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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.
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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
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