Langmuir 2002, 18, 7377-7383
7377
Transport Properties of Comb-Type Grafted and Normal-Type N-Isopropylacrylamide Hydrogel Masahiko Annaka,*,† Masaaki Sugiyama,‡ Masaki Kasai,† Takayuki Nakahira,† Toyoaki Matsuura,§ Hiroko Seki,| Takao Aoyagi,⊥ and Teruo Okano# Department of Materials Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan; Department of Physics, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka-shi, Fukuoka 812-8581, Japan; Department of Ophthalmology, Nara Medical University, 840, Shijyo-cho, Kashihara-shi, Nara 634-8522, Japan; Chemical Analysis Center, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan; Graduate School of Science and Engineering, Kagoshima University, Korimoto, kagoshima-shi, Kagoshima 890-0065, Japan; and Graduate School of Medicine and Institute of Biomedical Engineering, Tokyo Women’s Medical University, 8-1, Kawada-cho, Shinjyuku-ku, Tokyo 162-8666, Japan Received April 29, 2002. In Final Form: July 5, 2002 Transport properties of the comb-type grafted and normal-type N-isopropylacrylamide (NIPA) gel are investigated by gel filtration chromatography and a pulsed field gradient NMR technique with known standard probe molecules in terms of molecular weight cutoff and diffusion coefficient in and through the gels, respectively. The incorporation of NIPA grafted chains into NIPA gel makes the effective mesh size smaller and induces the additional friction between the probe molecule and gel network. One may control the maximum mesh size of the NIPA gel and thus the effective diffusivity of a probe molecule within a gel by incorporating grafted chains and by adjusting the temperature.
1. Introduction In recent years, considerable research attention has been focused on hydrogels that are able to alter their volume and properties in response to environmental stimuli such as temperature, pH, and ionic strength.1-11 Because of their drastic swelling and deswelling in response to environmental stimuli, these polymeric hydrogels have been investigated for many biomedical and pharmaceutical applications, including controlled drug delivery, molecular separation, tissue culture substrate, and materials for improved biocompatibility.12-16 Among these “intelligent” gels, temperature-sensitive hydrogels * To whom correspondence should be addressed. † Department of Materials Technology, Chiba University. ‡ Kyushu University. § Nara Medical University. | Chemical Analysis Center, Chiba University. ⊥ Kagoshima University. # Tokyo Women’s Medical University. (1) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820. (2) Tanaka, T.; Fillmore, D. J.; Sun, S. T.; Nishio, I.; Swislow, G.; Shah, A. Phys. Rev. Lett. 1980, 45, 1636. (3) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (4) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (5) Kiler, J.; Scranton, A. B.; Peppas, N. A. Macromolecules 1990, 23, 4944. (6) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345. (7) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Nature 1995, 374, 240. (8) Okano, T.; Bae, Y. H.; Kim, S. W. In Pulsed and Self-Regulated Drug Delivery System; Kost, J., Ed.; CRC Press: Boca Raton, FL, 1990; p 17. (9) Okano, T.; Yui, N.; Yokoyama, M.; Yoshida, R. Advances in Polymeric Systems for Drug Delivery; Gordon & Breach Science Publishers: Yverdon, Switzerland, 1994; p 67. (10) Hoffman, A. S. J. Controlled Release 1987, 6, 297. (11) Dusek, K., Ed. Responsive Gels: Volume Phase Transition II; Springer-Verlag: Berlin, 1993. (12) Okano, T., Ed. Biorelated Polymers and Gels: Controlled Release and Applications in Biomedical Engineering; Academic Press: New York, 2000. (13) Yoshida, R.; Kaneko, Y.; Sakai, K.; Okano, T.; Sakurai, Y.; Bae, Y. H.; Kiim, S. W. J. Controlled Release 1994, 32, 97.
are most widely investigated. Since the effective diffusivity of a certain solute within a hydrogel is proportional to the mesh size, quantitative estimation of the mesh sizes is of great interest. In particular, temperature-dependent mesh size and probe diffusion within the gel should be elucidated in detail for bioseparation process. One of the physical quantities which characterizes the transport properties in and though gels is the diffusion coefficient. The probe molecules thermally fluctuate in time and space in a simple fluid. The fluctuation and friction ζ between the probe molecule and the fluid determine the diffusion coefficient of the probe molecules in the solution.
D ) kBT/ζ
(1)
Here, kB and T are Boltamann’s constant and temperature, respectively. The hydrodynamic friction of the fluid of viscosity η that is experienced by a probe molecule of hydrodynamic radius Rh is given by the following equation:
ζ ) 6πηRh
(2)
The probe molecules, when introduced into the gel, however, experience additional friction by the gel network as well as the friction of the fluid. Therefore, the diffusion of the probe molecule largely depends on the mesh size of the gel network and gel architecture as well as the size of the probe molecule. The rate of gel swelling and shrinking is strongly dependent on the size of the gel. On the basis of the cooperative diffusion of a polymer network in a medium, (14) Aoki, T.; Kawashima, M.; Katono, H.; Sanui, K.; Ogata, N.; Okano, T.; Sakurai, Y. Macromolecules 1994, 27, 947. (15) Hoffman, A. S.; Afrassiabi, A.; Dong, L. C. J. Controlled Release 1986, 4, 213. (16) Peppas, N. A. Hydrogels in Medicine and Pharmacy; CRC Press: Boca Raton, FL, 1987; Vol. III.
10.1021/la025876t CCC: $22.00 © 2002 American Chemical Society Published on Web 08/15/2002
7378
Langmuir, Vol. 18, No. 20, 2002
Annaka et al.
Tanaka-Fillmore theory indicates that the characteristic time of gel swelling and shrinking is described by the following equation.17
τ≈
R2 Dcoop
(3)
where R and Dcoop are the size of the gel and the cooperative diffusion coefficient of the gel network, respectively. For typical polymer gels, Dcoop is on the order of 10-7 to 10-6 cm2/s, depending on polymer concentration, cross-linking density, and so forth. Since it is not easy to increase the value of Dcoop by a factor of 102 or more, a reduction of gel size has been considered to be the only way to achieve quick response. Recently Okano et al., however, have shown that the rate of gel shrinking could be accelerated by introducing comb-type grafted chains; the shrinking rate is 100 times faster than that for the conventional NIPA gel.7,18,19 Dangling chains in a gel can easily collapse on an external stimulus because one side of the chain is free, which induces a strong shrinking tendency in a gel. The unique architecture of a comb-type grafted gel is expected to be utilized for a number of applications in the area of biotechnology, that is, drug delivery systems, sizeselective bioseparation, and enzyme and cell immobilizations, soon. The grafted chains induce the additional friction between the probe molecule and the gel network; therefore, the transport properties in and though the combtype grafted gel are expected to be quite different from those in and though the normal-type gel. In this study, transport properties of the comb-type grafted and normal-type N-isopropylacrylamide (NIPA) gels are investigated. The mesh sizes of the gels are characterized using gel filtration chromatography in terms of molecular weight cutoffs with known standard probe molecules. The probe diffusion in the comb-type grafted NIPA gels is measured by a pulsed field gradient NMR technique under various experimental conditions: varying concentration of the gel and varying molecular weight of the probe molecules. Then the results are discussed together with those obtained form the normal-type NIPA gel. 2. Experimental Section 2.1. Materials. N-Isopropylacrylamide (NIPA; Kohjin Co.) was recrystallized form the mixture of toluene and n-hexane. Tetrahydrofuran (THF; Kanto Chemical Co., Ltd.), diethyl ether (Kanto Chemical Co., Ltd.), cyclohexane (Kanto Chemical Co., Ltd.), and acryloyl chloride (Aldrich) were purified just before use according to standard procedure. 2-Hydroxyethanethiol (Tokyo Kasei Kogyo Co., Ltd.), benzoyl peroxide (Nakarai Tesque Co.), N,N′-methylenebisacrylamide (BIS; Kanto Chemical Co.), N,N′tetramethylethylenediamine (TEMED; Kanto Chemical Co.), ammonium peroxide (APS; Kanto Chemical Co.), solbitane monooleate (Tokyo Kasei Kogyo Co., Ltd.), benzoic acid (Tokyo Kasei Kogyo Co., Ltd.), and poly(ethyleneglycol) (PEG; Tokyo Kasei Kogyo Co., Ltd.) were used as received. The probe molecules, vitamin B12, aprotinin, cytochrome c, ovalbumin, and blue dextran were obtained from Sigma. 2.2. Macromonomer Synthesis. A semitelechelic NIPA polymer with a terminal hydroxyl end group (PNIPA-OH) was prepared by radical telomerization of NIPA monomer using HESH as a chain transfer agent (Figure 1). NIPA (33.9 g, 0.3 mol), 2-hydroxyethanethiol (0.469 g, 6.0 mmol), and benzoyl peroxide (0.121 g, 0.5 mmol) were dissolved in THF (100 mL). (17) Tanaka, T.; Fillmore, D. J. Chem. Phys. 1979, 70, 1214. (18) Kaneko, Y.; Sakai, K.; Kikuchi, A.; Yoshida, R.; Sakurai, Y.; Okano, T. Macromolecules 1995, 28, 7717. (19) Kaneko, Y.; Nakamura, S.; Sakai, K.; Kikuchi, A.; Aoyagi, T.; Sakurai, Y.; Okano, T. Polym. Gels Networks 1998, 6, 333.
Figure 1. Preparation of N-isopropylacrylamide (NIPA) macromonomer.
Figure 2. Preparation of the comb-type grafted NIPA gel. The ampule containing the monomer solution was degassed by freeze-thaw cycles and was sealed in vacuo. Polymerization was carried out at 70 °C for 15 h. The reactant was pored into diethyl ether (2000 mL) to precipitate semitelechelic PNIPA-OH. Semitelechelic PNIPA-OH was collected by filtration and was purified by repeated precipitation in diethyl ether from acetone. Polymer was isolated by freeze-drying from aqueous solution. Semitelechelic PNIPA-OH was dissolved in acryloyl chloride (large excess), and the reaction was stirred at 40 °C for 2 h under nitrogen atmosphere. The reactant was poured into diethyl ether (2000 mL) to precipitate NIPA macromonomer. NIPA macromonomer was collected by filtration and was purified by repeated precipitation in diethyl ether from acetone. NIPA macromonomer was isolated by freeze-drying from aqueous solution. 2.3. Preparation of NIPA Gel Beads. NIPA beads for a gel filtration chromatography experiment were prepared by inverse suspension polymerization using cyclohexane as the continuous phase and solbitane monooleate as nonionic polymeric surfactant (Figure 2). x mol of NIPA, (1.38 - x) mol of NIPA macromonomer, and 0.013 g of BIS were dissolved in 5 mL of deionized, distilled water containing 10 mg of APS, and the mixture was bubbled with dry nitrogen to remove dissolved oxygen. This solution was immediately poured into 200 mL of cyclohexane containing 100 mL of solbitane monooleate, which was previously purged with dry nitrogen. The reaction mixture was stirred at 200 rpm under nitrogen atmosphere. After formation of aqueous droplets in continuous phase was confirmed, 30 mL of TEMED was added to the continuous phase to initiate polymerization. The polymerization was allowed to proceed for 3 h at 15 °C. After polymerization, the beads were separated from the oil phase and were washed several times with methanol and water. The feed compositions of monomers and other chemicals are listed in Table 1. The molar ratio of NIPA macromonomer based on total monomer was chosen to be 30 mol %, which is designated by GG30. 2.4. Swelling Experiments. A gel bead was placed in a glass cell, the temperature of which was controlled to within 0.1 °C of the desired temperature. The swelling degree of the gel, d/d0, was obtained by measuring the diameter of the gel bead, d, under a microscope. d0 is the diameter at gel preparation. 2.5. Gel Filtration Chromatography. We employed a gel filtration chromatography method to investigate mesh sizes of NIPA gels as reported by Park and Hoffman.20-22 1.5 g of freeze(20) Park, T. G.; Hoffman, A. S. Biotechnol. Prog. 1994, 10, 82. (21) Park, T. G.; Hoffman, A. S. Appl. Biochem. Biotechnol. 1988, 19, 1. (22) Dong, L. C.; Hoffman, A. S. In Reversible Polymeric Gels and Related Systems; Russo, P., Ed.; ACS Symposium Series 350; American Chemical Society: Washington, DC, 1987; p 236.
Transport Properties of N-Isopropylacrylamide Hydrogel
Langmuir, Vol. 18, No. 20, 2002 7379
Table 1. Feed Composition for Preparation of Normal-Type NIPA Gel (NG) and Comb-Type Grafted NIPA Gel (GG)a sample code
NG
GG30
NIPA macromonomer (mol %) NIPA monomer (mol/L) NIPA macromonomer (mol/L) BIS (g) TEMED (µL) APS (g) deionized, distilled water (mL) solbitane monooleate (µL) cyclohexane (mL)
0 1.38
30 0.96 0.42 0.013 30 0.010 5.0 100 200
0.013 30 0.010 5.0 100 200
a [NIPA macromonomer]/([NIPA macronomer] + [NIPA monomer]) ) 0.3. Mn ) 6940; Mw ) 5065; Mw/Mn ) 1.37.
Table 2. Feed Composition for Preparation of Normal-Type NIPA Gel and Comb-Type Grafted NIPA Gel for Self-Diffusion Measurementa sample code
NG
GG30
NIPA macromonomer (mol %) NIPA monomer (mol/L) NIPA macromonomer (mol/L) BIS (g) TEMED (µL) APS (g) D2O/H2O (10/90 in volume) (mL) probe molecule (PEG) (g)
0 1.38x 0 0.013x 30 0.010 5.0 0.5
30 0.96y 0.42y 0.013y 30 0.010 5.0 0.5
a The sample gel was prepared directly in the NMR tube (i.d. 5 mm). x, y ) 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0.
dried beads were allowed to swell for 12 h in 0.1 M phosphate buffer (pH 7.4) at 20 °C. The swollen gel beads were packed into a water-jacketed glass column (10 mm i.d. × 300 mm) and fully equilibrated with 0.1 M phosphate buffer (pH 7.4) at the desired temperature. A mixture of five probe molecules consisting of 100 µL each of 4.0 mg/mL ovalubumin, 8.0 mg/mL cytochrome c, 4.0 mg/mL aprotinin, and 4.0 mg/mL vitamin B12 was loaded onto the top of the gel bed surface. The flow rate of the elution buffer was set at 0.10 mL/min, and 30 drops of eluent were collected in each test tube. The absorbance of each fractional eluent was measured at 280 nm using a spectrophotometer (HITACHI U-3210). The void volumes of each column packed with different NIPA gel beads were determined separately with blue dextran (Mw 2 × 106). Here we defined the molecular weight cutoff as an exclusion limit of gel that is directly related to the mesh size. The molecular weight cutoff of each gel was determined by the plot of the molecular weight of the probe molecules, Mw against Ve/ V0, where Ve is the elution volume of a specific probe molecule in the profile and V0 is that of blue dextran. By the extrapolation to the axis of Ve/V0 ) 1, the molecular weight cutoff can be estimated from the intercept. 2.6. Self-Diffusion Measurements. Measurements of the probe diffusion in NIPA gels were performed on a JEOL LA-400 NMR spectrometer, equipped with a pulsed field gradient (PFG) apparatus (90 G/cm), at the frequency 400 MHz. The temperature was controlled at 15 ( 0.5 °C. The sample gels were prepared by radical polymerization of NIPA monomer, NIPA macromonomer, and BIS with probe molecules in D2O under the existence of 10% (w/v) probe molecule. The concentration of the NIPA monomer was changed from 0.69 to 6.9 mol/L, while the concentration of the cross-linker was fixed at 1.3 mol %. The probe molecules used in this study were water and PEG with different molecular weights: 200, 400, 1000, and 1500. The feed compositions of monomers and other chemicals are listed in Table 2. 2.7. Small-Angle X-ray Scattering (SAXS). SAXS experiments were carried out with the BL-10C installed at Photon Factory (Tsukuba, Japan). An incident X-ray beam from the synchrotron orbital radiation was monochromatized to 1.49 Å. The scattered X-ray was detected by a one-dimensional positionsensitive proportional counter positioned 1 m from the sample: the magnitude of the observed scattering vector ranged from 0.01 to 0.15 Å-1. The gel samples were sealed in a cell, the
Figure 3. Representative optical micrographs of (a) normaltype and (b) comb-type grafted NIPA gel beads in a swollen state obtained by the inverse suspension polymerization technique. temperature of which was controlled to within 0.1 °C of the desired temperature. The intensities were accumulated for 600 s in order to ensure sufficient statistical accuracy without degrading the gel samples by X-ray irradiation. The scattered intensities were corrected for the cell scattering and absorption, and then normalized with the thickness of the sample and irradiated time.
3. Results and Discussion 3.1. Preparation of NIPA Hydrogels. NIPA macromonomer was prepared by radical telomerization of NIPA monomer using 2-hydroxyethanethiol as a chain transfer agent. To confirm the polymerization of a semitelechelic PNIPA-OH, 1H NMR spectroscopy measurements were carried out. In 1H NMR spectroscopy (JEOL LA-400) measurements, the spectrum of semitelechelic PNIPAOH exhibited peaks at 1.1 ppm (-CH3) and 4.0 ppm (-CH-), while the peak at 2.7 ppm was due to chain transfer agent. Two broad peaks at 1.6 and 2.2 ppm due to methylene proton and methyne proton were observed, while the peaks of vinyl proton at 5.8-6.2 ppm were not detected. The molecular weight distribution of semitelechelic PNIPA-OH was determined by gel permeation chromatography apparatus23 in DMF with 10 mM LiCl. The number-average (Mn) and weight-average (Mw) molecular weights were 5065 and 6940, respectively (Mw/Mn ) 1.37). Comb-type grafted NIPA gels were prepared by radical polymerization of NIPA monomer and NIPA macromonomer in the presence of BIS as a cross-linker. The molar ratio of NIPA macromonomer to total amount of NIPA monomer (NIPA monomer + NIPA macromonomer) was chosen to be 30 mol %. Normal-type NIPA gel was also prepared without adding NIPA macromonomer. The weight conversions of graft-type gels and normal-type gel from monomers are 84% and 86%, respectively. Representative optical micrographs of the graft-type and normaltype NIPA gel beads are shown in parts a and b, respectively, of Figure 3. In the case of a water-in-oil system, it was reported that the monomer acted as a cosurfactant; for example, the stable dispersion domain in the phase diagram is considerably extended by addition of neutral or ionic monomers.24-26 NIPA macromonomer has a long side chain (Mn ) 5065) and is, therefore, expected to act as cosurfactant. It is, therefore, interesting to compare the distribution of the diameter of the combtype grafted NIPA gel to that of the normal-type NIPA gel (23) The molecular weight of semitelechelic PNIPA-OH was estimated by gel permeation chromatography, using a TSKgel GMPWHR column and a TOSOH RI-8022 detector. HPLC grade DMF with 10 mM LiCl was used as the mobile phase at the flow rate 1 mL/min. Calibration was carried out with monodisperse poly(ethylene glycol) standards purchased from TOSOH Corp. (24) Holtzscherer, C.; Candau, F. Colloids Surf. 1988, 29, 411. (25) Candau, F.; Leong, Y. S.; Pouyet, G.; Candau, S. J. Colloid Interface Sci. 1984, 101, 167. (26) Candau, F.; Zekhnini, Z.; Durand, J. P. Colloid Interface Sci. 1986, 104, 398.
7380
Langmuir, Vol. 18, No. 20, 2002
Annaka et al.
Figure 4. Equilibrium swelling degrees, d/d0, of normal-type gel and graft-type beads plotted as a function of temperature. Table 3. Probe Molecules for Gel Filtration Chromatography probe molecule
Mw
benzoic acid vitamin B12 aprotinin cytochrome c ovalbumin
122 1355 6500 12400 44000
prepared under the same conditions. The copolymerization of NIPA macromonomer results in a stable dispersion with a narrow size distribution: the average diameter is 330 µm with a standard deviation of 27.4 µm. On the other hand, without NIPA macromonomer, gel beads with a broad size distribution were obtained: the average diameter is 278 µm with a standard deviation of 68.3 µm. 3.2. Equilibrium Swelling Degree. Equilibrium swelling degrees of graft-type and normal-type gel beads are plotted as a function of temperature in Figure 4. The phase behaviors indicate that the graft-type gels show the same transition temperature (34 °C) as that of the normal-type gel. Graft-type gels exhibit higher swelling ratio than the normal-type gel in both swollen and collapsed states. As the grafted chains are structurally separated from the backbone cross-linked network, stronger hydration may be possible. This chain expansion is considered to result in increased hydration in grafttype gels. Controlling the amount of grafted chain can regulate the equilibrium swelling properties of hydrogels. 3.3. Gel Filtration Chromatography. The precise determination of mesh size in a hydrogel is quite difficult with the conventional electron microscope technique, since the inherent mesh structure cannot be preserved during the sample preparation. In this study, a gel filtration method was used to estimate the mesh size as reported by Park and Hoffman.20 The reswollen gel beads from the freeze-dried state are unchanged from intact gels. The reswollen gels regenerate the volumes before freezedrying, and exhibited the same temperature-sensitive properties. The elution profiles of six probe molecules having different molecular weights (Table 3) for normaltype and comb-type grafted NIPA gels at 15 °C are shown in Figure 5. In the case of normal-type NIPA gel, six peaks are clearly identified; they are void volume, ovalbumin, cytochrome c, aprotinin, and vitamin B12. This result suggests that the molecular weight cutoff value of the normal-type NIPA gel is at least larger than the molecular weight of ovalbumin (44000). On the other hand, in the case of comb-type grafted NIPA gel, four peaks are observedsvoid volume, aprotinin, vitamin B12, and benzoic acidseven in the swollen state at 15 °C. It can be seen that the elution profiles are largely dependent on the gel architecture. The molecular weight cutoffs for normaltype and comb-type grafted NIPA gels are estimated by
Figure 5. Elution profiles of six probe molecules having different molecular weights (as shown in Table 3) for (a) normaltype and (b) comb-type grafted NIPA gels at 15 °C.
Figure 6. Plot of molecular weight (Mw) vs Ve/V0 for normaltype and comb-type grafted NIPA gels at 15 °C. Ve is the elution volume of a specific probe molecule in the profile, and V0 is the void volume determined from the elution volume of blue dextran. The intercepts of the straight lines and the dotted line at Ve/V0 ) 1 correspond to the molecular weight cutoffs of the gels. Table 4. Molecular weight cutoffs for gels sample code
molecular weight cutoff
NG GG
1.27 × 105 1.27 × 104
plotting the molecular weight of probe molecules, Mw, against Ve/V0, where Ve is the elution volume of a specific probe molecule in the profile and V0 is that of blue dextran. Figure 6 shows the Mw versus Ve/V0 plot for normal-type and comb-type grafted NIPA gels at 15 and 35 °C. The estimated molecular weight cutoffs for the normal-type and comb-type grafted NIPA gels, respectively, are 1.27 × 105 and 1.25 × 104 at 15 °C (Table 4); the molecular weight cutoffs of the comb-type grafted NIPA gels are smaller than that of normal-type NIPA gel by a factor of 10. On the other hand, benzoic acid is eluted progressively earlier for both normal-type and comb-type grafted NIPA gels at 35 °C (above the transition temperature), all the probe molecules are not retained, and molecular weight cutoffs for both types of gel cannot be estimated. This is due to the collapse of pores in the gels hindering the diffusional penetration of the probe molecules. From these results, therefore, the incorporation of NIPA grafted chains into NIPA gel makes the effective mesh size smaller. The effective diffusivity of a probe molecule within a gel is proportional to the mesh size and amount of free water in the gel. One may control the maximum
Transport Properties of N-Isopropylacrylamide Hydrogel
Langmuir, Vol. 18, No. 20, 2002 7381
Figure 8. Diffusional spin-echo attenuation of HDO (circles) and the methylene proton of PEG (Mw ) 1000) (squares) in the normal-type (open symbols) and the comb-type grafted (solid symbols) NIPA gels by varying the field gradient duration (δ) at 15 °C. The straight line in this figure is obtained by leastsquares fitting.
Figure 7. SAXS scattered intensity profiles for NG (open circles) and GG (solid circles) at 15 °C: (a) linear plot; (b) Ornstein-Zernicke plot. The straight lines in part b indicate the fit with eq 4.
mesh size of the NIPA gel by incorporating grafted chains and also by varying the temperature. In this study, the average distance between cross-links is considered to be the same for both normal-type and comb-type grafted NIPA gels, assuming the same reactivity ratio of NIPA macromonomer, NIPA, and BIS. In this case, the combtype grafted NIPA gel is expected to have a homogeneous gel structure. We carried out an SAXS experiment to confirm the microscopic structure of the gels. Figure 7 shows (a) linear and (b) Ornstein-Zernicke plots of SAXS intensity profiles for NG and GG30 at 15 °C. As shown in Figure 7a, the scattered intensities for NG and GG decrease monotonically with the scattering magnitude q. The shapes of the scattering spectra of gels look rather similar to those of semidilute solutions, and they well support the absence of microphase separated structures for NG and GG under the conditions studied here. The increase in intensity at low q is due to the introduction of permanent cross-links to the system, which is generally observed in gels.27 Figure 7b shows that both NG and GG30 are well fitted to an Qrnstein-Zernicke form
I(q) )
I(0) (1 + ξ2q2)
(4)
where ξ is the correlation length.28 This was done in analogy with the procedure employed for semidilute solutions. From Figure 7b, the correlation lengths, ξ, for NG and GG30 at 15 °C are given: 28.8 Å and 47.6 Å, respectively. The difference in correlation length, ξ, is probably due to the freely mobile characteristics of NIPA graft chains. As the grafted chains are structurally separated from the backbone cross-linked network, they behave like a polymer solution, which makes the dynamic fluctuation of the polymer density for GG30 larger than that for NG. (27) Benoıˆt, H.; Picot, C. Pure Appl. Chem. 1966, 12, 545. (28) Hecht, A. M.; Duplessix, R.; Geissler, E. Macromolecules 1985, 18, 2167.
Figure 9. (a) Diffusion coefficients of the probe molecules in the normal-type NIPA gel as a function of the concentration of the gel at 15 °C. (b) Diffusion coefficients of HDO and PEG 1000 in the normal-type and comb-type grafted NIPA gels at 15 °C.
3.4. Self-Diffusion Measurements. The determination of diffusion coefficients by NMR using PFG is based on the work of Stejskal and Tanner29 in the basic gradient echo experiment (90°-t(PFG)-180°-t(PFG)-acq); the strengths of the PFG pulses are increased in the series of spectra. The intensity of the echo amplitude decreases as a consequence of the change in the spatial position of the molecule during the time interval between the two gradients. The change in signal intensity, I, is related to the translational diffusion coefficient D by the StejskalTanner equation
I ) I0 exp[-(γδG)2(∆ - δ/3)D]
(5)
where I0 is the signal intensity in the absence of the gradient pulses, γ is the gyromagnetic ratio, δ is the gradient duration, G is the gradient strength, and ∆ is the interval between PFG pulses. The ratio I/I0 of water proton and methylene proton of PEG (Mw ) 1000) obtained at 15 °C is plotted against (γδG)2(∆ - δ/3) in Figure 8 in a semilogarithmic manner. The diffusion coefficient of the probe molecule can be determined from the slope of the straight line in Figure 8. The diffusion coefficients of the probe molecules in the normal-type and comb-type grafted NIPA gels at 15 °C are respectively shown in parts a and b of Figure 9 as a function of the concentration of the gel. The diffusion coefficients of the probe molecules decrease with increasing concentration of the gel and with increasing molecular (29) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.
7382
Langmuir, Vol. 18, No. 20, 2002
Annaka et al.
weight of the probe molecules at a constant concentration of the gel. Although the diffusion coefficient of water is almost identical for both type gels, the diffusion coefficients of the PEG (Mw ) 1000) in the comb-type grafted NIPA gel decrease rapidly with gel concentration compared with those in normal-type NIPA gel. At 35 °C, the diffusion coefficients of water and PEG (Mw ) 1000), respectively, are 11.5 × 10-6 cm2/s and in the order of 10-9 cm2/s for both normal-type and comb-type grafted NIPA gels. It is clear that the diffusion coefficients of the probe molecules depend on the mesh size of the polymer network as well as the size of the probe molecule. The friction of the polymer network becomes dominant when the mesh size of the polymer network approaches the size of the probe molecules. In the case of the comb-type grafted NIPA gel, since the incorporation of graft chains induces the excess friction between the polymer network and the probe molecule, the diffusion coefficient of the probe molecule becomes smaller with the amount of grafted chain at a fixed concentration of the gel. Matsukawa and Ando30-33 reported that the diffusion coefficient for PEG, D, in the gel is followed by D/D0 ) exp(-κR), where D0 is the diffusion coefficient for PEG in water, k-1 is the dynamical screening length, R is the hydrodynamic radius of PEG, and k is proportional to the gel concentration, φ. Therefore, we empirically assume that the reduced diffusion coefficient, D/D0, is given by the following simple scaling function:
D/D0 ) exp(-Rh/ξ)
(6)
where D0 is the diffusion coefficient of probe molecules in water, ξ is the correlation length of the polymer network, and Rh is the hydrodynamic radius of the probe molecule. In the present study, since the measurements are carried out under isochoric conditions, the correlation length of normal-type NIPA gel is determined only by the predetermined concentration of NIPA monomer, φ. According to the scaling theory, the concentration dependence of the correlation length is well described by the following power law when the cross-linking density is low enough:34
ξ ∝ φ-3/4
(7)
Since the probe molecules used in this study are rather compact molecules, we simply assume a power law for the size of the probe molecule, Rh, and the molecular weight of the probe molecule, M.
Rh ∝ Μ1/3
(8)
Equations 7 and 8 yield the scaling variable of the form
Rh/ξ ∝ M φ
1/3 3/4
(9)
The results shown in Figure 9 are plotted in Figure 10 in terms of the scaling variable given in eq 9. It is found from Figure 10 that all the reduced diffusion coefficients of the probe molecules in the normal-type NIPA gel, D/D0, that were obtained at different concentrations of NIPA monomer are expressed by a single master curve. The results obtained from the normal-type gel are in good agreement with the scaling function of the diffusion coefficient of (30) Matsukawa, S.; Ando, I. Macromolecules 1996, 29, 7136. (31) Matsukawa, S.; Ando, I. Macromolecules 1997, 30, 8310. (32) Matsukawa, S.; Ando, I. Macromolecules 1999, 32, 1865. (33) Matsukawa, S.; Ando, I. Macromolecules 2001, 34, 1430. (34) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979.
Figure 10. Relationship between the normalized diffusion coefficients of the probe molecules, D/D0, in the normal-type and the comb-type grafted NIPA gels and the scaling variable M1/3φ3/4 at 15 °C. Symbols are same as those in Figure 8. The straight line in the figure represents the results of the leastsquares analysis.
Brownian spheres in semidilute polymer solution calculated by hydrodynamic theory.35,36 On the other hand, the simple scaling relationship does not hold for PEG (Mw ) 1000) in the comb-type grafted NIPA gel. In the case of normal-type NIPA gel, the translational motion of PEG is restrained by hydrodynamic interaction with the polymer network, which means that the polymer network works only as a spatial obstruction for the displacement of PEG. Although the reduced diffusion coefficient, D/D0, of PEG (Mw ) 1000) in the normal-type NIPA gel follows the scaling relationship, it does not always ensure that the same scaling variable is valid for the comb-type grafted NIPA gel. This may be due to the following reasons: (1) the incorporation of the graft chains decreases the apparent mesh size, which leads to induced additional friction between the probe molecule and the network, and (2) there exist other interactions between the polymer network and PEG, for instance, hydrophobic interaction and/or hydrogen bonding. These effects become dominant for the higher gel concentration. It should be noted that the scaling law used in this study does not appear to be on solid theoretical ground; that is, polymer concentration is assumed to be in the semidilute regime, and PEG is treated as a globular molecule. These issues and their implications on the model will need to be addressed in the future. The scaling model proposed here should provide a framework from which such models could be developed. More extensive study, however, is needed to identify the microscopic structure and the characteristics of the comb-type NIPA gel with due consideration for formation of polymer networks for the complete understanding of the transport properties of the NIPA gels. 4. Conclusion Transport properties of the comb-type grafted and normal-type NIPA gels are investigated by gel filtration chromatography and a pulsed field gradient NMR technique with known standard probe molecules. The estimated molecular weight cutoffs by gel filtration chromatography for the normal-type and comb-type grafted NIPA gels, respectively, are 1.27 × 105 and 1.25 × 104; the molecular weight cutoff of the comb-type grafted NIPA gels is an order of magnitude smaller than that of the normal-type NIPA gel. A PFG NMR spectrum reveals that the diffusion coefficient of PEG (Mw ) 1000) in the combtype NIPA gel is smaller than that of the normal-type NIPA gel by an order of magnitude. The incorporation of (35) Langevin, D.; Rondelez, E. Polymer 1978, 19, 875. (36) Cukier, R. I. Macromolecules 1984, 17, 252.
Transport Properties of N-Isopropylacrylamide Hydrogel
NIPA grafted chains into NIPA gel makes the effective mesh size smaller and induces the additional friction between the probe molecule and gel network. One may control the maximum mesh size of the NIPA gel and thus the effective diffusivity of a probe molecule within a gel by incorporating grafted chains and by adjusting the temperature.
Langmuir, Vol. 18, No. 20, 2002 7383
Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. M.A. is grateful to the Kumagai Memorial Foundation for Science & Technology for financial support. LA025876T
