Contact Angle, Wettability, and Adhesion

---

23 Surface Activity at Organic Liquid-Air Interfaces V. The Effect of Partially Fluorinated Additives on the Wettability of Solid Polymers N. L. JARVIS, R. B. F O X , and W. A. ZISMAN U. S. Naval Research Washington 25, D. C.

Laboratory,

The present investigation describes the suc­ cessful modification of the surface proper­ ties of polymeric solids by the adsorption of appropriate partially fluorinated compounds at polymer-air interfaces during the forma­ tion of the polymer surfaces. The extent of additive adsorption was found to be depend­ ent upon the molecular structure, fluorine content, and solubility of the additives in the solute—i.e., their organophilic-organophobic balance with respect to the solute. Certain effective additives were able to decrease the critical surface tension, γ , of such poly­ mers as poly(methyl methacrylate) and polyacrylamide to 20 and 11 dynes per c m . , r e ­ spectively. These low γ values correspond to surfaces containing closely packed CF and CF groups. c

c

2

3

Many surface properties of solid, smooth polymers are strongly dependent upon the chemical constitution of the surface layer of mole­ cules [25,26,27]; therefore, to modify a given surface property it is necessary to alter the chemical composition of the surface in someway. One example of such a modification is the commercial use of controlled oxidation of the surface of polyethylene foil to ensure wetting by and adhesion of polar printing inks [2]. A series of studies of surface activity of soluble and insoluble compounds at organic l i q u i d - a i r interfaces has been reported by Z i s ­ man, Ellison, Bernett, and Jarvis [4,10,11,16,17,18]. The most sur­ face active compounds were found to be various fluorocarbon deriva­ tives having the proper organophobic-organophilic balance. If one considers a plastic solid to be either a supercooled liquid or a liquid of very high viscosity, one would expect many of these partially fluorinated compounds also to manifest great surface activity when dissolved in 317

ADVANCES IN CHEMISTRY SERIES

318

various polymers, provided sufficient time is allowed for diffusion of the solute to the interface and for adsorption equilibrium to occur. The rate at which one of these surface active agents will adsorb at the polymer-air interface will be primarily determined by the rate of dif­ fusion in the bulk polymer phase. If added to a molten polymer or to a solution of the polymer in a volatile solvent, the surface active agent will be able to diffuse rapidly to the surface as the polymer solidifies or as the solvent evaporates. Once the polymer film is solid, however, it may take hours or days for a significant amount of the additive to migrate to the surface, depending upon the concentration of the additive and the nature of the polymer. The extent to which a surface active solute adsorbs at a polymer surface will be determined by essentially the same properties that are responsible for their effectiveness in organic liquids—namely, their concentration, ability to lower the surface energy of the system, fluo­ rine content, and solubility or extent of association with the polymeric substrate. By analogy with aqueous systems one might briefly call the combination of these properties in a fluorocarbon derivative its organophilic-organophobic balance. The presence of adsorbed and oriented fluorocarbon groups at the polymer-air interface will not only decrease the surface free energy of the polymer, and thus its wettabil­ ity, but should modify its coefficient of friction and decrease its adhe­ sive properties as well. It is the purpose of this report to demonstrate the effectiveness of a small concentration of a partially fluorinated additive in changing the surface constitution and reducing the wettability of various polymer surfaces. Experimental

Materials and Methods

The partially fluorinated compounds used in this investigation as surface active additives are listed in Table I. T A B L E I.

I II III IV V VI VII VIII

List of Partially Fluorinated Compounds Used as Surface Active Agents

Tris(lH,lH-pentadecafluoro-octyl) tricarballylate 3-(Hydroxymethyl)-l,5-pentanediol tris(heptafluorobutyrate) Bis(lH,lH-undecafluorohexyl)-3-methyl glutarate Bis(lH,lH-pentadecafluoro-octyl) tetrachlorophthalate ΙΗ,ΙΗ-Pentadecafluoro-octyl ethane sulfonate Bis(lH,lH-heptafluorobutyl) adipate 18,18,19,19,20,20,21,21,22,22,22-Undecafluorodocosanoic acid N,N,N-Dimethyl-3-(n-perfluoroheptanecarboxamido)propyl -3-aminopropionic acid, inner salt

Additives I through VI were prepared in high purity by O Rear and coworkers at this laboratory [12,19] and have been used in the previous studies of surface activity at organic liquid-air interfaces[4,10,11,16, 17,18], Additives VII and VIII are research preparations donated by the Organic Chemical Department, Ε. I. du Pont de Nemours & Co., and the Minnesota Mining and Manufacturing Co., respectively. The carefully purified liquids used in the determination of y were selected to give a wide range of surface tensions and a variety of structural types. Sources, methods of purification, and surface tensions of the majority f

c

23.

JARVIS

ET AL

Fluorinated

Additives

vs.

Wettability

319

of these liquids (listed in Tables II to V) have been given in previous reports from this laboratory [20,22]. The 1,1-diphenylethane was a product of Eastman Organic Chemicals, which had been freed from polar impurities by several percolations through a column of F l o r i s i l . Four polymers with different surface compositions were used in this study—polystyrene (PS), poly(methyl methacrylate) (PMMA), polyacrylamide (PAM), and a poly(vinylidene chloride) (PVeC) copolymer (containing 20% polyacrylonitrile). Polystyrene has essentially a hydrocarbon surface, whereas the surfaces of poly(methyl methacrylate) and polyacrylamide contain ester and amide groups, respectively. The surface of the poly(vinylidene chloride) copolymer on the other hand will contain a relatively large number of chlorine atoms. The presence of acrylonitrile in the poly(vinylidene chloride) copolymer improved the solubility characteristics of the polymer for the purposes of this study, but did not appreciably alter y , its critical surface tension of wetting. Values of y of these polymers ranged from 30 to 33 dynes per c m . for polystyrene to approximately 40 dynes per cm. for the poly(vinylidene chloride) copolymer. No attempt was made to determine the crystallinity of the polymer samples, or to correlate crystallinity with adsorption of the fluorocarbon additives. The poly(methyl methacrylate) used in these experiments was obtained from freshly distilled degassed monomer by bulk polymerization to 30% conversion at 60° C. with azodiisobutyronitrile as initiator. The polymer was purified by two reprecipitations from tetrahydrofuran solution with methanol. After drying _at room temperature under vacuum for 24 hours, the material had M = 7.76 x 10 . Mass spectroscopic analysis of the volatile material obtained by heating 0.1-gram film samples of the polymer at 110° C. for 4 hours showed 7.4 x 1 0 " mole of monomer and traces of methylene chloride solvent, no other volatile material having been evolved. F i l m s of P M M A about 15 m i crons thick, with and without additives, were prepared in shallow r e c tangular borosilicate glass dishes by the slow evaporation (for 24 hours or more) at room temperature of methylene chloride solutions of about 0.1 gram of the polymer and appropriate amounts of additive. During most of this time the surface-active compounds were free to diffuse to the polymer-air interface. Traces of solvent were removed by continuous pumping at room temperature an additional 16 hours. Finally, samples were stored in an evacuated desiccator until the contact angle measurements were made, usually within 3 days. In each case there appeared to be no significant change in contact angle with time after the third day. c

c

v

s

7

Polystyrene was prepared from freshly distilled and degassed styrène by bulk polymerization at 50° C. under nitrogen with benzoyl peroxide as the initiator: Conversion was about 50%. After precipitation with methanol, the polymer was purified by two reprecipitations from tetrahydrofuran solution with methanol, exhaustively extracted with methanol, and finally vacuum-dried at room temperature. The polymer had M = 1.8 x 10 . F i l m s of this material were prepared from methylene chloride solution in the same way as the P M M A films. Acrylamide (m.p. 85° C. from ethyl acetate) was polymerized in aqueous solution at about 75° C. in the presence of 2-propanol and potassium persulfate as described by Sorenson and Campbell [23]. The polyacrylamide was further purified by an additional precipitation from v

s

ADVANCES IN CHEMISTRY SERIES

320

aqueous solution with methanol and dried in vacuum. F i l m s were p r e ­ pared by the slow evaporation of water from aqueous solutions of the polymer and additive. The poly(vinylidene chloride) copolymer (Dow Chemical Co.) con­ tained 80% vinylidene chloride and 20% acrylonitrile. A small amount of a carbonyl- containing compound was the only impurity detected by means of the infrared spectra of a thin film of the copolymer. F i l m s were prepared by the slow evaporation of tetrahydrofuran solutions; otherwise they were handled in the same manner as the P M M A samples. Smooth surfaces of each polymer were also prepared (without ad­ ditives) by pressing samples of the powdered polymers against a highly polished stainless steel surface in a Carver press at 16,000 p . s . i . Circular disks 1 inch in diameter and weighing several grams were formed in this way. PS and the PVeC copolymer were compressed at room temperature, P A M at 120° C , and P M M A at 150° C. The pressure was maintained until smooth polymer surfaces were obtained. The mold and stainless steel piston were cleaned prior to use, so that con­ tact angles could be measured on the polymer surfaces without further surface treatment. The contact angles of the various liquids in Tables II to V on each polymer surface were determined while increasing the volume of the drop, and thus slowly advancing its periphery over the surface. The contact angles on each polymer surface prepared by solvent evapora­ tion were measured through the plane ends of the rectangular borosilicate glass dishes using the method and improved goniometer telescope described previously [13], A l l contact angles on P M M A , PS, and PVeC copolymer were measured in a i r , but to prevent the adsorption of water, contact angle measurements on P A M were made in a dry nitrogen atmosphere. Every contact angle reported is the average of the values obtained on at least three different drops on a plastic surface, and at least two independent samples of each film were prepared. Contact angles for successive drops on a given polymer surface seldom varied more than 4 ° ; however, somewhat larger differences were sometimes observed between independently prepared samples of the same polymer contain­ ing small amounts of certain additives. These variations may have been a function of the age of the solid—i.e., the longer the polymer specimen ages, the greater the amount of additive that may diffuse to the surface. Because of the difficulty involved in measuring small con­ tact angles through the plane ends of the glass sample dishes, contact angles of 5° or less were considered as essentially zero and indicative of spreading for the purposes of this study. Each prepared sample containing an additive was allowed to stand no longer than 4 days before contact angle measurements were made. A l l measurements were made at 25° ± 1° C. with the relative humidity varying from 15 to 30%, except for P A M . Experimental

Results

Polystyrene. Contact angles of the various pure liquids on poly­ styrene are given in Table II for surfaces of pure polystyrene prepared by solvent evaporation as well as by compression of the powdered polymer. Values of θ obtained in each case agree well with those

23.

JARVIS

ET AL

T A B L E II.

Fluorinated

Additives

vs. Wettability

321

Contact Angles for Various Liquids on Polystyrene Contact Angles, 0, Degrees Surface Tension, Dynes/Cm.

Liquid

,. , _ N

Surface by Solvent Evaporation

q

N

additive Water Glycerol Formamide Thiodiglycol Methylene iodide Arochlor 1242 (trichlorobiphenyl) 1 -Bromonaphthalene T r i c r e s y l phosphate Hexachloropropylene 1,1 -Diphenylethane tert-Butylnaphthalene Dicyclohexyl Bis(2-ethylhexyl) orthophthalate Squalane Hexadecane Tetradecane

72.6 63.4 58.2 54.0 50.8 45.3

18

44.6 40.9 38.1 37.7 33.7 32.8 31.3

18 14 11 12 Spr. Spr. 7

29.5 27.6 26.7

Spr. Spr.

93 82 76 a

b

4%

q

additive 96 84 80 63 a 16 15 18 14 12 Spr. Spr. 10 6 Spr. Spr.

a

d

d

*

i

10% v

e

a

d

d

£

i

v

96

97

16

18

12

14

6 Spr.

9 Spr.

e

P o l y m e r dissolved in methylene iodide, bContact angle less than 5 ° .

a

reported by Ellison and Zisman [9], as shown in Figure 1. This agree­ ment indicates that the polystyrene surfaces formed by solvent evapo­ ration were essentially free of contamination, for the presence of a modest fraction of a monolayer of residual methylene chloride solvent at the interface would have significantly decreased the contact angles [8] below those reported in Table Π. The plot in Figure 1 of cos θ vs. surface tension, ( y o ) , for various pure liquids indicates that y for polystyrene is between 30 and 35 dynes per c m . , the range previously reported as the best estimate of the critical surface tension of poly­ styrene [9, 24]. The fluorocarbon additives I through VII listed in Table I were added in varying concentrations to solutions of polystyrene in methylene chloride. Each was added in concentrations up to at least 1% by weight of polymer, while the pentanediol (Π) and 3-methylglutarate (ΠΙ) de­ rivatives were added in concentrations up to 10 and 4%, respectively. In every case where only 1% additive was present there was no p e r ­ ceptible change in the wetting behavior of the polystyrene surface. Even at the higher concentrations, additives II and ΙΠ had only a slight effect on the wettability of polystyrene, the most significant change being that hexadecane now gave definite, measurable, contact angles. The ineffectiveness of these additives in polystyrene is a result of their low solubilities in the polymer. Each additive contains one or more highly polar ester or acid group, which in combination with the high fluorine content of the molecule tends to cause low solubility in nonpolar L V

c

ADVANCES IN CHEMISTRY SERIES

322

1.0

0.8h

0.6h

Φ

0.4h

UJ

©

M I S C . LIQUIDS

ON

PS

#

MISC. LIQUIDS

ON

PS

ζ ο ο

(9)

0.2^ 0.0

-0.2

10

20

40

30

S U R F A C E

T E N S I O N

50

60

70

(DYNES/CMJ

Figure 1. Cos θ vs. y ° for various pure liquids on polystyrene surfaces L

V

liquids. In most instances the additives were so insoluble in the poly­ styrene that they either separated as another phase as the solvent evaporated or formed cloudy, opaque films. Evidently, in order to modify the surface of polystyrene by adsorption of surface active mole­ cules, it will be necessary to find partially fluorinated compounds that are more soluble in polystyrene. Poly (methyl Methacrylate), The contact angles of the various l i q ­ uids on P M M A surfaces are given in Table III. The surfaces of P M M A (without additives) prepared by the two independent techniques were initially studied exactly as prepared, without any attempt to clean the newly formed surfaces. Significant differences were observed between contact angles on these two surfaces, the specimen prepared by solvent evaporation exhibiting much higher contact angles with some liquids than the pressed disks. However, after the surface prepared by solvent evaporation was washed with detergent, rinsed profusely with distilled water, and dried, contact angles were obtained which agreed within the limits of experimental error with those measured on the pressed disk, and were in better agreement with values reported in the literature. Craig, B e r r y , and Peyton [7] have reported a contact angle of 78° for water on clean P M M A , in very good agreement with that reported here for the clean surface. Therefore, the surface of the P M M A prepared by solvent evaporation had been contaminated by an easily removed film. It is unlikely that the contamination came from the solvent, for samples of polystyrene prepared from the same batch of solvent and by the same technique did not show evidence of such contamination. It is concluded that the film removed by cleaning originated from a small

29 23

21.8

Octane

23

33 31

9

44 39

43 37

Spr.

Spr.

Spr.

25.4 23.9

36

Decane

39 15 48

Dodecane

45

Spr. 11

26.7

Tetradecane

47

40

51

19

21 52 48

Spr.

14

Spr.

27.6

Hexadecane

20

18 44

19 44

Spr.

6

8 26

Spr.

31.3 29.5

Squalane

6

Spr.

32

15

45

Bis(2-ethylhexyl) orthophthalate

Spr.

Spr.

Spr.

41

8

32.8

11

Dicyclohexyl

10

Spr.

Spr. Spr.

Spr. Spr.

Spr.

10

33.7

Spr.

Spr.

37.7

1,1 -Dipheny lethane

tert-Buty lnaphthalene

Spr.

Spr.

Spr.

Spr.

Spr.

38.1

Hexachloropropylene

18 26

17 24

18 24

16 20

19

40.9

16

44.6

1 -Bromonaphthalene

T r i c r e s y l phosphate

46

42

42

43

41

50.8

Methylene iodide

85 61

59

85

97 96

96

91

96

1.0% additive II

0.5% additive II

59

47

54.0

Thiodiglycol

89

96

0.5% additive I

79

64

58.2

Formamide

69

0.2% additive I

46

75 63

69

No additive (after cleaning) 76

Surface Formed by Solvent Evaporation

63

88

80

72.6

94

No additive

63.4

No additive

Pressed

Glycerol

Dynes/Cm.

Surface

Contact Angle, 0, Degrees

Contact Angles of Various Liquids on Poly (methyl Methacrylate)

Water

T A B L E ΙΠ.

ADVANCES IN CHEMISTRY SERIES

324

amount of impurity in the P M M A sample which had adsorbed at the polymer-air interface as the solvent evaporated. A similar accumula­ tion of impurity would not be expected to occur at the surface of the pressed disk, because of the slower rate of diffusion in the solid polymer. F r o m the contact angles in Table ΙΠ, cos θ vs. y was plotted in Figure 2 for clean P M M A . F r o m the intercept it is seen that y is approximately 39 dynes per cm., well within the range of 33 to 44 dynes per cm. recently reported by Wolfram [24]. Ellison and Zisman [9] also obtained a critical surface tension of between 39 and 40 dynes per cm. for poly (ethylene ter ephthalate), another polymer containing a large number of carboxylic ester groups. L

V

c

I-Oi

0.8

0.6

UJ

?

0.4|

if)

Ο o

0.2|

Ο

MISC. LIQUIDS ON PMMA

Δ

n-ALKANES ON PMMA + 0.5%

•

n-ALKANES ON PMMA* 10% ADDITIVE H

ADDITIVE

I

0.0

-0.2

10

20

40

30

S U R F A C E

T E N S I O N

50

60

70

(DYNES/CM.)

Figure 2. Cos θ vs. y for various pure liquids on poly(methyl methacrylate) surfaces L

y

0

The following fluorinated compounds were added to P M M A : t r i carballylate (I), pentanediol (II),and 3-methylglutarate (ΙΠ) derivatives. Additive I was added in concentrations up to 0.5% by weight of polymer, while Π and ΠΙ were added in concentrations up to 1.0 and 2.0%, r e ­ spectively. Contact angles were measured on P M M A films containing fluorinated additives without prior cleaning of the film surface, as in many cases washing with a detergent removed significant amounts of the adsorbed additive. The trace of impurity in the P M M A discussed in the previous paragraph was not considered important here, since in the competitive adsorption between it and one of the highly surface active fluorinated solutes, the latter would probably be the dominant species adsorbed. Therefore, an additive was considered effective only if it increased θ above the values reported in Table ΠΙ for P M M A

23.

JARVIS

ET AL

Fluorinated

Additives

vs.

Wettability

325

prepared without additives by solvent evaporation. Additive III ap­ peared to be so soluble in P M M A that even a 2.0% concentration r e ­ sulted in essentially no change in the wetting properties of the poly­ mer. However, additives I and II brought about significant changes in the wettability. The fact that identical concentrations of these additives in methylene chloride did not cause a similar change in the wettability of polystyrene indicates that this effect is truly related to the surface activity of the additives in the polymer and not to their surface activity in the evaporating solvents. The fluorinated tricarballylate (I) was very surface active in P M M A and at a concentration of only 0.2% had apparently reached its maximum adsorption at the polymer-air interface. As is shown in Table ΙΠ, θ did not increase further when the concentration was increased to 0.5%. However, significant changes in θ were observed only for the high sur­ face tension polar liquids and the low surface tension nonpolar liquids; those having intermediate properties exhibited contact angles identical to those measured on the additive-free P M M A . Previous work on the behavior of these additives when dissolved in organic liquids [17] indi­ cates that the observed effect arose from the ability of the sessile drop to dissolve the film of additive accumulated in the surface of the poly­ mer. Even if the entire fluorocarbon monolayer beneath a drop was dissolved, the concentration within the drop would still be too low to reduce the liquid surface tension. Essentially then we would have a situation analogous to a pure liquid drop resting on the additive-free PMMA. Additive Π did not show the same surface activity in P M M A as the tricarballylate (I), for 0.5% caused only moderate changes in Θ, and only when 1.0% had been added did θ approach values obtained with additive I. At the higher concentrations the polymer films containing the pentanediol derivative (Π) tended to wrinkle badly and pull away from the bottom of the glass dish; therefore reliable measurements of θ were difficult to make and hence fewer are reported. Again, the most significant changes in θ occurred for the very polar and nonpolar liquids. In many studies of wettability Zisman and coworkers have used the contact angles of a series of n-alkanes as a convenient means for de­ termining r for low energy solid surfaces [5, 6,13, 20]. In Figure 2 are plotted the cos θ vs. y o curves for the n-alkanes on P M M A sur­ faces containing 0.5% additive I and 1.0% additive II. The critical sur­ face tensions with additives I and Π were 19 and 20 dynes per c m . , respectively, representing a decrease of about 20 dynes per cm. from the value of y obtained with the additive-free surface. Since the y values of 19 and 20 dynes per cm. are very close to that of 18 dynes per c m . reported by Fox and Zisman [13] for the n-alkanes on poly­ tetrafluoroethylene surfaces, it is apparent that a number of perfluoroalkane groups are present in the outermost part of the surface phase with the principal axis of each carbon-carbon chain parallel to the surface. Poly(vinylidene Chloride) Copolymer. Contact angles observed on poly(vinylidene chloride) copolymer surfaces prepared by solvent evapo­ ration are given in Table IV, along with the values obtained on highly polished surfaces of compressed disks of the additive-free powdered polymer. Values of θ exhibited by the various liquids on each type of c

L

c

V

c

326

ADVANCES IN CHEMISTRY SERIES T A B L E IV. Contact Angles of Various Liquids on Poly(vinylidene Chloride) Copolymer Contact Angle, 0, Degrees Surface Tension, Dyne s / C m .

Liquid

Pressed disk IN υ

additive Water Glycerol Formamide Thiodiglycol Methylene iodide Arochlor 1242 (trichlorobiphenyl) 1 - B r omonaphthalene T r i c r e s y l phosphate Hexachloropropylene 1,1 -Dipheny lethane tert-Butylnaphthalene Dicyclohexyl Bis(2-ethylhexyl) orthophthalate Hexadecane Tetradecane Dodecane Decane Octane

Surface by Solvent Evaporation No additive

1.0% additive I

1.0% additive IV

72.6 63.4 58.2 54.0 50.8 45.3

81 67 65 42 27 11

85 72 70

86 72 71

100 92 82

27 Spr.

29 8

50 23

44.6 40.9 38.1 37.7 33.7 32.8 31.3

9 10 Spr. Spr. Spr. Spr. Spr.

9 11 Spr. Spr. Spr. Spr. Spr.

Spr. 12 Spr. Spr. Spr. Spr. Spr.

27.6 26.7 25.4 23.9 21.8

Spr.

Spr.

30

20 44 41 36 27 11

PVeC copolymer surface are in good agreement, generally within the limits of experimental e r r o r . These values of θ are very similar to those reported previously by Ellison and Zisman [8] on a poly(vinylidene chloride) surface, which for comparison are plotted in Figure 3 along with the present results. The plot of cos θ vs. Ύυγ° in Figure 3 shows the critical surface tension of the additive-free PVeC copolymer sur­ face to lie between 38 and 44 dynes per cm. Fluorinated compounds I, III, IV, and V were dissolved in tetrahydrofuran solutions of the PVeC copolymer in concentrations up to 1% by weight of the polymer. Of these additives, the 3-methyl glutarate (III), ethane sulfonate (V), and tricarballylate (I) derivatives failed to modify the wettability of the polymer surface. However, the tetrachlorophthalate derivative (IV) caused a marked decrease in the wetta­ bility of the polymer (see Table IV, last column). In Figure 3 the graph of cos θ vs. y o for the n-alkanes on the resulting surface shows that y is between 20 and 21 dynes per cm., which is close to the charac­ teristic 7 value of polytetrafluoroethylene), 18 dynes per cm. It is apparent that the polymer surface is rich in the fluorinated aliphatic chains of the solute and that the presence of the chlorine atoms in the tetrachlorophthalate molecule has increased the solubility of the addi­ tive in the highly chlorinated PVeC copolymer. The presence of the chlorine groups in the tetrachlorophthalate derivative thus gave it a more suitable organophilic-organophobic balance than was present in additives I, ΙΠ, and V . L V

c

C

23.

JARVIS

Fluorinated

ET AL

Additives

S U R F A C E

T E N S I O N

vs.

Wettability

327

(DYNES/CM.)

Figure 3. Cos θ vs. y for various pure liquids on surfaces of poly(vinylidene chloride) copolymer (containing 20% poly aery lonitrile) L

y

0

Polyacrylamide. In Table V are listed the contact angles of the various liquids on polyacrylamide. On account of the high water solu­ bility of this polymer, extremely low contact angles were observed for water, approaching zero with time. Glycerol and formamide also ap­ peared to interact with P A M , and failed to give reproducible contact angles. Contact angles of the remaining liquids, obtained in a dry nitrogen atmosphere, were reproducible (Table V). The contact angles observed on the clean P A M surface are somewhat larger than might have been predicted on the basis of the polymer s chemical composi­ tion. If the amide groups are exposed at the surface, one would expect the polymer to have a critical surface tension about as high as P M M A or nylon [9], whose y values are about 40 dynes per cm. On the other hand, if the amide groups are not exposed, the y for P A M would be closer to that of polyethylene, approximately 31 dynes per c m . The plot of cos θ vs. y ο for the pure liquids on additive-free surfaces does not give a well defined y . If a line is drawn through the points represent­ ing the liquids with surface tensions greater than 35 dynes per c m . , and is drawn parallel to corresponding lines in Figures 1 to 3, it indicates a y somewhere between 35 and 40 dynes per cm. However, if this is the true y of the surface, the liquids with surface tensions below 35 dynes per cm. should have spread. This anomalous wetting behavior of the low surface tension liquids on additive-free P A M may indicate that the polymer surface is con­ taminated. The most probable contaminant on the surface of this watersoluble polymer would be a layer of strongly adsorbed water, which is 1

c

c

c

c

c

ADVANCES IN CHEMISTRY SERIES

328

T A B L E V.

Contact Angles of Various Liquids on Polyacrylamide (Values obtained in dry N

2

Contact Angle, θ,

Surface Tension,

Water Glycerol Formamide Thiodiglycol Methylene iodide Arochlor 1242 (trichlorobiphenyl) 1 -Bromonaphthalene T r i c r e s y l phosphate Hexachloropropylene 1,1 -Dipheny lethane tert-Butylnaphthalene Dicyclohexyl Bis(2-ethylhexyl) orthophthalate Squalane Hexadecane Tetradecane Dodecane Decane Octane

No additive

a

72.6 63.4 58.2 54.0 50.8 45.3

28 47 33

44.6 40.9 38.1 37.7 33.7 32.8 31.3

33 31 Spr. 22 20 19 20

29.5 27.6 26.7 25.4 23.9 21.8

Degrees

Surface by Solvent Evaporation

Dyne s / C m . Liquid

atmosphere)

14 11

0.6% additive VIII

1.2% additive VIII

1.0% additiv

vni

a

91 87

84 93 86

95 83

86 87 39 82 80

87 87 42 82 78

85 85 59 82 78

78

77

76

76 71 68 66 62 57

74 71 68 65 62 56

74 70 67 64 60 54

Additive VIII added to monomer prior to polymerization.

not removed by drying under vacuum at room temperature. T o deter­ mine whether or not adsorbed water will influence the contact angle, surfaces of P A M prepared by solvent evaporation, and by pressing the dried powder into disks, were exposed for several hours to atmospheres of varying relative humidity. The contact angles of methylene iodide on both surfaces were observed to increase from 47° to 58° as the relative humidity rose from 0 to 99%, while the contact angles of hexa­ decane increased from 14° to 31° over the same relative humidity range. This behavior is analogous to the increase in methylene iodide contact angle with increasing hydration of a silicate surface that was observed by Shafrin [21]. On the basis of these experiments the finite contact angles of the low surface tension liquids on additive-free P A M surfaces may be explained by the presence of adsorbed water. The incorporation of a fluorocarbon additive in P A M presented a somewhat different problem than the previous polymers, inasmuch as it required a fluorinated additive that has some solubility in water. The partially fluorinated additives, I to VII, are insoluble in water and fail to disperse in the polymer when added to aqueous solutions; rather they form a separate phase and settle to the bottom of the sample dish. Additive VIII was therefore selected for use in this polymer, as Guenthner and Vietor [14] have shown that it was soluble in water up to 1% by weight. Each of the samples containing the additives was handled

23.

JARVIS

ET AL

Fluorinated

Additives

vs. Wettability

329

in the same manner as the additive-free surface, so that all would have about the same exposure to water vapor. In this way differences in wettability were observed, even though we were unable to determine the precise critical surface tension for the polymer itself. It subsequently was found that the contact angles on the films containing additive VIII did not seem to be susceptible to the presence of the small amounts of water vapor in the atmosphere, and did not change significantly upon standing in the room at 15 to 20% relative humidity up to several hours. The contact angles on P A M containing 0.6 and 1.2% of additive VIII given in Table V show that even 0.6% additive dramatically increased the contact angles of the liquids on the polymer surfaces. Contact angles for the n-alkanes approached those reported for surfaces con­ sisting largely of - C F groups [6,15,20,22]. A plot of cos θ vs. y o (see Figure 4) using the data for the n-alkanes shows that for this sur­ face y is 10.4 dynes per cm. There was no further decrease in y when the additive concentration was increased from 0.6 to 1.2%, indi­ cating that even at the lower concentration maximum adsorption of the additive had developed at the interface. 3

L

c

V

c

S U R F A C E

Figure 4.

T E N S I O N

(DYNES/CM)

Cos θ vs. y o for various pure liquids on polyacrylamide surfaces L

V

Besides being added to aqueous solutions of P A M , additive VIII was also added to a solution of the acrylamide monomer prior to polymerization. After polymerization, the P A M was purified as de­ scribed earlier and films were prepared by solvent evaporation with no further addition of surface active additive VIII. The contact angles given in the last column of Table V for the organic liquids on this film correspond, within the limits of experimental e r r o r , to those obtained

330

ADVANCES IN CHEMISTRY SERIES

when the additive was added to an aqueous solution of the polymer just prior to formation of the film. Discussion Small amounts of appropriate fluorine-containing compounds, 1% by weight or less, significantly modified the wetting properties of several polymer surfaces. The fluorine-containing surface active agents were equally effective when added to the monomer prior to polymerization or to solutions of the polymer in a volatile solvent. If one considers the polymer as a liquid of very high viscosity, it should be i m material how the additives are incorporated in the polymer, provided sufficient time is allowed for diffusion of the solute to the interface. The less viscous the polymer, the more rapidly adsorption equilibrium will be attained. One further property of a film formed by this technique is that it should be self-healing—that is, any surface active molecules lost from the film will be replaced by the diffusion of additional material into the interface. The rate of self-healing will be dependent upon the rate of diffusion of the fluorocarbon derivatives in the bulk polymer, and may be accelerated by heating the solid polymer or otherwise lowering its viscosity. It is apparent from this study that in many ways the problems involved in selecting surface active agents for polymeric systems are analogous to those of finding surface active agents for any organic liquid, the primary difference being the slow rate of diffusion of the additives in the polymer. The problem of surface activity in a polymeric system is thus a logical extension of the previous studies of surface activity in organic liquids, the effectiveness of a specific additive in a polymer being dependent upon its organophilic-organophobic balance with respect to that polymer. In the present study only fluorocarbon derivatives were used as surface active additives, but many hydrocarbons as well as silicone compounds would also be expected to show some surface activity in polymers. The primary advantage of a fluorine-containing compound is that it will cause a much greater decrease in surface free energy, and thus give a polymer surface a lower wettability. A critical surface tension of 10 to 11 dynes per cm. was observed for P A M containing only 0.6% by weight of a fluorinated additive, while P M M A and the PVeC copolymer had y values of approximately 20 dynes per cm. when small amounts of appropriate additives were present at the interface. The critical surface tensions of P M M A and the PVeC copolymer were thus reduced 20 dynes per c m . , while that of P A M was lowered at least 25 dynes per cm. These large decreases in critical surface tension reflect the change in surface composition which has taken place, the polymer molecules being replaced in the interface by closely packed C F 2 and C F 3 groups. The presence of a closely packed layer of adsorbed molecules at a polymer surface will have a marked effect on many of its surface properties, such as friction, adhesiveness, and wettability. These properties of an adsorbed film have already found some application in industry. Allan [l] has demonstrated that small amounts of oleylamide incorporated in polyethylene foil will diffuse to the surface of the foil and greatly reduce the friction and adhesion between sheets of the plastic. A committee from the Piedmont Section of the American c

23.

JARVIS ET AL

Fluorinated Additives vs. Wettability

331

Association of Textile Chemists and Colorists recently [3] described the incorporation of small amounts of fluorocarbon derivative in a polymeric material normally used to treat textiles for water repellency. They observed that the fluorocarbon preferentially adsorbed at the in­ terfaces and decreased the γ values to 16 to 18 dynes per cm. Their films clearly showed the ability to self-heal, for when the initially ad­ sorbed layer was deliberately scraped off, additional molecules quickly adsorbed at the interface when the polymer matrix was recured at an elevated temperature. The usefulness of adsorbed films of surface active molecules is thus apparent, and one may expect wide application of this technique to specific problems. The present study, in combina­ tion with previous investigations of wettability and surface activity in organic liquids, forms an excellent guide for the design and synthesis of further surface active agents for polymeric systems. c

Acknowledgment The authors thank L . G. Isaacs of this laboratory for preparing the polymer films that were studied. Literature Cited (1) Allan, A. J. G., J. Colloid Sci. 14, 206 (1959). (2) Allan, A. J. G., J. Polymer Sci. 38, 297 (1959). (3) American Association of Textile Chemists and Colorists, Piedmont Sec­ tion, Am. Dyestuff Reptr. 52, 83 (1963). (4) Bernett, M. K., Jarvis, N. L., Zisman, W. Α., J. Phys. Chem. 66, 328 (1962). (5) Bernett, M. K., Zisman, W. Α., Ibid., 64, 1292 (1960). (6) Ibid., 66, 1207 (1962). (7) Craig, R. G., Berry, G. C., Peyton, F. Α., Ibid., 64, 541 (1960). (8) Ellison, A. H., Zisman, W. Α., Ibid., 58, 260 (1954). (9) Ibid., p. 503. (10) Ibid., 60, 416 (1956). (11) Ibid., 63, 1121 (1959). (12) Faurote, P. D., Henderson, C. M., Murphy, C. M., O'Rear, J. G., Ravner, H., Ind. Eng. Chem. 48, 445 (1956). (13) Fox, H. W., Zisman, W. Α., J. Colloid Sci. 5, 514 (1950). (14) Guenthner, R. Α., Vietor, M. L., Ind. Eng. Chem. Prod. Res. Develop. 1, 165 (1962). (15) Hare, E. F., Shafrin, E. G., Zisman, W. Α., J. Phys. Chem. 58, 236 (1954). (16) Jarvis, N. L., Zisman, W. Α., Ibid., 63, 727 (1959). (17) Ibid., 64, 150 (1960). (18) Ibid., p. 157. (19) O'Rear, J. G., Sniegoski, P. J., Naval Research Lab., NRL Rept. 5795 (July 18, 1962). (20) Schulman, F., Zisman, W. Α., J. Colloid Sci. 7, 465 (1952). (21) Shafrin, E. G., private communication. (22) Shafrin, E. G., Zisman, W. Α., J. Phys. Chem. 66, 740 (1962). (23) Sorenson, W. R., Campbell, T. W., "Preparative Methods of Polymer Chemistry," p. 179, Interscience, New York, 1961. (24) Wolfram, E., Kolloid Z. 182, 75 (1962). (25) Zisman, W. Α., Advan. Chem. Ser., No. 43, 1 (1963). (26) Zisman, W. Α., "Constitutional Effects on Adhesion and Abhesion," Gen­ eral Motors Research Symposium on Adhesion and Cohesion, July 25, 1961. (27) Zisman, W. Α., "Decade of Basic and Applied Science in the Navy," NRL Rept. 4932, May 15, 1957, ONR Decennial Symposium, Washington, D. C., March 19, 1957. Received March 27, 1963.