Plasma Polymerization - American Chemical Society

2 Competitive Ablation and Polymerization (CAP) Mechanisms of Glow Discharge Polymerization H . K. YASUDA

Downloaded by UNIV OF MELBOURNE on March 28, 2016 | http://pubs.acs.org Publication Date: September 24, 1979 | doi: 10.1021/bk-1979-0108.ch002

Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, M O 65401

The formation of a polymer in a glow discharge is a complex phenomenon, and the interpretation of the formation mechanism is not at all clarified by the misapplication of the term "polymerization". The term "polymerization" is generally accepted to mean that the molecular units (monomers) are linked together by the"polymerization"process. Consequently, the resultant polymer is conventionally named by using the prefix "poly" plus the name of the monomer. For instance, the polymer formed by the polymerization of styrene is referred to as polystyrene. In other words, "polymerization" conventionally refers to molecular polymerization; that is, the chemical structure of a polymer can be derived from the chemical structure of the monomer. Strictly speaking, "polymerization" in the conventional sense does not represent the process of polymer formation that occurs in a glow discharge, because in glow discharge atomic polymerization may take place instead of molecular polymerization. Because of this atomic polymerization, some of the elements or fractions of the monomer are not incorporated into the polymer that is deposited but play an important role in sustaining the glow discharge of the polymer-forming plasma. Although the individual steps or reactions that are involved in the process of polymer formation in a glow discharge are extremely complex and are dependent on the system, several important types of phenomena can be identified for the purpose of constructing a general picture of the glow discharge polymerization. This process is represented by the Competitive Ablation and Polymerization (CAP) mechanism schematically shown in Figure 1. The direct route shown in the Figure is referred to as plasma-induced polymerization; the indirect route as plasma state polymerization.

0-8412-0510-8/79/47-108-037$05.00/0 © 1979 American Chemical Society Shen and Bell; Plasma Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

PLASMA

POLYMERIZATION

CAP (COMPETITIVE ABLATION AND POLYMERIZATION)


SCHEME OF GLOW DISCHARGE POLYMERIZATION

Figure

1.

Overall

mechanism

of glow discharge

polymerization

Shen and Bell; Plasma Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2.

YASUDA

Mechanisms

of Glow

39

Discharge

Plasma-induced polymerization is essentially conventional (mole cular) polymerization that is triggered by a reactive species created in an electric discharge. In order for one to form polymers by plasmainduced polymerization, the starting material must contain polymerizable structures, such as olefinic double bonds, triple bonds, or cyclic structures. Plasma state polymerization is an atomic process, which occurs only in a plasma state. This type of polymerization can be represented by the statements Downloaded by UNIV OF MELBOURNE on March 28, 2016 | http://pubs.acs.org Publication Date: September 24, 1979 | doi: 10.1021/bk-1979-0108.ch002

Initiation or Reinitation M. -> M.* ι ι M

M

k ~* k *

Propagation and Termination M

M

+ M

M

M

i * k* * i ~ k i *

M

+M

k*

i

-

M

k

in which i and k are the numbers of repeating units (i.e., i = k = 1 for the starting material), and M* represents a reactive species, which can be an ion of either charge, an excited molecule, or a free radical pro duced by M but not necessarily retaining the molecular structure of the starting material. That i s , M can be a fragment or even an atom de tached from the original starting material. In plasma state polymerization, the polymer is formed by the re peated stepwise reaction described above. It should be noted that plas ma-induced polymerization does not produce a gas phase by-product, because the polymerization proceeds via the utilization of a polymerizable structure. The process can be schematically represented by the following chain propagation mechanism: M* + M

MM* Propagation

M.* +M -> M* ι l+l M.* + M^* -+ M.-M

k

Termination

It should be emphasized that polymerization in a glow discharge con sists of both plasma-induced polymerization and plasma state polymeri zation. Which of these two mechanisms plays the predominant role de pends not only on the chemical structure of the starting material but on the condition of the discharge. Nonpolymer-forming gas products are produced where reactive



40

P L A S M A POLYMERIZATION

(polymer-forming) intermediates are formed when the polymer deposit or the substrate material is decomposed (etched). Because polymerforming species leave the gas (plasma) phase as the polymers are de posited, the major portion of the gas consists of the product gas when a high conversion ratio of a starting material to a polymer is obtained. Although this i s an extremely important factor, it has been dealt with lightly or neglected in most of the articles appearing in the literature. The characteristics of the product-gas plasma are the predominant factors in determining the extent of the ablation process, which i s shown in Figure 1. Because much of the work in glow discharge poly merization that appears in the literature deals with hydrocarbons, which produce hydrogen as the main product gas, the effect of ablation is not great. Consequently, the complete neglect of ablation does not make a significant difference in the overall picture of glow discharge polymeri zation; however, when a fluorine- or oxygen-containing compound i s used as the starting material, the extent of ablation becomes the domi nant factor, and the extent of polymer formation depends entirely on the extent of the product gas formation. Perhaps the most dramatic ablation effect for the glow discharge polymerization of CF4 was recently demonstrated by Kay (1). It had been thought that C F was one of the very few organic compounds that does not polymerize in a glow discharge. On the other hand, CF4 had been used as one of most effective gases in plasma etching. Kay ob served that no polymer deposition occurs under normal conditions in spite of the fact that C-F bonds are broken in the glow discharge, a fact which has been confirmed by mass-spectroscopic analysis of the gas phase. However, when a small amount of hydrogen is introduced into the discharge, a polymer is deposited. When the hydrogen flow is stopped, the polymer deposit ablates. The situation observed in the above example may be visualized by comparing bond energies. It should be noted that the energy level i n volved in a glow discharge is high enough to break any bond (2,3), i . e., C-F is broken, although C-F is stronger than C-H. The important fact is the stability of the product gas. The bond energy for F-F is only 37 kcal/mol, whereas H-F is 135 kcal/mol, which is higher than 102 kcal/ mol for a C-F bond. The introduction of H into the monomer flow evidently produces H F and removes F from the discharge system. This reduces the etching effect of the product gas (F ) plasma and shifts the balance between polymerization and ablation to polymerization. A l though the term F plasma is used to describe the effect of the detached F in plasma, F is not detected in the plasma state, perhaps because of its extremely high reactivity (4). It is interesting to note that F and Ο are the two elements that re duce the rate of polyer formation of compounds containing one or the 4

2

2

2

2


2.

YASUDA

Mechanisms

of Glow

Discharge

other of these elements. These two elements are the most electronega tive of all elements, but the bond energy itself is not a measure of the etching effect of the plasma. For instance, the N-N bond is only 32 kcal/mol, and the N plasma does not etch polymer surfaces. Instead, the incorporation of Ν into the surface predominates (5). Nevertheless, the importance of the ablation process shown in Figure 1 seems to be well demonstrated by the paucity of the polymer formation in the glow discharge polymerization of C F and of oxygen-containing compounds (6). The polymer formation and properties of polymers formed by glow discharge polymerization are controlled by the balance among plasmainduced-polymerization, plasma state polymerization, and ablation. That i s , polymer formation is a part of the CAP scheme shown in Figure 1. Because of this CAP scheme, gas evolved from substrate materials also plays an important role, particularly at the early stage of coating. It has been noticed that the deposition of a polymer from the glow dis charge of styrene onto plyoxymethylene, which is an oxygen-containing, plasma labile substrate, is significantly different from the deposits on more stable substrates and depends greatly on the condition of the glow discharge (7). A study of the properties of polymers formed by glow discharge polymerization from tetrafluoroethylene and investigated by electron spectroscopy for chemical analysis (ESCA) provides further evidence for the importance of the CAP mechanism (8). The ESCA C l s spectrum of commercially prepared polytetrafluoroethylene (Teflon) shows a single intense peak about 291 eV that corres ponds to -CF - carbons. The ESCA C l s spectrum of glow discharge polymerized tetrafluoroethylene is significantly different from that of Teflon and contains many peaks corresponding to -CF3, >CF-, -C-, and carbons bonded to other substituents, which includes nitrogen- and oxygen-containing groups, depending on the condition of glow discharge. I f a polymer is formed through plasma-induced polymerization from tetrafluoroethylene, the ESCA C l s spectrum should be identical to that of Teflon. Therefore, the fact that the ESCA C l s peaks are significant ly different from those in the spectrum of Teflon indicates that a major portion of the glow discharge polymerization is not plasma-induced polymerization. How the balance between plasma state polymerization and plasma-induced polymerization is influenced by the conditions of glow discharge and the location of polymer deposition within a reactor can be seen by comparing the ESCA C l s spectra shown in Figures 2-4. The characteristic shapes of the C l s peaks shown in Figure 2 indi cate that the polymers formed at locations on the upstream side and downstream side of the rf coil are quite different when a relatively low discharge power is used. The polymer formed on the upstream side contains considerable amounts of CF« (about 293 eV) and C F (about 289 2

4


41

2


POLYMERIZATION


PLASMA

Figure 2. Dependence of the ESCA C(ls) peaks of glow discharge polymers of tetrafluoroethylene on discharge conditions and the location of polymer deposi tion. Polymer deposits occur at two locations (a) before the rf coil and (b) after rf coil. Discharge power level is 1.9 Χ 10 J/kg. 7


2.

YASUDA

Mechanisms

of

Glow

Discharge

eV) besides the expected CF « This i s undoubtedly due to the atomic nature of glow discharge polymerization rather than conventional molecular polymerization. The polymer formed on the downstream side of the r f coil at this low discharge power contains much less F, i . e., much smaller peaks for a higher binding energy, and the peak of 284· 6 eV (carbon which i s not attached to F) becomes the major peak. This is a dramatic display of the effect of the energy input zone, where the tube is directly inside of the r f coil, on the properties of glow discharge polymers. As the discharge power is increased, this severe effect of the energy input zone expands eventually to the entire length of the tube, and at a high discharge power, the polymer formed on the upstream side of the rf coil becomes similar to the polymer formed on the downstream side, as can be seen in Figure 3. When a system in which the flow does not pass through the energy input zone and glow discharge polymerization is carried out in the t a i l flame portion of the glow discharge, the polymer formed at the downstream end of a reactor is not necessarily the same as that formed at the downstream end of a straight tube. The results depicted in Figure 4 show that the polymer formed in the nonglow region, although located at the downstream end of a reactor, is nearly identical to conventional polytetrafiuoroethylene. This means that polymers formed under such conditions are formed mainly by plasma-induced polymerization. With regard to the CAP mechanisms of glow discharge polymerization, it is important to recognize the following aspects that are inevitably involved. 1. The rate of polymer deposition as well as the rate of ablation vary according to the location of deposition within the reactor. 2. The properties of the polymer deposits are also different depending on the location of their deposition within the reactor. 3. The materials that are used in a reactor, such as the vessel walls and substrates, influence the glow discharge polymerization, i.e., the rate of polymer deposition and the properties of the polymer deposits. Because polymer formation can proceed through more than one major type of reaction, i . e., plasma-induced polymerization and plasma state polymerization, depending on the chemical structure of the monomer and also on the conditions of discharge, such as discharge wattage, flow rate, type of discharge, and geometrical factors of the reactor, the balance between polymer formation (polymerization) and ablation is for most cases extremely complicated. An attempt, the details of which are presented below, has been made to demonstrate the balance between polymerization and ablation by selecting a monomer system that can be manipulated to shift the balance. 2


43


PLASMA

POLYMERIZATION


44

Figure 3. ESCA C(ls) peaks of glow discharge polymers of tetrafluoroethylene in the same reactor shown in Figure 2 but at the higher discharge power level of 7.7Xl0 J/kg 8


Mechanisms

of Glow

Discharge


YASUDA

Figure 4. ESCA C(ls) peaks of glow discharge polymers of tetrafluoroethylene prepared in the reactor shown in the insert: (c) at the end of the glow region, and (d) at the end of the tube in the nonglow region.




46

The monomer chosen is hexafluoroethane, which cannot be polymerized by plasma-induced polymerization and which cannot be polymerized in a glow discharge under ordinary conditions presumably because the ablation process associated with the glow discharge is excessive. Attempts have been made to supress the ablation process and to shift the balance between plasma state polymerization, which is assumed to be present, and ablation. However, it has been observed that polymer formation for hexafluoroethane does occur when polyethylene is used as substrate. On the other hand, no polymer formation can be observed either with ESCA or by surface energy analysis when glass is used as a substrate. In an earlier study of glow discharge polymerization of ethylene in a magnetically enhanced discharge by a 10 kHz power source, the distribution of polymer deposition on the surfaces of electrodes and on an aluminum plate inserted between two electrodes was investigated (10). In a magnetically enhanced discharge, the power density or the intensity of glow is not uniform, because an intense glow occasioned by the magnetic field occurs near the surfaces of electrodes. This uneven distribution can be utilized to investigate how the formation of a polymer in a glow discharge is related to power density. In this study, the distribution of polymer deposition was compared for ethylene and hexafluoroehtane in identical systems with the addition of various amounts of Hg. The contributions of the component mechanisms, which are part of the CAP mechanism for these two monomers, are summarized in Table I. Table I Contributions of Different Mechanisms in Glow Discharge Polymerization Plasma-Induced Plasma State Polymerization Polymerization Ablation

Monomer Ethylene

C H

2

Hexafluoroethane CF

= C H

3

C F

2

3

Occurs

Occurs

Minimal

None

Increases as a function of H concentration

Decreases as a function of H g concentration

2

The distribution of polymer deposition from the glow discharge of ethylene is shown in Figure 5. In this figure, the deposition rate i s plotted against the distance from the center of the electrode. The deposition rate profile shows a strong peak at a location that corresponds with the intense glow. A much sharper peak occurs at the position of the electrode surface. The peaks for the deposition on an aluminum plate substrate placed between electrodes i s much less pronounced (Fig.



YASUDA

Mechanisms

of Glow

Discharge

Figure 5. Deposition rate profile on the electrode and substrate for a glow dis charge polymerization of ethylene activated at a frequency of 10 kHz with mag netic enhancement. The system pressure is 20 μm Hg. (Ό) 60 m A, (A) 200 mA.

American Chemical Society Library 1155 16th St. N. W. Shen and Bell; Plasma Polymerization Washington, D. C.Society: 20036 ACS Symposium Series; American Chemical Washington, DC, 1979.


48


5)· The sharp increase at the position of the intense glow indicates that the polymer formation from ethylene is proportional to the extent of ionization caused by electron bombardment. The deposition rate profile for hexafluoroethane is quite different as shown in Figures 6 and 7. The positive peak observed for ethylene at the location of the intense glow appears as a negative peak for hexa fluoroethane. As indicated in a previous study for tetrafluoroethy lene (11), the ablation effect of tetrafluoroethylene depends on the energy density of the glow discharge. It has been observed that when the energy density increases above a certain critical value, ablation becomes the predominant process, and a drastic decrease in polymer formation occurs. Therefore, the negative peak in the deposition profile can be ex plained by extensive ablation in the high energy density region. The relatively high deposition rates at the substrate surface, as compared to those at the electrode surface, can also be explained by the lower energy density in the middle of two electrodes, i.e., less ablation. As the concentration of H is increased to the stoichiometric equiv alent of the numbers of F atoms (H /H F Ε = 3), the negative peak ob served without H (Fig. 6) is changed to the positive peak (Fig. 7). This indicates that ablation is no longer dominant, and polymer forma tion prevails. This phenomenon can be explained by postulating that H reacts with F atoms emanating from the fluorine containing compound in the glow discharge and forms the more stable HF, which reduces the ablation in a dramatic manner. Because hexafluoroethane does not form a polymer by plasma-induced polymerization, the overall effect can be explained by the balance between plasma state polymerization and abla tion. This postulate of glow discharge polymerization can be extended to a case in which a fluorine containing compound is added to a hydrocar bon monomer. Kobayashi et al. (12) demonstrated that the addition of small amounts of Freon increases the deposition rate of ethylene and ethane in a glow discharge, and they reasoned that the increase is due to the enhanced abstraction of hydrogen from the monomers. However, when a similar experiment is conducted under the conditions in which plasma state polymerization rather than plasma-induced polymerization predominates and a very high yield of monomer to polymer is obtained, no such dramatic increase can be observed (13). Therefore, it can be stated that the addition of a fluorine containing compound enhances plasma state polymerization, which probably does not play a major role under the conditions employed in the experiment conducted by Kobayashi e t a l . (12). The above results demonstrate that CAP mechanisms may provide a reasonable explanation for glow discharge polymerization. Which reac2

2

2

2


YASUDA

Mechanisms

of Glow

49

Discharge

AF PLASMA, MAGNETICALLY ENHANCED

2 6 (H /C Fg) = 0 200 mA


F c

F

2

10

=

3

2

C

/

m

i

n

2

0

DISTANCE FROM ELECTRODE CENTER AXIS (cm)

Figure charge

6. Deposition rate profile on the electrode and substrate for a glow polymerization of tetrafiuoroethane activated at a frequency of 10 with magnetic enhancement. The system pressure is 20 ^m Hg.


diskHz

PLASMA

50

POLYMERIZATION


AF PLASMA, MAGNETICALLY ENHANCED

l— 0

1

1

1

1

I

5

1

I

•

•

•

I

10

0

I

I

I

I

I

I

1

1

5

DISTANCE FROM ELECTRODE CENTER AXIS (cm)

Figure charge

7. Deposition rate profile on the electrode and substrate for a glow dispolymerization of tetrafluoroethane with H activated at a frequency of 10 kHz with magnetic enhancement. System pressure is 50 ^m Hg. 2


I

1

10

2.

yasuda

51

Mechanisms of G l o w Discharge

tion p r e d o m i n a t e s i n the glow d i s c h a r g e p o l y m e r i z a t i o n of a p a r t i c u l a r m o n o m e r depends not only o n the c h e m i c a l s t r u c t u r e of the m o n o m e r but a l s o o n the conditions o f the d i s c h a r g e , p a r t i c u l a r l y the energy d e n sity at the location of deposition. Synopsis T h e formation o f p o l y m e r s i n the glow discharge of a n o r g a n i c c o m pound m a y b e explained by Competitive Ablation and P o l y m e r i z a t i o n (CAP) mechanisms.

A c c o r d i n g to t h i s p o s t u l a t e , p o l y m e r f o r m a t i o n


c o m p e t e s with ablation i n the p r e s e n c e o f n o n p o l y m e r - f o r m i n g g a s e s , which emanate f r o m the m o n o m e r i n the glow d i s c h a r g e .

The p o l y m e r

ization p r o c e s s i s not the s a m e a s conventional m o l e c u l a r p o l y m e r i z a tion.

It i s a n a t o m i c p o l y m e r i z a t i o n , i n w h i c h t h e m o l e c u l a r s t r u c t u r e

of t h e m o n o m e r i s n o t r e t a i n e d .

This can be determined by using e l e c

t r o n s p e c t r o s c o p y f o r c h e m i c a l a n a l y s i s d a t a o f the g l o w d i s c h a r g e p o l y m e r of tetrafluoroethylene f o r various discharge conditions, and byd e position rates of p o l y m e r s f o r m e d f r o m mixtures of hexafluoroethane and hydrogen. A c k n o w l e d g m ent s T h i s w o r k w a s supported i n p a r t b y the Office of W a t e r R e s e a r c h a n d T e c h n o l o g y o f t h e U . S . D e p a r t m e n t o f the I n t e r i o r u n d e r C o n t r a c t N o . 1 4 - 3 4 - 0 0 1 - 7 5 3 7 a n d t h e N a t i o n a l H e a r t , L u n g , a n d B l o o d Institute under Contract N o .

NIH-NOl-HV-3-2913.

References 1. Kay, Ε . , paper presented at International Round Table on Plasma Polymerization and Treatment, IUPAC Symp. on Plasma Chem., Limoges, France, July 13-19, 1977. 2. Clark, D. T. and Dilks, Α., Characterization of Metal on Polymer Surfaces, in "Polymer Surfaces", Academic Press, New York, 1977. 3. Wehner, G. K. and Anderson, G. S., in handbook of Thin Film Technology", Maissel, L. I. and Glang, R., eds., McGraw-Hill, New York, 1970. 4. Smolinsky, G., Bell Telephone Laboratories, private communica tion, August 1977. 5. Yasuda, H . , Marsh, H. C., Brandt, E. S.., and Reilley, C. Ν., J. Poly. Sci., Poly. Chem. Ed. (1977) 15 991. 6. Yasuda, Η., paper presented at International Round Table on Plasma Polymerization and Treatment, IUPAC Symp. on Plasma Chem., Limoges, France, July 13-19, 1977.


PLASMA

52

POLYMERIZATION

7. Yasuda, H . , Lamaze, C. E., and Sakaoku, K., J. Appl. Poly. Sci., (1973) 17, 137. 8. Yasuda, H. and Hirotsu, T., Surf. Sci. (1978) 76, 232.


9. Yasuda, H. and Hsu, T. S., J. Poly. Sci., Poly. Chem. Ed. (1977) 15, 2411. 10. Morosoff, N., Newton, W., and Yasuda, H., J. Vac. Sci. Tech. (March 1978) (submitted). 11. Yasuda, H. and Hsu, T. S., J. Poly. Sci., Poly. Chem. Ed. (1978) 16, 415. 12. Kobayashi, H . , Bell, A. T., and Shen, M . , J. Macromol. Sci. Chem. (1976) A10 (3), 491. 13. Yasuda, H. and Hirotsu, T., J. Poly. Sci., Poly. Chem. Ed. (1978) 16, 229. RECEIVED A p r i l 10, 1979.


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