Langmuir 1998, 14, 4475-4481
4475
Storage Stability and Rheological Behavior of Talc-Polyacrylylglycinamide Gelified Suspensions J. L. Trompette,* C. Charnay, and S. Partyka Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, ESA 5072, Universite´ Montpellier II, 2 Place Euge` ne Bataillon, Montpellier 34095 Cedex 05, France Received January 12, 1998. In Final Form: May 5, 1998 The rheological behavior of physically cross-linked polyacrylylglycinamide hydrogels at 4% (w/w), without and in the presence of a 10% (w/w) talc particle concentration, has been investigated. The thermoreversible gelation, studied in oscillatory shear regime, indicated for these systems melting temperatures below 310 K. The presence of the talc particles was found to lower the elastic contribution of the entangled network. The gelified systems were found to exhibit a comparable thixotropic behavior, allowing them to recover their initial structure after application of a structure-breaker mechanical treatment. The storage stability of the gelified talc suspensions was ascribed to the strong interaction between the constitutive entangled polymer chains of the network and the hydroxylated surface sites of the talc particles through hydrogen bondings.
Introduction Stability of aqueous colloidal dispersions is often desired in many industrial processes, such as paint, paper, or cosmetic industries. The storage conditions of the dispersions, the application of the dispersions on a surface, and their final properties all require that the system is stable kinetically. In the majority of these applications, the adsorption of specific surfactants and/or long-chain polymers is often resorted to in order to impart the stability of these systems.1 The role of polymers is to form a protective sheath around colloidal particles to prevent them from aggregating. For that purpose, diblock and triblock copolymers were found to exhibit the best steric stabilizing effects.2 Although at first, it seems desirable to have a totally deflocculated dispersion of particles in the medium, there exist some reasons why in some cases it may generate a problem. Indeed, if the dispersions are stored for a long time, unaggregated particles may settle into a close-packed “cake” at the bottom of the vessels, and these are difficult to redisperse simply by shaking. Moreover, in the case of aqueous suspensions of rather large particles, it is not easy to obtain stable systems since gravity is often the predominant force. As such, it may be envisaged that an alternative way for reducing sedimentation is for the particles to be trapped in a continuous macromolecular network extending throughout the whole volume, that is, a gel structure. However for specific applications, it may be necessary for these composite systems to present thixotropic properties. These systems are expected to flow when they are stirred moderately and to return quickly to their gel state after the end of the shearing process. Many natural and synthetic polymers are known to possess the ability for creating such thixotropic gels. Some of them have been largely widespread in many practical fields such as foods or pharmaceutics, like gels of ι-carrageenan in milk-based deserts3 or commercial hydrogels (constituted of an entangled network of two randomly grafted * To whom correspondence should 04.67.14.46.20; fax 04.67.14.33.04.
be
adressed:
tel,
(1) Tadros, T. F. The Effects of Polymers on Dispersion Properties; Academic Press: London, 1982. (2) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.
synthetic polymers) presenting interesting applications in the release of medications, due to their sensitivity to shear forces, adherence to biological tissues, and response to temperature.4 N-Acrylylglycinamide (AG) is a vinylic monomer whose synthesis has been described from several years.5 It is known to possess the ability for producing permanent (irreversibly cross-linked) gels upon its own free-radical polymerization in aqueous media. Various amounts of 2-propanol, acting as a chain-transfer agent, were added in these polymerization systems in order to obtain soluble polyacrylylglycinamide samples (PAG) and for molecular weight control. It has been found that these homopolymers yielded thermally reversible physical hydrogels. These systems were recognized to offer the opportunity for obtaining synthetic photographic gelatin substitutes and for understanding the mechanism of the gelation process.6 Equilibrium swelling and modulus measurements, connected with low values of the heat of gelation cross-linking (-∆hc) between 21 and 50.2 kJ/mol (determined by differential thermal analysis, DTA), and the absence of DTA-detectable fusion endotherms, supported the assumption of Hass et al.7 that for aqueous thermally reversible PAG gels, crystallite formation played no part in cross-link formation, unlike gelatin gels.8 In the present work, the rheological properties of physically cross-linked polyacrylylglycinamide hydrogels at 4% (w/w) have been studied. The results have been compared with those of a gelified suspension containing a 10% (w/w) talc particle concentration. The influence of aging time on the viscoelastic properties has been investigated. The resulting composite hydrogels have been expected to find some interesting applications due to their storage stability and flow properties under mechanical treatment. (3) Picullel, L. In Food Polysaccharides; Dekker, Inc.: New York, 1995. (4) Dagani, R. In Chem. Eng. News 1997, 75, 26. (5) Haas, H. C.; Schuler, N. W. J. Polym. Sci., Part B 1964, 2, 1095. (6) Haas, H. C.; Moreau, R. D.; Schuler, N. W. J. Polym. Sci., Part A-2 1967, 5, 915. (7) Hass, H. C.; Manning, M. J.; Mach, H. M. J. Polym. Sci.: Part A-1 1970, 8, 1725. (8) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992.
S0743-7463(98)00054-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998
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Experimental Section Preparation of a PAG Sample. Following the experimental procedure of Haas et al.,6 a polyacrylylglycinamide sample was synthesized by using a homemade N-acrylylglycinamide monomer,5 [AG] ) 0.5 mol/L, a free-radical initiator potassium persulfate (Fluka), [K2S2O8] ) 5 × 10-4 mol/L, and with a chaintransfer agent 2-propanol (Aldrich) at concentration [iPrOH] ) 1 mol/L, in an excess of water. The reaction was monitored at 358 K during 2 h. The soluble PAG sample was then precipitated in methanol and recrystallized twice in pure acetone.
Figure 1. Variation at 298 K of the reduced viscosity as a function of the concentration of the synthesized PAG sample in 2 M NaSCN.
Talc Sample. A mineral sample of talc (Mg3Si4O10(OH)2) was supplied by Luzenac Europe society (Trimouns, France). The specific surface area, measured by nitrogen gas adsorption at 77 K (BET method, am(N2) ) 16.2 Å2) was found to be 6 m2/g. The mean average particle size was found to be 3.3 µm in sedigraphic measurements (Sedigraph apparatus). The talc samples were used as received without any specific treatment. Talc is a trioctaedric phyllosilicate composed of three hydrated layers, where an octahedral plane of brucite (Mg(OH)2) is located between two tetrahedral sheets of silicate (interlayer distance 9.36 Å). The atoms within the layers are held together through ionic bonds whereas weak residual forces (van der Waals type) held the oxygen-oxygen interlayer atoms.9,10 Preparation of PAG Hydrogels. Four PAG hydrogel samples were prepared by dissolving, at 348 K during 20 min, a required amount of the synthesized polyacrylylglycinamide solute in pure water (10 mL), to obtain a concentration of 4% (w/w). In two of the warm PAG solutions, denoted as Test10 and Gel10, a given amount of talc particles was added to obtain the desired 10% (w/w) concentration. After agitation, all the solutions were cooled upon storage at 298 K up to their gel state. The resulting hydrogels, denoted as Gel0 (for PAG gel without any talc particle) and Gel10 (for PAG gel containing 10% talc particle concentration), were allowed to rest at 298 K for 1 month before any rheological study; whereas Test0 and Test10 samples were studied after 1 day at rest. Test0 and Gel0 samples were transparent clear hydrogels whereas Test10 and Gel10 samples were found to produce macroscopically homogeneous gelified suspensions. Adsorption Test. The adsorption experiments were carried out in stoppered glass tubes. Distilled and deionized water was taken as a solvent. Each test tube contained about 2 g of talc powder and the same mass (20 mg) of aqueous polymer solution of given concentration (stock solution 1 g/kg of solvent). The suspensions were equilibrated by slow agitation for 12 h at 298 K. After equilibrium, the solid samples were then separated from the supernatants by filtration through 0.45 µm cellulosefree acetate membrane filters (Millipore France). A finite volume of the clear supernatant was removed from each tube (using a long needle glass syringe) and analyzed for the solute PAG content by the combustion/nondispersive infrared gas analysis method with a total organic carbon analyzer (TOC-5000A, Shimadzu Japan). The amount adsorbed of the polymer sample, Γ, was calculated from the experimental data using the following relation
Γ)
m°1(C°2 - Cb2) mS
(in g/g of solid)
(1)
where m°1 is the initial mass of solvent (in kg), mS is the mass of solid salmple (in g), and C°2 and Cb2 are respectively the mass (9) Pask, J. F.; Warner, M. F. J. Am. Ceram. Soc. 1954, 37, 118. (10) Fuerstenau, M. C.; Lopez-Valdieso, A.; Fuerstenau, D. J. Miner. Process. 1988, 23, 161.
concentrations (in g/kg) of the initial polymer solution (before adsorption) and the equilibrium bulk solution (after attainment of adsorption equilibrium). Rheological Measurement. Dynamic moduli and steadystate shear experiments were measured by using the Physica UDS 200 rheometer (Paar Physica Instrument) with a coneplate geometry (plate diameter, 40 mm; cone angle, 2°; gap, 0.05 mm). Oscillatory shear measurements were performed at a maximum strain amplitude of 8%. The level of the strain was checked in order to ensure that all measurements were made within the linear viscoelastic regime. A controlled waterevaporation system was used for temperature-dependent experiments. A standard capillary viscosimeter (Bioblock apparatus) was used to determine the intrinsic viscosity from diluted solutions of the synthesized PAG sample in NaSCN, 2 M at 298 K.
Results and Discussion Characterization of the Synthesized PAG Sample. According to Haas et al.,6 2 M NaSCN was found to be a better solvent than pure water for polyacrylylglycinamide. The reported values of the intrinsic viscosity [η] and the Huggins constant k′, determined from viscosity measurements on various PAG samples, were lower in 2 M NaSCN than in water at 298 K. It was suggested that the presence of NaSCN could allow the disruption of soluble aggregates that might be present in water. They proposed6 a correspondence for [η] values obtained in water and in 2 M NaSCN at 298 K and a direct relationship between the intrinsic viscosity [η] and the number average molecular weight M h n of the samples, determined from osmotic measurements in 2 M NaSCN:
[η] ) (1.16 × 10-3)M h n0.52
(2)
Moreover, the obtained wide range of the k′ values was ascribed to the effect of pronounced branchings resulting from chain-transfer reactions with monomer and/or polymer;11 thus justifying the ability for these branched macromolecules to create gelified networks. The variation at 298 K of the reduced viscosity of 2 M NaSCN aqueous solutions versus the concentration of the synthesized PAG sample, is displayed in Figure 1. The intrinsic viscosity of the PAG sample was found to be [η] ) 0.39 dL/g. According to the correlations of Haas et al.,6 the obtained value corresponds to 0.5 dL/g in water and to a number average molecular weight about 72 000 g/mol. The concentration at the overlap threshold c*, for which the density of polymer chains is large enough to enable individual chains to overlap and for entanglements to begin to occur, may be evaluated by means of the (11) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
Behavior of Cross-Linked Hydrogels
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approximation c* ) 1/[η].12 It corresponds to a concentration of 2% (w/w) in water. Since PAG hydrogels are obtained from the cooling of semidiluted polymer solutions at a concentration of 4%, twice the critical overlap concentration c* (2%), sufficiently long and branched homopolymer chains are present so as to achieve the formation of a continuous macromolecular pathway in the whole volume. In such physical gels, crosslinks originate from low-energy physical interactions, like hydrogen bonds, and they are not limited to single points on the PAG chains but rather correspond to more or less extended junction zones. The cohesion of these physical networks is assumed to result from intermolecular entanglements between branched PAG chains, stabilized by numerous hydrogen bonds. Adsorption of PAG on the Talc Particles. To investigate more deeply the interactions that might exist between the talc particles and polyacrylylglycinamide chains, an adsorption isotherm was performed. Increasing amounts (from 0.05 to 1 g/kg) of the synthesized PAG sample were added in aqueous suspensions of talc particles at 10% (w/w). Within these conditions chain entanglements are not expected to occur, since the studied concentration range is well below the critical overlap concentration c* (20 g/kg). The adsorption isotherm at 298 K is shown in Figure 2. The adsorption curve is characterized by a steep initial rise for the lower equilibrium concentrations, and by a plateau region indicating saturation of the surface. The maximum amount adsorbed at the plateau corresponds to 6.4 mg/g or 1.07 mg/m2 in average. The pH of the equilibrium supernatants was found to be nearly constant around 7.5, whereas the pH value of the equilibrium supernatant for talc in pure water was about 9. This result suggests that the adsorption of the neutral PAG sample was accompanied by a charge regulation mechanism. The shape of the adsorption curve indicates that there is a rather strong affinity between the PAG chains and the surface of the talc particles. In a previous study,13 it has been already pointed out that the predominant force responsible for polyacrylamide adsorption (a polymer having the same amide functions) on oxide minerals was hydrogen bonding. It was emphasized that not only the electronegative CdO group could interact with the neutral proton-donating MOH group of the oxide surface but also the weakly acid NH2 function could form hydrogen bondings with the negative sites MO-. In the present case, the interaction between the NH2 groups and the negative sites MO- of the talc particles may induce a partial neutralization of the surface charge, leading to a
decrease in the negative surface potential. This effect may influence the surface amphoteric dissociation process to the formation of additional negatively charged sites in order to counteract the diminution in the surface potential.14 Consequently, changes in the surface charge are accompanied by a release of protons from the surface; it may explain the observed decrease of the pH value in the equilibrium phase from 9 to 7.5 in the presence of the adsorbed PAG molecules. Therefore, it can be expected that PAG chains are adsorbed onto the lateral surfaces of the talc particles through hydrogen bondings since the majority of the hydroxylated groups are present on these brucite layers.15 However it is possible that the adsorption of some chain segments onto the hydrophobic areas of talc surface plays a role. The adsorption of PAG is probably reinforced due to the fact that water is not a very good solvent for polyacrylylglycinamide, as already indicated.6 From the qualitative analysis of the turbidity of the supernatants and the aspect of the precipitates of the adsorption tubes, some information can be obtained concerning the behavior of the system. For the first three points of the isotherm, the supernatant was rather clear and the precipitate easily redispersable, whereas for the following points the supernatant became increasingly turbid and the precipitate more compact and difficult to redisperse by shaking. For lower polymer concentrations, incomplete surface coverage may ensure on each particle additional adsorption of branched segments of one or more PAG chains attached to other particles. The branched PAG chains are able to form floc structures consisting of many talc particles that may settle into voluminous openaggregates. These systems can redisperse readily on shaking. When a polymer chain adsorbs onto more than two particles and causes flocculation in colloidal suspensions, the effect is referred to as bridging flocculation.2 Although this phenomenon has been already described, it is amplified and becomes much more significant in the case of the branched PAG chains. At higher polymer concentrations, when the lateral surfaces of the particles become fully coated by segments of chains attached solely to one particle, a thick and strongly adsorbed polymer layer is produced so that steric stabilization can occur. The dispersion state of the system increases. However, when the unaggregated particles sediment, they contribute to form a more dense close-packed precipitate. As no phase separation nor any sedimentation of talc particles were observed for Test10 and overall Gel10 sample, it indicates that the presence of the PAG polymeric network has allowed the stabilization of the dispersed talc particles. Due to the high PAG chain concentration used, it can be assumed that the particles are dispersed in the suspension and that some of the adsorbed polymer segments participate to the formation of the surrounding entangled network of branched PAG molecules during the cooling and the aging at 298 K. That may explain why the Gel10 suspension is so kinetically stable; the talc particles should remain dispersed and grafted to the connected network created by the polymer chains. Viscoelastic Properties of PAG Gels. To characterize the viscoelastic behavior of each hydrogel sample, the storage modulus G′ (measuring the solidlike elasticity of the material) and the loss modulus G′′ (measuring the viscous flow properties of the material) were determined in oscillatory shear measurements. At 298 K, with a 1 Hz frequency and a 8% strain amplitude, both moduli were
(12) Durand, D. In Structure des Polyme` res et Me´ thodes d’Etudes, GFP Series, 1990; Vol. 8. (13) Lee, L. T.; Somasundaran, P. Langmuir 1989, 5, 854.
(14) Zajac, J.; Trompette, J. L.; Partyka, S. Langmuir 1996, 12, 1357. (15) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Langmuir 1997, 13, 6260.
Figure 2. Adsorption isotherm of PAG sample onto talc at 298 K.
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Table 1. Viscoelastic Parameters of PAG Hydrogel Samples strain amplitude, 8%; frequency, 1 Hz sample Test0 Test10 Gel0 Gel10
G′
(N/m2)
0.92 0.594 1.35 0.715
G′′
(N/m2)
0.264 0.233 0.275 0.240
|G*|
(N/m2)
0.957 0.638 1.378 0.754
found to be constant throughout the time analysis (15 min), and average values were taken. The norm of the complex shear modulus, |G*| ()(G′2 + G′′2)1/2), and the loss tangent value, tan δ ()G′′/G′), were also determined. The obtained values are reported in Table 1. The measurements indicate that the presence of the talc particles provokes a lowering of the rigidity of the gelified networks, with a significant decrease of the storage modulus. Cross-linked polymers have elastic properties that are adequately described by the simple theory of rubber elasticity. The dynamic storage modulus at the rubber plateau G°N is related to the number of elastically active network chains per unit volume ν by G°N ) υRT.11,12 In the case of physical gels, as trapped entanglements between polymer chains act as effective cross-links, it suggests that the talc particles have induced a reduction of the density of the entanglements between the constitutive polymer chains. However, there are still sufficient physical junctions to allow the polymer chains to ensure the cohesion of the gelified suspension. Moreover, the comparative study of the results obtained after 1 day and 1 month at rest (Test0 and Gel0 samples) indicate that the rigidity of the networks and their elastic contribution increase with aging time. It suggests that the gelified systems evolve by creating new, more stable structures by formation of a higher number of junction zones. However, this increase is less pronounced in the presence of the talc particles (Test10 and Gel10 samples). It may be assumed that the talc particles prevent the polymer chains from constituting such structured systems with aging time probably because, due to the adsorption of polymer molecules, the mobility of the constitutive PAG chains has been reduced. Measurements of the oscillatory shear moduli were frequently used to study continuously the viscoelastic properties of cross-linking systems from the sol (or gel) state through the transition to the gel (or sol) state.16-18 According to a traditional suggestion, the gel point was believed to correspond to the intersection of the storage (G′) and loss (G′′) moduli; that is, the point at which the loss tangent is tan δ ) 1. However more recently, experiment and theory indicated that at the gel point, G′ and G′′ exhibit a power-law dependence on the oscillation frequency and that gelation may occur before or after the crossover of G′ and G′′ at a specified frequency.19,20 Several theoretical analyses have developed expressions for the frequency dependence of G′ and G′′ at the gel point, using the fractal scaling concept to define the gel network structure.20-22 The results of these investigations indicated that at the gel point
tan δ )
G′′ nπ ) tan G′ 2
( )
(3)
The fractal exponent n was predicted theoretically and various experimental studies were aimed at testing the (16) Eloundou, J. P.; Feve, M.; Gerard, J. F.; Harran, D.; Pascault, J. P. Macromolecules 1996, 29, 6907. (17) Nystro¨m, B.; Walderhaug, H.; Hansen, F. K.; Lindman, B. Langmuir 1995, 11, 750. (18) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367.
melting point tan δ
t (°C)
tan δ
n value
0.287 0.392 0.204 0.336
33.8 30.9 35.3 31.6
1.423 1.522 0.573 0.573
0.61 0.63 0.33 0.33
validity of the theoretical models.23-30 Several gelling systems (especially chemical gels) supported the percolation model with n ) 0.72 or 0.67 in the Rouse limit.22 In previous studies,6 the melting point of various PAG hydrogels, differing from the molecular weight and concentration of the polymer chains, was estimated. The temperature at which the gels began to descend into inverted test tubes, which were immersed in a temperature-controlled water bath, was recorded as the melting point. As expected, the melting point was found to increase with the increase of the concentration and the molecular weight of the homopolymer chain. In the present study, the determination of the gel point for incipient PAG gel formation was not investigated. However the melting point of the various hydrogel systems was determined by observation of a frequency-independent value of tan δ, obtained from a plot of tan δ versus the temperature,17 at three measuring frequencies: 0.1, 0.5, and 1 Hz. The results are reported in Table 1. The typical behavior for Gel0 and Gel10 samples is shown in Figure 3. All the systems exhibit a similar behavior, a continuous increase in tan δ with increasing temperature where the increase is more pronounced for the lowest frequency. Both moduli G′ and G′′ decrease with the increase of the temperature; the reduction of the storage modulus is however much more significant (see Figure 4). With increasing temperature, the elasticity of the network decreases due to the reduction of the total number of effective cross-linking junctions and probably also to the release of some bound water molecules. Indeed, as it has been already suggested,8,12 water may exist in two different physical states in cross-linked polymeric gels. In the swollen state of the gel most of the water is in a free state; however some water molecules are assumed to be associated with the polymeric chains of the network and they may participate in the molecular structure of the physical junctions through hydrogen bondings with the CO and/or NH2 groups of the amide moieties. The obtained melting temperatures (see Table 1) reveal that these systems are able to flow easily for lowtemperature values. Moreover it indicates that the presence of talc particles increases the ability for these physical networks to transit to the sol state. This result is in accordance with the previous findings and confirms the influence of the talc particles on the reduction of the density of entanglements between the polyacrylylglyci(19) Winter, H. H. Polym. Eng. Sci. 1987, 27, 1698. (20) Derrida, B.; Stauffer, D.; Herrmann, H. J.; Vannimenus, J. J. Phys. Lett. 1983, 44, L701. (21) Muthukumar, J. J. Chem. Phys. 1985, 83, 3161. (22) Martin, J. E.; Adolf, D. Annu. Rev. Phys. Chem. 1991, 42, 311. (23) Durand, D.; Delsanti, M.; Adam, M.; Luck, J. M. Europhys. Lett. 1987, 3, 297. (24) Hodgson, D. F.; Amis, E. J. Macromolecules 1990, 23, 2512. (25) Rubinstein, M.; Colby, R. H.; Gillmor, J. R. Polym. Prepr. 1989, 30, 81. (26) Scanlan, J. C.; Winter, H. H. Macromolecules 1991, 24, 47. (27) Martin, J. E.; Adolf, D.; Wilcoxon, J. P. Phys. Rev. Lett. 1988, 61, 2620. (28) Hsu, S.; Jamieson, A. M. Polymer 1993, 34, 2602. (29) Michon, C.; Cuvelier, G.; Launay, B. Rheol. Acta 1993, 32, 94. (30) Lin, Y. G.; Mallin, D. T.; Chien, J. C.; Winter, H. H. Macromolecules 1991, 24, 850.
Behavior of Cross-Linked Hydrogels
Figure 3. Viscoelastic loss tangent as a function of temperature for (a) Gel0 sample and (b) Gel10 sample, at different frequencies: 0.1 Hz (9); 0.5 Hz (]); 1 Hz (O).
Figure 4. Variation of the logarithm of G′ and G′′ as a function of temperature for (a) Gel0 sample and (b) Gel10 sample, at 1 Hz frequency: log G′ ([), log G′′ (O).
namide chains. The comparative results between Test0 and Gel0 samples indicate that higher energy is required to melt the gels with aging time. It is in accordance with the observed increase of the gel rigidity with time. These results suggest that the systems become more structured; it may result from the formation of more dense packing of junction zones with aging time. From Figure 3, it can be observed that the loss tangent value for which the gelified systems melt is nearly the same for Gel0 and Gel10 samples, tan δ ) 0.573. It corresponds to a n value of 0.33. Since the n value has been shown to be correlated with the fractal dimension
Langmuir, Vol. 14, No. 16, 1998 4479
of a given gelified system,21,22 it suggests that although the presence of the talc particles at 10% (w/w) concentration contributes to lower the strength of the polymeric network, it does not modify to a great extent the way the constitutive aggregates occupy the volume to ensure gelation. The same trend is observed for Test0 and Test10 samples (see Table 1). However, the n value after 1 month is lower than that of Test samples and that predicted from dynamic scaling based on percolation theory.22 In fact, exponent values were found to be not universal. In the literature many experimental studies have reported that n values were located in the range 0.1-0.9, depending on the fractal dimension of the network, the stoichiometry of the gel, the strength of the hydrodynamic interaction between the polymer chain segments, and the molecular weight of the polymer chain.25-27 It is particularly the case for physical gels, for which the transient nature of the network junctions renders difficult to study these systems.12,28 For the thermoreversible gelling system of gelatin29 the values of n were shown to depend strongly on the concentration and on the thermal history of the gel (aging temperature and aging time). At a fixed concentration, the n values were found to decrease notably as the aging time increased. In the present case, the decrease of the n value to 0.33 may be correlated with this effect since the gels were stored at 298 K during 1 month. As suggested previously, it may correspond to the formation of more stable structures in which the junctions between the polymer chains are reinforced with aging time. According to the framework of the fractal model of Muthukumar,21 the observed decrease of the relaxation exponent n with time indicates that the fractal dimension increases. This could be interpreted as corresponding to the formation of a more tight network structure. Moreover in the case of the aging of physical gels, it can be assumed that the obtained n values are lower than that given by scaling predictions since in these models, chain entanglements and physical junctions are assumed to be motionless. From a more general point of view, these results confirm that in physical gels the cross-links or junction zones fluctuate with time and temperature. In summary, these results suggest that two main consequences should result when the particles are mixed with the polymer chains in the semidilute concentration domain. Since the PAG chains can interact strongly with the hydroxylated surfaces of the talc particles through hydrogen bondings, fewer entanglement junctions are available between the polymer chains in the network. Moreover it can be assumed that the extent of hydrogen bondings that might stabilize these entangled polymer chains is also reduced; thus resulting in a decrease of the global elasticity of the network. However, the stability of the gelified suspension suggests that the talc particles act as anchoring sites for branched PAG chains where some of them are involved into the formation of the entangled network, ensuring the cohesion of the system. These results would argue in favor of the picture of more labile and weak entangled systems in which the talc particles would be dispersed and trapped due to their adsorption with the constitutive polymer chains of the network, thus preventing them to sediment (see schematic representation in Figure 5). Thixotropic Behavior of PAG Gels. The shear rate dependence of the viscosity of Gel0 and Gel10 samples was investigated in permanent regime. The comparative curves are displayed in Figure 6. The results indicate that both gelified systems are broken down under the influence of the shear force, resulting in shear-thinning behavior within the investigated 0-800 s-1 shear rate
4480 Langmuir, Vol. 14, No. 16, 1998
Trompette et al. Table 2. Elasticity Ratio Values of Gel0 and Gel10 Samples Gel0 time (h)
G′(t) (N/m2)
G′′(t) (N/m2)
initial 0 0.5 1 1.5 2 2.5 3
1.33 0.628 0.646 0.738 0.851 0.976 1.15 1.194
0.275 0.299 0.257 0.245 0.237 0.235 0.235 0.234
Gel10 E(t)
G′(t) (N/m2)
G′′(t) (N/m2)
E(t)
0.979 0.903 0.929 0.949 0.963 0.972 0.978 0.981
0.715 0.3 0.283 0.365 0.435 0.565 0.709 0.72
0.24 0.233 0.21 0.205 0.207 0.21 0.215 0.221
0.948 0.79 0.803 0.871 0.903 0.937 0.957 0.95
Figure 5. Schematic representation of the gelified suspension in the presence of the talc particles.
Figure 7. Variation of the apparent viscosity of Gel0 sample ([) and Gel10 sample (O) as a function of time at 100 s-1 constant shear rate.
Figure 6. Variation of the apparent viscosity (in logarithmic scale) as a function of the shear rate for Gel0 sample ([) and Gel10 sample (O) at 298 K.
range. The polymer chains, which are all adhering to each other, give to the system a very high viscosity. However the presence of the talc particles reduces the density of entanglements between the polymer chains in the network, so that the resistance of the gelified suspension to flow is lower. At any shear rate, the apparent viscosity of Gel10 sample is lower than that of Gel0, which is consistent with the previous results. The thixotropic behavior of Gel0 and Gel10 hydrogels was evaluated by measuring at 298 K in dynamic regime (1 Hz frequency with a 8% strain amplitude) the variation of the viscoelastic properties of the samples at constant time intervals (30 min), once they have been sheared at a constant shear rate 100 s-1 during 4 min in the steadystate regime. Prior to application of this mechanical treatment, the viscoelastic parameters were measured. For such time-dependent experiments, an experimental elasticity ratio at time t, E(t), may be defined as
E(t) )
G′(t) |G*(t)|
(4)
where G′(t) is the storage (or elastic) modulus at time t and |G*(t)| is the norm of the complex shear modulus at the same time t. The evolution of the elasticity ratio as a function of time is assumed to allow appreciation of the ability for such physical hydrogels to recover a structure and to study the influence of the talc particles on the recovery of elasticity. The obtained values are listed in Table 2. Figure 7 shows the variation of the apparent viscosity for both samples during the mechanical treatment at 298 K. The applied shearing forces cause the gel structures to break down; they transform into a liquid whose apparent
Figure 8. Variation of the experimental elasticity ratio as a function of time for (a) Gel0 sample and (b) Gel10 sample.
viscosity decreases with shearing time. It can be noted that the major part of the destruction phase of the gel samples occurs during the primary 50 s of the shearing process (at 100 s-1), since the ratio ηapp(50)/η(0) is less than 1/2. The evolution of E(t) as a function of time, for Gel0 and Gel10 samples, is plotted in parts a and b of Figure 8, respectively. Time scale 0 corresponds to 5 min after the
Behavior of Cross-Linked Hydrogels
end of the mechanical treatment. The initial elasticity ratio value (see Table 2) is symbolized by the horizontal dashed line on the figures. The kinetics of the elasticity recovery are similar and are characterized by a rather rapid increase of E(t) after the end of the mechanical treatment and by a progressive slowdown to reach an apparent equilibrium, corresponding to the initial elasticity ratio. These results show that during the shearing process, the destruction phase of the gels is accompanied by a large decrease of the storage modulus since the values of G′(initial) are greater than twice that of G′(0) for both samples (see Table 2). However the begining of the reconstitution of the gelified structures is rather fast since G′ is greater than G′′ at time scale 0. These results indicate that the gel structure is able to re-form. The suspension was found to return to having a high viscosity and to be still macroscopically homogeneous, denoting the storage stability of the talc particles. Figure 8 reveals clearly the mechanical reversibility of these systems since they recover their initial viscoelastic properties after less than 3 h. Moreover, the results show that the mechanical reversibility of PAG hydrogels has not been altered by the presence of a 10% talc particle concentration. It suggests that the coated talc particles do not prevent the constitutive polymer chains to re-form a comparable entangled
Langmuir, Vol. 14, No. 16, 1998 4481
system as that before application of the mechanical treatment. Conclusions Although PAG gels were known from several years, the study of their rheological behavior was practically nonexistant, especially in oscillatory shear regime. The thermoreversible gelation of the studied gel samples indicated that the melting occurred for low temperatures (
