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Langmuir 1999, 15, 7076-7079
Surface Energy Characteristics of Polymer Film Structures: a Further Insight into the Molecular Design Requirements John Tsibouklis,* Maureen Stone, Adrian A. Thorpe, Paul Graham, Thomas G. Nevell, and Richard J. Ewen† School of Pharmacy and Biomedical Sciences, University of Portsmouth, St. Michael’s Building, White Swan Road, Portsmouth PO1 2DT, U.K. Received April 9, 1999. In Final Form: June 16, 1999 The significance of the Liftshitz/van der Waals, Lewis-acid, and Lewis-base contributors to the total surface energy of a material is discussed in terms of the molecular design features and surface organization phenomena characterizing film structures prepared from four homologous series of polymeric compounds, namely the poly(methylpropenoxyperfluoroalkylsiloxane)s, the poly(methylpropenoxyalkylsiloxane)s, the poly(perfluoroalkylacrylate)s, and the poly(alkylacrylate)s.
1. Introduction The significance of current research efforts aimed at the development of nonwettable, nonstick polymeric materials with good film-forming characteristics is appreciated when the lack of universally applicable and environmentally friendly protection against bacterial and other biological fouling is considered.1,2 Potential uses in stain-resistant fabrics and self-lubricating surfaces provide an additional impetus for the development of such materials.3 In general, nonstick materials are characterized by a low surface energy. The surface energy (γ) of a material is expressed by:4
γ ) γ°Fβ where γ° and β are temperature-independent constants, and F is the specific gravity of the material. For polymeric materials, the value of γ° is determined by the chemical structure at the surface: It is now established that the surface energy of constituent groups decreases in the order CH2 (36 dyn cm-1) > CH3 (30 dyn cm-1) > CF2 (23 dyn cm-1) > CF3 (15 dyn cm-1).5-8 The value of β (the Macleod exponent), which is normally in the range 3.0-4.5, is determined by the overall structure of the macromolecule. Finally, the entropy of the surface is of some significance: Amorphous materials exhibit lower * Corresponding author. Tel ++44 (0)1705 843606; fax ++44 (0)1705 843565; e-mail
[email protected]. † Faculty of Applied Sciences, University of the West of England, Frenchay Campus, Coldharbor Lane, Bristol BS16 1QY U.K. (1) Kobayashi, H.; Owen, M. J. Trends Polym. Sci. 1995, 3, 10. (2) Schmidt, D. L.; Coburn, C. E.; DeKoven, B. M.; Potter, G. E.; Meyers, G. F.; Fischer, D. A. Nature 1994, 368, 39. (3) Cuddihy, E. F. In Particles on Surfaces 1: Detection, Adhesion and Removal, Mittal, K., Ed.; Plenum Press: New York, 1988; pp 91111. (4) Wu, S. H. Polymer Interfaces and Adhesion; Marcel Dekker: New York, 1982. (5) Lau, Y. W.; Burns, C. M. J. Polym. Sci., Polym. Phys. Ed. 1974, 12, 431. (6) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1960, 64, 1292. (7) Dettre, R. H.; Johnson, Jr., R. E. J. Colloid Interface Sci. 1966, 21, 367. (8) Dettre, R. H.; Johnson, Jr., R. E. J. Colloid Interface Sci. 1969, 31, 568.
surface energy values than crystalline counterparts.9 As a general rule, amorphous, comblike polymers possessing a flexible linear backbone onto which side-chains with low intermolecular interactions are attached, exhibit low γ values.10 As part of our work on new polymers with low-surfaceenergy properties, we have recently reported on the synthesis, characterization, film-forming characteristics, and surface energies of two classes of materials which may comply with the molecular design requirements imposed by the above considerations, namely the poly(perfluoroalkylacrylate)s11 and the poly(methylpropenoxyperfluoroalkylsiloxane)s.12 For purposes of comparison, the hydrocarbon analogues of the same classes of materials were also considered.13 All fluoro-substituted materials were found to exhibit surface energies which were markedly lower than those of corresponding alkylsubstituted compounds. Contrary to expectation,1,9 however, the more flexible siloxanes were found to exhibit higher surface energy values than their acrylate counterparts. In an effort to understand the underlying reasons for this behavior, we have undertaken a study of the surface composition of these materials using X-ray photoelectron spectroscopy (XPS); the results of our investigations are reported here. 2. Experimental Section 2.1. Materials. Both classes of materials are readily accessible: The poly(perfluoroalkylacrylate)s and the poly(alkylacrylate)s are prepared by the bulk polymerization of corresponding acrylate esters,11 whereas the synthesis of the poly(methylpropenoxyperfluoroalkylsiloxane)s12 and poly(methylpropenoxyalkylsiloxane)s13 involves a hydrosilylation reaction (Figure 1). Films of these materials were deposited onto glass and/or poly(methyl methacrylate) supporting substrates (10 × 10 × 1 mm). (9) Glennon, D.; Smith, J. R.; Cox, P. A.; Ewen, R.; Nevell, R. T.; Nevell, T. G.; Tsibouklis, J. J. Mater. Sci. 1998, 33, 3511. (10) Owen, M. J. Comments Inorg. Chem. 1988, 7, 195. (11) Stone, M.; Nevell, T. G.; Tsibouklis, J. Mater. Lett. 1998, 37 (1-2), 102. (12) Thorpe, A. A.; Young, S. A.; Nevell, T. G.; Tsibouklis, J. Appl. Surf. Sci. 1998, 136 (1-2), 99. (13) Thorpe, A. A.; Nevell, T. G.; Tsibouklis, J. Appl. Surf. Sci. 1999, 137 (1-4), 1.
10.1021/la990411x CCC: $15.00 © 1999 American Chemical Society Published on Web 09/09/1999
Surface Energy of Polymer Film Structures
Langmuir, Vol. 15, No. 20, 1999 7077 Table 1. Surface Roughness (Ra) and Advancing Contact Angles for Water, Diiodomethane (DIM) and Ethylene Glycol (EG) on Poly(perfluoroalkylacrylate) (PFA), Poly(methylpropenoxyperfluoroalkylsiloxane) (PFE), Poly(octylacrylate) (PFOA5), and Poly(methylpropenoxyalkylsiloxane) (PES) Film Structures* contact angle, θ° (H°) surface energy (mJ m-2) sample Ra/nm
H2O
DIM
EG
γS+
γS-
γS
5.0
0.1
1.6
5.6
6.2
0.1
2.2
6.9
6.9 10.2
0.0 0.4
3.7 4.3
7.0 12.9
γSLW
poly(perfluoroalkylacrylate)s11 PFA9
11.0
PFA7
9.6
PFA5 PFA3
5.1 3.1
The acrylates were deposited from the melt or from CF2ClCFCl2 (0.1% w/w) solution (dipping speed: 10 mm s-1), whereas the liquid silicone samples formed into glassy structures after thermal cross-linking at 105 °C for 16 h; the mechanism of the crosslinking process is currently under investigation. Surface roughness profiles of film structures were obtained by atomic force microscopy (AFM); studies were performed in air, under ambient conditions, using a Discoverer TopoMetrix TMX2000 scanning probe microscope (SPM) which was mounted on a custom-built mass/spring antivibration rig with a lateral natural frequency of 0.40 Hz and a vertical natural frequency of 0.52 Hz. 2.2. Goniometry. The surface free energies of polymer samples were determined by contact angle goniometry using a manual Kernco instrument or a Kruss G10 goniometer interfaced to image-capture software; both instruments were fitted with an enclosed thermostated cell. In the case of silicones, cross-linked film structures that had been annealed at 30 °C for 4 months were utilized for the contact angle experiments. Measurements of droplets (2-10 µL) were recorded at 25 ( 1 °C using doubly distilled water (surface tension γL ) 73.4 mN m-1 at 18.75 °C, literature value14,15 ) 73.05 mN m-1 at 18 °C), diiodomethane (>99%; γL ) 48.7 mN m-1 at 18.8 °C, literature value14,15 ) 50.76 mN m-1 at 20 °C), and 1,1-ethanediol (ethylene glycol, >99%; γL ) 47.7 mN m-1 at 18.8 °C, literature value14,15 ) 48.40 mN m-1 at 20 °C). 2.3. Surface Energy Calculations. Surface energies were evaluated using the surface-tension-component theory.16,17 According to this approach, the surface energy of a solid, γS, combines three contributions, eq 1:
γS ) γSLW + (γS+ γS-)1/2
(1)
where γLW Liftshitz/van der Waals component, γ+ is the Lewisacid component, and γ- is the Lewis-base component. For a drop of a liquid at equilibrium with a solid surface, γSLW, γS+ and γS- can be calculated (eq 2) by performing liquid-solid (14) Kaye, G. W. C., Laby T. H., Eds.; Table of Physical and Chemical Constants, 15th ed.; Longman Scientific and Technical: Harlow, 1992. (15) Lide, D. R., Ed.; Handbook of Chemistry and Physics, 76th ed.; CRC Press: 1995. (16) Good, R. G.; van Oss, C. J. In Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1991; pp 1-27. (17) Good, R. G.; Chaudhury, M. K.; van Oss, C. J. In Fundamentals of Adhesion; Lee, L. H., Ed.; Plenum Press: New York, 1991; pp 153172.
112 (16) 112 (12) 108 105
120 (25) 108 (17) 108 110
PFE7 PFE5 PFE3
poly(methylpropenoxyfluoroalkylsiloxane)s12 3.1 109 95 94 10.6 1.3 0.5 (3) (12) (8) 2.3 106 96 91 10.4 1.3 1.1 1.0 103 87 83 14.1 0.8 1.7 0.5 105 89 79 13.0 0.1 4.6
PFOA5
10.2
PFE9
Figure 1. The four homologous series of polymers considered: (a) poly(methylpropenoxyperfluoroalkylsiloxane)s; (b) poly(methylpropenoxyalkylsiloxane)s; (c) poly(perfluoroalkylacrylate)s; and, (d) poly(alkylacrylate)s. The value of n is denoted by the numerical suffix in the code name for each material.
125 (4) 117 (8) 114 113
PES13 PES11 PES9 PES7 PES5 PES3 PES1
96
poly(alkylacrylate)s11 52 70 33.0
12.2 12.6 16.4 14.2
0.01
1.5
33.1
poly(methylpropenoxyalkylsiloxane)s13 100.0 106.9 73.4 81.8 21.0 0.1 115.5 84.7 72.1 75.6 21.7 0.0 0.9 85.2 70.0 75.4 22.9 0.0 5.6 80.5 66.8 73.8 24.6 0.1 8.2 72.2 51.6 61.2 33.3 0.1 3.5 91.0 62.1 82.0 27.4 0.5 2.0 79.6 55.2 70.9 31.3 0.3
0.3 12.8 12.0 16.4 18.5 8.8 14.9
21.3 21.8 23.4 26.6 35.2 31.6 35.4
a The corresponding surface energies are also presented. Each contact angle value is the mean of six drops on two independently prepared polymer samples; standard deviations were in the range 0.62 to 1.68.
contact angle measurements (θ).
γL (1 + cos θ) ) 2[(γSLW γLLW)1/2 + (γS+ γL-)1/2 + (γS- γL+)1/2] (2) where γL is the surface tension (surface energy) of the liquid; subscripts: S ) solid, L ) liquid. By measuring contact angles for three well-characterized (in terms of γLLW, γL+ and γL-)14,15 liquids, three equations with three unknowns are generated; water, diiodomethane, and ethylene glycol were employed. For comparison, the surface energy of poly(tetrafluoroethylene), Teflon, was also determined using these liquids; the value of 21 mJ m-2 was obtained.18 2.4. X-ray Photoelectron Spectroscopy. XPS was carried out using a VG Scientific ESCALAB Mk.II employing a nonmonochromatized AlKR source (1486.6 eV) operating at a power of 250 W. In all cases, the takeoff angle between electron exit and the sample surface was kept constant at 75°. The analyzer was operated at a constant pass energy of 20 eV. Line shape analysis was performed on each peak, and atomic percentages were calculated from the peak areas using standard atomic sensitivity factors.19
3. Results and Discussion 3.1. Surface Energies. Consideration of the surface energy components associated with each member of the three homologous series of materials (Table 1) reveals that, in addition to surface roughness (the effects of which can be assessed by combining advanced contact angle measurements with receding contact angle determinations), the wetting behavior is determined by four struc(18) Drummond, C. J.; Chan, D. Y. C. Langmuir 1997, 13, 3890. (19) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
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Table 2. Binding Energies and Atomic Proportions of Elements at the Surface of Representative Poly(methylpropenoxyperfluoroalkylsiloxane), Poly(methylpropenoxyalkylsiloxane), and Poly(perfluoroalkylacrylate) Film Structures Eb/eV
atomic % found (calc)
2 1 2 1 1 7 1 1 21 1 1 1
PFE9 284.7 285.7 287.2 287.6 291.3 292.6 293.5 294.8 690.2 533.2 534.4 102.5
poly(methylpropenoxyfluoroalkylsiloxane)s PFE7 3.3 (5.0) 2 284.8 4.8 (7.1) 1.6 (2.5) 1 285.7 2.6 (3.6) 3.2 (5.0) 2 287.3 4.8 (7.1) 1.6 (2.5) 1 287.8 2.5 (3.6) 1.6 (2.5) 1 291.5 2.0 (3.6) 11.3 (17.5) 3 292.6 7.5 (10.7) 1.6 (2.5) 1 293.1 2.4 (3.6) 1.5 (2.5) 1 294.8 2.3 (3.6) 39.9 (52.5) 13 690.4 36.4 (46.4) 11.5 (2.5) 1 533.5 14.0 (3.6) 3.8 (2.5) 1 534.9 2.9 (3.6) 6.0 (2.5) 1 102.7 6.2 (3.6)
C (1s) C (1s)
2 14
PES11 284.1 285.0
poly(methylpropenoxyalkylsiloxane)s PES3 6.4 (9.5) 2 284.0 43.6 (66.7) 6 285.0
5.2 (15.4) 46.3 (46.2)
C (1s) O (1s) O (1s) Si
2 1 1 1
286.7 532.7 534.1 102.0
5.6 (9.5) 24.4 (4.8) 3.0 (4.8) 11.6 (4.8)
4.4 (15.4) 25.6 (7.7) 2.4 (7.7) 11.4 (7.7)
C (1s)
1
PFA9 285.0
poly(perfluoroalkylacrylate)s PFA7 1.1 (2.6) 1 285.0
1.8 (3.1)
C (1s)
1
286.3
1.9 (2.6)
1
286.3
2.1 (3.1)
C (1s) C (1s) C (1s) C (1s) C (1s) C (1s) C (1s) F (1s) O (1s) O (1s)
1 1 1 1 7 1 1 21 1 1
286.5 286.8 289.5 291.1 292.5 293.4 294.8 690.4 533.8 535.2
2.6 (2.6) 2.8 (2.6) 2.1 (2.6) 3.4 (2.6) 14.9 (18.4) 2.8 (2.6) 2.4 (2.6) 56.4 (55.2) 3.1 (2.6) 3.0 (2.6)
1 1 1 1 5 1 1 17 1 1
286.5 286.8 289.5 291.1 292.6 293.4 294.8 690.3 533.8 535.2
2.6 (3.1) 3.2 (3.1) 2.0 (3.1) 3.0 (3.1) 16.0 (15.6) 2.2 (3.1) 2.4 (3.1) 54.8 (53.1) 3.3 (3.1) 3.1 (3.1)
element
C (1s) C (1s) C (1s) C (1s) C (1s) C (1s) C (1s) C (1s) F (1s) O (1s) O (1s) Si
no. of atoms
no. of atoms
tural features of the macromolecule, namely: (1) nature of the pendent chain; (2) length of the pendent chain; (3) nature of the linking moiety, and, (4) nature of the polymer backbone. The results presented in Table 1 demonstrate that the nature of the pendent chain has a most profound effect in determining the surface energy of the material, with all fluoro-substituted compounds seen to exhibit surface energies which are markedly lower than those of corresponding alkyl-substituted macromolecules. The influence of the length of the pendent fluorocarbon or hydrocarbon moiety is also evident: A general trend of lower surface energy with increasing chain length is observed. However, as the length of the pendent chain increases, the average surface roughness of the corresponding film structures is seen to follow the same trend. This effect, which is reflected by the magnitude of the observed hysteresis values (H), may exaggerate the differences in contact angles observed between adjacent members of each homologous series. Nonetheless, the incorporation of a long perfluorocarbon side chain is an essential element of the molecular design for such materials, as it serves to inhibit the absorption of liquids by the bulk sample; only the materials for which hysteresis values are reported (PFE9, PFE7 and PFA9) were found to be resistant to penetration by the liquids considered.11-13 The effects of the linking moiety become apparent when the individual contributors to the total surface energy of
2 1 1 1
Eb/eV
286.9 532.8 534.6 102.0
atomic % found (calc)
assignments
CH3-Si-CH2-CH2-CH2-O-CH2-CH2-O-CH2-CH2-CF2-CF2-CH2-CF2-CF2-(CF2)x-CF2-CF3 -CF3 -Si-O-CH2-O-CH2-
CH3-Si-CH2-CH2-CH2-O-CH2-CH2-(CH2)x-CH3 -CH2-O-CH2-Si-O-CH2-O-CH2-
-CH2-CHbackbone -CH2-CHbackbone -O-CH2-CH2-CH2-CF2CdO -CH2-CH2-CF2-(CF2)x-CF2-CF3 -CF3 -O-CH2CdO
each type of material are considered. In particular, the Liftshitz/van der Waals component (γSLW) determined for fluorosilicones is significantly greater than that of corresponding fluoroacrylates, indicating that different molecular segments are responsible for the observed surface energies in each class of material. By contrast, the γSLW values determined for the nonfluorinated macromolecules PFOA5 and PES5 are essentially identical. For all polymers considered, the contribution of the Lewis-acid component (γS+) to the total surface energy is negligible, indicating that these materials have little affinity for bases. The mild ability of the higher fluorosiloxanes (n ) 5-9) to accept electron pairs may be due to the polarization of the O-C bond of the ether linkage as a result of the strong inductive effect relayed by the fluorocarbon chain. This effect is, presumably, of lesser significance in the case of fluoroacrylates, which contain the more electronrich ester linkage. The Lewis-base component (γS-) is quite well-defined in all cases, reflecting the presence of nonbonded electron pairs on the oxygen atom of both types of linking group. The relative ability to donate these electron pairs is reflected by the magnitude of this component: The higher fluorosilicones (n ) 5-9) exhibit suppressed values of this parameter, whereas PFE3 has a γS- value which is very similar to that of the corresponding acrylate, PFA3. Characteristically, the higher analogues within each class of material exhibit the lowest γS- values, reflective of the reduced availability of the
Surface Energy of Polymer Film Structures
nonbonded electron pairs due to the masking effects of the long carbon chains. Comparison of the total surface energy values characterizing each of the four homologous series of macromolecule considered (Table 1) highlights the relative significance of the two main molecular design requirements for the construction of low-surface-energy polymers, namely fluorosubstitution and backbone flexibility. Typically, the more flexible fluorosiloxanes exhibit higher surface energy values (ca. 12-17 mJ m-2 for n ) 3-9) than their acrylate counterparts (ca. 6-13 mJ m-2 for n ) 3-9); the same effect, although less pronounced, is observed with the n ) 5 nonfluorinated materials, PFOA5 and PES5. This behavior, which is in marked contrast to expectation on the basis of previous literature reports,1,9 may be rationalized at the submolecular level in terms of two possible phenomena, namely: (1) thermal cross-linking, which makes the polysiloxane backbone more rigid than that of polyacrylates (it is worth noting that thermally crosslinked PFE9 exhibits a total surface energy value which is ca. 3 mJ m-2 greater than that of a parallel sample which was not subjected to such treatment);12 and (2), the different molecular organization characteristics displayed by the two classes of materials at the interface. In an effort to identify the dominant mechanism responsible for the observed differences in surface energies between the two classes of materials, representative film structures from each class of material were subjected to XPS analysis. 3.2. XPS Studies. The XPS data associated with film structures prepared using the higher fluorosilicones (PFE9 and PFE7; Table 2) reveal that, for both materials, there is a preferential distribution of the siloxane backbone nearer to the surface with the pendent perfluorocarbon chains oriented toward the bulk of the sample: The atomic percentages for silicon and backbone oxygen are significantly higher than those theoretically calculated for the bulk sample, whereas the corresponding values for fluorine and carbon are proportionately suppressed. The surface organization of the nonfluorinated silicones, PES11 and PES3, is analogous to that adopted by their fluorinated counterparts (Table 2). In particular, the atomic percentages associated with the Si-O backbone
Langmuir, Vol. 15, No. 20, 1999 7079
and those obtained for the two carbons which are attached onto silicon, CH3-Si-CH2-, are markedly greater than values calculated for the bulk sample. In parallel, the determined atomic percentages for side-chain oxygen, -CH2-O-CH2-, and, in the case of the higher homologue, side-chain carbon, are correspondingly subdued. The surface arrangements of the poly(perfluoroacrylate)s, PFA9 and PFA7, appear to be diametrically opposite to those of the polysiloxane films considered. The results (Table 2) provide significant evidence to suggest that, for this type of material, the perfluorocarbon side-chains segregate preferentially at the surface. More specifically, the atomic percentages of the two types of backbone carbon, -CH2-CH-, are significantly lower than those calculated for the bulk sample. By contrast, the atomic percentages for fluorine and oxygen, both of which are associated with the side-chain, are mildly enhanced; in view of the large number of side-chain atoms relative to the number of backbone atoms, the observed increase in atomic percentages is only marginal. Since the lowest energy surfaces are those containing perfluoroalkyl groups which are oriented to yield exposed -CF3 moieties,20 the observed surface organization phenomena for all polymers considered are consistent with the trends in surface energy values summarized in Table 1. 4. Conclusions The relative merits of the molecular design features of four homologous series of polymeric compounds are considered in terms of their ability to determine surface organization phenomena and their influence on the magnitude of the Liftshitz/van der Waals, Lewis-acid, and Lewis-base contributors to the total surface energy. Acknowledgment. The financial support of this work by EPSRC/MTD Ltd. (research studentships to M.S. and A.T.) and NERC (research studentship to P.G.) is gratefully acknowledged. LA990411X (20) Wang, J.; Ober, C. K. Macromolecules 1997, 30, 7560.
