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Langmuir 1999, 15, 634-637
Surface Tension Relaxation of Poly(N-isopropylacrylamide) at the Air/Water Solution Interface As Probed by Surface Laser Light Scattering Q. R. Huang and C. H. Wang* Department of Chemistry, University of NebraskasLincoln, Lincoln, Nebraska 68588-0304 Received January 29, 1998. In Final Form: October 15, 1998 Surface laser light scattering (SLLS) is used to investigate the surface relaxation dynamics at the interface of the air/poly(N-isopropylacrylamide) (PNIPAM)-water solution. The SLLS data yields a result consistent with a model invoking three consecutive regions in the surface relaxation process. In the region where the surface tension undergoes a rapid change, a line width maximum is found. The line width maximum is due to a strong coupling of the capillary wave with the surface longitudinal wave associated with the surface dilational modulus, as a result of rapid segmental motion of the polymer chains at the interface.
Introduction When a fresh air/surfactant water solution interface is first formed, the surface tension closely corresponds to that of the air/pure water interface. The surface tension subsequently decreases with time due to surfactant adsorption at the interface; it eventually reaches an equilibrium value upon the completion of surfactant adsorption. The time dependent surface tension is known as the “dynamic surface tension.”1 The average time constant at which the surface tension approaches the equilibrium value is determined by polymer chain relaxation processes taking place near the air/water interface. The study of surface relaxation processes of surface active polymers (or polymer surfactants) has attracted a considerable attention in recent years because of relevant industrial applications.2 Technological processes, such as foaming, emulsification, and detergency, are more affected by the dynamic than equilibrium surface tension.3,4 The surface adsorption properties of a polymer solution strongly depend on the chain conformation. A change in the conformation of the adsorbed polymer surfactant is expected to affect the surface tension and the thickness of the adsorbed layer. The relationship between the surface relaxation and the conformation change is not clear at present; more experimental and theoretical efforts are needed to clarify their relation. Experimental studies of the surface relaxation process have so far been carried out by the pendant drop apparatus,5,6 the oscillating jet technique,7,8 and the inclined plane method,9 etc. The main drawback of these techniques (except for the pendant drop technique) is that they perturb the air/liquid interface and, hence, are not convenient for probing the dynamic process of thin fragile films at the interface. Using the experimental data * To whom correspondence should be addressed. (1) Ward, A. F.; Tordai, L. J. J. Chem. Phys. 1946, 14, 453. (2) Florence, A. T.; Attwood, D. Physicochemical Principles of Pharmacy; Macmillan: New York, 1988. (3) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1988, 124, 652. (4) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1990, 139, 397. (5) Nahringbauer, I. J. Colloid Interface Sci. 1995, 176, 318. (6) Bonfillon, A.; Sicoli, F.; Langevin, D. J. Colloid Interface Sci. 1994, 168, 497. (7) Bleys, G.; Joos, P. Colloid Polym. Sci. 1983, 261, 1038. (8) Bleys, G.; Joos, P. J. Phys. Chem. 1985, 89, 1027. (9) Vanden Bogaert, R.; Joos, P. J. Phys. Chem. 1979, 83, 2244.
obtained by the pendant drop apparatus plus additional data previously obtained by other surface techniques, Nahringbauer5 has proposed a model that invokes three consecutive kinetic regions to describe the surface relaxation process. These kinetic regions are the induction, the surface coverage, and the mesophase regions. Surface laser light scattering (SLLS) is another useful technique that can be used to probe the surface dynamics through the investigation of the surface wave propagation behavior of fluids.10 The dynamics of capillary waves propagating on the fluid interface is determined by surface tension, surface dilational modulus, and the shear viscosity of the underlying liquid.11 Although SLLS has been used in the studies of various physical properties of liquid surfaces, such as insoluble biomembranes and polymer films,12,13 liquid-crystal surfaces,14 liquid thin films,15,16 and other surfaces related to various supramolecular systems, it has not been used to probe the surface tension relaxation phenomenon. Since the SLLS is a surface nonperturbative technique, it is ideally suited for investigating the relaxation phenomenon that takes place at the air/solution interface. This paper demonstrates the application of SLLS for the study of the surface tension relaxation process, with poly(N-isopropylacrylamide) (PNIPAM) at the air/water interface as a example. PNIPAM is a polymer surfactant. Since a finite time is required for adsorption, one expects to observe the surface relaxation process associated with the adsorption of PNIPAM at the air/aqueous solution interface. It should be noted that the same system has recently been studied by the pendant drop17 and the Wilhelmy plate method.18 To gain additional insight, we use the SLLS technique to (10) For a review of capillary waves and surface laser light scattering, see Light Scattering by Liquid Surfaces and Complementary Techniques; Langevin, D., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1992. (11) Lucassen-Reynders, E. H.; Lucassen, J. Adv. Colloid Interface Sci. 1969, 2, 347. (12) Sauer, B. B.; Griffin, W. G.; Yu, H. Macromolecules 1989, 22, 786. (13) Mann, E. K.; Langevin, D. Langmuir 1991, 7, 1112. (14) Shih, L. B.; Mann, J. A.; Brown, S. H. Mol. Cryst. Liq. Cryst. 1973, 22, 317. (15) Huang, Q. R.; Wang, C. H. Langmuir 1996, 12, 2679. (16) Joosten, J. G. H. in Thin Liquid Films; Ivanov, I. B., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1988. (17) Zhang, J.; Pelton, R. Langmuir 1996, 12, 2611. (18) Kawaguchi, M.; Hirose, Y.; Kato, T. Langmuir 1996, 12, 3523.
10.1021/la9801209 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/31/1998
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investigate the surface tension relaxation dynamics by probing the capillary wave propagating behavior as a function of the aging time. Experimental Section Poly(N-isopropylacrylamide) or PNIPAM was synthesized in our laboratory following the procedure previously described,19,20 with some modifications. N-Isopropylacrylamide was purchased from Aldrich Chemical Co., Inc., and was recrystallized three times from a 65/35 mixture of hexane and benzene (both from Fisher Scientific Co.). Acetone (Fisher Scientific Co., ACS grade) was carefully dried by refluxing over CaH2 powder to remove moisture. The purified monomer with 1 mol % of purified 1,1′azobis(cyclohexanecarbonitrile) (Aldrich Chemical Co., Inc., recrystallized twice from ethanol) used as initiator was dissolved in benzene. The solution was then degassed through several freezing and thawing cycles. The polymerization was carried out in an oil bath, controlled at about 50 °C for 24 h under a positive argon pressure. After the completion of the polymerization process, the solvent was evaporated with a rotary evaporator. The solid PNIPAM polymer was then dissolved in dried acetone and precipitated by the addition of hexane. After being vacuumdried, the PNIPAM polymer was then twice precipitated from the acetone solution at room temperature by the stepwise addition of hexane. The polymer was further vacuum-dried for 72 h at 40 °C. Using static light scattering and gel-permeation chromatography techniques, we found that the PNIPAM fraction finally used in this experiment has Mw )1.6 × 106 with dispersity equal to 1.3. The PNIPAM solutions were prepared with 18.3 MΩ cm water from a Millipore Milli-Q system. The stock PNIPAM solution with bulk concentration equal to 4 × 10-6 g/mL was poured into a clean glass cell for the SLLS experiment. All measurements were carried out at 20 ( 0.2 °C. The surface laser light scattering setup used in our laboratory has been described elsewhere.15,21,22 Light from an Ar+ laser (λ ) 488 nm) passing through a transmission grating was incident (at an incident angle of about 64°) upon the liquid surface. Light scattered at a given wave vector q defined by the grating was mixed with the diffracted light at the same wave vector for the heterodyne detection. The q value, ranging from 159 to 616 cm-1 (q being the amplitude of the scattering wave vector q), was detected with a low-noise photomultiplier (PMT). The photocurrent from PMT, after passing through a current-voltage converter and a high-pass filter-amplifier (Ithaco 4302 dual filter) to cut off the noise signals below 500 Hz, was amplified 10 times before it was routed to a fast Fourier transform spectrum analyzer (Stanford Research Systems, Model SR760). The equilibrium of the polymer solution at the air/water interface was disturbed by violent shaking. The data accumulation time for each measurement was about 8 s. Since the surface tension relaxation time is much longer, this enables us to study the surface relaxation process. The power spectrum is fit to a Lorentzian function to obtain the peak frequency fs. The corrected spectral line width, ∆fs,c, is obtained by subtracting off the instrumental width ∆fi, using the following expression:10 ∆fs,c )∆fs - (∆fi2/∆fs). The peak frequency is related to the propagation velocity of the capillary waves, and the halfwidth at half-height is related to the damping constant. To certify reproducible results, after the capillary wave frequency had reached an equilibrium value, we again disturbed the surface by violent shaking and repeated the same SLLS measurement procedure. The repeated measurements gave consistent results.
Results and Discussion PNIPAM molecules adsorbed at the air/water interface result in the reduction of the surface tension. One can investigate the surface tension reduction process by monitoring the behavior of the capillary wave by using the nonperturbative surface laser light scattering (SLLS) (19) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381. (20) Schild, H. G.; Tirrell, D. Langmuir 1991, 7, 665. (21) Huang, Q. R.; Wang, C. H. J. Chem. Phys. 1996, 105, 6546. (22) Huang, Q. R.; Wang, C. H.; Deng, N. J. J. Chem. Phys. 1998, 108, 3827.
Figure 1. Experimental SLLS spectra obtained from the air/ PNIPAM water interface for the solution with PNIPAM bulk concentration equal to 4 × 10-6 g/mL. Spectra with four different amplitudes of the scattering vector are (a) q ) 159.2 cm-1, (b) q ) 312.7 cm-1, (c) q ) 464.3 cm-1, and (d) q ) 616.5 cm-1.
technique. The polymer solution at very dilute concentration can be treated as a Newtonian fluid, and the theory of SLLS for the Newtonian fluid is well-established.10 For the surface of a Newtonian fluid, one finds that the peak frequency fs and the line width ∆fs of the SLLS spectrum associated with the capillary wave are given by (neglecting the effect of surface dilation)10
fs )
1 2π
x
∆fs )
γq3 F
ηq2 πF
(1) (2)
where γ is the surface tension; F is the density of the polymer solution; q is the amplitude of the scattering vector, and η is the shear viscosity of the underlying bulk liquid. One notes that in the simplest case the peak frequency is associated with the surface tension, and the line width with the viscosity of the bulk liquid. Figure 1 shows four SLLS spectra of the PNIPAM/water solution after the surface is aged to t ) 3127 s obtained with q ) 159.2, 312.7, 464.3, and 616.5 cm-1. The spectral intensity diminishes with increasing q, accompanied by line width broadening. This is consistent with the theoretical prediction for the propagating capillary wave in the Newtonian fluid. Shown in Figure 2 is the log-log plot of the experimentally observed peak frequency verses q; the slope of 1.5 confirms the q dependence (fs ∝ q3/2) for the capillary wave. The result suggests that the surface tension can be deduced from eq 1. It also suggests that the spectrum at any q can be used to monitor the surface tension relaxation process. We choose to use the high q value (616.5 cm-1) for the surface tension relaxation study because the spectral change at the high q value is easier to discern. Figure 3 shows the SLLS spectrum obtained at q ) 616.5 cm-1 with different aging times. All spectra have an approximate Lorentzian shape. These spectra show the existence of three conspicuous regions: (1) As the aging time increases from 0 to 470 s (Figure 3A), the peak
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Huang and Wang
Figure 4. Aging time dependence of the half width at halfheight ∆fs,c at q ) 616.5 cm-1 for PNIPAM aqueous solution. The solid line is used to guide the eyes. Figure 2. Log-log plot of the peak frequency versus q. The solid line is the linear fit, which gives the slope of 1.5.
Figure 3. Experimental SLLS spectra obtained for q ) 616.5 cm-1 for different aging times t: (A) (b) t ) 96 s, (9) t ) 470 s, (2) t ) 916 s; (B) (b) t ) 1000 s, (9) t ) 1239 s, (2) t ) 1775 s; (C) (b) t ) 2743 s, (9) t ) 3073 s, (2) t ) 3401 s.
frequency changes very little, but the spectral line width broadens. However, upon a further increase of the aging time to 916 s, the peak frequency decreases, accompanied by a further line width broadening and an intensity decrease. (2) On the other hand, as the aging time is further increased to 1775 s, the line width narrows with increasing aging time, accompanied by a dramatic peak frequency decrease and an intensity increase. This behavior suggests the setting-in of surface coverage at this stage (Figure 3B). (3) Finally, upon further increasing the aging time to more than 3000 s, the spectral intensity, peak frequency,
and line width reach steady-state values (Figure 3C), indicating that an equilibrium state has been achieved. The asymptotic peak frequency of 15.5 kHz corresponds to a static surface tension of 40.8 mN/m, which is consistent with the value measured with the pendant drop method. Figure 4 shows the variation of the line width ∆fs,c (the subscript c denotes that the line width has been corrected from the instrumental contribution) with the aging time. At the early stage, the line width increases with increasing the aging time and exhibits a maximum at around t ) 1000 s. At the maximum, the damping is about 2.6 times larger than that of the free water surface. The change in the line width is not predicted by eq 2, which is obtained by neglecting the effect of the surface dilational modulus on the capillary wave. Since the surface dilational modulus arises from surface adsorption, this renders the surface tension inhomogeneous. The effect of dilational modulus on the line width has been observed in spread films10 and soluble block copolymer surfactant.15 The time dependence of the line width clearly suggests the significant polymer segmental arrangements on the surface occur before the adsorption process reaches the steady state. The time dependence of the peak frequency (O) and the surface tension calculated by using eq 1 (b) are shown in Figure 5. In the region from 0 (the fresh surface) up to 850 s, these is little change in the surface tension. Following this is a region associated with rapid reduction of the surface tension. The surface tension reduction is clearly due to an increase in surface coverage. It eventually reaches a steady state, with an equilibrium surface tension value equal to about 40.8 mN/m, which, as mentioned above, is very close to the values measured by pendant drop17 and the Wilhelmy plate technique.18 The change of the surface tension thus corresponds approximately to the regions proposed by Nahringbauer.5 Following Hua and Rosen,3 we fit the dynamic surface tension data shown in Figure 5 to an empirical equation, given by
γ(t) ) γm + (γ0 - γm)/[1 + (t/τ)n]
(3)
where γ(t) is the surface tension at aging time t, γ0 is the surface tension of pure water, γm is the equilibrium surface tension, τ is the time for the surface pressure, given by [γ0 - γ(t)], to attain the half of its equilibrium value, and n is a dimensionless parameter. Using a nonlinear least-
Surface Tension Relaxation As Probed by SLLS
Figure 5. Peak frequency (O) and surface tension (b) calculated by using eq 1 plotted versus the aging time. The solid line is the fit to the surface tension (b) according to eq 3.
squares minimization method for the best fit, we obtain τ ) 1604 s and n ) 2.96. The fitting curve is shown by the solid curve in Figure 5. One notes that the fitted curve qualitatively describes the experimental data; however, there is a significant discrepancy between the fitted curve and the experimental data in the initial induction region. The discrepancy may be due to the use of the Kevin
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equation (eq 1) to obtain γ. As mentioned above, the surface dilational modulus needs also to be included to account for the capillary wave propagation dynamics. The inclusion of the surface dilation modulus significantly modified the dispersion equation of the capillary wave dynamics.10 It is expected that its inclusion would modify the value of γ that is extracted from the SLLS technique. In addition, the effect of diffusional exchanges between surface and bulk molecules also needs to be included to properly account for the result. The study of these additional effects are in progress in our laboratory and will be a subject of further publications. In summary, we have demonstrated, for the first time, that the surface light scattering technique can be used to investigate the surface tension relaxation phenomenon. We have used as an example the PNIPAM/H2O solution with the polymer concentration equal to 4 × 10-6 g/mL. The SLLS data yield a result consistent with a model involving three consecutive regions in the surface tension relaxation process. In the region where the surface tension undergoes a rapid decrease, we have found a line width maximum. The line width maximum is believed to be involved with a strong coupling of the capillary wave to the surface longitudinal wave associated with the surface dilational modulus. Such a strong coupling is probably due to the result of rapid segmented motion of PNIPAM chains at the interface. Acknowledgment. The partial financial support of the Center of Material Research and Analysis at the University of NebraskasLincoln is acknowledged. LA9801209
