+
+
Langmuir 1996, 12, 2611-2612
2611
Poly(N-isopropylacrylamide) at the Air/Water Interface Ju Zhang and Robert Pelton* McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 Received November 1, 1995. In Final Form: February 1, 1996
Poly(N-isopropylacrylamide), PNIPAM, is a watersoluble polymer with a lower critical solution temperature, LCST, of 32 °C in water.1 The potential utility of temperature-sensitive polymer solutions and gels has stimulated many papers involving PNIPAM. It is generally accepted that the phase separation is driven by the entropy gained by the release of water molecules which are partially immobilized by the hydrophobic propyl groups. The isopropyl groups also cause PNIPAM to be surface active in water. It seems reasonable to propose that for polymer molecules at the air/water interface, the methyl groups will preferentially be oriented toward the air. Furthermore, one might expect that water molecules will not be associated with protruding methyl groups. If these speculations are correct, PNIPAM located at the air/water interface should not demonstrate a lower critical solution temperature because there is no associated water and thus no driving force for phase separation. Fujishige and co-workers reported surface tension results, using the Whilhelmy method, for 0.25-1% (w/v) PNIPAM.2 At an unspecified temperature above the LCST, the surface tension was 41.9 mJ/m2 compared with 47.8 mJ/m2 at 16 °C. Thus, it appears that when a surface covered with PNIPAM is heated above the LCST, the surface tension behavior does not reflect the phase separation exhibited by the polymer in solution. However, the Whilhelmy method is problematic because phaseseparated polymer deposits on the plate The objective of the work described in this note was to extend Fujishige’s results to higher temperatures and to compare the behavior of surfaces generated at low temperature and heated above the LCST with surfaces formed above the LCST. For this, surface tension values of PNIPAM (890 000 Da) solutions were measured using the pendant drop method under a variety of conditions in two concentration ranges, dilute (10 mg/L) and concentrated (10 g/L). Figure 1 shows the surface tension versus pendant drop age for dilute solutions at two temperatures. At 25 °C the surface tension dropped to a constant value of 42.42 mJ/ m2. By contrast at 40 °C, the polymer phase separated before the drop was formed and there was no indication of adsorption at the air/water interface. Dynamic light scattering indicated that the phase separated polymer was present as 59 nm diameter particles at 40 °C. The results in Figure 1 show that after 30 min the surface tensions were constant indicating the approach to equilibrium. Such steady-state values have been called “mesoequilibrium surface tensions” recognizing that very long times may be required for polymers to achieve true equilibrium at an interface.3 Curves similar to those in (1) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (2) Fujishige, S.; Koiwai, K.; Kubota, K.; Ando, I. Kenkyu Hokokuseni Kobunshi Zairyo Konkyusho 1991, 167, 47.
Figure 1. Surface tension of 10 mg/L PNIPAM as a function of time for pendant drops formed at two temperatures.
Figure 2. Surface tension as a function of temperature for two PNIPAM concentrations. Each point was for a drop formed at that temperature and aged to give a steady state reading (about 30 min.).
Figure 3. Surface tension of a pendant drop of 10 mg/L PNIPAM solution. The drop was formed at 25 °C (curve 1) after which it was heated to 40 °C (curve 2) followed by cooling to 25 °C (curve 3). The drop was again heated to 40 °C for the final data set (curve 4).
Figure 1 were obtained over a range of temperatures and at two polymer concentrations. The mesoequilibrium surface tension values are shown as a function of temperature in Figure 2. PNIPAM at low concentration (10 mg/L) did not influence surface tension above 35 °C which is consistent with Figure 1. On the other hand, the concentrated polymer (10 g/L) gave low surface tensions under conditions where the polymer solution had phase separated. The implication is that phase-separated polymer, present as colloidal particles, diffused to the surface and the particles unfolded to give an adsorbed polymer layer. If this explanation is correct, the dilute solution should also have given low surface tensions after very long times. The results in Figure 2 suggest that PNIPAM can adsorb at the air/water interface above the cloud point temperature. Thus, polymer adsorbed at low temperature should remain at the interface after the temperature is raised irrespective of the bulk concentration. Figure 3 shows surface tension results for a single pendant drop of dilute polymer solution taken through temperature cycles. The drop was formed at 25 °C giving surface tension data (curve 1) similar to those in Figure 1. However, when the drop (3) Tripp, B. C.; Magda, J. J.; Andrade, J. D. J. Colloid Interface Sci. 1995, 173, 16.
+
+
2612
Langmuir, Vol. 12, No. 10, 1996
already supporting interfacial polymer was heated to 40 °C (curve 2), there was a slight decrease in surface tension. Thus, there was no phase separation or desorption of the interfacial polymer between 25 and 40 °C as evidenced by surface tension. Subsequent cooling (curve 3) and heating (curve 4) caused a shift to slightly lower surface tension, perhaps indicating more efficient packing of propyl groups on the interface. In summary, it appears that PNIPAM on the air/water interface does not undergo a phase transition below 40 °C. Since the molecular weight of PNIPAM was high (890 000 Da), one might expect that portions of the polymer molecule would exist as loops in the water phase. It is remarkable that upon heating, the loops in the water did not drag the interfacial polymer segments down into a coacervate phase to give higher surface tension when phase separation occurred in solution. Indeed, Figure 2 suggests that the reverse occurs. That is, phase-separated polymer in solution can diffuse to the interface and spread. The PNIPAM system is not unique. Chang and Gray reported the surface tension behavior of hydroxypropyl cellulose solutions, another LCST system.4 Although they dealt primarily with time dependence of surface tension, their results show that surface tension decreased slightly with increasing temperature above the LCST. The conclusions from this work are as follows: 1. PNIPAM at the air/water interface did not exhibit a phase change over the temperature range 25-40 °C, whereas PNIPAM in solution has a LCST of 32 °C. 2. Aqueous PNIPAM was present as colloidal particles (4) Chang, S. A.; Gray, D. G. J. Colloid Interface Sci. 1978, 67.
Notes
above the LCST. The particles diffused to a newly formed air/water interface, unfolded, and adsorbed to lower surface tension. Experimental Section N-Isopropylacrylamide (40 g) (Kodak Co.), double-recrystallized from toluene/hexane, was dissolved in 400 mL of Milli Q water at room temperature under N2. The polymerization was initiated with 40 mg of ammonium persulfate and 40 mg of sodium metabisulfite. After 25 h the solution was heated to about 40 °C to precipitate the polymer. The polymer was dissolved and precipitated from water twice after which the polymer solution was dialyzed (12000-14000 molecular weight cutoff), until the conductivity of the dialysate was less than 1 µS/cm. Finally, the water was evaporated and the polymer was dried to constant weight at 60 °C under vacuum. The intrinsic viscosity of the polymer was measured and the corresponding viscosity average molecular weight was 8.9 × 105 based on the Mark Houwink coefficients reported by Chiantore et al.5 Dynamic surface tensions were measured by pendant drop experiments using axisymmetric drop shape analysis (ADSA) in the Surface Science Lab of University of Toronto. Details of the methodology can be found elsewhere.6
Acknowledgment. The authors acknowledge Professor W. Neumann and his colleagues at the University of Toronto for making available their facilities for measuring surface tensions. The work was funded by the Canadian Natural Science and Engineering Research Council. LA950548X (5) Chiantore, O.; Guaita, M.; Trossarelli, L. Makromol. Chem. 1979, 180, 968. (6) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169.
