Chemical Reactions on Polymers - American Chemical Society


Chemical Reactions on Polymers - American Chemical Societyhttps://pubs.acs.org/doi/pdf/10.1021/bk-1988-0364.ch025Similar...

0 downloads 118 Views 1MB Size

Chapter 25 3

Reaction of Atomic Oxygen [O( P)] with Polybutadienes and Related Polymers Downloaded via TUFTS UNIV on July 21, 2018 at 20:18:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Morton A. Golub, Narcinda R. Lerner, and Theodore Wydeven Ames Research Center, National Aeronautics and Space Administration, Moffett Field, CA 94035

Thin films of closely related polymers were exposed at ambient temperature to ground-state oxygen atoms [O( P)], generated by a radio-frequency glow discharge in O . The polymers were cis- and trans-1,4-polybutadienes (CB and TB), atactic 1,2-polybutadiene (VB), polybutadienes with different 1,4/1,2 contents, trans polypentenamer (TP), cis and trans polyoctenamers (CO and TO), and ethylene-propylene rubber (EPM). Transmission infrared spectra of CB and TB films exposed to O( P) revealed extensive surface recession (etching), unaccompanied by any microstructural changes within the films; this demonstrated that the reactions were confined to the surface layers. There was no O( P)-induced cis-trans isomerization in CB or TB. From weight-loss measurements, etch rates for elastomeric polybutadienes were found to be very sensitive to the vinyl content, decreasing by two orders of magnitude from CB (2% 1,2) to structures with≥20-40%1,2 double bonds, thereafter remaining substantially constant up to VB(97%1,2). Relative etch rates for EPM and the elastomeric polyalkenamers were in the order: EPM > CO > TP > CB. The highly crystalline TB had an etch rate about six times that of CB, ascribable to a morphology difference, while the partially crystalline TO had an etch rate somewhat higher than that of amorphous CO. Cis/trans content had little or no effect on the etch rate of the polyalkenamers. A mechanism involving crosslinking through vinyl units is proposed to explain the unexpected protection imparted to vinylene-rich polybutadienes by the presence of 1,2 double bonds. 3

2

3

3

This chapter is not subject to U.S. copyright. Published 1988, American Chemical Society

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

25.

Reaction of Atomic Oxygen

GOLUBETAL.

Recently,

much i n t e r e s t

has been shown i n the e f f e c t s

gen on v a r i o u s p o l y m e r i c m a t e r i a l s environment

(1-4).

l o s s or s u r f a c e

This

observed

3

[0( P)],

subjected has f o c u s e d

with s c a r c e l y

analogous

reactions

i n v o l v i n g polymers.

With the p o s s i b l e

exception

What i s

+

0",

0 O D ) , free

t o s i n g l e t oxygen

known (12)

L

[0 ( Ag),

(leading

l

2

3

and 0 ( P ) .

(10)

1

Q

a r e c o n f i n e d t o the s u r f a c e

it

is

a family of c l o s e l y

Thus,

from t h a t o f s i n g l e t

initial

paper examines ent cial

reactions

3

of 0( P)

well-

double is

1

2

3

role of

0( P)

polymers

oxygen. 3

i n degradation o f those polymers.

reactions

identify

the

interest

from the f a c t

oxygen atom a t t a c k and c u l m i n a t i n g As a s t a r t

in this

with polybutadienes

direction, having

Spe­

derives

t h a t oxygen a t o m - o l e f i n

the

reactions

reactions

constitute

(5,16),

tation products.

Thus,

the m a c r o m o l e c u l a r c o u n t e r p a r t s o f

fragmen­ these

r e a c t i o n s might be more r e a d i l y o b s e r v e d and i n t e r p r e t e d than produced i n s a t u r a t e d p o l y m e r s w h i c h , by a n a l o g y s h o u l d i n v o l v e m a i n l y p r o c e s s e s ensuant T h e r e i s added i n t e r e s t

cis-1,4-polybutadiene that t h i s with i t s

since

a process

those

alkanes

(17,18)

i n the c o r r e s p o n d i n g cis-2-butene

with

reported

i s o m e r i z a t i o n on exposure

l o w - m o l e c u l a r weight a n a l o g u e ,

hydro­

3

reaction of 0( P)

Rabek and c o - w o r k e r s

not observed

to simple

on a b s t r a c t i o n o f

i n e x a m i n i n g the

polymer underwent c i s - t r a n s

most

c h a r a c t e r i z e d by

the f o r m a t i o n o f e p o x i d e s and c a r b o n y l compounds as w e l l as

a t o m i c oxygen,

this

differ­

i n s t u d y i n g u n s a t u r a t e d h y d r o c a r b o n polymers 3

(5),

with

various

1 , 4 / 1 , 2 c o n t e n t s and w i t h t h e i r p o l y a l k e n a m e r homologues.

extensively studied class of 0( P)

gen.

to

molecular

while 0

the

the mechanisms o f 0 ( P )

r e l a t e d polymers i n o r d e r t o

s t e p s commencing w i t h t h e

design

possible

i n promoting c h e m i c a l changes i n u n s a t u r a t e d o r s a t u r a t e d a need t o s t u d y

2

i n the c a s e o f

(13>14)),

(15).

0 ,

2

with u n s a t u r a t e d polymers are

2

(9), have

0 ,

By s u i t a b l e

two s p e c i e s and g r o u n d - s t a t e

toward s a t u r a t e d polymers

There i s

the

(6-9).

radiation, in addition

t o a l l y l i c h y d r o p e r o x i d e s and s h i f t e d

s h o u l d be d i s t i n g u i s h a b l e

of

known i s m a i n l y

bonds but w i t h no c h a i n s c i s s i o n — p r o c e s s e s w h i c h , unreactive

the

with

studies

w i t h polymer f i l m s

and u l t r a v i o l e t

or 0 ]

2

Reactions o f

polymer f i l m s ,

3

of 0( P)

i n oxygen plasmas which c o n t a i n

electrons,

e l i m i n a t e a l l but the l a t t e r (11).

to

o f the work o f MacCallum and Rankin

and o p e r a t i o n o f an oxygen d i s c h a r g e a p p a r a t u s , oxygen

given

Although there

on a v a r i e t y o f polymers

the o t h e r s t u d i e s on a t o m i c oxygen r e a c t i o n s i n v o l v e d t h e i r exposure

weight

to ground-

any a t t e n t i o n

t h e r e have been no m e c h a n i s t i c

o f w e i g h t - l o s s measurements

oxy­

orbital

on s i g n i f i c a n t

i n p o l y m e r s exposed

l i t e r a t u r e on the c h e m i c a l r e a c t i o n s

o r g a n i c compounds ( 5 ) ,

0 ,

o f atomic

t o low E a r t h

c h e m i c a l changes i n the p o l y m e r s t h e m s e l v e s .

is a substantial

result

interest

recession

s t a t e oxygen atoms detailed

343

to

reaction

(19,20).

Experimental Polymers.

The polymers used

1,4-polybutadienes

in this

study comprised c i s -

and t r a n s -

(CB and T B ) , amorphous 1 , 2 - p o l y b u t a d i e n e

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

(VB), a

344

CHEMICAL REACTIONS ON POLYMERS 1,4/1,2 contents,

number o f p o l y b u t a d i e n e s w i t h d i f f e r e n t pentenamer

(TP), cis

ethylene-propylene

and t r a n s p o l y o c t e n a m e r s

r u b b e r (EPM),

trans

the s t r u c t u r e s and s o u r c e s

a r e i n d i c a t e d i n T a b l e s I and I I .

poly-

(CO and T O ) , and o f which

A l l o f t h e s e p o l y m e r s were

elasto­

mers e x c e p t f o r the h i g h l y c r y s t a l l i n e TB and the p a r t i a l l y ( 3 3 ? ) c r y s t a l l i n e TO. benzene

The polymers were p u r i f i e d

T B , c a s t from benzene KBr d i s k s

stock solutions

to a thickness 3

ature to 0 ( P )

o f -5-15

3

and

ym, were exposed a t ambient b e f o r e and a f t e r

an IBM IR/85 S p e c t r o m e t e r w i t h ATR a t t a c h m e n t respectively.

above were a l s o weight,

Thin films of a l l 3

to 0 ( P )

Cahn e l e c t r o b a l a n c e 0( P)

Reactor.

(99.99? 0 ) 2

r a d i o - f r e q u e n c y power s u p p l y .

3

of

each

the

beyond the The

i n a p a r a l l e l - p l a t e plasma

in Figure

flow

1) d r i v e n by a 13.56-MHz

Power, s u p p l i e d t h r o u g h a m a t c h i n g ( 3 . 9 7 cm χ 1.59 To p r e v e n t

Additionally,

cm; 1.59

cm a p a r t )

i n - l i n e exposure o f

the samples were l o c a t e d

the

t a i l o f any v i s i b l e glow,

which ended b e f o r e

At an 0

flow r a t e o f 6 . 5 χ 10~

2

r e a c t o r p r e s s u r e o f 73 Pa ( 0 . 5 5 power l e v e l χ 10-

sion of 0

2

2

of cm

3

the b e n d .

3

times

described

(STP)/s,

t o r r ) w i t h the d i s c h a r g e o f f ,

( S T P ) / s ; the l a t t e r

to 0 atoms.

(0.8

cm

15 W, the flow r a t e o f 0 atoms was found t o

the polymer samples least eight

2

discharge

12.7 cm

0 atom f l o w r a t e was measured by NO2 t i t r a t i o n as

elsewhere ( 2 1 ) .

and a

be

f i g u r e r e p r e s e n t e d an 18? c o n v e r ­ 3

Assuming complete 0 ( P ) - i n d u c e d o x i d a t i o n

to C 0

2

and H 0 , the f l o w r a t e o f 0 atoms was 2

t h a t r e q u i r e d to m a i n t a i n the h i g h e s t

etch

of at

rate

2

mg/cm -h, in T B ) .

Polymer sample t e m p e r a t u r e , measured w i t h a thermocouple a t e d beneath a t h i n g l a s s p l a t f o r m s u p p o r t i n g the s a m p l e , e t c h r a t e o f the p o l y m e r ; the maximum t e m p e r a t u r e r i s e any

cover

was measured w i t h a

i n the plasma r e a c t o r between the o r i g i n o f the

the s a m p l e .

observed

The w e i g h t

to 0 ( P )

The

of

t o u l t r a v i o l e t r a d i a t i o n from the p l a s m a , a r i g h t - a n g l e bend

was l o c a t e d

2.4

weight

t o the e x t e r i o r w a l l o f the g l a s s r e a c t o r , was measured by a

B i r d Model 43 r f - w a t t m e t e r .

and

constant

Oxygen atoms were produced from Matheson Gas P r o d u c t s

network to copper e l e c t r o d e s

samples

cm .

exposure

( i l l u s t r a t e d schematically

attached

selenide

t o a p r e c i s i o n o f ± 0 . 0 1 mg.

u l t r a - h i g h p u r i t y oxygen reactor

d r i e d to

initial

2

a r e a was 2.5

s l i p p l u s f i l m b e f o r e and a f t e r

spectrophotometer (zinc

f o r w e i g h t - l o s s measurements.

weight o f each c o v e r s l i p was -100 mg; the f i l m was ~10 mg, and i t s

r e a c t i o n with

the p o l y m e r s mentioned

c a s t onto g l a s s c o v e r s l i p s ,

and s u b j e c t e d

temper­

T r a n s m i s s i o n and ATR

were o b t a i n e d w i t h a P e r k i n - E l m e r Model 621

crystal),

3

F i l m s o f C B , VB, and

onto 0 . 5 - i n c h d i a m e t e r NaCI o r

f o r various periods of time.

i n f r a r e d s p e c t r a o f these f i l m s , 0( P),

by r e p r e c i p i t a t i o n from

s o l u t i o n u s i n g methanol as p r e c i p i t a n t .

run was 9 K , e x h i b i t e d by TB a f t e r

20 min e x p o s u r e

situ­

depended on

encountered 3

to 0 ( P ) .

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

in The

25. G O L U B E T A L . temperature r i s e longed

Reaction of Atomic Oxygen

345

f o r most samples was i n s i g n i f i c a n t

even a f t e r

pro­

exposure.

R e s u l t s and D i s c u s s i o n Infrared

Indications of Etching.

undergoes 3

0( P),

substantial

t a i n l y no c i s - t r a n s

isomerization.

bands, w h i l e

1,4 bands a t

Thus,

intensities

the r e l a t i v e

(or

the

transmission

IR s p e c t r a

increased transmittance)

intensities

of

13.6 and 10.3 ym, r e s p e c t i v e l y ,

are unchanged.

that

the r e a c t i o n o f 0 ( P )

3

w i t h polymer f i l m s a t ambient layers.

of

the c i s - 1 , 4 and t r a n s -

i m p l i e s what had been n o t e d by p r e v i o u s w o r k e r s ( 6 , 9 ) «

c o n f i n e d t o the s u r f a c e

to

and c e r ­

also is

CB f i l m

on p r o l o n g e d e x p o s u r e

w i t h no i n d i c a t i o n s o f any m i c r o s t r u c t u r a l changes

show g r e a t l y d e c r e a s e d all

As may be seen i n F i g u r e 2,

t h i n n i n g , or e t c h i n g ,

T h i s view

is

Figure 2 namely,

temperature

r e i n f o r c e d by

the

c o r r e s p o n d i n g ATR IR s p e c t r a which r e v e a l e d no m i c r o s t r u c t u r a l changes

i n the e t c h e d CB f i l m

spectra are e s s e n t i a l l y i s no need t o p r e s e n t

to a depth o f

the same as

them h e r e .

1

0 -reacted 2

which would i n d i c a t e

3

oxygen

-100 nm; s i n c e

i n the 0 ( P ) - i n d u c e d

etching,

involvement

of of

(13»14).

from F i g u r e 2c i n R e f e r e n c e t o a t o m i c oxygen g e n e r a t e d

photosensitized

decomposition o f N 0 .

recognized that

t h e i r observed c i s - t r a n s

2

been due i n s t e a d

2

They s u p p o r t e d t h i s 15 min t o N 0

17) which i n d e e d showed N 0 with c i s - t r a n s

F i g u r e 3 which p r e s e n t s exposure

to N 0

2

for

2

(!)

2

of

of

view by e x h i b i t i n g

1 atm ( F i g . 4 i n

T h a t view

is

changes

c o n f i r m e d by

o u r IR s p e c t r a o f CB f i l m b e f o r e and a f t e r

15 s

N 0 - r e l a t e d bands a t 6 . 1 2 ,

at

reduced p r e s s u r e

6.45,

7.40,

isomerization (strong

i n he t r e a t e d C B ) .

at

2

groups

2

bands as w e l l as s p e c t r a l

isomerization.

i s v e r y s i m i l a r t o Rabek and c o - w o r k e r s with evidence

Actually,

1

atm); Figure 3

(0.42

Figure 4 in that

7.80,

it

and 11.8 ym,

1 0 . 3 - and weak

shows

together

13.6-ym bands

the N 0 - i n d u c e d c i s - t r a n s

isomerization

2

CB had been r e p o r t e d 25 y e a r s ago by S o v i e t w o r k e r s ( 2 2 ) .

ever,

the p r e s e n t work c l e a r l y demonstrates

3

that 0 ( P )

polymer nor any c i s - t r a n s shows t h a t TB l i k e w i s e 3

film,

i s a new,

surface

i n the

i s o m e r i z a t i o n ; i n the v e r y t h i n ,

weak band a t

12.6 ym, the a s s i g n m e n t

bulk

Figure 4

For completeness,

undergoes e x t e n s i v e e t c h i n g on e x p o s u r e

w i t h no t r a n s -»· c i s there

isomerization.

How­

causes

e t c h i n g o f C B , w i t h no o b s e r v a b l e m i c r o s t r u c t u r a l changes

0( P),

since

2

IR s p e c t r a o f CB exposed

Ref.

(17)

i s o m e r i z a t i o n might have

t o N 0 , a b y p r o d u c t o f the N 0 d e c o m p o s i t i o n ,

a t t a c h e d t o the CB b a c k b o n e .

associated

17,

by mercury

Rabek and c o - w o r k e r s

the c i t e d f i g u r e showed v a r i o u s bands a t t r i b u t a b l e t o N 0 the

there

a l t h o u g h ATR s p e c t r a

CB f i l m s have d i s p l a y e d s u c h a b s o r p t i o n

Figure 2 c e r t a i n l y d i f f e r s r e p o r t e d t o be CB f i l m exposed

the ATR

transmission spectra,

N e i t h e r o f t h e s e s p e c t r a show any

new a b s o r p t i o n a t 2 . 8 ym ( - 0 0 H ) , singlet

the

to

e t c h e d TB o f which

i s as y e t unknown, i n d i c a t i n g minor m i c r o s t r u c t u r a l m o d i f i c a t i o n o f this

p o l y m e r , presumably a t o r near the s u r f a c e .

sion

IR s p e c t r a o f the o t h e r polymers s t u d i e d show o n l y the

of

f i l m - t h i n n i n g or etching,

S i n c e the

they were o m i t t e d from t h i s

transmis­ effects

paper.

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

346

CHEMICAL REACTIONS ON POLYMERS

ELECTRODES

Figure 1.

3

Apparatus f o r exposure o f polymer films to 0 ( P ) .

Λ

τΛ/

3500

3000

2500

r

WAVELENGTH, μηι 6 7 1 r

1800

1600

1400 1200

1000

800

600

WAVENUMBER, cm"

1

Figure 2.

Transmission IR spectra o f CB f i l m on KBr before ( and a f t e r (—) exposure to 0 ( P ) for 16 h. 3

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

)

25.

G O L U B E T AL.

347

Reaction of Atomic Oxygen

WAVELENGTH, μτη 6 7

100,

8

9 10

12 15 —ι—ι ι ι ι

80 λ

i j»

Ι» '

60 < t 40 h CO Ζ < 20 oc ι

3500

ι

I

ι ι ι ι

3000

I/LJ

2500

V6.45

I

I

1800

U L L

1600

1

1

L

1400

I

1200

L

1000

800

600

WAVENUMBER, cm"

1

Figure 3 .

Transmission and a f t e r

IR s p e c t r a o f CB f i l m on NaCl b e f o r e

(—) exposure t o N 0 a t reduced 2

(

)

pres­

s u r e f o r 15 s .

WAVELENGTH, μ η

I

3500

ι

3000

2500

l

V

1800

I

I

1600

I

I

1400

I

I

1200

I

A 10.3

Li—I

1000

1

800

1

600

WAVENUMBER, cm"

1

F i g u r e 4.

Transmission and a f t e r

IR s p e c t r a o f TB f i l m on KBr b e f o r e 3

(—) exposure t o 0 ( P ) f o r 4 . 5 h .

American Chemical Society Library 1155 16th St., N.W.

Benham and Kinstle; Chemical Reactions on Polymers Washington, D.C.Society: 20036Washington, DC, 1988. ACS Symposium Series; American Chemical

(

)

CHEMICAL REACTIONS O N POLYMERS

348 Weight-Loss I n d i c a t i o n s o f E t c h i n g . butadienes

f o r 0 ( P ) - i n d u c e d weight

and p o l y a l k e n a m e r s .

T a b l e s I and I I .

loss in various

From s u c h p l o t s ,

obtained for a family o f c l o s e l y in

F i g u r e 5 shows t y p i c a l z e r o -

3

order k i n e t i c plots

r e l a t e d polymers and a r e summarized

F o r unknown r e a s o n s ,

the k i n e t i c p l o t s

e r a l o t h e r polymers (not shown i n the f i g u r e ) periods,

b u t the p l o t s were a l m o s t always

e r o s i o n commenced.

exhibited

example may be found i n F i g u r e

l i n e a r once the of

this

paper.

1 o f R e f e r e n c e 9,

sev­

surface

p l o t w i t h an i n t e r c e p t on the

Another

which shows z e r o -

t h r o u g h the o r i g i n f o r f o u r d i f f e r e n t

straight-line

for

induction

An example o f a p l o t w i t h an i n d u c t i o n p e r i o d may

be seen i n F i g u r e 4 o f the p r e p r i n t (23) order p l o t s

poly-

e t c h r a t e d a t a were

p o l y m e r s , and a

time-axis

for a

fifth

polymer. A l t h o u g h e t c h r a t e d a t a f o r a p a r t i c u l a r polymer f i l m straight-line any by

kinetic plots,

g i v e n polymer e x h i b i t e d c o n s i d e r a b l e s c a t t e r ; the l a r g e s t a n d a r d d e v i a t i o n s

scatter,

this

i n T a b l e s I and I I .

the cause o f which i s under i n v e s t i g a t i o n ,

d a t a r e p o r t e d h e r e have o n l y s e m i q u a n t i t a t i v e As may be seen from the d a t a o f T a b l e u n i t s has a marked e f f e c t

yielded

the d a t a from one f i l m t o a n o t h e r

for

indicated

Because o f the e t c h

the

rate

significance.

I,

on the e t c h r a t e s

is

the p r e s e n c e

for

of

vinyl

polybutadienes,

d e c r e a s i n g by about two o r d e r s o f magnitude from CB ( w i t h 2% v i n y l units)

to V22-V40,

thereafter

VB ( w i t h 97% v i n y l u n i t s ) . 3

effects of 0( P)

remaining s u b s t a n t i a l l y

T h i s demonstrates

r e a c t i o n w i t h the

1,4

constant

a t once t h a t

and 1,2

up

d o u b l e bonds a r e

not

a d d i t i v e and t h a t the v i n y l groups i m p a r t a s p e c i a l p r o t e c t i o n polybutadienes,

to

the

u n f o r t u n a t e l y , a sample o f CB w i t h no v i n y l

to

double

bonds c o u l d n o t be o b t a i n e d f o r comparison w i t h the v i n y l - f r e e

poly­

alkenamers

have

(Table I I ) ,

but such a polymer would be e x p e c t e d

an e t c h r a t e somewhat h i g h e r than the v a l u e Table I. cis

i n d i c a t e d f o r CB i n

T h a t the e t c h r a t e f o r TB i s about s i x

isomer,

CB, i s

believed

between the r e s p e c t i v e

times

to be due t o a morphology

polymers:

to

that of

scanning e l e c t r o n micrographs

showed the h i g h l y c r y s t a l l i n e TB f i l m t o have a much g r e a t e r roughness The

than the amorphous o r e l a s t o m e r i c d a t a o f T a b l e II

indicate that

EPM was chosen fully

in vinylene

the e t c h r a t e s

(-CH=CH-) u n s a t u r a t i o n .

CB t o a v o i d a morphology f a c t o r

was o b s e r v e d w i t h c r y s t a l l i n e T B .

The d i f f e r e n c e

the p a r t i a l l y c r y s t a l l i n e TO and the e l a s t o m e r i c 1.2:1.0)

rates

than t o the d i f f e r e n c e

C i s / t r a n s content

had l i k e w i s e

The e l a s t o m e r i c

in etch rates,

was a s m a l l e f f e c t , the v i n y l

for

about

between

in their cis/trans (see

the as

in etch rates CO ( r a t i o o f

no p e r c e p t i b l e e f f e c t

i n the v i n y l - c o n t a i n i n g p o l y b u t a d i e n e s

its

monotonically

as a model f o r

i s a t t r i b u t a b l e more t o a morphology d i f f e r e n c e

these polyoctenamers tent.

f o r CB and

increase

instead of c r y s t a l l i n e polyethylene

'saturated'

surface

CB f i l m .

"homologues"—TP, CO ( o r T O ) , and EPM—tend t o with a decrease

its

difference

Table I ) ;

con­

on

etch

if

there

i t was c e r t a i n l y masked by the dominant e f f e c t

groups.

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

of

GOLUB ET AL.

0

Figure 5.

Reaction of Atomic Oxygen

1

2 3 4 5 TIME OF EXPOSURE, h

Typical kinetic

6

7

3

p l o t s f o r 0 ( P ) - i n d u c e d weight

in various polybutadienes

and p o l y a l k e n a m e r s .

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

loss

350

CHEMICAL REACTIONS ON POLYMERS

Table I .

E t c h Rate Data f o r V a r i o u s

? Double bonds Polymer**

c i s - 1 ,4

trans-1,4

Polybutadienes

Etch rate

a

Source

2

mg/cm -h

Vinyl

0

TB

2

96

2

0.80

± 0.40

Goodrich

CB

96

2

2

0.132

± 0.011

Polysar

d

V4

20

76

4

0.093

± 0.011

GenCorp

e

V7

48

45

7

47

10

± 0.031 ± 0.020

V11

11

0.092

± 0.050

Firestone**

V22

47 40

43 42

0.077 0.026

GenCorp

V10

38

22

0.0016 ± 0.0012

V33 V40

31 28

36

33

32

40

0.0046 ± 0.0053 0.0028 ± 0.0016

V70

15

15

70

V82

9 2

9 1

82

0.0015 ± 0.0006 0.0020 ± 0.0012

97

0.0022 ± 0.0007

VB

a

-CH CH=CHCH -...-CH CH(CH=CH )-...

b

V4

2

2

2

Aldrich GenCorp Firestone GenCorp Goodyear^ Firestone Firestone

2

t h r o u g h V82 d e n o t e

polybutadienes

with indicated

vinyl

contents. c

Mr.

J . J . Shipman ( d e c ) ,

BFGoodrich Research C e n t e r , B r e c k s v i l l e ,

OH. d

Dr.

e

Dr.

I. G. Hargis,

f

Dr.

T . A . A n t k o w i a k , The F i r e s t o n e T i r e & Rubber C o . , A k r o n , OH.

g

Dr.

A . F . H a l a s a , Goodyear T i r e & Rubber C o . , A k r o n , OH.

S . E . H o m e , P o l y s a r I n c . , Stow, OH. GenCorp,

A k r o n , OH.

A sample o f p a r t i a l l y h y d r o g e n a t e d V70 (same s o u r c e a s V70; see Table I ) , hydrogenated with a p r o p r i e t a r y c a t a l y s t , ined. and

T h i s polymer, designated

10? 1,2 d o u b l e b o n d s ,

p r i s i n g 5? h y d r o g e n a t e d The

exam­ trans-1,4

1,2 d o u b l e b o n d s .

reduced v i n y l

2

found t o be 0.010 m g / c m - h , o r a b o u t seven t i m e s is consistent

12?

t h e r e m a i n i n g s a t u r a t e d monomer u n i t s com­

1,4 and 60? h y d r o g e n a t e d

e t c h r a t e f o r HV70, w i t h i t s g r e a t l y

result

was a l s o

HV70, had 13? c i s - 1 , 4 ,

content,

that o f V70.

with the above-mentioned o b s e r v a t i o n t h a t

d o u b l e bonds p r o t e c t p o l y b u t a d i e n e a g a i n s t

3

0( P)-induced

was This 1,2

etching.

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

25. G O L U B E T A L . Table I I .

Reaction of Atomic Oxygen

351

E t c h Rate Data f o r c i s - 1 , 4 - P o l y b u t a d i e n e Homologues

and

its

a

Double bonds/

Distribution of

carbon Polymer

atom

CB

0.25 0.20

TP CO

0.125

TO

0.125 0

EPM

a

d o u b l e bonds % cis

x

2

2

0.132

±

0.011

Polysar**

17 81

83

-

0.240 ±

0.016

Goodyear

0.293 ± 0.009

Huls

-

0.355 ± 0.049 0.347 ± 0.014

Huls Polysar

20

19 80

-

-

n

-

-

saturated

0

d

χ = 2 ( C B ) , 3 ( T P ) , 6 (CO, T O ) , » (EPM,

with 2.4:1.0 ethylene-propylene fully

Source

2

96

-[CH=CH-(CH ) ] -; 2

Etch rate mg/cm -h

% Vinyl

% trans

molar r a t i o — a q u a s i model f o r

the

polyalkenamer).

b

Dr.

S.

c

Dr.

E . A . O f s t e a d , Goodyear T i r e & Rubber C o . , A k r o n , OH.

d

Dr.

E . 0.

E . Home, Polysar Corp., E . Siebert,

A k r o n , OH.

Huls C o r p . ,

Piscataway, N J .

Mechanistic Considerations To account f o r the major f i n d i n g s tive

effect of vinyl units

in this

work, namely,

in 1,4-/1,2-polybutadienes

the

and

increased etch rate with decrease in vinylene unsaturation p o l y a l k e n a m e r s , we i n v o k e the g e n e r a l l y 3

reaction of 0( P)

with simple o l e f i n s .

C v e t a n o v i c some 25 y e a r s ago 3

addition of 0( P)

the

T h i s mechanism,

and r e c e n t l y

updated ( 1 6 ) ,

pressure-independent

The "hot" p r o d u c t s i n t u r n a r e e i t h e r (PDF).

biradical

collisionally

PDF, which i s

or c a r ­

deacti­

pressure-

completely

suppressed

a t h i g h p r e s s u r e o r i n condensed m e d i a , would be u n i m p o r t a n t i n c a s e o f polymer f i l m s .

P I F , however,

which i s

i n condensed media a t c r y o g e n i c t e m p e r a t u r e s tant

3

in 0( P)

ture, tions.

reactions

i f PIF o c c u r s

completely

(),

d o u b l e bonds i s assumed

initially

at

scheme:

-CHo—CH -

-CHo—CH -

I •O—CH—CH » 2

(MINOR) -CH

—ÇH—CHO

2

+

»CH 2

(CHAIN R U P T U R E )

PIF

OR

-CHo —CH -

-CHo—CH —C—CH

C = 0 + Η·

I CH « 2

(NO C H A I N R U P T U R E )

3

Whereas 0 ( P )

(NO C H A I N R U P T U R E )

CROSSLINKS

a d d i t i o n t o the

1,2

c h a i n r u p t u r e , a d d i t i o n t o the

(CROSSLINKING POTENTIAL)

d o u b l e bond has no d i r e c t r o u t e

1,4

d o u b l e bond d o e s .

to

Such c h a i n

r u p t u r e would be a p r e c u r s o r t o polymer f r a g m e n t a t i o n and w e i g h t loss, the

while c r o s s l i n k i n g — a p o t e n t i a l

1,2

d o u b l e bond—would c o u n t e r a c t

t e c t " t h e polymer a g a i n s t lower e t c h r a t e than the and 1,2

the

the b i r a d i c a l

unit,

1,4-polybutadiene

CB.

for

formed from the

thus

VB had a much

For polybutadienes

1,4

- C H - (generated 2

1,2

The l e v e l i n g o f f

e x c e s s o f -20? s u g g e s t s t h a t

t h a t can p r o p a g a t e the

d o u b l e bond c o n t e n t of the

i n PIF

d o u b l e bond) o n t o a nearby

T h i s can a c c o u n t f o r the s h a r p d r o p i n e t c h

p o l y b u t a d i e n e s as the

to - 2 0 - 4 0 ? .

to "pro­

d o u b l e b o n d s , a d d i t i o n a l c r o s s l i n k i n g can o c c u r

f o r m i n g a new p o l y m e r i c r a d i c a l

linking process.

addition

T h i s scheme can

1,2-polybutadiene

t h r o u g h a t t a c k o f the m a c r o m o l e c u l a r r a d i c a l of

3

from 0 ( P )

f r a g m e n t a t i o n and t h e r e b y

surface erosion.

a c c o u n t f o r the f i n d i n g t h a t w i t h b o t h 1,4

result

is

rates

i n c r e a s e d from 2

the e t c h r a t e f o r v i n y l "protective

vinyl

cross-

contents

c a p a c i t y " o f the

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

in

vinyl

25. G O L U B E T A L .

Reaction of Atomic Oxygen

groups by c a p t u r i n g the n a s c e n t vinyl

-CH radicals is

reached a t

2

that

content. A l t h o u g h hydrogen a b s t r a c t i o n 3

factor ing

353

in 0( P)

reactions

could r e s u l t

vinyl unit.

is

not expected

with polybutadienes,

from a b s t r a c t i o n o f

The r e s u l t i n g ,

the

resonating

-CH -C-CH=CH 2

t

2

t o be an i m p o r t a n t

additional

crosslink-

t e r t i a r y hydrogen i n

the

radical:

-CH -C=CH-CH 2

2

I

I

could readily attack a v i n y l

d o u b l e bond i n a n o t h e r polymer

molecule

t o produce a c r o s s l i n k and a new p r o p a g a t i n g polymer r a d i c a l . process

would have the e f f e c t

mentation,

since

of protecting

c r o s s l i n k e d polymers a r e more r e s i s t a n t

than a r e t h e i r u n c r o s s l i n k e d c o u n t e r p a r t s To e x p l a i n the p r o g r e s s i v e

increase

i n u n s a t u r a t i o n i n the p o l y a l k e n a m e r s , m e n t a t i o n p r o c e s s e s subsequent efficient

than f r a g m e n t a t i o n

d o u b l e bonds. the

3

rate constant

for 0( P)

(6). in etch

rate with decrease

we p o s t u l a t e

that 3

p r o c e s s e s ensuant

-2.6

from the

on 0 ( P )

fact

times that

that

amers,

the

f o r C B , even though

«135-175

t i m e s the

rate constant

for

(5,16).

increasing etch rate

for

the in

hydro­ Thus,

translates

i n the homologous

The hydrogen a b s t r a c t i o n

to

polyalken­

i n c r e a s i n g l i k e l i h o o d f o r hydrogen a b s t r a c t i o n

χ = 2 to » .

frag­

addition

d o u b l e bonds become f a r t h e r a p a r t i n the

into a progressively from

the

the e t c h r a t e

gen a b s t r a c t i o n from a t e r t i a r y C - H bond ( i n a l k a n e s ) the -CH=CH-

frag­

etching

a d d i t i o n t o the v i n y l e n e d o u b l e bond (as

c i s - 2 - b u t e n e ) a t 298 Κ i s as

to

to hydrogen a b s t r a c t i o n a r e much more

T h i s view f o l l o w s

f u l l y s a t u r a t e d EPM i s

This

the polymer a g a i n s t

series

reaction:

0 + RH -> · 0 Η + Rmust be r a p i d l y

followed

by: R. + 0

R0-*

S i n c e a l k o x y r a d i c a l s a r e known p r e c u r s o r s t o c h a i n s c i s s i o n autoxidation

(24),

undergo f a c i l e is

the

in

"hot" a l k o x y r a d i c a l s formed as shown s h o u l d

chain s c i s s i o n

or fragmentation.

i l l u s t r a t e d f o r the a l k o x y r a d i c a l

e t h y l e n e o r p r o p y l e n e monomer u n i t -CH -C(R)(0-)-CH -* 2

2

The c h a i n

d e r i v e d from e i t h e r

scission the

i n EPM: _* - C H - C ( = 0 ) - R + - C H -

where R i s

2

Η or C H

2

3

Conclusions The major f i n d i n g s o f t h i s protective

effect

study are that v i n y l

in polybutadienes

against

3

groups e x e r t

0( P)-induced

a strong

etching,

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

CHEMICAL REACTIONS O N POLYMERS

354

that

the e t c h

r a t e s f o r the p o l y a l k e n a m e r s

-CH=CH- u n s a t u r a t i o n , surfaces, diene

and t h a t

films

that

there

the

reactions

i s no c i s - t r a n s

i n the b u l k on e x p o s u r e 3

mechanism f o r r e a c t i o n s

of 0( P)

the u n s a t u r a t e d p o l y m e r s . polybutadienes double bonds,

is

isomerization of

with simple o l e f i n s effect

is

applied

to in

through a b s t r a c ­

i n the v i n y l monomer u n i t s .

r a t e d a t a f o r the p o l y a l k e n a m e r s a r e a c c o u n t e d

a high

polybuta-

The C v e t a n o v i c

i n p a r t by c r o s s l i n k i n g t h r o u g h t h e s e

t i o n o f t e r t i a r y hydrogen atoms

the d o u b l e b o n d , the

in

polymer

o f the v i n y l s

and i n p a r t by c r o s s l i n k i n g i n i t i a t e d

increasing competition

t o the

t o a t o m i c oxygen.

The p r o t e c t i v e

explained

increase with decrease

are confined

The e t c h

f o r on the b a s i s

of

between hydrogen a b s t r a c t i o n and a d d i t i o n

former p r o c e s s

giving rise

to fragmentation

to with

efficiency.

Acknowledgments The a u t h o r s a r e g r a t e f u l T a b l e s I and II in

this

t o the

for their g i f t s

i n d i v i d u a l s mentioned of

in footnotes

the v a r i o u s polymer samples

of

used

study.

Literature Cited 1. Visentein, J. T.; Leger, L. J.; Kuminecz, J. F.; Spiker, I. K. Paper 85-0415, AIAA 23rd Aerospace Sciences Meeting, January 1985, and references cited therein. 2. Zimcik, D. G.; Tennyson, R. C.; Kok, L. J.; Maag, C. R. Eur. Space Agency Spec. Publ., 1985, ESA SP-232; Chem. Abstr. 1986, 104, 149894, and references cited therein. 3. Banks, Β. Α.; Mirtich, M. J.; Rutledge, S. K.; Swec, D. M. Thin Solid Films 1985, 127, 107. 4. Arnold, G. S.; Peplinski, D. R. AIAA J. 1985, 23, 1621. 5. Huie, R. E.; Herron, J. T. Prog. Reaction Kinetics 1975, 8, 1. 6. Hansen, R. H.; Pascale, J. V.; De Benedictis, T.; Rentzepis, P. M. J. Polym. Sci. 1965,A3,2205. 7. Taylor, G. N.; Wolf, T. M. Polym. Eng. Sci. 1980, 20, 1087. 8. Ueno, T.; Shiraishi, H.; Iwayanagi, T.; Nonogaki, S. J. Electrochem. Soc. 1985, 132, 1168. 9. MacCallum, J. R.; Rankin, C. T. Makromol. Chem. 1974, 175, 2477. 10. Lawton, E. J. J. Polym. Sci., A-1, 1972, 10, 1857. 11. Westenberg, Α. Α.; de Haas, Ν. J. Chem. Phys. 1964, 40, 3087. 12. Golub, M. A. Pure Appl. Chem. 1980, 52, 305. 13. Kaplan, M. L.; Kelleher, P. G. J. Polym. Sci., A-1, 1970, 8, 3163; Rubber Chem. Technol. 1972, 45, 423. 14. Rabek, J. F.; Ranby, B. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 1463. 15. Carlsson, D. J.; Wiles, D. M. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 2217.

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

25.

GOLUB ET AL.

Reaction of Atomic Oxygen

355

16. Cvetanović, R. J . ; Singleton, D. L. Rev. Chem. Intermed. 1984, 5, 183. 17. Rabek, J. F.; Lucki, J . ; Rånby, B. Eur. Polym. J. 1979, 15, 1089. 18. Rabek, J. F.; Ranby, B. Photochem. Photobiol. 1979, 30, 133. 19. Klein, R.; Scheer, M. D. J. Phys. Chem., 1966, 72, 616. 20. Cvetanovio, R. J. Advan. Photochem., 1963, 1, 115. 21. Kaufman, F.; Kelso, J. R. In 8th Symposium (International) on Combustion; The Combustion Institute, Williams and Wilkins: Baltimore, 1960, p. 230. 22. Ermakova, I. I.; Dolgoplosk, Β. Α.; Kropacheva, Ε. N. Dokl. Akad. Nauk. USSR 1961, 141, 1363; Rubber Chem. Technol. 1962, 35, 618. 23. Golub, Μ. Α.; Lerner, N. R.; Wydeven, T. Polym. Prepr. 1986, 27 (2), 87. 24. Shelton, J. R. Rubber Chem. Technol. 1983, 56, G71. R E C E I V E D August 2 7 , 1987

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.