WETTABILITY OF POLYETHYLENE SINGLE CRYSTAL AGGREGATES
since we have begun with (16). We should note, however, that in the case of the stochastic nonequilibrium thermodynamics of chemical reaction, the coefficients
381 1
gu are explicitly expressed in terms of fluctuations with
respect to the numbers of molecules in equilibrium state.
Wettability of Polyethylene Single Crystal Aggregates
by Harold Schonhorn and Frank W. Ryan Bell Telephone Laboratories, Incorporated, Murray
Hill,New Jersey (Received M a y 4, 1966)
~~
The importance of describing fully the detailed physical properties (e.g., density, degree of crystallinity, and molecular weight distribution) of materials to be classified with respect to their critical surface tension of wetting (yo) is stressed. This is illustrated by determining the yofor a well-characterized preparation of polyethylene single crystal aggregates. The yc of the crystalline polyethylene is shown to be 53.6 dynes/cm compared to the generally accepted value of 31 dynes/cm. An analysis based on Fowked approach to wettability data is consistent with our results.
Investigations in surface chemistry as applied to the wettability of polymers' have failed generally to specify with any precision the detailed physical properties (e.g. density, degree of crystallinity, and molecular weight distribution) of the materials to be classified with respect to their critical surface tension of wetting (yo). In this communication we shall endeavor to demonstrate the importance of describing fully the preparation of samples employed in wettability studies. We shall demonstrate that, for example, a variation in the surface density ( p s ) of a polymer will change the critical surface tension of wetting. Recently, Roe,2 and Lee, Muir, and Lyman3 have called attention to the concept of the density of the surface layer of polymers as being important in determining their ultimate wettability. To obtain agreement between the accepted critical surface tensions of wetting ( y o ) at 20" and empirical calculations based on the parachor concept, the above authors2s3had to employ the amorphous densities of the polymers. However, there is no a priori reason for choosing the amorphous density since polymers may assume a range of densities depending upon their molecular weight and degree of
crystallinity, while retaining their chemical constitution. Polyethylene, for example, the subject of this investigation, has an amorphous bulk density ( p ~ of ) 0.855 g / ~ m and ~ , ~a crystalline bulk density of 1.OOO g / ~ m at ~ , 20". ~ Therefore, in principle, polyethylene should assume a spectrum of surface densities and yo values depending upon the ratio of amorphous to crystalline polymer present in the surface layer of the specimen. Invariably, the polymer specimens which are employed in wettability experiments are of the meltcrystallized variety. That is, they are molded in the melt against a smooth surface, then cooled. Polymer molecules which cannot be accommodated into the crystal lattice during crystallization are rejected to the (1) E. Wolfram, Kolloid-Z., 182, 75 (1962); K. L. Wolf, 2. Physik. Chem. (Leipzig), 2 2 5 , 1 (1964); V. R. Gray, Forest Prod. J., 1 2 , 452 (1962); A. V. Neumann and P. J. Sell, 2. Physik. Chem. (Frankfurt), 41, 183, 191 (1964). (2) R. J. Roe, J. Phys. Chem., 69, 2809 (1965). (3) I. J. Lee, W. M. Muir, and D. J. Lyman, ibid., 69, 3220 (1965). (4) G. Allen, G. Gee, and G. J. Wilson, Polymer, 1, 456 (1960). (5) P. H. Geil, "Polymer Single Crystals," Interscience Publishers, Inc., New York, N . Y., 1963.
Volume 70,Number 12 December 1966
3812
HAROLD SCHONHORN AND FRANK W. RYAN
surface region and reside there in an amorphous or liquid-like state.6 Apparently, polymers which have been melt crystallized have surface regions which have densities corresponding to the amorphous solid. Therefore, we decided to observe the effect of density on the wettability of polyethylene by using single crystal aggregates to determine yo. I n this case, we would expect that the surface density would more closely approximate the bulk density, that is, ps ,OB. Since the wettability of a polymer is governed solely by the nature of the outermost functional group,' orientation effects are probably unimportant with respect to the critical surface tension of wetting. However, orientation effects are important when solid-liquid interfacial tensions are ~onsidered.~For example, single-crystal aggregates of polyethylene have both a lateral and fold surface structure associated with them. The fold surface interfacial tension that exists a t the crystal-melt interface has been estimated to be about 70 ergs/cmZl5 while the lateral surface interfacial tension is about 10 ergs/~m~.~ Since Roe2 has indicated that the critical surface tension of wetting is proportional to the fourth power of the amorphous density for polyethylene (yo p 4 ) , we should expect to obtain greater values of yc for polyethylene single-crystal aggregates where the surface density more closely epproximates the bulk density of the polymer. If a surface density of 1.00 g/cm3 for polyethylene were achieved, then we should expect a value of yo > 60 dynes/cm. If this were the case, it would show that an important parameter in wettability studies is the number of functional groups per square centimeter of polymer surface that interact with the wetting liquid. I n effect, both the surface density of these functional groups and their chemical nature would govern the wettability with respect to a given liquid.
Experimental Section I . Preparation of Polyethylene Xingle Crystals. Aggregates of crystals of linear polyethylene, having one branch per 1000 carbon atoms, = 66,000 as determined from both light scattering and gel permeation chromatography (gpc) and 2, = 6000 as determined from gel permeation chromatography (Marlex 6000 series, Type 50, Phillips Petroleum Co., Bartlesville, Okla.) grown from 0.04% solution in xylene (Fisher, Certified ACS grade) have been studied. While gpc may not ordinarily be an absolute technique, it is here because of calibration with known fractions of linear polyethylene. Crystals were prepared by dissolving a portion of polyethylene in boiling xylene then pouring this solution into a larger volume (1OX) of xylene thermostated at 85' to give a final concentration of
aw
The Journal of Physical Chemistry
0.04%. The solution was allowed to crystallize overnight at 85' and then was filtered at 85'. Essentially none of the starting material remained in solution after filtration. Therefore, the molecular weight distribution of the single crystal aggregates was similar to the original material. When the solution was allowed to crystallize at 85' for 1 hr and then filtered at 85', only 50% of the starting material was recovered. I n this case, the molecular weight distribution was still broad, but the intrinsic viscosity of this preparation was greater than the original polymer. However, the wettability results obtained with both preparations were essentially identical. Crystals formed under both these conditions at 85' were essentially monolayer truncated lozenges while those formed at 7 5 O and below are ridged true lozenges or dendrites.8 Films formed by filtering crystals from suspension were dried at 40' in a vacuum oven for a minimum of 16 hr. The amount of solvent retained in a specimen was obtained by mass spectrophotometric analysis of gases evolved on melting samples at 150°.9 I n these thin films of single crystals, no xylene was noted after drying in the vacuum oven. The mats of single crystals prepared in the above manner were slowly formed into thin disks at pressures of 20,000 psi and a temperature of 20' in a die having a specularly smooth finish. Pressure was maintained for a period of several minutes. As a precaution, the die was cleaned scrupulously and air dried. This produced a glossy almost clear specimen suitable for wettability studies. X-Ray diffraction and infrared transmission analysis revealed no apparent changes in crystallinity before and after pressing the polymer mat into disks. The densities of the filtered mat and the molded polyethylene single crystals were both 0.972 g/cm3, as measured with a density gradient column. 2. Contact Angle Measurements. For the contact angle measurements, the polyethylene film composed of single-crystal aggregates was mounted on a standard microscope slide, employing double-backed adhesive tape. The advancing contact angles were measured directly by employing a telescopic device equipped with an OCUlar protractor which was built by the Gaertner Scientific Corp., Chicago, Ill. Three separate drops of the wetting liquid were placed on the polymer surface with opposite edges of the drops being measured. The surface tensions of the wetting liquids were measured prior to ~
~
~
(6) H. D. Keith and F . J. Padden, J . A p p l . Phys., 35, 1270, 1286
(1964). (7) W. A. Zisman, Advances in Chemistry Series, No. 43, American
Chemical Society, Washington, D. C., 1964, p 1. (8) D. C. Bassett and A. Keller, Phil.Mag., 7, 81 (1962). (9) R. Salovey and D. C. Bassett, J. Appl. Phys., 35, 3216 (1964).
WETTABILITY OF POLYETHYLENE SINGLE CEYSTAL AGGREQATES
the wetting experiments. Excellent agreement with literature values for the Y L V of the test liquids were obtained. The drops were equilibrated for a minimum of 10 min prior to reading the contact angle. The reproducibility of the readings was about k 2 O . Subsequently, the pressed crystal aggregate was washed alternately in hexane and detergent, dried, and the wettability redetermined for both cleaning procedures. No significant changes in the contact angles were observed. Analysis of Wettability Theory It has been suggested thatlo (YSVC
-
YSLC)
= (Wac-
YSLBC)
E (YSVS - Y SLB)
(1)
-
when o
ps = ps
ac
=
=
YLV
-
YSL?
2
(YSVSC
=
(9)
($%a)
it is plausible to state that
Combining eq 9 and 10 yields
Substituting eq 11 into the geometric mean term of eq 7 we obtain (Ysva,ac,c
-
YsLa,ac,c
) =
[(yLvd)pYLvd]L/2 - YLV
(12)
PSS
= Yo
(3)
where ysva is the surface free energy of the amorphous solid. If there 1s a difference in the density of the crystalline and amorphous states, then eq 1 may no longer be valid. Therefore, we suggest that (YSVC
4
YLV
2( $)2
(2) where the superscripts a, ac, and c refer to amorphous, partially crystalline, and crystalline, respectively. I n addition, it has been suggested thatlo YSVB
3813
- YsLao) 2
(YSVB
where the subscript p refers to the polymer. Equation 12 is equivalent to eq 6 when p ~ = ~psa. ~ ' ~ To estimate the contact angle of a wetting liquid on the crystalline or partially crystalline polymer surface, eq 12 is employed with the Young equation. The general form of the Young equation for polymers becomes (Ysva,aw - YsLa,aw) = YLV(COS e)a,ac,c (13) Combining eq 12 and 13 yields
- YSLB) (4)
is probably true for the more general case when PSO
1 psac 2
(5)
PSS
However, there may possibly be exceptions as in the case of poly-4 methylpentene-1, where it has been reported that p s B > ps'." We shall attempt to obtain a more useful form of eq 4 by employing the Fowkes expression12
- 'YSL) = 2(YSVdYLVd)*'*-
(6) where the superscript d refers to the dispersion component of the surface free energy of the solid. Previously, we suggested that'" (YSV
(rLvd)p,5
T,
YLV
= (Y*Lv)ac,od z
VLV,
VLVd,
dynes/ om
dynes/ om
Ocaiodl
Oexpti,
deg
deg
20
93
Glycerol-Polyethylene Single Crystals 63.4 37.0 60.6
67
Formamide-Polyethylene Single Crystals 58.2 39.5 47.1
55
a
g/cm*
dynes/ om
Water-Polyethylene Single Crystals 0.972 0.855 36.2 72.8 2 1 . 8 90.3
(7)
(8) where ( ~ * L vis) ~ the dispersion component of the surface tension for a liquid having a density equivalent to psC or psae. Since Roe2 and Lee, Muir, and Lyman3 have shown that eq 9 occurs in the following relation
paat
g/om:
> psar we have ysv(ao,c)d
Pac,
OC
YLV
(Ysv&lacIc- YsLa~ac,o) = 2(YSVBdYLVd)l/*When pat*'
Table I : The Calculated Contact Angles at 20" of Polar Liquids on Polyethylene Single Crystals Employing Eq 14
Reference 15.
yLVd
(10) H. Schonhorn, J. Phys. Chem., 69, 1084 (1965). (11) J. H.Griffith and B. G. Ranby, J. Polymer Sci., 44, 360 (1960). (12) F. M. Fowkes, J. Phys. Chem., 66, 1863 (1962); 67, 2538 (1963); Advances in Chemistry Series, No. 43, American Chemical Society, Washington, D. C., 1964,p 99.
Volume 70, Number 19 December 1966
HAROLD SCHONHORN AND FRANX W. RYAN
3814
Table 11: Wettability of Polyethylene at 20' YLV,
yLVd,
Liquid
dynea/cm
dynedcm
Water Glycerol Formamide Methylene iodide a-Bromonaphthalene
72.8 63.4 58.2 50.8 44.6
21.8 37.0 39.5 48.5 44.6
d/rLvd/r~v,
l./(dynes/cm)'/z
0.0641 0.0959 0.1080 0.1404 0.1497
Y S i n g l e crystal6, deg cos e
93 67 55 40 Spreads
-Melt BPa deg
-0.052 0.391 0.574 0.760 1.000
crystallized-
cos ff
94 79 77 52 35
-0.070 0.191 0,225 0.616 0.818
Reference 7.
The calculated values for the contact angles of several liquids on the polyethylene single crystal aggregates at 20' are shown in Table I.
Results The wettability data for the pressure-molded aggregates of the polyethylene single crystals are presented in Table 11. There is a significant difference in the advancing contact angles between the usual melt-crystallized polymer and the compressed single crystal aggregates for all the liquids employed except water. If we mold the single crystal aggregates of polyethylene above their melting point and cool rapidly or slowly, we again have the situation of a low ye, indicative of an amorphous surface layer. A Zisman? type plot of the data in Table I1 for cos 6 us. ~ L Vis shown in Figure 1. A narrow rectilinear band is obtained with an extrapolated range for yc of 44.6-47.2 dynes/cm. A Fowkes-type plot is shown in Figure 2. In this representation, cos B is plotted as a
-
0.4
-
0.2
-
-0.2
I
0
I
10
I
20
I
30
Y,,
I I 40 50 (DYNESKM)
I 60
Figure 1. The critical surface tension of wetting for polyethylene single crystals is determined in a Zisman-type plot. A narrow rectilinear band results in an extrapolated range for yo between 44.6 and 47.2 dynes/cm.
The Journal of PhysacaZ Chemistry
I
70
I
80
a
I
0.12
0.16
function of ( ~ L v ~ ) ' / ' / ~ L v . Table I1 itemizes the accepted values of ~ L for V ~the wetting liquids. ApV methylene ~ iodide used in plotting parently, the ~ L for Figure 2 is too high. The normal behavior of the Zisman plot strongly suggests that this is the case. The value proposed by Fotvkes13for ~ L ofVmethylene ~ iodide is 48.5 f 9 dynes/cm. A value for ~ L ofV about ~ 40 dynes/cm would give better agreement with the linear representation in Figure 2. When cos B = 1, then YLV/ ( y ~ v ~ ) " =' (ysvd)'/'. The value of ysvd obtained from Figure 2 is 53.6 dynes/cm. It should be noted in this connection that the Fowkes-type plot yields a higher value for ysvd or yc than the plot of Zisman. For melt-crystallized polyethylene, yo = 31 dynes/cm,' while employing the same contact angle data in a Fowkes-type representation ysvd % 35 dynes/cm. l 3
0.6
0-
I
0.08
YLV
L -
U
I 0.04
Figure 2. The wettability of polyethylene single crystals is shown in a Fowkes-type representation. The extrapolated value of 53.6 dynes/cm is similar to the values obtained in Figure 1.
"O 0.8
e
8
-IV 0
Discussion One implication of the present work is that wettability is not dependent solely upon the constitution of (13) F. M. Fowkes, ASTM Special Technical Publication No. 360, 1964, p 20.
WETTABILITY OF POLYETHYLENE SINGLECRYSTALAGGREGATES
3815
Table 111: A Comparison of the Extrapolated Valuas for the Surface Tension of Several Polymers and Their Critical Surface Tension of Wetting YLe,
Polymer
Polyethylene Polypropylene Poly(ch1orotrifluoroethylene) Poly(dimethylsi1oxane) Polystyrene
YLV(DYNESICM) Figure 3. The density of the homologous series of n-hydrocarbons is plotted against their respective surface tensions as calculated employing the parachor concept. Two cuwes corresponding to parachor values of 39.0 and 40.0 are represented.
the surface. As evidenced from this investigation, polyethylene may assume a yo greater than that obtained for many polar polymers if the surface is crystalline. This investigation indicates the necessity to characterize fully any polymer studied with respect to preparation, degree of crystallinity, and history of the sample. The liquid-like behavior in the surface region of meltcrystallized polymers is shown in Table 111. The Y,, for several polymers are compared to the extrapolated values of their melt surface tensions. This suggests that eq 3 is appropriate for melt-crystallized nonpolar polymers. Apparently, this is why the parachor concept has been useful in analyzing wettability data. Figure 3 illustrates the effect of density on the surface tension for a homologous series of n-hydrocarbons. The density is plotted against the calculated surface tensions for two parachor values. Parachors of 39.0 and 40.0 were chosen to be representative of the n-hydroearbon series.'* The density of the vapor at 20' is insignificant with respect to PL. The density of 0.855
YC20.
dynes/cm
dynes/om
36.2" 28.0" 30.gd
31' 2Qb 3 lb
20.6" 21.6' 32.4O
22' 33b
'Reference 15. 'Reference 7. H. Schonhorn and L. H. Sharpe, J. Polymer Sci., B3, 235 (1965). d H . Schonhorn, F. W. Ryan, and L. H. Sharpe, J. Polymer Sci., A2,538 (1966). H. Tarkow, J . Polymer Sci., 27,35 (1958). T = Reference 2. 30'. J. E. Marian, ASTM Special Technical Publication No. 340, 1963, p 122.
'
g/cm3 for amorphous polyethylene yields a calculated surface tension of 35.4 dynes/cm for P = 40.0 and 33.6 dynes/cm for P = 39.0. Apparently, P = 40.0 is more appropriate for amorphous polyethylene since yLv20 is 36.2 dynes/cm.15 It is obvious from Figure 3 that for a hypothetical n-hydrocarbon liquid having a density of 1.000 g/cm3, the projected density for an ideal single cryst,alof polyethylene at 20'. Y*LV
= 66.1 dynes/cm = yo
If the surface of the polymer consisted of both amorphous and crystalline regions, then we should expect a spectrum of yo values ranging from 35.4 dynes/cm for the completely amorphous surface layer to 66.1 dynes/ cm for the completely crystalline surface layer. Consequently, it appears that 35.4 dynes/cm
6
yo
6
66.1 dynes/cm
is appropriate for polyethylene. (14) 0 . R. Quayle, Chem. Rev., 53, 439 (1953). (15)H. Schonhorn and L. H. Sharpe. J. Polymer Sei., A3, 569 (1965).
Volume 70, Number 1.2 December 1966
