Langmuir 2008, 24, 12627-12635
12627
Gelation Mechanism of Poly(N-isopropylacrylamide)-Clay Nanocomposite Hydrogels Synthesized by Photopolymerization Bernhard Ferse,† Sven Richter,*,‡ Franziska Eckert,† Amit Kulkarni,§ Christine M. Papadakis,§ and Karl-Friedrich Arndt† Professur fu¨r Spezielle Physikalische Chemie/Physikalische Chemie der Polymere, Technische UniVersita¨t Dresden, D-01062 Dresden, Germany, Leibniz-Institut fu¨r Polymerforschung Dresden e. V., Hohe Str. 6, D-01069 Dresden, Germany, and Technische UniVersita¨t Mu¨nchen, Physikdepartment E13, D-85747 Garching, Germany ReceiVed July 8, 2008. ReVised Manuscript ReceiVed August 19, 2008 The gelation process of poly-(N-isopropylacrylamide)-clay nanocomposite hydrogels (PNIPAAm-clay NC gels) was investigated by dynamic and static light scattering (DLS and SLS), as well as by fluorescence correlation spectroscopy (FCS). The photopolymerization method chosen for the radical polymerizing system ensured that, when the irradiation is removed, the reaction stopped immediately. Experiments showed that shortly before the gelation threshold is reached, no changes in the DLS autocorrelation functions appear, while the monomer conversion can be observed by 1H NMR spectroscopy. These results correspond to the formation of microparticles, in which the PNIPAAm chains are closely attached to the clay platelets. During the further polymerization process, clay clusters are developed before the sol-gel threshold is reached. FCS measurements were performed to obtain information on the motion of the clay platelets inside the NC gel. The DLS method gives only an average of the motions in the gel. In a time window between 10 µs and 1 s, the clay sheets labeled with Rhodamine B show no characteristic motions.
Introduction In the last three decades, hydrogels have attracted increasing attention in many applications.1-3 Conventional hydrogels crosslinked with organic cross-linkers, e.g. N,N′-methylenebisacrylamide, show serious disadvantages for technical and medical usage. In particular, the low mechanical toughness, limited swelling ratio at equilibrium, and poor transparency of hydrogels with high cross-linking agent content are undesirable. For this reason, polymer-clay nanocomposites have been developed in recent years.4 Haraguchi et al. synthesized novel polymer-clay nanocomposite gels (NC gels), which possess very large deformability, amazing toughness, and high optical transparency.5-9 They consist of poly-(N-isopropylacrylamide), a temperature-sensitive polymer, and the synthetic clay Laponite. The inorganic clay, a synthetic hectorite, belongs to the 2:1 phyllosilicates (Figure 1). An octahedral layer is surrounded by two tetrahedral layers. Upon dispersion in water, the lamellar crystal structure is swollen and gradually cleaved into discrete disklike particles. Monolayer clay particles are anisotropic platelets of 25 nm in diameter and ca. 1 nm in thickness. The resulting aqueous clay suspensions, composed of exfoliated clay particles, are homogeneous and transparent.8 * Corresponding author. E-mail:
[email protected]. † Technische Universita¨t Dresden. ‡ Leibniz-Institut fu¨r Polymerforschung Dresden e. V. § Technische Universita¨t Mu¨nchen.
(1) Thiel, J.; Maurer, G.; Prausnitz, J. M. Chem. Ing. Technik 1995, 67, 1567. (2) Richter, A.; Paschew, G.; Klatt, S.; Lienig, J.; Arndt, K.-F.; Adler, H.-J. Sensors 2008, 8, 561. (3) Richter, A.; Tu¨rke, A.; Pich, A. AdV. Mater. 2007, 19, 378. (4) Novak, B. M. AdV. Mater. 1993, 5, 422. (5) Haraguchi, K.; Takehisa, T. AdV. Mater. 2002, 14, 1120. (6) Haraguchi, K.; Takehisa, T.; Fan, S. Macromolecules 2002, 35, 10162. (7) Haraguchi, K.; Farnworth, R.; Ohbayashi, A.; Takehisa, T. Macromolecules 2003, 36, 5732. (8) Haraguchi, K.; Li, H.-J.; Matsuda, K.; Takehisa, T.; Elliot, E. Macromolecules 2005, 38, 3482. (9) Haraguchi, K.; Taniguchi, S.; Takehisa, T. ChemPhysChem 2005, 6, 238.
Figure 1. Size and shape of a hectorite particle with the exact structure (right).
An understanding of the gelation mechanism and the properties of NC gels is important for their application in industry. The rheological properties of NC pregel solutions were already investigated by Haraguchi et al.8 They observed a decrease of viscosity of the aqueous clay suspension after adding Nisopropylacrylamide (NIPAAm) monomer. This is caused by the shielding of the physical interactions between the clay platelets and the surrounding monomer, resulting in a steric stabilization. Further, a sharp and short reduction of the optical transmittance during the polymerization to NC gels is reported. In standard reactions of conventional hydrogels, like PNIPAAm cross-linked with N,N′-methylenebisacrylamide (OR gels), this behavior is not typical. They assumed a formation of so-called “clay-brush” particles during the first period of the polymerization, which reflects the visible light because the formation of a number of grafted chains on the clay surfaces would cause a local change in the density. An aggregate of such particles with a size of several tens of nanometers would scatter light. This phenomenon is not yet sufficiently clarified and needs further investigation. Furthermore, Shibayama et al. extensively examined the gelation mechanism and the microscopic structure of NC gels with DLS,10,11 the deformation process under uniaxial stretching by small angle neutron scattering (SANS),12 and the clay concentration dependence by contrast-matched SANS.12 They assumed that the clay particles disperse homogeneously within the PNIPAAm matrix and act as a multifunctional cross-linker, but the polymer chains between two neighboring cross-links are
10.1021/la802162g CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
12628 Langmuir, Vol. 24, No. 21, 2008
Ferse et al.
Figure 2. Norrish type I cleavage of the photoinitiator 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone.
significantly longer than is usual for conventionally cross-linked OR gels. Additionally, after a 10-fold stretching of these NC gels, the original form was recovered.14 Obviously the PNIPAAm chains have to be strongly bound to the clay surface.15-17 The present investigations were carried out on NC gels, which were synthesized by a thermal initiation by redox systems consisting of potassium peroxodisulfate (KPS) and N,N,N′,N′tetramethylethylenediamine (TEMED) as an initiator and an accelerator, respectively. This work presents a study of the gelation mechanism and the properties of photopolymerized NC gels. Additionally, a photopolymerization at a wavelength of approximately 360 nm in this given case is an appropriate technique for obtaining welldefined hydrogel structures. For applications of these NC gels (e.g. in tactile communications, microvalves), these hydrogels must be synthesized by a patterning process. 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone was used as photoinitiator. By absorption of a photon, the ketone of the photoinitiator can be converted into a photoactivated species. It is promoted to the singlet excited (S1) state, from which it can reach the triplet excited (T1) state by intersystem crossing. The homolytic Norrish type I cleavage may occur from either or both states and leads to the formation of two radicals. Aromatic ketones generally undergo the photolytic cleavage from the triplet excited state, since the intersystem crossing is usually fast in those cases (Figure 2).13 A further major benefit of this radical initiating system consists in the immediate termination of the polymerization process when the UV light is switched off. Therefore, the different stages of the gelation course can be investigated separately by different scattering methods. With a usual thermal initiated radical polymerizing system, e.g. KPS and TEMED, it is not feasible to control or terminate the polymerization. In this case, TEMED is used as an accelerator, allowing polymerization at room temperature. This condition is necessary to avoid the phase transition at around 32 °C of the PNIPAAm. DLS and SLS measurements are helpful to obtain information about the gelation mechanism, the structures, and the dynamics (10) Shibayama, M.; Suda, J.; Karino, T.; Okabe, S.; Takehisa, T.; Haraguchi, K. Macromolecules 2004, 37, 9606. (11) Miyazaki, S.; Endo, H.; Karino, T.; Haraguchi, K.; Shibayama, M. Macromolecules 2007, 40, 4287. (12) (a) Miyazaki, S.; Karino, T.; Endo, H.; Haraguchi, K.; Shibayama, M. Macromolecules 2006, 39, 8112. (b) Shibayama, M.; Karino, T.; Miyazaki, S.; Takehisa, T.; Haraguchi, K. Macromolecules 2005, 38, 10772. (13) Laue, A.; Plagens, A. Namens und Schlagwortreaktionen in der Organischen Chemie; Teubner: Wiesbaden, 2000. (14) Haraguchi, K.; Li, H.-J. Macromolecules 2006, 39, 1898. (15) Nie, J.; Du, B.; Oppermann, W. Macromolecules 2004, 37, 6558. (16) Nie, J.; Du, B.; Oppermann, W. Macromolecules 2005, 38, 5729. (17) Nie, J.; Du, B.; Oppermann, W. J. Phys. Chem. B 2006, 110, 11167.
in the pregel solutions. FCS measurements were carried out, resulting in specific information on the diffusivity of the clay platelets in the NC gel when they are fluorescence labeled. In contrast, DLS only provides an average of the motions of all scattering objects, i.e., PNIPAAm and clay.
Theoretical Background DLS Measurements. In general, light-scattering techniques are very suitable to study the gelation process without disturbing the gelling system. Previous publications18,19 have shown that four methods of dynamic light scattering (DLS) are applicable for the determination of the gelation threshold: (i) the change in the scattered intensity (occurrence of speckle patterns), (ii) the power-law behavior in the time-intensity correlation function (TCF), (iii) a characteristic broadening of the decay time distribution function, and (iv) the suppression of the initial amplitude of the TCF. Each of these methods is a phenomenon based on the characteristic features of gels, i.e., (i) inhomogeneity, (ii, iii) connectivity divergence, and (iv) nonergodicity. The principal focus of our work is method ii. In DLSsat the gelation thresholdsa power law behavior for the TCF is revealed.
g2(t) - 1 )
〈I(0) I(t)〉 - 1 ∝ t-µ 〈I〉2
0.19 e µ e 0.9
(1)
Here 〈I(t)〉 is the scattering intensity at a time t with respect to t ) 0 and 〈...〉 denotes a time average. When approaching the gel point, the slow modes become dominant and a power-law function appears. The power-law behavior, which has no characteristic relaxation time, is self-similar. The incipient gel is a self-similar (fractal) distribution of fractal clusters of all sizes, from monomer to the infinite cluster. For many gelling systems, a clear power-law behavior in the time-intensity correlation function at the gelation threshold has been reported. For more details and references, see the recent review by one of the authors.20 For large macromolecules and polydisperse systems, one can expect a more complex decay. In this case, g1(q,t) is described by a sum of exponential functions reflecting the distribution of relaxation times G(Γ), which can be calculated by the Laplace inversion (the CONTIN procedure)21,22 of the following expression: (18) Shibayama, M.; Norisuye, T. Bull. Chem. Soc. Jpn. 2002, 75, 641. (19) Shibayama, M. Bull. Chem. Soc. Jpn. 2006, 79, 1799. (20) Richter, S. Macromol. Chem. Phys. 2007, 208, 1495. (21) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (22) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229.
Gelation of Polymer-Clay Nanocomposite Hydrogels
g1(q, t) )
Langmuir, Vol. 24, No. 21, 2008 12629
∫ G(Γ) exp(-Γt) dΓ
(2)
For spherical particles the hydrodynamic radius (or the dynamic correlation length ξ for concentrations well above the chain overlapping concentration c*) can be calculated by means of the Stokes-Einstein equation
Rh(ξ for c g c*) )
kT 6πη0Dz
(3)
with Dz, kB, T, and η0 being the translational diffusion coefficient, the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. SLS Measurements. With SLS it is possible to obtain both the weight-average molar mass Mw and the z-average radius of gyration Rg of scattering objects in an extremely dilute solution by measuring the angular dependence of excess of absolute timeaveraged scattering intensity (the Rayleigh ratio Rθ) in the range of the Rayleigh-Debye approximation
(
)
(qRg)2 Kc 1 ) 1+ + 2A2c Rθ MW 3
(4)
In this equation, the contrast factor K ) 4π2n2(dn/dc)2/(NAλ04) and the scattering vector q ) (4πn/λ0)sin(θ/2) are used, with n, NA, λ0, θ, c being the solvent refractive index, the Avogadro number, the wavelength of the incident light in vacuum, the scattering angle, and the concentration, respectively. For large macromolecules, eq 4 can be transformed into
( ) Kc Rθ
) cf0
1 ¯ MWP(q)
(5)
with P(q) the particle scattering factor. FCS Measurements. To focus on one particular species of fluorescent particles an autocorrelation analysis can be performed. Intensity fluctuations are due to number fluctuations of fluorescent particles in the detection volume, which is as small as 1 µm3. The typical decay time of the autocorrelation function of the fluorescence intensity is related to the diffusion time of the particles through this volume. The fluorescence intensity, I(t), emitted by fluorescent molecules in the observation volume is measured and autocorrelated with the following equation23,24
G(τ) - 1 )
〈δI(t) δI(t + τ)〉 〈I(t)〉2
(6)
δI(t) ) I(t) - 〈I(t)〉 is the instantaneous deviation of the measured intensity from its time average, 〈I(t)〉. For an ensemble of uniformly fluorescing, freely diffusing, and monodisperse particles of different diffusion coefficients, the correlation function has the following analytical form24-27
G(τ) ) 1 +
[
( )]
TT 1 τ exp × 1+ N 1 - TT τT n
∑ i)1
(
τ 1+ τD,i
× Fi
)
()
τ z0 / 1+ τD,i w0
(7) 2
N is the average number of particles in the observation volume. The effective observation volume, V ) π3/2z02ω0, is approximated by a three-dimensional Gaussian profile with z0 and ω0 being the (23) Hess, S. T.; Webb, W. W. Biophys. J. 2002, 83, 2300. (24) Krichevesky, O.; Bonnet, G. Rep. Prog. Phys. 2002, 65, 251. (25) Hess, S. T.; Huang, S. H.; Heikal, A. A.; Webb, W. W. Biochemistry 2002, 41, 697.
focal depth along the optical axis and the radius of the focused beam spot, respectively. τD,i is the average diffusion time of the fluorescent particles of species i through the detection volume. When the observation volume is calibrated so that ω0 and ω0 are known, the diffusion coefficient DS of the fluorescent species can be determined by25
τD )
ω02 4DS
(8)
The calibration for a particular experimental setup is usually done by measuring the characteristic diffusion time of molecules or particles whose diffusion coefficients are exactly known from literature data.
Experimental Section Samples. N-Isopropylacrylamide (NIPAAm) monomer was purchased from Acros Co.. It was purified by recrystallization from n-hexane, followed by drying under vacuum. The inorganic clay, a synthetic hectorite called “Laponite XLS” (92.32 wt % [Mg5.34Li0.66Si8O20(OH)4]Na0.66 + 7.68 wt % Na4P2O7) from Rockwood Specialties Inc. was used after washing and freeze-drying. The photoinitiator 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) from Aldrich and the fluorescence dye Rhodamine B were used as received. For all investigations, Millipore water (with a conductivity of 18.2 µΩ/cm2) was used. The solution was degassed for more than 2 h with argon to remove to oxygen. Throughout all experiments, oxygen was excluded from the reaction system. To receive hydrogels by photopolymerization, a transparent aqueous solution consisting of water, inorganic clay (Laponite XLS), NIPAAm, and photoinitiator was prepared. The concentrations of NIPAAm and photoinitiator (PI) were kept constant with 10 wt % monomer and 0.3 wt % PI, respectively. The amount of clay was varied over a wide range between 1.5 and 4 wt % in the aqueous solution. The sample codes were defined by the concentration of clay in water. (“NC2” is equal to a 2 wt % aqueous clay suspension). After dissolving, the solution was filtered quickly, using a 0.45 µm nylon-membrane filter. The photopolymerization was carried out in an argon atmosphere through UV irradiation (λ > 360 nm, mercury UV lamp of 900 W, (“LOT Oriel”) approximately 5 mW/cm2 at a wavelength of 360 nm) at different exposure times. To avoid inhomogeneities in the structure of hydrogels due to the UV irradiation, NMR tubes (with an outside diameter of 5.0 mm) were used for the photopolymerization and the following DLS and SLS investigations. We observed no perceptible intensity gradient of the radiation across the sample tubes caused by the used wavelength of 360 nm (which is not the absorption maximum of the photoinitiator) and the very thin NMR tubes. Furthermore, during UV exposure we rotated the tubes in order to preclude a surface reaction to occur. It was shown that UV light with a wavelength of 360 nm will almost not be absorbed by the NMR tubes. DLS. An ALV DLS/SLS-5000 light-scattering system with a He-Ne laser (Uniphase 1145P, output power 22 mW and λ0 ) 632.8 nm) as the light source was used. This laser is equipped with an ALV-5000/EPP multiple digital time correlator. The DLS experiments were carried out between angles of θ ) 30° and θ ) 150° and with an acquisition time of 3 min. The NMR tubes were immersed in a toluene bath and thermostated at 20.0 ( 0.1 °C. SLS. SLS experiments were performed at a temperature of 20 °C by using a modified, “SOFICA” (SLS Systemtechnik G. Baur, Denzlingen, Germany) with a He-Ne laser (λ0 ) 632.8 nm) in a range of the scattering angle from 45° to 145°, in steps of 3°. FCS. For FCS, a ConfoCor 2 from Carl Zeiss Jena GmbH was used together with an Ar+ laser (λ ) 540 nm), a pinhole with a diameter of 80 µm, a BP 530-600 emission filter, and an HFT 543 (26) Eigen, M.; Rigler, R. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 5740. ¨ .; Rigler, R. J. Chem. Phys. 1995, 99, 13368. (27) Widengren, J.; Metz, U
12630 Langmuir, Vol. 24, No. 21, 2008
Ferse et al.
plate beam splitter. The autocorrelation functions of the fluctuations of the fluorescence intensity, G(τ), were analyzed by fitting the expression in eq 7.27 N is the total number of fluorescent particles in the observation volume, τD,i the diffusion time of the ith type of fluorescent particles, Fi the relative amplitude of the decay of the ith species, and w0 and z0 the half-width and half-height of the observation volume, respectively. TT and τT are the triplet fraction and time. w0 was determined before each session by measuring the diffusion time of Rhodamine 6G (Sigma-Aldrich, DRh6G ) 2.8 × 10-10 m2 s-1),28 τD,Rh6G, and by using w0 ) (4DRh6GτD,Rh6G)1/2. A value of w0 = 0.2 µm was obtained. The ratio z0/w0 determined from the fit to the Rhodamine 6G correlation function in water was typically 5-6. Aqueous solutions of silicates labeled with Rhodamine B having concentrations between 0.03 and 10 mg/mL and swollen hydrogels cross-linked by labeled silicates were prepared in deionized and filtered water. As for the hydrogel, a small piece of the dry gel was kept in deionized and filtered water for 1 h. This fully swollen piece of gel was mounted in the FCS chamber for measurements. All measurements were carried out at T ) 27 °C. Ten measurements with a duration of 10 s were carried out for each sample. The average diffusion time of the silicates in solution was determined from fits of eq 7 to the average correlation function with n ) 2. Using the relation D ) DRh6GτD,Rh6G/τD, the diffusion coefficients D of freely diffusing Rhodamine B molecules, DRhB, as well as the fluorescencelabeled silicate particles, Dclay, were determined, which together with the Stokes-Einstein relation, Rh ) kBT/6πηD (kB is Boltzmann’s constant and η ) 0.95 × 10-3 Pa s, the viscosity of water at 27 °C), lead to the hydrodynamic radii of Rhodamine B, Rh,RhB and to the fluorescence-labeled particles, Rh,clay. NMR. A Bruker DRX 500 P spectrometer was used at 500 MHz. For these measurements, the nanocomposite hydrogels were synthesized in D2O. The same tubes were used for DLS as well as for NMR investigations to guarantee the comparability of the experiments. To calculate the conversion of the monomer NIPAAm, the signals (s, 1 H, N-CH) and (s, 1 H, CH2-CH) were used.
Results and Discussion Figure 3a shows the time-intensity DLS correlation functions of PNIPAAm in the absence of any cross-linker. Consequently, during the polymerization process only linear PNIPAAm chains were synthesized. All samples were irradiated with different exposure times. For each new exposure, we took a new sample. For these measurements, the same concentrations were chosen as in the gelation studies. With increasing exposure times, the TCFs are shifted to higher decay times. Additionally, in Figure 3b the distribution functions are presented, which were determined by the CONTIN procedure. Usually the polymer chains are formed in milliseconds and consequently only one TCF has to be obtained independently from the exposure time. Only under controlled radical conditions is continuous polymer growth possible. An increase of the hydrodynamic radius (Rh) in this case may be explained by aggregation of the polymers. This is the reason for the broadening of the size distributions with increasing irradiation time. During this reaction we did not reach the chain overlapping concentration, c*, so only the Rh value can be stated. We are sure that c* is reached closely before 105 s exposure time; see Figure 5b. The hydrodynamic radii are only apparent, because the DLS measurements were carried out at a single angle (90°). However, in this case, it is not necessary to obtain exact hydrodynamic radii as only the variation is important. The gelation process of the NC gels by photopolymerization was monitored by 1H NMR. The dependence of the conversion of the monomer NIPAAm upon the exposure time is shown in Figure 4. After irradiation, the samples were measured by 1H (28) Bonne´, T. B.; Lu¨dtke, K.; Jordan, R.; Sˇteˇpa´nek, P.; Papadakis, Ch. M. Colloid Polym. Sci. 2004, 282, 833.
Figure 3. (a) TCFs of PNIPAAm at different exposure times without cross-linker (clay). (b) Size distribution calculated from the TCFs by the CONTIN procedure.
Figure 4. Determination of the conversion of the NIPAAm double bond vs the exposure time by 1H NMR spectroscopy of the gelation process.
NMR. A significant increase of the conversion can be monitored. After 4 h, the measurements were repeated on the same irradiated samples. During this time interval, the samples were kept in darkness. Nearly no change of the conversion can be observed. This means that the polymerization stops immediately after the UV light is switched off. The ketone of the photoinitiator will be converted into photoactivated singlet and triplet species by
Gelation of Polymer-Clay Nanocomposite Hydrogels
Langmuir, Vol. 24, No. 21, 2008 12631
Figure 6. Data evaluation of the TCFs in Figure 5b according to Kohlrausch-Williams-Watts and the mean relaxation time 〈τ〉.
this stage of the polymerization the clay platelets do not change their size. The solution remains liquid. With increasing exposure time, the sol-gel transition is reached at 105 s of irradiation. A typical power law can be obtained at this time. With a linear fit in the decay time window from 0.3 to 100 ms, an exponent µ ) 0.5 (solid line in Figure 5a) of the power law in the TCF can be calculated. Afterward, the TCFs exhibit only a small relaxation around 0.5 ms, which is typical for gels. During the sol-gel transition the solution becomes more viscous, and as the gelation threshold is reached, the solution converts completely to a mechanically stable gel. A tilting test showed the formation of a stable gel at the gelation threshold after 110 s of exposure time at 5 mW/cm2. The TCFs could be well-described by a KohlrauschWilliams-Watts stretched exponential function29 Figure 5. (a) TCFs of the polymerizing system, obtained with 50 mW/ cm2. (b) TCFs of the polymerizing nanocomposite system obtained with 5 mW/cm2. The solid line indicates the power law on the gelation threshold.
absorption of a photon of light. The homolytic Norrish type I cleavage into the initiating radicals may occur from both activated states, but the lifetime of these activated states is only a few nanoseconds. A stationary state of the activated species is reached during the irradiation. Afterward, the conversion into the excited states is stopped directly because no radicals can be formed anymore. For this reason, it is possible to freeze the radical polymerizing system at any time and a separate investigation of the different stages of the gelation process is feasible. For investigations of the gelation with DLS, the concentration of clay was fixed at 2 wt %. Figure 5a shows the TCFs of the irradiated samples, synthesized with a specific power of 50 mW/cm2. We assume that the polymerization is too fast for investigations in the region near the gelation threshold. It seems that the sol-gel transition takes place abruptly. To slow down the system behavior, it is essential to reduce the polymerization rate. It is not possible to decrease the concentration of the photoinitiator, because a smaller amount of initiator does not lead to a gelation. A reduction of the specific power to approximately 5 mW/cm2 leads to a continuous course of the TCFs (Figure 5b). Under these conditions, during the first 80 s of irradiation, no change in the shape of the TCFs could be observed (Figure 5b). In contrast, the TCFs, which were measured in the absence of cross-linker (Figure 3), show a characteristic shift to higher decay times due to the aggregation of polymer chains. Obviously, in
[ ( )]
g2(t) - 1 ) A exp -
t τ1
β
+C
(9)
in which A and C are constants corresponding to the correlation strength and the gel plateau, respectively. The two other parameters, β and τ, characterize the mean relaxation time, which is given by 〈τ 〉
)
∫ exp[-(t/τ1)β] dt ) (τ1/β)Γ(1/β)
(10)
with Γ(x) being the γ-function. The inverse relaxation time is proportional to the translational diffusion coefficient at exposure times below the gel point, and it is the cooperative diffusion coefficient of the gel mode above the gelling exposure time, because the dynamics is governed by the mesh size of the network. Divergence of 〈τ〉 can be expected at the gel point.30 As the cross-linking proceeds, the mesh size becomes smaller and the corresponding diffusion faster, and consequently, the relaxation time decreases again. Figure 6 exhibits the dependence of the mean relaxation time as a function of exposure time. The maximum is close to 105 s. Further evidence for the sol-gel threshold is a simple sol-gel analysis. Figure 7 illustrates the course of the sol content and gel content, respectively. The apparent (the samples were not diluted) weight average molecular masses, Mw,app, which were obtained (29) (a) Williams, G.; Watts, D. C. Trans. Faraday Soc. 1970, 66, 80. (b) Williams, G.; Watts, D. C.; Worth, A. M. Trans. Faraday Soc. 1971, 67, 1323. (c) Kohlrausch, R. Poggendorff’s Anal. Chem. 1854, 91, 179. (d) Kohlrausch, R. Poggendorff’s Anal. Chem. 1863, 119, 337. (30) Rodd, A. B.; Dunstan, D. E.; Boger, D. V.; Schmidt, J.; Burchard, W. Macromolecules 2001, 34, 3339.
12632 Langmuir, Vol. 24, No. 21, 2008
Ferse et al.
Figure 7. Analysis of the sol content of the irradiated samples shows the sol-gel transition; the apparent weight average molecular masses increase near the gelation threshold.
Figure 8. Angular dependent relaxation times τ(q) for pregel solutions at different exposure times, as measured by DLS.
by SLS measurements, increased to infinity at the gelation threshold. The sol-gel analysis was done by weighting and extracting of the samples. For the region of interest between 0 and 100 s of exposure time, the irradiated samples were measured at different angles. An investigation of the nature of the relaxation processes gives evidence of the structure in the pregel solution. The calculated relaxation times τ ()1/Γ1) were plotted as a function of the scattering vector q, which is shown in Figure 8. The characteristic size of the fluctuating units can generally be estimated by analyzing the correlation functions in terms of the first cumulant31
1 Γ1 ) - d[ln(g2(t) - 1)]/dttf0 2
can be identified, the only characteristic length that can define the dynamics is the resolution of the observation, i.e., 1/q. The dependency of the relaxation rate on q3 is a characteristic of microgels,32,33 where internal motions dominate the dynamic response.34 The slight increase of the slope to m ) 2.3 for the sample irradiated for 100 s thus is indicative of the formation of larger polydisperse polymer-clay clusters (microgels) in a low concentration regime. The value of this exponent of m ) 2.3 is attributed to a superposition of internal relaxations inside these microgels and their diffusive motion. These microgels are formed during the polymerization with exposure times of 80-100 s. At lower irradiation times, the characteristic size of the microgels is too small to observe internal relaxations. SLS measurements of the same irradiated samples led to further information about the structure of the pregel solution. Figure 9 shows Guinier plots of samples, which were irradiated up to 80 s. At the beginning an increase of the scattering intensity can be observed due to the formation of clay-polymer particles, which scatter visible light because the formation of a number of grafted chains on the clay surfaces causes a change in the local density.8 After 80 s the scattering intensity decreases, because the overlap concentration c* is reached. Before 60 s almost no angular dependency appears and the plots are similar to the nonirradiated sample. This indicates that all solutions contain particles with a similar dimension of about Rg ) 20 nm. This value corresponds to the size of clay platelets, which means that the growing polymer chains are closely attached to the clay surface. At 80 s of irradiation, an angular dependency appears, because larger microgels or clusters, consisting of a few clay platelets and polymers, are formed. These larger clusters can be observed especially at small angles, which is the reason for the curvature of the response. The phase transition temperature (LCST) of an aqueous polymer-clay solution was measured after an irradiation of 100 s using DLS with a waiting time of 15 min at each measuring point to make sure that the thermodynamic equilibrium was reached. The result is shown in Figure 10. The clusters consist of two, three, or more particles, which are cross-linked with PNIPAAm. A sharp phase transition at the LCST of around 30 °C is visible, while the apparent hydrodynamic radii show only a slight decrease of 16 nm. The clusters shrink only up to 25% of their size, because the clay platelets prevent further shrinking of the polymer chains.
(11)
Γ1 obeys a power law with the form
Γ1 )
kBT m q 6πη
(12)
For the samples irradiated for 0 s (i.e., nonirradiated) and 60 s, an exponent of m ) 2 was found. For an irradiation of 80 s, this exponent increased to 2.2, while after exposure times of 90 and 100 s this value slightly increases further to 2.3. In systems where no length scale smaller than 1/q intrinsic to the specimen (31) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814.
Figure 9. Guinier plots of the irradiated samples until 80 s of exposure time, obtained by SLS.
Gelation of Polymer-Clay Nanocomposite Hydrogels
Langmuir, Vol. 24, No. 21, 2008 12633
Figure 10. Temperature-responsive properties of PNIPAAm-clay microgels. The hydrodynamic radii are apparent, because they were measured at only a scattering angle of 90°. As an example of the influence of the angular dependence of the translational diffusion coefficient on that system, the real hydrodynamic radius at 20 °C was estimated to 140 nm.
With dynamic light scattering it is only possible to obtain the sum of all motions in the gel. One cannot decide if the relaxation has its origin in the polymer, in the clay, or in collective movement. For this purpose, it was necessary to develop a suitable synthesis to label the clay platelets with a fluorescent dye. Due to the negatively charged surface of the clay platelets, a cation exchange of the Na+ ions with Rhodamine B could be carried out to label the inorganic particles. Rhodamine B possesses a positively charged amino group. The cation exchange was performed in dichloromethane with a concentration of Rhodamine B of 10 µg per 1 g of Laponite XLS. However the Rhodamine B dye is destroyed when it is irradiated with UV light. For this reason NC gels were synthesized by thermal initiation with KPS and TEMED. Since both NC gels (synthesized by photopolymerization or thermal polymerization) show the same swelling properties, a comparison between this two synthesized gels is possible. The correlation curves of two aqueous solutions of fluorescencelabeled silicate particles as well as the one of the swollen hydrogel are shown in Figure 11. At times below ∼10 µs, the triplet decay of the Rhodamine B label is observed with triplet fractions TT ) 0.15. The characteristics of the two diffusional decays are given in Table 1. In Figure 11b the correlation functions are shown with higher clay contents. These concentrations are quite similar to the clay concentrations in the gel. The triplet decay of Rhodamine B disappears. In the correlation curves of the solutions having concentrations of 0.03 and 0.04 mg/mL (Figure 11 a), two diffusional decays are present, with decay times of 11-22 µs and ∼1.6 ms, resulting in hydrodynamic radii of 0.3-0.6 and 41-44 nm, respectively. The fast decay is attributed to the diffusion of free Rhodamine B dye molecules; i.e., in solutions with low concentration, a certain fraction of Rhodamine B is not bound to the clay particles. The slow process is due to hydrated clay particles. The values of Rh,clay are compatible with their geometric size of the Laponite particles (25 nm × 1 nm). Increasing the clay concentration of the solutions to 5 and 10 mg/mL results in much higher noise in the correlation curves (32) Wu, C.; Zhou, S. Macromolecules 1996, 29, 1574. (33) Boyko, V.; Richter, S.; Burchard, W.; Arndt, K.-F. Langmuir 2007, 23, 776. (34) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell: Ithaca, NY, 1979.
Figure 11. FCS correlation curves of aqueous solutions of clay fluorescence labeled with Rhodamine B and of a PNIPAAM-clay hydrogel cross-linked with these layered silicates. (a) Low concentrations: open red circles, 0.03 mg/mL; open blue triangles, 0.04 mg/mL; filled black circles, gel. (Inset) Illustration of the clay platelets labeled with Rhodamine B; for simplification only one negative charge on the clay surface is drawn. (b) High concentrations: open red circles, 5 mg/mL; open blue triangles, 10 mg/mL; filled black circles, gel. The lines are fits of eq 7. As evident from eq 7, the higher the average number of fluorescent particles in the FCS detection volume, the lower the ordinate. In Figure 11a, the concentration of fluorescent clay particles (0.03 or 0.04 mg/mL) was chosen to be much lower than in Figure 11b (5 or 10 mg/mL), hence the difference in the ordinate. The concentration of the suspensions in Figure 11b was chosen to be so high to resemble the one in the gel. This proves that correlation functions can be measured even at this high concentration.
(Figure 11b). The decay at high times (diffusion of fluorescencelabeled clay particles) still can be discerned at ∼103 µs, whereas no clear decay is present at lower times. This may be due to the poor statistics or to the shift of the equilibrium between free Rhodamine B molecules and those bound to clay particles. Due to the significantly higher concentration of clay particles, we speculate that the largest fraction of the Rhodamine B molecules are bound to clay particles. The hydrodynamic radius of the clay particles is determined at 31 nm, a value smaller than at lower concentrations. Bleaching is usually evident from a steady decrease of the fluorescence intensity. This was not observed in the experiment, so we conclude that bleaching can be neglected. The high ratio of the hydrodynamic radius and the radius of gyration may be due to the highly anisotropic shape of the disklike particles. In contrast, the correlation curve of the PNIPAAm hydrogel, which was cross-linked with the fluorescence-labeled clay particles, shows only the fast triplet decay. No diffusional decay is observed in the time range of the experiment (up to ∼1 s). This means that the cross-linker clay particles do not diffuse over the length scale of the detection volume (∼1 µm3), but their position is fixed within this volume. These results indicate that the majority of the silicate particles participate in cross-linking of the polymer
12634 Langmuir, Vol. 24, No. 21, 2008
Ferse et al. Table 1. Results from FCS on the Clay Solutions
concn (mg/mL)
τD,RhB (µs)
τD,clay (ms)
DRhB (10-10 m/s2)
Dclay (10-12 m/s2)
Rh,RhB (nm)
Rh,clay (nm)
0.03 0.04 5 10
11 ( 1 22 ( 3
1.50 ( 0.05 1.59 ( 0.96 0.976 0.976
7.4 ( 0.7 3.8 ( 0.5
5.6 ( 0.4 5.3 ( 0.3 7.5 7.5
0.31 ( 0.03 0.61 ( 0.08
41 ( 3 44 ( 3 31 31
network and that the amount of free silicate particles is negligible. No free Rhodamine B is observed either. The concentration of clay particles in the gel is estimated at 10 mg/mL, thus similar to the concentrated solutions. Still, no decay is observed. This means that the diffusion can be measured at this concentration, but since it is not observed in the gel, it is a clue for arrested diffusion. As a summary, Figure 12 illustrates schematically the gelation mechanism of photopolymerization for NC gels. Due to the scattering of the small clay platelets (stage I), the pristine sample shows one correlation rate in the TCF at the beginning. The monomer does not scatter light. In the next step (stage II) no changes in the TCFs appear, although NMR studies verified a conversion of monomer at this exposure times. The growing polymer chains are closely attached to the surface and surround the clay platelets completely, due to the strong physical interactions between polymer and clay. The high concentration in the system leads to a broad size distribution and therefore very small changes in the size of the particles cannot be measured by DLS. In these models, only a small number of PNIPAAm chains
and clay particles are drawn for the purpose of simplicity. With increasing exposure times the polymer chains become larger, the surface of the clay sheets are totally occupied, and consequently, “clay-brushes” (clay-brush particles)8 are formed, which are connected to clay clusters consisting of two, three, or more clay platelets (stage III). In this state, the gelation threshold is reached and the concentration is high enough for the formation of a percolated network (stage IV). The power-law behavior, having no characteristic relaxation time, is self-similar. The incipient gel is a self-similar (fractal) distribution of fractal clusters of all sizes, from monomers to the infinite cluster. Afterwards, the clay clusters interlink and the NC hydrogel is formed. The multifunctional cross-linker, clay, is in the gel completely surrounded by a PNIPAAm layer. From this layer the growing polymer chains extend to the next surrounded clay platelet and cross-link the particles. The PNIPAAm chains are closely attached to the clay surface, so the polymer displaces solvent and any reagents due to the strong physical interactions between polymer and clay. That is why it is not possible to destroy the physical interactions between PNIPAAm and clay by dissolving the NC
Figure 12. Gelation mechanism of NC gels in different stages of polymerization; the middle figures are the TCFs (Figure 5) of the photopolymerization of the irradiated samples. Blue dots, red disks, and blue lines indicate the NIPAAm monomer, clay sheets, and polymer chains, respectively.
Gelation of Polymer-Clay Nanocomposite Hydrogels
gel with reagents like urea and guanidine hydrochloride, which are usually used to cleave inclusion bodies of proteins formed by hydrogen bonds. Interestingly, with addition of guanidine hydrochloride in the polymerizing system, no gelation could be observed.
Conclusion The photopolymerization process of the system (NIPAAm/ Laponite XLS/Irgacure 2959) to produce bulk hydrogels is described for the first time. The resulting gels showed thermosensitive behavior and improved mechanical properties. Photopolymerization is a suitable technique to investigate the gelation process of a free radical polymerizing system. With 1H NMR measurements, it was shown that the conversion is stopped immediately when the UV light is switched off, thus making it possible to observe the gelation process at different polymerization stages separately. A continuous transition from sol to gel could be observed with DLS on irradiating with 5 mW/cm2. With additional static light
Langmuir, Vol. 24, No. 21, 2008 12635
scattering measurements, a mechanism of the gelation process was determined, in which the PNIPAAm chains are closely attached to the surface in the early stages of polymerization. As a result, it is not possible to destroy the physical interactions between PNIPAAm and clay to dissolve the NC gel. For FCS measurements, it was possible to label the clay platelets with the fluorescent dye Rhodamine B. Consequently, only the movements of the labeled clay platelets could be monitored. In a time window between 10 µs and 1 s, no FCS correlation was observed, implying that the clay platelets inside the gel show no movement and seemed to be arrested due to the polymer chains. Acknowledgment. We thank Dr. M. Gruner (TU Dresden) for recording the NMR spectra. This work was supported by the research fund of the SFB 287 (DFG). We thank Rockwood Inc. for the Laponite samples. A.K. and C.M.P. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft within the priority program SPP 1259 (Pa771/4). LA802162G
