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Notes The Effects of Temperature and Methanol Concentration on the Properties of Poly(N-isopropylacrylamide) at the Air/Solution Interface Robert Pelton,*,† Robert Richardson,‡ Terence Cosgrove,§ and Robert Ivkov| McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7, H. H. Wills Physics Laboratory and School of Chemistry, University of Bristol, Tyndall Avenue, Bristol, U.K. BS8 1TL, and NIST Center for Neutron Research, 100 Bureau Drive, Stop 3460, Gaithersburg, Maryland 20899-3460 Received March 13, 2001. In Final Form: May 25, 2001
Introduction Perhaps one of the most interesting properties of the much-studied poly(N-isopropylacrylamide) (polyNIPAM),1-3 is cononsolvency in alcohol/water mixtures.4,5 The polymer is less soluble in mixtures of alcohol (methanol, ethanol, and isopropyl alcohol) and water than in either pure water or pure alcohol. This extreme nonideal behavior is reflected in both the cloud point temperature versus solvent composition curves for linear polyNIPAM4,5 and the swelling behavior of cross-linked polyNIPAM macro-6 and microgels.7,8 Recently, Zhang and Wu reported the light scattering behavior of dilute polyNIPAM solutions at 20 °C as a function of methanol content.9,10 They postulated that the polyNIPAM solubility was an indication of water/ methanol complex formation. Although the swelling behavior of polyNIPAM gels in water has been modeled, no quantitative theories predicting cononsolvency for polyNIPAM have been published. Both Schild et al.4 and Winnik et al.5 emphasized that multicomponent variations of Flory-Huggins theory cannot explain cononsolvency. However, an extension of the Hino and Prauznitz11 model, which accounts for hydrogen bonding, would probably predict cononsolvency. A number of authors qualitatively explain the cononsol* Author to whom correspondence should be addressed. E-mail:
[email protected]. † McMaster University. ‡ H. H. Wills Physics Laboratory, University of Bristol. § School of Chemistry, University of Bristol. | NIST Center for Neutron Research. (1) Heskins, M.; Guillet J. E. J. Macromol. Sci., Chem. 1968, A2 (8), 1441. (2) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (3) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (4) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (5) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1990, 23 (8), 2415. (6) Amiya, T.; Hirokawa, Y.; Hirose, Y.; Li, Y.; Tanaka, T. J. Chem. Phys. 1987, 86, 2375. (7) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (8) Crowther, H. M.; Vincent, B. Colloid Polym. Sci. 1998, 276, 46. (9) Zhang, G.; Wu, C. J. Am. Chem. Soc. 2001, 123, 1376. (10) Zhang, G.; Wu, C. Phys. Rev, Lett. 2001, 86, 822. (11) Hino, T.; Prausnitz, J. M. Polymer 1998, 39, 3279.
vency in terms of competitive interactions between alcohol, water, and polymer.4,5,8 In previous work, we have shown that, although polyNIPAM is quite surface-active, the surface tension of aqueous polyNIPAM is remarkably insensitive to the solution behavior of polyNIPAM in water. For example, when the temperature is increased through the cloud point (32 °C), all solution properties show a marked discontinuity, whereas the surface tension does not.12 In related studies, we employed neutron reflectivity to characterize the behavior of polyNIPAM at the air/water interface with and without SDS.13 Described in this paper are the results of a neutron reflectivity study of the properties of polyNIPAM on the surface of methanol/water mixtures as functions of composition and temperature. We show that the concentration of polyNIPAM at the solution/air interface is sensitive to the conditions in solution. Experimental Section Materials. PolyNIPAM-D7, based on deuterated propane, was purchased from Polymer Source, Dorval, QU, Canada.14 According to the supplier, the polyNIPAM-D7 sample had values of Mn ) 358 000 and Mw/Mn ) 2.6, and its intrinsic viscosity in methanol was 0.905 dL/g. The neutron reflection experiments were performed using the NG7 reflectometer at the NIST Center for Neutron Research, Gaithersburg, MD. The reflectivity was recorded over a range of scattering vector, Q, from 0.025 to 0.2 Å-1. The solutions were contained in a PTFE trough that was enclosed by a temperaturecontrolled environmental box that has been described in detail elsewhere.1515 For each angle of incidence, the reflectivity was determined by subtracting the nonspecular background and normalizing to the incident beam intensity using the beam monitor. All of the reflectivity measurements were made using null reflection solutions. The compositions of these solutions were determined by calculating the mole fractions of CH3OH, H2O, and D2O with the required ratio of methanol to water and zero scattering length density.
F ) nCH3OHbCH3OH + nH2ObH2O + nD2ObD2O
(1)
The number density, ni, for each component was calculated from the partial molar volume, vi, calculated from density data.
ni )
NA vi
(2)
All solvent compositions are presented as apparent volume percentages of methanol, which are based on the volume of methanol added to the water divided by the sum of the two puresolvent volumes. This was done because previous literature (12) Zhang, J.; Pelton, R. Langmuir 1996, 12, 2611. (13) Richardson, R.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 33 6269. (14) Certain commercial material and equipment are identified in this publication in order to specify adequately the experimental procedure. In no case does such identification imply recommendation by the National Institute of Standards and Technology, nor does it imply that either the material or the equipment identified is necessarily the best available for this purpose. (15) Clifton, B. J.; Cosgrove, T.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P. Macromolecules 1998, 248, 289.
10.1021/la010383u CCC: $20.00 © 2001 American Chemical Society Published on Web 07/12/2001
Notes
Langmuir, Vol. 17, No. 16, 2001 5119
describing polyNIPAM in water/methanol mixtures was presented in this way. The solutions, however, were prepared based on masses for the calculations for null reflecting water (see above). Theoretical Background. Because the solution was contrastmatched to air, the reflectivity depends on the distribution of the adsorbed polymer layer, and because the absolute value of the reflectivity is less than 1%, the kinematic approximation works very well. In this approximation, the reflectivity from a film on a contrast-matched subphase is given by the formula16,17
R(Q) )
16π2bN2 Q2
|∫
∞
0
|
2
nN(z)eiQz dz
(3)
where bN is the scattering length of a polymer segment (90.5 × 10-5 Å for polyNIPAM-d7) and nN(z) is the number density of polymer segments as a function of depth. At low Q, a Guiniertype approximation holds, which gives the adsorbed amount and the root-mean-square deviation of the adsorbed layer, σ, which is a model-free estimate of the thickness.
R(Q) ≈
16π2bN2Γ2 Q2
exp(-Q2σ2)
Figure 1. Measured reflectivity from polyNIPAM-d7 in null reflecting solution for several temperatures in 20% methanol (points) and Gaussian model fits to the data (lines). At temperatures at and below 26 °C, the parameters for the two sides of the Gaussian are equal at 8 to 13 Å. At 29 °C, the total thickness has increased to about 60 Å.
(4)
where Γ is the adsorbed amount in segments per unit area. Thus, any distribution that fits the reflectivity from the low-Q part of the reflectivity profile (i.e., Q < 1/σ) will give the correct values of the adsorbed amount and the root-mean-square (rms) thickness, although the details of the distribution might not be accurate. We found that a two-sided Gaussian profile for the distribution of segments gave a reasonable fit to the reflectivity from polyNIPAM under most conditions, and so, it was chosen as the function to analyze these data.
nN(z) )
xπ2 (σ +Γ σ ) exp(- 2σz 1
nN(z) )
) ( ) 2
2
x
2
∀z0
(5)
where the overall rms thickness measurement, σ, is given by
σ13 + σ23 2 σ2 ) - (σ2 - σ1)2 σ1 + σ2 π
(6)
The reflectivity was calculated numerically from profiles described in eq 5 using the standard method due to Abe´le`s.18 The thickness parameters of the two-sided Gaussian, σ1 and σ2, and the adsorbed amount, Γ, were adjustable. For all of the thinner polyNIPAM layers, it was found that a symmetric Gaussian (i.e., σ ) σ1 ) σ2) gave an excellent fit over the entire Q range of the data. The adsorbed amount was converted into mass units by the formula
Γm )
ΓM NA
(7)
where M is the relative molar mass of one segment of NIPAM and NA is Avogadro’s number. A possible complication is the preferential solvation of polyNIPAM with methanol. However, solvation would only make a small change to the adsorbed amounts of NIPAM that we have determined. This is because the scattering length density of CH3OH (0.04 × 10-5 Å-2) is very much closer to that of the null solution (0 × 10-5 Å-2) than that of pure NIPAM-D7 (0.48 × 10-5 Å-2). Thus, if each polymer molecule had an equal volume of methanol in its solvation shell rather than a solvation shell of (16) Penfold, J.; R. K. Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369-1412. (17) Phipps, J. S.; Richardson, R. M.; Cosgrove, T.; Eaglesham, A. Langmuir 1993, 9, 3530-3537. (18) Abele`s, F. Ann. Phys. (Paris) 1948, 3, 504.
Figure 2. Experimental conditions for the reflectivity experiments (points) and LCST of polyNIPAM as a function of solvent composition (line). The size of the data points indicates the quantity of adsorbed polymer at the air/solution interface. null solution, then the measured adsorbed amounts would be 10% too high.
Results and Discussion Figure 1 shows a typical set of reflectivity curves that were measured from solutions containing 20% methanol. The lines are the calculated reflectivities that have been fitted to the data by adjusting the thickness and adsorbedamount parameters. It can be seen from the reflectivity data that there is an abrupt change when the temperature of the solution increases above 26 °C. For other solution compositions, this jump took place at different temperatures. Reflectivity measurements were made as functions of the volume fraction of methanol and the temperature. Figure 2 summarizes the temperatures and solution compositions for the reflectivity experiments. The corresponding lower critical solution temperature (LCST) versus methanol content curve taken from Winnik et al.5 is shown for reference. The size of the data points gives a relative indication of the amount of adsorbed polymer at the solution/air interface. For a given methanol concentration, the adsorbed amounts are low below the LCST and high above it. Figure 3 shows adsorbed amounts as a function of temperature. The adsorbed amounts below the LCST
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Figure 3. Concentrations of adsorbed polyNIPAM at the solution/air interface as functions of temperature and composition. Methanol concentrations are expressed as apparent volume percentages, defined as the volume of added methanol divided by the sum of the volume of water and methanol used to make the solutions.
values are about 1 mg/m2, typical of an adsorbed polymer single layer, and the corresponding error bars are small. At high temperatures, the adsorbed amounts are 8 times greater, with broad error bars indicating multilayer adsorption. The relationship between surface concentration and solution temperature relative to the LCST is further illustrated in Figure 4, where the adsorbed amounts are plotted as functions of the temperature minus the corresponding LCST at that methanol concentration. The data are separated into two groups. The diamonds correspond to the steep section of the LCST vs methanol curve in Figure 2 where the LCST values are not accurate; the squares represent more reliable results. Below the LCST (negative values on the x axis), the adsorbed amounts decrease with distance from the LCST. By contrast, adsorption increases rapidly above the LCST. The basic behavior seen in this system is analogous to that found in polymer adsorption studies at the solid/ solution interface on approach to the bulk theta temperature. In both cases, phase separation in the solution is the driving force for multilayer adsorption. Although only one molecular weight was studied here, by analogy, the adsorption close to the cloud point temperature is likely to be strongly molecular-weight-dependent. Note that, at high (65-75%) methanol concentrations, the LCST versus methanol plot is very steep (see Figure 2). Furthermore, the LCST curve was taken from the literature and thus corresponds to a different molecular weight than used here. Therefore, both the LCST values
Notes
Figure 4. Adsorbed polyNIPAM as a function of the solution temperature minus the corresponding LCST. Data from a range of methanol concentrations collapse onto a single master curve when plotted in this way. Data points plotted as diamonds correspond to the steep section of the LCST vs percent methanol curve and so are less reliable.
and the corresponding reduced temperature values are not reliable for the 65-75% solutionssthese points are shown as diamonds in Figure 4. In other words, the horizontal error bars on the diamonds are large and unknown. Conclusions In conclusion, the interfacial behavior of polyNIPAM reflects the state of the solution. Three adsorption regimes are apparent. When the methanol concentration is above 65%, the surface concentration of polyNIPAM is essentially zero, which is consistent with the fact that pure methanol is a good solvent for polyNIPAM,19,20 The second regime occurs at lower alcohol concentrations and at temperatures below the LCST. Under these conditions, conventional adsorbed monolayers form. The third regime occurs above the LCST, where thick layers form in what is essentially the deposition of phase-separated polyNIPAM at the air/ solution interface. Acknowledgment. We acknowledge Sushil Satija for help with the use of the neutron facilities at NIST. R.P. was funded by the Canadian Natural Science and Engineering Research Council. LA010383U (19) Chiantore, O.; Guaita, M.; Trossarelli, L. Makromol. Chem. 1979, 180, 968. (20) Meewes, M.; Ricka, J.; de Silva, M.; Nyffenegger, R.; Binkert, Th. Macromolecules 1991, 24, 5811.
