Poly(Î±-methylstyrene-dimethylsiloxane) Block Copolymers. The

26 Poly(α-methylstyrene-dimethylsiloxane) Block Copolymers. The Effects of Microstructure on Properties ,1

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch026

ANDREW H. WARD, THOMAS C. KENDRICK

and JOHN C. SAAM

Dow Corning Corp., Midland, Mich. 48640

A highly regular microstructure of rod-like microdomains of poly(α-methylstyrene) in a polydimethylsiloxane matrix forms in extruded plugs or films. When compared with solution cast films, the extrudedfilmshave anisotropic mechanical properties and in creased oxygen permeability. Extruded and blownfilmsshow no definable structure, whereas solution-castfilmshave a morphology of islands of poly(α-methylstyrene) in the siloxane matrix in some regions and the inverse structure in others.

C

o n s i d e r a b l e a t t e n t i o n h a s b e e n d e v o t e d t o t h e effects of m o l e c u l a r s t r u c t u r e o n p r o p e r t i e s a n d m o r p h o l o g y of b l o c k c o p o l y m e r s w h e r e h a r d g l a s s y b l o c k s b o u n d soft r u b b e r y b l o c k s . S t u d i e s of t h e effects o f s a m p l e h i s t o r y a n d f a b r i c a t i o n , h o w e v e r , h a v e so f a r b e e n l i m i t e d t o cases w h e r e p o l y s t y r e n e is the h a r d b l o c k , a n d p o l y b u t a d i e n e or p o l y i s o p r e n e are t h e r u b b e r c o m p o n e n t s ( I , 2 , 3 ) . T h e s e s t u d i e s h a v e s h o w n t h a t t h e l e v e l of s h e a r a p p l i e d d u r i n g m e l t f a b r i c a t i o n h a s p r o f o u n d effects o n b o t h m o r p h o l o g y a n d m e c h a n i c a l properties i n the fabricated sample. T h e p r e s e n t s t u d y seeks t o c o r r e l a t e s a m p l e h i s t o r y a n d f a b r i c a t i o n w i t h m o r p h o l o g y a n d s u b s e q u e n t effects o n m o d u l u s a n d p e r m e a b i l i t y t o o x y g e n i n a n a n a l o g o u s s y s t e m w h e r e t h e r u b b e r b l o c k is p o l y d i m e t h y l s i l o x a n e (4). T h i s i s of i n t e r e s t n o t o n l y b e c a u s e of t h e s i g n i f i c a n t l y l a r g e r d i s p a r i t y i n s o l u b i l i t y , b u t also b e c a u s e of t h e u n i q u e l y h i g h gas p e r m e a b i l i t y of p o l y d i m e t h y l s i l o x a n e . P e r m e a b i l i t y as w e l l as m e c h a n i c a l p r o p e r t i e s m i g h t b e e x p e c t e d to b e s e n s i t i v e t o c h a n g e s i n m i c r o s t r u c t u r e b r o u g h t a b o u t d u r i n g f a b r i c a t i o n . Permeability i n related poly (sulfone-siloxane) block copolymers was unaffected b y s o l v e n t h i s t o r y i n s o l u t i o n - c a s t films ( 5 ) , b u t t h e effects of m e c h a n i c a l shear d u r i n g f a b r i c a t i o n h a v e n o t been e x a m i n e d i n b l o c k c o p o l y m e r s based o n siloxanes. T h e s y s t e m c h o s e n f o r s t u d y h a s p o l y ( α - m e t h y l s t y r e n e ) as i t s g l a s s y c o m p o n e n t ; i t d i f f e r s f r o m t y p i c a l b l o c k c o p o l y m e r s i n t h a t t h e h a r d a n d soft 1

Present address: D o w C o r n i n g Ltd., B a r r y , W a l e s CF6 7YL, U.K. 300

Platzer; Copolymers, Polyblends, and Composites Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

WARD E T A L .

Microstructure

v s . Properties

301


26.

Figure 1.

Electron micrographs showing effect of block size on morphology

A , B, and C : surface of thin films cast from toluene of block copolymers containing 40% poly(amethylstyrene) [A, poly(a-methylstyrene) block—4000 g/mole; B, 8000 g/mole; and C , 18,000 g/mole]; D: thin section perpendicular to the surface, composition as in Β but thicker film

b l o c k s a l t e r n a t e six t o e i g h t t i m e s a n d a r e t e r m i n a t e d w i t h t h e s i l o x a n e b l o c k s . T h e h a r d b l o c k s i n t h i s s y s t e m , as i n o t h e r s t h a t w e r e b a s e d o n p o l y s t y r e n e ( 6 ) , are n e c e s s a r i l y s m a l l ( m o l e c u l a r w e i g h t 4 0 0 0 - 1 5 , 0 0 0 ) c o m p a r e d w i t h t y p i c a l t r i b l o c k c o p o l y m e r s . L a r g e r b l o c k sizes i n t h i s s y s t e m r e s u l t i n m a t e r i a l s t h a t have poor strength a n d are difficult to fabricate f r o m the melt. A n i n c i d e n t a l


302

COPOLYMERS,

POLYBLENDS,

AND

COMPOSITES

a d v a n t a g e is t h a t s t a i n i n g o f t h e r u b b e r y p h a s e i s n o t n e c e s s a r y i n t h e e l e c t r o n microscopy ( 7 ). Experimental W i t h t h e exception of t h e sequence s h o w n i n F i g u r e 1, a l l data w e r e obtained o n a single block c o p o l y m e r of the f o r m u l a [ B A B ] where A repre sents p o l y ( a - m e t h y l s t y r e n e ) b l o c k s o f m o l e c u l a r w e i g h t 6 0 0 0 , a n d Β p o l y dimethylsiloxane blocks of molecular w e i g h t 4500. Synthesis a n d methods of c h a r a c t e r i z a t i o n h a v e a l r e a d y b e e n r e p o r t e d ( 4 , 6). Tensile m o d u l u s w a s measured o n a n Instron under the conditions given i n T a b l e I. E l e c t r o n m i c r o g r a p h s w e r e o b t a i n e d w i t h a H i t a c h i H S - 7 5 electron m i c r o s c o p e . S o l u t i o n - c a s t films w e r e p r e p a r e d b y r e m o v a l o f s o l v e n t a t r o o m t e m p e r a t u r e f r o m 1 0 % t o l u e n e s o l u t i o n s f o l l o w e d b y final s o l v e n t r e m o v a l a t 7 0 ° C i n v a c u u m . S a m p l e s w e r e p r e p a r e d f o r t h e m i c r o g r a p h s as i n d i c a t e d i n F i g u r e s 1, 2, a n d 3. N o special staining techniques w e r e r e q u i r e d . P e r m e a b i l i t i e s o f 0 . 5 - m m s o l u t i o n - c a s t films a n d 0 . 2 5 - m m e x t r u d e d films w e r e m e a s u r e d b y a t e c h n i q u e based o n t h e mass spectrometer (8).


7

Table I.

Effect of Sample H i s t o r y on M e c h a n i c a l Properties Tensile

Initial Method

3

of

Fabrication

Modulus,

b

Solution-cast from toluene Sample annealed at 150°C Extruded, measured parallel to flow Sample annealed at 135°C Extruded, measured perpendicular to flow Sample annealed at 135°C Extruded and blown

cd

e

psi

Properties

Stress,

psi

0

at

Break

Strain,

4350 1300 4500 3800

1150 1850 1600 1500

930 825 660 370

490 480 16500

990 1700

260 420

1

%

D a t a obtained on polymer described in Figure 2. R u n at 10 in. /min. D a t a subject to error due to high degree of dependence on the angle of applied stress (Equation 1); maximum values reported. Brabender extruder, head temperature 2 3 0 - 2 5 0 ° C ; draw ratio 4:1, extruded as tubing, 0.010 in.-thick wall. Draw and blow ratios 4:1 ; 0.002-in. film. a

6

c

d

e

Morphology F i g u r e s 1 A , I B , a n d 1 C represent a sequence of electron p h o t o m i c r o g r a p h s of films cast f r o m t o l u e n e . T h e c o p o l y m e r s w e r e o f t h e s a m e c o m p o sition but increasing block size. T h e dark d o m a i n s correspond to p o l y d i m e t h y l siloxane, t h e lighter domains to p o l y ( α-methylstyrene ). T h e c o p o l y m e r w i t h s m a l l e s t b l o c k s i z e h a d w h a t c a n best b e d e s c r i b e d as a t e x t u r e o b s e r v a b l e a t the l i m i t s of resolution. C o p o l y m e r s w i t h larger b l o c k size, h o w e v e r , h a d a discrete m o r p h o l o g y reminiscent of p o l y ( styrene-butadiene ) triblock copoly mers ( 2 ) despite t h e differences i n composition a n d architecture. T h e p o l y ( a m e t h y l s t y r e n e ) t e n d e d t o f o r m i r r e g u l a r i s l a n d s i n s o m e areas w h i l e i n o t h e r areas i t a p p a r e n t l y c o m p o s e d t h e c o n t i n u o u s p h a s e . O f t e n t h e i s l a n d s a p p e a r e d t o m e r g e at t h e i r b o r d e r s . D o m a i n s i z e t e n d e d t o i n c r e a s e w i t h m o l e c ular w e i g h t of t h e blocks, i n d i c a t i n g a h i g h e r degree of a g g r e g a t i o n i n t h e d o m a i n s as m o l e c u l a r w e i g h t i n c r e a s e d . F i g u r e I D is a t h i n s e c t i o n t a k e n p e r p e n d i c u l a r t o t h e s u r f a c e o f a t o l u e n e cast film o f a p o l y m e r s i m i l a r t o t h a t i n F i g u r e I B . T h e m o r p h o l o g y w a s similar w h i c h confirms the i s l a n d or sphere t y p e m o r p h o l o g y i n thicker, solutioncast s a m p l e s .


WARD E T A L .

Microstructure

vs.

Properties

303


26.

Figure 2.

Electron micrograph of extruded copolymer freeze-sectioned perpendicular to direction of extrusion [copolymer, 40 wt % poly(a-methylstyrene)]

M e l t extrusion a n d melt extrusion followed b y d r a w i n g p r o d u c e d a n e w , h i g h l y o r g a n i z e d m i c r o s t r u c t u r e . W h e n v i e w e d p e r p e n d i c u l a r to t h e d i r e c t i o n of e x t r u s i o n ( F i g u r e 2 ) , l o c a l l y r e g u l a r h e x a g o n a l a r r a y s o f c i r c u l a r d o m a i n s c o m p o s e d of p o l y (α-methylstyrene) surrounded b y a b a c k g r o u n d matrix of p o l y d i m e t h y l s i l o x a n e w a s v i s i b l e i n s o m e r e g i o n s o f t h e field. O b s e r v a t i o n p a r a l l e l to t h e d i r e c t i o n of e x t r u s i o n r e v e a l e d r e g u l a r b a n d s a l t e r n a t e l y c o m -


COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES


304

Figure 3.

Electron micrograph of same copolymer as in Figure parallel to direction of extrusion

2, but sectioned

p o s e d of p o l y d i m e t h y l s i l o x a n e a n d p o l y ( « - m e t h y l s t y r e n e ) (Figure 3). The distance b e t w e e n b a n d s corresponds to t h e distance b e t w e e n circles i n F i g u r e 2. T h e light bands were approximately 60 A w i d e , a n d they were separated b y ^ 6 5 A of i n t e r v e n i n g siloxane. These dimensions correspond roughly to t h e u n p e r t u r b e d c h a i n e n d - t o - e n d distances c a l c u l a t e d f o r h o m o p o l y m e r s o f each block.


26.

WARD E T A L .

vs.

Microstructure

305

Properties

T h e o b s e r v a t i o n s s u g g e s t a n o r d e r e d m i c r o s t r u c t u r e c o n s i s t i n g of a r r a y s of p o l y ( α - m e t h y l s t y r e n e ) r o d s i n a p o l y d i m e t h y l s i l o x a n e m a t r i x . T h e r o d s w e r e o r i e n t e d i n t h e d i r e c t i o n o f e x t r u s i o n . M a n y of t h e r o d s i n F i g u r e 3 appear to terminate a n d b e d i s p l a c e d b y their neighbors. T h i s a n d s i m i l a r irregularities were probably caused b y the freeze-sectioning. L i t t l e , i f a n y , d i s t i n c t i v e m o r p h o l o g y w a s o b s e r v e d w h e n s a m p l e s of b l o c k c o p o l y m e r w e r e b i a x i a l l y stressed w h i l e at a n e l e v a t e d t e m p e r a t u r e . T h i s w a s d o n e b y t h e b u b b l e p r o c e s s of film b l o w i n g i n w h i c h a n e x t r u d e d t u b e i s simultaneously d r a w n a n d e x p a n d e d r a d i a l l y . B o t h d r a w a n d expansion ratios were 4 : 1 .


Mechanical

Properties

T h e effects of s a m p l e h i s t o r y o n m e c h a n i c a l p r o p e r t i e s a r e s u m m a r i z e d i n T a b l e I. I n a l l b u t t h e e x t r u d e d s a m p l e t h a t w a s m e a s u r e d p e r p e n d i c u l a r

ο

I 0

I 10

I 20

I 30

I 40

I 50

I 60

I 70

I 80

1 90

0°

Figure 4.

Plot of elastic modulus vs. angle of applied stress


306

COPOLYMERS,

T a b l e II.


a

POLYBLENDS,

AND

COMPOSITES

Effect of D r a w Ratio on E x t r u d e d R o d s "

Draw Ratio

Initial Modulus, psi

2.0 1.8 1.4 1.2

2040 1750 690 480

Same polymer as i n Figure 2 extruded through a 1/4 in.-diameter circular die at 2 3 0 - 2 5 0 ° C .

to t h e d i r e c t i o n o f flow, a d e c r e a s e i n i n i t i a l m o d u l u s w a s o b s e r v e d w h e n t h e samples were annealed. T h i s decrease w a s especially large i n t h e solution-cast films. T h e s e films c a n b e d i s s o l v e d after a n n e a l i n g a n d re-cast to give t h e o r i g i n a l p r o p e r t i e s ; this p r o c e d u r e w a s r e p e a t e d u p to three cycles. T h e d r o p i n m o d u l u s w a s a c c o m p a n i e d b y increased tensile properties. T h e initial m o d u l u s of extruded a n d d r a w n film h a d a h i g h angular d e p e n d e n c e . T h e s e f i l m s t e n d e d t o b e stiff w h e n stressed i n t h e d i r e c t i o n o f e x t r u s i o n a n d v e r y r u b b e r y p e r p e n d i c u l a r t o t h e e x t r u s i o n d i r e c t i o n (see Table I ) . F i g u r e 4 shows t h e same effect i n m o r e d e t a i l . A n n e a l e d samples f e l l o n t h e s a m e c u r v e . S i m i l a r effects w e r e n o t e d b y F o l k e s a n d K e l l e r ( I ) w h o w e r e a b l e t o fit t h e i r d a t a t o a n e q u a t i o n d e r i v e d b y c o n s i d e r i n g t h e d e f o r m a t i o n of a r u b b e r m a t r i x filled w i t h g l a s s y r o d s .

1/Ee

= l/#90° (sin θ + 4 sin θ cos Θ) 4

2

(1)

2

w h e r e θ is t h e a n g l e o f a p p l i e d stress r e l a t i v e t o t h e d i r e c t i o n o f e x t r u s i o n , E o is t h e m o d u l u s p e r p e n d i c u l a r t o t h e d i r e c t i o n o f f l o w , a n d Ε i s t h e m o d u l u s a t a n g l e Θ. T h e b r o k e n l i n e i n F i g u r e 4 i s t h e p l o t o f E q u a t i o n 1 u s i n g t h e v a l u e for E o that w e f o u n d w i t h o u r system. T h e samples used for F i g u r e 4 w e r e o b t a i n e d at a d r a w r a t i o o f 3 : 1 . O t h e r s a m p l e s d r a w n a t 1 . 5 : 1 f e l l o n a s i m i l a r curve, b u t the m o d u l u s was significantly l o w e r (at θ = 0, Ε = 1 4 9 0 p s i ) . T h e e f f e c t o f d r a w r a t i o o n m o d u l u s w a s s i m i l a r w h e n s a m p l e s w e r e e x t r u d e d as r o d s (see T a b l e I I ) . 9

0

θ

9

0

θ

T h e i n i t i a l m o d u l u s o f films d r a w n b i a x i a l l y , i.e. b l o w n films, i n c r e a s e d m a r k e d l y (see T a b l e I ) . P r o p e r b a l a n c e o f d r a w r a t i o a n d b u b b l e d i a m e t e r gives isotropic materials.

T a b l e III. T h e Effect of Sample History on Permeability to O x y g e n i n a Poly(a-methylstyrene-dimethylsiloxane) Block C o p o l y m e r 0

Method of Fabrication

Polydimethylsiloxane, compression-molded and lightly crosslinked Poly (a-methylstyrene) Block copolymer solution-cast extruded film extruded and blown film b

c

α b c

Permeability at 25°C, ml in./100 in. 24 hr atm 2

1.2 1.4

Χ

10 10

1.7

Χ

10

3.3

Χ

10

Χ

Same copolymer as in Figure 2. See Ref. 9. A n estimate based on polystyrene (10).


5

2

4

4

26.

WARD E T A L .

vs.

Micro structure

307

Properties

Permeability Blown film

film

h a d a p e r m e a b i l i t y to o x y g e n r o u g h l y t w i c e t h a t of t h e

cast f r o m t o l u e n e .

p e n d i c u l a r to t h e e x t r u s i o n d i r e c t i o n . Discussion

of

same

I n these studies, p e r m e a b i l i t y w a s m e a s u r e d o n l y p e r T h e d a t a are s u m m a r i z e d i n T a b l e

III.

Results

T h e s e m i - e m p i r i c a l m o d e l d i s c u s s e d b y T a k a y a n a g i et al. i n collating observations

considering modulus i n a composite, t r i b u t e d so t h a t o n e

is u s e f u l

(11)

of m o d u l u s a n d p e r m e a b i l i t y i n o u r s y s t e m .

field

When

t h e m o d e l is s u c h t h a t stresses a r e

dis

of f o r c e passes t h r o u g h o n l y t h e r u b b e r p h a s e w h i l e

a s e c o n d passes t h r o u g h b o t h p h a s e s .

E q u a t i o n 2 is a p p r o p r i a t e f o r s u c h

a


m o d e l w h e n t h e r u b b e r p h a s e is c o n t i n u o u s .

=

( ΐ - χ ) ί ,

χ * .

+

T h e m o d u l u s of t h e g l a s s y p h a s e , E , g

b y three decades i n our copolymer.

E, r

4

+

r

(

e x c e e d s t h a t of t h e r u b b e r y

T h e m i x i n g parameters, φ a n d λ, approach equality i n

systems.

)

phase,

T h e r e f o r e the o b s e r v e d m o d u l u s of the

c o p o l y m e r m i g h t b e e x p e c t e d to b e m o r e r e s p o n s i v e to c h a n g e s i n t h e phase.

2

finely

glassy

dispersed

T h e n u m e r i c a l v a l u e of φ i n t h i s d i s c u s s i o n i n d i c a t e s t h e d e g r e e

to

w h i c h a s i n g l e field of f o r c e passes t h r o u g h b o t h p h a s e s — t h e d e g r e e of series mixing.

T h e v a l u e of λ i n d i c a t e s t h e d e g r e e of p a r a l l e l m i x i n g .

M i x i n g parameters

c a n also b e d e t e r m i n e d f r o m o b s e r v a t i o n s

of

p e r m e a b i l i t y b y t h e a p p l i c a t i o n of a n a n a l o g o u s m o d e l w h e r e p r e s s u r e a n d flow replace a force

field

a n d strain.

I n this m o d e l , flow i n the

oxygen gradient rubber

p h a s e is i n f l u e n c e d b y b o t h p h a s e s w h i l e f l o w i n t h e g l a s s y p h a s e is u n i q u e . P e r m e a b i l i t y i n t h e r u b b e r y p h a s e , P , e x c e e d s flow i n t h e g l a s s y p h a s e , P , r

by

g

t h r e e d e c a d e s , a n d o b s e r v a t i o n s of p e r m e a b i l i t y , P , a r e v e r y h i g h l y r e s p o n s i v e to c h a n g e s i n t h e r u b b e r y p h a s e . the m i x i n g parameters,

Therefore,

to o b t a i n m e a n i n g f u l v a l u e s

φ' a n d λ ' , it is n e c e s s a r y to a s s i g n t h e r o l e of

for

discrete

p h a s e to t h e r u b b e r a r b i t r a r i l y .

^ Table IV.

-

(I-X')P.

+

X'P, +

( 3 )

E f f e c t of S a m p l e H i s t o r y o n M i x i n g

Parameters"

Modulus Method

of Sample

Fabrication

Solution cast Above sample annealed Extruded, measured parallel to flowAbove sample annealed Extruded, measured perpendicular to flowExtruded and blown

Permeability

M easurements Measurements

Measurements

b

b

Φ

λ

4.46 8.05 4.40 4.77 13.21 2.32

.090 .050 .091 .084 .030 .17

1

φ'

λ'

1.63

.37

.992

.605

" B y the approach of Takayanagi et al. (11). Calculated presuming the rubbery phase continuous, Er = 200 psi, and Eg = 200,000 psi. Calculated presuming the glassy phase continuous, and using data in Table III.

b

c

The

s i g n i f i c a n t l y h i g h e r v a l u e s of φ a n d φ ' o v e r λ a n d λ ' i n T a b l e

i n f e r a h i g h e r d e g r e e of series m i x i n g .

T h i s is i n t e r p r e t e d as a g g r e g a t i o n

IV of

t h e m i c r o d o m a i n s as o p p o s e d to a u n i f o r m d i s t r i b u t i o n . E s s e n t i a l l y t h i s is a l s o



308

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

what is observed in electron photomicrographs of solution-cast as well as ex truded films viewed perpendicular to the direction of extrusion. Extruded film stressed parallel to the extrusion direction approaches solution-cast film in mechanical equivalence. This is apparent from Table IV where φ and λ values for extruded and solution-cast films are similar. The extremely high values of φ noted for the extruded sample tested perpendicular to the direction of extrusion is ascribed to a nearly ideal state of series mixing. This is substantiated by electron micrographs (Figures 2 and 3) that show the arrays of long rods which in this case run perpendicular to the field of applied stress. In such an arrangement, the applied field of force must run through both domains—the glassy rods and the rubbery matrix. Blown films, on the other hand, have little detectable microstructure. They would be expected to have values of φ and λ approaching one another more closely than do those for solution-cast films because of better dispersion. Although we do not attempt to explain the driving force leading to the formation of the various observed microstructures, it seems clear that, in addi tion to block dimensions and insolubility of the phases, shearing forces play a major role. The magnitude of fabrication-induced changes in basic properties reported here is certainly remarkable. Similar changes and anisotropic effects probably effect other properties as well. Literature Cited 1. Folkes, J. M., Keller, Α., Polymer (1971) 12, 222. 2. Aggarwal, S. L., Lavigni, R. Α., Marker, L. F., Dudek, T. J., "Block and Graft Copolymers," J. J. Burke and V. Weiss, Eds., p. 157, Syracuse University, Syracuse, 1973. 3. Beecher, J. F., Marker, L., Bradford, R. N., Aggarwal, S. L.,J.Polym. Sci. Part C (1969) 26, 117. 4. Saam, J. C., Ward, A. H., Fearon, F. W. G., ADVAN. C H E M . SER. (1973) 1 2 9 , 239. 5. Roberson, L . M., Nosha, Α., Matzner, M., Merriam, C. N., Angew. Makromol. Chem. (1973) 2 9 / 3 0 , 47. 6. Saam, J. C., Gordon, D. J., Lindsey, S. L., Macromolecules (1970) 3, 1. 7. Saam, J. C., Fearon, F. W. G., Ind. Eng. Chem. Prod. Res. Develop. (1971) 10, 10. 8. Dow Chemical Co., "Modern Methods of Research and Analysis," Interpretive Analytical Services (1973) p. 66. 9. Flaningham, O., private communication. 10. Brandup, J., Immergut, Ε. H., "Polymer Handbook," Interscience, New York, 1966. 11. Takayanagi, M., Harima, H., Iwata, Y., Mem. Fac. Eng. Kyushu Univ. (1963) 2 3 C 1 , 1. RECEIVED June 11,

1974.


Poly(Î±-methylstyrene-dimethylsiloxane) Block Copolymers. The

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