pubs.acs.org/Langmuir © 2009 American Chemical Society
Hybrid Thermoreversible Gels from Covalent Polymers and Organogels† D. Dasgupta,‡ S. Srinivasan,§ C. Rochas,^ A. Ajayaghosh,§ and J. M. Guenet*,‡ ‡
Institut Charles Sadron, CNRS UPR22, 23 rue du Loess, BP 84047 F-67034 Strasbourg Cedex 2, France, § National Institute for Interdisciplinary Science and Technology (NIIST), CSIR Industrial Estate P.O, etrie Physique, Pappanamcode Trivandrum 695019, India and ^Laboratoire de Spectrom CNRS-UJF UMR5588, F-38402 Saint Martin D’Heres Cedex, France Received December 20, 2008. Revised Manuscript Received February 10, 2009
This paper reports on experiments intended for investigating the feasibility of preparing hybrid thermoreversible gels from covalent polymers and noncovalent self-assembling π-conjugated molecules. The formation and the degree of dispersion of these hybrid gels have been studied with polystyrenes of various tacticities and oligo(p-phenylenevinylene) molecules (OPV) in different nonpolar organic solvents. Detailed investigations of the systems have been carried out by DSC, SAXS, and AFM. It is shown that no liquid-liquid phase separation is involved, indicating that the systems are highly compatible, and that the growth of one type of gel does not interfere with the other. These studies reveal that the resultant hybrid gels are composed of the intermingled fibrillar architectures of both gels.
Introduction The present paper examines the possibility of designing through physical processes novel hybrid nanomaterials with specific functionalities from hybrid gels of supramolecular polymers1 and covalent polymers. Supramolecular polymers are obtained from the self-assembly of small molecules via reversible weak interactions such as H-bonds, van der Waals, and the like.1 Organogels, which can be obtained from assemblies of such supramolecular polymers, have received considerable attention in the past few years mainly as they possess a fibrillar morphology that was so far the attribute of thermoreversible gels from covalent polymer.2-4 Recently, Ajayaghosh et al. have succeeded in obtaining a novel class of organogels obtained from all-trans-oligo (p-phenylenevinylene)s (OPVs) in aliphatic hydrocarbon solvents that display interesting optical properties as they change emission color during the sol-gel transition.5,6 The gelation occurs as a result of the H-bond-assisted supramolecular polymerization and the π-stacking interaction (see Figure 1). The supramolecular nanotapes of 3-20 nm thickness, 50-200 nm width, and several micrometers in length form a fibrillar network whose mesh size is concentration-dependent and lies in the micrometer range. As a result of gelation, strong perturbation of optical properties such as red shift in the emission is observed which may be of interest in the design of lightharvesting materials and light-emitting diodes.7 These molecules have, however, not led so far to large-scale applications as functional materials, chiefly due to their poor † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. E-mail:
[email protected].
(1) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH-Weinheim: New York, 1995. (2) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer-Verlag: Dordrecht, 2006. (3) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (4) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992. (5) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (6) Ajayaghosh, A.; George, S. J. Angew. Chem. 2003, 115, 346. (7) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644.
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mechanical properties,8 although some progress was made recently through the chemical approach.9,10 As for the gels considered here, it is readily obvious that OPV organogels are very weak materials unlike covalent polymer gels: the former are not self-supporting unlike the latter. As a rule, materials are not made up with only one component but are rather composites. One possible path for making such composite materials is the use of a covalent polymer matrix that would guarantee improvement of the mechanical properties. The path we intend to follow will consist in preparing hybrid gels, namely, organogel + polymer gel. As a matter of fact, covalent polymers, especially steroregular polymers, are known to form thermoreversible gels in a large variety of organic solvents.4,11 These gels also possess a fibrillar morphology whose characteristics are quite similar to those of organogel formed by small molecules. It is therefore expected to succeed in preparing intermingled networks with a high dispersion of one component into the other. The advantage of these ternary systems over mixture of covalent polymers is the total compatibility observed in solution. As recently reported for mixtures of isotactic polystyrene + bicopper complex molecules,12,13 homogeneous solutions are obtained at temperatures where the bicopper complex molecules are not self-assembled. Decreasing the temperature triggers selfassembling, thus allowing the bicopper complex to form long threads, which are still compatible to an extended degree with the polymer.14 In view of these findings, high molecular dispersion, and therefore the formation of intermingled networks, should become an achievable goal, which is expected (8) Dammer, C.; Maldivi, P.; Terech, P.; Guenet, J. M. Langmuir. 1995, 11, 1500. (9) Diaz, D. D.; Rajagopal, K.; Strable, E.; Schneider, J.; Finn, M. G. J. Am. Chem. Soc. 2006, 128, 6056. (10) Diaz, D. D.; Cid, J. J.; Vazquez, P.; Torres, T. Chem.;Eur. J. 2008, 14, 9261. (11) Guenet, J. M. Polymer-Solvent Molecular Compounds; Elsevier: London, 2008. (12) Lopez, D.; Guenet, J. M. Eur. Phys. J. B 1999, B12, 405. (13) Lopez, D.; Guenet, J. M. Macromolecules 2001, 34, 1076. (14) Poux, S.; Thierry, A.; Guenet, J. M. Polymer 2003, 44, 6251.
Published on Web 03/18/2009
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Figure 1. Left top: the chemical structure of the OPV16 molecule. Left bottom: a schematic representation of the same molecules: the black bar stands for the phenyl rings core and the gray bars for the aliphatic wings. Right: the way OPV16 organogel fibrils are built up with the corresponding distances observed by X-rays.6 to offer an efficient and simple way to prepare materials with improved mechanical properties. This paper is therefore focused on the feasibility of such hybrid gels and particularly on the investigation of the degree of mutual dispersion of both gels. For this purpose polystyrenes of various tacticities have been used with the hydroxymethyl end-functionalized OPV molecules in different solvents. The resulting systems have been subjected to detailed studies by DSC, SAXS, and AFM.
Experimental Section 1. Materials. The Covalent Polymers. The isotactic polystyrene sample (iPS) was synthesized following the method devised by Natta.15 After removal of the atactic fraction due to thermal polymerization, 1H NMR characterization showed that the content of isotactic triads was over 99%. The different moments of the molecular weight distribution were determined by GPC in THF at room temperature. The following values were obtained: Mw = 2.0 105 with Mw/Mn = 2.4. The syndiotactic polystyrene (sPS) sample was synthesized following the method devised by Zambelli and co-workers.16 The fraction of syndiotactic triads characterized by 1H NMR was found to be over 99%. Molecular weight characterization was performed by SEC in dichlorobenzene at 140 C and yielded the following data: Mw = 1.0 105 with Mw/Mn = 4.4. In all the cases, the molecular weight measurements were done using polystyrene as standard. The atactic polystyrene sample was kindly provided by Dr. P. Lutz from ICS. It was synthesized by means of anionic polymerization. Its weight-averaged molecular weight is Mw = 1.96 105 with a polydispersity Mw/Mn = 1.034. The Organogel Bricks. The fibrillar organogel is formed through the predominantly 1-D aggregation of an oligo(pphenylenevinylene) (designated as OPV16 in what follows) whose chemical structure is shown in Figure 1 where R = n-C16H33. The way the molecules pack to form the organogel fibrils is shown in the same figure. The synthesis and properties of these molecules are fully described in ref 5. The Solvents. The solvents were chosen thanks to their properties of favoring the formation of thermoreversible gels with the stereoregular polystyrenes: cis-decalin and trans-decalin for isotactic polystyrene and benzene for syndiotactic polystyrene. All the solvents, of high purity grade (described by Aldrich as “GC 99%”), were purchased from Aldrich and were used without further purification. 2. Techniques. Differential Scanning Calorimetry. The gel formation and melting were investigated by means of the (15) Natta, G. J. J. Polym. Sci. 1955, 16, 143. (16) Grassi, A.; Pellecchia, C.; Longo, P.; Zambelli, A. Gazz. Chim. Ital. 1987, 117, 249.
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Diamond DSC from Perkin-Elmer. Different heating and cooling rates were used, ranging from 2.5 to 20 C/min. Homogeneous solutions were prepared by heating at desired temperature in hermetically closed test tube that contained a mixture of appropriate amount of polymer and solvent. Gels were obtained by a subsequent cooling of these solutions at room temperature. Pieces of gel of about 20 mg were then transferred to into a “volatile sample” pan that was hermetically sealed. The weight of the sample was checked after completing the different cycles to make sure no solvent loss occurred. Optical Microscopy. The samples used for optical observation were prepared by remelting between glass slides those samples prepared beforehand in a test tube. In order to minimize solvent evaporation, the edge of the thin upper glass slide was glued with solvent-resistant epoxy resin. The phase contrast optical investigations were carried out with a NIKON Optiphot-2 equipped with a CCD camera and using LUCIA, a software developed by Laboratory Imaging for image processing and analysis. Atomic Force Microscopy. AFM experiments were carried out at room temperature in air using a Nanoscope III instrument (Digital Instruments, Santa Barbara, CA). The image was taken by means of a silicon nitride cantilever (Scientec, France) having a spring constant of 25-50 N/m and a rotating frequency of 280-365 kHz. Films were prepared by deposition of a drop of hot, homogeneous solution onto a glass slide, and then the solvent was allowed to evaporate. The observation of the surface topography and phase images of the films was performed with a scanning rate varying from 1 to 2 Hz using tapping mode. X-ray Diffraction. The X-ray experiments were performed on beamline BM2 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The energy of the beam was 15.8 keV, which corresponds to a wavelength of λ = 7.86 10-2 nm. At the sample position the collimated beam was focused with a typical cross section of 0.1 0.3 mm2. The scattered photons were collected onto a two-dimensional CCD detector with typical acquisition times of about 10-20 s, which allows timeresolved experiments to be carried out at 2 C/min, a value similar to the heating rate used for DSC experiments. For investigating the small-angle scattering range the sampleto-detector distance was set to D = 1.52 m. Under these conditions, the scattering vectors range extended from q = 0.15 nm-1 to q = 2 nm-1, with q = (4π/λ) sin (θ/2), and where λ and θ are the wavelength and the scattering angle, respectively (further information is available on http://www.esrf.fr). The scattering intensities obtained were corrected for the detector response, dark current, empty cell, sample transmission, and sample thickness. To obtain a one-dimensional X-ray pattern out of the two-dimensional digitalized pictures, the data were radially regrouped and a silver behenate sample was used for determining the actual values of the momenta transfer q. The intensity scattered Is(q) by the species reads Is ðqÞ ¼ Isample ðqÞ -½ð1 -jÞIsol ðqÞ
ð1Þ
where Isol(q) is the scattering by the solvent and j is the volume fraction of the scattering species. Is(q) is in turn expressed as Is ðqÞ ¼ KX Cs Ss ðqÞ
ð2Þ
where Cs is the concentration of scattering species, Ss(q) the structure factor of these species, and KX the contrast factor. The latter parameter is expressed in the case of small-angle scattering X-ray scattering through17 2 4:76 v0 ð3Þ KX ¼ Z Z 0 m m0 2 vm (17) Kirste, R. G.; Wunderlich, W. Z. Phys. Chem. (Munich) 1968, 58, 133.
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where Z0, v0 and Zm, vm are the number of electrons in the polymer and in the solvent and their molar volume, respectively, and m0 is the monomer unit molecular weight. Values of the different contrast factors are gathered in Table 1.
Table 1. Values of KX Calculated from Relation 3 for the Different Species in Decalin (Cis or Trans) and Benzene solvent
OPV
polystyrene
The gels were prepared beforehand in test tubes and then transferred into cylindrical thin-walled (≈0.1 mm thick) glass tubes of 3 mm inner diameter. These tubes were finally sealed, and the system was reheated in order to obtain homogeneous solutions and so apply the same thermal treatment as that used for the DSC experiments. Note that, due to the small beam cross section, these tubes are equivalent to containers with parallel walls. Such geometry clearly simplifies data processing. We have been in a position to use glass tubes instead of mica cells since for a wavelength of λ = 7.86 10-2 nm glass is virtually transparent to X-ray radiation.
decalin (cis or trans) benzene
0.012 0.028
0.016 0.033
Results and Discussion
Figure 2. Temperature-concentration phase diagram for OPV16
We shall first present the results obtained on the three solvents used in this study for pure OPV16, then mixtures of OPV16 + aPS, and finally the results obtained on the hybrid gels with iPS and sPS. Note that low covalent polymer and OPV concentrations were used on purpose. As a result, the mesh size of the gels is always in the micrometer range, which, provided no liquid-liquid phase separation occurs, should allow one system to grow within the other with the minimum hindrance. 1. OPV Gels. DSC traces of OPV16 solutions (see Supporting Information Figure 1) have provided one with the temperature-concentration phase diagram drawn in Figure 2 in the low concentration range. As can be seen, there is a marked hysteresis between the gelation temperature and the gel melting temperature (about 30 C). Also, the observation of a latent heat for the sol-gel and the gel-sol transitions points to a first-order transition phenomenon, which, in view of the accepted definition, means that OPV16 gels are not strictly speaking self-assembled systems.18 As can be observed from Figure 2, the behavior of the OPV16 solutions is independent of whether cis- or trans-decalin is used as the gel formation and gel melting temperatures, and the formation/melting enthalpies are virtually identical. Conversely, the gel formation and gel melting temperatures are significantly lower in benzene. As was reported for other solvents,7 the morphology as observed by optical microscopy, a nondestructive technique, of the as-prepared samples reveals a fibrillar morphology (inset Figure 3). Further investigations by AFM on the dried samples show the detail of the fibrillar morphology. Other AFM images shown in Figure 3 and in the Supporting Information (Figures 2 and 3) suggest that the fibrillar cross section are significantly larger in cis-decalin and benzene (with typical cross-section radii of r = 60 ( 20 nm) than in trans-decalin (with typical cross-section radii of r = 10 ( 4 nm). Also, in cis-decalin and benzene OPV16 fibrils are made up through the bunching of a discrete number of smaller fibrils something which is less apparent with trans-decalin. It ought to be stressed that this cross-section discrepancy between cisand trans-decalin has no noticeable effect on the gel melting
organogels: (b) gel melting in cis-decalin; (O) gel melting in transdecalin; (2) gel melting in benzene; (9) gelation in cis-decalin; (0) gelation in trans-decalin; (4) gelation in benzene. Inset: the corresponding enthalpies.
Figure 3. AFM image of organogel prepared from OPV16/ cis-decalin solution. Full scale = 900 nm; COPV = 0.004 g/cm3. Inset: optical micrograph of organogel prepared from OPV16/cisdecalin solution with COPV = 0.004 g/cm3. temperature unlike what could have been expected on the basis of Gibbs theory. 2σS Tm ¼ Tm0 1 ΔHm V
wherein Tm0 and Tm are the melting temperatures of the “infinite” 3-D system and of the finite system, respectively, σ the surface free energy, ΔHm the melting enthalpy, and S and V are the surface and the volume of the system. For an infinitely long cylinder-like object of cross-section radius r, the following relation holds:
(18) Steed, J. W.; Turner, D. R.; Wallace, K. J. Core Concepts in Supramolecular Chemistry and Nanochemistry; Wiley: New York, 2007.
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Tm ¼
Tm0
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That the melting temperature is nearly independent of the fibril cross section suggests that Tm ≈ Tm0. This subsequently implies that the term σ/ΔHmr in relation 5 has to be negligible. The melting enthalpy of the pure OPV16 as calculated by extrapolation of the DSC data obtained in either cis- or trans-decalin amounts to about ΔHm ≈ 240 J/g, a rather high value which most probably accounts for the σ/ΔHmr term being negligible and therefore Tm being independent of the value of r. It is worth emphasizing that Tm0 is both solvent- and concentration-dependent. Increasing the solute concentration entails an increase of Tm0, while improving the solvent quality produces a decrease of Tm0. Consequently, the lower value of the gel melting temperature in benzene arises simply from the fact that this solvent is a better solvent to the supramolecular polymer made up from OPV16 molecules than are cis- or trans-decalin. For cis- and trans-decalin solutions, the SAXS experiments were chiefly performed at a concentration of COPV = 0.004 g/ cm3 and a quenching temperature of Tq = 0 C. A few SAXS experiments were carried out at different concentrations and at Tq = 20 C. For benzene solutions, the OPV16 concentration was also COPV = 0.004 g/cm3, and only one quenching temperature, Tq = 20 C, was considered. As presented in Figure 4, the SAXS experiments reveal a conspicuous maximum at q = 1.34 ( 0.02 nm-1, corresponding to a Bragg distance of d = 4.68 ( 0.08 nm, which stands for the spacing of the OPV16 molecules within the fibrils as portrayed in Figure 1. As was already mentioned by Ajayaghosh et al., this distance value suggests that the aliphatic arms are totally extended.7 Interestingly, SAXS patterns obtained at differing OPV16 concentrations and quenching temperatures with OPV16/ cis-decalin organogels reveal a change in the full width at half-maximum (FWHM) of the Bragg peak (Figure 4). As shown in Table 2, the FWHM decreases when increasing the OPV16 concentration as well as when increasing the quenching temperature. As the FWHM is inversely proportional to the size of the diffracting plane of the investigated structure, this means that the fibrils cross section increases when increasing the OPV16 concentration and/or the quenching temperature. This effect can also be evidenced in the very low-q range of the SAXS curves. In the case of fibrillar gels, it has been shown that the distribution in fibril cross-section radii obeys a variation of the type19 wðrÞ∼r -λ
ð6Þ
where λ is an exponent such as 0 e λ e 3 and with two cutoff radii rmax and rmin. It turns out that the case λ = 1 usually pertains to fibrillar gels. In addition to the Porod regime, for which the intensity varies as q-4 due to the compactness of the fibrils, a so-called transitional domain can also be observed, with no restriction on the value of qr. The absolute scattered intensity under a q4I(q) form is written in this domain: q4 IðqÞ 2F rmax ð7Þ ¼ 2π πFq log -1 C rmax rmin where F is the fibrils density. (19) Guenet, J. M. J. Phys. II 1994, 4, 1077.
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Figure 4. SAXS data plotted by means of q2I(q) vs q for OPV16 organogels prepared from cis-decalin solutions at different concentrations and temperatures: (O) COPV = 0.017 g/cm3, Tq = 0 C; (b) COPV= 0.01 g/cm3, Tq = 0 C; (9) COPV = 0.004 g/cm3, Tq = 0 C (0); COPV = 0.004 g/cm3, Tq = 20 C. Inset: lowest SAXS data for OPV16 organogels prepared from cis-decalin solutions plotted with q4I(q) vs q: (b) COPV = 0.004 g/cm3, Tq = 20 C; (O) COPV = 0.004 g/ cm3, Tq = 0 C. Table 2. FWHM for OPV Organogels in cis-Decalin for Different Concentrations and Preparation Temperature sample
Δq (nm-1)
COPV = 0.017; T = 0 C COPV = 0.01; T = 0 C COPV = 0.004; T = 0 C COPV = 0.004; T = 20 C
0.14 ( 0.02 0.45 ( 0.05 0.51 ( 0.05 0.26 ( 0.03
Extrapolation to q4I(q) = 0 from this domain yields a value q0 from which rmax can be derived through rmax ¼
2 πq0
ð8Þ
Such an analysis has been applied to OPV16/cis-decalin organogels (see inset of Figure 4). As can be seen, a linear variation can be observed in the low-q range. Extrapolation to q4I(q) = 0 yields rmax ≈ 170 nm for Tq = 20 C against rmax ≈ 6 nm for Tq = 0 C. This again highlights the occurrence of larger fibrils cross section when quenching at higher temperature. Note also that the slope is proportional to 1/log(rmax/rmin). The slope is significantly lower at Tq = 20 C with respect to that at Tq = 0 C, which is expected as rmax(20 C) > rmax(0 C). 2. The OPV/Atactic Polystyrene Gels. DSC experiments highlight that the presence of atactic polystyrene, a nongelling polymer in the present solvents, does change neither the organogel formation threshold nor the organogel melting temperature (see Supporting Information Figure 1). This behavior is the same independent of the solvent used in this study. This therefore entails that aPS and the supramolecular polymer from OPV16 are totally compatible throughout the gelation process. The SAXS experiments are consistent with this outcome since the scattering curve of OPV/aPA organogel is, within experimental error, simply the sum of the scattering curves of the pure OPV16 organogel and of the pure aPS chains solution (Figure 5). This holds in the three solvents used in this study (see Supporting Information Figure 4). Note that the aPS chains scattering intensity for large q is close to 1/q2, yet there is a slight departure from this behavior in the case of aPS/cis-decalin solutions. The ratio signal/noise is sufficiently high so that this behavior cannot be attributed to the background subtraction. A possible explanation may be found in the solvent properties: benzene is a good solvent to Langmuir 2009, 25(15), 8593–8598
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Figure 5. SAXS data plotted by means of q2I(q) vs q for systems
prepared at Tq = 20 C: (O) OPV16/benzene organogels, COPV = 0.004 g/cm3; () aPS/benzene solutions, CaPS = 0.04 g/cm3; (9) organogels obtained from OPV16/aPS/benzene solutions, COPV = 0.004 g/cm3, CaPS = 0.04 g/cm3; (0) stands for the sum of () and (O).
aPS, and so at CaPS = 0.04 g/cm3 the chains are well above their overlap concentration, hence the 1/q2 behavior20 (strictly speaking, I(q) ∼ (q2 + ξ-2)-1 in which ξ is the screening length). Conversely, the θ-point in cis-decalin is θ = 6 C,21 which means that the chains are in a good solvent situation at T = 20 C but below their overlap concentration as their expansion coefficient is certainly lower than that in benzene. As a result, the expanded chain behavior, i.e., I(q) ∼ q-5/3, can be observed.20 The AFM experiments further confirm that the OPV fibrils cross section are about the same with and without the presence of aPS chains (see Supporting Information Figure 5). The deceiving impression that the fibril density is lower with respect to pure OPV16 arises from the fact that solid aPS covers part of the fibrils after drying and so hinders their observation. Removal by a solvent of this excess of atactic polymer was not attempted as the OPV fibrils may be also damaged. 3. The OPV/Isotactic Polystyrene Hybrid Gels. As was already mentioned in the Introduction, isotactic polystyrene (iPS) gels produced in cis- and trans-decalin possesses differing properties; in particular, iPS/trans-decalin gels melt some 25 C higher than their cis-decalin counterpart.4 In addition, iPS/cis-decalin gels possess a melting temperature close to that of pure OPV16 gels while iPS/trans-decalin gels melt at a significantly higher temperature. These unusual characteristics led us to select these two solvents for the present study. In both cases, however, the OPV16/decalin gel forms at the same temperature, yet at a higher temperature than the iPS/ decalin gel. DSC traces are displayed in Figure 6 for the ternary systems OPV16/iPS/cis-decalin. On cooling two exotherms occur: the first one is located at Tgel = 38 ( 2 C, near the OPV16 gelation temperature, and is most probably the gelation threshold of the OPV16 organogels; the second one is located at much lower temperature (Tgel = 11 ( 2 C) and corresponds most certainly to the gelation of iPS. On heating only one endotherm is observed at T = 60 ( 2 C, which suggest that both gels melt concurrently. DSC traces for the ternary systems OPV16/iPS/trans-decalin are shown in Figure 6. Here again, on cooling two exotherms occur: the first one at Tgel = 43 ( 2 C certainly stands for the OPV16 gelation temperature although it is some 5 C above the gelation temperature observed in the (20) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (21) Berry, G. C. J. Chem. Phys. 1966, 44, 4450.
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Figure 6. DSC thermograms of ternary systems: (a) OPV16/iPS/ trans-decalin, CiPS = 0.07 g/cm3; (b) OPV16/iPS/cis-decalin, CiPS = 0.025 g/cm3; (c) OPV16/sPS/benzene, CsPS = 0.07 g/cm3. In all cases COPV = 0.004 g/cm3. Heating and cooling rates 2 C/min. OPV/trans-decalin binary system. The second exotherm at lower temperature, i.e., Tgel = 16 ( 2 C, arises from the gelation of iPS. Unlike the previous system, two endotherms are seen, which again coincide with the melting of the OPV16 gel at T = 65 ( 2 C (low-temperature endotherm) and the iPS gel melting at T = 85 ( 2 C (high-temperature endotherm). SAXS investigations reveal that the curves observed for the iPS gels are similar to those already reported from smallangle neutron scattering investigations4 (see Supporting Information Figures 6 and 7). The upturn at small angle is related to the existence of fibrils with cross section in the nanometer range,11 while the upturn at large angle is due to the observation of the rigid, helical structure of the iPS chains. This is possible in the latter case because the helices are solvated, namely are sheathed with a layer of solvent molecules, and thus are spaced enough so as to scatter as “single chains”. Interestingly, the SAXS results for the OPV16/iPS hybrid gels turn out to be chiefly the sum of the scattering by the two binary systems (see Supporting Information Figures 6 and 7). This indicates that both gels are perfectly intermingled for this range of distances. From the AFM picture displayed in Figure 7 one can clearly distinguish the OPV16 organogel (the large crosssection fibrils) and the iPS gel (the very small cross-section fibrils). The magnitude of fibrils cross section of the OPV16 organogel has been hardly altered in both ternary systems (Figure 7 in text and Figure 8 of the Supporting Information). Note that the AFM image can be deceiving as the polymer gels may appear as having a very small mesh size, which is actually not the case.4 These results altogether show that there is no phase separation of the liquid-liquid type prior to the hybrid gel formation. The systems are truly intermingled at all distance range while nearly keeping the structure they have in the binary systems. 4. The OPV/Syndiotactic Polystyrene Hybrid Gels. The DSC traces for the ternary systems OPV/sPS/benzene reveal two exotherms on cooling (Figure 6). The first exotherms peaking at Tgel = 44 ( 2 C stands for the gelation of sPS11,22 while the second one at Tgel = 37 ( 2 C is most probably due to the gelation of OPV although it occurs at a higher temperature than has been observed for pure OPV in benzene (Tgel = 26 ( 1 C; see Figure 2). On heating two endotherms are clearly observed: at T = 45 ( 2 C, which certainly stands for the OPV gel melting, and at T = 70 ( 2 C, which corresponds to the sPS gel melting. It is again worth (22) Daniel, C.; Brulet, A.; Menelle, A.; Guenet, J. M. Polymer 1997, 38, 4193.
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Figure 7. AFM picture obtained from OPV16/iPS/cis-decalin hybrid gels (COPV = 0.004 g/cm3, CiPS = 0.025 g/cm3). Full scale = 3 μm. noting that the melting temperatures are virtually the same as those of the binary systems (namely, OPV/benzene and sPS/ benzene of the same concentration). The higher gelation temperature observed for OPV in these ternary systems may be due to some nucleation effect by the sPS gel fibrils. There is therefore a noticeable effect of one component, sPS, on the gelation of the other, OPV. Yet, the final thermal properties are not significantly altered. The SAXS investigations again show that the scattered intensity of the OPV16/sPS hybrid gel turns out to be the sum of the intensities scattered by each binary system (Supporting Information Figure 9). This implies that in the range of distances probed by the SAXS experiments the gels are perfectly intermingled. Note that the sPS/benzene gel produces expected scattering curves. Here the terminal behavior is of the 1/q2 type as, unlike iPS, sPS produces rows of polymer stems alternating with rows of solvent.23,24 The 1/q2 scattering behavior is therefore that of a sheet.25 AFM investigations further support and extend this assumption. The OPV gel fibrils and the sPS fibrils can be easily distinguished in the tapping mode phase image (see Supporting Information Figure 10). Interestingly, a change of color can be obtained by raising the temperature above the organogel melting point but still below the sPS network melting point as is shown in Figure 8. Unlike what is observed with a pure OPV16 organogel, where the change of color corresponds to the sol-gel transition, here the whole systems remains a solid gel. This effect of temperature is perfectly reversible which again emphasizes the intermingling of both gels and the absence of a significant interference of one gel on the formation process of the other.
Figure 8. Change of color occurring in sPS/OPV16/benzene gel (COPV = 0.004 g/cm3, CsPS = 0.07 g/cm3) when the temperature is raised above the OPV16 organogel melting point but below the sPS gel’s. The phenomenon is perfectly reversible. affecting the organogel formation. In no case large-scale phase separation of the type liquid-liquid phase separation has been observed, and the growth of OPV organogel is not impeded by the presence of the polymer. To be sure, this statement is valid for rather low concentrations, and the compatibility remains to be tested for higher concentrations. When using stereoregular polystyrenes, which are intrinsically capable of forming fibrillar thermoreversible gels, hybrid gels are obtained with a very high degree of dispersion of one component into the other. Again, the use of higher concentrations of either OPV or of the polymer or both may affect this finely intermingled structure. For instance, increasing the polymer concentration, and correspondingly decreasing the resulting polymer gel mesh size, is likely to alter significantly the growth of the organogel fibrils when the polymer gel forms first. The making of such intermingled gels obtained from the thermoreversible gelation of a covalent polymer together with an oligomolecule certainly opens possibilities for preparing materials possessing the mechanical properties of polymers and the functionality of the self-assembled molecules. The possibility of making porous, functional materials through solvent extraction by supercritical carbon dioxide can also be envisaged.
In this paper we have shown that OPV molecules and atactic polystyrene are compatible to a very high extent without
Acknowledgment. This collaborative research work was supported by a grant from the Indo-French Centre for the Promotion of Advanced Research (IFCPAR-CEFIPRA, Grant No. 3808-2). D. Dasgupta and S. Srinivasan are also indebted to IFCPAR-CEFIPRA, the former for a postdoctoral fellowship and the latter for a short-visit allowance at Institut Charles Sadron, under the same grant. Thanks are also due to Cathy Saettel and Christophe Contal for their continuous technical assistance with the DSC experiments and for the AFM characterization, respectively.
(23) Chatani, Y.; Shimane, Y.; Inagaki, T.; Ijitsu, T.; Yukinari, T.; Shikuma, H. Polymer 1993, 34, 1620. (24) Petraccone, V.; Tarallo, O.; Venditto, V.; Guerra, G. Macromolecules 2005, 38, 6965. (25) Porod, G. Kolloid-Z. 1952, 125, 51.
Supporting Information Available: DSC traces, AFM images, and SAXS plots of OPV16 organogels and their hybrid gels with aPS, iPS, and sPS. This material is available free of charge via the Internet at http://pubs.acs.org.
Concluding Remarks
8598 DOI: 10.1021/la804185q
Langmuir 2009, 25(15), 8593–8598
