Contact Angle, Wettability, and Adhesion

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21 The Wettability of a Homologous Series of Nylon Polymers

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TOMLINSON F O R T , J R . Fiber Surface Research Section Textile Fibers Department Ε. I. du Pont de Neumours & Co., Inc. Kinston, N. C.

The wettability of a homologous series of nylon polymers was studied. Measured con­ tact angles and test liquid surface tensions were used to estimate the critical surface tension of wetting, and the approximate work of adhesion of each test liquid, for each nylon homolog. Polymer wettability decreased with increasing separation of the amide groups in the solid surfaces. However, odd and odd­ -odd nylons were more easily wet than their even and even-even homologs. Within the odd and even series, wettability was determined by the average nylon number and not by the spacing of the polar groups along the poly­ mer chains. Results are interpreted in terms of the constitutive law of wettability and the morphologies and crystal structures of the nylons.

Some nine years ago, Ellison and Zisman [4] reported their study of the wettability of 6-6 nylon (polyhexamethylene adipamide) and com­ pared their results to those previously obtained on polyethylene [7]. They found that the nylon was more easily wet than the polyethylene, especially by hydrogen-bonding liquids, and attributed these findings to the presence of amide groups in the nylon surface. Since the time that study was made, a large body of experimental data has accumulated which shows that regular and predictable changes in solid-liquid contact angles result from changes in the outermost layer of atoms in the solid surface. A "wettability spectrum" has been pub­ lished [14],and the utility of the "critical surface tension of wetting" as an index of solid surface energy has been well established. This paper uses these concepts to correlate wettability with the surface constitution of a homologous series of nylons. These materials 302

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

27.

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Polymers

are of special interest not only because of their commercial importance (at least four members of the series are produced on a large scale), but also because characteristic differences in crystal structure and molecular orientation occur among the different polymer homologs. These differences, as well as the different spacings of the amide groups along the polymer chains, might be expected to lead to significant variations in the contact angles exhibited by hydrogen-bonding liquids on the various nylon surfaces.

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Experimental Polymers. The nylon 11 and polyethylene were commercial materials (Société Organico, P a r i s , France, and Ε. I. du Pont de Nemours & C o . , Inc., Wilmington 98, Del.). A l l the other polymers were prepared from the dry salt by bulk melt polymerization. Pertinent data for the samples used are collected in Table I. Amine ends were determined by titration with perchloric acid in 85% phenol-methanol solution and carboxyl ends by titration with sodium hydroxide in benzyl alcohol. Other analyses were by standard methods. Elemental analyses confirmed the identity of the polymers. Each was a pure sample, except nylon 11, which contained 0.26% of an unidentified inorganic material. Table I. Polymer Analyses Melt Temp., ° C .

Amine Ends

Carboxyl Endsb

Inherent Viscosity

6-6

252

47.3

75.0

1.13

7-7

201

38.1

91.9

1.23

8-8

206

36.6

78.5

1.08

9-9

182

36.0

93.5

1.01

10-10

192

20.5

6

216

31.8

11

183

56.5

Polymer

a

b

182 33.0 287

0

0.80 1.26 1.07

I n designating nylons, the first number refers to the number of carbon atoms in the diamine component, the second number to the number of carbon atoms in the dicarboxylic acid component. Single numbers refer to poly-Ω-amino acids. hNumber of ends per million grams of polymer. 0.5% solution in m - c r e s o l .

a

c

Wetting Liquids. The wetting liquids were all reagent grade chem­ icals, further purified either by distillation or by percolation through silica-alumina adsorption columns until their surface tensions checked reliable literature values [1]. Sample Preparation. Samples of polymer flake were baked in a vacuum oven for 4 hours at 110°C. to rid them of sorbed water. They were then formed into smoot! continuous films by pressing small quan­ tities in a Carver press (Fred S . C a r v e r , I n c . , 1 Chatham Road,Summit, N . J.) between a stainless steel disk (Planchets C o . , Chelsea, Mich.)

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

ADVANCES IN CHEMISTRY SERIES

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304

1.25 inch in diameter and 0.012 inch thick and a thin sheet of bright aluminum foil (Aluminum C o . of A m e r i c a , 123 South Broad St., P h i l ­ adelphia 9, Pa.). The pressing temperature was 10° higher than the polymer's melting point, pressing time was 45 seconds, and the pres­ sure was 4 tons p.s.i.g. The disk-polymer foil "sandwich" was p r o ­ tected by Ferrotype (Sears, Roebuck and C o . , Inc., Greensboro, N . C.) plates throughout the film-forming step. The films were quenched by placing the hot sandwiches on a cold stone laboratory bench top. The aluminum foil was then peeled away, exposing a smooth polymer surface. Measurements were made with the nylons still adhering to the steel disk, since the rigidity of the disk prevented wrinkling of the thin polymer film. Several methods of cleaning the films were tested. Either washing in a concentrated solution of the detergent Tide, followed by thorough rinsing, or extracting in a Soxhlet apparatus with carbon tetrachloride gave polymer surfaces yielding equal and reproducible contact angles. However, neither treatment was necessary if all metal surfaces which touched the polymer were thoroughly cleaned by solvent extraction prior to the pressing step, and most films were prepared using this latter technique. Samples were stored in a desiccator, since Ellison and Zisman [4] have shown that nylon wettability is much influenced by moisture sorbed from the atmosphere. Apparatus, Technique, and Conditions of Measurement. Most con­ tact angles were measured by observing light reflected from the surface of sessile drops of the various test liquids, using a special goniometer and technique [5]. Those few angles which were greater than 90° were measured by drawing tangents to the drop profiles projected onto a screen, as described by Kneen and Benton [12]. In all cases, the r e ­ corded data are the maximum reproducible advancing contact angles measured at 22°C. and 65% relative humidity, for each solid-liquid system. Experiments were performed at least in duplicate, on different days and using different pieces of polymer film. Angles were read to the nearest degree, and precision was better than ± 2 ° . Results The results are presented graphically in Table II. The wettability of a given polymer decreased as the surface tension of the test liquid was raised and with a given liquid, wettability decreased as the average nylon number (average number of carbon atoms per amide group) rose. However, significant differences were found between the odd- and evennumbered nylon homologs. The critical surface tensions of wetting, 7 ,of the nylons by hydro­ gen bonding liquids were estimated by linear extrapolation of graphs of the contact angle cosines vs. wetting liquid surface tensions to cos Θ = 1 [7]. These estimates, collected in Table III, range from 43 dynes per c m . for 7-7nylon to 28 dynes per cm. for polyethylene; the latter figure is 3 dynes per cm. lower than that found by Fox and Zisman [7] but is within the 7 range of 26 to 37 dynes per cm. recently reported for polyethylene by Wolfram [18]. Values for the approximate work of adhesion, WA, of each polymer for each test liquid are indicated in Table IV. They were calculated C

C

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

21.

FORT

Wettability

of

Nylon

305

Polymers

Table II. Wettabilities of Homologous Nylons Contact Angle, Degrees Water

Glycerol

Formamide

6

70

63

51

6-6

73

61

52

7-7

70

60

50

8-8

86

78

68

9-9

82

69

62

10-10

94

82

72

11

89

79

69

101

86

79

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Nylon

olyethylene)

Table III. Critical Surface Tensions of Wetting Nylon

X , Dynes/Cm. c

6

42

6- 6

42

7- 7

43

8- 8

34

9- 9

36

10-10

32

11

33

(Polyethylene)

28

from the Young-Dupré equation,WA = f o + r o (1 + cos 0), assuming that f o , the free energy of immersion of the solid in the vapor, was negligible [6] and that the measured contact angles, 0, approximated the true equilibrium values, 0 . WA varied as the separation of the amide groups in the polymers changed and was lowest for polyethylene and highest for 7-7 nylon. s

v

L

V

s v

E

Discussion It was expected that as the average nylon number increased, the contact angles exhibited by hydrogen-bonding liquids on the various surfaces would rise, approaching the wettability of polyethylene as an asymptotic limit (polyethylene might be regarded as nylon oo). Such

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

306

ADVANCES IN CHEMISTRY SERIES Table IV. Approximate Works of Adhesion Approximate WA, E r g s . / S q . C m . Water

Glycerol

Formamide

6

97.0

92.5

94.8

6-6

93.4

94.4

93.5

7-7

96.9

95.4

95.1

8-8

76.1

76.8

79.6

9-9

82.3

86.2

85.1

10-10

67.3

72.4

75.8

11

73.5

75.5

78.6

)lyethylene)

58.4

67.4

68.9

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Nylon

behavior would be predicted from consideration of the "constitutive law of wettability" [14] and the Young equation [19], cos θ = ^sv " ^ S L since decreasing the surface density of amide groups in the nylons should reduce the difference between the solid-vapor, y , and solidliquid, y , interfacial tensions. This behavior was found. However, when y , WA, or θ for any given wetting liquid was plotted as a function of average nylon number (Figure 1), the data fell on not one but two curves. It was reasoned that the indicated alternation of polymer wettabili­ ties had to result from differences in density, orientation, and/or bond­ ing of the amide groups in the nylon surfaces. Consequently,an attempt was made to relate the observed wettabilities to polymer morphology. A literature survey revealed that each class of nylon crystallizes to form sheets of polymer chains, with 100% hydrogen bonding between amide groups on adjacent chains in the sheets [10]. To achieve this complete bonding, different crystal habits must be taken by the differ­ ent classes, as shown in Table V . Diagrams of the hydrogen-bonded sheets for representative members of each class are indicated by F i g ­ ure 2. The plane of the hydrogen-bonded sheets in thin nylon films might be expected to correspond to the plane of the films. Diffraction studies have confirmed this arrangement as correct for oriented films made by squashing nylon samples under the wheels of a railway train [2] or by evaporating the solvent from a formic acid solution of 6-6 nylon [9,13]. However, studies by Barriault and Gronholz [1], Burnett and McDevit [3], and Jenckel and coworkers [8] have shown that melt-quenched 6-6 nylon is spherulitic and that the surface of this nylon normally consists of a thin "transcrystalline skin" in which the polymer chains are o r ­ iented parallel to the surface but with their hydrogen-bonded planes normal to it. It is significant that the " s k i n " has been observed in nylon films prepared by melt-pressing between aluminum plates [8], as were s v

S L

c

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

27.

FORT

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Nylon

307

Polymers

6-6

and

6

.15

ε .io

h

10-10

h"

/ οο (Polyethylene ) J I I I L 40 35 30

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3

Ζ

c ο

>s

Ζ

φ σ»

2 .05 φ > <

.00 25

Figure 1.

50

45

Variation in wettability with average nylon number

Table V . Structure of Crystalline Polyamides Class of Polyamide

a

Crystal Habit

a

Polymer Chains

Reference

Even-even

Triclinic

Planar zigzag

[2]

Odd-odd

Hexagonal

Twisted

[10,

Even (4 and 6)

Monoclinic

Planar zigzag

[2, 17]

Even(>6)

Hexagonal

Twisted

[16,17]

Odd

Triclinic

Planar zigzag

[10, 16]

E v e n and even-even nylons are polymorphic. Crystal habit stable structure at room temperature.

listed

is

15]

normal

the samples used in this study. The transe r y stall ine region is formed as a result of growth from spherulite nuclei which exist at the surface of the polymer. Since the density of these nuclei is always high, cover­ age of the nylon by the transcrystalline layer is complete, unless quench­ ing is extremely rapid, even if the bulk of the polymer is amorphous in character.

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

ADVANCES IN CHEMISTRY SERIES

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Figure 2.

Hydrogen-bonded sheets in representative nylons Ο Carbon atoms • Oxygen atoms Q Nitrogen atoms Hydrogen bonds

Contact angle measurements showed that the odd and odd-odd ny­ lons wet easier than their even and even-even homologs, a differentia­ tion not consistent with the crystal structures listed in Table V . How­ ever, if the transcrystalline layer extends all the way to the surface of the polymers, so that the edges of the hydro gen-bonded sheets dia­ grammed in Figure 2 make up the interface, the even and even-even homologs expose, alternately, a keto oxygen and imido hydrogen function, while the odd and odd-odd homologs expose one or the other, but not both. The wettability of the even polyamides would then represent an average for the two kinds of polar groups, while that of the odd polyamides would represent only one of them. Since these would logically be different, the observed variation in wettability could be explained.

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

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Wettability of Nylon Polymers

309

Exposure of the plane of hydrogen-bonded sheets cannot explain the observed wettability differences. If this orientation were taken, the energy of the odd-odd nylons would be increased slightly, relative to that of the other polymers tested, because of the approximately 5% greater amide group density in the "pleated" odd-odd nylon sheets [10]. However, this increase would not seem large enough to account for the wetting results, and the orientation is further disqualified by the fact that nylon 11, which crystallizes with fully extended polymer chains, behaves like its odd-odd and not its even-even homologs. Finally, it is necessary to consider the effects on wettability of the different separations of the hydrogen-bonded sheets in the extended and twisted chain polymers. These differences are caused by the greater space requirements of the twisted chain structures. Thus, in nylon 6-6 the interplanar distance is 3.7 A . [2] and in nylon 7-7 it is 4.2 A. [11]. Zisman and coworkers [7] have shown that closer packing of hydrocar­ bon chains leads to more difficult wetting, while closer packing of polar functions has the opposite effect. Since nylons 7-7, 9-9, and 11 all be­ haved so similarly, the two effects must come near to canceling in the polymers tested. Acknowledgment The author thanks I. A. David, Carothers Research Laboratory, Du Pont Co., for preparing the nylon polymers used in this work, and the Du Pont Co. for permission to publish. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

Bariault, R. J., Gronholz, L. F., J. Polymer Sci. 18, 393 (1955). Bunn, C. W., Garner, Ε. V., Proc. Roy. Soc. London 198A, 39 (1947). Burnett, B. B., McDevit, W. F., J. Appl. Phys. 28, 1101 (1957). Ellison, A. H., Zisman, W. Α., J. Phys. Chem. 58, 503 (1954). Fort, Tomlinson, Jr., Patterson, H. T., J. Colloid Sci. 18, 217 (1963). Fox, H. W., Zisman, W. Α., Ibid., 5, 514 (1950). Ibid., 7, 428 (1952). Jenckel, Von Earnst, Teege, Ernst, Hinricks, Walter, Kolloid Z. 129, 19 (1952). Keller, Α., J. Polymer Sci. 36, 361 (1959). Kinoshita, Y., Makromol. Chem. 33, 1 (1959). Ibid., p. 21. Kneen, E., Benton, W. W., J. Phys. Chem. 41, 1195 (1937). Scott, R. G., J. Appl. Phys. 28, 1089 (1957). Shafrin, E. G., Zisman, W. Α., J. Phys. Chem. 64, 519 (1960). Slichter, W. P., J. Polymer Sci. 35, 77 (1958). Ibid., 36, 259 (1959). Vogelsong, D. C., Ibid., in press. Wolfram, E., Kolloid Z. 182, 75 (1962). Young, T., Phil. Trans. Roy. Soc. (London) 95, 65 (1805).

Received April 1, 1963.

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.