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J. Phys. Chem. 1996, 100, 17316-17322
Effects of Temperature and Shear History on the Network Properties of Amphiphilic Hydrogels Shun-Yuan Wu,† Ramesh Varadaraj,‡ and Carol A. Steiner*,† Department of Chemical Engineering, The City College of New York, 140th Street and ConVent AVe., New York, New York 10031, and Exxon Research and Engineering Co., Rte. 22E, Clinton Township, Annandale, New Jersey 08801-0998 ReceiVed: June 25, 1996; In Final Form: August 13, 1996X
We report on thermally irreversible hydrogels made from un-cross-linked hydrophobically modified (hydroxyethyl)cellulose in solution with sodium dodecyl sulfate (SDS). The effects of temperature and shear history on bulk and microstructural properties of these gels were investigated over a temperature range 2570 °C. The bulk properties investigated were composition, gel volume, and dynamic storage and loss moduli. Microstructural properties of the networks were investigated using two spectroscopic probes, pyrene and ET(30), which gave complementary information about the packing density and surface hydrophobicity, respectively, of the hydrophobic microdomains in the gels. The hydrogels exhibit an irreversible transition in both their bulk properties and the structure of the microdomains at a temperature in the range 35-50 °C. Below the transition, thermal gelation is observed as the polymer chains expand with temperature and SDS is driven to associate with the side chains. Above the transition temperature the gels collapse, and the dynamic storage modulus of the gels goes down. At the same time the hydrophobic microdomains become increasingly porous. A mechanism for these changes is proposed, and the properties of these materials are contrasted with those of other temperature-sensitive hydrogels.
Introduction Temperature-sensitive hydrogels have received much attention in recent years because their properties may be exploited in numerous separations and controlled release applications. When a hydrogel expands in an aqueous solution, water and small solutes will partition into the gel while macromolecules such as proteins and enzymes will be excluded and will form a concentrated supernatant phase. Thus, these materials can be used to extract water from aqueous solutions of macromolecules.1 Conversely, when a gel collapses, water and small solutes are released into the aqueous supernatant. Gels that undergo steep volume transitions at relatively mild temperatures are particularly attractive for these purposes, as the energy required for the separation or release is small. Reversible gels are desirable if they are to be regenerated for reuse. Most of the research on temperature-sensitive hydrogels that has appeared to date has focused on chemically cross-linked gels such as those made from poly(N-isopropylacrylamide) (NIPA),1-6 which collapse at 34 °C, cellulose ethers with some incorporated hydrophobe,7 which collapse in the range 35-60 °C, and poly(2-hydroxyethyl methacrylate) (poly-HEMA).8 All of these transitions are reversible, reflecting the fact that the polymer dimensions govern the gel volume. The relatively low transition temperatures of these materials may be attributed to the amphiphilic structure of the polymers, which also gives rise to lower critical solution temperature (LCST) behavior. Moreover, the transition temperatures of hydrogels correspond within a few degrees to the LCST of the component macromolecules. Two un-cross-linked polymer/surfactant systems have also been found to form reversible hydrogels at elevated temperatures. High molecular weight ethyl (hydroxyethyl)cellulose (EHEC) (Mv ) 660 000) in the presence of both anionic and †
The City College of New York. Exxon Research and Engineering Co. X Abstract published in AdVance ACS Abstracts, October 1, 1996. ‡
S0022-3654(96)01897-7 CCC: $12.00
cationic surfactants changes reversibly from a homogeneous viscous solution to a homogeneous viscoelastic hydrogel on increasing the temperature, with a peak in low shear viscosity at 60 °C.9 Thermal gelation does not occur in solutions of EHEC in water or in surfactant solutions containing relatively low molecular weight EHEC (Mv ) 150 000), all of which exhibit a drop in viscosity with temperature consistent with the collapse of marginally soluble individual polymer chains. It was concluded that gelation occurs because the surfactant forms mixed micelles with the pendant ethyl groups on the polymer, bridging macromolecular chains. This association is favored at high temperatures where the polymer becomes dehydrated, hence more hydrophobic; the fact that the phenomenon is reversible is presumably a consequence of the rehydration of the chains accompanied by a change in their conformation on cooling.10,11 Gelation does not occur with the low molecular weight polymer because it is too hydrophilic, as indicated by its relatively high cloud point in water. The gel point is shifted toward lower temperatures with increasing polymer concentration.12 Reversible thermal gelation has also been reported in solutions of hydrophobically modified poly(sodium acrylate) and nonionic surfactants.13,14 In these systems the gelation temperature coincides with the temperature at which the surfactant aggregates change from micelles to bilayers. It was postulated that the resulting surfactant vesicles are large and widespread enough in the solution to incorporate hydrophobic side chains from numerous polymer molecules, resulting in the formation of a strong network. On cooling, the surfactant aggregates revert to small micelles, and the network structure breaks down. Our group has been investigating viscoelastic hydrogels made from a water-insoluble hydrophobically modified (hydroxyethyl)cellulose (HMHEC) in aqueous systems.15-18 The hydrogels precipitate out of solution and form a separate phase in equilibrium with a Newtonian supernatant containing little or no polymer. They are physical gels, in that the cross-linking interactions that define the three-dimensional network structure (i.e., the hydrophobic clusters) are physical, rather than chemical, © 1996 American Chemical Society
Network Properties of Amphiphilic Hydrogels in nature. Nevertheless, they exhibit rheological properties comparable to those of permanent networks, with high (103104 dyn/cm2) plateau values of the dynamic storage modulus, G′. This indicates that the hydrophobe-hydrophobe interactions tend to persist longer than any other type of interaction in the system.19 The activation energy for disengagement of these clusters has been estimated to be of order 0.9kT per methylene group for pairwise interactions.20 Thus, the strength of the gel will depend on the number and composition of the microdomains. The hydrogels may be dried into films and reswollen to a constant volume without dispersing.16 Viscoelastic hydrogels from HMHEC have been made to date using two different solvents, ethanol/water15,16 and aqueous sodium dodecyl sulfate (SDS) below the critical micelle concentration (cmc) of the surfactant.17,18 The function of the ethanol and SDS, respectively, is to solubilize the hydrocarbon side chains that render the polymer insoluble in water alone. However, in the ethanol/water system the side chains are solubilized within the bulk solution and can aggregate intermolecularly. Thus, the solvent composition at which the polymer solubility (as measured by the Hildebrand solubility parameter) is optimized also produces the strongest gels. On the other hand, in the SDS/water system the side chains are solubilized in mixed micelles with the surfactant. Below the cmc each mixed micelle contains side chains from several different polymer molecules, and viscoelastic hydrogels result. As the surfactant concentration is increased, the average number of side chains per aggregate goes down. The plateau storage modulus of the hydrogels drops, while the viscosity of the supernatant (and also the polymer concentration in that phase) climbs. At surfactant concentrations above the cmc the polymer is completely soluble, with the solution viscosity equal to that of control (backbone) polymer in water. Thus, in this system the weakest gels are formed in the best solvent, exactly the opposite of what we found in HMHEC/ethanol/water systems. These two examples illustrate that care must be taken in attempting to predict the behavior of the hydrogel from the apparent “goodness” of the solvent or the rheological properties of the precursor solution. In the ethanol/water case, the strongest gels arise when hydrophobic side chains are solubilized in the bulk solution. The polymer chains in both the solution and the resulting gel are maximally extended, and the probability of intermolecular interactions involving the side chains is highest. In the surfactant systems above the cmc, the surfactant has the effect of shielding the side chains from any interactions with the surroundings, so intermolecular hydrophobic interactions are precluded. Thus, the strength of the gel depends simultaneously on the equilibrium dimensions of the polymer in the precursor solution and on the extent to which the hydrophobic side chains are free to aggregate intermolecularly. Two factors that influence the polymer dimensions and dimensional stability in polymer/surfactant solutions, and hence, we expect, in the resulting hydrogels, are the thermal and shear histories of the system. The processing temperature will determine the dimensions of the polymer backbone in solution, its level of hydration (i.e., its hydrophobicity), and the extent of interaction between the surfactant and the side chains. The shear history will influence the conformation of the chains in solution, before the gel sets. If the gel sets while the polymer is in an unstable conformation, the stability of the gel might be compromised as well. On the other hand, if that unstable conformation exposes more side chains to favorable intermolecular interactions, the gel might be forced permanently into a relatively highly cross-linked state. We present below the results
J. Phys. Chem., Vol. 100, No. 43, 1996 17317 of our studies on the effects of temperature and shear history on HMHEC/SDS hydrogels. Experimental Section Hydrophobically modified (hydroxyethyl)cellulose (HMHEC) was a gift from Hercules, Inc. (Wilmington, DE). The HMHEC used here, which has been used by us in several other studies,15-18 has a molecular weight of 106, a hydroxyethyl molar substitution (HEMS) of 3.7 per anhydroglucose unit, and a hydrophobe degree of substitution (DS) of 1.33 wt % C12H25. This polymer forms cloudy solutions in water at room temperature. Its (hydroxyethyl)cellulose (HEC) backbone is soluble in water but insoluble in SDS solutions both below and above the cmc. The polymer was washed with ethanol to remove all conducting material; this extraction process was repeated until the specific conductance of the solvent was
