The Modification, Degradation, and Synthesis of Polymer Surfaces

17 The Modification, Degradation, and Synthesis of Polymer Surfaces Studied by ESCA D. T. CLARK

Downloaded by EAST CAROLINA UNIV on January 3, 2018 | http://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch017

Department of Chemistry, University of Durham, South Road, Durham City, England

Organic based polymers represent extremes in terms of both complexity of structure and sensitivity to interrogation and the single most powerful tool which has emerged for these investigations in terms of surface structure has proved to be ESCA (1). It is now over a decade since the first experiments on polymers were documented (2) and the intervening period has seen an explosive growth in the literature such that most of the requisite experiments for delineating the information levels and derived areas of applicability of the technique have broadly been accomplished. It is the wide ranging nature of these information levels which imbues the technique with capabilities far higher than might, naively have been thought from a consideration of the sum of each of the single information levels available from the ESCA experiment. For the sake of completeness we should perhaps at this stage list the available information levels in each case indicating how recent developments have expanded the scope for studies of the surface structure and its modification and for studies of the synthesis and reactions at polymer surfaces. Figure 1 shows schematically the typical data levels and variable experimental parameters which can be employed in the application of ESCA to polymeric systems. Figure 2 indicates how, many of the information levels available from the ESCA experiment may be used to study the time and temperature dependent behavior of polymer surface phenomena. Many of the contributions to this book deal with specific applications with an emphasis on one or other of the information levels available from the ESCA technique. Before considering specific applications of our own we may profitably highlight recent developments which point the way forward for applications in the next decade. Absolute Binding Energies. A s u b s t a n t i a l l i b r a r y o f both e x p e r i m e n t a l and t h e o r e t i c a l d a t a i s now a v a i l a b l e and i t i s n o r m a l l y a matter o f r o u t i n e to i d e n t i f y g i v e n s t r u c t u r a l f e a 0097-6156/81/0162-0247$ 11.25/0 © 1981 American Chemical Society

American Chemical Society Library 1155 s t . of N Polymer . w. Structure and Properties Dwight et al.; Photon, Electron, and 16th Ion Probes Washington, D . C .Society: 20036 ACS Symposium Series; American Chemical Washington, DC, 1981.

248

PHOTON,

ELECTRON,

A N D ION

PROBES

D a t a level Experimental

Absolute

parameters

Auger

Shake up

BE.

states

Valence

parameters

Structure and in the outermost

Bonds

Rel Int of

Charging

Core levels

phenomena

Bonding and changes therein 5 - 300 A

Sample mhomogeneities (lateral, vertical etc) Biasing Experiments


Flood gun

Figure 1. Data levels in the ESCA investigation of polymers

Manifestation

Surface hydrogen bonding M2O

in ESCA H0

OH

OH

2

DO

NH

C-0 -J

2

J

Change in Q-\ line shape and signal intensity s

Migration Angular dependent behaviour changes in relative intensities

Elimination e.g. Intensity changes in 0 i s

Intensity changes appearance of shake up

Reorganisation

OH

^OO

_J

X

L_

c--o OH

Figure 2.

x

Angular dependent behaviour changes in relative intensities

Changes in surface chemistry as a function of temperature and time studied by means of ESCA

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


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Studies of Polymer Surfaces

t u r e s p a r t i c u l a r l y from d e t a i l e d a n a l y s e s o f the t h e o r e t i c a l l y and e x p e r i m e n t a l l y w e l l u n d e r s t o o d C i s l e v e l s . W i t h good sample h a n d l i n g c a p a b i l i t i e s (_3) i t i s no p r o b l e m t o s t u d y l o w m o l e c u l a r w e i g h t model s y s t e m s i n t h e s o l i d s t a t e and for such systems i t i s o f t e n p o s s i b l e to c a r r y out d e t a i l e d n o n e m p i r i c a l LCAOMO SCF c o m p u t a t i o n s t o p r o v i d e a f u n d a m e n t a l i n t e r pretation of substitutent effects (4). T h e o r e t i c a l s t u d i e s are a l s o o f p r i m e i m p o r t a n c e i n d e f i n i n g d a t a f o r s y s t e m s w h i c h may be d i f f i c u l t t o s t u d y e x p e r i m e n t a l l y o r w h i c h may s u f f e r s i g n i f i c a n t r a d i a t i o n damage. A recent compilation of t h e o r e t i c a l data r e f e r e n c e d f o r b o t h gas and s o l i d s t a t e s t u d i e s i s d e p i c t e d i n F i g u r e 3 (5). The d a t a r e f e r to c o m p u t a t i o n s i n the ASCF f o r m a l i s m i n STO 4 . 3 1 G b a s i s s e t s and p a r t i c u l a r l y n o t e w o r t h y a r e t h e d a t a f o r o x y g e n and n i t r o g e n f u n c t i o n a l i t i e s . Taken i n c o n j u n c t i o n w i t h t h a t p r e v i o u s l y p u b l i s h e d (6) a r a t h e r c o m p l e t e s e t o f b a c k g r o u n d d a t a i s now a v a i l a b l e f o r t h e i n t e r p r e t a t i o n o f t h e o x i d a t i o n and n i t r a t i o n o f p o l y m e r s and we w i l l r e t u r n t o t h i s aspect i n a l a t e r section. P h o t o n E n e r g y Dependent S t u d i e s . To d a t e most a p p l i c a t i o n s i n t h e ESCA f i e l d have b e e n r e s t r u e t e d t o t h e commonly e m p l o y e d M g K ^ ^ X - r a y s o u r c e s where t h e e s c a p e and s a m p l i n g d e p t h s f o r t h e c o r e l e v e l s o f u s u a l i n t e r e s t t y p i c a l l y r a n g e from 8 - 30& and 25 - 90X r e s p e c t i v e l y . Most i n s t r u m e n t a t i o n now has " p u s h b u t t o n " f a c i l i t y f o r c h a n g i n g p h o t o n e n e r g y and t h e v i r t u e s o f u s i n g harder X-ray sources i s becoming a p p a r e n t . Of c o n s i d e r a b l e i n t e r e s t i s t h e T i K X - r a y s o u r c e where t h e p h o t o n e n e r g y o f ^ 4 5 1 0 eV e n a b l e s d e p t h s ^ 300A t o be sampled and t h i s i s i n d i c a t e d schematically i n Figure 4. This provides a convenient step i n d e p t h p r o f i l i n g to MATR ( M u l t i p l e A t t e n u a t e d T o t a l R e f l e c t i o n I n f r a r e d S p e c t r o s c o p y ) where u n d e r n o r m a l c o n d i t i o n s t h e s a m p l i n g d e p t h m i g h t be o»0.5y. As an example o f t h e use o f a T i K X - r a y s o u r c e F i g u r e 5 shows t h e c o r e l e v e l s p e c t r a f o r a n y l o n f i l m . The 01s and N i s l e v e l s s t u d i e d w i t h t h e h a r d e r X - r a y s o u r c e c l e a r l y show t h a t t h e i n h e r e n t w i d t h o f t h e T i K r a d i a t i o n i s ^ 3 eV and c o n s i s t s o f a d o u b l e t o f i n t e n s i t y r a t i o 2 : 1 and s p l i t t i n g 5 . 8 e V . The l a r g e i n h e r e n t w i d t h o f the X - r a y source i s advantageous i n t h i s l i n e shape a n a l y s i s s i n c e t h e a n a l y z e r c o n t r i b u t i o n and any c h a r g i n g inhomogeneity c o n t r i b u t i o n i n l i n e w i d t h are n e g l i g i b l e . The w i d t h o f the components o f t h e C i s , 0 1 s and N i s l e v e l s are t h e r e f o r e i d e n t i c a l and i t i s a s t r a i g h t f o r w a r d manner t o a n a l y z e t h e C i s s p e c t r u m i n terms o f a s u p e r p o s i t i o n o f a 3 component l i n e s h a p e as i n d i c a t e d i n t h e f i g u r e . The s p e c t r a , r e c o r d e d w i t h t h e f i x e d r e t a r d a t i o n r a t i o (FRR) mode, show t h a t t h e d i f f e r e n t i a l s e n s i t i v i t i e s ( a c o n v o l u t i o n o f i n s t r u m e n t a l and c r o s s s e c t i o n f a c t o r s ) change by a s i g n i f i c a n t b u t n o t d r a s t i c amount on g o i n g from M g K t o T i K . The d a t a a r e a l s o i n t e r e s t i n g i n t h a t they r e v e a l the presence o f a s m a l l e x t e n t o f hydrocarbon contami n a t i o n on t h e v e r y s u r f a c e s i n c e t h e MgK intensity ratios a

a

a

a

a

1

9


250

PHOTON,

5

NH

7

5

45

5

3

E L E C T R O N ,

*17 « 5 413 411 409

5

f. . . ^.V

A N DION

300 298. 296. 294

1

3

HCN HNC £H CN CH NC CH NH CH3NH2 (CH ) N CH3-O-NH2

I,

/ I I I I

3

3

2


PROBES

3 3

H-0-NH

I

2

CH3NO2 CH3QNO CH3QNO2

M 1

I I

I

I

I

I

HC^°

^ NH2

3

^NH (CH^NH*

2

I

ICH3) N0 3

, I

HCNfcJH H CNfcj 2

H' ^ 0 0—0

I I

Hy/ H

Q

H

537

535

533

531

408

Ois

404 N

Corrected

Figure 3.

406

402

1 s

400

291

289 C

287 1

285

s

Binding Energies (eV)

Theoretically computed (A SCF) calculations of C-ls, N-ls, O-ls core level binding energies



17.

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CLARK

251


Escape

and

Mg ^ K

100 •

sample

and

Ti

K q L

depths Photon

sources

(Schematic)

90-

k

, C i

1

s

sampling depth

x

80 hv 7

1253-7ev

222

P ]

Samplmcfdepths

Q .

"

2 5 0

A

very similar hv

4510 ev

60-

X

A 50-

Sampling -50

40-

depth

A

m

30-

2010 -

Ois N i s i

1

^_ Sampling depths J " — vary more than for T i ^ proportionally

*

3-0

3-5

a

4-0

log K E

Figure 4.

Schematic of mean-free paths as a function of KE (MgK ,t and TiK sources) ai


aij2


Figure 5. Core-level spectra for Nylon 6,6 with two different photon sources


m

ο w

α ο

>

r w ο H Ο

M

Η Ο

a ο

to

17.

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i n d i c a t e a component a t 285 eV i n e x c e s s o f t h a t f o r t h e r e p e a t unit. This i s not evident f o r the TiKa data s i n c e the c o n t r i b u t i o n t o t h e t o t a l s i g n a l from t h e v e r y s u r f a c e i s v e r y s m a l l . Such l o w l e v e l s u r f a c e c o n t a m i n a t i o n f o r a f i l m c a n o f c o u r s e more r e a d i l y t o be d e t e c t e d by a n g u l a r d e p e n d e n t s t u d i e s however f o r powder samples where s u c h s t u d i e s a r e n o t a p p l i c a b l e t h e c a p a b i l i t y o f i n t e r r o g a t i n g v e r t i c a l i n h o m o g e n e i t i e s by means o f p h o t o n energy dependent s t u d i e s i s o f g r e a t i m p o r t a n c e . A n g u l a r Dependent S t u d i e s . We have r e c e n t l y p o i n t e d o u t t h e c o n s i d e r a b l e c a r e w h i c h must be e x e r c i s e d i n i n t e r p r e t i n g a n g u l a r ( t a k e o f f a n g l e ) dependent d a t a i n d e l i n e a t i n g u n a m b i g u o u s l y l a t e r a l and v e r t i c a l sample i n h o m o g e n e i t i e s ( 7 ) . The u n r i v a l l e d c a p a b i l i t y o f ESCA i n p r o v i d i n g d a t a on v e r t i c a l i n h o m o g e n e i t i e s has a l r e a d y p r o d u c e d new i n s i g h t s i n t h e s u r f a c e science of p o l y m e r s on b o t h an a c a d e m i c and t e c h n o l o g i c a l f r o n t . Before the advent o f ESCA t h e q u e s t i o n o f t h e initial r e a c t i o n o f a s o l i d s u r f a c e w i t h a g i v e n environment c o u l d n o t be addressed. The c o m p l e x i t i e s o f s t r u c t u r e s f o r p o l y m e r s w h i c h a r e p a r t i a l l y a t l e a s t d i c t a t e d by p r o c e s s i n g c a n c l e a r l y l e a d t o c o r r e s p o n d i n g l y complex s u r f a c e s t r u c t u r e s f o r systems w h i c h have undergone r e a c t i o n a t a g a s / s o l i d o r l i q u i d / s o l i d i n t e r f a c e . A t y p i c a l s i t u a t i o n m i g h t be t h a t d e p i c t e d i n F i g u r e 6 . The s u r f a c e , s u b - s u r f a c e and b u l k may have c r y s t a l l i n e and amorphous regions. For a r e a c t i v e species M, for which the a c t i v a t i o n e n e r g y i s s m a l l , r e a c t i o n may o c c u r f o r e v e r y c o l l i s i o n and t h e r e a c t i o n may t h e n o c c u r r a t h e r u n i f o r m l y b o t h l a t e r a l l y and vertically. I f the a c t i v a t i o n energy i s h i g h , however, then b o t h l a t e r a l and v e r t i c a l i n h o m o g e n e i t i e s may o c c u r . The s i t u a t i o n i s a l s o c o m p l i c a t e d by t h e f a c t t h a t t h e c r y s t a l l i n i t y o f a g i v e n p o l y m e r s y s t e m may be s t r o n g l y i n f l u e n c e d by p r o c e s s i n g c o n d i tions. T h u s , u n i a x i a l and b i a x i a l o r i e n t a t i o n o f a g i v e n p o l y m e r s y s t e m may w e l l a l t e r q u i t e d r a s t i c a l l y t h e c r y s t a l l i n i t y o f a g i v e n s y s t e m and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 7 . A s we have n o t e d w i t h c a r e f u l c o n t r o l o f a l l t h e v a r i a b l e s a c o m b i n a t i o n o f a n g u l a r and p h o t o n e n e r g y d e p e n d e n t s t u d i e s p r o v i d e s ESCA w i t h u n r i v a l l e d c a p a b i l i t y f o r depth p r o f i l i n g . In appropriate c a s e s i t i s a l s o p o s s i b l e t o s t u d y l a t e r a l i n h o m o g e n e i t i e s and w i t h i m p r o v e d s e n s i t i v i t y s m a l l a r e a a n a l y s i s becomes a d i s t i n c t possibility. T h i s c a n be a c c o m p l i s h e d e i t h e r by means o f t h e s p e c t r o m e t e r l e n s s y s t e m o r by means o f a p p r o p r i a t e s l i t s y s t e m s . A t t h i s s t a g e however i t w o u l d seem t h a t i n t h e n e x t t e n y e a r s we a r e u n l i k e l y _ £ o see s p e c t r o m e t e r s w i t h a s p o t r e s o l u t i o n s i z e o f l e s s t h a n 10 s q . cm. w h i c h i s s t i l l r a t h e r g r o s s compared w i t h SAM. Sample C h a r g i n g . We have shown t h a t sample c h a r g i n g i m p o r t a n t i n f o r m a t i o n l e v e l i n i t s own r i g h t and w i t h t h e b i l i t y o f " t u n i n g " s a m p l e c h a r g i n g by means o f a UV f l o o d the p a r t i c u l a r case o f s t u d i e s i n v o l v i n g a monochromatised

i s an possigun i n source


254

PHOTON,


M

ELECTRON,

A N DI O N PROBES

Reactive species M

Diffusion in amorphous region

crystalline region

(?) Low activation energy:- uniform reaction at surface (ii) High

" . - diffusion controlled, non uniform reactivity of various surface and subsurface regions. Figure 6.

Figure 7.

Surface reactions of polymers

Fringed micelle structure of polymers and orientation effects on bulk morphology


17.

CLARK

ESCA


255

t h e r e w i l l u n d o u b t e d l y be an i n c r e a s i n g a w a r e n e s s o f t h i s i m p o r t a n t i n f o r m a t i o n l e v e l w h i c h i s t r a d i t i o n a l l y t h r o w n away ( 8 ) . I n t h e t h r e e s e c t i o n s b e l o w we c o n s i d e r i m p o r t a n t a r e a s o f a p p l i c a t i o n s o f ESCA i n ; m i g r a t i o n phenomena, i n t h e s y n t h e s i s o f p o l y m e r s u r f a c e s by means o f p l a s m a t e c h n i q u e s and i n r e a c t i o n s i n i t i a t e d at the surface o f c o t t o n f i b r i l s i n the n i t r a t i o n o f cellulose.


M i g r a t i o n Phenomena (9^) A s we have i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 2 t h e m i g r a t i o n and s e g r e g a t i o n o f s m a l l m o l e c u l e s a t p o l y m e r s u r f a c e s c a n b e m o n i t o r e d by i n t e n s i t y changes o f a p p r o p r i a t e c o r e l e v e l s as a f u n c t i o n o f take o f f angle. A particularly intriguing applicat i o n o f t h i s genre i n v o l v e s the m i g r a t i o n o f the low m o l e c u l a r w e i g h t s i l i c o n e m a t e r i a l u s e d as a r e l e a s e a g e n t i n d o u b l e s i d e d S c o t c h t a p e commonly used f o r m o u n t i n g p u r p o s e s i n ESCA e x p e r i ments. A p r i o r i m i g r a t i o n o f low m o l e c u l a r w e i g h t m a t e r i a l c o u l d c o n c e i v a b l y o c c u r by two d i s t i n c t mechanisms and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n F i g u r e 8 . On t h e l e f t hand s i d e m i g r a t i o n from one s u r f a c e o f a p o l y m e r t o t h e o t h e r i s by b u l k m i g r a t i o n i n v o l v i n g permeation through the b u l k . The a l t e r n a t i v e i n v o l v e s m i g r a t i o n a l o n g s u r f a c e s as i n d i c a t e d s c h e m a t i c a l l y on t h e r i g h t hand s i d e o f t h e f i g u r e . A n ESCA e x a m i n a t i o n o f a f r e s h l y e x p o s e d s u r f a c e o f d o u b l e s i d e d S c o t c h t a p e ( 3 M ' s company e l e c t r i c a l i n s u l a t i n g t a p e ) r e v e a l s a f a i r l y i n t e n s e s i g n a l from S i 2 and 0 1 s l e v e l s a p p r o p r i a t e to s i l i c o n e type m a t e r i a l . To d e m o n s t r a t e t h e m i g r a t i o n o f t h i s s i l i c o n e m a t e r i a l a p i e c e o f t a p e was a t t a c h e d t o a s t r i p o f p o l y m e r and t h e r e s u l t s f o r h i g h and l o w d e n s i t y polyethylene samples (HDPE LDPE) a r e shown i n F i g u r e 9 . By t a k i n g s a m p l e s f r o m d i f f e r e n t s e c t i o n s o f t h e p o l y m e r s t r i p a f t e r a p e r i o d o f 10 days a t a m b i e n t t e m p e r a t u r e two f e a t u r e s a r e e v i d e n t . F i r s t l y LDPE h a s a d i f f e r e n t b e h a v i o r f r o m HDPE. S e c o n d l y b o t h p o l y m e r f i l m s show evidence that the s i l i c o n e w i l l migrate along the s u r f a c e . This i s n o t e n t i r e l y u n e x p e c t e d on t h e b a s i s o f t h e s u r f a c e e n e r g e t i c s and t h e l i k e l y d e g r e e o f c r y s t a l l i n i t y e t c . o f t h e s u r f a c e s o f h i g h and l o w d e n s i t y p o l y e t h y l e n e . I n o r d e r t o d i f f e r e n t i a t e between m i g r a t i o n a l o n g a s u r f a c e v e r s u s p e r m e a t i o n t h r o u g h t h e b u l k as mechanisms f o r s i l i c o n e t o move from one s u r f a c e t o a n o t h e r we h a v e c o n s t r u c t e d t h e a p p a r a t u s shown i n F i g u r e 1 0 . By m o u n t i n g p o l y m e r f i l m s i n a c l a m p i n g r i n g and by m o n i t o r i n g d i f f e r e n t portions o f t h e f i l m i t becomes p o s s i b l e to d i f f e r e n t i a t e between the main a l t e r n a t i v e s for mig r a t i o n mechanisms and t h i s i s i n d i c a t e d s c h e m a t i c a l l y i n t h e figure. The c o r e l e v e l d a t a f o r two o f t h e p o s i t i o n s o f i n t e r e s t f o r a LDPE f i l m o f ^ 1 0 0 u t h i c k n e s s a r e shown i n F i g u r e 1 1 . C o n s i d e r i n g f i r s t l y t h e room t e m p e r a t u r e d a t a t h e i n t e n s i t i e s f o r t h e S i 0



256

PHOTON,

ELECTRON,

A N D

ION PROBES

Prototype system Low MWt silicone on HDPE and LDPE

Figure 8.

Migration phenomena: possible extremes for migration of low-molecular weight materials.

HDPE

tape LDPE

Figure 9.

The C-ls, O-ls, and Si-2p core levels for strips of LDPE film with Scotch tape attached (diffusion studies, 10 days RT)



POLYMER STUDIED-

escapes vessel

if it escapes vessel

Diffusion studies apparatus and its application (1:1 scale diagram)

diffusion through lateral migration

migration around edge of polymer

Figure 10.

Shows extent of Shows extent to which

MODES OF SAMPLING

CROSS SECTION

Sample to show extent of lateral

MEDIUM

DIFFUSION

PLAN VIEW


polymer


Figure 12.

Comparison of C-ls and Si-2p levels for HDPE and LDPE samples mounted in diffusion study apparatus


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and 0 1 s s i g n a l s a r e somewhat s i m i l a r f o r b o t h p o s i t i o n s and i n d i c a t e t h a t m i g r a t i o n a l o n g the s u r f a c e i s d r a s t i c a l l y c u r t a i l ed by t h e c l a m p i n g r i n g a r r a n g e m e n t . A f t e r 11 d a y s a t 75 C however t h e s i t u a t i o n i s s u b s t a n t i a l l y d i f f e r e n t t h e m i g r a t i o n through the b u l k b e i n g s u b s t a n t i a l l y f a s t e r than along the s u r face. The d i f f e r e n c e b e t w e e n HDPE and LDPE i s s t i l l evident however f o r t h e b u l k m i g r a t i o n and t h i s i s c l e a r from t h e d a t a g i v e n i n F i g u r e 12. These d a t a show t h e p o t e n t i a l f o r s t u d y i n g t h e mode o f m i g r a t i o n o f s m a l l m o l e c u l e s b o t h a l o n g and t h r o u g h s u r f a c e s and a l s o p o i n t t o t h e d a n g e r o f l o n g t e r m e x p o s u r e o f samples t o S c o t c h t a p e as m i g h t be u s e d f o r example i n s e c u r i n g samples f o r t r a n s p o r t i n g purposes. The P l a s m a S y n t h e s i s O f U l t r a T h i n F l u o r o p o l y m e r F i l m s The s y n t h e s i s o f u l t r a t h i n p o l y m e r f i l m s by p l a s m a t e c h n i q u e s i s an a r e a o f c o n s i d e r a b l e c u r r e n t i n t e r e s t from s e v e r a l v i e w p o i n t s ( H ) , 11). Thus t h e c a p a b i l i t y o f t a k i n g b u l k p o l y m e r s o f d e s i r a b l e b u l k p r o p e r t i e s and s p e c i f i c a l l y m o d i f y i n g t h e s u r face to a l t e r the c h e m i c a l , p h y s i c a l , m e c h a n i c a l or e l e c t r i c a l p r o p e r t i e s h a s much t o commend i t and f i e l d s o f a p p l i c a t i o n w h i c h have a l r e a d y r e c e i v e d a t t e n t i o n i n c l u d e g r a f t i n g p o l y a c r y l i c a c i d o n t o p o l y e s t e r f i b r e t o i m p r o v e t h e c o m f o r t f a c t o r and d e p o s i t i o n o f low f r i c t i o n c o a t i n g f l u o r o p o l y m e r f i l m s o n t o e l a s t o m e r s e a l s f o r use i n the o i l d r i l l i n g i n d u s t r y . Plasma d e p o s i t i o n o f u l t r a t h i n polymer f i l m s onto metals i s o f i n t e r e s t f o r b i o m o n i t o r i n g e l e c t r o d e s and p l a s m a c o a t i n g s a r e a l s o u n d e r i n v e s t i g a t i o n f o r laser fusion targets. Plasma polymers p r e p a r e d from h e x a m e t h y l d i s i l o x a n e have b e e n shown t o be amongst t h e most b i o c o m p a t i b l e o f p o l y m e r i c f i l m s w h i l s t the d e p o s i t i o n o f v e r y t h i n u n i f o r m p i n h o l e f r e e f i l m s o f u n u s u a l e l e c t r i c a l p r o p e r t i e s makes p l a s m a polymer f i l m s o f i n t e r e s t i n device a p p l i c a t i o n s . The s p e c i f i c m o d i f i c a t i o n o f p o l y m e r s u r f a c e s by " c o o l " p l a s m a s i s a l s o an a r e a of considerable current interest. Thus d e t a i l e d k i n e t i c s t u d i e s have b e e n made o f d i r e c t and r a d i a t i v e e n e r g y t r a n s f e r t o p o l y m e r s u r f a c e s by means o f i n e r t gas p l a s m a s , a t o p i c o f i n t e r e s t i n t h e s u r f a c e c r o s s - l i n k i n g o f p o l y m e r s and r e m o v i n g weak b o u n d a r y layers for adhesive bonding. The s u r f a c e f u n c t i o n a l i s a t i o n o f p o l y m e r s c a n a l s o be a c c o m p l i s h e d i n a c o s t e f f e c t i v e manner by means o f p l a s m a t e c h n i q u e s . The m o t i v a t i o n f o r s t u d y i n g t h e f u n d a m e n t a l s o f p l a s m a p o l y m e r i z a t i o n s and m o d i f i c a t i o n s o f s u r f a c e s i s t h e r e f o r e n o t d i f f i c u l t t o u n d e r s t a n d and we c o n s i d e r h e r e one a s p e c t o f t h e t o p i c , t h e p l a s m a p o l y m e r i z a t i o n o f s i m p l e fluoropolymers. The e x c i t a t i o n o f a p l a s m a i n a t y p i c a l o r g a n i c m o l e c u l e g i v e s r i s e t o a number o f r a d i c a l c a t i o n s , and e x c i t e d s t a t e s and a n i o n r a d i c a l s a r i s i n g from e l e c t r o n a t t a c h m e n t . I t i s not s u r p r i s i n g t h e r e f o r e t h a t t r a n s f o r m a t i o n s i n v o l v i n g such s p e c i e s and r e a c t i o n s w i t h t h e o r i g i n a l m o l e c u l e e i t h e r i n t h e gas p h a s e



260

PHOTON,

ELECTRON,

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PROBES

o r at the g a s - s o l i d i n t e r f a c e g i v e s r i s e t o a r e a c t i o n pathway which i s t y p i c a l l y q u i t e complex. F o r v i n y l monomers i t may b e shown t h a t u n d e r a p p r o p r i a t e c o n d i t i o n s p l a s m a i n d u c e d p o l y m e r i z a t i o n s may accompany t h e more g e n e r a l p r o c e s s o f p l a s m a p o l y m e r i z a t i o n , the former g i v i n g r i s e to l i n e a r systems, the l a t t e r cross-linked products. 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 where t h e i o n d e n s i t y i s h i g h e s t polymer d e p o s i t i o n a r i s e s from the competi t i v e p r o c e s s o f p o l y m e r i z a t i o n and a b l a t i o n , t h e s o - c a l l e d CAP scheme ( 1 0 ) and t h e g r e a t v e r s a t i l i t y o f t h e t e c h n i q u e i n b e i n g a b l e t o p r o d u c e t h i n f i l m s f r o m v i r t u a l l y any o r g a n i c p r e c u r s o r makes t h e s t u d y o f t h e p r o c e s s o f d e p o s i t i o n and t h e s t r u c t u r e o f t h e a c c o m p a n y i n g f i l m s one o f g r e a t i n t e r e s t . The f a c t t h a t t h e f i l m s o f i n t e r e s t a r e t h i n ( g e n e r a l l y i n t h e r a n g e 5 - 5000X) and t h e r e f o r e need t o be s t u d i e d i n s i t u and a r e i n s o l u b l e r e n d e r s ESCA t h e t e c h n i q u e o f c h o i c e f o r s t u d y i n g d e t a i l s o f s t r u c t u r e and bonding. I n d e e d i t i s t r u e t o say t h a t t h e m e c h a n i s t i c s t u d y o f t h e f o r m a t i o n o f s u c h f i l m s has o n l y made s i g n i f i c a n t p r o g r e s s s i n c e t h e a d v e n t o f ESCA. I n r e c e n t p a p e r s we have shown how ESCA may be u s e d t o establish r e l a t i o n s h i p between the s t r u c t u r e o f the initial m o l e c u l a r s y s t e m i n w h i c h t h e d i s c h a r g e i s e x c i t e d and t h a t o f t h e p o l y m e r d e p o s i t e d i n e i t h e r g l o w o r n o n - g l o w r e a t i o n s (1_2). The o b j e c t i v e s o f t h e p r e s e n t programme o f w o r k on p l a s m a p o l y m e r s may b e summarized as f o l l o w s : (i) To e s t a b l i s h t h e o v e r a l l s t o i c h i o m e t r y and s t r u c t u r a l f e a t u r e s present i n a polymer f i l m . (ii) To s t u d y the v a r i a t i o n i n ( i ) as a f u n c t i o n o f g e o m e t r i c f a c t o r s , power, p r e s s u r e e t c . (iii) To s t u d y r a t e s o f d e p o s i t i o n as a f u n c t i o n o f t h e gas and t h e s i t e o f d e p o s i t i o n . F o r a v a r i e t y o f r e a s o n s t h e work r e p o r t e d h e r e p e r t a i n s t o i n d u c t i v e l y c o u p l e d RF p l a s m a s i n t h e p r e s s u r e r a n g e 50 - 2 0 0 u . Such e x p e r i m e n t s a r e p a r t i c u l a r l y e a s y t o c a r r y o u t and c l o s e c o n t r o l c a n be m a i n t a i n e d on t h e i m p o r t a n t v a r i a b l e s s u c h as p r e s s u r e f l o w r a t e , power i n p u t e t c . Y a s u d a ( 1 0 ) has p o i n t e d o u t the i m p o r t a n c e o f W/FM (where W i s t h e power i n p u t , F i s t h e f l o w r a t e and M the m o l e c u l a r w e i g h t ) i n d e t e r m i n i n g d e p o s i t i o n r a t e and n a t u r e o f t h e m a t e r i a l ( v i z . p o w d e r , powdery f i l m , f i l m , o i l ) . Under t h e c o n d i t i o n s o f work d e s c r i b e d h e r e p o l y m e r s were found t o d e p o s i t as u n i f o r m f i l m s b o t h on g o l d , p y r e x and s i l i c a substrates. T h r e e d i s t i n c t a p p a r a t u s ' s have been employed i n t h i s work. A schematic o f a t y p i c a l free s t a n d i n g r e a c t o r f o r the d e p o s i t i o n of f i l m s which are subsequently t r a n s f e r r e d to the s p e c t r o m e t e r i s shown i n F i g u r e 1 3 , w h i l s t t h e r e a c t o r u s e d f o r i n s i t u d e p o s i t i o n o f p o l y m e r f i l m s w h i c h may t h e n be d i r e c t l y i n s e r t e d i n t o t h e s p e c t r o m e t e r h a s b e e n d e s c r i b e d i n some d e t a i l p r e v i o u s l y (11). I n o r d e r t o s t u d y the dependence o f p o l y m e r s t r u c t u r e and r a t e o f d e p o s i t i o n as a f u n c t i o n o f d e p o s i t i o n s i t e a s p e c i a l l o n g r e a c t o r has been c o n s t r u c t e d and t h i s i s shown s c h e m a t i c a l l y i n F i g u r e 14.


CLARK

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261

TEFLON VALVES

PUMPING

CAJON ULTRA TORR

VITON 0 RING SEALS


COUPLINGS 1"dia.

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'0' ring seal on ground glass faces REACTOR * 2

Figure 13. Apparatus for studying RF plasma polymers

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coil only In turns -soft copper)

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Figure 14. Apparatus for studying site dependence of structure of plasma polymers



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I t i s known t h a t p l a s m a p o l y m e r s o f t e n c o n t a i n u n p a i r e d s p i n s and as s u c h may r e a c t on e x p o s u r e t o a t m o s p h e r e o x y g e n t o produce s u r f a c e o x i d a t i v e f u n c t i o n a l i z a t i o n . I n g e n e r a l t h i s has n o t p r o v e d t o be o f any m a j o r s i g n i f i c a n c e as f a r as t h e f l u o r o p o l y m e r s s t u d i e d i n t h i s work a r e c o n c e r n e d and t o e m p h a s i z e t h i s p o i n t F i g u r e 15 shows a w i d e s c a n ESCA s p e c t r u m o f a t h i n p l a s m a p o l y m e r f i l m p r o d u c e d from p e r f l u o r o b e n z e n e . The s t o i c h i o m e t r y o f t h i s f i l m i s c l o s e t o t h a t o f t h e i n i t i a l "monomer" and by g o i n g t o a h i g h t a k e o f f a n g l e we c a n d e t e c t a v e r y l o w l e v e l o f o x y g e n s i g n a l f o r t h i s f i l m p r o d u c e d i n an a p p a r a t u s as i n F i g u r e 13. A s w e l l as h a v i n g i n t e r e s t i n g c h e m i c a l , p h y s i c a l , e l e c t r i c a l and m e c h a n i c a l p r o p e r t i e s f l u o r o p o l y m e r s p r o d u c e d by p l a s m a means a r e p a r t i c u l a r l y s u i t a b l e f o r s t u d y by ESCA n o t t h e l e a s t b e c a u s e o f the v e r y d i s t i n c t i v e n a t u r e o f the C i s l e v e l s i n such systems. F i g u r e 16 d e p i c t s t y p i c a l b i n d i n g e n e r g y r a n g e s f o r a v a r i e t y o f s t r u c t u r a l f e a t u r e s which might a r i s e i n f l u o r o p o l y m e r systems and w i t h a b a c k g r o u n d o f s u c h d a t a we may s t r a i g h t f o r w a r d l y a s s i g n s p e c t r a for plasma polymer systems. A p a r t i c u l a r l y s t r i k i n g example o f t h i s g e n r e i s p r o v i d e d by the d a t a g i v e n i n F i g u r e 17. The s t o i c h i o m e t r y o f f i l m s c a n b e d e t e r m i n e d from t h e i n t e g r a t e d C l s : F l s i n t e n s i t y r a t i o s and f r o m t h e components o f t h e C i s l e v e l s . From t h i s d a t a i t i s c l e a r t h a t plasma p o l y m e r i z a t i o n i n v o l v e s s u b s t a n t i a l rearrangement s i n c e t r i f l u o r o m e t h y 1 groups form a s i g n i f i c a n t f e a t u r e o f the s p e c t r a . F o r p e r f l u o r o c y c l o h e x a n e , n o r m a l l y c o n s i d e r e d an e x t r e m e l y i n e r t f l u o r o c a r b o n , the f l u o r o p o l y m e r e x h i b i t s a l a r g e p e r c e n t a g e o f p e r f l u o r o e t h y l g r o u p s and has a s t o i c h i o m e t r y l o w e r i n f l u o r i n e than the s t a r t i n g m a t e r i a l . W i t h the advent o f ESCA as a s t r u c t u r a l t o o l i t has b e e n shown f o r t h e f i r s t t i m e t h a t t h e d e p o s i t i o n i n the glow r e g i o n o f a plasma can produce w e l l d e f i n e d p o l y m e r i c p r o d u c t s o v e r a r a n g e o f p r e s s u r e and power i n p u t r a n g e s (12) . The n a t u r e o f t h e c o m p e t i t i v e p r o c e s s o f p o l y m e r i z a t i o n and d e p o s i t i o n i s s u c h t h a t we m i g h t a n t i c i p a t e t h a t a t t h e edges and o u t s i d e o f the glow r e g i o n l o n g e r l i v e d r e a c t i v e s p e c i e s m i g h t w e l l l e a d t o the f o r m a t i o n o f d i f f e r e n t p o l y m e r i n r e g i o n s r e m o t e f r o m t h e c o i l r e g i o n i n an i n d u c t i v e l y c o u p l e d p l a s m a . Indeed i n t h e c a s e o f c o n v e n t i o n a l l y p o l y m e r i z a b l e "monomers" s u c h as t h e f l u o r i n a t e d a l k e n e s t h e p o l y m e r i z a t i o n mechanism i n t h e n o n - g l o w r e g i o n may w e l l be d o m i n a t e d by c o n v e n t i o n a l a d d i t i o n p o l y m e r ization: t h e so c a l l e d p l a s m a i n i t i a t e d p o l y m e r i z a t i o n . There are two i m p o r t a n t q u e s t i o n s w h i c h may r e a d i l y b e a d d r e s s e d by ESCA. F i r s t l y , what i s t h e r a t e o f d e p o s i t i o n o f t h e p o l y m e r f i l m a t a g i v e n s i t e i n a p l a s m a r e a c t o r and s e c o n d l y how d o e s t h e s t r u c t u r e depend on t h e s i t e o f d e p o s i t i o n ? To i l l u s t r a t e t h e g r e a t power o f t h e t e c h n i q u e i n a n s w e r i n g t h e s e q u e s t i o n s we c o n s i d e r h e r e a r e c e n t d e t a i l e d i n v e s t i g a t i o n o f t h e i n d u c t i v e l y coupled plasma p o l y m e r i z a t i o n o f pentafluorobenzene (13) .



CLARK

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297

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295

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293

291

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289

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Figure 16. The C-ls core binding energies forfluorocarbonstructural features


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C i s c o r e l e v e l s p e c t r a as a f u n c t i o n o f two t a k e o f f a n g l e s f o r the p l a s m a p o l y m e r i z a t i o n i n the g l o w r e g i o n o f a free s t a n d i n g r e a c t o r a r e shown i n F i g u r e 1 8 , a t a p r e s s u r e o f 200u and power l o a d i n g o f 10 w a t t s . The s p e c t r a show c o n s i d e r a b l e f i n e s t r u c t u r e and c o m p a r i s o n w i t h d a t a s u c h as t h a t i n F i g u r e 16 allows a s t r a i g h t f o r w a r d assignment. The m a j o r components o f t h e l i n e p r o f i l e c o n s i s t s o f two p e a k s c e n t e r e d ^ 2 8 6 . 7 eV and 2 8 8 . 7 eV c o r r e s p o n d i n g t o T T * s h a k e up satellite of the component centered ^ 2 8 8 . 7 eV t h u s i n d i c a t i n g t h e p r e s e n c e o f a c o n j u g a t e d system. The s m a l l component a t 285 eV a l m o s t c e r t a i n l y c o r r e s ponds to a low l e v e l o f h y d r o c a r b o n c o n t a m i n a t i o n s i n c e the r e l a t i v e i n t e n s i t y i n c r e a s e s s l i g h t l y on g o i n g t o a h i g h e r t a k e off angle. W i t h i n e x p e r i m e n t a l e r r o r t h e s t o i c h i o m e t r i e s f o r t h e two t a k e o f f a n g l e s a r e t h e same and s l i g h t l y l o w e r i n f l u o r i n e c o n t e n t t h a n the s t a r t i n g m a t e r i a l . The p r e s e n c e o f C F ^ and C F ^ s t r u c t u r a l f e a t u r e s p r o v i d e s s t r o n g e v i d e n c e f o r the e x t e n s i v e m o l e c u l a r rearrangement accompanying the p o l y m e r i z a t i o n r e a c t i o n . The s t r u c t u r e and c o m p o s i t i o n o f t h e p o l y m e r d e p o s i t e d i n t h e g l o w r e g i o n r e m a i n s e s s e n t i a l l y c o n s t a n t as a f u n c t i o n o f power and p r e s s u r e o v e r a c o n s i d e r a b l e r a n g e o f t y p i c a l o p e r a t i n g parameters. T h i s F i g u r e 19 shows t h e component a n a l y s i s f o r f i l m s d e p o s i t e d on g o l d s u b s t r a t e s i n t h e g l o w r e g i o n a t a fixed p r e s s u r e o f 200u and power i n p u t s o f 1 0 , 20 and 30 w a t t s . W i t h a k n o w l e d g e o f mean f r e e p a t h s as a f u n c t i o n o f k i n e t i c e n e r g y i t becomes p o s s i b l e t o s t u d y t h e r a t e o f f i l m d e p o s i t i o n as a f u n c t i o n o f the o p e r a t i n g parameters. T y p i c a l d a t a f o r the i n s i t u d e p o s i t i o n o f polymer i n a s m a l l r e a c t o r a t t a c h e d to the s p e c t r o m e t e r a r e g i v e n i n F i g u r e 2 0 . By m o n i t o r i n g t h e s u b s t r a t e A u ^ „ , l e v e l s (MgK . ^ r a d i a t i o n ) t h e r a t e s o f d e p o s i t i o n may be s t u d i e d and t h e u n i f o r m n a t u r e o f t h e d e p o s i t i o n i s e v i d e n t f r o m this data. F o r a f i v e f o l d i n c r e a s e i n power t h e d e p o s i t i o n r a t e c h a n g e s f r o m ^ 2A p e r s e c . t o per s e c . i n d i c a t i n g t h a t the r a t e o f d e p o s i t i o n i s a p p r o a c h i n g t h e maximum a t t h e p r e s s u r e and f l o w r a t e employed f o r t h e s e e x p e r i m e n t s . We now t u r n t o t h e i m p o r t a n t q u e s t i o n o f t h e s t r u c t u r e o f t h e p o l y m e r as a f u n c t i o n o f s i t e d e p o s i t i o n . Employing the r e a c t o r c o n f i g u r a t i o n d e p i c t e d i n F i g u r e 14, the polymer d e p o s i t e d at the r e a c t o r w a l l and a t t h e c e n t e r o f t h e r e a c t o r h a s b e e n i n v e s t i g a t e d as a f u n c t i o n o f p o s i t i o n as i n d i c a t e d . The g l o w r e g i o n f o r a p l a s m a e x c i t e d a t 200u and 10 w a t t s i n p u t power e x t e n d s f o r a r e g i o n ^ 1 5 cms. i n from o f and ^ 1 0 cm. t o t h e r e a r o f t h e c o i l region. F i g u r e 21 shows t h e F l s , C i s and F 2 s l e v e l s f o r the polymer d e p o s i t e d i n the glow r e g i o n i n f r o n t o f the c o i l system. The s p e c t r a a r e e s s e n t i a l l y i d e n t i c a l t o t h o s e i n F i g u r e 1 8 . The c o r e l e v e l s p e c t r a f o r t h e f i l m d e p o s i t e d a t t h e edge o f t h e g l o w r e g i o n 45 cms. from t h e f r o n t o f t h e r e a c t o r a r e d i s t i n c t i v e l y n >


2



266

PHOTON,

ELECTRON,

A N D ION

PROBES

297 295 293 291 289 287 285 Binding energy ev

Figure 18.

The C-ls core levels as a function of take-off angle for RF plasma polymer from pentafluorobenzene (10 W, 200 fi)

H RF

plasma polymerization

40Q"CF

n

35- A CF - C 30%

C:F stoichiometry 25-

contribution

Cis components

to C i profile

1:075

1:078

1^078

1:072

1 072

s

20-

1 5

.

micro analysis 1 :071

-» CF3 " * IT-^Tf* shake up IQF) 20

30

power Iwatts)

Figure 19. Component analysis of structural features in fluoropolymer films from RF plasmas excited in pentafluorobenzene (free-standing reactor (200p))



17.

CLARK

ESCA

0

267


5

10

15

5

10

15

20 tfsec)

25

30

20

25

30

35

tlsec)

Figure 20. Rates of deposition of plasma polymerS from pentafluorobenzene



268

PHOTON,

ELECTRON,

A N D

ION

PROBES

Figure 21. The F-ls, C-ls, and F-2s levels for fluoropolymer produced from pentafluorobenzene (site of deposition 10 cm from input end of reactor in Figure 14)



17.

CLARK

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269


d i f f e r e n t as i s a p p a r e n t f r o m a c o m p a r i s o n o f F i g u r e 22 a n d F i g u r e 21. C l e a r l y t h e c o n t r i b u t i o n from C F ^ s t r u c t u r a l f e a t u r e s h a s s i g n i f i c a n t l y i n c r e a s e d and t h i s becomes more p r o n o u n c e d f o r f i l m s d e p o s i t e d i n t h e non g l o w r e g i o n 60 cms. f r o m t h e f r o n t o f t h e r e a c t o r ( F i g u r e 2 3 ) . C F ^ s t r u c t u r a l f e a t u r e s now d o m i n a t e t h e s p e c t r a and F i g u r e 24 shows how t h e d e r i v e d s t o i c h i o m e t r y o f t h e p o l y m e r f i l m depends on s i t e o f d e p o s i t i o n . I t may a l s o b e shown t h a t the r a t e o f d e p o s i t i o n i n the glow r e g i o n i s extremely r a p i d compared w i t h t h e non g l o w r e g i o n s . With the type o f i n f o r m a t i o n p r o v i d e d by t h e s e ESCA s t u d i e s t h e s t o i c h i o m e t r y and m a i n s t r u c t u r a l f e a t u r e s f o r a g i v e n p o l y m e r f i l m d e r i v e d from a g i v e n s t a r t i n g m a t e r i a l may b e s e l e c t e d f o r a g i v e n a p p l i c a t i o n . An ESCA I n v e s t i g a t i o n O f The S u r f a c e C h e m i s t r y O f The N i t r a t i o n And D e n i t r a t i o n O f C e l l u l o s e M a t e r i a l s Introduction. The s t u d y o f t h e n i t r a t i o n o f c e l l u l o s e p a r t i c u l a r l y i n t h e f o r m o f c o t t o n l i n t e r s has a l o n g and c h e q u e r ed h i s t o r y (_14). I n d e e d t h e d i s c o v e r y i n 1833 by B r a c c o n n o t ( 1 5 ) that the r e a c t i o n o f n i t r i c a c i d w i t h c o t t o n produced inflammable m a t e r i a l s and t h e s u b s e q u e n t p a t e n t g r a n t e d t o S c h o n b e i n (16) i n 1846 s p e c i f y i n g t h e p r o d u c t i o n o f n i t r o c e l l u l o s e s from c o t t o n and m i x e d a c i d s c a n be s a i d t o h a v e m a t e r i a l l y changed t h e c o u r s e o f history. A f t e r the f i r s t (unscheduled) l a r g e s c a l e e x p l o s i o n i n 1 8 4 7 , work on t h e t e c h n i c a l p r o d u c t i o n o f n i t r o c e l l u l o s e s was somewhat r e s t r a i n e d , a f t e r t h i s m i s h a p however t h e p o s s i b i l i t y o f m o d i f y i n g b u r n r a t e f o r p r o p u l s i o n p u r p o s e s was r e c o g n i z e d and by 1867 t h e b a s i s o f t h e p r o d u c t i o n o f s m o k e l e s s p r o p e l l a n t s f o r g u n s , r i f l e s and s h e l l s was e s t a b l i s h e d and m a t e r i a l l y changed the course o f h i s t o r y n o t o n l y i n the m i l i t a r y sense b u t a l s o i n the p i o n e e r i n g days o f the w i l d w e s t . The i n n o v a t i v e u s e o f camphor as a p l a s t i c i z e r f o r n i t r o c e l l u l o s e b y P a r k e s i n 1862 (_17) i n t h e p r o d u c t i o n o f c e l l u l o i d r e p r e s e n t s an i m p o r t a n t l a n d m a r k i n t h e emergence o f p o l y m e r s c i e n c e and t h e o b s e r v a t i o n o f t h e s o l u b i l i t y o f n i t r o c e l l u l o s e s and t h e p r o d u c t i o n o f C o l l o d i o n and t h i n f i l m s u n d o u b t e d l y a c c e l e r a t e d t h e d e v e l o p m e n t o f p h o t o g r a p h y as we now know i t . A s an amusing s i d e l i g h t the p r o d u c t i o n o f c e l l u l o i d a l l o w e d the w i d e r d e v e l o p m e n t o f s n o o k e r and b i l l i a r d s . The p l a s t i c i z i n g e f f e c t o f camphor on n i t r o c e l l u l o s e u n d o u b t e d l y p r o v i d e d c o n s i d e r a b l e i m p e t u s t o t h e d e v e l o p m e n t o f d o u b l e and t r i p l e b a s e d propellants b a s e d on n i t r o g l y c e r i n e / n i t r o c e l l u l o s e f o r m u l a t i o n s . The f o u n d a t i o n s o f t h e p r o d u c t i o n methods u s e d f o r n i t r o c e l l u l o s e s ( F i g u r e 25) were l a r g e l y l a i d down i n t h e e a r l y t w e n t i e t h c e n t u r y and d e s p i t e t h e t e c h n o l o g i c a l , m i l i t a r y and academic i m p o r t a n c e o f t h e w h o l e f i e l d and d e s p i t e a v o l u m i n o u s l i t e r a t u r e much o f i t c l a s s i f i e d t h e r e a r e a number o f unanswered q u e s t i o n s . This i s p a r t i c u l a r l y so when t h e i m p o r t a n t q u e s t i o n o f s u r f a c e c h e m i s t r y is addressed. Thus t h e i n i t i a l i n t e r a c t i o n o f c e l l u l o s e f i b r i l s w i t h n i t r a t i n g media, the burn i n i t i a t i o n , the s t a b i l i z a t i o n ,


270

PHOTON,

ELECTRON,

A N DION PROBES


Cis


18

H 1-6

p*]

RF plasma 10 watts 200 u

U 1-2 10 Average C/F Stoichiometry

0

.

j

8

0-6

-];

. . . . \ stoichiometry of monomer

11 11111 i, 11

0* coil region 0-2

+. flow direction 0

10

20

30

£0

50

60

position in reactor (at wall)

Figure 24.

Average carbon to fluorine stoichiometry as a function of site of deposition for fluoropolymers produced from pentafluorobenzene



17.

CLARK

ESCA


271

r e p r e s e n t a few amongst many o f t h e i m p o r t a n t a s p e c t s o f t h e n i t r o c e l l u l o s e f i e l d w h i c h depend on s u r f a c e phenomena. The w i d e r a n g i n g n a t u r e o f b o t h t h e t e c h n o l o g i c a l and a c a d e m i c i n v e s t i g a t i o n s o f t h e c e l l u l o s e and n i t r o c e l l u l o s e f i e l d o v e r t h e p a s t h u n d r e d y e a r s o r so p o s e s c e r t a i n d i f f i c u l t i e s s i n c e r e l e v a n t d a t a i s w i d e l y d i s p e r s e d o v e r a number o f d i s c i p l i n e s . The most c o m p l e t e p u b l i s h e d s u r v e y ( 1 4 ) i s now some 30 y e a r s o l d and p r e - d a t e s most o f t h e r e l e v a n t d e v e l o p m e n t s i n b o t h s u r f a c e c h e m i s t r y and m e c h a n i s t i c o r g a n i c c h e m i s t r y w h i c h a r e e s s e n t i a l to the d e t a i l e d u n d e r s t a n d i n g o f heterogeneous p r o c e s s e s at the surfaces o f c e l l u l o s e based m a t e r i a l s . The n i t r a t i o n o f c e l l u l o s e i n t h e form o f c o t t o n l i n t e r s i s u s u a l l y i n m i x e d a c i d and on a t h r e e component d i a g r a m t h e zone o f t e c h n i c a l n i t r a t i o n i s d e l i n e a t e d by c o m p o s i t i o n s i n t h e r a n g e H N 0 17-27%, H S 0 66-50% and H O 17-23% ( 1 4 ) . A l t h o u g h n i t r a t i o n s c a n be c o n v e n i e n t l y e f f e c t e d on t h e l a b o r a t o r y s c a l e w i t h n i t r i c p h o s p h o r i c m i x e s r e c o v e r y expense and c o r r o s i o n p r o b l e m s n o r m a l l y p r e c l u d e t h e u s e o f s u c h a c i d m i x e s on a c o m m e r c i a l scale. N i t r i c a c i d - water mixes a r e not used s i n c e n i t r a t i o n i s u n e v e n and d e g r a d a t i o n and g e l a t i n i z a t i o n o f p r o d u c t a r i s e a t t h e h i g h percentage n i t r i c mixes r e q u i r e d f o r r e a c t i o n . Technical n i t r a t i o n s as p r e v i o u s l y n o t e d a r e c a r r i e d o u t i n n i t r i c s u l p h u r i c m i x e s s i n c e t h i s a f f o r d s a cheap method f o r p r o d u c t i o n o f h i g h n i t r o g e n c o n t e n t m a t e r i a l w i t h c o n v e n i e n t r e c o v e r y and w i t h a lack o f c o r r o s i o n problems. The m a i n p o i n t o f i n t e r e s t i n t e c h n i c a l n i t r a t i o n i n m i x e d a c i d i s t h e q u e s t i o n o f t h e d e g r e e o f s u b s t i t u t i o n and how t h i s r e l a t e s to the a c i d mix c o m p o s i t i o n . Thus a l t h o u g h F i g u r e 26 shows a maximum d e g r e e o f s u b s t i t u t i o n o f 3 , i n p r a c t i c e t h i s i s n e v e r a t t a i n e d and t h e maximum d e g r e e o f s u b s t i t u t i o n i s % 2 . 8 . A d e f i n i t i v e answer as t o why t h i s s h o u l d be so h a s n o t t h u s f a r b e e n g i v e n s i n c e i n a heterogeneous p r o c e s s the d e l i n e a t i o n between the p o s s i b l e e x p l a n a t i o n s r e q u i r e s a t e c h n i q u e w h i c h c a n c l e a r l y d i s t i n g u i s h s u r f a c e from b u l k phenomena ( c f . F i g u r e 2 7 ) . The q u e s t i o n o f t h e l i m i t i n g d e g r e e o f s u b s t i t u t i o n ( u s u a l l y e s t a b l i s h e d by a b u l k n i t r o g e n d e t e r m i n a t i o n u s i n g a m i c r o K j e l d a h l technique) i n n i t r o c e l l u l o s e s could a p r i o r i be r a t i o n a l i z e d i n terms o f two e x t r e m e m o d e l s . The f i r s t c a n be a t t r i b u t e d t o t h e m i c r o and m a c r o s c o p i c s t r u c t u r e o f the c e l l u l o s e . Thus i n h o m o g e n e i t i e s i n t h e b u l k structure c o u l d c o n c e i v a b l y g i v e r i s e t o a c c e s s i b l e and i n accessible regions. S i n c e n i t r a t i o n must depend on t h e d i f f u s i o n of reagent throughout the b u l k s t r u c t u r e a f u r t h e r c o n s i d e r a t i o n i s t h a t t h e r e may w e l l be a c o n c e n t r a t i o n p r o f i l e t h r o u g h o u t t h e structure. On t h i s b a s i s t h e l e s s t h a n maximum d e g r e e o f s u b stitution is attributable t o an inhomogeneous b u l k structure c o r r e s p o n d i n g t o r e g i o n s o f c o m p l e t e l y n i t r a t e d m a t e r i a l and unreacted i n a c c e s s i b l e regions. A n a l t e r n a t i v e and somewhat more p l a u s i b l e a l t e r n a t i v e i s that since n i t r a t i o n i s a r e v e r s i b l e e s t e r i f i c a t i o n process then 3

2

4


PHOTON,

A N D ION

Uneven nitration

Rel. high nitrogen

High nitrogen content

degradation

content. Cheap, ease

with small amount of

gelatinisation of

of recovery and lack

degradation. Corrosion

product.

of corrosion problems.

and recovery

Figure 25.

PROBES

Nitric- Phosphoric

Nitric - Sulphuric

Nitric acid


ELECTRON,

problems

Nitration of cellulose: possible reaction pathways

Degree of Substitution

Nitrogen Content 14-4 Theoretical

27-2-9

12-6 - 13.4 Gun Cottons

2-5-26

11.8-12.4 Photographic films

10.6-124

2-25-2.6

Nitro lacquers

10.6-11-2 Celluloid

2-25-2-4

Figure 26.

1. Maximum (a)

Nitrocelluloses DOS as a function of application

Degree

of Substitution

Inhomogeneities (i) In bulk

is

^

2-8

why ?

?

structure

Iii) In nitrating mix (iii) Combination of (i) and (ii)

(b) Figure 27.

Kinetic and Thermodynamic

reasons.

Points of interest in the technical nitration in mixed acid of cellulose


17.

CLARK

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the the

273

^ 2 . 8 degree o f s u b s t i t u t i o n t y p i c a l l y observed r e p r e s e n t s equilibrium situation ( c f . Figure 28). I n o r d e r t o shed l i g h t on t h i s s i t u a t i o n we p r e s e n t h e r e a c o m p a r i s o n o f t h e s u r f a c e and b u l k n i t r a t i o n o f c e l l u l o s e i n t h e form o f l i n t e r s p a p e r (_17). B e f o r e d i s c u s s i n g t h e d a t a however i t i s worthwhile b r i e f l y c o n s i d e r i n g the background concerning the b u l k s t r u c t u r e o f c e l l u l o s e and n i t r o c e l l u l o s e . The s t r u c t u r e o f c e l l u l o s e o r n i t r o c e l l u l o s e may b e d i s c u s s ed on t h r e e d i s t i n c t l e v e l s ( 1 4 , 1 8 ) . (i) The s t r u c t u r e o f the macroscopic f i b r e generally about 20u i n d i a m e t e r . S t r u c t u r a l B o t a n y h a s shown t h a t t h e f i b r e s a r e c h a r a c t e r i z e d by a number o f d i s t i n c t l a y e r s t h e r e s u l t o f t h e g r o w t h mechanism w i t h i n the l i v i n g p l a n t . (ii) The n a t u r e o f t h e s e l a y e r s w h i c h i n t h e c a s e o f c e l l u l o s e a p p e a r t o be composed o f f i b r i l s within f i b r i l s , r a n g i n g f r o m a b o u t 1 u i n d i a m e t e r down t o t h e so c a l l e d u l t i m a t e m i c r o f i b r i l o f a b o u t 35& d i a m e t e r and t h o u g h t t o c o n s i s t o f c h a i n s o f D g l u c o s e m o l e c u l e s 12 x 8 u n i t s i n c r o s s s e c t i o n . (iii) The m o l e c u l a r l e v e l i n c l u d i n g t h e u n i t c e l l o f t h e c r y s t a l l i n e r e g i o n s , molecular weight d i s t r i b u t i o n etc. F o r b o t h c e l l u l o s e and n i t r o c e l l u l o s e a c o n s i d e r a b l e body o f e v i d e n c e h a s b e e n g e n e r a t e d on a s p e c t s ( i i ) and ( i i i ) h o w e v e r c o n s i d e r a b l e c o n t r o v e r s y surrounds the i n f o r m a t i o n p e r t a i n i n g t o (ii). L e w i s ( 1 8 ) has s u c c i n c t l y d e s c r i b e d t h e c u r r e n t s t a t e o f k n o w l e d g e i n w h i c h t h e t h r e e m o d e l s d e p i c t e d i n F i g u r e 29 have been d i s c u s s e d . Most o f t h e o l d e r l i t e r a t u r e d i s c u s s e s t h e c h e m i s t r y o f c e l l u l o s e i n terms o f t h e w e l l known f r i n g e d m i c e l l e m o d e l ( a ) . The m a i n f e a t u r e o f t h i s m o d e l i s an e x t e n d e d c r y s t a l l i n e s t r u c t u r e i n t e r s p e r s e d w i t h amorphous r e g i o n s . X - r a y s t u d i e s and more r e c e n t c h e m i c a l e v i d e n c e p r o v i d e l i t t l e support f o r such a model and an a l t e r n a t i v e , m i c r o f i b r i l m o d e l ( b ) h a s b e e n d e v e l o p e d . The f u n d a m e n t a l a s p e c t o f t h i s m o d e l i s t h a t v i r t u a l l y 100% o f t h e c o t t o n i s i n t h e c r y s t a l l i n e f o r m and c o n s i s t s o f u l t i m a t e f i b r i l s 3-5 u i n d i a m e t e r . The amorphous o r non c r y s t a l l i n e r e g i o n s i n t h i s model are a s s o c i a t e d w i t h the s u r f a c e r e g i o n s o f these microfibrils. The v a r y i n g d e g r e e o f a c c e s s i b i l i t y t o c h e m i c a l r e a g e n t s i s e x p l a i n e d by p o s t u l a t i n g t h a t t h e number o f a c t i v e c e n t e r s on any p a r t i c u l a r c e l l u l o s e m o l e c u l e w i l l depend on whether i t l i e s a l o n g the c o r n e r , edge, the face or the i n t e r i o r o f the m i c r o f i b r i l . The l a r g e r t h e f i b r i l d i a m e t e r t h e s m a l l e r t h e p r o p o r t i o n o f non c r y s t a l l i n e m a t e r i a l p r e s e n t . I n the case o f n i t r o c e l l u l o s e e x p e r i m e n t a l e v i d e n c e has been p r e s e n t e d which i s b a s i c a l l y i n c o m p a t i b l e w i t h s u c h a m o d e l . A compromise b e t w e e n t h e f r i n g e d m i c e l l e and t o t a l l y c r y s t a l l i n e m i c r o f i b r i l theory has r e c e n t l y b e e n s u g g e s t e d , t h e m a i n f e a t u r e o f t h e m o d e l ( c ) b e i n g t h a t amorphous r e g i o n s b e t w e e n c r y s t a l l i t e s r u n i n c o n t i n u -


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

274

Older Theory


In) Amorphous regions

Crystalline

Mix proceeds

H 0|

HN0 | 3

2

tendency to lower nitration

Accessible and Inaccessible

Nitrating as Nitration

level and denitration

regions.

of

first formed nitrate.

Figure 28.

Inhomogeneities leading to reduced DOS for bulk materials

(b) Ki

0 - NO2 + HNO3. 2 ° H

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o-H

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2

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Kinetics and equilibria involved in nitrations in mixed acids



17.

CLARK

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ous v e i n s t h r o u g h t h e p o l y m e r and a r e n o t r a n d o m l y d i s t r i b u t e d as i n the f r i n g e d m i c e l l e t h e o r y . The a v a i l a b l e d a t a p a r t i c u l a r l y that r e l a t i n g to p r o c e s s i n g o f n i t r o c e l l u l o s e i s compatible w i t h s u c h a m o d e l w i t h t h e m i c r o f i b r i l s as i n ( b ) b e i n g t h e c r y s t a l l i t e s w i t h t h e i n t e r v e n i n g r e g i o n s b e i n g f i l l e d w i t h amorphous m a t e r i a l w h i c h d i f f e r s l i t t l e i n d e n s i t y from t h a t o f t h e c r y s t a l l i n e r e g i o n s ( s e e F i g u r e 30) . I n any h e t e r o g e n e o u s p r o c e s s i t i s t h e s u r f a c e w h i c h p r o v i d e s a " w i n d o w " on t h e r e a c t i o n however as we have p r e v i o u s l y n o t e d t h e s t u d i e s t o d a t e have f o c u s s e d on t h e b u l k c h e m i s t r y . M i l e s ( 1 4 ) has summarized much o f t h e i m p o r t a n t l i t e r a t u r e i n t h e p e r i o d t o c i r c a 1950 and e v i d e n c e h a s b e e n p r e s e n t e d t h a t u n d e r c e r t a i n c o n d i t i o n s e q u i l i b r i u m b e t w e e n n i t r a t i o n and d e n i t r a t i o n i s e s t a b l i s h e d on t h e t y p i c a l t i m e s c a l e i n v o l v e d f o r n i t r a t i o n . V e r y r e c e n t ( 1 3 ) C nmr s t u d i e s have c o n f i r m e d ( 1 9 ) an e a r l i e r c h e m i c a l l y based i n f e r e n c e (20) t h a t i n a n i t r a t e d c e l l u l o s e o f a g i v e n d e g r e e o f s u b s t i t u t i o n (DOS) t h e p a r t i a l esterification r a t i o s ( p r i m a r y v s secondary 0N0 ) are i n the order 6 > 3 > 2. I t i s important to recognize that t n i s almost c e r t a i n l y corresponds t o an e q u i l i b r i u m o r d e r o f r e a c t i v i t y s i n c e i t i s known t h a t i n homogeneous r e a c t i o n s t h e r a t e c o n s t a n t s f o r d e n i t r a t i o n o f p r i mary e s t e r s i s s u b s t a n t i a l l y s m a l l e r t h a n f o r n i t r a t i o n u n d e r a g i v e n s e t o f c o n d i t i o n s (2_1). The c h e m i c a l r e a c t i o n s e q u e n c e involving treatment o f the n i t r a t e d material with Nal i n a c e t o n y l a c e t o n e p r o v i d e s i n f o r m a t i o n on t h e t o t a l l e v e l o f p r i mary n i t r a t e e s t e r g r o u p s i n t h e sample ( 2 0 ) . By c o n t r a s t t h e h i g h r e s o l u t i o n (1_3) C nmr s t u d i e s r e l a t e o n l y t o t h e s o l u b l e fraction of nitrated material. The i n f o r m a t i o n d e r i v e d f r o m t h e s e c h e m i c a l and s p e c t r o s c o p i c s t u d i e s a r e v a l u a b l e b u t s t i l l l e a v e i m p o r t a n t gaps i n t h e i n f o r a m t i o n r e q u i r e d t o f u l l y u n d e r s t a n d t h e n i t r a t i o n o f c e l l u l o s e i n m i x e d a c i d s . I n o r d e r t o shed new l i g h t on t h i s t e c h n o l o g i c a l l y and a c a d e m i c a l l y i m p o r t a n t a r e a we p r e s e n t h e r e a d e t a i l e d c o m p a r i s o n o f t h e s u r f a c e and b u l k n i t r a t i o n and d e n i t r a t i o n o f c e l l u l o s e m a t e r i a l s i n terms o f d e g r e e o f s u b s t i t u t i o n and r e l a t i v e r a t e s o f r e a c t i o n s . ?

Experimental. S i n c e the n i t r a t i o n o f c e l l u l o s e i n the form o f l i n t e r s p a p e r may w e l l b e u n f a m i l i a r t o many p e o p l e we p r e s e n t here a b r i e f synopsis o f the procedures i n v o l v e d . The g e n e r a l procedures are i n d i c a t e d schematically i n Figure 31. Commerciall y p r o d u c e d l i n t e r s p a p e r ( H e r c u l e s Powder C o . ) o f S h i r l e y f l u i d i t y 8 . 8 and a p p r o x i m a t e d e g r e e o f p o l y m e r i z a t i o n 1100 was vacuum d r i e d i n an oven a t 60 f o r 2 h r s . and s t o r e d o v e r P « 0 , - . This p r o v i d e s a s t a r t i n g c e l l u l o s e sample w i t h < 2% w a t e r . Nitrations and d e n i t r a t i o n s were a c c o m p l i s h e d by i m m e r s i o n i n t h e a p p r o p r i a t e a c i d m i x ( o f sample s t r i p s ( 4 cm x 4 cm) f o r a g i v e n p e r i o d o f t i m e u s i n g an a p p a r a t u s s u c h as t h a t d e p i c t e d i n F i g u r e 3 2 . B u l k n i t r o g e n d e t e r m i n a t i o n s have b e e n c a r r i e d o u t u s i n g a m i c r o Kjeldahl.



Figure 30. Possible models for the structure of cellulose and nitrocellulose (after Ref. 18;

c)

THE VEINED MODEL

Cellulose paper type 1 Ex M.O.D. dried at 60° in vacuum oven. 2hrs.

I Stored over P2O5 4x4cm. strips immersed in Nitrating acid by trome or Mitchell method.

As for Nitration always Mitchell method.

Atter specfic time, strips are removed from mix, squeezed free of acid on a Buchner funnel and quenched in 1 litre of water of 5°C.

Nitrocellulose is washed in 3» 500 mis. of cold water and further boiled for 2-3 hours.

Nitrocellulose (high N content) placed in spent acrd of mix used to Nitrate Cellulose to a low D.O.S.

Soxhlet washing hos also been tried.

Work up as for Nitrotion

Drying proceedure as above. ESCA and Kjeldahl analysis carried out.

Figure 31. Nitration and denitration procedures (nitrations according to Mitchell procedure) (22)



17.

CLARK

ESCA


277

Preliminary Analysis. The c o r e l e v e l ( C i s , O l s , N i s ) s p e c tra for a typical nitrocellulose sample which has been c o m m e r c i a l l y p r o d u c e d i s shown i n F i g u r e 3 3 . The C i s s p e c t r u m shows 3 d i s t i n c t i v e components ( o n c e a s t a n d a r d l i n e shape a n a l y s i s h a s b e e n c a r r i e d o u t ) . Thus t h e c e n t r a l component a t ^287.4 eV a r i s e s p r e d o m i n a n t l y from t h e c a r b o n s C 2 , C3 and C6 b e a r i n g t h e n i t r a t e e s t e r f u n c t i o n a l i t y w i t h c o n t r i b u t i o n s from C4 and C 5 . C I u n i q u e l y a t t a c h e d t o two oxygens i n t h e c y c l i c h e m i a c e t a l f o r m u l a t i o n o f the 3-Dglucopyranose r i n g i s at h i g h e s t b i n d i n g energy. The component a t 285 eV a r i s e s f r o m e x t r a n e o u s h y d r o c a r b o n w h i c h we w i l l show l a t e r i s c o n f i n e d t o t h e v e r y s u r f a c e o f t h e s a m p l e . I n c e l l u l o s e samples i r r e s p e c t i v e o f source o r t y p e ( e . g . l i n t e r s or s t a p l e f i b r e s ) t h e h y d r o c a r b o n s i g n a l o b s e r v e d i s v a r i a b l e b u t inevitably present. The N i s s i g n a l c o n s i s t s o f an i n t e n s e h i g h b i n d i n g e n e r g y component and ( c f . F i g . 3) c o m p a r i s o n w i t h m o d e l s y s t e m s u n a m b i g u o u s l y i d e n t i f i e s t h i s as o r i g i n a t i n g f r o m t h e n i t r a t e e s t e r groups -O-NO^. I n i n d u s t r i a l s c a l e n i t r a t i o n s a low b i n d i n g e n e r g y component i s o f t e n o b s e r v e d a t ^ 4 0 6 eV and t h u s i s associated with n i t r i t e ester groups. I n the l a b o r a t o r y s c a l e n i t r a t i o n s where c o n d i t i o n s c a n p e r h a p s be more p r e c i s e l y c o n t r o l l e d such s t r u c t u r a l f e a t u r e s a r e a t a much l o w e r l e v e l . Conventional n i t r i t e traps (urea, ascorbic acid e t c . ) dissolved i n the n i t r a t i n g m i x to o b v i a t e the p o s s i b i l i t y o f f o r m a t i o n o f n i t r i t e e s t e r s does n o t seem t o remove t h e v e r y l o w l e v e l s o f s u c h s t r u c t u r a l f e a t u r e s and i t c o u l d be t h a t t h e y a r i s e as a r e s u l t o f r e a c t i o n s o c c u r r i n g d u r i n g s t a b i l i z a t i o n and s t o r a g e . The O l s l e v e l s a r e an u n r e s o l v e d b r o a d p e a k . With a knowledge o f s e n s i t i v i t y f a c t o r s f o r the v a r i o u s core l e v e l s i t i s p o s s i b l e t o s t r a i g h t f o r w a r d l y work o u t t h e d e g r e e o f s u b s t i t u t i o n DOS ( a v e r a g e number o f n i t r o - e s t e r functionalities per glucose r e s i d u e ) . Thus t h e i n t e g r a t e d C l s / N l s a r e a r a t i o s ( e x c l u d i n g t h e e x t r a n e o u s h y d r o c a r b o n component) y i e l d s a DOS o f 2.3 f o r the p h o s p h o r i c / n i t r i c n i t r a t i n g m i x i d e n t i c a l w i t h t h a t d e t e r m i n e d from m i c r o K j e l d a h l b u l k a n a l y s i s . The t o t a l C l s / O l s r a t i o i n d i c a t e s that there i s l i t t l e r e s i d u a l water i n the n i t r a t e d sample. W i t h f i b r i l l a r samples s u c h as l i n t e r p a p e r s any i n f o r m a t i o n on v e r t i c a l i n h o m o g e n e i t i e s i n t o t h e sample may o n l y be i n f e r r e d by l o o k i n g a t d i f f e r e n t l e v e l s c o r r e s p o n d i n g to d i f f e r e n t e s c a p e depths. C l e a r l y t h e good agreement b e t w e e n b u l k and s u r f a c e a n a l y s e s s u g g e s t s t h a t t h e sample i s u n i f o r m a l l y n i t r a t e d and t h e e x t r a n e o u s h y d r o c a r b o n must t h e r e f o r e be l o c a l i z e d a t t h e s u r f a c e p e r h a p s i n t h e form o f a p a t c h e d o v e r l a y e r s i n c e i n most c a s e s t h e r e may w e l l be s i g n i f i c a n t l y l e s s t h a n a m o n o l a y e r p r e s e n t . One way o f e s t a b l i s h i n g t h e p u r e l y s u r f a c e n a t u r e o f t h i s h y d r o c a r b o n ( s i n c e the u s u a l a n g u l a r dependent s t u d i e s a r e n o t f e a s i i b l e ) i s t o compare DOS a v e r a g e d o v e r d i f f e r e n t s a m p l i n g d e p t h s . Whereas f o r M g K t h e t y p i c a l s a m p l i n g d e p t h w i l l b e s a y ^ 5 0 A f o r T i K w i t h p h o t o n e n e r g y 4510 eV a f i g u r e o f ^ 3 0 0 S w o u l d b e more appropriate. a

a


278

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

CLAMP

SS SAMPLE HOLDER


STIRRER

CONSTANT TEMP. BATH ACID BATH

Figure 32.

Nitrating apparatus used in ESCA studies


17.

CLARK

ESCA


279

F i g u r e 34 shows N i s and C i s s p e c t r a f o r c e l l u l o s e l i n t e r s p a p e r n i t r a t e d f o r 300 s e e s , i n a l o w n i t r a t i n g m i x . Whereas t h e M g K s p e c t r a f o r t h e C i s l e v e l s show a s i g n i f i c a n t c o n t r i b u t i o n from t h e e x t r a n e o u s hydrocarbon the c o r r e s p o n d i n g lineshape a n a l y s i s f o r t h e T i K s p e c t r a i s d e s c r i b e d w i t h no c o n t r i b u t i o n from h y d r o c a r b o n s i n c e even monolayer coverage 5$) w o u l d c o n t r i b u t e a n e g l i g i b l e c o n t r i b u t i o n to the C i s l e v e l s f o r e l e c t r o n s h a v i n g a mean f r e e p a t h a t l e a s t an o r d e r o f m a g n i t u d e greater 9 0 £ ) . T h i s shows t h e g r e a t v a l u e o f h a v i n g a v a r i a b l e photon source r o u t i n e l y a v a i l a b l e . The r e l a t i v e s e n s i t i v i t y o f n i t r o c e l l u l o s e s t o p h o t o c h e m i c a l d e c o m p o s i t i o n s i s known and i t i s t h e r e f o r e n e c e s s a r y t o i n v e s t i g a t e t h e s e n s i t i v i t y t o i n t e r r o g a t i o n by means o f ESCA o f n i t r a t e d and d e n i t r a t e d c e l l u l o s e s a m p l e s . I t may r e a d i l y b e shown t h a t on t h e t y p i c a l t i m e s c a l e f o r t h e ESCA i n v e s t i g a t i o n s photochemical degradation i s n e g l i g i b l e . A t t h e t y p i c a l dose r a t e s i n v o l v e d ( t y p i c a l X - r a y power ^ 150 w a t t s ) significant signs o f decomposition require i r r a d i a t i o n periods o f > 2 hours. The m a i n r e a c t i o n a p p e a r s t o be p h o t o - r e d u c t i o n . Thus t h e h i g h b i n d i n g e n e r g y component i n t h e N i s s p e c t r u m a p p r o p r i a t e t o -O-NO^ s t r u c t u r a l f e a t u r e s i s a c c o m p a n i e d i n t h e c a s e o f m a t e r i a l s u b j e c t e d t o i r r a d i a t i o n f o r e x t e n d e d p e r i o d s by a s m a l l peak a t low b i n d i n g e n e r g y a t t r i b u t a b l e t o - C - N H f u n c t i o n a l i t y . Even after 5 h o u r s i r r a d i a t i o n however t h i s component s t i l l only r e p r e s e n t s a s m a l l f r a c t i o n o f the t o t a l N i s spectrum. We may c o n c l u d e from t h i s t h a t X - r a y d e g r a d a t i o n i s n e g l i g i b l e d u r i n g t h e t i m e s c a l e o f a t y p i c a l ESCA i n v e s t i g a t i o n . a


a

Detailed S t u d i e s O f N i t r a t i o n And D e n i t r a t i o n . For a n i t r a t i n g m i x o f a g i v e n c o m p o s i t i o n one o f t h e most i m p o r t a n t f e a t u r e s o f i n t e r e s t i s t h e q u e s t i o n as t o how r a p i d l y an e q u i l i b r i u m DOS i s e s t a b l i s h e d i n t h e s u r f a c e r e g i o n s a c c e s s i b l e t o ESCA. A s an e x a m p l e F i g u r e 35 shows c o r e l e v e l s p e c t r a f o r samples o f t h e same b a t c h o f l i n t e r s p a p e r s n i t r a t e d f o r d i f f e r i n g p e r i o d s i n a n i t r a t i n g m i x c o n s i s t i n g o f 75% H ^ S O ^ : 22.2% HNO^: 2.8% H ^ O . The N i s l e v e l s , w h i c h p r o v i d e a r e a d y means o f f o l l o w i n g t h e n i t r a t i o n , r e m a i n c o n s t a n t i n i n t e n s i t y from r e a c t i o n times o f 1 sec. to 1 hour. T h i s i n d i c a t e s t h a t on t h e ESCA d e p t h scale, equilibrium i s very r a p i d l y established. This i s not e n t i r e l y unexpected s i n c e the d i f f u s i o n o f n i t r a t i n g mix i n t o the o u t e r m o s t 50& o r so o f t h e c e l l u l o s e f i b r i l s i s e x p e c t e d t o o c c u r rapidly. A s i m i l a r c o m p a r i s o n i s shown i n F i g u r e 36 f o r s a m p l e s s t u d i e d by means o f t h e h a r d e r T i K X-ray source. Here the s a m p l i n g d e p t h w i l l be s e v e r a l h u n d r e d A n g s t r o m s y e t t h e s p e c t r a r e c o r d e d a f t e r r e a c t i o n t i m e s o f 1 s e c . and 300 s e c . a r e c l o s e l y similar. It is clear therefore t h a t t h e e q u i l i b r i u m DOS i s r a p i d l y e s t a b l i s h e d i n the surface r e g i o n s . a

The g r o s s f e a t u r e s o f t h e e q u i l i b r i a w h i c h a r e i n v o l v e d i n d e t e r m i n i n g t h e o v e r a l l DOS i n t h e s u r f a c e r e g i o n s o f c o t t o n f i b r i l s i s o u t l i n e d s c h e m a t i c a l l y i n F i g u r e 3 7 . F o r mixed a c i d


280

PROBES


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

Figure 34.

The N-ls and C-ls spectra for nitrocellulose produced in mixed acid (MgKai, and TiK spectra) 2

ai>2



17.

CLARK

Figure 35.

ESCA


281

Core-level spectra as a function of time for nitration in mixed acid



282


416

412

408 404

293

PROBES

289 285

Figure 36. The TiK spectra for nitration of cellulose for 1 s and 300 s in mixed acid a



17.

ESCA

CLARK


283

n i t r a t i n g m i x e s i t h a s been p r o p o s e d t h a t t h e s u l p h u r i c a c i d component does n o t d i f f u s e i n t o t h e b u l k ( 1 4 ) and t h i s c o u l d o b v i o u s l y l e a d t o d i f f e r e n c e s i n DOS f o r t h e s u r f a c e and b u l k r e g i o n s o f samples s i n c e o n l y i n the s u r f a c e r e g i o n s i s s u l p h o n a tion competitive with n i t r a t i o n . An important q u e s t i o n which ESCA i s p o t e n t i a l l y c a p a b l e o f a n s w e r i n g i s w h e t h e r sulphate e s t e r s may be d e t e c t e d i n t h e s u r f a c e r e g i o n s o f n i t r a t e d m a t e rial. M g K ^ £ s p e c t r a f o r t h e N i s , S2p and C i s r e g i o n s o f c o t t o n l i n t e r s for* n i t r a t e d m a t e r i a l a r e shown i n F i g u r e 3 8 . A l o w l e v e l S2p s i g n a l i s d e t e c t e d and t h e h i g h b i n d i n g e n e r g y o f ^ 169 eV i d e n t i f i e s t h i s as a r i s i n g from s u l p h a t e e s t e r g r o u p s . It is clear therefore that sulphate e s t e r s a r e formed i n t h e v e r y surface regions. Comparable s t u d i e s w i t h t h e T i K X - r a y source w i t h a l a r g e r s a m p l i n g d e p t h shows v i r t u a l l y no e v i d e n c e f o r s u l p h a t e e s t e r s and ESCA t h e r e f o r e u n i q u e l y demonstrates the s u r f a c e n a t u r e o f such g r o u p s . The f i n a l DOS f o r a n i t r o c e l l u l o s e , a t l e a s t as f a r as t h e s u r f a c e r e g i o n s i s c o n c e r n e d , depends on n i t r a t i o n , d e n i t r a t i o n , sulphonation e q u i l i b r i a . The f a c t that t h e DOS i s r a p i d l y e s t a b l i s h e d suggests that d e n i t r a t i o n i s competitive w i t h n i t r a t i o n w h i l s t t h e f a c t t h a t e v e n i n h i g h s u l p h u r i c m i x e s t h e DOS i s s t i l l appreciable i l l u s t r a t e s that n i t r a t e ester formation i s more f a c i l e t h a n s u l p h a t e e s t e r f o r m a t i o n . The r a p i d i t y w i t h which d e n i t r a t i o n - n i t r a t i o n e q u i l i b r i a are e s t a b l i s h e d i n the s u r f a c e r e g i o n s i s n i c e l y i l l u s t r a t e d by t h e d a t a d i s p l a y e d i n F i g u r e 3 9 . Thus f o r s t a r t i n g m a t e r i a l , DOS 2 . 7 d e n i t r a t i o n o f t h e o u t e r m o s t few t e n s o f a n g s t r o m s i s r a p i d and d e p e n d e n t on a c i d m i x . I n 79.1% HNO^ d e n i t r a t i o n i n 1 s e c . i s t o DOS 2 . 3 s i n c e t h i s m i x i s c l o s e t o t h e minimum n e c e s s a r y f o r n i t r a t i o n t o be e f f e c t e d (77.8% H N 0 c o r r e s p o n d s t o t h e m o n o h y d r a t e ) w h i l s t f o r 84.4% H N 0 d e n i t r a t i o n i s t o a DOS o f 2 . 5 on a 1 s e c . t i m e s c a l e . Corresponding s p e c t r a w i t h a T i K X - r a y source i n d i c a t e s t h a t the degree o f d e n i t r a t i o n i s s l i g h t l y lower than for the M g K X - r a y source. The e x t r e m e s e n s i t i v i t y o f ESCA i n t h e d e t e c t i o n o f t h e i n i t i a l stages o f r e a c t i o n s i s n i c e l y d i s p l a y e d by t h e comp a r a t i v e d a t a g i v e n i n F i g u r e 4 0 . T h i s shows t h e n i t r a t i o n and d e n i t r a t i o n o f c e l l u l o s i c samples. The s u b s t a n t i a l secondary s h i f t o f t h e n i t r a t e e s t e r group i s shown by t h e s h i f t t o h i g h b i n d i n g e n e r g y compared w i t h t h e e x t r a n e o u s h y d r o c a r b o n peak (AE 2 . 3 eV f o r t h e C-O-NO^ component compared w i t h AE 1.7 eV f o r cellulose itself. The ESCA d a t a p r e s e n t e d , thusfar therefore establishes that n i t r a t i o n - d e n i t r a t i o n - s u l p h o n a t i o n e q u i l i b r i a are r a p i d l y e s t a b l i s h e d i n the surface r e g i o n s . S y s t e m a t i c s t u d i e s have been made o f n i t r a t i o n s i n b o t h n i t r i c - p h o s p h o r i c and n i t r i c - s u l p h u r i c m i x e s , b o t h from a s u r f a c e and b u l k p o i n t o f v i e w . F o r n i t r i c p h o s p h o r i c m i x e s t h e DOS depends on t h e c o m p o s i t i o n and t h e r e i s l i t t l e e v i d e n c e f o r formation o f phosphate e s t e r s . The DOS i n t h e s u r f a c e r e g i o n t h e r e f o r e r e p r e s e n t s t h e e q u i l i b r i u m b e t w e e n n i t r a t i o n and d e a

3

3

a

a



Figure 38. The N-ls, C-ls, and S-2p core-level spectra for nitrated material (detection of sulfate esters)



17.

CLARK

Figure 39.

ESCA


285

Denitration of nitrocellulose DOS 2.7 as a function of denitrating mix (MgKaij)



286


Figure 40.

PROBES

Comparison of surface nitration and denitration of cellulose



Figure 41.

3

9

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2

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o BULK

60

TIME

Mins.

90

' Ahrs.

11-2

11-9

1275

13.0

%N

Comparison of surface and bulk DOS as a function of time for the nitration of cellulose in nitric-phosphoric acid mixes

30

. . : . ?. . ! . . . . . *5-8%. . P 0 . .2.0.3%. ,H .0. (EX. M,0D.). . ® I ^ A C E

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b

7 5 % H3SO4

70°/o_^°k

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Figure 42.

2-5-

Degree of Substitution

30

HNpJ H

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The Modification, Degradation, and Synthesis of Polymer Surfaces

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