Thermal Stability of Electron-Irradiated Poly(tetrafluoroethylene)

Chapter 16

Thermal Stability of Electron-Irradiated Poly(tetrafluoroethylene)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 22, 2016 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch016

X-ray Photoelectron and Mass Spectroscopic Study Donald R. Wheeler and Stephen V. Pepper Lewis Research Center, National Aeronautics and Space Administration, Cleveland, OH 44135

Polytetrafluoroethylene (PTFE) was subjected to 3 keV electron bombardment and then heated in vacuum to 300 C. The behavior of the material as a function of radiation dose and temperature was studied by x-ray photoelectron spectroscopy (XPS) of the surface and mass spectroscopy of the species evolved. Lightly damaged material heated to 300 C evolved saturated fluorocarbon species, whereas unsaturated fluorocarbon species were evolved from heavily damaged material. After heating the heavily damaged material, those features in the XPS spectrum that were associated with damage diminished, giving the appearance that the radiation damage had annealed. The observations were interpreted by incorporating mass transport of severed chain fragments and thermal decomposition of severely damaged material into the branched and cross-linked network model of irradiated PTFE. The apparent annealing of the radiation damage was due to covering of the network by saturated fragments that easily diffused through the decomposed material to the surface region upon heating. In many applications, polymers are exposed to radiation, high temperature or a combination of both. Satellite thermal blankets, wiring harnesses and components are irradiated on passage through the radiation belts (1.2). while being exposed to the varying thermal environment of space. In the nuclear industry, materials suffer both intense radiation and high temperature. U.V. radiation is known to contribute to the surface modification of polymers during plasma treatment (3) And in the electronics industry, electron, U.V. and x-ray radiation are used to treat mask, insulator, packaging and circuit board materials(4), a l l of which can be exposed to high temperatures either during treatment or service. Polytetrafluoroethylene (PTFE) is a candidate material in many of these applications because of its superior thermal and chemical stability. However, its well known sensitivity to radiation (5) limits its use in many cases. On This chapter not subject to U.S. copyright Published 1990 American Chemical Society Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

224

METALLIZATION OF POLYMERS


the other hand, low energy, high dose i r r a d i a t i o n of PTFE can improve the adhesion of coatings (6) or i n h i b i t the action of chemical etch treatments used to prepare the surface f o r coating (7). The l a t t e r e f f e c t has been proposed as a method f o r preparing printed c i r c u i t s on PTFE substrates (8). Thus, f o r many p r a c t i c a l reasons, the behavior of i r r a d i a t e d PTFE during heating i s of potential interest. The objective of t h i s study was to use both X-ray photoelectron spectroscopy (XPS) and mass spectroscopy to examine the e f f e c t s of temperatures from 20 C to 300 C on PTFE i r r a d i a t e d with 3 keV electrons. EXPERIMENT Apparatus. The experiments were performed i n a vacuum system con s i s t i n g of a preparation chamber and an analysis chamber separated by a gate valve. The preparation chamber was pumped by a 150 1/s turbo pump to a base pressure of 1x10" mbar. A quadrupole residual gas analyzer (RGA) was connected to the pumping l i n e of the chamber. It did not have a d i r e c t l i n e - o f - s i g h t to the specimen. Gases could be admitted to the chamber as desired through a v a r i a b l e leak valve. In the preparation chamber, the specimen could be mounted i n a heatable support where i t could be i r r a d i a t e d with an electron gun. A trans fer device allowed the specimen to be moved to the analysis chamber without exposure to a i r . The analysis chamber was equipped with d i f f u s i o n and T i sublimation pumps and had a base pressure of 1x10" mbar It was also equipped with an RGA i d e n t i c a l to the one on the preparation chamber. In t h i s chamber the specimen could be analyzed by XPS and heated by a r e s i s t i v e element incorporated i n the specimen mount. The temperature of the specimen was determined with an infrared pyrometer (9). The unmonochromated x-ray source was Mg Κα. To minimize x-ray damage to the PTFE specimen (10), the source was operated at only 7.5 kV and 5 mA and was retracted as f a r from the specimen as p o s s i ble. Under these conditions, no change i n the Cls l i n e of v i r g i n PTFE could be detected after several hours of x-ray exposure, and more importantly, there was no detectable outgassing of the specimen at the 1x10" mbar l e v e l . A l l spectra were obtained under the same conditions: Data were taken with the specimen normal at 80° to the axis of the analyzer i n put lens (grazing electron exit angle). The analyzed spot on the specimen was, then, 6 mm by 5 mm. The acceptance angle of the input lens was ±6°. The analyzer was a Vacuum Generators ESCALab Mk II op erated with 0.5 eV resolution. Data were taken at 0.1 eV steps. The actual width of the Cls l i n e from v i r g i n PTFE was 1.6 eV as a result of the x-ray l i n e width and d i f f e r e n t i a l charging of the specimen. The data were smoothed using a 15 point cubic-quartic Savitzky and Golay (Γ1) algorithm, the x-ray s a t e l l i t e s and a S h i r l e y background were subtracted using computer routines a v a i l a b l e i n the Vacuum Gen erators data analysis software. Only the treated data are presented here. Specimens. The PTFE specimens used i n a l l the experiments reported here were 76 μιη thick sheets of pure, unsintered PTFE (Fluorglas #R126, Fluorglas, Hoosick F a l l s , NY 12090). The material was mounted

Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


16.

WHEELER AND PEPPER

Electron-Irradiated Poly(tetrafluoroethylene)

on a 1 cm diameter, Mo sample stub. The stub's surface was ground on SiC paper to give i t a s l i g h t curvature and was then polished with 6 μιη diamond paste. An oversize piece of the PTFE sheet was gently stretched over the curved surface and fastened by wrapping a wire around the shank of the stub. The stub was then heated i n flowing nitrogen u n t i l the PTFE f i l m changed from white to clear. After cooling, the PTFE sheet was i n intimate contact with the heatable stub. The fastening wire was removed and the excess PTFE sheet was cut away with a scalpel. An area about 2 mm by 4 mm at the edge of the specimen was painted with an isopropyl alcohol suspension of graphite (DAG) to serve as the target of the infrared pyrometer dur ing temperature measurement (9). Procedure. The experiment consisted of f i r s t i r r a d i a t i n g the PTFE i n the preparation chamber, then either 1) heating i n the analysis cham ber and acquiring XPS spectra after each temperature step or 2) heat ing i n the preparation chamber and acquiring the mass spectrum of the gas evolved from the damaged material. Each of these experiments was separate, with new PTFE specimens used at each r a d i a t i o n damage l e v e l and f o r XPS and RGA analysis. In the following sections, the proce dure used to condition the PTFE f o r the experiments i s described, first. Then the i r r a d i a t i o n conditions are described. F i n a l l y , the thermal treatment and d e t a i l s of the XPS and mass spectroscopy are described. Specimen Conditioning. The sample was f i r s t placed i n the prepara tion chamber and heated to 350 C. During t h i s f i r s t heating i n vacuum, there was normally some evolution of gas. The mass spectrum of the gas c l o s e l y matched that of the tetrafluorethylene monomer. No gas was evolved during subsequent heating of the specimen to temp eratures below 350 C. An XPS spectrum of the specimen was obtained to check i t s i n i t i a l condition. The specimen prepared i n t h i s manner exhibited only XPS features from F and C. The Cls l i n e was a single feature i d e n t i c a l to the dashed peak l a b e l l e d CF i n Figure 1. Its 2

binding energy was 397.0 eV to 397.2 eV below the F i s l i n e , and the r a t i o of the F i s to Cls peak areas, i n our spectrometer, was 7.1±0.2. Irradiation. Samples were irradiated with electrons i n the prepara tion chamber. The electron accelerating voltage was 3 keV. The beam current was 0.7 μΑ and was measured by d i r e c t i n g the beam into a hole in the sample support which was biased 30 v o l t s p o s i t i v e . After the beam current was stable, the specimen was placed on the support and the beam was rastered over an area of 1.3 cm which covered the en t i r e PTFE surface, and an absorbed-current image of the specimen was displayed. It i s l i k e l y that the specimen charged during i r r a d i a t i o n either due to embedded charge or excess secondary electron emission. How ever, charging was never severe enough to produce v i s i b l e d i s t o r t i o n of the absorbed-current image, and the XPS spectrum was stable imme d i a t e l y after i r r a d i a t i o n . If charging of the specimen affected the electron beam intensity, the e f f e c t was quite reproducible, since the same changes i n the XPS spectrum were observed f o r the same i r r a d i a tion times on many d i f f e r e n t specimens.


22


226


Heating and X-ray Photoelectron Spectroscopy. After i r r a d i a t i o n f o r the desired time, the specimen was moved to the a n a l y t i c a l chamber where i t was heated i n successive 50 C steps from 100 C to 300 C. The desired temperature could be achieved i n 3 to 5 min, and was maintained for 20 min. During the f i r s t 5 min of heating there was a r i s e i n pressure i n the system as gas was evolved from the specimen, but by the end of the 20 min interval the pressure was close to the background l e v e l . In preliminary experiments, i t was determined that any changes i n the XPS spectrum were complete before the end of the 20 min of heating. Before heating and a f t e r each heating step, Cls and F i s XPS spectra were obtained. Before i r r a d i a t i o n , the XPS spectrum consisted of single Cls and F i s l i n e s . The Cls l i n e occurred at approximately 292.0 eV binding energy. It i s by now well established that, on i r r a d i a t i o n of PTFE, the r a t i o of the F i s to the integrated Cls XPS i n t e n s i t i e s decreases (10-14). while the Cls l i n e develops a complex structure. Figure 1 shows the Cls l i n e from PTFE irradiated f o r 100 min. Since the specimens charged during analysis, and the degree of charging varied with i r r a d i a t i o n time and subsequent heating, the spectra were a l l s h i f t e d to a l i g n the component of the Cls spectrum l a b e l l e d CF i n Figure 1 2

and corresponding to unirradiated PTFE. The binding energy of that peak has been set to 292.0 eV. The change i n the spectrum consisted of the growth of three components other than the o r i g i n a l l i n e . The structure could, i n fact, be f i t reasonably well with four i d e n t i c a l Gauss-Lorentz l i n e s as shown, i n Figure 1. The width of the component peaks varied from spectrum to spectrum, but was the same f o r a l l components of one spectrum. The four components have previously been assigned to carbon atoms i n CF CF and CF groups and C with no p r i mary f l u o r i n e bonds (10,15). In a l l the following results, the Cls spectrum was characterized by the r e l a t i v e areas of these four components and by the r a t i o of the F i s to Cls integrated peak areas. 3>

2

Heating and Mass Spectroscopy. Specimens were prepared as described above but using a Ni stub which had an attached thermocouple. The specimens were placed on a heatable sample mount i n the preparation chamber and irradiated. The sample temperature was then raised at a rate of 20 C/min to 30 C/min from 100 C to 300 C. The RGA continuously recorded the spectral i n t e n s i t i e s at 19, 24, 31, 50, 69, 81, 93, 100 and 119 AMU. The mass spectra of several test gases were recorded i n preliminary experiments, so that they could be compared to the gas evolved from the specimen. These test-gas spectra are shown i n Table I. The r e l a t i v e peak heights i n the spectra were somewhat pressure dependent, but the data i n Table I were taken at approximately the same pressure as the evolved gas. Table I shows two c h a r a c t e r i s t i c d i f ferences between saturated and unsaturated fluorocarbons. F i r s t , the 69/31 r a t i o i s greater than one f o r the saturated gases and less than one f o r the unsaturated gas. Second, the m/e = 81 feature i s absent in the saturated gases but present i n the unsaturated gas. The sample of gases i n Table I i s limited, however these features are also c h a r a c t e r i s t i c of mass spectra reported i n the l i t e r a t u r e (16). Therefore, these two c h a r a c t e r i s t i c s are used, here, to d i s t i n g u i s h saturated from unsaturated fluorocarbon gases evolved from PTFE.


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WHEELER AND PEPPER

Table I.

Mass Spectra of Several Fluorocarbon Gases at 1x10" r e l a t i v e to the largest component i n each test gas

Torr,

Test Gas m/e

Species

C F 2

19

F

24

C

31

CF

50

CF

69

CF

81

C F

100

C F

119

C F

C F 6

3.3

3

C F

8

4

0. 0

C F 10

2

0.,0

4

1.4

0.3

0. 1

0.,0

2.2

48.5

31. 1

26.,1

100.0

14.2

6. 2

4. 2

23.0

100.0

100. 0

100. 0

1.7

0.0

0. 0

0. 0

23.7

0.2

1. 8

2. 2

6.5

5.4

0. 9

1. 6

0.0


2

2 3 2 2 2

3 4 5

RESULTS For surface analysis, the specimens were heated stepwise a f t e r i r r a d i a t i o n , and after each temperature an XPS spectrum was recorded. These results are described f i r s t . A l t e r n a t i v e l y , i r r a d i a t e d PTFE specimens were heated by ramping the temperature, as described above, while the mass spectrum of the evolved gas was recorded. These res u l t s are described i n the second section. X-ray Photoelectron Spectra. Heating produced changes i n both the Cls structure and the F l s / C l s peak area r a t i o such that those f e a tures associated with radiation damage were reduced. The change i n the C l s structure i s i l l u s t r a t e d i n Figure 2 f o r a specimen i r r a d i a t ed 100 min. It i s apparent that between 150 C and 200 C the components of the l i n e attributed to CF and C decreased r e l a t i v e to the other two components. The result was a r e l a t i v e recovery of the CF 2

(undamaged) component and a change i n the Cls l i n e shape to one chara c t e r i s t i c of less damaged PTFE. This change i n the CF component 2

upon heating i s shown f o r samples i r r a d i a t e d f o r other times i n F i g ure 3. The recovery was only observed f o r i r r a d i a t i o n times greater than 30 min, while the samples irradiated f o r the shortest times may have a c t u a l l y exhibited some decrease i n CF during the i n i t i a l 2

stages of heating. The F l s / C l s peak area r a t i o increased on heating. This p a r t i a l recovery of the f l u o r i n e concentration i s shown by the data of Figure 4 which shows that, l i k e the change i n the Cls structure, the change in the F l s / C l s r a t i o began above 150 C, and only occurred f o r i r r a d i ation times greater than 30 min. Mass Spectra of Evolved Gas. The mass spectra of the gas evolved while heating the irradiated PTFE varied with temperature and i r r a d i ation time. Figure 5(a-d) shows the i n t e n s i t i e s of the mass spectro-


22


228


280

285

290 295 Binding Energy, eV

300

305

Figure 1. Four-component synthesis of C l s XPS feature from PTFE i r r a d i a t e d with 3 keV, 0.5 μΑ/cm electrons f o r 100 min. F i l l e d c i r c l e s are o r i g i n a l data; dotted curves are component peaks. S o l i d l i n e i s sum of component peaks. Shirley background i s shown.

280

285

290 295 Binding Energy, eV

300

305

Figure 2. E f f e c t of 20 min heating on C l s XPS feature from PTFE i r r a d i a t e d with 3 keV, 0.5 μΑ/cm electrons f o r 100 min. F u l l l i n e , no heating. Dashed l i n e , 150 C. Dot-Dashed l i n e , 200 C. Dotted l i n e , 300 C. 2


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WHEELER AND PEPPER


80

Temperature CZ3 20 C [3D 100 C 150 C 7ΖΔ 200 C 250 c • • 300 C

σ 70


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Thermal Stability of Electron-Irradiated Poly(tetrafluoroethylene)

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