ESCA and SEM Studies on Polyurethanes for Biomedical Applications

22

Photon, Electron, and Ion Probes of Polymer Structure and Properties Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 04/11/17. For personal use only.

ESCA and SEM Studies on Polyurethanes for Biomedical Applications B. D. RATNER Department of Chemical Engineering and Center for Bioengineering, BF-10, University of Washington, Seattle, WA 98195

Polyetherurethanes (PEU's) have long been considered for use in biomedical applications because of their excellent mechanical properties, their resistance to hydrolysis and degradation, and in some instances, their good biocompatibility. Assuming the absence of toxic, leachable, low molecular weight components, the biocompatibility (and, in particular, the blood compatibility) of PEU's w i l l be strongly influenced by the surface properties of the polymers. Techniques which have been used to study polyurethane surfaces include contact angle measurements (2), attenuated total reflectance IR (ATR-IR) (1, 2, 3), Auger spectroscopy (4), and electron spectroscopy for chemical analysis (ESCA) (5-9). One of the most important observations from previous characterization studies is that for solvent-cast PEU films, the air and the casting surface sides differ significantly in average chemical structure (3-7). The significance of differences in surface structure of polyurethanes on biological reactivity has received relatively little study. Many vena cava ring test studies were performed on PEU's (10), but no clear-cut structure-blood compatibility relationships were arrived at. Lyman, et al. (7), noted differences in the ratio of adsorbed albumin to other plasma proteins on two PEU's differing in polyether chain length. They also observed that platelet adhesion to these materials decreased with increasing albumin fractions at the surface. Stupp, et al. (2), found differences in the amount of adsorbed fibrinogen and possibly i n the conformation of the adsorbed fibrinogen on PEU's cast against glass and poly(ethylene terephthalate). They related this to differences in the aromatic and polyether contents of the surface as observed by ATR-IR. This study represents a preliminary investigation on the chemistry and morphology of polyurethane surfaces with long term goals directed toward relating these factors both to the bulk structure of the polyurethanes and to their blood and tissue 0097-6156/81/0162-0371 $05.00/ 0 © 1981 American Chemical Society

372

PHOTON, ELECTRON,

A N D ION PROBES

compatibility. The p r i m a r y a n a l y t i c a l t e c h n i q u e u s e d was ESCA b e c a u s e o f i t s s u r f a c e s e n s i t i v i t y ( 1 0 - 1 0 0 8) and b e c a u s e o f t h e h i g h i n f o r m a t i o n c o n t e n t from h i g h r e s o l u t i o n C i s s p e c t r a .


Methods The ESCA a n a l y s i s o f t h e p o l y m e r s was p e r f o r m e d on a H e w l e t t P a c k a r d M o d e l 5950B ESCA s y s t e m . A 0.8 kwatt monochromatized X r a y beam from an a l u m i n u m anode was u s e d f o r a l l s p e c t r a . An e m i s s i o n from an e l e c t r o n f l o o d gun was u s e d to n e u t r a l i z e c h a r g e build-up. C i s peaks a s s o c i a t e d w i t h h y d r o c a r b o n - l i k e e n v i r o n ments were a s s i g n e d a b i n d i n g e n e r g y o f 2 8 5 . 0 eV t o c o r r e c t f o r the e n e r g y s h i f t r e s u l t i n g f r o m the e l e c t r o n f l o o d g u n . Areas under the v a r i o u s o v e r l a p p i n g peaks i n t h e C i s s p e c t r a were d e t e r m i n e d u s i n g a Dupont 310 c u r v e r e s o l v e r . A l l p o l y u r e t h a n e m a t e r i a l s used i n t h i s s t u d y were e i t h e r i n t h e form o f e x t r u d e d t u b e s ( T y g o t h a n e , S u p e r t h a n e , P e l l e t h a n e ) o r as f i l m s c a s t on c l e a n g l a s s from r e a g e n t g r a d e d i m e t h y l a c e t a m i d e (DMAC). O n l y t h e l u m i n a l s u r f a c e s o f t h e t u b e s and t h e g l a s s f a c i n g s i d e s o f the c a s t f i l m s were o b s e r v e d by ESCA. Results

and D i s c u s s i o n

The a p p r o a c h i n t h i s s t u d y to the a n a l y s i s o f p o l y u r e t h a n e s u r f a c e s was b a s e d upon the i d e a t h a t the s u r f a c e properties r e p r e s e n t a summation o f t h e e f f e c t s o f a l l c h e m i c a l g r o u p s a t t h e surface. The v a r i o u s s t r u c t u r a l u n i t s w h i c h m i g h t be e x p e c t e d i n a p o l y u r e t h a n e were r e p r e s e n t e d by m o d e l compounds. ESCA s p e c t r a o f t h e s e m o d e l compounds r e v e a l e d the c h e m i c a l s h i f t s f o r e a c h t y p e o f g r o u p and the peak w i d t h s to be e x p e c t e d ( F i g u r e 1 ) . Unambiguous c u r v e r e s o l u t i o n c a n be p e r f o r m e d u s i n g t h e s e two pieces of information (11). I n i t i a l ESCA a n a l y s i s o f a s e r i e s o f c o m m e r c i a l l y a v a i l a b l e p o l y u r e t h a n e s r e v e a l e d a wide range o f s u r f a c e s t r u c t u r e s with l a r g e v a r i a t i o n s i n the p r o p o r t i o n s o f the v a r i o u s i m p o r t a n t s t r u c t u r a l g r o u p s at the s u r f a c e o f the m a t e r i a l s ( F i g u r e 2 ) . S i n c e the p o s s i b i l i t y o f s u r f a c e c o n t a m i n a n t f i l m s o b s c u r i n g t h e t r u e p o l y u r e t h a n e s u r f a c e was c o n s i d e r e d , and a l s o s i n c e l e a c h a b l e components f r o m p o l y m e r i c m a t e r i a l s f r e q u e n t l y i n d u c e u n d e s i r a b l e b l o o d and t i s s u e r e s p o n s e s , v a r i o u s e x t r a c t i o n p r o c e d u r e s were i n v e s t i g a t e d . F i g u r e 3 shows C i s ESCA s p e c t r a o f t h e p o l y e s t e r u r e t h a n e Tygothane (Norton P l a s t i c s ) a f t e r a s e r i e s o f washes and e x t r a c t i o n s . F i g u r e s 4 and 5 show s i m i l a r r e s u l t s w i t h the p o l y e t h e r u r e t h a n e s S u p e r t h a n e (Newage I n d u s t r i e s , I n c . ) and P e l l e t h a n e 2 3 6 3 - 8 0 A ( U p j o h n , I n c ) . D a t a on peak a r e a s and s h i f t s are t a b u l a t e d i n Table I for Tygothane. The I v o r y soap s o l u t i o n wash ( w i t h s o n i c a t i o n ) p r o b a b l y b e g i n s t o remove o n l y a s u r f a c e c o n t a m i n a n t f i l m . Evidence for t h i s i s s u g g e s t e d by t h e peak s h i f t t o h i g h e r b i n d i n g e n e r g i e s o f the h i g h e s t b i n d i n g e n e r g y component o f t h e C i s s p e c t r a f o r t h e

RATNER

Polyurethanes for Biomedical Applications

373

-CH; POLYETHYLENE


1

[•

-*CH CH f 2

2 n

POLYETHYLENE • POLY (ETHYLENE GLYCOL) -tCHjCH^-tCHjCHjOf,

DIETHYL UREA 0 CHjCH NCNCH CH, I I H H 0 2

NI C=0 N

2

CH, 40% 40% 20% 43% 41% 16%

N

C=0

t- BUTYLCARBAMATE

,

0 NH -C-0-t-Bu 2

CH.-CO- NCO Expected

60% 20% 20% 67% 12% 21%

N N ' ' C=0 C=0 0

287' "C0--CH3 285eV

283eV

N —

BINDING ENERGY (eV)

Figure 1. The C-ls ESCA spectra of model compounds used to obtain peak widths and peak shifts for functional groups expected in polyurethanes

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


SRI 3-1000-IE

289

287

285

ESTANE 58409

283

289

287

PELLETHANE

285

binding energy

Figure 2.

PROBES

283

289

287

285

283

(eV)

The C-ls ESCA spectra of three polyurethanes showing the variation in peak shapes that can be expected for polyurethanes

\ \ \ MEOH/ACETONE

1 \ \ MeOH-2 WEEKS

Ld 1 \ MeOH-l WEEK

\ \ l V O R Y SOAP X^UNWASHED 291

289 -*

Figure 3.

287

285

283

281

Binding Energy (eV)

The C-ls spectra for Tygothane after a series of washes and extractions


RATNER


MEOH + ACETONE

289

287

285

283

289

287

UNWASHED

3 DAYS - MEOH

I WEEK MEOH

285

289

287

375

285

283 289

287

285

283

binding energy (eV)

Figure 4.

The C-ls spectra for Superthane after a series of extraction procedures

MEOH + ACETONE

289

287

UNWASHED

3 DAYS-MEOH

285

283 289

287

285

283 289

287

285

283

binding energy

Figure 5.

The C-ls spectra for Pellethane after a series of extraction procedures

376

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

Table

I

Relative Concentrations o f C a r b o n S p e c i e s at the Tygothane Tubing A f t e r V a r i o u s C l e a n i n g Procedures Percent


-NH

Unwashed Ivory 1 Week - MeOH 2 Week - MeOH 2 Week - MeOH - Acetone

of

o f C i s Spectrum 0

I -0-C=0 Tygothane: Tygothane: Tygothane: Tygothane: Tygothane: + 3 Days

Surface

II -C-N-

— — 16 16 9

5 5 -

-C-0

-C-H

26 11 25 25 20

69 84 59 59 71

unwashed and I v o r y soap washed T y g o t h a n e s ( T a b l e I ) a f t e r s o l v e n t treatment. The peak p o s i t i o n p r i o r to s o l v e n t t r e a t m e n t i s i n d i c a t i v e o f an amide group (288 eV) r a t h e r t h a n the expected c a r b a m a t e l i n k a g e ( 2 8 9 . 2 e V ) . T h u s , t h e s u r f a c e o b s e r v e d may be t h a t o f a r e l a t i v e l y low m o l e c u l a r w e i g h t s u r f a c t a n t / a m i d e o f t h e type frequently used as an e x t r u s i o n l u b r i c a n t . After the m e t h a n o l e x t r a c t i o n ( i n a s o x h l e t e x t r a c t o r ) i n c r e a s e s i n compone n t s a s s o c i a t e d w i t h h i g h e r b i n d i n g e n e r g y peaks a r e o b s e r v e d . A c e t o n e e x t r a c t i o n o f the m e t h a n o l - e x t r a c t e d T y g o t h a n e r e s u l t s i n a p a r t i a l l o s s o f t h e s e h i g h e r b i n d i n g e n e r g y components a t t h e surface. Upon c o o l i n g , t h e m e t h a n o l and a c e t o n e u s e d i n the e x t r a c t i o n were found to c o n t a i n w h i t e , f l o c u l a n t p r e c i p i t a t e s . The amount o f p r e c i p i a t e was a l w a y s g r e a t e r w i t h a c e t o n e . Gravim e t r i c e x t r a c t i o n data is l i s t e d i n Table I I . Table E x t r a c t i o n Data For

Tygothane Pellethane Superthane

II Polyurethanes

% Wt. L o s s A f t e r Methanol E x t r a c t i o n

% Wt. Loss A f t e r M e t h a n o l + Acetone E x t r a c t i o n

1.31 + 0 . 0 1 1.24 + 0 . 0 2 1.56+0.03

2.54 + 0.08 2.49 + 0.12 5.25+0.14

F o r S u p e r t h a n e , t h e e x t r a c t p r e c i p i t a t e from m e t h a n o l had a C / 0 r a t i o ( a s d e t e r m i n e d by ESCA) o f 3 . 7 1 . T h i s v a l u e s h o u l d be compared t o C / 0 v a l u e s f o r t h e l u m i n a l s u r f a c e o f t h e t u b i n g a f t e r methanol e x t r a c t i o n (3.39-3.83) and a f t e r acetone e x t r a c t i o n (4.47). The a n a l y s i s o f the C i s s p e c t r u m o f the Superthane e x t r a c t ( F i g u r e 6) i n d i c a t e s t h e p o s s i b i l i t y o f a h i g h p r o p o r t i o n


22.

RATNER


—i

291

1

1

1

289

287

285

377

1—

283

binding energy (eV)

Figure 6. The C-ls spectrum of the precipitate from the methanol used to extract Superthane. The component peaks were resolved using a peak width at half height of 1.2 eV.

378

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

o f C-0 t y p e bonds. The e x t r a c t has b e e n found by GPC t o be o f s u b s t a n t i a l l y lower m o l e c u l a r weight than the o r i g i n a l polymer (Table I I I ) . Table


Gel Permeation tracts

Chromatographic

III Data

for

Polyurethanes

RETENTION TIME with LiBr Untreated Pellethane Extracted Pellethane Methanol E x t r a c t o f Pellethane Acetone E x t r a c t o f P e l l e t h a n e P o l y s t y r e n e (MW = 3 , 0 0 0 ) P o l y s t y r e n e (MW = 3 7 , 0 0 0 ) P o l y s t y r e n e (MW = 8 0 , 0 0 0 ) P o l y s t y r e n e (MW = 2 3 3 , 0 0 0 ) * DMF, F l o w r a t e = 0 . 7 5 m l / m i n , c olumns.

(Minutes)* without

32.4 31.3 44.1 35.0 — — — — 10

X , 10

and E x -

LiBr

31.7 32.3 44.2 36.0 54.7 43.5 40.5 34.2 8,

and 10

8 Styragel

A h y p o t h e s i s c o n c e r n i n g the s u r f a c e s t r u c t u r e o f t h e s e m a t e r i a l s has b e e n c o n s t r u c t e d b a s e d upon t h i s d a t a . I t would appear t h a t the p o l y u r e t h a n e s o b s e r v e d c o n t a i n p o l y e t h e r or p o l y e s t e r e n r i c h e d o l i g o m e r s . These have r e l a t i v e l y l o w s o l u b i l i t y i n warm m e t h a n o l and m i g r a t e t o t h e s u r f a c e o f t h e p o l y m e r t u b e where t h e y slowly leach into s o l u t i o n . In acetone, these low m o l e c u l a r w e i g h t components a r e r a p i d l y removed. Thus, the s u r f a c e o f the tube observed a f t e r ( i n c o m p l e t e ) methanol e x t r a c t i o n i s r i c h i n t h e o l i g o m e r phase ( i . e . , t h e C / 0 r a t i o o f t h e e x t r a c t i s a p p r o x i m a t e l y e q u a l to t h a t o f the t u b i n g s u r f a c e ) , w h i l e a f t e r a c e t o n e e x t r a c t i o n , the C / 0 r a t i o i s h i g h e r i n d i c a t i n g l o s s o f m a t e r i a l high i n oxygen. The d i s a p p e a r a n c e o f C - 0 t y p e bonds i n t h e C i s s p e c t r a a f t e r acetone e x t r a c t i o n a l s o supports t h i s h y p o t h e s i s . Experiments with p r e c i p i t a t i o n - p u r i f i e d polyetherurethanes of known s t r u c t u r e p r e p a r e d by c a s t i n g on g l a s s f u r t h e r s u p p o r t t h e hypothesis. These p o l y m e r s , p r e p a r e d b y S R I I n t e r n a t i o n a l and containing poly(oxypropylene glycol) blocks of various molecular w e i g h t s a l w a y s showed, b y ESCA, a s u r f a c e e n r i c h m e n t o f the p o l y e t h e r component compared t o what w o u l d be e x p e c t e d b a s e d upon t h e s t o i c h o m e t r y . O t h e r i n v e s t i g a t o r s have a l s o r e c e n t l y r e p o r t ed p o l y e t h e r s u r f a c e e n r i c h m e n t or d e p l e t i o n f o r P E U ' s (_12, 1 3 ) . D e t a i l e d r e s u l t s w i t h t h e s e p o l y m e r s w i l l be r e p o r t e d e l s e w h e r e . D i s t i n c t m o r p h o l o g i c a l changes i n the s u r f a c e s t r u c t u r e at each s t a t e o f t h e w a s h i n g / e x t r a c t i o n p r o c e s s a r e o b s e r v e d by scanning electron microscopy. F i g u r e 7 shows r e s u l t s f o r T y g o -


22.

RATNER


379

Figure 7. Scanning electron micrographs of Tygothane surfaces: (a) unwashed, 1500X; (b) Ivory soap washed with sonication, 750X; (c) extracted one week in methanol, 750X; (d) after methanol extraction, 3 days acetone extraction, 750X •


380

PHOTON,

ELECTRON,

A N D ION PROBES

^ 3C-,

Percent of surface carbon atoms forming hydrocarbon (C-H) bonds

Figure 8.

Platelet consumption of polyurethanes as a function of percent hydrocarbon component in the C-ls spectra (from Ref. 9)

22.

RATNER


381

thane. S i m i l a r o b s e r v a t i o n s were made f o r S u p e r t h a n e and P e l l e t hane. I n t h e e a r l i e s t wash s t a g e s , i n c l u s i o n s , w h i c h a p p e a r a l i g h t c o l o r , seem to be imbedded i n a d a r k e r m a t r i x . Extraction w i t h m e t h a n o l may remove t h i s " m a t r i x r e v e a l i n g t h e n a t u r e o f t h e substructure. One c a n e a s i l y s p e c u l a t e on t h e n a t u r e o f t h e s e m o r p h o l o g i c a l c h a n g e s b a s e d upon t h e c h e m i c a l e v i d e n c e p r e s e n t e d previously. H o w e v e r , f u r t h e r e x p e r i m e n t s must be p e r f o r m e d b e f o r e the p r e c i s e n a t u r e o f t h e s e m o r p h o l o g i c a l changes c a n be clarified. Photon, Electron, and Ion Probes of Polymer Structure and Properties Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 04/11/17. For personal use only.

1 1

Conclusions U s i n g m o d e l compounds, t h e s u r f a c e c o m p o s i t i o n o f p o l y u r e t h a n e m a t e r i a l s o f unknown c o m p o s i t i o n c a n be i d e n t i f i e d . Large d i f f e r e n c e s i n surface s t r u c t u r e are observed for p o l y u r e t h a n e s . A l s o , the s u r f a c e s t r u c t u r e i s s e n s i t i v e to e x t r a c t i o n and c l e a n ing procedures. Some o f t h e s e changes may be r e l a t e d t o p r o c e s s i n g a d d i t i v e s i n the c o m m e r c i a l grade p o l y u r e t h a n e s used or to low molecular weight polyurethanes. F u t u r e e x p e r i m e n t s w i l l l o o k at c a r e f u l l y s y n t h e s i z e d p o l y e t h e r u r e t h a n e s o f known c o m p o s i t i o n t o r e l a t e b u l k and s u r f a c e s t r u c t u r e . A l s o , e x t r a c t s w i l l be f u r t h e r a n a l y z e d by E S C A , GPC and IR t o d e t e r m i n e t h e i r s t r u c t u r e . The b i o l o g i c a l s i g n i f i c a n c e o f t h i s work i s r e v e a l e d i n r e c e n t e x p e r i m e n t s i n w h i c h the s u r f a c e p r o p e r t i e s o f v a r i o u s p o l y u r e t h a n e s were d i v i d e d i n t o n o n - d i s p e r s i v e and d i s p e r s i v e f o r c e components by c o n s i d e r i n g , as a s i m p l e f i r s t - o r d e r a p p r o x i m a t i o n , t h a t the h y d r o c a r b o n - t y p e C i s peak r e p r e s e n t s c o n t r i b u t i o n s to o n l y t h e d i s p e r s i v e f o r c e component, and a l l o t h e r C i s peaks indicate contributions to non-dispersive-type surface forces. When p l a t e l e t c o n s u m p t i o n as m e a s u r e d i n a b a b o o n A - V s h u n t m o d e l i s p l o t t e d a g a i n s t the f r a c t i o n o f the C i s ESCA s p e c t r a r e p r e s e n t a t i v e o f hydrocarbon-type groups ( d i s p e r s i v e f o r c e component), a l i n e a r r e l a t i o n s h i p i s o b t a i n e d ( F i g u r e 8, reference 9 ) . R e c e n t l y , ESCA and p l a t e l e t c o n s u m p t i o n d a t a from e x t r a c t e d and u n e x t r a c t e d T y g o t h a n e have been found t o f i t t h i s trend. This i s p a r t i c u l a r l y i n t e r e s t i n g since i t i n d i c a t e s that b o t h p o l y e s t e r u r e t h a n e s and p o l y e t h e r u r e t h a n e s b e h a v e i n a m e c h a n i s t i c a l l y s i m i l a r f a s h i o n w i t h r e s p e c t to t h e i r r e a c t i o n w i t h blood. A l s o , f u r t h e r a n a l y s i s o f t h e ESCA d a t a has i n d i c a t e d t h a t p l a t e l e t consumption c o r r e l a t e s s t r o n g l y w i t h the c o n c e n t r a t i o n o f C-CK t y p e l i n k a g e s a t t h e s u r f a c e and n o t w i t h t h e c o n c e n t r a t i o n o f c a r b a m a t e - t y p e g r o u p s . These c o n c l u s i o n s do n o t a g r e e w i t h t h e c o n c l u s i o n s a r r i v e d a t by a n o t h e r r e s e a r c h g r o u p i m p l i c a t i n g the h a r d segment ( i . e . c a r b a m a t e - t y p e f u n c t i o n a l i t i e s ) as t h e i n i t i a t o r of blood r e a c t i o n (140. A d e t a i l e d s t u d y on t h e p l a t e l e t c o n s u m p t i o n o f p o l y u r e t h a n e s w i l l be p u b l i s h e d s h o r t l y . Acknowledgement This 22163.

work was

supported

by NHLBI

Program P r o j e c t

G r a n t HL

382

PHOTON, ELECTRON, AND ION PROBES


Literature Cited 1.

Boretos, J.W., Pierce, W.S., Baier, R.E., Leroy, A . F . , Donachy, H . J . , J . Biomed. Mater. Res., 1975, 9, 327.

2.

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RECEIVED

March 10, 1981.

1980,

ESCA and SEM Studies on Polyurethanes for Biomedical Applications

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