Surface Characterization of Plasma-Fluorinated Polymers - ACS

21 Surface Characterization of Plasma-Fluorinated Polymers M. ANAND, R. E. COHEN, and R. F. BADDOUR

Downloaded by CORNELL UNIV on June 7, 2017 | http://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch021

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

Previous Work. In previous publications (1, 2, 3) we have reported on the surface fluorination of low density polyethylene using a low pressure atmosphere of a dilute mixture of fluorine in helium and using a cold plasma generated from this same gas mixture. It was found that in the glow discharge, perfluorination of the surface was accomplished readily, whereas with elemental fluorine treatment, the reaction was slow and did not lead to complete surface fluorination under the conditions employed (1). In dilute elemental fluorine reactions, several partially fluorinated species, such as CR -CHF, CHF-CHF, were formed on the surface as determined by X-ray photoelectron spectroscopy measurements (XPS), while in the case of plasma treatment, largelyCF type groups were generated. Presence of CF groups on the surface was also shown by Fourier Transform Multiple Internal Reflectance (FTMIR) infrared studies and by advancing contact angle measurements. Previous experiments also showed that, for reaction conditions under which complete surface fluorination was not obtained, a post-reaction with oxygen and oxygen-containing species (largely moisture from the air) occurred, resulting in large oxygen signals in XPS measurements and in sharp decreases in contact angles (1, 3). However, for samples which were fully fluorinated at the surface, very small oxygen levels were found, and this favorable condition persisted indefinitely. No reaction or post-reaction with oxygen or water is observed for reactions that go to completion. In previous fluorination studies in which elemental fluorine was employed, several oxygen-containing species including acylfluoride groups, carboxyl and hydroxyl groups, and hydrogen-bonded water have been found in the surface and sub-surface layers (4, 5, 6, 7). Reaction conditions which influence the fluorination of LDPE in the fluorine plasma have also been discussed previously (2). High pressures and flow rates favored the fluorination reaction kinetics. There was a competition between ion-assisted etching 2

2

2

0097-6156/81/0162-0353$05.00/0 © 1981 American Chemical Society Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

354

P H O T O N , E L E C T R O N , A N D ION PROBES


and c h e m i c a l r e a c t i o n r e s u l t i n g i n s m a l l (~40A) d e p t h s o f f l u o r ination. H o w e v e r , t h e d e p t h o f f l u o r i n a t i o n was i n c r e a s e d i n r e a c t i o n s c a r r i e d out under i o n - d e p l e t e d c o n d i t i o n s . T h i s was a c h i e v e d a t the e x p e n s e o f t h e r e a c t i o n r a t e w h i c h was r e d u c e d by a f a c t o r o f a b o u t two (_3). E v e n u n d e r t h e s e r e a c t i o n c o n d i t i o n s , an u p p e r l i m i t on t h e d e p t h o f f l u o r i n a t i o n (~60A) was o b s e r v e d p o s s i b l y due t o a d i f f u s i o n - c o n t r o l l e d r e a c t i o n . P r e s e n t Work. I n t h i s p a p e r we r e p o r t the r e s u l t s of r e a c t i o n s c a r r i e d out under c o n d i t i o n s o f h i g h e r f l u o r i n e c o n c e n t r a t i o n s w i t h t h e aim o f i n c r e a s i n g t h e d e p t h o f f l u o r i n a t i o n . R e s u l t s for r e a c t i o n s w i t h s e v e r a l other polymers are a l s o r e p o r t e d . F u r t h e r , a d i s c u s s i o n o f t h e f l u o r i n a t i o n o f LDPE powders i n a f l u i d i z e d b e d r e a c t o r and t h e m o l d i n g p r o p e r t i e s o f t h e s e powders i s a l s o i n c l u d e d . The a p p a r a t u s b e i n g u s e d t o t r e a t p o l y m e r f i l m s i s shown i n F i g . 1 ( a ) and f o r t r e a t m e n t o f powders i n F i g . K b ) . The s u r f a c e a n a l y t i c a l t o o l s employed f o r c h a r a c t e r i z a t o n a r e X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y ( ~ 5 0 A ) , FTMIR ( 10000A) and c o n t a c t a n g l e measurements ( f i r s t m o l e c u l a r l a y e r ) , t h e d e t a i l s o f w h i c h h a v e been p r e v i o u s l y d e s c r i b e d ( 3 ) . I n the r e s u l t s b e i n g p r e s e n t e d h e r e , 5% and 15% p r e m i x t u r e s o f f l u o r i n e w i t h h e l i u m a r e u s e d i n the f l u o r i n e glow d i s c h a r g e . The g a s e s , o b t a i n e d from M a t h e s o n G a s , w e r e p a s s e d t h r o u g h a s o d i u m f l u o r i d e c o l u m n t o remove t r a c e amounts o f h y d r o g e n f l u o r ide. No e f f o r t was made t o remove s m a l l amounts o f w a t e r and o x y g e n t h a t may have been p r e s e n t i n t h e i n c o m i n g g a s e s . The p o l y m e r f i l m s were t r e a t e d a t t h e p l a s m a b u l k gas t e m p e r a t u r e w h i c h was c l o s e t o a m b i e n t f o r a l l r e a c t i o n s . Visual observation o f t h e f i l m s d i d n o t show any m e l t i n g o r d e f o r m a t i o n and hence i t was assumed t h a t i f the s u r f a c e temperatures d i d v a r y , the f l u c t u a t i o n s were s m a l l . R e s u l t s and D i s c u s s i o n R e a c t i o n s Under I o n - S h i e l d i n g C o n d i t i o n s . F l u o r i n e - c o n t a i n i n g p l a s m a s a r e known t o c a u s e e t c h i n g i n b o t h o r g a n i c and i n o r g a n i c s y s t e m s ( 8 , 9 , 1 0 , 11). I n the case o f h y d r o c a r b o n p o l y m e r s , i o n a s s i s t e d e t c h i n g i s a p p a r e n t l y c a u s e d by f l u o r i n e i o n s s u c h as , F„ , F i n the plasma. Our e x p e r i m e n t s i n t h e g l o w r e g i o n o f t h e p l a s m a have c l e a r l y shown e t c h i n g t a k i n g p l a c e as e v i d e n c e d by the a p p a r e n t u p p e r l i m i t on d e p t h o f f l u o r i n a t i o n (3). The amount o f e t c h i n g c o u l d be r e d u c e d by e l i m i n a t i o n o f i o n s f r o m the r e a c t i o n z o n e . T h i s c a n be a c h i e v e d e i t h e r by i s o l a t i n g t h e samples from t h e p l a s m a by e n c l o s i n g them i n a F a r a d a y cage ( a n e l e c t r i c a l l y g r o u n d e d m e t a l s c r e e n ) i n s i d e t h e g l o w r e g i o n o r by c a r r y i n g out t h e r e a c t i o n s i n t h e d a r k r e g i o n d o w n s t r e a m o f the p l a s m a . I n o u r s t u d y we have u s e d a m o d i f i e d F a r a d a y cage i n w h i c h t h e m e t a l s c r e e n was n o t g r o u n d e d . The m e t a l a c t s as a s i n k f o r t h e i o n s and p e r m i t s o n l y f r e e r a d i c a l s

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ANAND E T A L .

Plasma-Fluorinated Polymers

TO FLUORINE TRAP, VACUUM PUMP AND EXHAUST


GASKET 8 PLATE

VACUUM — I 1_ GAUGE L H

QUARTZ SAMPLE HOLDER

MASS FLOW METER FLUORINE/ HELIUM PREMIXTURE

RADIO-FREQUENCY GENERATOR

TO FLUORINE TRAP, VACUUM PUMP AND EXHAUST

POWDERS FOR TREATMENT FRITTED GLASS SUPPORT VACUUM GAUGE MASS FLOW METER I FLUORINE/HELIUM PREMIXTURE

Figure 1. Schematic of experimental setup: (a) reactor for film treatment; (b) fiuidized bed for treatment of powders


356

PHOTON, ELECTRON,

A N D ION PROBES


w i t h mean f r e e p a t h s s m a l l e r t h a n t h e s c r e e n o p e n i n g i n t o t h e r e a c t i o n zone. T h i s was c l e a r l y o b s e r v e d by a g l o w o u t s i d e t h e screen but a dark r e g i o n i n s i d e i t . I n the r e s u l t s p u b l i s h e d e a r l i e r (_3 ) , we showed t h a t w i t h F ^ / H e m i x t u r e s c o n t a i n i n g 5% f l u o r i n e , t h e d e p t h o f f l u o r i n a t i o n was i n c r e a s e d from 40A i n t h e c a s e o f r e a c t i o n i n t h e g l o w t o a b o u t 60A f o r r e a c t i o n s i n t h e d a r k r e g i o n . H o w e v e r , we c o u l d n o t a c h i e v e t r e a t m e n t s t o d e p t h s much beyond 60A p o s s i b l y due t o inadequate c o n c e n t r a t i o n o f f l u o r i n e s p e c i e s at the s o l i d subs t r a t e or due t o e x t r e m e l y s l o w p r o p a g a t i o n r e a c t i o n i n t o t h e bulk. To t e s t t h i s h y p o t h e s i s , e x p e r i m e n t s were c a r r i e d o u t w i t h a F ^ / H e m i x t u r e c o n t a i n i n g 15% F ^ i n t h e d a r k r e g i o n i n s i d e t h e metal cage. The r e s u l t s o f t h e s e e x p e r i m e n t s a r e shown i n F i g u r e s 2 , 3 , 4 , 5 and T a b l e s 1 and 2 . TABLE

I

DEPTH OF FLUORINATION FOR VARIOUS PLASMA TREATMENTS Conditions

Time Of T r e a t m e n t , m i n

3 . 0 mm, 40 c c / m i n , 50W A l u m i n u m Cage (5% F )

2 5 15 60

30 38 >60 -60

3 . 0 mm, 40 c c / m i n , 50W No Cage (5% F )

1 2 60

22 30 35

3 . 0 mm, 40 c c / m i n , 50W A l u m i n u m Cage (15% F )

2 5 15

>60 ^60 >60

2

2

Depth, A

T a b l e 1 compares d e p t h s o f f l u o r i n a t i o n c a l c u l a t e d ( 4 ) f r o m XPS measurements f o r r e a c t i o n s i n t h e g l o w and i n t h e d a r k r e g i o n w i t h b o t h 5% and 15% F ^ p r e m i x t u r e s . I n t h e c a s e o f 15% f l u o r i n e , t h e d e p t h o f f l u o r i n a t i o n i s b e y o n d t h a t w h i c h c a n be measured by XPS e v e n a f t e r o n l y 2 m i n u t e s o f t r e a t m e n t . An e s t i m a t e o f the d e p t h f l u o r i n a t i o n was o b t a i n e d by a s s u m i n g t h a t t h e i n c r e a s e i n w e i g h t o f t h e t r e a t e d p o l y m e r was due t o s u b s t i t u t i o n o f h y d r o g e n s i n t h e p o l y m e r by f l u o r i n e . The r e s u l t s o f t h e s e c a l c u l a t i o n s a r e shown i n T a b l e 2 . The e x a c t v a l u e s o f the d e p t h s o f f l u o r i n a t i o n must be v i e w e d c a u t i o u s l y b e c a u s e o f c o m p l i c a t i n g f a c t o r s s u c h as a d s o r p t i o n and c h e m i s o r p t i o n ( e s p e c i a l l y f o r p o l y m e r s n o t p e r f l u o r i n a t e d ) o f w a t e r on t h e s u r f a c e i n t h e t i m e b e t w e e n t a k i n g t h e sample o u t o f t h e r e a c t o r and w e i g h i n g i t . However, t r e n d s a r e c l e a r e s p e c i a l l y f o r t h e c a s e o f 15% F t r e a t m e n t where l a r g e ?


21.

357


ANAND E T A L .

i n c r e a s e s i n weight are observed. These r e s u l t s s u g g e s t much g r e a t e r depths o f f l u o r i n a t i o n w i t h i n c r e a s e d treatment time, s u p p o r t i n g the h y p o t h e s i s t h a t o b s e r v e d l i m i t a t i o n s i n t h e d e p t h o f f l u o r i n a t i o n a r i s e from a d i f f u s i o n c o n t r o l l e d r e a c t i o n . A d d i t i o n a l evidence i n support of t h i s increase i n f l u o r i n a t i o n d e p t h w i t h t r e a t m e n t t i m e i s p r o v i d e d by t h e o b s e r v e d i n c r e a s e w i t h t i m e i n t h e i n t e n s i t y o f t h e i n f r a r e d band c o r r e s p o n d i n g t o CF^ s p e c i e s , as shown i n F i g . ( 2 ) . TABLE

II

DEPTH OF FLUORINATION ESTIMATED FROM WEIGHT


Reaction conditions:

3 . 0 mm, 40 c c / m i n ,

INCREASES

50W, A l u m i n u m Cage

Time o f Treatment

% of Increase 5%F , 2

95%He

i n Weight

Depth o f F l u o r i n a t i o n , A

1 5 % F , 85%He

5%F

2

2>

95%He

1 5 % F , 85%He 2

2 min

0.136

0.113

235

195

5 min

0.096

0.170

160

290

15 m i n

0.155

0.276

260

475

The i n c r e a s e i n c o n c e n t r a t i o n o f f l u o r i n e from a 5% t o a 15% m i x t u r e s h o u l d a c c e l e r a t e the r e a c t i o n because o f i n c r e a s e d r e a c t i v e species a v a i l a b l e for r e a c t i o n . T h i s i s c l e a r l y seen i n F i g . (3) i n the p l o t o f c o n t a c t angle a g a i n s t time o f t r e a t m e n t . C u r v e (b) f o r t r e a t m e n t w i t h 15% F s u g g e s t s t h a t the surface l a y e r i s p e r f l u o r i n a t e d ( f o r m a m i d e has an a d v a n c i n g c o n t a c t a n g l e o f 92 w i t h PTFE) w i t h i n 5 m i n u t e s o f t r e a t m e n t , w h i c h i s n o t t h e c a s e f o r t r e a t m e n t w i t h t h e 5% f l u o r i n e p l a s m a t r e a t m e n t . H i g h e r o x y g e n c o n t e n t i s a l s o s e e n f o r the 5% F~ p l a s m a at 5 m i n u t e treatment t i m e , s u g g e s t i n g a case i n which the s u r f a c e i s not f u l l y f l u o r i n a t e d and i s s u s c e p t i b l e t o c h e m i s o r p t i o n o f w a t e r . However, F i g . 4 , a p l o t o f atomic p e r c e n t f l u o r i n e v s . time o f t r e a t m e n t does n o t s u g g e s t a f a s t e r r e a c t i o n w i t h the 15% F mixture. T h i s r e s u l t may be e x p l a i n e d i f t h e a c t i v e f l u o r i n e f i r s t a t t a c k s the amorphous z o n e s o f the p o l y e t h y l e n e and t h e r e a c t i o n p r o p a g a t e s b e y o n d t h e f i r s t 60A b e f o r e t h e c r y s t a l l i n e r e g i o n s are f l u o r i n a t e d . I n s u c h a c a s e , the XPS s i g n a l s f r o m F , and F w o u l d s u g g e s t a d e p t h o f f l u o r i n a t i o n o f g r e a t e r t h a n 60A b e c a u s e the c a l c u l a t i o n assumes a homogeneous m a t e r i a l . Such a s i t u a t i o n p o s s i b l y e x i s t s i n o u r c a s e b e c a u s e t h e C ^ XPS s p e c t r a o f F i g . 5 c l e a r l y show a r e s i d u a l C H s i g n a l f o r t h e c a s e o f t h e 2 minute treatment. The r e a c t i o n i n t h e g l o w r e g i o n o f t h e p l a s m a i s c l e a r l y f a s t e r t h a n i n t h e d a r k r e g i o n o f t h e p l a s m a as s e e n 2

2

2

g

g

?


358

PHOTON,

E L E C T R O N , A N D ION

PROBES


A

1600

1400

1200

1000

Wave number cm

800

600

-1

Figure 2. FTMIR spectra of LDPE treated with 15% fluorine mixture with helium under ion-depleted conditions: (A) 2-min, (B) 5-min, and (C) 15-min treatments. Reaction conditions: 40 cc/min, 3.0 mm, 50 W; spectra at 45° incidence.



21.

ANAND E T A L .

0

Plasma-FHuoiincited Polymers

1 2

3 4

5 6 7 8 9 10 II 12 13 14 15 TIME OF TREATMENT, MIN

359

60

Figure 3. Variation of contact angle and atomic percent oxygen as a function of time of treatment (YH, \J) 5% F , no cage; (A, A) 5% F , aluminum cage; (%, O) 15% F, aluminum cage; reaction conditions: 3.0 mm, 40 cc/min, 50 W). 2

2

Figure 4. Build-up of fluorine on the surface of LDPE with treatment time (({J) 5% F , no cage; (A) 5% F , aluminum cage; (O) 15% F , aluminum cage; reaction conditions: 3.0 mm, 40 cc/min, 50 W). 2

2

2


PHOTON,

ELECTRON,

A N D ION PROBES


PLASMA TREATMENT UNDER ION-DEPLETED CONDITIONS

i 1i i i i 1 i i i i 1 i i i iI 295 290 285 2 80 BINDING ENERGY, eV

Figure 5. The C-ls XPS spectra for LDPE film treated with 5% and 15% mixtures of fluorine in helium. Reactions carried out in the dark region of the plasma.


21.

ANAND E T A L .

Plasma-Fluorincited Polymers

361

from b o t h F i g u r e s ( 3 ) and ( 4 ) and from t h e XPS s p e c t r a r e p o r t e d e a r l i e r (1_, _3). Another important f e a t u r e o f the r e a c t i o n s i n the i o n d e p l e t e d zone i s t h a t " C F ^ t y p e g r o u p s a r e formed p r e d o m i n a n t l y as i n t h e c a s e o f r e a c t i o n s i n the g l o w r e g i o n o f t h e p l a s m a . This i s seen by the c h e m i c a l s h i f t or a b o u t 6eV f r o m t h e o r i g i n a l C H C H - peak i n t h e XPS s p e c t r a ( F i g . 5 ) . The d e t a i l s o f s u c h assignments have b e e n d i s c u s s e d e a r l i e r (_3). This is very different from r e a c t i o n w i t h e l e m e n t a l f l u o r i n e ( 3 , 4) where s e v e r a l p a r t i a l l y f l u o r i n a t e d s p e c i e s a r e formed b e f o r e g e n e r a t i o n of - C F groups. B a s e d on XPS d a t a , i t a p p e a r s t h a t t h e - C F g r o u p s formed i n t h e c a s e o f t r e a t m e n t w i t h and w i t h o u t t h e m e t a l screen are i d e n t i c a l . H o w e v e r , FTMIR s p e c t r a r e v e a l t h a t t h e r e a r e d i f f e r e n c e s i n t h e t y p e o f ~ C F g r o u p s as shown i n F i g s . ( 2 ) and ( 6 ) . The p r e d o m i n a n t ~CF« s p e c i e s formed from t r e a t m e n t i n t h e a l u m i n u m cage appear^ a t 1230 cm as o p p o s e d t o b u i l d u p o f C F s p e c i e s a t 1090 cm f o r treatment i n the glow r e g i o n . The bands a s s o c i a t e d w i t h f l u o r i n e - c o n t a i n i n g s j x e c i e s a p p e a r i n o u r s p e c t r a i n t h e rai^ge o f 1000 cm t o 1300 cm . The b a n d s a t 1090 cm and 1250 cm are p o s s i b l y a s s o c i a t e d w i t h - C F symmetrical ( E , ) and - C F a s s y m e t r i c a l ( E ^ ) s t r e t c h e s r e s p e c t i v e l y (12^, 1 3 , 14;. (The band due t o ~ C F s y m m e t r i c a l s t r e t c h i s a c t u a l l y r e p o r t e d t o o c c u r a t 1141 cm ~1 (_12., J ^ , 14) b u t i t seems t o m i x w i t h FCF b e n d , CC s t r e t c h and CCC bend as a c o n s e q u e n c e o f w h i c h t h e p o s i t i o n m i g h t be s h i f t e d t o a l o w e r v a l u e . F l u o r i n e c o n t a i n i n g CF u n s a t u r a t e ^ and CF s a t u r a t e s a r e a l s o known t o o c c u r i n t h e r a n g e o f 1000 cm - 1200 cm . I t i s p o s s i b l e t h a t i n t h e d e p t h b e i n g s a m p l e d , some o f t h e s e g r o u p s maybe p r e s e n t e s p e c i a l l y i f t h e r e i s a g r a d i e n t i n t h e p o l y m e r c o m p o s i t i o n from a c o m p l e t e l y C H - C H ^ - s t a t e i n t h e b u l k t o a c o m p l e t e l y - C F - C F - s t a t e on the surface). This suggests subtle differences i n the two f l u o r i n a t e d polymers. F u r t h e r , i n f r a r e d s p e c t r a r e v e a l t h a t i n the case o f plasma t r e a t m e n t i n t h e g l o w , t h e r e i s some carbon-carbon_unsaturation p r e s e n t on t h e s u r f a c e e v i d e n c e d by band a t 890 cm (Fig. 6). These a r e n o t p r e s e n t i n t h e c a s e o f t r e a t m e n t i n t h e d a r k r e g i o n o f the p l a s m a . A l s o , t h e amorphous n a t u r e o f t h e f l u o j i n a t e d p o l y m e r l a y e r i s e x h i b i t e d by t h e s m a l l band a r o u n d 740 cm (12). 2

2

2

2


2

2

2

2

2

2

2

2

F l u o r i n a t i o n o f Other Polymers Surface f l u o r i n a t i o n of s e v e r a l polymers other than p o l y e t h y l e n e has b e e n a t t e m p t e d t o show t h e e f f e c t i v e n e s s o f t h i s technique i n generating perfluorinated surfaces. Results of formamide c o n t a c t a n g l e e x p e r i m e n t s a r e s u m m a r i z e d i n T a b l e ( 3 ) . I t appears from these r e s u l t s t h a t under the c o n d i t i o n s s t u d i e d , p o l y p r o p y l e n e and n y l o n 6 , 6 c a n be p e r f l u o r i n a t e d b u t p o l y m e t h y l m e t h a c r y l a t e does n o t p e r f l u o r i n a t e i n t h e t i m e s c a l e shown. The C, XPS s p e c t r a and t h e r e s p e c t i v e c h e m i c a l c o m p o s i t i o n s are shown i n F i g u r e ( 7 ) and T a b l e ( 3 ) . E v e n t h o u g h PMMA shows h i g h


362

PHOTON,


UNTREATED LDPE

ELECTRON,

A N D ION PROBES

FLOURINE PLASMA 5 min

1 1 J L . 2000 1800 1600 1400 1200 1000 800 600 2000 1800 1600 1400 1200 1000 800 600 2000 1800 1600 1400 1200 1000 800 600

WAVENUMBER , cm"

1

5 MIN DARK REGION

J I I I 1600 1400 1200 1000 800 600 cm" 1

Figure 6.

1600 1400 1200 IOOO 800 600 cm" 1

( 6 0

o 1400 1200 1000 800 600 cm -1

FTMIR spectra comparing treatments in the glow and the dark region of the plasma (spectra at 30° incidence)


21.

ANAND E T A L .

Plasma-Fluorincited Polymers

363

f l u o r i n e contents and r e a s o n a b l e d e p t h s o f f l u o r i n a t i o n , the c o n t a c t a n g l e w i t h formamide i s l o w , s u g g e s t i n g t h a t t h e o x y g e n must r e s i d e i n t h e f i r s t m o l e c u l a r l a y e r . It is p o s s i b l e that o x y g e n i n the p o l y m e t h y l m e t h a c r y l a t e m o l e c u l e i s d i s a s s o c i a t e d from t h e m o l e c u l e c h a i n and p o s t - r e a c t s w i t h t h e s u r f a c e a f t e r t h e p l a s m a i s t u r n e d o f f . R e s u l t s o f c o n t a c t a n g l e measurements from n y l o n 6 , 6 and p o l y p r o p y l e n e a r e s u r p r i s i n g b e c a u s e the C ^ XPS s p e c t r a suggests p a r t i a l l y f l u o r i n a t e d s u r f a c e s . However, the f i r s t m o l e c u l a r l a y e r s must be p e r f l u o r i n a t e d as s u g g e s t e d by c o n t a c t a n g l e measurements and c a l c u l a t i o n s o f d e p t h o f f l u o r i n a tion. g


TABLE I I I COMPOSITION OF FLUORINATED FILMS OF VARIOUS POLYMERS

Treatment* Time

Polymer

F

Atomic % 0 c

N

F'

A

Contac Angle,

Polymethylmethacrylate

0

-

25.2

74.8

-

-

56°

Polymethylmethac r y l a t e

2

44.6

13.2

42.2

-

20

15°

52.7

29

40°

Polymethylmethacrylate

5

N y l o n 6,6

0

-

N y l o n 6,6

3

49.6

Polypropylene

0

-

Polypropylene

2

49.3

LDPE

0

-

5.0

LDPE

*

2

A l l w i t h 5% F

2

56.5

7.8

39.5

-

81.6

3.0

-

39.4

1.7

5

7.0

93.0

-

-

7.2

43.5

-

26

90

95.0

-

-

71

38.3

-

37

15.4 9.3

5.2

55 92-93° 78°

92-93°

m i x t u r e a t 2 . 0 mm, 40 c c / m i n . , 50W power

** w i t h Formamide

T r e a t m e n t Of P o l y m e r Powders Low d e n s i t y p o l y e t h y l e n e was c r y o g e n i c a l l y g r o u n d t o y i e l d a powder w i t h a s p e c i f i c s u r f a c e a r e a o f a b o u t 0 . 1 5 m / g as d e t e r m i n e d by BET m e a s u r e m e n t s . We found t h a t f o r powders w h i c h were n o t f u l l y f l u o r i n a t e d ( s h o r t r e a c t i o n t i m e s ) s i g n i f i c a n t amounts o f w a t e r were a d s o r b e d on t h e s u r f a c e . As a consequence o f t h i s , t h e powders seemed s i m i l a r i n a p p e a r a n c e t o m o i s t s a l t . Further, a d e c r e a s e i n pH was o b s e r v e d when t h e s e powders were s u s p e n d e d i n d i s t i l l e d w a t e r , s u g g e s t i n g the presence o f a c i d i c g r o u p s . Also, when t h e s e s a m p l e s were mounted f o r XPS e x p e r i m e n t s , t h e w a t e r a p p a r e n t l y d i d n o t c o m p l e t e l y d e s o r b ; when t h e X - r a y s o u r c e was


364

PHOTON,

ELECTRON,

A N D ION PROBES


POLYMETHYLMETHACRYLATE

1 i i i i Ii i i i 1 290 285 280

I i i i i I i i i i

300

295

BINDING ENERGY . eV

Figure 7.

The C-ls XPS spectra for polypropylene, polymethylmethacrylate, and Nylon 6,6 treated in the plasma



21.

ANAND E T A L .


365

moved t o s u c h a s a m p l e , o u t g a s s i n g was o b s e r v e d as i n d i c a t e d by an i n c r e a s e i n p r e s s u r e i n t h e XPS c h a m b e r . Thus we c o n c l u d e d t h a t w a t e r was c h e m i s o r b e d t o t h e a c i d i c s i t e s on t h e p o l y m e r s u r f a c e . Most s i g n i f i c a n t l y , f o r t h e s e s p e c i m e n s , v e r y l a r g e amounts o f o x y g e n were found on the s u r f a c e w i t h v e r y s m a l l c a r b o n s i g n a l s . F u r t h e r m o r e , t h e sample c h a r g i n g p r o p e r t i e s were a l t e r e d so d r a m a t i c a l l y t h a t t h e p e a k s were s h i f t e d t o l o w e r b i n d i n g e n e r g i e s by as much as 20 - 30eV. The s i g n a l s t r e n g t h s and w i d t h s v a r i e d d u r i n g the e x p e r i m e n t s w h i c h made i t i m p o s s i b l e t o t a k e any meaningful spectra. I t i s p o s s i b l e t h a t i f 2-3 l a y e r s o f w a t e r a r e c h e m i s o r b e d , l i t t l e or no c a r b o n s i g n a l w i l l be seen and when the X - r a y s o u r c e i s d i r e c t e d on t h e s a m p l e , t h e e n e r g y may be h i g h enough t o debond some w a t e r r e s u l t i n g i n a c o n t i n u o u s change o f the s u r f a c e . T h i s e x p l a n a t i o n i s c o n s i s t e n t w i t h our o b s e r v a tions. H o w e v e r , f o r powders w h i c h had undergone significant e x t e n t s o f f l u o r i n a t i o n , XPS s p e c t r a c o u l d be o b t a i n e d e a s i l y . The r e s u l t s p r e s e n t e d b e l o w a r e f o r s u c h c a s e s . As shown i n F i g . ( 8 ) , f l u o r i n a t i o n r e a c t i o n s o f p o l y e t h y l e n e powders i n t h e f l u i d i z e d bed were s l o w e r compared t o t h o s e o f f i l m s i n the p l a s m a . T h i s may be r e l a t e d t o t h e greater s u r f a c e a r e a i n v o l v e d i n the^ t r e a t m e n t o f powders ( ~ 0 . 1 5 m / g f o r powder compared to 0 . 0 3 m / g f o r f i l m ) . Further, for treatment o f p o w d e r s , t h e o v e r a l l e x t e n t o f f l u o r i n a t i o n i s l o w e r compared t o t h a t o f f i l m s - 52% F as o p p o s e d t o 60% f o r f i l m s ( F i g . 8 ) . This i s a l s o s u p p o r t e d by the s p e c t r a ( F i g . 9) i n w h i c h r e s i d u a l C H s i g n a l s are p r e s e n t at ^ 8 5 . O e V . T h i s r a i s e s q u e s t i o n s about u n i f o r m i t y o f the t r e a t m e n t and t h e d e p t h t o w h i c h f l u o r i n a t i o n has p r o g r e s s e d . On t h e b a s i s o f c a l c u l a t i o n s s i m i l a r t o t h o s e u s e d f o r f i l m s , we f o u n d t h a t t h e d e p t h o f f l u o r i n a t i o n ( i n t h e c a s e s where a t o m i c p e r c e n t f l u o r i n e was o f t h e o r d e r o f 52%) was a b o u t 4 0 - 5 0 A w h i c h s u g g e s t e d t h a t i f t h e t r e a t m e n t were u n i f o r m , t h e r e s i d u a l - C H ^ s i g n a l s h o u l d have been v e r y s m a l l . It is p o s s i b l e t h a t t h e t r e a t m e n t i s n o t u n i f o r m , w h i c h c o u l d be r e l a t e d t o p o o r m i x i n g due t o i n a d e q u a t e f l u i d i z a t i o n or due t o z o n e s w h i c h m i g h t n o t be a c c e s s i b l e f o r f l u o r i n a t i o n b e c a u s e o f f o r m a t i o n o f s t a g n a n t l a y e r s , e s p e c i a l l y o f HF and H ^ , w h i c h p r e v e n t further reaction. F i g u r e (9) i s a l s o u s e f u l i n i d e n t i f y i n g c h e m i c a l s p e c i e s t h a t a r e formed on t h e p o l y m e r p o w d e r . On t h e b a s i s o f c h e m i c a l s h i f t s as i n the c a s e o f p o l y m e r f i l m s , the s p e c i e s generated are l a r g e l y - C H ^ . 2

Q

D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (20 K / m i n o v e r t h e r a n g e 350 K t o 650 K ) o f the t r e a t e d powders showed no change i n t h e m e l t i n g p o i n t o f p o l y e t h y l e n e , and d i d n o t r e v e a l any s i g n i f i c a n t f e a t u r e s a t t r i b u t a b l e to the f l u o r i n a t e d p o l y m e r . The t r e a t e d powders c o u l d n o t be m o l d e d u n d e r c o n d i t i o n s w h i c h were s u i t a b l e f o r the u n t r e a t e d p o l y e t h y l e n e ( 1 5 0 ° , 30 M P a ) . However, the t r e a t e d powders c o u l d be p r e f o r m e d a t 95 C and 60 MPa t o a l l o w f o r c o l d c o m p a c t i n g i n an i s o s t a t i c p r e s s . The c o l d c o m p a c t i n g i m p r o v e d t h e m e c h a n i c a l s t r e n g t h o f t h e molded p o l y m e r s b u t n o t t o an e x t e n t where t h e y c o u l d be p r a c t i c a l l y u s e d . The c o n d i t i o n s o



366

PHOTON,

I

0

I

2

I I

ELECTRON,

A N D ION

PROBES

I I I I I I I I I I I

4 6 8 10 12 TIME OF TREATMENT, min

14

Figure 8. Comparison of fluorination of LDPE films and powders ((V) 40 cc/ min, 2.0 mm, 50 W, film; (O) 40 cc/min, 9.0 mm, 25 W, powder; (A) 30 cc/min, 9.0 mm, 25 W, powder)


ANAND E T A L .



^^^^

i ,, .ii, i, i 300

Figure 9.

295 290 285 BINDING ENERGY, eV

The C-ls XPS spectra for fluorinated powders (reaction conditions: 9.0 mm, 50 W, 40 cc/min)


PHOTON,

368

ELECTRON,

AND

ION

PROBES

f o r s i n t e r i n g o f t h e s e powders i s b e i n g i n v e s t i g a t e d w i t h t h e a i m to i m p r o v e t h e m e c h a n i c a l p r o p e r t i e s w i t h o u t damaging t h e f l u o r inated layer.


Summary R e a c t i o n s i n t h e p l a s m a c a n be c a r r i e d o u t i n t h e g l o w r e g i o n i n the p r e s e n c e o f i o n s or i n t h e d a r k r e g i o n i n t h e a b s e n c e o f ions. I n the f l u o r i n e p l a s m a t r e a t m e n t s we found t h a t for e x p e r i m e n t s c a r r i e d out i n t h e g l o w , t h e r e was a c o m p e t i t i o n between c h e m i c a l r e a c t i o n and a b l a t i o n o f m a t e r i a l from t h e s u r f a c e o f the p o l y m e r t h u s l i m i t i n g t h e d e p t h o f f l u o r i n a t i o n . An i n c r e a s e i n t h e d e p t h o f f l u o r i n a t i o n was a c h i e v e d by s h i e l d i n g the s p e c i m e n s f r o m i o n s . T h i s r e s u l t e d i n a drop i n the r a t e o f c h e m i c a l r e a c t i o n on t h e s u r f a c e . I n both these cases - C F ^ groups were r a p i d l y formed on the s u r f a c e as d e t e r m i n e d by XPS m e a s u r e ments. FTMIR e x p e r i m e n t s s u g g e s t t h a t some d i f f e r e n c e s do e x i s t i n the o r g a n i z a t i o n o f the " C F ^ groups i n the f l u o r i n a t e d polymer s u r f a c e s o b t a i n e d from t h e s e r e a c t i o n s . An a p p a r e n t u p p e r l i m i t on t h e d e p t h o f f l u o r i n a t i o n was o b s e r v e d even f o r r e a c t i o n s u n d e r i o n - d e p l e t e d c o n d i t i o n s . This was a t t r i b u t e d t o an i n s u f f i c i e n t f l u o r i n e g r a d i e n t from the s u r f a c e o f the p o l y m e r i n t o the b u l k . An i n c r e a s e in this g r a d i e n t o b t a i n e d by an i n c r e a s e o f f l u o r i n e c o n c e n t r a t i o n i n t h e plasma r e s u l t e d i n g r e a t e r depths o f f l u o r i n a t i o n s u g g e s t i n g a mechanism c o n t r o l l e d by d i f f u s i o n o f f l u o r i n e s p e c i e s i n t o t h e polymer. P e r f l u o r i n a t i o n on t h e s u r f a c e s o f p o l y p r o p y l e n e and n y l o n 6,6 c o u l d be a c h i e v e d b u t n o t w i t h p o l y m e t h y l m e t h a c r y l a t e u n d e r the r e a c t i o n c o n d i t i o n s e m p l o y e d . I t i s p o s s i b l e t h a t the oxygen from t h e PMMA c h a i n i s d i s l o d g e d from t h e m o l e c u l e and p o s t - r e a c t s w i t h t h e s u r f a c e when t h e p l a s m a i s t u r n e d o f f . LDPE powders t r e a t e d w i t h t h e f l u o r i n e p l a s m a i n a f l u i d i z e d bed d i d n o t show p e r f l u o r i n a t i o n on t h e s u r f a c e e v e n t h o u g h t h e c h e m i c a l g r o u p s g e n e r a t e d were l a r g e l y - C F ^ . T h i s c o u l d be a s s o c i a t e d w i t h n o n - u n i f o r m t r e a t m e n t c a u s e d by i n a d e q u a t e m i x i n g i n the f l u i d i z e d bed or due t o f o r m a t i o n o f s t a g n a n t HF and l a y e r s i n a r e a s o f the b e d . The p r e - f o r m i n g c o n d i t i o n s for m o l d i n g o f t h e s e powders were i d e n t i f i e d ; s i n t e r i n g c o n d i t i o n s w i t h o u t d i s r u p t i n g the f l u o r i n a t e d l a y e r are b e i n g i n v e s t i g a t e d . Acknowledgement This research i s supported D i v i s i o n of Engineering.

by N a t i o n a l S c i e n c e

Foundation,

Abstract A study of fluorination of low density polyethylene in the glow and dark regions of a low temperature plasma is reported.



21. ANAND ET AL.


The gas feed to the plasma consisted of dilute mixtures of fluorine in helium. Reactions which were carried out in the glow region of the plasma and which employed a fluorine/helium ratio of 0.05 revealed that the depth of fluorination was limited to about 40A owing to ion assisted etching. This depth was increased to about 60A by carrying out the reactions under ion-depleted conditions but the small fluorine concentration retarded the advance of the fluorinated layer into the bulk. This process was accelerated by increasing the fluorine/helium ratio to 0.15. The kinetics of these fluorination processes are discussed qualitatively. Results of plasma fluorinations of polymers other than low density polyethylene are also reported. Fluorine plasma treatment of low density polyethylene powders in a fluidized bed reactor is also described along with the molding properties of these powders.

Literature Cited 1.

Anand, Μ., Cohen, R.E. and Baddour, R.F., ACS Polymer Pre prints, 1979, 20(2), 507.

2.

Anand,Μ., Cohen, R.E. and Baddour, R.F., ACS Polymer Pre prints, 1980, 21(1), 139.

3.

Anand, Μ., Cohen, R.E. and Baddour, R.F., Polymer, in press.

4.

Clark, D.T., Feast, W.J., Musgrave, W.K.R. and Ritchie, I . , J. Polym. S c i . , Polym. Chem. Ed., 1975, 13, 857.

5.

Florin, R.E. and Wall, L . A . , J. Chem. Phys., 1972, 57(4), 1791.

6.

Shinohara, H . , Iwasaki, M., Tsujimura, T., Watanabe, Κ., and Okazaki, S., J . Polymer S c i . , 1972, A-l, 10, 2129.

7.

Shimada, J. and Hoshino, M., J. Appl. Polymer S c i . , 1975, 19, 1439.

8.

Masuoka, T., Yasuda, H. and Morosoff, Ν., ACS Polym. Pre prints, 1978, 91(2), 498.

9.

Yasuda, H., ACS Polymer Preprints, 1978, 19(2), 491.

10.

Hatsuo, Seitaro and Takehara, Yumido, Japan J . Appl. Phys., 1977, 16, 175.

11.

Coburn, J.W., Winters, H.F. and Chuang, T . J . , J . Appl. Phys., 1977, 48, 3532.

12.

Giegengack, H. and Hinze, D., Phys. State Sol. (A), 1971, 8,

513.


369

370

PHOTON, ELECTRON, AND ION PROBES

13.

Harmon, M.J., Boerio, F . J . and Koenig, J . L . , J . Chem. Phys., 1969, 50(7), 2829.

14.

Liang, C.Y. and Krimm, S., J . Chem. Phys., 1956, 25(3), 503. January 27, 1981.


RECEIVED


Surface Characterization of Plasma-Fluorinated Polymers - ACS

Recommend Documents