Surface Chemical Properties of Highly Fluorinated Polymers

14 Surface Chemical Properties o f H i g h l y Fluorinated

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 13, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch014

Polymers MARIANNE K. BERNETT and W. A. ZISMAN Laboratory for Chemical Physics, Naval Research Laboratory, Washington, D.C. 20375

Introduction Highly fluorinated linear polymers are characterized by a low free surface energy and concomitant low wettability, as evidenced by the large contact angles of drops of organic and aqueous liquids. A comprehensive set of workable principles had been built up by Zisman and co-workers relating the chemical and spatial constitution in the outermost surface of a polymer with its surface energy (1, 2). During the last few years Wall, Brown, and Lowry of the National Bureau of Standards synthesized several new highly fluorinated ethylene polymers and copolymers (3-8) and established (4) that according to Wunderlichs "bead" theory and "rule of constant heat increment" (9, 10), the ethylenic polymers with fluorinated side groups generally contribute two carbon atoms to the backbone chain. This 2-carbon moiety plus the sub stituent constitute the smallest unit whose oscillations affect surface lattice equilibrium in a polymeric material, which repre sents the lowest free energy configuration. This paper discusses the critical surface tensions of several of these radiation-induced polymers and copolymers, and compares them to those of polymers with related structures and surface constitutions. '

Critical Surface Tensions of Wetting Table I lists the experimental fluoropolymers and the data on their structural formulae, intrinsic viscosities [η] in hexafluorobenzene, and glass transition temperatures Tg (3-8), along with the code numbers used for this report. By casting a polymer from solution in hexafluorobenzene as a film on a clean glass slide, a smooth surface is obtained which is characterizable for wetting properties by contact angle (θ) measurements with freshly percolated liquids. Table II shows the average values of the advancing contact angles (± 1°) obtained from such liquids of two homologous series, the n-alkanes and the 199 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

3

2

3

n

2

61

4 2

6

i n hexafluorobenzene;

2

6

5

5

n

+ [-CF^CF,,-]^

b e i n acetone; i n benzene

n

n

[-CF -CF(C F )-]

2

XIV

1 1

[-CF -CF(C H )-]

5

XIII

2

[-CH -CH(C F )-] 2 6 ρ η

a

3

[-CF -CF(n-C F )-]

3

XII

XI

n

[-CHF-CH(CF )-]

X

3

n

2

[-CH -CF(CF )-]

2

IX

2

3

+ [-CF^CF^]^

16.5 16.5 18.8 17.5

29 21 49 87

1.63 1.30

e

0.27

1.0

4.0

α 25000

0.23

25

2

2

194

25.4

202

17.8

22.5

14.1 105

235

16.2

45

[-CH -CH(CF CF CF )-]

n

15.5

58

VIII

2

3

3

52

2

2

2


2

2

VII

2

2

0.74

VI

V 79

Μ

16.3

41

0.33

18.0

9

2.10

21.1

19

21.5

5.00


+ [-CFg-CFg-]^

2

2

0.63


2

2

[-CF -CF -]

+ C-CF -CF -]

[-CH -CH(CF CF )-]

2

+

c

7 , dyn/cm 27

a

1.10

[η], d l / g

n

IV

39

[-CH -CH(CF )-]

III

3

5Ô

2

[-CH -CH(CF )-]

n

II

3

[-CH -CH(CF )-]

2

Polymer and Composition. mol%

I

Code

Table I . P h y s i c a l P r o p e r t i e s of E t n y l e n i c Fluoropolymers



n-ALkanes Hexadecane Tetradecane Trideoane Dodecane Undecane Decane Nonane Octane .methylsiloxanes DC200 3.0 c S t DC200 2.0 c S t DC200 1.5 c S t 12 (DM3) 10 (OMS) 9 (DMS) 8 (DMS) 7 (DMS) 6 (DMS) 5 (DMS)

Liquids

19.4 18.9 18.1 19.7 19.5 19.3 19.1 19.0 18.6 18.2

27.8 26.7 26.0 25.4 24.6 23-9 23.1 21.8

(dyn/cm)

56

42 35 29

45

36 29

50 47

63

III

51

II

50

I

41 40 39 38 37 33 32

50 46 42

55

61 58

IV

54 51

44 40 36

43 40 36

59

64

VI

55 51

61

67

V

Code

48 44

37 33 29

37 33 28

53

59

48 45

55

61

VII VIII

19 16 14 10 7 6 5

39 36 30

45

52 49

IX

30 29 28 26 24 22 18

44 41 36

49

56 53

Χ

51 50 49 48 47 45 43

55 53 49

60

66 64

XI

< 5

22 13 9

31

41 37

XII

Table I I . Advancing Contact Angles (Deg) of L i q u i d s on Fluoropolymers (25°C)


< 5

< 5

14 10

XIII

40 38 36 34 33 29 27

32 28 25

46

52 50

XIV


202

ADSORPTION A T INTERFACES

d i m e t h y l s i l o x a n e s , where the l a t t e r were e i t h e r the Dow Corning 200 s e r i e s o r the w e l l - c h a r a c t e r i z e d s e r i e s c o n t a i n i n g from 6 t o 12 d i m e t h y l s i l o x a n e (DMS) -units. (11, 12). When the cos θ of each member of such a homologous s e r i e s of l i q u i d s on a smooth, c l e a n , s o l i d , low-energy surface i s p l o t t e d a g a i n s t the s u r f a c e t e n s i o n (ïjJ) f o r each of those l i q u i d s , a s t r a i g h t l i n e r e s u l t s ; the i n t e r c e p t a t cos θ = 1 (θ = 0°) i s r e f e r r e d t o as the c r i t i c a l s u r f a c e t e n s i o n of w e t t i n g (7 ) f o r t h a t p a r t i c u l a r s u r f a c e (13). Figure 1 shows such a grapn f o r polymer IV, [-CHp-CHiCpF^)-] , and i s r e p r e s e n t a t i v e o f the graphs f o r t h e other polymers w i t h the e x c e p t i o n of the p e r f l u o r o p h e n y l s u b s t i t u t e d polymers X I I and XIV. Here the s t r a i g h t l i n e f o r the n-alkanes d i s p l a y s a marked d i s c o n t i n u i t y i n the r e g i o n of 7 = 24-25 dyn/cm, r e s u l t i n g i n two values of 7 , d i f f e r i n g by 1 dyn/cm. Experimental phenomena suggest t h a t tne s m a l l e r mole c u l e s a r e capable of s l i p p i n g i n t o the i n t e r s t i c e s o f the p o o r l y a d l i n e a t e d s t r u c t u r e s o f polymers X I I and XIV, whereas t h e l a r g e r and b u l k i e r molecules are r e t a i n e d on the s u r f a c e , thus b e i n g more r e l i a b l e r e p r e s e n t a t i v e s f o r the t r u e value of 7 . Values of 7 thus obtained f o r each polymer f i l m are given i n Table I . Wettability f o r low-energy s u r f a c e s , as d e f i n e d by 7 , i s determined e s s e n t i a l l y by the nature and packing of the exposed surface atoms of t h e s o l i d and i s otherwise independent o f the nature and arrangement of the u n d e r l y i n g atoms and molecules ( l , 2). The arrangement o f the s u r f a c e atoms, of course, must represent the lowest f r e e energy c o n f i g u r a t i o n f o r a given s e t of r e s t r a i n i n g c o n d i t i o n s such as the nature, s i z e of the u n d e r l y i n g atoms, l e n g t h of the c h a i n , e t c . I n p a r t i c u l a r , f o r polymeric m a t e r i a l s i t depends on d e f i n i t i o n of the s m a l l e s t u n i t , which f o r ethylenic polymers c o n s i s t s o f 2 carbons i n the backbone (Sb IQ) p l u s the s u b s t i t u e n t . Table I I I l i s t s 7 values of f l u o r i n e - c o n t a i n i n g e t h y l e n i c homopolymers, wherS the formulae shown are the r e p e a t i n g u n i t s i n the polymer s t r u c t u r e , as they would appear i f a l l c o n s t i t u e n t s were present i n the s u r f a c e . The order o f s t r u c t u r e s i s arranged t o show p r o g r e s s i v e s u b s t i t u t i o n s of e i t h e r a hydrogen o r a f l u o r i n e atom i n the backbone c h a i n by e i t h e r a f l u o r i n e atom or a p e r f l u o r o group. S e v e r a l observations can be made from i n s p e c t i o n of values of 7 : (a) When the ethylene c h a i n i s f u l l y hydrogenated, replacement of one hydrogen by a - C F group lowers 7 a p p r o x i mately 10 dyn/cm. (Compare XV and i f XVI and IX, and XVI and X.) (b) When one carbon atom i n the 2-carbon moiety of the ethylene c h a i n i s f u l l y f l u o r i n a t e d , replacement of a hydrogen by a -CF^ group on the other carbon atom lowers 7 o n l y about 5 dyn/cm. (Compare XVII and XIX.) (c) Nearly equSl y values a r e observed on s e v e r a l p a i r s of monomers of u n l i k e molecular c o n s t i t u t i o n s , such as I and X V I I , I X and X V I I I , and X and XIX. I n s p e c t i o n o f S t u a r t - B r i e g l e b molecular models shows t h a t , i n c e r t a i n s t e r i c arrangements, the 7 -determining packing of f l u o r i n e atoms a t the s u r f a c e o f these paîrs could be v e r y s i m i l a r , s i n c e the pendant ?

Q


14.

BERNETT

AND

Highly Fluorinated Polymers

zisMAN

203

Table I I I . E f f e c t on C r i t i c a l Surface Tensions of Wetting by Replacement of Hydrogen w i t h F l u o r i n e and/or Pendant Group


Code

7 (dyVcm)

Polymer

31

XV

[CH -CH -]

I

[CH -CH(CF )-]

IV

[-CH -CH(CF -CF )-]

V

[-CH -CH ( C F ^ C F ^ C F ^ ) -]

15.5

XVI

[-CH -CFH-]

28

IX

[-CH -CF(CF )-]

n

18.8

X

[-CH(CF )-CFH-]

n

17.5

XVII

[-CF -CFH-]

n

22

XVIII

C-CF -CF -]

n

18.5

XIX

[-CF -CF(CF )-]

XI

[-CF -GF(nC F

XX

[-CH -CH(C H )-]

XII

[-CH -CH(C F )-]

XIII

[-CF -CF(C H )-]

n

25.4

XIV

C-CF -CF(C F )-]

n

17.8

2

2

2

n

3

2

21.5

n

2

3

2

2

n

3

3

2

2

2

2

3

2

2

2

2

2

Reference 14;

16.3

n

n

2

5

6

6

6

6

i:L

17

n

)-]

5

5

5

5

a

n

n

n

b

b

C

d

14.1 33-35

e

22.5

Reference 15; °Reference 13; Reference 16;

References 17, 18



Macromolecules

Figure 2. Various configurations of polymer [—CHz—CHtCeFs)—']». (a) (top left) syndiotactic, exposure of flat side; (b) (top right) syndiotactic, exposure of edge; (c) (bottom left) isotactic (12).



14.

BERNETT

AND

Highly Fluorinated

zisMAN

205

Polymers

-CF^ groups may be so l o c a t e d as t o p a r t i a l l y obscure the hydro gen atoms . The t o t a l e f f e c t achieves a balance between the hydrogen and the -CF- c o n t r i b u t i o n s which approximates the s u r face c o n s t i t u t i o n of the l i n e a r unbranched c o n f i g u r a t i o n . (d) Polymers IX and X demonstrate a c o n f i r m a t i o n of the c o n c l u s i o n of Pittman and co-workers ( 1 £ ) , t h a t i n a p a r t i c u l a r f l u o r i n a t e d polymer y i s not n e c e s s a r i l y dependent on the t o t a l f l u o r i n e content: deSpite the i d e n t i c a l f l u o r i n e content, the molecular s t r u c t u r e s d i f f e r s u f f i c i e n t l y t o r e s t r i c t f r e e r o t a t i o n of the -CF~ groups i n polymer X w i t h the n e t r e s u l t of c l o s e r surface packing of the f l u o r i n e atoms and thus a lower y . (e) As a n t i c i p a t e d , the lowest y , 14-1 dyn/cm, i s obtained f o r polymer X I which i s not only fulîy f l u o r i n a t e d but a l s o d i s p l a y s the l o n g e s t pendant p e r f l u o r i n a t e d group. The i n c r e a s e i n l e n g t h from 1 carbon t o 5 carbons decreases 7 about 3 dyn/cm (XIX and XI) because of b e t t e r a d l i n e a t i o n and îess r e s t r i c t i o n i n the longer c h a i n , ( f ) For aromatic s u b s t i t u t i o n s , replacement of the hydrogen w i t h f l u o r i n e i n e i t h e r the phenyl group, X I I , or the backbone c h a i n , X I I I , lowers 7 about 10 dyn/cm from the 33-35 dyn/cm of p o l y s t y r e n e . V a r i a t i o n s o r spread of y values f o r a given polymer can be now explained by the v a r i o u s o r i e n t a t i o n s of the phenyl group i n the s u r f a c e , such as exposure of the f l a t s i d e o r the edge, or the t a c t i c i t y of the arrangement (Figure 2 a, b, c ) . T o t a l f l u o r i n a t i o n , as i n polymer XIV of course r e s u l t s i n even lower 7 , s i n c e only f l u o r i n e atoms a r e exposed i n the s u r f a c e . An i n t e r e s t i n g p a r a l l e l can be observed and i s demonstrated i n F i g u r e 3: When the hydrogen atoms i n the e t h y l e n i c backbone are r e p l a c e d by f l u o r i n e atoms, r e g a r d l e s s of whether the pendant group i s the a l k y l -CF~ or the aromatic -CvF , 7 i s lowered by about 4 · 5 dyn/cm. When, on the other hand, ?he S l k y l -CFo group i s r e p l a c e d by the aromatic -C,F group, whether on a f u l l y hydrogenated or f u l l y fluorinaïed backbone, 7 i s r a i s e d by approximately 1 dyn/cm. Q

c

5

6

0

S t e r i c Configurations The problem o f determining the arrangement of the atoms and s u b s t i t u e n t s of the f i r s t l a y e r of the s o l i d s u r f a c e of a polymer or copolymer needs t o be s o l v e d before we can r e l a t e the observed value of 7 t o the most probable surface composition of the polymeric s o l i d . F o r simple u n s u b s t i t u t e d polymers such as [-CF -CF -] o r [-CH ~CH -] the surface conformation can be r a t i o n a l i z e d by S t u a r t - B r i e g l e b molecular models arranged i n p o s s i b l e conformations on a f l a t t a b l e . However, where s i d e chains a r e introduced i n t o the model, s t e r i c hindrances are a l s o i n t r o d u c e d . I t i s obvious from Figure 4, where only some of the p o s s i b l e arrangements of the atoms and s u b s t i t u e n t s i n the f i r s t l a y e r of the s o l i d [-CH -CH(C F^)-] polymer surface a r e shown, t h a t i n a three-dimensional a r r a y a m u l t i t u d e of a l t e r n a t e 2

2

2

2

2

n



206

- CH, - CH(CF ) -

- CF - CF(CF,) -

}

2

21-5

17.0

+0.8 I


l+l.O

4.5


14.

BERNETT AND

zisMAN


207


conformations are p o s s i b l e , l i m i t e d o n l y by s t e r i c considerations. Lacking s p e c i f i c i n f o r m a t i o n on the t a c t i c i t y of the r a d i a t i o n induced polymer, we assumed them t o be a t a c t i c , i . e . , randomly o r i e n t e d . Any p r e d i c t i o n of surface p a c k i n g i s thus p r o b l e m a t i c . In a d d i t i o n t o the s p a t i a l arrangements, polymers such as X a l s o e x h i b i t c i s and trans isomerism w i t h respect t o the o r i e n t a t i o n of the -F and -CF^ s u b s t i t u e n t s on the backbone, f u r t h e r compli c a t i n g and expanding the number of p o s s i b l e c o n f i g u r a t i o n s . Electrostatic

Dipoles

A t the juncture of the -CH CH< backbone and the - ( C F j F s i d e group there exists an uncompensated e l e c t r o s t a t i c d i p o l e . I f one assumes t h a t the s i d e group i s d i r e c t e d away from the polymer s o l i d surface i n t o the w e t t i n g l i q u i d , the d i p o l e i s closer t o the interface the lower the value of x. Experiments have shown t h a t as χ becomes s m a l l e r , θ a l s o becomes s m a l l e r . Thus, when χ changes from 1 t o 3, 7 i s lowered from 21.5 t o 15.5 dyn/cm (Figure 5), w i t h the l a r g e r Secrease of 5.2 dyn/cm when 1 < χ < 2 and the s m a l l e r decrease of 0.8 dyn/cm when 2 < χ < 3· I n t h e i r study of adsorbed monolayers of p r o g r e s s i v e l y f l u o r i n ated f a t t y a c i d s of the general formula F ( C F ) ( C H j ^ C O O H and 2

p

F

C F

C H

C 0 Q H

&

Γ

Γ

Ϊ

η

z

i

s

m

a

n

^ 2^ 2^10 ' ^ · ^ (20)/sSowed TOat the uncompensated d i p o l e has a l a r g e e f f e c t on w e t t i n g when χ < 7, but becomes l e s s s i g n i f i c a n t when χ ^ 7. Measurements of e l e c t r i c a l and mechanical p r o p e r t i e s of i n s o l u b l e monolayers on water by Bernett and Zisman (21), supported t h e i r view a l s o . S h a f r i n and Zisman a l s o n o t i c e d an abrupt r e v e r s a l of the e f f e c t of homology a t 2 < χ < 3 where the d i f f e r e n c e i n y was only 0.8 dyn/cm because of random t i l t i n g , of the fluoroSarbon group. A s i m i l a r abrupt d i s c o n t i n u i t y a t 2 < χ < 3 f o r the e t h y l e n i c polymers can be explained by r e s t r i c t i o n of r o t a t i o n of the s u b s t i t u e n t and the subsequent s h i e l d i n g of the e l e c t r o s t a t i c d i p o l e . The l a t t e r i s accentuated by the f a c t t h a t the e t h y l e n i c hydrocarbon, t o which the p e r f l u o r o a l k y l s i d e groups are connected, imposes r e s t r i c t i o n s on the p a c k i n g of these s u b s t i t u e n t groups, n e c e s s i t a t i n g p r o g r e s s i v e l y l a r g e r i n t r a molecular r o t a t i o n s and bending w i t h i n the c h a i n . This r e s u l t s i n exposure of the -CF - atomic grouping i n an outermost surface of randomly o r i e n t e d perfluoroethyl o r -propyl groups. I t would be i n t e r e s t i n g t o study polymers w i t h p r o g r e s s i v e l y longer p e r f l u o r o a l k y l s i d e groups t o a s c e r t a i n whether r e g u l a r decreases i n y can be observed or whether a l i m i t i n g value has been approacheS. When the ethylene backbone i s f u l l y f l u o r i n a t e d , no l a r g e uncompensated d i p o l e s are present, and a gradual and u n i n t e r r u p t e d decrease i n y i s observed w i t h i n c r e a s e i n χ (Figure 5 ) . The shape o f tne two curves i n Figure 5 seems t o p o i n t t o an eventual asymptotic approach t o the y -vs-x curve obtained from adsorbed monolayers o f f u l l y f l u o r i n a t e d c a r b o x y l i c ?


ADSORPTION AT

208

INTERFACES

a c i d s , as shown i n Figure 6 (20). Since t o date no curve of anyother f a m i l y or s e r i e s of r e l a t e d compounds (22), i n c l u d i n g the two present ones, has crossed or gone beyond the curve of the p e r f l u o r o a c i d s , i t i s suggested t h a t the l a t t e r represents an envelope of l i m i t i n g v a l u e s .


Copolymers

I n t h e i r study on copolymers, Hu and Zisman (11) p l o t t e d y f o r each polymer a g a i n s t the mole % of [-CF -CF -] i n the copolymer (Figure 7 ) . The three graphs show the comparative case of p r e d i c t i n g the e f f e c t on y of c o p o l y m e r i z a t i o n of [ C F - C F - ] w i t h [-CH -CH -] (upper curve) (22) and the greater d i f f i c u l t y (middle graph) of p r e d i c t i n g the e f f e c t on y of c o p o l y m e r i z a t i o n w i t h [-CH -CH(CF^)-] and the intermediate c

2

2

C

2

2

n

2

2

n

2

n

F

e f f e c t w i t h [ - C H - C H ( C o ) - ] ("bottom c u r v e ) . I t should be evident t h a t we are d e a l i n g w i t h s e v e r a l e f f e c t s of adding f l u o r i n a t e d carbon atoms: (a) those due t o London d i s p e r s i o n f o r c e changes (upper graph) and (b) those combining e l e c t r o s t a t i c e f f e c t s and s t e r i c hindrance e f f e c t s (lower two graphs). I t i s obvious from the multitude of p o s s i b l e s t e r i c c o n f i g u r a t i o n s and d i p o l e c o n t r i b u t i o n s t h a t we face the d i f f i c u l t problem of r a t i o n a l i z i n g how t o compute the c o r r e c t molecular conformation t o a r r i v e a t a minimum s u r f a c e energy f o r s u b s t i t u t e d ethylene f luoropolymers and copolymers. 2

7

n

Summary

Wetting p r o p e r t i e s of new, w e l l - c h a r a c t e r i z e d , h i g h l y f l u o r i n a t e d l i n e a r e t h y l e n i c polymers and copolymers w i t h t e t r a f l u o r o e t h y l e n e were i n v e s t i g a t e d . F l u o r i n a t i o n i n the p o l y ethylene backbone was v a r i e d by degree of f l u o r i n e atom s u b s t i t u t i o n ; n - a l k y l s i d e chains of i n c r e a s i n g number were f u l l y f l u o r i n a t e d , whereas phenyl s i d e groups were e i t h e r f u l l y or nonf l u o r i n a t e d . C r i t i c a l s u r f a c e tensions of w e t t i n g obtained on t h i n c a s t f i l m s of these fluoropolymers were compared t o those of polymers w i t h r e l a t e d s t r u c t u r e s and surface c o n s t i t u t i o n s . Because of the presence of the b u l k y f l u o r i n e atoms and aromatic s i d e groups, some of these molecules are extremely s t e r i c a l l y blocked, which makes the p r e d i c t i o n of an e q u i l i b r i u m surface conformation very d i f f i c u l t . The r e s u l t s are discussed i n terms of s o l i d s u r f a c e c o n s t i t u t i o n , s t e r i c hindrance, and e l e c t r o s t a t i c dipole contribution. Literature Cited

1.

Zisman, W. Α., in "Contact Angle Wettability and Adhesion," Adv. Chem. Ser., No. 43, p. 1, Am. Chem. Soc., Wash., D.C., 1964.


BERNETT

AND ZISMAN



14.

0

J

I 2

L

4

J 6

1

ι 8

209

Ο

F(CF ) C00H

Δ

F ( C F ) ( C H ) C 0 0 H (FROM SOLUTION)]

•

F ( C F ) ( C H 2 ) C 0 0 H (FROM MELT)

•

H(CH ) C00H

ι

2

2

x

2

2

X

2

I 10

(FROM SOLUTION)

X

| 6

|6

(FROM SOLUTION)

| 7

I

l 12

L

X=NUMBER OF FLUORINATED CARBON ATOMS PER

14

I

I 16

I

18

MOLECULE Journal of Physical Chemistry

Figure 6.

Effect of fluorination of the adsorbed acid monolayer on y ( 20 )


c



Macromolecules

Figure 7. Effect of copolymerization with [—CF -CF —] on y { 11) ?r

2

n

c



14.


BERNETT AND ZISMAN

211

2. Zisman, W. Α., J. Paint Tech., (1972), 44, 42. 3. Brown, D.W., Wall, L. Α., J . Polym. Sci. A-1, (1968), 6, 1367. 4. Brown, D. W., Wall, L. Α., J . Polym. Sci. A-2, (1969), 7, 601. 5. Brown, D. W., Lowry, R. E . , Wall, L. Α., J . Polym. Sci. A-1, (1970), 8, 2441. 6. Brown, D. W., Lowry, R. E . , Wall, L. Α., J. Polym. Sci. A-1, (1970), 8, 3483. 7. Brown, D. W., Lowry, R. E . , Wall, L. Α., J . Polym. Sci. A-1, (1971), 9, 1993. 8. Brown, D. W., Lowry, R. E . , Wall, L. Α., J. Polym. Sci. (Polym. Chem. Ed.) (1973), 11, 1973. 9. Wunderlich, B., J. Phys. Chem., (1960), 64, 1052. 10. Wunderlich, B., Bodily, D. M., Kaplan, H. M., J . Appl. Phys., (1964), 35, 95. 11. Hu, W. Κ. H., Zisman, W. Α., Macromolecules, (1971), 4, 688. 12. Bernett, M. K., Macromolecules (In Press) (Sep 1974), 7. 13. Fox, H. W., Zisman, W. Α., J. Colloid Sci., (1950), 5, 514. 14. Fox, H. W., Zisman, W. Α., J. Colloid Sci., (1952), 7, 428. 15. Ellison, A. H., Zisman, W. Α., J . Phys. Chem., (1954), 58, 260. 16. Bernett, M. K., Zisman, W. Α., J . Phys. Chem., (1961), 65, 2266. 17. Ellison, A. H., Zisman, W. Α., J. Phys. Chem., (1954), 58, 503. 18. Fox, R. F., Jarvis, N. L . , Zisman, W. Α., in "Contact Angle Wettability and Adhesion," Adv. Chem. Ser., No. 43, p. 317, Am. Chem. Soc., Wash., D.C., 1964. 19. Pittman, A. G., Sharp, D. L . , Ludwig, Β. Α., J. Polym. Sci. A-1, (1968), 6, 1729. 20. Shafrin, E. G., Zisman, W. Α., J . Phys. Chem., (1962), 66, 740. 21. Bernett, M. K., Zisman, W. Α., J . Phys. Chem., (1963), 67, 1534. 222. Pittman, A. G. "Fluoropolymers," p. 419, Wiley Interscience Press, N.Y., 1972. 23. Fox, H. W., Zisman, W. Α., J . Colloid Sci., (1952), 7, 109. ;


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