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Structured Polymer Networks from O/W-Microemulsions and Liquid Crystalline Phases Wolfgang Meier Institut fu¨ r Physikalische Chemie, Departement Chemie, Universita¨ t Basel, Klingelbergstr. 80, CH-4056 Basel Received July 22, 1996. In Final Form: September 23, 1996X The droplet and the lamellar liquid crystalline phase of a pentaethylene glycol monodocecyl ether (C12E5)/ decane/water system is used as a matrix to create new amphiphilic polymer network structures from R,ω-hydrophobically modified poly(oxyethylene) bearing polymerizable end groups and hydrophobic comonomers. The influence of the individual “subnetworks” on the structure and phase behavior of the system is discussed. The fraction of bridge- and loop-forming polymer chains as a function of the droplet concentration in the system can be estimated from mechanical data. The number of bridges decreases considerably below a certain threshold concentration corresponding to the root-mean-square end-to-end distance of the poly(oxyethylene) chains.
Introduction Microemulsions are thermodynamically stable mixtures of oil, water, and surfactant. One of their characteristic features is the compartmentalized structure of nanometersized hydrophilic and hydrophobic domains. In a certain range of their phase diagram they consist, for example, of surfactant-stabilized droplets of the minor phase dispersed in a continuous major phase. This structure makes them preeminently suitable as a solvent for polymers consisting of water- and oil-soluble segments or blocks whereby the different parts of the polymer are selectively dissolved in the corresponding domains of the microemulsion.1-3 It is well established that such polymers may bridge different domains of the same type. As a consequence, increasing polymer concentration leads to the formation of transient polymer network structures in the microemulsion which display interesting dynamic and rheologic properties.4-7 The chain ends of the polymers (soluble in the droplet medium) can be functionalized with reactive groups. Due to the rather high local concentration of chain ends in the interior of the nanodroplets, the formation of chemical bonds between different chains can be achieved.8 The resulting covalently cross-linked polymer gels (“microemulsion elastomers”) combine for the first time solid state properties like elasticity or stability of shape with the structure and the phase behavior of microemulsions.8 For example in oil-in-water microemulsions, consisting of nanometer-sized oil droplets in water, an R,ω-hydrophobically end-capped poly(oxyethylene) carrying additional polymerizable methacrylate groups at each hydrophobic X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Eicke, H.-F.; Quellet, C.; Xu, G. Colloids Surf. 1989, 36, 247. Quellet, C.; Eicke, H.-F.; Hauger, Y. Macromolecules 1990, 23, 3347. Eicke, H.-F.; Hilfiker, R.; Xu, G. Helv. Chim. Acta 1990, 73, 213. Vollmer, D.; Hofmeier, U.; Eicke, H.-F. J. Phys. II 1992, 2, 1677. (2) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509. (3) Bagger-Jo¨rgensen, H.; Olsson, U.; Iliopoulos, I. Langmuir 1995, 11, 1934. (4) Zo¨lzer, U.; Eicke, H.-F. J. Phys. II 1992, 2, 2207. (5) Stieber, F.; Hofmeier, U.; Eicke, H.-F. Ber. Bunsenges. Phys. Chem. 1993, 97, 812. (6) Gradzielski, M.; Rauscher, K.; Hoffmann, H. J. Phys. IV, Suppl. J. Phys. II, 1993, 3, 65. (7) Odenwald, M.; Eicke, H.-F.; Meier, W. Macromolecules, 1995, 28, 5069. (8) Meier, W.; Falk, A.; Odenwald, M.; Stieber, F. Colloid. Polym. Sci. 1996, 274, 218.
S0743-7463(96)00718-4 CCC: $12.00
Figure 1. Schematic representation of a covalently cross-linked polymer network structure (formed by an R,ω-hydrophobically modified poly(oxyethylene) bearing polymerizable end groups and hydrophobic comonomers) in the L1 droplet phase of a microemulsion.
chain end can be used as bridging polymer. It has been shown that the cross-linking reaction can be performed using a light-induced free radical polymerization in the interior of the droplets.8 It is straightforward that the oil phase of the droplets can be successively replaced by oil-soluble comonomers for the reactive chain ends. For a mixture of lipophilic monomers bearing one and more than one polymerizable groups subsequent polymerization leads to the formation of a hydrophobic nanogel in the interior of the droplet (see Figure 1). These nanogels serve as a cross-links for an otherwise hydrophilic polymer network (formed by the water soluble poly(oxyethylene) chains). The resulting structured gels can be regarded as being composed of a hydrophilic and a hydrophobic subnetwork in series and therefore can be expected to exhibit exceptional mechanical properties. The nanogel in the interior of the droplet as © 1996 American Chemical Society
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Scheme 1. Structure of the End Group Functionalized Polymer 1
well as the system-spanning network formed by the bridging polymer must significantly influence the whole system. In this paper we describe the preparation of a series of new amphiphilic polymer gels. The influence of the structure and composition of the individual subnetworks on the phase behavior of the underlying microemulsion is investigated. Cryo-electron microscopy and mechanical measurements are used to characterize the structure of these new materials. Experimental Section Materials. Pentaethylene glycol monododecyl ether (C12E5) and decane were obtained from Fluka (purity g99%) and used without further purification. Water was doubly distilled. The functionalized R,ω hydrophobically modified poly(oxyethylene) 1 was prepared from poly(oxyethylene) with a molecular weight of 35 000 g mol-1 (Mw/Mn ) 1.08, using GPC) and 4-[(11methacryloylundecyl)oxy]benzoyl chloride following the procedure of ref.]8 The degree of conversion was found to be larger than 98% by 1H-NMR, so that the fraction of polymer molecules bearing only one hydrophobic end group can be neglected in our considerations. The acid chloride was prepared starting from 11-bromoundecanol and 4-(hydroxymethyl)benzoate9 and subsequent azeotropic esterification of the resulting 4-[(11-hydroxyundecyl)oxy]benzoic acid with methacrylic acid.10 Reaction with SOCl2 led then to the corresponding acid chloride. Assuming Gaussian statistics for the polymer coil and excluded volume interaction, the root-mean-square end-to-end distance of the poly(oxyethylene) is calculated to be 29 nm.11 The overlap concentration c* of the poly(oxyethylene) chains in the water phase can be estimated from c* ≈ M/NARG3 12 with NA the Avogadro number and RG the radius of gyration. We obtain c* ≈ 37 g L-1 which is significantly higher than the highest polymer concentrations used in our experiments (∼25 g L-1), i.e., the concentration of the polymer in the water phase remains dilute. Dodecyl methacrylate and octane-dimethacrylate were stirred with CaH2 and subsequently distilled under high vacuum. The purified monomers were stored in an ampoule under slight overpressure of dry Ar at -20 °C between the experiments. Microemulsions were prepared by mixing weighed amounts of the individual components. Mass fraction of the droplets are given by co ) (ms + mo)/(ms + mo + mw), with mi the mass fraction of surfactant (i ) s), water (i ) w), and oil (i ) o). The droplet size is determined by ro ) mo/ms and was kept constant at ro ) 1 in all experiments. The diameter of the droplets was determined by dynamic light scattering to d ) 20 nm in good agreement with ref 13. Replacing the decane of the droplets by the mixture of the hydrophobic monomers did not alter the droplet diameter. The concentration of the R,ω hydrophobically modified poly(oxyethylene) 1 is given by the number ratio R ) (polymer molecules/droplet). The number of nanodroplets in the microemulsion was calculated to be 1.4 × 1017 cm-3. Microemulsion Elastomers. The preparation followed the procedure described in ref 8. To create a hydrophobic subnetwork in the interior of the droplets the decane of the oil droplets was partly replaced by a monomer mixture consisting of 5 wt % octane dimethacrylate and 95 wt % dodecyl methacrylate keeping the total amount of the oil components constant. This mixture has (9) Houben-Weyl, 6/3; Georg Thieme Verlag: Stuttgart, 1965; p 55. (10) Organikum; VEB Deutscher Verlag der Wissenschaften: Berlin, 1970. (11) Polymer Handbook, 2nd ed.; J. Wiley & Sons: New York, 1974; p IV4 ff. (12) Graessley, W. W. Polymer 1980, 25, 5283. (13) Olsson, U.; Schurtenberger, P. Langmuir 1993, 9, 3389.
Figure 2. Typical microemulsion elastomer (sample dimensions about 1.5 cm diameter and 3cm height) with co ) 0.2. About 10 wt % of the oil phase of this sample is replaced by a hydrophobic nanogel. the advantage that whole the decane of the droplets can be replaced without changing the phase diagram prior to polymerization. Measurements. Determination of the phase diagrams required thermostating of the samples in a water bath to within 0.02 K. Phase boundary temperatures were determined by visual inspection in transmitted light, in scattered light, and between crossed polarizers. The kinetics of phase separation and the reverse process, solubilization, was very slow (in the order of several days) in the covalently cross-linked samples. Surface tension σ was measured using a Kru¨ss Interfacial Tensiometer K8. The critical micelle concentration (cmc) of the hydrophobically modified polymer 1 in water was deduced from the discontinuity in the σ(ln cpolymer) curve and found to be 1.0 × 10-2 g L-1 (or 2.8 × 10-7 mol L-1). In order to evaluate the mechanical properties of the polymer networks, rheological measurements were performed. They were carried out with a Carrimed CRSH 100 controlled stress rheometer with a laboratory made cone-cone geometry and a thermostatable cap to isolate samples from outer atmosphere and prevent solvent evaporation (detailed description see refs 4 and 7). Within the experimentally accessible frequency range for the covalently cross-linked samples, only a plateau in G′(ω) corresponding to the shear modulus Gp of the sample was found (see also ref 8). Therefore the discussion of the mechanical properties is mainly restricted to variations of Gp. Freeze-Fracture Replication Transmission Electron Microscopy. A small piece (∼10 µL) of the sample was brought onto a gold platelet at room temperature and was quenched by hand plunging into a mixture of 15% 2-methylbutane and 85% propane at 83 K. After quenching the sample was transferred into liquid nitrogen and clamped on a brass block (Balzer). It was mounted on a Balzer freeze edge device (BAF300), and subsequently the pressure was reduced to 5 × 10-7 mbar. After evacuation the sample was fractured with a liquid nitrogen cooled microtome. To enhance the contrast of the surface structure, the sample was warmed up to 153 K and edged for 10 min. Thereafter the sample was cooled again with liquid nitrogen and shadowed with W/Ta under an angle of 30°. After the samples were warmed up to room temperature and brought to atmospheric pressure, the replica was washed with chloroform, put on a 400 mesh copper TEM grid, and examined with a Hitachi H-8000 electron microscope operating at 100 kV.
Results and Discussion Free radical polymerization of the end group functionalized polymers 1 in the comonomer containing nanodroplets of a microemulsion leads to the formation of a covalently cross-linked transparent polymer gels (see Figure 2). Figure 3 shows a micrograph of a covalently cross-linked oil-in-water microemulsion system with a mass fraction of droplets of co ) 0.2. The figure demonstrates that the droplet structure of the microemulsion is
Structured Polymer Networks
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Figure 4. Phase diagram of a chemically crosslinked O/Wmicroemulsion (co ) 0.2, ro ) 1; 10 wt % of the oil phase replaced by hydrophobic nanogel) as a function of polymer concentration R ) number of hydrophobically modified polymers/droplet). Dotted line indicates transition from a liquid L1 phase to a solid L1 network. Figure 3. Micrograph of a microemulsion elastomer (same sample as in Figure 2) in the L1 phase region. The length of the bar is 100 nm.
preserved despite the cross-linking reaction and the formation of the hydrophobic nanogel in the interior of the droplets. The average droplet diameter can be directly measured from the micrograph yielding 20 ( 3 nm in good agreement with light-scattering results on the pure polymer free microemulsion (see above). The center-tocenter droplet distance is found to be 35 ( 5 nm compared to 33.4 nm calculated from the droplet volume fraction and diameter, i.e., the polymerization does not measurably alter structural parameters of the microemulsion. The polymer network seems to induce, however, a certain ordering of the nanodroplets compared to pure polymer free microemulsion where the arrangement of the droplets is more irregular.14-16 The errors for the determined diameter and droplet-droplet distance are mainly due to a slight polydispersity of the droplets, a restricted visibility of the droplet partially buried after the fracture, and difficulties in distinguishing them from their surrounding. For the center-to-center distance additionally defects in the ordering of the droplets play a role. With the structure of the underlying microemulsion also, at least qualitatively, the phase behavior is preserved. Temperature variation still may cause droplet aggregation, phase transitions to liquid crystalline phases, or phase separation. The influence of the concentration of the bridging polymer on the phase behavior of a microemulsion elastomer (with oil droplets containing 10 wt % of the hydrophobic nanogel) is shown in Figure 4. Compared to the pure microemulsion (R ) 0) the formation of a covalent polymer network considerably stabilizes the L1 droplet phase. For polymer concentrations larger R ) 4 the homogeneous one phase domain extends over a temperature interval of more than 40 K compared to 8 K for the underlying pure microemulsion. This is probably the result of an elastic stabilization of singly dispersed nanodroplets.17 To characterize the mechanical properties of the materials, rheological measurements have been performed. Because of the covalently cross-linked network structure, only a plateau in the dynamic storage modulus G′(ω) is observed. The plateau value of G′(ω), the shear modulus (14) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294. (15) Bodet, J.-F.; Bellare, J. R.; Davis, H. T.; Sciven, L. E.; Miller, W. G. J. Phys. Chem. 1988, 92, 1898. (16) Vinson, P. K.; Sheehan, J. G.; Miller, W. G.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1991, 95, 2546. (17) Meier, W.; Eicke, H.-F.; Odenwald, M. Colloid Surf. A 1996, 110, 287.
Figure 5. Shear modulus Gp as a function of polymer concentration R for several weight fractions of droplets co. ([) co ) 0.3; (O) co ) 0.25; (9) co ) 0.2; (2) co ) 0.15; (1) co ) 0.1; (+) co ) 0.05; (4) co ) 0.04; (]) co ) 0.03.
Figure 6. Shear modulus Gp at fixed polymer concentration (R ) 4) as a function of droplet concentration co. Solid line: fit according to Gp ) (constant) co.
Gp, is shown in Figure 5 as a function of the polymer concentration R for several microemulsion systems containing various droplet concentrations co. The fraction of the hydrophobic monomers in the droplets was kept constant at 10 wt %. It has to be emphasized that the number of cross-links of the network structure in our systems is given by the number of nanodroplets of the microemulsion (i.e., proportional to co).1 Consequently a variation in the concentration R of the bridging polymer increases only the functionality of the cross-links and not their number. Coincident with our findings (see Figure 5) and previous measurements7 theoretical considerations predict for this case Gp ∝ R.18 For a given functionality (R ) constant), however, classical rubber theory yields Gp ∝ co (i.e., the number of the cross-links).19 Figure 6 shows that also this relation holds pretty well for the microemulsion elastomers, at least for droplet concentrations co > 0.1. For smaller concentrations Gp decreases faster. Prior to the chemical cross-linking the droplets are able to exchange the hydrophobic chain ends from droplet to (18) Scaling Concepts in Polymer Physics; de Gennes, P. G., Ed.; Cornell University Press: Ithaka, 1979. (19) Principles of Polymer Chemistry; Flory, P. J., Ed.; Cornell University Press: Ithaka, 1953.
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Figure 7. Fraction of bridge forming polymer fb ) 1/R0 as a function of the droplet shell-to-shell distance δ (b). Solid lines represent fit according to eq 1. Average droplet-droplet distance δ between the shells of the nanodroplets in dependence of droplet concentration co (O).
droplet. Therefore an equilibrium exists between bridges (polymers inserting their hydrophobic chain ends into different droplets), loops (polymers inserting both chain ends into the same droplet), dangling ends (one hydrophobic end exposed to water), and free chains (both hydrophobic ends exposed to water).20,21 This equilibrium depends sensitively on the root-mean-square end-to-end distance r0 of the polymer and the average droplet-droplet distance in the microemulsion. Only when both are compatible can a significant fraction of bridging polymers be formed.20,21 Decreasing droplet concentration increases their average distance and therefore influences this equilibrium. Subsequent polymerization freezes in that equilibrium. Since only the bridge-forming fraction of the polymer contributes to the shear modulus, the deviations in Gp(co) directly reflect a decreasing number of bridging chains. To analyze this effect in more detail, we have to go back to Figure 5 which contains additional information directly allowing to determine the fraction of bridge forming polymer. Extrapolation of the straight lines for different co to Gp ) 0 yields R0 the minimum polymer concentration (i.e. functionality of the cross-links) necessary for the formation of an elastically active polymer network. While for higher droplet concentrations the extrapolated straight lines intersect the R-axis only slightly above R ) 1, for concentrations co < 0.1 clearly higher R0 values are required. According to Flory-Stockmayer theory19 this minimum polymer concentration is expected to correspond to a functionality of 2, i.e., R0,theoretical ) 1. Consequently the fraction of bridge forming polymer fb is directly given by fb ) R0,theoretical/R0. Figure 7 shows fb as a function of the calculated shell-shell distance δ ()(center-center distance) - 2(droplet radius)) of the nanodroplets. In the same figure, the dependence of δ on co is shown (dotted line). As long as the average droplet-droplet distance δ is smaller than the root-mean-square end-to-end distance of the poly(oxyethylene) r0 ) 29 nm (see the Experimental Section) fb remains constant at fb ≈ 0.8. For δ > r0, the bridge-forming fraction of the modified polymer decreases drastically. The cmc of the hydrophobically end-capped poly(oxyethylene) was found to be cmc ) 2.8 × 10-7 mol L-1, i.e., very low. Dangling ends and free chains with one or both hydrophobic end groups exposed to water are therefore energetically very unfavorable and their concentration in the microemulsion networks is consequently negligible. In this case only bridges and loops have to be considered in the equilibrium mentioned above. If one assumes a certain energy b(δ) necessary for the formation of a polymer bridge and l for the formation of (20) Misra, S.; Mattice, W. L. Macromolecules 1994, 27, 2058. (21) Misra, S.; Nguyen-Misra, M.; Mattice, W. L. Macromolecules 1994, 27, 5037.
Figure 8. Phase diagram of the pure, polymer free microemulsion (O) and a covalently cross-linked microemulsion elastomer (R ) 2; b) as a function of the weight fraction of methacrylate monomers cm ) mmethacrylate/moil in the oil droplets.
a loop, the fraction of bridges fb is given by
(
) ) ( )
exp fb )
(
b(δ) kT
b(δ) 1 exp + exp kT kT
(1)
with k the Boltzmann constant and T the temperature. For the bridging energy b two situations have to be considered: (i) in the region δ e r0 the polymer is always able to form bridges without changing its natural random coil conformation (e.g., by bridging not nearest neighbor droplets), i.e., b ) 0 and (ii) for δ > r0 bridges can only be formed if the polymer chain adopts a more stretched conformation, i.e., b ) b(δ) is must be a monotonically increasing function of the droplet-droplet distance δ. It is obvious that in contrast to polymer bridging the energy l required for loop formation must be independent from the droplet-droplet distance in the diluted concentration regime of the polymer relevant for our samples. Since in our system the droplet diameter of 20 nm is smaller than r0, loop formation always requires a certain energy related to the corresponding deformation of the polymer chain. For the region δ e 29 nm (b ) 0!) where fb is independent from δ (see Figure 7) one obtains with help of eq 1 l to be l ) 1.3 kT and the region δ g 29 nm yields b(δ) ) (0.1 kT/1 nm)δ which seem to be rather reasonable values for energies mainly correlated with deformations of the polymer chains.18,19 Another interesting feature of these microemulsion elastomers arises directly from the compartmentalized structure of the polymer network. As already mentioned the decane of the oil droplets of the underlying microemulsion can be successively replaced by our lipophilic monomer mixture. After polymerization an increasing part of the nanodroplets is therefore occupied by the hydrophobic nanogel, and this influences considerably the phase behavior of the underlying system. For example, in the pure (uncross-linked) microemulsion with increasing nanogel content the system undergoes a transition from a thermodynamically stable microemulsion (e.g., pure decane in the droplets) to an unstable latex dispersion (e.g., pure nanogel). Figure 8 shows the phase behavior as a function of the mass fraction cm ) mmethacrylate/ moil of methacrylate comonomers in the oil phase of the droplets. If one assumes the monomers to be quantitatively converted during polymerization, about 25 wt % of the oil phase of the underlying microemulsion can be replaced by the resulting nanogel until the L1 droplet phase becomes unstable and phase separation in a nanogel containing oil phase and a water phase occurs.
Structured Polymer Networks
The covalently cross-linked network structure formed by the bridging polymer 1 (R ) 2 in Figure 8), however, tries to hold the droplets or particles always at a certain equilibrium distance compatible with the natural random coil conformation of the network chains. As a consequence polymerization leads always to transparent solid gels even for pure nanogel particles (see Figure 8). Above about 45 °C the gels become turbid, probably due to aggregation of the nanoparticles and a phase separation into a (turbid) gel in equilibrium with excess water. The gel swells again upon equilibration at room temperature and becomes homogeneous and transparent within several days. Subsequent extraction of the polymer gels with methanol and CH2Cl2 allows the soluble components (surfactant, unreacted monomers, and polymers) to be removed. The weight of the dried residue was always in good agreement with a complete conversion during polymerization, i.e., corresponds to the weighed-in amount of polymer 1 together with the monomer mixture.22 The gels, however, exhibit after swelling again with water nearly the same phase behavior as before. Obviously in these systems the phase behavior is mainly determined by the polymer network structure. The surfactant plays only a minor role. During the cross-linking reaction the network stores information about the structure of the system. This structure is compatible with the random coil conformation of the network chains of the gel and each deviation from this favored state forces the chains to adopt a less favorable conformation and costs therefore elastic energy. As a result the system always returns to the original structure. This phenomenon can be exploited for the preparation of other interesting new polymer gels. As can be read of the phase diagram of Figure 4, the underlying microemulsion undergoes with increasing temperature a phase transition into a lamellar liquid crystalline (LR) phase which extends from 30 to 39°C. Here the system consists of alternating oil and water layers separated by surfactant. Polymerization in the lamellar phase at 35 °C consequently must lead to the formation of a layered polymer network structure (see Figure 9). If the liquid crystalline phase is macroscopically uniform ordered (e.g., by a magnetic field) prior to polymerization, the resulting layered gel exhibits anisotropic physical properties.23 For our preliminary experiments we did not apply such an external field so that the resulting gel possesses a polydomain structure and is therefore comparable to a polycrystalline material. It is straightforward, however, that the layered structure of alternating hydrophobic and hydrophilic subnetworks does not fit any more in the nanodroplet structure of a L1 phase. This is has serious consequences for the resulting phase behavior of the cross-linked samples. This is shown in Figure 10. Although the samples have completely the same composition as the ones of Figure 4 and differ only in the temperature (and therefore the structure) at which the crosslinking process was performed, really no L1 phase is observed. The gels exhibit now only a lamellar liquid crystalline phase which extends from below room temperature up to more than 50 °C. Conclusions Using the complex structure of microemulsions or lyotropic liquid crystalline phases as a matrix, it is possible to create new covalently cross-linked amphiphilic polymer network structures. An essential feature of these materials is that the complex solvent and the network structure mutually influence each other in a unique way. While (22) Meier, W. In preparation. (23) de Gennes, P. G. Phys. Lett. 1969, 28A, 725.
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Figure 9. Schematic representation of an amphiphilic polymer network crosslinked in the LR phase of the system.
Figure 10. Phase diagram of the LR elastomer as a function of polymer concentration R (cm ) 10 wt %). Dotted line indicates transition from a liquid LR phase to an solid LR network.
the phase structure of the underlying microemulsion or liquid crystalline phase is stabilized by the network, the polymer gel itself is able to remember the structure of the system existing during the cross-linking process. It preserves certain features of the solvent even after washing out the components not covalently attached to the polymer network (e.g., surfactant, oil, water, ...). This allows preparation of new structured elastomers, consisting of two subnetworks in series, which are expected to exhibit interesting mechanical characteristics or even anisotropic physical properties (if cross-linked in an ordered liquid crystalline phase). Acknowledgment. The author wants to thank Prof. Dr. H.-F. Eicke for many stimulating discussions. The author is indebted to M. Ha¨ner for the measurements at the Maurice E.-Mu¨ller Institute for High-Resolution Electron Microscopy and Dr. M. Odenwald for help with the mechanical investigations. Financial support from the Swiss National Science Foundation is gratefully acknowledged. LA960718L
