Plasma Polymerization - American Chemical Society

11 Plasma Deposition of Fluorinated Compounds ATTILA Ε. PAVLATH and A L L E N G. PITTMAN

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: September 24, 1979 | doi: 10.1021/bk-1979-0108.ch011

Western Regional Research Center, Science and Education Administration, U.S. Department of Agriculture, Berkeley, CA 94710

The unusual surface p r o p e r t i e s o f p o l y ( t e t r a f l u o r o e t h y l e n e ) have i n i t i a t e d many attempts t o cover organic and i n o r g a n i c sur faces with f l u o r i n a t e d coatings possessing s i m i l a r characteristics ( l ) . For a number o f reasons, vapor phase d e p o s i t i o n could have p r a c t i c a l advantages. However, the p o l y m e r i z a t i o n o f most o f the h i g h l y f l u o r i n a t e d compounds, needed f o r d e s i r a b l e surface proper t i e s , i s q u i t e d i f f i c u l t and slow i n the vapor phase. Polymeric f i l m s and non-polymeric coatings can be obtained through vapor phase r e a c t i o n s i n an e l e c t r i c glow discharge {2) and t h e r e f o r e t h i s method appears t o be an e x c e l l e n t t o o l f o r preparing f l u o r i n e c o n t a i n i n g s u r f a c e s . In previous s t u d i e s , we were able t o o b t a i n f l u o r i n e c o n t a i n i n g f i l m s on glass even from monomers such as o c t a fluorobutene-2 (_3). T e t r a f l u o r o e t h y l e n e alone (k) or ethylene with fluorocarbons (5_,6) r e s u l t e d i n polymeric f i l m d e p o s i t i o n . Hexafluorοethane, a saturated and r e l a t i v e l y i n e r t f l u o r o c a r b o n , r e s u l t e d i n a f l u o r i n a t e d surface on wool (7.) · The adhesion o f these f i l m s t o substrate depends mostly on the chemical nature o f the s u b s t r a t e . Fluorocarbon deposited on an inorganic s u r f a c e , such as g l a s s , i s g e n e r a l l y e a s i l y removable with the a i d o f solvents and/ or s w e l l i n g agents, however, organic surfaces can y i e l d a f l u o r o carbon-substrate g r a f t . In t h i s paper, we r e p o r t the formation o f f l u o r i n a t e d surfaces from v a r i o u s saturated fluorocarhon i n a glow discharge and compare the r e s u l t s with those obtained u s i n g unsa t u r a t e d fluorocarbons. Experimental The glow discharge was created through e x t e r n a l c a p a c i t i v e c o u p l i n g . In order t o minimize polymer d e p o s i t i o n on the r e a c t o r w a l l , the f l u o r i n a t e d compounds were introduced i n the glow d i s charge immediately above the sample. F o r t h i s purpose a s p e c i f i c a l l y designed r e a c t o r was used (3.). Even so, a build-up o f a f l u o r i nated compounds was observed on the r e a c t o r w a l l s with c e r t a i n un saturated monomers. The discharge was created i n an argon flow. The radiofrequency ( r f ) generator (13.56 MHz) had a maximum c a p a c i t y This chapter not subject to U S copyright. Published 1979 American Chemical Society

In Plasma Polymerization; Shen, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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of 100 W, but the highest power a t t a i n a b l e i n the system i n the presence of a fluorocarbon was 70 W. Tfre lowest power s e t t i n g r e quired t o maintain a steady glow discharge was 10 W. The exposure time v a r i e d from 5 seconds to 5 minutes at 1 Hgmm pressure. Gases were introduced through a micrometer v a l v e to c o n t r o l the flow. L i q u i d f l u o r i n a t e d compounds were f i r s t v o l a t i z e d by bubbling a r e g u l a t e d flow of argon through them at room temperature. Compounds t e s t e d were p e r f l u o r o e t h y l e n e , -propylene and -butene-2, h e p t a f l u o r o i s o p r o p y l a l l y l e t h e r , p e r f l u o r o e t h a n e , a com m e r c i a l mixture of isomeric C y F ^ (b.p. = 60-70 ° ), hexafluoroacetone, CF3(CF2) CH 0H and e t h y l p e r f l u o r o b u t y r a t e . The isomeric perfluoroheptane mixture was manufactured by P i e r c e Co., others were obtained from r e g u l a r l a b o r a t o r y chemical o u t l e t s . Substrate m a t e r i a l s were g l a s s microscope s l i d e s , p o l y ( e t h y l e n e ) and p o l y e s t e r f i l m s , undyed wool f a b r i c . A l l substrates were cleaned by Soxhlet e x t r a c t i o n i n C2F3CI3, acetone and f i n a l l y r i n s i n g i n d i s t i l l e d water. The same procedure was used,when i n d i c a t e d , a f t e r glow discharge treatment. The a n a l y s i s of the surfaces was made u s i n g X-ray photoelectron spectroscopy (XPS) (DuPont 650 E l e c t r o n Spectrometer) and surface w e t t a b l i t y v i a the measurement o f the advancing l i q u i d contact angle. The XPS a n a l y s i s i n v o l v e d the de termination of the c a r b o n , f l u o r i n e and oxygen b i n d i n g energy s p e c t r a . The f l u o r i n e and oxygen s p e c t r a were simple, though some what wider than the average s i n g l e peak observed f o r model com pounds. The h a l f w i d t h was i n the range of 2 Λ - 2 . 6 i n s t e a d o f 2 . 3 2.k eV. The carbon Is spectrum was wide and complex i n each case, the b i n d i n g energy ranged from 296 t o 2Qk eV. The number o f major components and t h e i r approximate p o s i t i o n was determined by a r e c e n t l y developed method u s i n g the second d e r i v a t i v e of the s p e c t r a ( 8_). Using these data the carbon I s s p e c t r a were deconvo l v e d by a non-linear l e a s t square curve f i t t i n g program (9). Caution was a p p l i e d i n i n t e r p r e t i n g the q u a n t i t a t i v e r e s u l t s o f the deconvolution of such complec s p e c t r a , s i n c e small changes i n the c h a r a c t e r i s t i c s of the instrument, can cause considerable changes i n the c a l c u l a t e d r a t i o of the components. Curve f i t t i n g was a p p l i e d i n case of surface f l u o r i n a t i o n of p o l y ( e t h y l e n e ) t o i d e n t i f y s h i f t s of 0 . 5 - 1 . 0 eV ( l O ) . While the method could provide d e c e i v i n g l y good f i t s i n the case o f numerous and c l o s e peaks, c r i t i c a l e v a l u a t i o n could p o i n t t o c e r t a i n shortcomings which may i n v a l i d a t e the q u a n t i t a t i v e data. For t h i s reason, the presence of only f i v e c h a r a c t e r i s t i c carbon components was assumed i n t h i s a n a l y s i s : 2 8 5 . 0 - 2 8 5 . 5 , 2 8 7 . 0 - 2 8 8 . 0 , 2 8 9 . 0 - 2 9 0 . 0 , 2 9 1 . 5 - 2 9 2 . 0 and 293.2-29^.0 eV. The f i r s t b i n d i n g energy i s f o r the normal a l i p h a t i c carbon, while the l a s t three were assigned t o CF, CF and CF3 r e s p e c t i v e l y . The second range, between 287-0-288.0 eV, could be a j o i n t peak of C - 0 , C=0 and a quaternary carbon surrounded by four C F groups.


2

2

2

X

Surface w e t t a b i l i t y was determined by the measurement of ad vancing contact angles of hvdroeen bonding l i q u i d s (ethylene g l y c o l , formamide, g l y c e r o l and water) and non-hydrogen bonding


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P A V L A T H AND Ρ ί τ τ Μ Α Ν

liquids

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(decane, dodecane, tetradecane and hexadecane) ( l l ) .


Results and D i s c u s s i o n Unsaturated f l u o r i n a t e d compounds r e a d i l y polymerized when i n j e c t e d i n the glow discharge and a f i l m d e p o s i t i o n , g e n e r a l l y co l o r e d brown-yellow, was e q u a l l y v i s i b l e on a l l s u b s t r a t e s . Rapid polymer formation occurred with the h e p t a f l u o r o i s o p r o p y l a l l y l e t h e r monomer. Other o l e f i n s with f l u o r i n e atoms on the double bond (perf l u o r o e t h y l e n e , -propylene and - b u t e n e - 2 ) , polymerized more slowly, but s t i l l f a s t enough t o o b t a i n substrate coverage at an exposure time as short as 5 seconds. While v a r y i n g the r e a c t i o n time from 5 seconds t o 5 m i n u t e s , i t was found t h a t the f l u o r i n e t o carbon r a t i o on a p o l y ( e t h y l e n e ) surface was 1.2:1 t o 1.5:1 a f t e r a 15 second treatment, and t h a t a d d i t i o n a l exposure d i d not s i g n i f i c a n t l y change t h i s r a t i o . S i m i l a r l y , t h e advancing contact angles o f hydrophobic and h y d r o p h i l i c l i q u i d s d i d not s i g n i f i c a n t l y change a f t e r 15 seconds of fluorocarbon-plasma treatment. XPS-analysis o f surface exposed t o the p e r f l u o r i n a t e d monomers showed the presence of a l l p o s s i b l e C F species r e g a r d l e s s o f mono mer s t r u c t u r e . F i g . 1 shows the deconvoluted carbon Is XPS spectrum of a p o l y ( e t h y l e n e ) surface a f t e r exposure o f hexafluoropropylene i n a glow discharge. A 1:1:1 r a t i o of the CF3, C F and CF groups was found suggesting the s t r u c t u r e of a normal a d d i t i o n polymer. The evidence f o r fragmentation was more obvious with the t e t r a f l u oroethylene monomer. F i g . 2 i s a carbon Is XPS spectrum obtained a f t e r exposing wool t o CF2=CF2 i n a glow discharge. The same three groups were present although not i n a 1:1:1 r a t i o . Difluoromethylene was, as expected, the major component. We observed that as the power and exposure time were i n c r e a s e d , fragmentation i n c r e a s e d as d i d the oxygen c o n c e n t r a t i o n . The appearance of oxygen i s a t t r i b u t e d t o the formation of l o n g - l i f e f r e e r a d i c a l s on the s u r f a c e , and t h e i r r e a c t i o n with oxygen a f t e r t h e i r removal from the r e a c t o r chamber. The f i l m s deposited i n v a r i o u s substrates a f t e r plasma t r e a t ment i n the presence o f p e r f l u o r i n a t e d o l e f i n s were subject t o p a r t i a l removal w i t h e x t r a c t i o n . G e n e r a l l y , a f t e r e x t r a c t i o n , the f l u o r i n e t o carbon r a t i o , determined through XPS a n a l y s i s , decrea sed t o 0.2:1 t o 0.3:1 r e g a r d l e s s of the a p p l i e d power and residence time used during the formation o f the f i l m . In a l l cases a moderate but c o n s i s t e n t amount o f oxygen was detected i n the f l u o r o c a r b o n f i l m . The oxygen t o carbon r a t i o 0.12:1 t o 0.1*1:1 before e x t r a c t i o n and 0 . 0 8 : 1 t o 0.1:1 after extraction. Saturated fluorocarbons do not polymerize under normal c o n d i t i o n s , although t h e i r p y r o l y s i s at h i g h temperatures (500-800 ) i s well-known. However, they can e x h i b i t s u r p r i s i n g r e a c t i v i t y i n a glow discharge. The r a t e o f polymer d e p o s i t i o n decreased with i n c r e a s i n g number of f l u o r i n e atoms i n unsaturated compounds. However the r a t e of f l u o r i n a t e d f i l m d e p o s i t i o n i n c r e a s e d w i t h i n c r e a s i n g f l u o r i n e content i n s a t u r a t e d compounds. The presence o f C-H i n t e r X

2

1


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BINDING ENERGY (EV) Figure

1. C(ls) XPS spectrum of hexafluoropropylene deposit Power, 50 W; time, 1 min. Dotted line is the calculated

xlO

on polyethylene. composite.

4

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2.

C (Is) XPS spectrum of tetrafluoroethylene deposit on wool. 50 W; time, 1 min. Dotted line is the calculated composite.


Power,

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P A V L A T H AND p i T T M A N

Deposition

of

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185

f e r e d with the formation of fluorocarbon r a d i c a l s necessary f o r f i l m formation. For example, when the alcohol,CF3(CF ) CH 0H,was i n j e c t e d i n t o a glow discharge, no fluorocarbon d e p o s i t i o n was de t e c t a b l e by XPS-analysis. "When the e s t e r , C F 3 ( C F 2 ) 2 2 5 > used the d e p o s i t i o n of a brown f i l m occurred, but the amount of f l u o r i n e was very low even before e x t r a c t i o n (F:C = 0.1:1 t o o.2:l) Presumably, the e s t e r decomposes t o form 0 Η^· r a d i c a l s or ethylene which i s known t o polymerize r a p i d l y i n glow discharge (12). The i n i t i a l concentration of fluorocarbon deposited on the various substrates was l e s s a f t e r glow discharge exposure with hexafluoroethane than a f t e r exposure with t e t r a f l u o r o e t h y l e n e . For example, the F:C r a t i o ranged θΛ:1 t o 0.55:1 on wool and 0.6:1 t o 0.8:1 on p o l y ( e t h y l e n e ) and p o l y e s t e r surface f o l l o w i n g exposure to hexafluoroethane, compared t o about 1.2-1.5*1 a f t e r t e t r a f l u o roethylene treatment. However, successive e x t r a c t i o n s with 0^3013, acetone and water d i d not remove as much of the fluorocarbon which was deposited using hexafluoroethane, as i t d i d with the f l u o r o carbon deposited using t e t r a f l u o r o e t h y l e n e i n the glow discharge. The F:C r a t i o , f o r exaple, of p o l y ( e t h y l e n e ) t r e a t e d with h e x a f l u oroethane was 0.U:1 t o 0.55:1 ( f o l l o w i n g e x t r a c t i o n ) compared t o the values of 0.2:1 t o 0.3:1 f o l l o w i n g e x t r a c t i o n of a t e t r a f l u o r o ethylene t r e a t e d s u r f a c e . F i g . 3 and h show the deconvoluted carbon Is XPS spectrum of a p o l y ( e t h y l e n e ) sample t r e a t e d i n a glow d i s charge with hexafluoroethane before and a f t e r e x t r a c t i o n respec t i v e l y . I t can be seen t h a t e x t r a c t i o n decreases the CF3 s i g n a l very l i t t l e compared t o the s i g n a l s of the other c a r b o n - f l u o r i n e groups. We assume that CF3 groups are introduced through d i r e c t coupling of CF3 r a d i c a l s with the substrate surface and are thus r e s i s t a n t to e x t r a c t i o n procedures. S i m i l a r t o the f i n d i n g s with unsaturated f l u o r o c a r b o n s , i n c r e a s i n g the r f power or extending the residence time d i d not have s i g n i f i c a n t e f f e c t on the F:C r a t i o . An increase i n the 0:C r a t i o d i d occur with i n c r e a s i n g power or residence time with both s a t u r a t e d and unsaturated f l u o r o carbons. 2

C00C

2

2

H

v

a

s


2

When a commercially a v a i l a b l e mixture of perfluoroheptane i s o mers was used i n s t e a d of perfluoroethane, the amount of f l u o r i n e detected by XPS a n a l y s i s was much higher. The F:C r a t i o was 0.9:1 t o 1:1 f o r wool and ΐ Λ : 1 t o 1.5:1 f o r s y n t h e t i c polymer substrates before e x t r a c t i o n . F i g . 5 and 6 show the deconvoluted carbon Is s p e c t r a f o r both examples. E x t r a c t i o n reduced the f l u o r i n e content uniformly by 25-h0% The F:C values a f t e r e x t r a c t i o n are higher than f o r hexafluoroethane. F agmentation o f perfluoroheptane i s o mers can create a wide v a r i e t y of p e r f l u o r o a l k y l r a d i c a l s which may couple with the s u r f a c e . Hexafluoroacetone i s not known t o polymerize by a f r e e r a d i c a l mechanism, however, i t r e a c t s r e a d i l y with n u c l e o p h i l i c reactants and can form copolymers v i a n u c l e o p h i l i c intermediates (13.) · Many of i t s d e r i v a t i v e s create surfaces with low c r i t i c a l surface t e n sions (l). I t was t h e o r i z e d that i n a m i l d glow discharge r e a c t i o n , the carbonyl group could be a c t i v a t e d and the C3Fg0 group might


PLASMA


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BINDING ENERGY (EV)

Figure 3. C(ls) XPS spectrum of hexafluoroethane fore extraction. Power, 50 W; time, 1 min. Dotted

xlO

deposit on polyethylene beline the calculated composite.

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4.

Same as Figure

3, after

extraction


PAVLATH

AND ρ ι τ τ Μ Α Ν

Deposition

of

Fluorinated

292

288

Compounds


xlO*

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5. C (Is) XPS spectrum of perfluoroheptane deposit Power, 50 W; time, 1 min. Dotted line is the calculated

Figure

6.

Same as Figure

on polyethylene. composite.

5, on wool


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P L A S M A POLYMERIZATION

188

become attached t o the surface without fragmentation. The presence of f l u o r i n e was r e a d i l y detected a f t e r a 30 second exposure o f a l l substrates a t 25W power. Deconvolution o f t h e carbon I s XPS spec trum, however, r e v e a l e d t h a t the fragmentation o f hexafluoroacetone i n glow discharge i s s i m i l a r t o that o f hexafluoroethane. F i g . 7 and 8 show the spectrum before and a f t e r e x t r a c t i o n f o r the deposi t i o n o f hexafluoroacetone obtained on polyethylene. E x t r a c t i o n e l i minated some o f the fluorocarbon d e p o s i t i o n , decreasing the F:C r a t i o from 0·9-1·ο:1 t o 0.5-0.6:1. I n t e r e s t i n g l y , the e x t r a c t i o n e l i m i n a t e d a l a r g e r percentage o f the CF3 group than that o f other C F groups. This could i n d i c a t e that some o f the hexafluoroacetone might have been deposited on the surface i n some non-fragmented but e a s i l y removable form. The degree o f surface coverage and w e t t a b i l i t y o f t h e f l u o r o carbon d e p o s i t i o n s were examined by l i q u i d contact angle measure ment. When the cosine o f these contact angles was p l o t t e d against the surface t e n s i o n o f the t e s t l i q u i d s , a s t r a i g h t l i n e p l o t was obtained f o r the hydrogen bonding and non-hydrogen bonding l i q u i d s s e p a r a t e l y . Due t o the high e l e c t r o n e g a t i v i t y o f the f l u o r i n e atom i t has a quenching e f f e c t on discharges a c t i n g as an e l e c t r o n s i n k . In order t o keep the c h a r a c t e r i s t i c s o f the glow discharge as s i m i l a r as p o s s i b l e , hexafluoropropylene,hexafluoroacetone and hexafluoroethane were s e l e c t e d f o r comparative study s i n c e they con t a i n the same number o f f l u o r i n e atoms. I t i s evident from F i g . 9 that the hydrophobic character o f the p o l y ( e t h y l e n e ) surface i n creased a f t e r exposure t o these fluorocarbons i n glow discharge, s i n c e i n c r e a s i n g contact angles, i . e . small or negative cosine v a l u e s , i n d i c a t e d higher water r e p e l l e n c e . However, e x t r a c t i o n e l i minated t h i s improvement except f o r the f i l m obtained f o r h e x a f l u oroethane. This suggests t h a t the g r a f t i n g was more prevalent with hexafluoroethane than w i t h the other two compounds. I n order t o evaluate the e f f e c t o f the f l u o r i n e d e p o s i t i o n we must take i n t o c o n s i d e r a t i o n the f a c t that a glow discharge alone w i l l a l s o have an e f f e c t on the surface w e t t a b i l i t y . F i g . 11 i l l u s t r a t e s t h a t the hydrophylic nature o f p o l y ( e t h y l e n e ) i s much higher a f t e r glow discharge treatment. Increased h y d r o p h y l i c i t y i s b e l i e v e d t o be the r e s u l t o f f r e e r a d i c a l formation on t h e p o l y ( e t h y l e n e ) surface during glow discharge treatment, followed by o x i d a t i o n when the surface i s exposed t o a i r . The w e t t a b i l i t y o f plasma deposited coatings can be compared with p o l y ( t e t r a f l u o r o e t h y l e n e ) and p o l y ( e t h y l e n e ) . When t h e cosine of contact angles obtained with non-hydrogen bonding l i q u i d s are p l o t t e d against t h e i r surface t e n s i o n , e x t r a p o l a t i o n o f t h e r e s u l t ing s t r a i g h t l i n e t o cos 0=1 y i e l d s a term c a l l e d the c r i t i c a l sur face t e n s i o n (CST) which i s c h a r a c t e r i s t i c o f that surface ( l l ) . CST i s expressed i n terms o f the surface t e n s i o n o f a l i q u i d which j u s t wets the s u r f a c e , i . e . has a 0 contact angle. F i g . 12 gives CST p l o t s f o r t h e same surfaces shown i n F i g . 9 and 10 "before and a f t e r e x t r a c t i o n . I t can be seen that a l l f i l m s have lower CST values than p o l y ( t e t r a f l u o r o e t h y l e n e ) "before e x t r a c t i o n , but a f t e r


X


P A V L A T H AND

xlO


Deposition

piTTMAN

of

Fluorinated

Compounds

189

4

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BINDING ENERGY (EV) Figure 7. C(ls) XPS spectrum of hexafluoroacetone deposit on polyethylene before extraction. Power, 50 W; time, 1 min. Dotted line is the calculated composite.

xlO

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8.

Same as Figure

7, after

extraction


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Plasma Polymerization - American Chemical Society

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