Langmuir 1998, 14, 3435-3439
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Viscoelastic Behavior of Polymer Surface during Wetting and Dewetting Processes E. Tomasetti,†,‡ P. G. Rouxhet,† and R. Legras*,‡ Unite´ de chimie des interfaces and Unite´ de chimie et de physique des hauts polyme` res, Research Center on Advanced Materials, Universite´ Catholique de Louvain, Place Croix de Sud 1, 1348 Louvain-la-Neuve, Belgium Received January 2, 1998. In Final Form: March 6, 1998 The contact angle of water was measured with the Wilhelmy plate method on apolar polymers: soft ethylene-propylene copolymer (EP), rigid polypropylene (PP), and their blends PP/EP (80 wt % PP) and EP/PP (20 wt % PP). Upon liquid displacement, the contact angles of the soft materials, EP and EP/PP, depended on viscoelastic energy dissipation due to solid deformation. This was observed in different situations: wetting and dewetting, liquid displacement due to the relative motion of the plain liquid with respect to the solid, and meniscus relaxation when that motion was stopped. The wetting hysteresis of polymer blends may be understood by considering viscoelastic energy dissipation and the static contact angles. The former depends on the elastic modulus while the latter is determined by the more hydrophobic phase and the less hydrophobic phase, for advancing and receding contact angles, respectively.
Introduction Spreading of a liquid drop on an elastomeric solid was investigated by Shanahan and Carre´.1-3 If the solid is soft enough, a local deformation of the surface may occur at the wetting front. This deformation, following the liquid front movement, may lead to local strain/relaxation cycles with viscoelastic energy dissipation in the solid. Therefore, liquid spreading is slowed. The Wilhelmy plate method is widely used to study wetting of solids in dynamic conditions: the sample plate is immersed in and withdrawn from a liquid at a constant rate and a balance records the capillary force, providing advancing and receding contact angles. A hysteresis is frequently obtained between records of advancing and receding contact angles, even at low displacement rates.4 It is usually interpreted in terms of roughness, chemical heterogeneity, deformation, reorientation, and swelling of the surface;5-9 however it is difficult to evaluate the respective influences of these factors.9-12 Wetting and dewetting by a moving liquid are not instantaneous due to viscous dissipation in the liquid,13 amplifying the difference between advancing and receding contact angles and thus the hysteresis; however, this phenomenon is only significant at high liquid rates. * Corresponding author. † Unite ´ de chimie des interfaces. ‡ Unite ´ de chimie et de physique des hauts polyme`res. (1) Shanahan, M. E. R.; Carre´, A. Langmuir 1994, 10, 1647. (2) Carre´, A.; Shanahan, M. E. R. Langmuir 1995, 11, 24. (3) Shanahan, M. E. R.; Carre´, A. Langmuir 1995, 11, 1396. (4) Mahale, A. D.; Wesson, S. P. Colloids Surf., A 1994, 89, 117. (5) Morra, M.; Ochiello, E.; Garbassi, F. J. Colloid Interface Sci. 1991, 149, 84. (6) Yasuda, T.; Okuno, T.; Yasuda, H. Langmuir 1994, 10, 2435. (7) Andrade, J. D.; Smith, L. M.; Gregonis, D. E. In Surface and interfacial aspects of biomedical polymers; Andrade, J. D., Ed.; Plenum Press: New York and London, 1985; p 249. (8) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (9) Shanahan, M. E. R. Macromol. Symp. 1996, 101, 463. (10) Hayes, R. A.; Rolston, J. Colloids Surf., A 1994, 93, 15. (11) Schroeder, L. W. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Zeist, The Netherlands, 1993; p 349. (12) Vergelati, C.; Perwuelz, A.; Vovelle, L.; Romero, M. A.; Holl, Y. Polymer 1994, 35, 262. (13) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.
The present work is a first attempt to detect viscoelastic energy dissipation at soft thermoplastic polymer surfaces and during both wetting and dewetting processes. Wetting with water was studied using both soft and rigid apolar polymers and their blends. The use of blends threw further light on the origin of wetting hysteresis with heterogeneous surfaces and pointed out that the determination of viscoelastic behavior is important in the field of adhesion. Background The model describing the viscoelastic energy dissipation within a soft solid when a liquid spreads on its surface was developed by Shanahan.14,15 As long as the contact angle has not reached its stationary value (θe), a driving force appears, generating a work (per unit time and per unit length) given by
Pt ) γ‚Vt(cos θe - cos θt)
(1)
where Vt is the displacement rate of the liquid front, θt is the contact angle at time t, and γ is the liquid surface tension. Moreover, the component of the liquid surface tension perpendicular to the surface leads to a deformation of a solid zone which moves along with the liquid front (Figure 1). If the solid is perfectly elastic, the energy released by one zone is used to deform the next zone; thus, the liquid front does not spend any energy to move and spreads quasi-instantaneously. Conversely, if the solid is viscoelastic, a fraction of energy stored in the deformation is dissipated. Thus, the liquid front spends some work to advance, and consequently it is slowed. Shanahan and Carre´1-3 calculated the energy (per unit time and per unit length) required to displace the solid deformation for an elastomer surface (Poisson coefficient ) 0.5):
Wt ) 3γ2Vt/2πG
(2)
(14) Shanahan, M. E. R. J. Phys. D: Appl. Phys. 1988, 21, 981. (15) Carre´, A.; Shanahan, M. E. R. Langmuir 1995, 11, 3572. (16) Carre´, A.; Gastel, J.-C.; Shanahan, M. E. R. Nature 1996, 379, 432.
S0743-7463(98)00003-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998
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Tomasetti et al.
log(cos θe - cos θt) ) n log Vt + log 3Kγ/2πG (5)
Figure 1. Solid surface deformation due to liquid surface tension.
where G is the shear modulus of the solid and is a cutoff width under the three-phases contact line, within which the behavior of the solid may be considered to be no longer linearly elastic. For a viscoelastic solid, a fraction R of Wt is dissipated and balances the work of capillary force (Pt):
Pt ) RWt
(3)
It is assumed that R ) K(Vt)n, where K and n are constants characteristic of the system.1-3 Then, eq 1 becomes
Pt ) γ‚Vt(cos θe - cos θt) ) 3K(Vt)nγ2Vt/2πG (4) which may be put in logarithmic form as follows
A similar phenomenon is the viscoelastic braking of a rigid cylinder rolling down an inclined elastomeric track. In this case values of n were found in the range 0.5-0.6, which is the same as that obtained for spreading of a liquid drop on identical elastomers.1-3 Recently, Carre´ et al.16 validated the theoretical profile of the deformation provoked by a tricresyl phosphate drop on a piece of silicon rubber by means of a white-light interferometric microscope. The height of the deformation decreased according to the log of the distance from the liquid front, and at the three-phases contact line it was about equal to
d ) γ sin θ/G
(6)
The determination of the viscoelastic behavior of polyolefin blend surfaces is important in the field of adhesion. Indeed, the work of adhesion measured between two solids, at least one of which is elastomeric, is greater than the energy required to create new interfaces. This excess energy corresponds essentially to viscoelastic dissipation occurring in the elastomer during separation of the two phases. Maugis et al.17 showed that excess energy depends on separation rate (Vt) according to (aTVt)n with n ) 0.6 and aT the WLF shift factor at temperature T. This value of n links together wetting and adhesion phenomena.
Figure 2. SEM images obtained on PP/EP (a) and EP/PP (b) blends.
Polymer Surface during Wetting and Dewetting Processes
Langmuir, Vol. 14, No. 12, 1998 3437
Figure 3. Dynamic wetting on EP: (a) wetting cycle at 50 µm/s with stops at the end of receding (A to B) and advancing (C to D) curves; (b) variation of cos θ during the stops; (c) variation of log |cos θe - cos θt| as a function of the log of the liquid displacement rate Vt, (O, variation during a stop after the advancing run; b, variation during a stop after the receding run; *, variation during plain liquid surface displacement).
Materials and Methods Materials. Materials used were pellets of polypropylene (PP, Shell LY 6100, elastic modulus about 1500 MPa) and ethylenepropylene copolymer (EP, Exxon Vistalon 719, elastic modulus about 20 Mpa, glass transition temperature -42 °C). Two physical blends of these materials (called PP/EP and EP/PP) were prepared by mixing in a brabender at 60 rpm and 180 °C for 5 min. The weight percentages in the blends were 80 wt % PP for the PP/EP blend and 20 wt % of PP for the EP/PP blend. The blends were ground in liquid nitrogen. PP, EP, and the blends were compression-molded on polyimide surfaces (45 000 kg/m2, 2.5 min, 220 °C, thickness 100 µm). Figure 2 presents SEM images of the PP/EP and EP/PP blend surfaces (acceleration voltage 15 kV, current 100 mA, Au-Pd coating 15 nm, Hitachi s-570 microscope). It shows that the minor phase is present as nodules (≈1-3 µm) at the surface. Dynamic Study of Wetting. Wetting of the films with water was investigated using the Wilhelmy plate method, with a Cahn dynamic contact angle balance (DCA 322) at room temperature. Successive cycles of immersion and emersion (advancingreceding) were performed, and the contact angle (θadv, θrec) was deduced from the following equation:
F ) mg + pγ cos θt - Fb
(7)
where F is the force measured by the balance, recorded every second during the wetting cycle; m and p are the mass and the perimeter of samples, respectively; and g is the gravity constant. The buoyancy Fb was computed at each position using the relation
Fb ) FgAZ
(8)
where F is the water-specific weight, A is the plate cross-section, (17) Maugis, D.; Barquins, M. J. Phys. D: Appl. Phys 1978, 11, 1989.
and Z is the immersed depth, given by the distance between the bottom of the plate and the free liquid surface. The position of the three-phases contact line at the sample surface X was obtained by accounting for the height of the meniscus (h):18
X)Z+h where
ht )
x 2γFg(1 - sin θ ) t
(9)
This equation implies that the meniscus shape is always in equilibrium with the real contact angle, supposing that wetting is not perturbed by viscous dissipation in the liquid, which is a reasonable assumption in our operating conditions (maximum displacement rates 50 µm/s). The liquid displacement was stopped at the end of advancing and receding runs. During stops, F variations were recorded as a function of time t and cos θt variations were deduced. As Z was constant, the height of the meniscus ht was computed by eq 9. The mean value of cos θt and the rate of the three-phases contact line were calculated over 4 s.
Results Figure 3a presents the second wetting cycle with stops at the end of receding and advancing runs performed on the ethylene-propylene copolymer (EP) film at 50 µm/s. During the stops, cos θadv (C to D) and cos θrec (A to B) evolved, revealing meniscus relaxation. Their variations (18) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley & Sons: New York, 1990.
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Tomasetti et al. Table 1. Differences between the cos θ Values at the Beginning and at the End of the Stops for Advancing and Receding Wetting Runs Recorded at 50 µm/s
Figure 4. Wetting curves recorded on EP at 2 (A), 20 (B), and 50 (C) µm/s.
as a function of time are given in Figure 3b. The displacement rate of the liquid was computed as described above. Figure 3c reports the variation of the log of the absolute value of the difference between cos θt and cos θe as a function of the log of the displacement rate Vt. The same linear relationship was obtained for the data of receding and advancing phases. Figure 4 shows wetting curves recorded on EP at 2, 20, and 50 µm/s. Relation 5 was also applied, taking θt as the contact angle recorded during displacement of the plain liquid surface, Vt as the rate of displacement, and θe as the contact angle measured at the end of the stops in Figure 3a. This was done for both advancing and receding runs, providing the stars in
sample
∆cosadv
∆cosrec
EP EP/PP PP/EP PP
0.35 0.15 0.05 0.05
0.35 0.30 0.05 0.05
Figure 3c, which fit well the data resulting from meniscus relaxation experiments. Wetting cycles were recorded at 50 µm/s on EP and PP and on PP/EP and EP/PP blends with stops at the end of receding and advancing runs. Figure 5a presents the evolution of cos θ vs time during the stops. It shows that cos θadv and cos θrec at the end of the stop are higher for EP than for PP; the cos θadv values of the two blends are close to the PP values while cos θrec is close to the EP values. The differences between the cos θ values at the beginning and at the end of the stops are presented in Table 1. Figure 5 presents also the plots of log |cos θe - cos θt| vs log Vt for PP, PP/EP, and EP/PP, as shown in Figure 3c for EP. When the data cover an appreciable range of cos θ, for EP and EP/PP, the regression lines show little dispersion and their slopes are close to 0.5. However, the range of cos θ during meniscus relaxation on EP/PP is smaller upon advancing (0.15) compared to receding (0.3). For PP and PP/EP, the variation of cos θ during stops is much smaller and the data are more scattered in the log/ log plot. The receding and advancing data do not superpose anymore.
Figure 5. Evolution of cos θ during the stops of wetting curves recorded at 50 µm/s (a) for PP (placed dotted line), EP (thin full line), PP/EP (near dotted line), and EP/PP (thick full line). Variation of log |cos θe - cos θt| as a function of the log of the liquid displacement rate Vt for PP (b), PP/EP (c), and EP/PP (d): (O) variation during a stop after the advancing run; (b) variation during a stop after the receding run.
Polymer Surface during Wetting and Dewetting Processes
Discussion For EP, when the plain liquid surface is stopped, the displacement rate of the three-phases contact line during meniscus relaxation follows relation 5 with n close to 0.5. This indicates that the rate of the process is controlled by viscoelastic energy dissipation in the solid.1-3,15 Upon forced displacement of the plain liquid surface, relation 5 is expected to also be followed, as the work generated by the driving force is still balanced by the energy dissipation. In Figure 3, the difference between meniscus relaxation (circles in Figure 3c) and meniscus forced flow (stars in Figure 3c) is not significant in view of the precision of the results. The fact that both processes follow the same relationship indicates that the assumption involved in relation 9 (no appreciable viscous dissipation in the liquid) is satisfactory. These observations hold for both wetting and dewetting processes. Viscoelastic energy dissipation thus controls the rate of the liquid front displacement, whether it is due to meniscus relaxation or to continuous motion of the plain liquid surface. EP/PP follows the same behavior as EP in terms of meniscus relaxation. This may be attributed to low elastic modulus, which allows large solid deformation at the threephases contact line. For PP and PP/EP, meniscus relaxation is very weak and the precision of the data does not allow us to draw any conclusion. Application of relation 6 to EP (E ) 20 MPa) with cos θ ) 0.35 indicates that the height of the deformation at the three-phases contact line d is about 10 nm. Figure 5a shows that, even after a prolonged stop, i.e., at extremely slow water front displacement, the advancing and receding angles remain different. Both indicate that EP is less hydrophobic than PP. For the blends, it appears that the stationary advancing contact angle is close to that of PP, the more “hydrophobic” component, while the receding contact angle is close to that of EP, the less “hydrophobic” component. This is in agreement with literature data,4,19,20 showing that the stationary contact angle is determined by the phase which is less favorable to further liquid displacement. This simple behavior holds (19) Johnson, R. E.; Dettre, R. H. Surf. Colloid Sci. 1969, 2, 85. (20) Wesson, S. P.; Kamath, Y.-K.; Mahale, A. D. Colloids Surf., A 1994, 89, 133.
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only if the size of dispersed particles (1-3 µm in this case) is much smaller than the difference of meniscus heights of the two individual phases (600 µm between EP and PP). When the liquid moves, the contact angle is determined by two factors: the stationary contact angle of the phase which is less favorable to further liquid displacement and the slowing down of liquid movement by viscoelastic dissipation in the solid. For the EP/PP blend, the second factor is important and the first factor is controlled by the PP phase upon advancing and by the EP phase upon receding. This explains why the range of cos θ during meniscus relaxation on EP/PP is smaller upon advancing compared to receding (see Table 1). Conclusion The variation of EP contact angle according to the rate of liquid displacement is controlled by viscoelastic energy dissipation in the solid. This occurs upon both wetting and dewetting, whether the liquid displacement is due to meniscus relaxation or to relative motion of the plain liquid surface with respect to the solid. A similar behavior is observed for a dispersion of PP in EP but not for PP or a dispersion of EP in PP. Indeed, a soft matrix is required. The behavior of a dispersion of PP in EP demonstrates that the variation of the wetting-dewetting hysteresis loop results from the conjunction of two factors: the static contact angle and the viscoelastic energy dissipation. The former is determined by the more hydrophobic component upon advancing and the less hydrophobic component upon receding, whatever are the continuous and the dispersed phases. The latter is responsible for the dependence on the liquid displacement rate and is determined mainly by the elastic modulus of the material. Acknowledgment. The support of Fonds pour la Formation a` la Recherche dans l’Industrie et l’Agriculture (F.R.I.A.), of Fonds National de la Recherche Scientifique (F.N.R.S.), and of the Department of Scientific Policy (PAISupramolecular Chemistry and Catalysis) is gratefully acknowledged. LA980003D
