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Swelling Behavior and Microstructure of Poly(12-acryloyloxydodecanoic acid-co-acrylic acid) Gels in the Ethanol/Water System Fumiyoshi Ikkai,*,† Naoki Masui,‡ Takeshi Karino,§ Sachio Naito,† Kimio Kurita,⊥ and Mitsuhiro Shibayama*,# L’ORE Ä AL RechercheTsukuba, 5-5 Tokodai, Tsukuba, Ibaraki 300-2635, Japan; Department of Polymer Science & Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan; Department of Materials Structure Science, The Graduate University for Advanced Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan; College of Science and Technology, Nihon University, Kandasurugadai, Chiyoda-ku, Tokyo 101-8308, Japan; and The Institute for Solid State Physics, The University of Tokyo, Tokai, Naka-gun, Ibaraki 319-1106, Japan Received September 25, 2002. In Final Form: December 12, 2002 The swelling behavior and hierarchical structures of poly(acrylic acid) (AA) homopolymer gels and poly(12-acryloyloxydodecanoic acid (ADA)-co-AA) gels carrying ionizable groups as well as hydrophobic groups have been investigated by equilibrium swelling measurement, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The equilibrium swelling degree (Sv) and the structure factors of the copolymer gels were obtained as a function of the ethanol (EtOH)/water (W) solvent composition, x () vol % of EtOH). Sv showed a convex-upward function of x. That is, the gels swelled with increasing x until x ) 50-60 vol % for AA gels and x ) 70-80 vol % for ADA/AA gels and then decreased gradually. This phenomenon is explained by hydrophobic association of long alkyl chains of ADA and a cosolvency effect of poly(AA) in EtOH and W. The DLS results also indicated that the major component of light scattering was frozen inhomogeneities. SANS intensity functions, I(q), for the copolymer gels when x ) 25 vol % showed the presence of an ordered structure in water with a long spacing of ca. 46 Å. These experimental pieces of evidence suggest a strong correlation between the macroscopic properties, such as Sv, and the microscopic structure as well as the dynamics. The diffusion coefficients were obtained by DLS as well as SANS, of which physical meanings are discussed by comparing the x dependence of Sv.
Introduction Hydrophobic interaction is one of the most important interactions in biological systems because it is involved in structure formation of protein and DNA and in biochemical reactions, such as enzyme reactions.1 To understand the role of hydrophobic interaction in structure formation, it is useful to study the structure of suitable model systems mimicking protein molecules.2-7 Poly(12acryloyloxydodecanoic acid (ADA)-co-acrylic acid (AA)) gel is such an interesting system, which consists of hydrophobic groups, ionizable groups, and groups capable of L’ORE Ä AL Recherche Tsukuba. Kyoto Institute of Technology. § The Graduate University for Advanced Studies. ⊥ Nihon University. # The University of Tokyo. * To whom correspondence should be addressed. F.I.: e-mail
[email protected], fax +81-298-47-7985, tel +81-29847-7984. M.S.: e-mail
[email protected], fax +81-29283-3922, tel +81-29-287-8904. † ‡
(1) Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J. D. Molecular Biology of the Cell; Garland Publishing Inc.: New York, 1994. (2) Uchida, M.; Kurosaka, M.; Osada, Y. Macromolecules 1995, 28, 4583. (3) Miyazaki, T.; Kaneko, T.; Gong, J. P.; Osada, Y. Macromolecules 2001, 34, 6024. (4) Matsuda, A.; Sato, J.; Yasunaga, H.; Osada, Y. Macromolecules 1994, 27, 7695. (5) He, X.; Oishi, Y.; Takahara, A.; Kajiyama, T. Polym. J. 1996, 28, 452. (6) He, X.; Takahara, A.; Kajiyama, T. Polym. Gels Networks 1996, 4, 315. (7) Masui, N.; Ikkai, F.; Karino, T.; Naito, S.; Kunugi, S.; Kurita, K.; Shibayama, M. Langmuir 2002, 18, 5092.
hydrogen bonding.2,7 In a previous paper,7 we discussed the effects of hydrogen bonding and hydrophobic interaction on the macroscopic equilibrium swelling behavior and the microscopic SANS profiles by changing the solvent composition of 1-propanol/water or propionic acid/water. The main conclusion was that, in addition to hydrophobic interaction between ADA side chains in water, hydrogen bonding took place between the carboxyl terminal groups and played an essential role in microstructure formation. In this work, we investigated how the hydrophobic interactions together with hydrogen-bonding effects contributed to the micro- and macrostructure in water/alcohol solutions. We used the ADA/AA gel system, having a 33: 67 (ADA/AA molar ratio), and an AA homopolymer gel swollen in ethanol (EtOH)/water (W) mixtures, where ADA and AA polymers behave as hydrophobic and hydrophilic polymers, respectively. It is noteworthy that ADA monomer is hydrophobic and hardly dissolves in water or methanol, while the hydrophilic poly(AA) is scarcely soluble in 1-propanol. The ADA/AA gels studied here were synthesized in EtOH. This means that the ADA/AA gels were not expected to have any aggregated structure in the preparation stage because of neutrality of EtOH toward both components. As will be shown later, the swelling behavior of ADA/AA copolymer gel or AA homopolymer gel was strongly correlated with monomer preferential solubility. As a matter of fact, it was found that in EtOH/W mixtures the AA part in the ADA/AA gels dissolved and swelled irrespective of solvent composition whereas the ADA part dissolved in mixtures with higher EtOH contents. In this paper, we discuss the changes in (i) the equilibrium swelling degree, (ii) the microstructure,
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(iii) the diffusion coefficient, and (iv) the degree of hydrophobicity of ADA/AA gels in various circumstances as a function of solvent composition.
〈IF〉T 〈I〉T,p 2 ) 〈I〉T,p DA,p DDLS DDLS
Theoretical Background
The slope and intercept correspond to 〈IF〉T and DDLS, respectively. The correlation length, ξDLS, is evaluated from DDLS via the following relation:14
DLS. Polymer gels are nonergodic systems in which polymer chains cannot travel freely in the phase space due to cross-links connecting the chains.8,9 Hence, thermal concentration fluctuations are partially frozen in space. Because of this nature, the ergodicity, normally applied to any thermodynamic system, is no more valid in polymer gels. Here, the ensemble average of a physical variable X, 〈X〉E, is not the same as that of time average, 〈X〉T, where 〈 〉E and 〈 〉T denote ensemble and time averages, respectively. The time-average intensity-time correlation func(2) (τ), tion (ICF) of polymer gels at a sample position p, gT,p is defined by (2) gT,p (τ) - 1 )
〈Ip(t) Ip(t+τ)〉T 〈Ip(t)〉T2
-1
∫0∞Gp(Γ) exp(-Γτ) dΓ]2
(2)
The ensemble average ICF is also defined by averaging (2) gT,p (τ) with a weight of 〈I(t)〉T,p, i.e.,10
g(2) E (τ) ≡
∑p 〈I(t) I(t+τ)〉T,p ∑p 〈I(t)〉T,p2g(2) p (τ) )
∑p 〈I(t)〉T,p2
∑p 〈I(t)〉T,p2
(3)
(7)
I(q) )
IOZ(0) 1 + ΞOZ2q2
+ Iexcess(q)
(8)
where ΞOZ is a static correlation length, corresponding to the averaged gel network size. Here, we used a symbol Ξ instead of ξ in order to discriminate the static correlation length from that of dynamic correlation length due to thermal fluctuations, ξ. For slightly cross-linked gels, the contribution of the excess scattering appears only at the low-q region and can be neglected for the analysis of ΞOZ. The static correlation function, G(r), is obtained from a SANS intensity function, I(q), as follows:
sin(qr) 4πq2 dq qr ) G(r) ) sin(qr) ∞ 2 4πq dq lim 0 I(q) rf0 qr 1 ∞ I(q) sin(qr)q dq r 0
∫0∞I(q) ∫
Because of nonergodicity, the collective diffusion coefficient obtained from the first cumulant is also position-dependent. Hence (2) (τ) - 1]/∂τ|τ)0 DA,p ) -(1/2q2) ∂ ln[gT,p
kT 6πηDDLS
where k, T, and η are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. SANS. The scattered intensity function, I(q), for polymer gels is given by a sum of an Ornstein-Zernike scattering function, valid for polymer solutions in semidilute regime, and an excess scattering function characteristic of gels, Iexcess(q), i.e.9
(1)
where t is the time and τ is the time lag. If there are many (2) (τ) is given with the decay-rate relaxation processes, gT,p distribution function, G(Γ), where Γ is the decay rate (2) (τ) - 1 ) [ gT,p
ξDLS ≈
(6)
(4)
Here, the subscripts A and p in DA,p indicate the apparent diffusion coefficient dependent on the sample position. The “true” collective diffusion coefficient obtained from DLS, DDLS, gives the lower and upper bounds of DA,p with the inequality DDLS/2 < DA,p < DDLS.11 For polymer gels well above the gelation threshold,10 DDLS is given by8,12 (2) (τ) - 1 ) Xp2 exp(-2DDLSq2τ) + gT,p
2Xp(1 - Xp) exp(-DDLSq2τ) (5) where Xp (≡ 〈IF〉T/〈I〉T,p) is the ratio of the intensity from the pure homodyne component (dynamic part) to the total intensity. Note that 〈IF〉T is independent of the sample position. DDLS can be extracted by plotting 〈I〉T,p/DA,p as a function of 〈I〉T,p13 (8) Pusey, P. N.; van Megen, W. Physica A 1989, 157, 705. (9) Shibayama, M. Macromol. Chem. Phys. 1998, 199, 1. (10) Takeda, M.; Norisuye, T.; Shibayama, M. Macromolecules 2000, 33, 2909. (11) Shibayama, M.; Fujikawa, Y.; Nomura, S. Macromolecules 1996, 29, 6535. (12) Horkay, F.; Burchard, W.; Hecht, A. M.; Geissler, E. Macromolecules 1993, 26, 3375. (13) Shibayama, M.; Norisuye, T.; Nomura, S. Macromolecules 1996, 29, 8746.
∫
∫0∞I(q)q2 dq
(9)
where r is the distance in the real space. Owing to the mode-mode coupling theory which allows one to link spatial correlations, G(r), with the dynamic properties, i.e., the diffusion coefficient, we can evaluate the collective diffusion coefficient from the SANS result.15 As is wellknown, the mode-mode coupling theory assumes decoupling two correlations, i.e., correlation of concentration fluctuations and velocity-velocity correlation. The former is given by G(r), and the latter ends up to the StokesEinstein relation; i.e., r ) kT/6πηD, where D is the diffusion coefficient. The cooperative diffusion coefficient, DSANS, of the gel network is defined by
kT dr ∫0∞4πr2G(r)6πηr DSANS ) ∫0∞4πr2G(r) dr
(10)
This relation is valid for polymer solutions in the semidilute regime, where G(r) is given by (14) Tanaka, T. In Dynamic Light Scattering; Pecora, R., Ed.; Plenum Publishing: New York, 1985; pp 347-362. (15) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University: Ithaca, NY, 1979.
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G(r) )
(
Ikkai et al.
)
1 r exp r ΞSANS
(11)
However, no attempts have been made for polymer gels that contain frozen concentration fluctuations. We try to examine the validity of eq 10 in this work. Experimental Section Preparation of ADA/AA and AA Gels. The ADA/AA gel was prepared by dissolving ADA monomer (0.5 M) and acrylic acid (AA) (4.5 M) in EtOH, followed by addition of 0.24 mol % of R,R′-azobis(isobutyronitrile) (initiator) and 1 mol % of N,N′methylenebis(acrylamide) (cross-linker). ADA monomer synthesis was described in detail in a previous paper.7 ADA/AA copolymerization was carried out for 24 h at 60 °C. Then the gel obtained was rinsed and conditioned with excess amount of EtOH and water, where the ratio was changed from EtOH-rich to W-rich compositions. The ADA content in the ADA/AA gel was determined by 1H NMR to be 32.7 mol %. The total monomer concentration was kept at 5.0 M. The AA homopolymer gel was prepared following a similar procedure. After drying, ADA/AA and AA gels were put under stirring in EtOH/W mixture with varying EtOH/W ratio for swelling, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) experiments. Swelling Measurement. Swelling measurements of the ADA/ AA gels were made with a cathetometer (Nippon Optical Works, Co.). The size of the gel was converted to the equilibrium swelling degree, Sv, according to eq 12:
Sv ≡
()
l volume of swollen gel ) volume of dried gel l0
3
(12)
where l and l0 are the lengths of the gel after swelling and of dried gel, respectively. DLS Experiments. DLS experiments were conducted using a DLS/SLS-5000 (ALV, Co. Ltd., Langen, Germany) with a 22 mW He-Ne laser (wavelength, λ ) 632.8 nm). The sample in an 8 mm test tube was placed in a decahydronaphthalene (decalin) bath thermostated at 25 ( 0.1 °C. The time-averaged scattered intensity, 〈I(q)〉T,p, and its time correlation, i.e., time-averaged intensity correlation function (ICF), g(2) T,p(τ) - 1, were obtained at a scattering angle of 90°, where p is the sample position arbitrarily chosen in the sample. q is the scattering vector, and the 90° angle corresponds to q ≈ 1.87 × 10-3 Å-1 () (4nπ/λ) sin(θ/2)), where n is the refractive index of solvent (n ) 1.33 for water and 1.36 for ethanol at 25 °C). The gel samples were put into a test tube and supported by a straw tube and then used for experiment. SANS Experiment. Small-angle neutron scattering (SANS) experiments were carried out using a two-dimensional SANS spectrometer (SANS-U) of Institute for Solid State Physics, University of Tokyo, Tokai, Japan. The SANS intensity function was collected at room temperature after 1.0 or 1.5 h respectively for 1 and 4 m sample-to-detector distance conditions. Gel samples in quartz cells with 4 mm optical path were irradiated by a neutron beam with a wavelength of 7.0 Å. The solvents were replaced by deuterated ones, i.e., combinations of D2O and deuterated ethanol (d-EtOH). The scattered intensity was circularly averaged and rescaled to the absolute intensity scale with a polyethylene secondary standard. The incoherent background subtraction was made by measuring dried ADA/AA and AA gels. The solvent correction of scattered intensity was made by using the following equation:
I(q) ) I(q)sample - (1 - 1/Sv)[yI(q)organic + (1 - y)I(q)D2O] - Iincoh(q)/Sv (13) where I(q)k denotes the observed scattered intensity in absolute scale of the kind k () sample, organic solvent, D2O, and incoherent of the bulk polymer) and y is the weight fraction of the organic component in the solvent.
Results and Discussion Swelling Behavior of AA and ADA/AA Gels in Alcohol/Water Solutions. Figure 1 shows the swelling
Figure 1. Swelling degree, Sv, as a function of solvent composition, x.
behavior of AA and ADA/AA gels as a function of alcohol content, x, (i.e., methanol, ethanol, or 1-propanol)/water mixtures. First of all, let us discuss the swelling behavior of AA gels. As a common feature, AA gels showed a bellshape swelling curve as a function of x. This phenomenon is explained as follows. For AA gels, the AA monomer consists of two parts, i.e., the hydrophilic carboxy group and hydrophobic ethylene backbone. The former part is weakly charged at neutral condition and more familiar with water than alcohols. On the other hand, the latter part has a higher solubility in alcohols than water. Therefore, in lower x, the solubility of AA monomer increases by the weakly charged carboxy part and decreases by the ethylene backbone. As a result, the “cosolvency” effect is expected when a mixed solvent is used as a swelling medium. It is essential to compare the cosolvency effect of the poly(AA) and poly(ADA/AA) gels with various types of alcohol.The AA gel in the methanol/water system (Figure 1a) swelled at lower x values, and the gel in the 1-propanol/ water system (Figure 1c) remarkably shrank at higher x values, i.e., x ≈ 100 vol %. The AA gel in the ethanol/water (EtOH/W) system (Figure 1b) showed the lowest Sv change with x. As shown in these figures, the cosolvency effect is different from one to another and is complicated. The peak position in Sv moves exclusively in the case of AA gels as the number of carbon in the alcohol increases. However, since our purpose in this study was to investigate structural changes related to hydrophobic ADA in ADA/
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Figure 2. Solvent composition, x, dependence of the ensemble average scattered light intensity, 〈I(q)〉E, for ADA/AA and AA gels. The inset shows speckle pattern for ADA/AA gels.
Figure 4. Variations of (a) DDLS and (b) ξDLS as a function of x.
(2) Figure 3. (a) Time-intensity correlation functions, gT,p (τ) 1, for ADA/AA gels and (b) linearization plot for estimation of the cooperative diffusion coefficient, DDLS, and the fluctuating component of the scattered intensity, 〈I F(q)〉T.
AA gels, a smaller contribution of AA parts to the gel structural changes was desirable. Therefore, in the following sections, we focus on structural changes of ADA/ AA gels in EtOH/W mixtures because of the following reason. Compared to the case of the poly(AA) gel, Figure 1 indicates that the ADA/AA gels have almost the same swelling curves irrespective of the type of alcohol used.
That is, the gel is almost in a shrunken state in water (i.e., x ≈ 0), where the hydrophobic long alkyl side chains of the ADA component tend to form intermolecular and/ or intramolecular hydrophobic aggregated structures. However, the gels gradually swelled with increasing x, indicating a disorganization of hydrophobic aggregations. A further increase in x results in reshrinking. This is likely to be due to the same features as seen in AA gels, i.e., the nonsolvency effect of EtOH to the AA side chains. DLS. Figure 2 shows the x dependence of the ensembleaveraged scattered intensity, 〈I(q)〉E, for AA and ADA/AA gels. The 〈I(q)〉E was obtained by averaging light scattered intensities at 100 different sample positions arbitrarily chosen in the gel. The inset shows the variation of scattered intensity with sample position, i.e., a speckle pattern, for ADA/AA gels at x ) 0 vol %. The horizontal solid line denotes the ensemble-averaged scattered intensities, 〈I(q)〉E. The 〈I(q)〉E of AA gels is almost independent of x, although that of ADA/AA gels is strongly x dependent and increases with lowering x, indicating the formation of a hydrophobic aggregated structure. According to the q range of observation, the size of aggregation is speculated to be over more than a few thousand angstroms. For each of the data points in the inset of Figure 2, we obtained the time-correlation function, g(2) T,p(τ) - 1. Figure 3a shows the representative 10 correlation functions at 10 different sample positions out of 100 speckles. Although no vertical shift was taken for each of the correlation function, the magnitudes of g(2) T,p(τ) - 1 are scattered large enough to avoid overlap. This is due to strong nonergodicity of the system. Note that the initial amplitude of g(2) T,p(τ) - 1, i.e.,
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Figure 5. SANS intensity functions, I(q), for (a) AA and (b) ADA/AA gels.
g(2) T,p(0) - 1, is strongly dependent on the sample position. Figure 3b shows the plots on the basis of eq 6,11 from which 〈IF(q)〉T and DDLS were evaluated. Figure 4 shows the x dependence of (a) DDLS and (b) ξDLS obtained by DLS. DDLS for the AA gels is less dependent on x compared to that of ADA/AA gels. Since the AA gels are highly swollen in EtOH/W mixtures, the relative variation of DDLS is much smaller than the ADA/AA systems. On the other hand, DDLS of the ADA/AA gels increases with x. This indicates that the higher the x, the smaller the frozen inhomogeneities due to disorganization of ADA-rich aggregates. Because of the strong x dependence of the solvent viscosity, ξDLS cannot be simply obtained via eq 7 by taking an inverse of DDLS. Indeed, η of the water/EtOH mixture has a bell-shape curve as a function of x, showing a maximum at around x ) 50 vol %, e.g., η ) 0.001 × 10-3 N s/m 2 (x ) 0), 0.002807 × 10-3 N s/m 2 (x ) 50), and 0.001201 × 10-3 N s/m 2 (x ) 100).16 This is partially due to densification of the solvent when mixing dissimilar molecules, i.e., water with EtOH. It is rather surprising that the viscosity changes by 2-fold by mixing. This strong solvent dependence has to be taken into consideration in the analysis of dynamic light scattering data. As a result, the obtained ξDLS is a monotonically decreasing function of x for the case of ADA/AA, while that of AA gel is rather constant. This strong x dependence of ξDLS for the ADA/ AA gels indicates that by decreasing x (i.e., increasing water content) the gel approaches spinodal due to reduction of the miscibility of ADA in water. (16) Wolf, A. V., Brown, M. G.,Prentiss, P. G., Eds.; CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1986; Vol. 66.
Figure 6. Spatial correlation function, G(r), for (a) AA and (b) ADA/AA gels.
SANS. Figure 5 shows SANS intensity functions, I(q), of (a) AA gels and (b) ADA/AA gels in EtOH/W with various values of x. I(q)s of AA gels were monotonically decreasing functions of the scattering vector, q. On the other hand, I(q)s of ADA/AA gels had a long spacing correlation peak at q ) ca. 0.1 Å-1. This characteristic scattering peak became broader with increasing x and disappeared for x > 25 vol %. Changes in the microstructure are more clearly shown by a correlation function analysis as shown in Figure 6a,b. The correlation function, G(r), for ADA/AA gels is found to oscillate at x ) 0 and 25 vol %. Though the peak positions at r ) 46 Å were independent of x, the peak height decreased with x. This peak is ascribed to a microphase separation between the hydrophobically aggregated part and the hydrophilically swelling part. Except for these two G(r)s for ADA/AA gels that show a peak, all of the other G(r)s are monotonically decreasing functions of r, indicating the absence of any aggregatedtype structure. Hence, we estimated the correlation length by two approaches. One is to use the Ornstein-Zernike scattering function, given by the first term on the righthand side of eq 8. Figure 7 shows the Ornstein-Zernike plots, i.e., 1/I(q) vs q2, for (a) AA and (b) ADA/AA gels. There appeared a linear region in 1/I(q) around 0.0001 < q2 < 0.0005 Å-2. From the slope of the linear regression of this region, ΞOZ was estimated, which is displayed in Figure 8. The shape of the plot of ΞOZ vs x is very similar to the swelling behavior of the EtOH case as shown in Figure 1. This indicates
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Figure 9. Variations of DSANS as a function of x for ADA/AA and AA gels.
Figure 7. Ornstein-Zernike plots of (a) AA and (b) ADA/AA gels.
Figure 8. Variations of ΞOZ as a function of x for ADA/AA and AA gels.
that an analogy holds between the microscopic and macroscopic swelling when changing solvent composition. The lack of data points of ΞOZ of ADA/AA gels at x ) 0 and 25 vol % is due to the appearance of the broad peak in I(q). A second approach to estimate the static correlation length was to evaluate the diffusion coefficient from the SANS static structure factor on the basis of the modemode coupling theory as discussed in the theoretical section. Figure 9 shows the x dependence of DSANS evaluated with eq 10. Interestingly, DSANS is a concave function of x, as opposed to the x dependence of Sv. The magnitude of DSANS is significantly lower than DDLS by a factor of ca. 3 although their x dependence is similar to
Figure 10. Schematic diagram showing the scattering vector ranges for evaluation of two different types of diffusion coefficients and correlation lengths.
each other. Since G(r) used for evaluation of DSANS contains contribution of frozen concentration fluctuations, DSANS might be underestimated. Comparison of Diffusion Coefficients Obtained by the Two Methods. Let us discuss the physical meaning of DDLS and DSANS. First of all, the q range is very different between DLS and SANS as shown in Figure 10. The q value employed in the DLS measurement was 0.001 Å-1, where only the translational mode of polymer chains is observed. On the other hand, the SANS covered 0.008 < q < 0.2 Å-1, corresponding to the mesh and/or blob size of polymer network. Furthermore, since DDLS is evaluated from the dynamic component of the scattered intensity, it corresponds to the dynamics of the polymer chains. On the other hand, DSANS contains information about frozen inhomogeneities since it is obtained from G(r), which has both dynamic and static contributions in the case of polymer gels.9 These facts lead to different physical meanings in diffusion coefficient and correlation length. As a result, this may be why DDLS is larger than DSANS. In other words, faster dynamics is observed by DLS than by SANS. However, for x g 50 vol %, both DDLS and DSANS increase with x. This indicates that the system recovers the original structure as was prepared in ethanol. This also leads to a constant value of ξDLS and ΞOZ. It is clear that the maximum in ΞOZ is due to strong swelling and increase in inhomogeneities as reported by Mendez et al.17 and Shibayama et al.18 However, such a (17) Mendes, E.; Oeser, R.; Hayes, C.; Boue, F.; Bastide, J. Macromolecules 1996, 29, 5574. (18) Shibayama, M.; Shirotani, Y.; Shiwa, Y. J. Chem. Phys. 2000, 112, 442.
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strong x dependence does not exist in ξDLS and is simply decreasing with x for both gels. This may suggest that the correlation length obtained from DLS means a range of concentration fluctuations rather than the range of spatial correlation. From this respect, it should be stressed that DLS and SANS provide different types of information. Conclusion The swelling behavior and structure of AA and ADA/ AA gels in EtOH/W mixtures have been investigated from microscopic points of view using small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The gels swelled with increasing x (vol % of EtOH) until x ) 50-60 vol % for AA gels and x ) 70-80 vol % for ADA/AA gels and then decreased gradually. This phenomenon was explained by hydrophobic association of long alkyl side chains of ADA and a cosolvency effect of poly(AA) in EtOH and W. A strong correlation was observed between macroscopic swelling degree and the microscopic correlation lengths. Evaluation of collective diffusion coefficients and correlation lengths from both DLS and SANS disclosed characteristic features of gels different from corresponding polymer solutions. Those are originated from the presence of cross-links and inhomogeneities. The diffusion coef-
Ikkai et al.
ficient obtained from DLS is about 3 times larger than that from SANS. This is explained that the DSANS is suppressed by inhomogeneities, while DDLS only corresponds to the dynamics of mobile chains like those in polymer solutions. It is also noted that the mode-mode coupling theory was successfully employed to evaluate DSANS. The macroscopic swelling properties of copolymer gels in a mixed solvent, such as ADA/AA gels studied in this work, are very complicated. However, it is essential to understand structure formation of higher-order structure of protein molecule in native state since protein is a heteropolymer and is immersed in various environments. This study demonstrates the close relationship among the swelling behavior (i.e., conformation), local structure, and local chain dynamics of heteropolymer. Acknowledgment. This work is supported by the Ministry of Education, Science, Sports and Culture, Japan (Grant-in-Aid, Nos. 14045216 and 14350493). This work was performed with the approval of Institute for Solid State Physics, The University of Tokyo (Proposal No. 011596), at Japan Atomic Energy Research Institute, Tokai, Japan. LA026605L
