Thermally stimulated depolarization current behaviour of 75 MeV oxygen ion irradiated Kapton-H polyimide film

Abstract

The thermally stimulated depolarization current (TSDC) technique has been used to investigate the various dielectric relaxation processes in high-energy ion irradiated Kapton-H polyimide. The 25 µm thick Kapton-H samples were irradiated with 75 MeV oxygen ions (fluences: 1.88 × 10 11 , 1.88 × 10 12 , 1.88 × 10 13 , 5.63 × 10 12 ions/cm 2 ) at the Nuclear Science Center (Pelletron Facility), New Delhi. The TSDC characteristics were obtained at a linear heating rate of 2 • C/min for different poling temperatures varying from 80 to 250 • C and poling fields from 160 to 400 kV/cm. The TSDC spectra mainly consist of four current maxima, viz. γ , β, α and ρ in the increasing order of temperature. The γ -relaxation has been attributed to the increased water absorptivity in Kapton-H due to irradiation. The demerization of carbonyl groups, cross-linking of imide groups and formation of free radicals due to energetic heavy ion irradiation in Kapton-H polyimide have been held responsible to govern the behaviour of β, α and ρ relaxations. In the vicinity of the β-relaxation, a new relaxation process, termed as δ-relaxation, is also observed. The activation energy values (0.78, 0.60, 1.04 and 1.32 eV for γ , β, α and ρ relaxations, respectively) for different relaxations were calculated by the Bucci plot method.

TSDC spectroscopy of relaxational and interfacial phenomena

“…Polymers with imide [338][339][340]343,[345][346][347][348][349][350][351][352]) and amide [351,353] structures (which are of interest from a practical point of view), demonstrate certain regularities in their relaxation phenomena. The much larger polarity and polarisability of C-N and N-H bonds in comparison with C-C and C-H provide the relaxation effects of several mechanisms.…”

“…It is to be mentioned that the distribution of traps vs distribution of attempt frequencies in these polymer systems cannot be determined exclusively from TSDC results [338]. Additionally, the electret behaviour and multiple relaxation processes in Kapton-H polyimide irradiated [344][345][346][347][348][349] and thermally treated [350] were studied by the TSDC method. Kripotou et al [351] investigated molecular mobility, morphology and crystallization/melting events of PEG in poly (imide-amide)-polyethylene glycol hybrid networks (PIA-PEG) with short (M n = 1000 g/mol) and long (M n = 3400 g/mol) PEG crosslinks using TSDC (F p = 600 kV/m, t p = 5 min, T p = 300 K, T 0 = 88 K, T = 77-300 K), broadband DRS (complex dielectric permittivity ε⁎ = ε′ − iε″ was determined at 0.1-10 6 Hz and 130-350 K), DSC (120-500 K) and to a lesser extent water uptake measurements.…”

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

“…Besides the apparent peak, two other relaxations are observed. A first one is observed as a very weak shoulder of the apparent peak around 337 K. For T c = 313 the apparent peak is around 331 K and for T c = 333 the apparent peak is around 356 K. The peak at 337 K represents the β 1 relaxation [27][28][29]. This relaxation is very weak.…”

“…Beside the apparent peak, a weak relaxation is entering around 337 K. This fact is better observed for E c = 18 MV/m and t c /t di = 30 min/30 min. A peak was observed in tan δ around this temperature [13] and it was attributed to absorbed water [27][28][29]. Because our measurements are carried out in a vacuum we cannot attribute the peak to absorbed water (see below).…”

“…(3) to determine the thermal activation energy W = 0.36 eV. A search of literature shows that, using the TSDC method, a peak was reported at 390 K (W = 0.19 eV) and at 420 K (W = 0.4 eV) [18], at approximately 363 K (W = 0.60 eV) and it was attributed to electric dipole orientational polarization resulting from the co-operative molecular motion of the residual reactive groups C=O [27], a peak ranging between 398 and 463 K [28]. From mechanical and dielectric measurements the activation energies for the β relaxation of polyimides have been reported in the range of 0.5-1 eV [13,17,27] .…”