polyimide as well as simplification of production technology. This paper presents the effects of certain modifications on the dielectric relaxation process in polyimide film. Test Material and Methods Tests were carried out for five types of polyimide foil samples, which differed either in their molecular structure (P1, P2) or in the amounts of additives used (P5, P7, P8). The basic material used was film with the trade name KAPT (sample P1 Fig.1.a). During the modification process the sample was structurally altered by the use of two methods: - modification of the main chain (sample P2 Fig.1.b), - - physical addition of different amounts of fullerenes 60 (P5-0,1% 60, P7-1% 60, P8-2% 60 ). The differences in the main chain structures of sample P1 and P2 are given in Fig.1. All rights reserved. o part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,23:38:35)

2 158 Functional anomaterials for ptoelectronics and other Applications a) b) n n Fig.1. hain structure of (a) Kapton and (b) Upilex. The measurements of frequency correlations between the dielectric loss factor tan δ and the relative dielectric permitivity ε r were taken in the temperature range 160 K to 380 K and the frequency range 100 Hz 1 MHz using a bridge type RL HP 4284A. The analysis of the resulting data was undertaken using ovocontrol software, i.e. the program RIGI v.6.1 and WinFit v.2.9. The frequency correlations of complex dielectric permitivity ε*(ω) were approximated using Havriliaka-egami s formula in the following form [5]: σ 2 0 ε K ε( ω ) = j + + ε ε0ω K = 1 α βk ( 1+ ( jωτ ) K K ) K (1) where:`σ 0 - D conductivity, - exponent, ε - polarizability, ε 0 - dielectric vacuum permitivity, τ - relaxation time, α and β - H- constants. Results and Discussion The approximation of frequency correlations of complex dielectric permitivity of the samples (Fig.2) using the equation (1) has proved that all the samples, tested within the temperature and frequency range conditions given above, show three relaxation types of different time constants. Their temperature changes can be expressed in form of a general relation as follows: = W exp KT τ τ 0 (2) where: W - activation energy, K Boltzmann's constant. onsidering conditions of high frequencies and low temperatures, β-relaxation could be observed, whose time constants ranged s, and possibly the secondary relaxation γ whose time constants were lower than 10-8 s. The activation energy values (0,36 0,45 ev), calculated from (2) confirm that the β-relaxation is practically independent of both the fullerene content and the modification caused by introducing an additional benzene ring (sample P2) Fig.3a. However, it seems that the stronger inter-molecular interaction and more rigid structure of terminal groups in sample P1 causes the relaxation time of polar groups to be shorter than that of sample P2.

3 Solid State Phenomena Vols However, higher mobility of the entire benzene-carbonyl segment in P2 is responsible for the increase in dielectric permitivity and polarizability Δε 1. Fig. 2. Frequency-temperature relationships of a real part (a) and imaginary part (b) of complex dielectric permitivity in sample P2. The essential differences in relaxation were observed for the low frequency range and high temperature range. Although the relatively steady dipole structure of activation energy at 0,1 ev of KAPT can be observed, the addition of fullerenes induces charge relaxation whose time constants and activation energies depend on the amount of 60 when a wide range of changes in activation energy ranging from 0,45 ev up to 1,6 ev (Fig.3b) is particularly noticeable. As the process of charge polarization formation seems be insufficiently clear, further tests and investigations are recommended, concerning the charge dissemination within the sample volume. Fig.3. Influence of temperature on the time constants of β-relaxation and secondary relaxation (a) and charge relaxation (b) in the modified PI.

4 160 Functional anomaterials for ptoelectronics and other Applications Fig.4. Effect of temperature on the dielectric loss (a) and permitivity (b) of modified PI-films. The values are given for f = 10 khz. onclusions The effect of the presence of 60 -carbon in KAPT is a clear reduction in dielectric permitivity ε r for the entire range of temperature considered and a decrease in the dielectric loss factor tan δ at temperatures below 300 K (Fig.4). A constant temperature of extreme tan δ and a decrease in their values along with higher fullerene contents can indicate that the additions of 60 -carbon do not generate qualitative changes in the main dipole relaxation of time constant τ 1 ; however they cause a reduction inthe main dipole relaxation volume. It can indicate that initially the dipole relaxations fill the free spaces in the polyimide PI whereas the space charge is likely to be formed at the phase boundary between the main polyimide chain and the fullerene References [1] M. I. Bessonov, M. M. Koton and L. A. Laius: Polyimides. Thermally Stable Polymers., ew York [2] S.Bauer-Gogonea, R.Gerhard-Mulhaupt: L ptical Polymer Electes, IEEE Transactions on Dielectrics and Electrical Insulators, v (1996). [3] Motyl E.: Przewodnictwo elektryczne w poliimidach., III Seminarium Techniczne., Materiały i układy elektroizolacyjne w przemyśle elektrotechnicznym., Ustroń-Jaszowice [4] Motyl E.: mechanizmie przewodnictwa elektrycznego w Kaptonie H., Konferencja naukowa, Szklarska Poręba [5] Havriliak.S. Jr., Dielectric and Mechanical Relaxation in Materials. Analysis, Interpretation, and Application to Polymers, Hanser Publishers, Munich Vienna, ew York, 1997.

5 Functional anomaterials for ptoelectronics and other Applications / Dielectric haracteristics of Polyimides Modified by Additions of 60 -Fullerene /