Langmuir 1991, 7, 1975-1980
1975
Direct Measurement of Molecular Level Adhesion Forces between Biaxially Oriented Solid Polymer Films William W. Merrill and Alphonsus V. Pocius* 3M, 3M Center, Bldg 236-GA-03, St. Paul, Minnesota 55144
Bimal V. Thakker and Matthew Tirrell Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received December 14, 1990. I n Final Form: April 8, 1991 We report the first measurements of the solid surface energy of a polymer film by means of the Israelachvili surface forces apparatus (SFA). To this end, poly(ethy1ene terephthalate) (PET) films were generated in 3.5-6 pm thick samples, which could be silvered and used in the SFA. Adhesive contact pull-off forces were measured in air for unmodified PET as well as for PET modified by direct fluorination. The solid surface energy of the polymer films was determined by application of the Johnson-KendallRoberts (JKR) theory or the Derjaguin-Muller-Toporov (DMT) theory. Measurements of the critical wetting tension as well as surface analysis by XPS were done on the PET surfaces. The value of solid surface energy for untreated PET predicted by the DMT theory agrees well with the critical wetting tension determined by contact angle measurements,but the details of the shape of the contactinginterface as well as the radius at pull-off are best represented by the JKR theory. With increasing fluorination of the PET surface, the wettability by polar liquids increases while the SFA solid surface energy decreases. This is ascribed to a weak boundary layer generated by the fluorination.
Introduction The surface free energy, y, is the reversible work required to create a unit area of new surface from the bulk material. For a completely brittle material, it is sensible to relate surface energy to the work of cohesion, W,. If one considers an arbitrary plane through the bulk, a separation of material across a unit area on this plane creates two new surfaces, hence
w, = 27
(1)
When the plane separates two different materials across an interface, the work of adhesion is the reversible work to form two new surfaces across a unit area of this plane
w, = Y 1 +
72
- 712
where y1 and y2 are the surface energies of the two materials and 7 1 2 is the additional intermolecular interaction between these different materials acting across the surface, the so-called “interfacial energy”. The quantities W, and W, differ remarkably from the apparent work of cohesion or adhesion, Wapp,measured in practice since measurements of W,, are not normally accomplished reversibly. Since real materials are not completely brittle, energy dissipation can result from the local deformation of intermolecular juxtapositions and intramolecular conformations. For example, polymer chains straddling the interfacial zone will conformationally stretch and eventually pull out of one side or the other, or break, during interfacial separation. Thus, Wapp is a combination of the reversible work needed to create the new surface locallyand the irreversible dissipation, P,lost in deforming the material surrounding the new surfaces. If the reversible work combines additively with the work dissipated, Wp, then the value of the thermodynamic work becomes moot because Wp is much greater than W, or W,. However, a host of experiments on adhesive peel suggests a multiplicative relation 0743-7463191J 2407-1975$02.50/0
Wapp = WaP (3) where P is a universal dissipation function.l-3 Relation 3 suggests an important role for the thermodynamic work W, or W, in adhesion and fracture mechanics, even in test environments where energy dissipation is significant. From the preceding discussion, we conclude that it is important for the science of adhesion to have a way to measure W,reversibly so that one may gain an understanding of the relationship of Wapp to the more basic functions of W, and P. The surface forces apparatus (SFA) has the potential to provide a measure of W, and, hence, y reversibly. Conceived by Israelachvili and Tabor in 1972: the SFA combines a direct and precise measure of interfacial forceswith a simultaneous optical measurement of surface profile in the zone of contact between two surfaces. The SFA provides a means to approach the measurement of the reversible thermodynamic work of adhesion since the deflection of the polymer in the contact area is only on the order of 1000 A units over a thickness of polymer of about 5 pm, a small preturbation from equilibrium. The SFA has enabled an extensive investigation of forces between fluid and adsorbed phases such as that between mica and various liquids? polymers adsorbed on mica and various liquids6as well as contact forces between adsorbed but dried polymer^.^ Until recently, measurements obtained by using the SFA have been limited to those that could be done on mica or materials that could be adsorbed (1) Gent, A. N.; Schultz, J. J. Adhes. 1972,3,281-294. (2) Andrews, E. H.; Kinloch, A. J. Proc. R. SOC.London, A 1973,332, 385-399,401-414. (3) Andrews, E. H. J. Mater. Sci. 1976,11, 1354-1361. (4) Ieraelachvili, J. N.; Tabor, D. Proc. R. SOC.London, A 1972,311, 19. (5) (a) Israelachvili, J. N. Chemtracts: Anal. Phys. Chem. 1989, 1, 1-12. (b)Israelachvili,J. N.;Adams,G. E.J. Chem. Soc.,Faraday Tram. 1 1978, 74, 975-1001. (6) Patel, S. S.; Tirrell, M. Annu. Reu. Phys. Chem. 1989,40,597-635. (7) Watanabe, H.; Tirrell, M. Polym. Prepr. 1989,30, 387-388.
0 1991 American Chemical Society
1976 Langmuir, Vol. 7, No. 9, 1991 on mica. Some measurements have been carried out on glass and sapphire?^^ Industrially important polymer films have not been amenable to direct study with the SFA. We report t h e first measurements of ysobtained by using the SFA for a polymer film. Our polymer of choice, poly(ethy1ene terephthalate) (PET), is an important substrate for products ranging from computer disks a n d audio/video tapes t o photographic films. At room temperature, the typical condition for SFA measurements, biaxially oriented PET is semicrystalline with glassy noncrystalline microdomains. This glassy state assures negligible interpenetration during contact a t room temperature. In contrast to t h e recent work of Watanabe e t al.7 on polystyrene/isoprene block copolymers adsorbed on mica from solution and later dried, our samples are preformed biaxially oriented films, directly silvered, a n d glued onto the glass lenses of the SFA. This opens several new avenues of exploration with t h e SFA. First, we can study a n important polymer, PET, in an industrially useful state. Second, we avoid t h e restrictive a n d sometimes cumbersome use of mica. Third, our solvent-free, glassy film should minimize dissipation mechanisms because the polymer chains remain essentially fixed in their initial configurations with negligible interfacial penetration. This enhances our approach to a truly reversible measurement of W,. Fourth, preformed films can allow us, in principle, to study the changes in surface energy with orientation and processing of the PET. This will be t h e subject of future work. We present here results of investigation of t h e adhesion force between films of PET a n d PET films t h a t were modified by direct fluorination.
Experimental Section Film Preparation. In order to provide samples suitable for the SFA,polymer films must be molecularlysmoothand optically transparent. Thus, slip agents, such as are used to generate commercially useful PET film, must be avoided. In addition, the samples must be substantially free of dust or gel particles. Finally, the films needed to be self-supporting and thin enough to allow easy optical separation of the fringes of equal chromatic order (discussed below). The appropriate thickness is about 5 pm. Unfilled PETresin (number average molecular weight 24 0o0 with a polydispersity of about 2) was extruded on a conventional polyester maker to generate a 2 mil (50.8 pm) thick film of essentially unoriented, amorphous PET. Samples 4.5 in. square were cut and stretched on a T. M. Long laboratory film stretcher to achieve final samples thin enough ( l),the JKR surface profile and pressure distribution obtains except in a vanishinglysmall boundary layer about the perimeter of the contact area. This DMTlike boundarylayer removes the singularities of the original JKR theory. A crude calculation shows that p is much greater than 100 for our system. Thus MYD theory predicts JKR-like surface profiles, in accord with our observations. However, the MYD theory also predicts JKR-like surface energies. So, either the JKR energy values are correct and therefore a t variance with the critical wetting tension values or the theories are inadequate in some other way. Two possible sources for the discrepancy between SFA measurements and contact angle measurements are surface roughness and polymer interdiffusion. The latter can likely be discounted because the polymer under study is at least 40 “C below its glass transition temperature. Therefore, the large scale segmental motion necessary for interdiffusion is not available to this polymer under our experimental conditions. In addition, there was no obvious effect of contact time on the measurement of surface energy, albiet all our contact times were short (
