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Chemical Etching Behaviour of a Polyimide

Wissenschaftliche Studie 2009 22 Seiten

Physik - Kernphysik, Teilchenphysik, Molekularphysik, Festkörperphysik


Table of contents


1.1.1 Synthesis
1.1.2 Important properties





Investigations were carried out on etching behaviour of an engineering polymer Kapton- H (4-4'-oxydiphenylene pyromellitimide). Kapton-H samples were subjected to etching in 4N NaOH at 400 C and at 500 C temperatures in pristine as well as irradiated form. Irradiation of pristine Kapton-H specimen was done using 75 MeV/nucleon O+ ion beam of fluence 1.875 x 1012 ions/cm2. The specimen was exposed to etchant for a period of 150 minutes. The effect of etching was observed as half layer thickness removed. Thickness measurements were made at etching cycles of 15 minutes each. It was observed that temperature and the irradiation has their effect on etching behaviour of Kapton-H. Study showed that the temperature results in the increased average bulk etch rate. The average bulk etch rate was also observed to increase with the irradiation of the sample.1


1.1 Kapton-H

Polyimides constitute an important class of materials because of their many desirable characteristics viz. excellent mechanical properties, low dielectric constant , low relative permittivity, high breakdown voltage, low dielectric loss over a wide range of frequency, good polarization, good processing capability, wear resistance, radiation resistance, inertness to solvents, good adhesion properties, low thermal expansion, good hydrolytic stability and long term stability. Because of these traits, polyimides have found applications in a host of technologies as inter-metal dielectric, high temperature adhesive, photoresist etc. the applications of polyimides range from aerospace to microelectronics besides optoelectronics and composites.

The synthesis of an aromatic polyimide was first reported in 1908 but it was not until the late 1950s that DuPont developed a successful commercial route to high molecular weight polyimide. DuPont introduced the first commercial polyimide in the late 1960s.since then the field has blossomed and there has been a high tempo of R & D activity in synthesizing an array of polyimides with requisite properties for a given application and in devising new methods of characterization.

The oxydianiline (ODA) - pyromellitic dianhydride (PMDA) polyimide, with a commercial name Kapton-H, attracted the attention of researchers over other polyimides because ether structure of ODA would enhance possibilities for a moulding resin. Soon it was discovered that the ODA based polymers had real potential for moulding, superior toughness and hydrolytic stability as compared to other polyimide which were MPD (m- phenylene diamine) and PPD (p-phenylene diamine) based. Therefore Kapton-H polyimide, which is superior to other polyimides, has been used in the present investigations. Poly(4-4’ oxydiphenylene pyromellitimide), chemical name of Kapton-H , is available in thin film form in standard thickness of 7.5, 12.5 ,25 ,50 ,75 and 125 µm.

it is a linear polymer comprising of heterocyclic rings linked together by one or more covalent bonds.

In the polyimide crystal, the chain assumes a planer zigzag conformation, with an oxygen ether angle of 1260. Conformational analysis indicates that this is the most probable conformation of an isolated chain and would, therefore, exist in the amorphous as well as in crystalline phase. The driving force for this conformation as well as for the remarkable stability of the film is the maximum degree of electron delocalization of the overlapping pie- orbital, with resultant energy stabilization. The films are largely amorphous, having an as received crystallinity of 5%. These results are also in conformity with Laue transmission. A degree of ordering lower than the three dimensional crystallinity i.e. paracrystalline nature of Kapton-H was also found by Sacher et al. paracrystalline ordering in Kapton-H was confirmed by the increase in density from 1.418 to 1.424 gm cm-3 and crystallinity from 5% to 9% following annealing of the films at 2400 C for 24 hours. The outstanding thermal resistance of the material can be assigned to (a) strong interchain forces (b) the conformational flexibility of the pyromellitic acid , di-imide units and phenyl groups, which can oscillate in phase with the equivalent groups of the surrounding chains without a sensible loss of packing energy. The imide bonds in its aromatic heterocyclic structure cause the excellent mechanical properties and thermo- oxidative stability.1

1.1.1 Synthesis

Polyimides are produced by synthesizing the soluble polymer precursor namely “poly (amic acid)’ and converting it to the final polyimide. Polyamic acid, prepared by the polycondensation of PMDA and ODA in N-methylpyrrolidinone (NMP), reacts by condensation cyclization to form an imide. This highly elegant process made it possible to bring the first significant commercial polyimide product into the market and it is still the method of choice in majority of applications.1

1.1.2 Important properties

Kapton-H exhibits marvelous electrical, dielectric, physical and mechanical properties retainable over a wide range of frequency and temperature, extending from 4 K 2

to 600 K. The electrical properties include high resistivity, low dielectric loss over a wide frequency range and fairly high dielectric strength. It shows no glass transition degradation up to 600k. In addition, the material is resistant to radiation damage to a large extent and is hardly affected by various kinds of chemicals .There is no known organic solvent for the material. It is infusible and flame resistant. The Kapton-H polyimide (chemical name: poly 4-4 Oxydiphenylene Pyromellitimde, PMDA-ODA) used in the present study was procured from DuPont in film form. This polymer is well known for its outstanding electrical properties, solvent resistance, abrasion resistance, flame resistance and exceptional heat resistance. It has a monomeric unit of composition (C22H10N2O5) possessing the rigid chain structure. Figure 2.3 represents the structural unit of Kapton-H polyimide. Pristine Kapton-H (C22H10N2O5) is a linear polymer comprising of heterocyclic rings linked together by one or more covalent bonds. Its chain assumes a plane zigzag conformation with an oxygen ether angle of 1260. Conformational analysis indicates that this is the most probable conformation of an isolated chain and would therefore exist in the amorphous as well as in crystalline phase. Figure 1.1 represent the molecular structure of repeating unit for Kapton-H.

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Figure 1.1


The possible etching mechanism of Kapton-H with NaOH on the basis of Dine Hart et al. 2 may be given in the following steps:

a. During hydrolytic reaction the imide bonds get ruptured and as a result partial positive charge on the carbon atoms and partial negative charge on nitrogen atoms appear. The H+ of water shifts to nitrogen atom and OH part to carbon atom. This hydrolytic reaction is shown in Figure 1.2.
b. Compound A reacts with NaOH and results in the formation of carboxylate salt named as sodium benzene-1, 2, 4, 5-tetra carboxylate (following the IUPAC norms). The reaction is shown in Figure 1.3.
c. Compound B named as 4, 4’-biz amino phenylene oxide undergoes alkaline hydrolysis to form 4-aminonitrophenol as shown in Figure 1.4.

The presence of 4-aminonitrophenol was detected by treating the hydrolytic product with neutral FeCl3 solution. The change in the colour of neutral FeCl3 into dark violet confirms its presence.

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Figure 1.2

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Figure 1.3

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Figure 1.4


Solid state nuclear track detectors (SSNTDs) when used for detection and identification of ionizing particles need to be calibrated using known ions. Whatever may be the mechanism for production of latent tracks the basis for identification lie in the data acquisition on the various etched-track parameters viz., track length L, residual range R, track etch rate VT, etch rate ratio, etch-pit diameter and growth profile besides REL, (d E/dx) and primarly ionization data. Many methods such as L-R plots, track profiling, VT vs R, VT vs (Z */ β ), mean etch rate ratio vs mean total energy etc. are available [3-5].

Almost all of these methods require rigorous geometrical measurements on the track parameters and hence are time consuming. Ruddy et al6 and Grabez et al7 using the concept of etch-induction time (Tind the time interval which elapses between the start of the etching and the first appearance of a microscopically observable track in the given detector) suggested a correlation between Tind and characteristics like Z and β of the incoming ion. However this concept does not appear to be justified as it means that an etched track of size 0.3µm (which is invisible through an optical microscope) would be ignored and hence the measured value of Tind would be larger than the actual value when the track-size grows 10 dimensions comparable with the mean wavelength of the light used. One can, therefore conclude that this concept cannot be used for sub-microscopic track events. Again the observations on etch induction time measurements made by Schwenck et al8 on Fe-ion (500 MeV/n) tracks in Daicel cellulose nitrate suggested that there existed a 1-2 µm thick surface crust in which track etching does not take place, leading to a delay between the start of the track etching and the immersion of the detector in the etchant. All the methods used for identification of particles using solid state nuclear track detectors are based upon the acquisition of data on the various etched track geometrical parameters, as well as REL and (d E/dx) values. Chakarvart y et al9 has attempted to explore the possibility of making use of breakthrough time information obtained during electrolytically controlled etching of nuclear track filter foils of Cellulose

Nitrate (CN), (Daicel and Kodak) and Cellulose Acetate (CA), (Daicel) for discriminating heavy energetic ions viz.,208 Pb (17.1 MeV/n),238 U (13.64 MeV/n, 16.34 MeV/n) and132 Xe (14 MeV/n). They found that the breakthrough time in a given detector and under given etch conditions depends upon (d E/dx) and also is function of (Z */ β ) of the particle. The method was non microscopic and useful only for charged panicles of range ˃ thickness of detector foil. Zhu et al 10 have reported the bulk-etch rate value for Kapton as 0.88 µm/h with 10% NaOCl at 70°C, Vater et al 11 have reported the bulk-etch rate value for Kapton as 1.3 µm /h with 13% NaOCl as the etchant at 70°C. It has not been mentioned whether the etching was carried out under no-light or light- exposed conditions. Chakarvarty & Mahna12 have reported bulk-etch rate value to be about 1.6 times the reported values of Vater et al. 11 under no-light exposure and with stirring conditions with a NaOCl solution with an even lower amount of Cl (4%). At 30°C it was found that average bulk-etch rates for Kapton under light exposure are reduced to ca 50% of the value obtained under no-light conditions. In the case of Thermalimide, the corresponding values of Vb have an insignificant difference up to the first 60 hrs duration of etching. The etching tends to become slower for longer (> 60 hrs) etch-times. At higher temperatures (50°C, 60°C and 70°C), Thermalimide is found to have relatively higher bulk-etch rates under no-light and no-stirring conditions.



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chemical etching behaviour polyimide



Titel: Chemical Etching Behaviour of a Polyimide