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Ind. Eng. Chem. Res. 1996, 35, 3100-3107
Polymer Precipitation Using a Micellar Nonsolvent: The Role of Surfactant-Polymer Interactions and the Development of a Microencapsulation Technique Sukanta Banerjee, Ramannair Premchandran, Murthy Tata, Vijay T. John,* and Gary L. McPherson*,† Departments of Chemical Engineering and Chemistry, Tulane University, New Orleans, Louisiana 70118
Joseph Akkara and David Kaplan U.S. Army Soldier Systems Command, Natick RDE Center, Natick, Massachusetts 01760
Conjugated phenolic polymers are precipitated from solution using a nonsolvent system containing sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles. When contacted with nonpolar fluids, these polymers coil-up at low concentrations as a result of intramolecular hydrogen bonding. Precipitation using a micellar nonsolvent results in highly dispersed polymeric particles whose internal voidage can be controlled by the water content of the micelles. Precipitation in such fluids also results in an encapsulation of intramicellar solutes (enzymes and/or ferrite nanoparticles), leading to the formation of microspherical composites with biocatalytic and/or magnetic properties. Introduction The interaction of surfactants with polymers is an area of great interest for applications ranging from enhanced oil recovery (Nagarajan and Harold, 1982) to emulsion polymerization (Sun and Ruckenstein, 1993) and polymer composite development (Qutubuddin et al., 1994). The present work reports observations on the influence of surfactant during phase separation and polymer precipitation. Specifically, we examine the role of surfactant when a polymer solution is contacted with a nonsolvent system containing surfactant aggregates in the form of water-in-oil microemulsions (the term reversed micelles, although somewhat incorrect, is used interchangeably here). The objective of the work is to determine whether intramicellar solutes, either inorganic nanoparticles or macromolecules such as proteins, can be entrapped within the polymer matrix during precipitation. Additionally, the objective is to determine if surfactant-polymer interactions influence the morphology of the precipitated polymer, specifically the formation of polymeric microspheres. The overall applied objective of the research is to develop a microencapsulation technique where the encapsulate confers its specific property to the composite. For example, ferrite particles encapsulated in polymeric microspheres confer magnetic properties to the composite. Similarly, encapsulated enzyme molecules confer biocatalytic properties to the composite. The research follows from our earlier observations on the enzymatic synthesis of phenolic polymers in reversed micelles (Karayigitoglu et al., 1995). Such reactions have their biological basis in the synthesis of lignin in woody tissues, where the substrate is more complex, typically a p-hydroxy- and methoxy-substituted cinnamyl alcohol. In our research, the substrate is simply an alkyl-substituted phenol. Figure 1 illustrates a simplified mechanism behind polymer synthesis. Upon addition of H2O2, phenoxy radicals are formed; * To whom correspondence should be addressed. Phone: 504-865-5883. Fax: 504-865-6744. E-mail:
[email protected]. edu. † Department of Chemistry.
S0888-5885(95)00788-3 CCC: $12.00
Figure 1. Simplified schematic of the polymerization reaction. The bold arrows (in the phenoxy radical) indicate coupling in the ortho positions. The resonance structures also imply the possibility of para coupling.
the radical centers migrate to the ortho-positions, following which coupling occurs. Polymer growth occurs by such stepwise condensation. Phenolic polymers are useful for a variety of applications as resins in coatings technologies (Kopf, 1985). The enzymatic route to these polymers avoids the use of formaldehyde typically used in the traditional chemical route to these polymers and is therefore an environmentally benign approach. But, additionally, the absence of formaldehyde as a reactant makes the enzymatically synthesized material somewhat different in that the polymers lack intervening methylene bridges between the rings. The direct ringto-ring coupling as shown in Figure 1 leads to a polymer that is conjugated and therefore electrooptically active. These polymers therefore have applications as conductive polymers and in nonlinear optics (Akkara et al., 1991). The synthesis of phenolic polymers in aqueous systems has limitations due to the low solubility of the monomer and the even lower solubility of the chain. However, synthesis in an organic medium that sustains chain solubility implies the necessity of enzyme activity © 1996 American Chemical Society
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Figure 2. (a) Chemical structure of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate. (b) Schematic of enzyme encapsulation in the micelle and monomer partitioning to the micelle interface. The arrow here refers to the monomer.
maintenance, and much work has focused on the activity of peroxidase in organic solvents (Akkara et al., 1991; Dordick et al., 1987). An effective compromise is the use of reversed micelles or water-in-oil microemulsions for enzymatic polymer synthesis. Here the enzyme is encapsulated in the micellar water pools and maintains catalytic activity. In earlier work, we have shown the feasibility of phenolic polymer synthesis sodium bis(2ethylhexyl)sulfosuccinate in (AOT) reverse micelles (Rao et al., 1993, Karayigitoglu et al., 1995). An interesting aspect of the reaction is the fact that the monomer, being somewhat amphiphilic, partitions to the oil-water interface, leading to a case of interfacial polymerization. The situation is shown schematically in Figure 2b; the head of the arrow represents polar hydroxyl groups of the phenol. The penetration of the monomer to the interfacial region can be clearly seen by monitoring hydrogen-bond interactions between the phenol hydroxyl groups and the surfactant polar head group, as will be discussed shortly. The interesting aspect of enzymatic polymerization in reversed micelles is the observation that the polymer precipitates out as both distinct and interconnected microspheres when there is sufficient surfactant in the system (Karayigitoglu et al., 1995). The possibility arises that there is a templating effect based on surfactant-monomer interactions that leads to such morphology. As a conservative estimate a 3/1 surfactant/ monomer ratio is necessary prior to synthesis to generate polymer in microsphere morphology. A second interesting aspect of the polymerization is that the precipitating polymer entraps intramicellar solutes. This includes both the catalytic enzyme, peroxidase, other cosolubilized enzymes, and nanoparticles such as ferrites initially solubilized in the micelles (Kommareddi et al., 1995). The present work is complementary and based on questions seeking to elucidate surfactant interactions with the monomer and polymer leading to morphology development. The experiments involve taking presynthesized polymer and dissolving it in a suitable solvent to destroy all gross traces of morphology. Subsequently, the polymer is reprecipitated by contact with a large excess of a reversed micellar solution which is a nonsolvent for the specific polymer (poly(p-ethylphenol)). Our objective again was to observe polymer morphology and to determine if the precipitating polymer entraps intramicellar solutes. The use of a nonsolvent to precipitate polymers is well-known. Some recent and fascinating aspects of
such work include phase separation by thermal or nonsolvent-induced entry into the spinodal region to produce bicontinuous precipitated phases (Aubert and Sylvester, 1991; Dixon et al., 1993), the rapid expansion of supercritical solutions to form a nonsolvent (Lele and Shine, 1992), and polymer precipitation into a supercritical fluid nonsolvent (Dixon et al., 1993). But the use of reversed micelles in the nonsolvent phase is rather interesting, especially when studied in the applied context of micellar solute encapsulation. The specific polymer used here, poly(p-ethylphenol), is also rather interesting due to its “sticky” nature as a consequence of its strong tendency to hydrogen bond. As described in this paper, it is possible to probe polymer conformation and interactions with the surfactant, through the infrared spectra of hydrogen-bonding chacteristics. Materials and Methods All chemicals were purchased from Sigma-Aldrich (St. Louis, MO) in analytical-grade purity. For this study, we have focused on one specific monomer, pethylphenol. The surfactant used is AOT, or sodium bis(2-ethylhexyl)sulfosuccinate, shown in Figure 2a. The organic bulk phase of reversed micellar systems is isooctane. The specific polymer used here is poly(p-ethylphenol) that is synthesized enzymatically using horseradish peroxidase, as shown in the schematic of Figure 1. Details of the polymerization are found in our earlier paper (Karayigitoglu et al., 1995), but the essence of the procedure is the assembling of all reaction constituents with the exception of H2O2, followed by reaction initiation by stepwise addition of H2O2. All reactions were carried out at ambient conditions. As shown in Figure 1, there is a 1:1 correspondence between monomer conversion and H2O2 added; i.e., each monomer unit added to the chain requires one molecule of H2O2. The precipitated polymer is washed several times with isooctane to remove all traces of adsorbed surfactant, dried, and stored. This is the polymer used in our studies with dissolution and reprecipitation. The polymer has negligible solubility in water and in aliphatic hydrocarbons, typically isooctane. It is moderately soluble in CCl4 and is very soluble in polar solvents (e.g., acetone, THF). The polymer has an average molecular weight between 1500 and 2000, as determined using gel permeation chromatography (polystyrene standards, and elution in a Jordi DVB column using tetrahydrofuran as the eluent). Here we must correct an error in our previous work (Rao et al., 1993; Karayigitoglu et al., 1995) where much higher molecular weights (90 000-400 000) were postulated. This may have been due to the fact that polymer aggregates were not broken, leading to rapid elution of aggregates and an apparent high molecular weight. A recent paper (Ayyagari et al., 1995) describes the use of LiBr in a DMF solvent to obtain accurate values of phenolic polymer molecular weights. Our results using a redesigned GPC system and THF as the solvent are now in agreement with these reported results. Since there were several different experiments dealing with polymer precipitation, the specific methodologies employed are described together with the observations to relate the observations to the methodology. All precipitation steps were carried out at ambient conditions. In characterizations, the polymer morphology was characterized by electron microscopy, using a JEOL 820 scanning electron microscope operating at 15 -30 kV
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Figure 3. IR spectra of the hydroxyl stretch of the monomer and polymer: (a) 0.1 M p-ethylphenol (EP) in CCl4. (b) 1 M EP in CCl4. (c) 12 mg/mL of poly(p-ethylphenol) (PEP) in CCl4. (d) 12 mg/mL of PEP in acetone.
and a Phillips 410 transmission electron microscope operating at 60 kV. Aluminum stubs (SEM) and carbon-coated copper grids (TEM) were used for mounting the samples. FTIR spectra were recorded using a ATI/Mattson Galaxy 6021 spectrometer. A liquid sample cell (SpectraTech) with CaF2 windows and path length 0.3 mm was used for liquid samples. UV-vis spectra were recorded using a Shimadzu UV-160 instrument. Results and Discussion Surfactant Interactions with the Monomer and the Polymer. Some fundamental understanding of AOT interactions with the monomer (p-ethylphenol) and the polymer can be gained through the FTIR spectra of these species in solution. Figure 3 illustrates control experiments where the monomer and polymer spectra were recorded in the absence of AOT. Trace a in Figure 3 illustrates the hydroxyl stretch of the monomer (pethylphenol) at a concentration of 0.1 M in CCl4, a nonpolar solvent that does not interact with either the monomer or polymer. The sharp peak at 3600 cm-1 indicates free hydroxyl, and it can be concluded that the monomer has minimal self-association at this concentration. However, at a concentration of 1 M (trace b), a second broad band appears centered at 3300 cm-1; the free phenol peak at 3600 cm-1 is also significantly lowered. This is an indication that, at higher concentrations, self-association takes place as a result of intermolecular hydrogen bonding. Trace c is interesting in that it represents the hydroxyl stretch of the polymer at a concentration of 0.012 g/L, which is the monomer equivalent of 0.1 M. In other words, the concentration of OH groups in the system of trace c is the same as that in trace a. It is therefore evident that the shift to low frequency indicates hydrogen bonding between phenolic groups of the polymer. Interestingly, the situation has been reported as far back as 1962 for novolaks (chemically synthesized phenol-formaldehyde polymers, where the phenol units are linked through methylene groups) dissolved in CCl4 (Cairns and Eglinton, 1962). The implication is that the hydrogen bonding is not intermolecular at these concentrations but rather intramolecular. A similar reasoning applies here, and the conclusion is reached that in nonpolar media, at low concentrations, intramolecular hydrogen
Figure 4. Space-filling models of a pentamer: (a) Coiling due to intramolecule interactions; (b) open conformation present in polar solvents. The dark spheres represent oxygen atoms, the smaller gray spheres, hydrogen, and the larger gray spheres, carbon.
bonding is prevalent. The broad band is the consequence of n hydroxyl interactions, n varying from 2 to 6. Trace d represents the case where the polymer is dissolved in acetone. Interestingly, all the hydroxyls now behave equivalently, with a sharp peak centered around 3410 cm-1. This must mean the loss of intramolecular hydrogen bonding and the interaction of the hydroxyls with the solvent CdO groups, which fully solvate the hydroxyls. A representation of the situation is shown in the space-filling models of Figure 4. Figure 4a represents a model of a pentamer where all OH groups are positioned together, situated on the same side of the polymer backbone. This may be the case with the polymer in CCl4. It is seen that the polymer acquires a natural curvature as a result of such interactions in nonpolar solvents. Approximately six monomer units are sufficient to complete one pitch of a possible helical conformation. In polar solvents such as acetone, solvent hydrogen bonding with the polymer may allow other configurations. Figure 4b represents a situation where the hydroxyl groups are randomly oriented due to interaction with the solvent. The results of this simple but interesting analysis do provide the information that when the polymer is placed in nonpolar media at low concentrations, intramolecular hydrogen bonding will result in a coiling of the chain. This may be a first step in the nucleation process when supersaturation occurs upon contact with a nonsolvent. At higher concentrations, it is possible that chain-chain interactions may compete with the intramolecular interactions. Figure 5 illustrates AOT-monomer interactions. The AOT concentrations here on the order 10-1-10-2 M, are higher than the critical micellar concentration of AOT in CCl4 (10-4 M). Although residual water levels even in dry AOT (Ueda and Schelly, 1988) could conceivably complicate spectra interpretation, the observed OH shift to the low-frequency band with increasing amounts of AOT clearly indicates hydrogen bonding between the monomer and the surfactant head group. The precise nature of monomer-AOT interactions as affecting micellar structure at these concentrations is not fully known at this time. We are conducting detailed NMR studies to look at the AOT conformation in the presence of the monomer, as probed by spin coupling between the ABX protons of the succinic bridge in AOT (Tata et al.,
Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3103
a
Figure 5. AOT effect on the monomer hydroxyl stretch. The monomer (p-ethylphenol, EP) concentration is kept constant at 0.1 M. The solvent is CCl4, and w0 ) 0.
1994a). In earlier studies, we have found that a dramatic phase change to an organogel occurs when more acidic phenols (e.g., p-chlorophenol) are contacted with AOT in CCl4 in an equimolar ratio (Xu et al., 1993; Tata et al., 1994b). With p-ethylphenol, similar gels are obtained in nonpolar hydrocarbon solvents, but not in CCl4. Monomer and polymer interactions with AOT can be compared by examining perturbations to the surfactant CdO stretch, a band specific to the surfactant. Parts a and b of Figure 6 illustrate a qualitative comparison between binding affinities of the monomer and polymer to the surfactant. If the extent of shift to lower frequency is considered a measure of interaction, it appears that interaction between the monomer and AOT in CCl4 is considerably greater than that between the polymer and AOT. Perhaps this observation could be rationalized as a consequence of the tendency of the polymer to intramolecularly hydrogen bond and to inhibit full AOT access to the polymer. To summarize, the FTIR spectra of monomer and polymer in CCl4 reveal the tendency of the polymer to intramolecularly hydrogen bond and thereby fold to perhaps more compact structures. AOT does interact both with the monomer and with the polymer, but monomer-AOT interactions are more pronounced than polymer-AOT interactions in nonpolar solvents. From the characterizations of interactions between the surfactant and the polymer/monomer, the molecular aspects, we proceed to the colloidal and particulate aspects of polymer precipitation upon contact with nonsolvent. Polymer Precipitation Using Reversed Micelle Containing Nonsolvent. Experiments were carried out where 1 mL of a solution containing 0.1 mg of poly (p-ethylphenol) dissolved in acetone was contacted with 20 mL of a nonsolvent, either neat isooctane or 0.5 M AOT dissolved in isooctane (method I). Acetone was used as the solvent for the polymer rather than CCl4, to completely solvate the polymer and to minimize intramolecular hydrogen bonding and polymer-polymer interactions. It must be reiterated, however, that this is just intuitive; we have been unable to distinguish polymer self-interactions and polymer-polymer interactions from polymer-solvent interactions in a polar solvent. Nevertheless, the tendency of phenolic hydroxyls to hydrogen bond with acetone carbonyl groups
b
Figure 6. (a) Monomer (EP) effect on the AOT CdO stretch. In trace a the AOT concentration is 0.1 M. In traces b and c the EP concentration is constant at 0.1 M and AOT is adjusted accordingly. The spectra are overlaid to avoid concentration effects on intensity. (b) Polymer (poly-EP) effect on the AOT CdO stretch. Here the AOT concentration is kept constant at 0.05 M, and the polymer level is adjusted accordingly.
indicates a strong possibility that the polymer attains a more random configuration in this solvent, as illustrated in Figure 4b. In conducting the precipitation, the only agitation provided was the initial contact of the excess antisolvent to the polymer solution. No additional stirring was done. As observed, a turbid layer develops due to particle nucleation; the turbid layer slowly compacts into a precipitated polymer phase. The gross observation is that a finely dispersed polymer precipitate is observed in the case where a reversed micellar solution is used as the antisolvent. In the case where neat isooctane is the antisolvent, although some finely dispersed particles are observed, most of the precipitate exists as nondispersed chunks of material. Scanning electron micrographs of the two cases are shown in parts a and b of Figure 7. In Figure 7a where the micellar solution was used as the antisolvent, in addition to discrete spherical particles there are irregularly shaped particles which seem to be the result of the coalescence of 2-3 spherical particles. Figure 7b is also interesting; what appears to be a chunk of material does contain a fair amount of coalesced spherical particles as can be
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Figure 7. Scanning electron micrographs of precipitated polymer morphology. Precipitation under nonagitated conditions (method I): (a) in the presence of AOT; (b) in the absence of AOT.
Figure 8. Scanning electron micrographs of precipitated polymer morphology. Precipitation with stirring (method II). (a) in the presence of AOT; (b) in the absence of AOT.
seen by the spherical outlines on the top surface. Indeed, sonication does result in the breakoff of some of these particles. It therefore appears that AOT does have an effect on particle nucleation and growth. Perhaps one effect of AOT micelles is to compartmentalize the system so that particle coalescence is inhibited. Although the FTIR evidence indicates the presence AOT-polymer interaction (Figure 6b), it is hard to define the role of this interaction in morphology development, whether AOT actually directs polymer chain folding or simply inhibits coalescence. An alternate method of precipitation (method II), where the polymer solution is introduced in small aliquots into the nonsolvent and the mixture stirred vigorously, results in an even better dispersion. Parts a and b of Figure 8 illustrate the cases with micellar nonsolvent and neat isooctane nonsolvent, respectively. As intuitively expected, precipitation in low concentrations with stirring results in minimization of particle coalescence during phase separation and the expulsion of solvent from the polymer phase. It is interesting to note that precipitation using the micellar solvent results in a broader size distribution, with a significant fraction of extremely small particles (
