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J. Phys. Chem. B 2001, 105, 2091-2098

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ARTICLES Morphological Characteristics of 1,3:2,4-Dibenzylidene Sorbitol/Poly(propylene glycol) Organogels Debra J. Mercurio† and Richard J. Spontak*,†,‡ Departments of Materials Science & Engineering and Chemical Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27695 ReceiVed: June 21, 2000; In Final Form: NoVember 12, 2000

The organic gelator 1,3:2,4-dibenzylidene sorbitol (DBS) self-organizes to form a 3-D network stabilized by hydrogen bonds at relatively low concentrations in a variety of nonpolar organic solvents and polymers. The resultant network consists of nanofibrils and is responsible for the physical gelation of the matrix component. In this work, the morphological characteristics of organogels composed of DBS and poly(propylene glycol) (PPG) are investigated as functions of DBS concentration, PPG molecular weight, and temperature through the use of polarized light microscopy, transmission electron microscopy, X-ray diffractometry, and spectrophotometry. Polarized light microscopy reveals thermally reversible features that become increasingly more pronounced with increasing DBS concentration. Electron microscopy verifies that these features arise due to the presence of a DBS nanofibrillar network, with nanofibrils measuring ca. 10 nm in diameter. Comparison of X-ray diffraction patterns of pure DBS crystals and DBS networks from which PPG is removed by supercritical fluid extraction reveals that the DBS nanofibrils are crystalline, differing slightly from the structure of pure DBS. Spectrophotometry is used to probe the temperature-dependent development of the molecular network in DBS/PPG organogels.

Introduction Organic molecules capable of self-organizing into molecular networks within organic media continue to gain attention due to their ability to physically gel the liquid matrix in which they reside, thereby yielding organogels.1,2 One class of organic gelators of technological interest in the development of personalcare products3 and electrochemical devices4,5 is based on 1,3: 2,4-dibenzylidene sorbitol (DBS), a butterfly-shaped amphiphile derived from the sugar alcohol D-glucitol. The chemical structure of DBS is displayed in Figure 1 and reveals that the molecule possesses several sites ideally suited for specific chemical interactions: (i) one pendant hydroxyl group, (ii) one terminal hydroxyl group, and (iii) four acetal oxygen moieties. Infrared spectroscopy,6 as well as molecular modeling calculations,7,8 of DBS molecules in organic solvents indicates that the pendant hydroxyl group tends to hydrogen-bond intramolecularly to its nearest-neighbor acetal oxygen, whereas the terminal hydroxyl group readily establishes intermolecular hydrogen bonds with the acetal oxygens on adjacent molecules. Hydrogen bonding due to the terminal hydroxyl group is therefore presumed to be principally responsible for molecular network formation and, as a consequence, gelation. Studies6,9-11 seeking to identify the factors governing the DBS-induced physical gelation of low-molar-mass organic solvents confirm that it is generally sensitive to DBS concentration, temperature * To whom correspondence should be addressed. † Department of Materials Science & Engineering. ‡ Department of Chemical Engineering.

Figure 1. Chemical structure of 1,3:2,4-dibenzylidene sorbitol (DBS) illustrating the unique “butterfly” shape of this molecule. Infrared spectroscopy6 and molecular modeling7,8 reveal that these molecules self-organize in nonpolar media through intermolecular hydrogen bonding between the terminal hydroxyl group of one molecule and an acetal oxygen of a neighboring molecule.

and solvent polarity. These reported findings are consistent with the expectation that the DBS network is stabilized by intermolecular hydrogen bonds. Complementary efforts9,10 employing transmission electron microscopy (TEM) have shown that, after solvent removal, the DBS network consists of helical nanofibrils typically measuring ca. 10 nm in diameter. The gelation efficacy of DBS is not, however, limited to organic solvents. Recent studies have likewise demonstrated that DBS can self-organize into a molecular network in, and physically gel, a variety of macromolecules (polyolefins,12,13 silicones,14-17 and polyalkylene oxides17-19) in the melt. Unlike conventional polymer gels, the molten polymer constitutes the major (matrix) component in DBS/polymer organogels. If a macromolecule possesses repeat units that are geometrically and dimensionally similar to the butterfly shape of DBS (e.g.,

10.1021/jp002247o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/21/2001

2092 J. Phys. Chem. B, Vol. 105, No. 11, 2001 isotactic polypropylene), DBS and its chemical derivatives can serve another valuable purpose as crystal nucleating agents.2,9,12,13,20-29 Structural analyses of DBS/polymer organogels, performed by either (i) removing the polymer through solvent/supercritical fluid extraction14-17 or (ii) sectioning a DBS-filled polymer below its glass transition temperature (Tg),2,13,27 have shown that the DBS network responsible for polymer gelation also consists of highly connected nanofibrils measuring about 10 nm in diameter. Dynamic rheological testing17-19 has verified that the stability and properties of DBS/ polymer organogels depend sensitively on DBS concentration, polymer polarity, and polymer molecular weight, as well as on factors such as temperature, the extent of initial DBS dispersion, the external shear field imposed during mixing, and the time over which quiescent recovery occurs. Despite these attempts to establish structure-property relationships in DBS/polymer organogels, however, little is known about the nature or development of the DBS network comprising such an organogel. In the present work, we report on the structure and temperature response of DBS organogels composed of poly(propylene glycol). Experimental Section Materials. Three grades of poly(propylene glycol) (PPG) were obtained in liquid form from Aldrich Chemical Co. (Milwaukee, WI) and were used as-received in this study. Their number-average molecular weights were provided as 425, 1000, and 4000, in which case these polymers are designated here as PPG425, PPG1000, and PPG4000, respectively. The DBS was kindly supplied in powder form by Milliken Chemicals (Spartanburg, SC) and also used as-received. Methods. Physical gels of DBS and PPG were prepared by dissolving DBS powder at a specific concentration (expressed in mass percent) in a given PPG at 190 °C. Each mixture was continuously stirred until it appeared clear, which required from 10 to 60 min, depending on DBS concentration. The solution was then cooled quiescently to ambient temperature, during which time gelation occurred. For a given PPG molecular weight, the time corresponding to the onset of gelation was inversely proportional to DBS concentration. To ensure that the organogels were provided ample time for network development, they were stored in a dark location at ambient temperature for 1 week prior to analysis. Specimens examined by light microscopy were prepared by depositing a drop of molten DBS or DBS/PPG4000 gel between a heated glass slide and a cover slip. The resulting assembly was subsequently allowed to cool slowly to ambient temperature. Images of such samples were recorded under crossed polars on a Nikon Optiphot light microscope as they were heated at 2 °C/min in a Mettler PM400 hotstage. Organogels investigated by TEM were sectioned at -100 °C in a Reichert-Jung Ultracut S cryoultramicrotome. [The Tgs of the PPG homopolymers are about -70 °C, according to differential scanning calorimetry (DSC) results reported18 at a heating rate of 10 °C/min.] Thin sections measuring ca. 100 nm thick were collected on carbon-coated TEM grids and subsequently exposed to the vapor of 2% RuO4(aq) for 7 min. Zero-loss images of the stained sections were collected on a Zeiss EM902 electron spectroscopic microscope operated at an accelerating voltage of 80 kV and an energy-loss (∆E) setting of 0 eV. Specimens subjected to X-ray diffractometry (XRD) were composed of different PPG425 concentrations. Some of these gels were dried through the use of supercritical Freon 13 extraction (at 42 °C and 9.31 MPa).14,15 X-ray scattering patterns

Mercurio and Spontak

Figure 2. Polarized light micrograph of pure DBS after recrystallization from the melt. Note the spherulitic texture present in this image.

were collected in transmission from both wet gels (held between two sheets of Mylar) and dry gels (inserted into capillary tubes) on a 3-circle X-ray diffractometer that employed Cu KR radiation with a wavelength (λ) of 0.154 nm. The step size (∆q, where q is the scattering vector determined from the scattering angle by Bragg’s law) used here was 0.2 nm-1, and the temperature was maintained at 25 °C. Additional DBS organogels were remelted and poured into either plastic or quartz cuvettes, in which the gels were permitted to cool and remain quiescent for a period of 3-4 days prior to analysis in a Shimadzu UV-2101 PC UV-vis scanning spectrophotometer. Spectrophotometry was performed with λ ranging from 400 to 700 nm on organogels differing in DBS concentration and PPG molecular weight. Absorption data acquired at ambient temperature were normalized with respect to the parent (ungelled) PPG. Spectrophotometry measurements were also acquired at temperatures ranging from 40 to 90 °C. In these experiments, selected specimens were heated to 100 °C and then cooled at an average rate of 0.6 °C/min, with a dwell time of 1 min prior to data collection at each selected temperature ((0.5 °C). Results and Discussion Figure 2 shows a polarized light micrograph of pure DBS at ambient temperature after it was melted (Tm ) 225 °C, measured18 by DSC at 10 °C/min) and recrystallized between a glass slide and a glass cover slip. The spherulitic texture evident in this image is distorted, but nonetheless retained, in comparable images of 1.5 and 2.0% DBS in PPG4000 (Figures 3a and 4a, respectively) at ambient temperature. The spherulitelike morphologies apparent in these figures are qualitatively similar to those reported elsewhere15,16 for DBS in siliconebased polymers and are due to large-scale organization of the DBS nanofibrillar network. Such structural hierarchy in DBSinduced organogels has been previously evidenced.15 The observation that these morphologies closely resemble impinging spherulites in semicrystalline polymers indicates that the mechanism by which this large-scale microstructure develops is most likely nucleation and growth. As the DBS concentration is increased from 1.5% (Figure 3a) to 2.0% (Figure 4a), the features of the PPG-containing spherulites become sharper, whereas a reduction in DBS concentration is accompanied by

Characteristics of DBS/PPG Organogels

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Figure 3. Polarized light micrographs of 1.5% DBS in PPG4000 showing a spherulite-like morphology and its evolution at several temperatures (in °C): (a) 25, (b) 100, (c) 115, and (d) 120. Upon subsequent recooling, the morphology is recovered, verifying that these gels are thermally reversible.

less order and the eventual disappearance of a discernible spherulitic morphology. According to prior results obtained19 from dynamic rheological analysis, the DBS concentration identifying the onset of gelation (in a rheological sense) is not strongly dependent on PPG molecular weight (0.25-0.60% in the current series). Since the spherulites observed in Figures 3 and 4 develop at substantially higher DBS concentrations, it immediately follows that the nanofibrillar network can develop at low DBS concentrations without organizing into large-scale structural elements such as spherulites. The concentration range over which organogels are stable (without evidence of DBS precipitation) is, on the other hand, sensitive to PPG molecular weight, increasing with decreasing PPG molecular weight. This trend is attributed to the increase in PPG polarity that accompanies a reduction in PPG chain length. Increased solvent polarity has been found6 to prevent DBS from precipitating at high concentrations (as high as about 9.5% DBS in PPG42519). One of the characteristics of a physical gel, which is stabilized by noncovalent cross-link sites, is that it can be heated into a homogeneous (solution) state and returned to its gel state upon subsequent cooling. Included in Figures 3 and 4 are polarized light micrographs collected at several elevated temperatures to demonstrate that, as the temperature is increased in Figures 3b-d and 4b-d, the spherulite-like morphologies seen in Figures 3a and 4a become less distinct and birefringent, eventually disappearing altogether at some temperature between 115 and 120 °C in Figure 3. The disappearance of birefringent microstructure is presumed to coincide with dissolution of the DBS network

within the PPG4000 matrix. At 120 °C, the microstructure in Figure 4d remains barely visible, indicating that the dissolution temperature increases slightly as the DBS concentration is increased from 1.5 to 2.0% in PPG4000. It is interesting to note at this juncture that the formation temperature (Tf) of the DBS network in PPG4000, as measured18 rheologically, is about 122 °C, in favorable agreement with the image series presented in Figure 4. Equally good qualitative agreement is obtained at lower DBS concentrations (down to 0.5% DBS) when the dissolution temperatures (Td) obtained here for PPG4000 are compared to the Tf results recently provided by Fahrla¨nder et al.18 for PPG5000 (no direct comparison is possible except for organogels with 2% DBS). As the DBS concentration is reduced in a given PPG, Td decreases (abruptly as the concentration signifying the onset of gelation is approached). At high DBS concentrations, however, Td eventually attains a limiting value, which is slightly dependent on PPG molecular weight (due to the matrix polarity considerations discussed earlier). As the organogels featured in Figures 3 and 4 are cooled again, the spherulite-like texture is recovered, verifying that each organogel is thermally reversible. As alluded to earlier, the DBS network responsible for the spherulite-like microstructure in Figures 3 and 4 has been found to consist of nanofibrils measuring about 10 nm in diameter. Shown in Figure 5 is a TEM image collected from a thin section of one of the DBS/PPG organogels. Prior TEM studies of DBS organogels have relied upon solvent evaporation6,9 (in organogels with a low-molar-mass solvent) or selective DBS stain-

2094 J. Phys. Chem. B, Vol. 105, No. 11, 2001

Mercurio and Spontak

Figure 4. Series of polarized light micrographs of 2.0% DBS in PPG4000 showing a spherulite-like morphology and its evolution at several different temperatures (in °C): (a) 25, (b) 100, (c) 115, and (d) 120. Note that this microstructure is more sharply defined than the one shown in Figure 3.

Figure 5. Transmission electron micrograph of a cryosectioned/RuO4stained PPG4000 organogel with 2% DBS. This image confirms that this gel consists of DBS nanofibrils measuring about 10 nm in diameter. The nanofibrils appear electron-transparent (light) due to one or both of the explanations offered in the text.

ing2,13,27 (in systems with a polymer such as isotactic polypropylene). Since RuO4 is known30 to oxidize phenyl rings, RuO4stained DBS molecules have thus far been observed to appear electron-opaque (dark) in TEM images. The TEM image

displayed in Figure 5 reveals, however, that the DBS gel network produced by blending PPG4000 with 2.0% DBS is electron-transparent (light) relative to the PPG background. This observation is unexpected in light of the prior findings described above. Two possible explanations are offered for this result. The first is that the phenyl rings are stacked so tightly within the nanofibrils as to preclude incorporation of RuO4 molecules. Morel and Grubb31 have used RuO4 to examine the morphology of isotactic polystyrene and report that the amorphous, not crystalline, regions appear stained (thereby providing sufficient contrast by which to identify the crystals). If the nanofibrils are highly crystalline and the phenyl rings of the DBS molecules lie in registry (as predicted by molecular modeling7,8), then it is plausible that the nanofibrils would not be highly stained. A ramification of this possibility is that the nanofibrils (detected2,13,27 at concentrations typically 400 nm range. At 50 °C, the absorbance curve in Figure 9 is shifted to higher values and becomes more sensitive to λ in the low-λ range (as is generally seen in Figure 7). This change in absorbance response indicates that a DBS microstructure primarily consisting of small structural features forms between 80 and 50 °C upon cooling. The absorbance curve measured at 25 °C reveals that the DBS microstructure continues to develop as the temperature is decreased further. We continue to explore these considerations s i.e., temperature-induced microstructural formation and development s in the next section. The dependence of absorbance in the PPG425 organogel (2% DBS) on reciprocal temperature is presented on semilogarithmic coordinates in Figure 10 at four different λ, with the wavelength increment (∆λ) arbitrarily set to 100 nm. As seen in Figure 9, the measured absorbance values remain nearly λ-independent (except at λ ) 400 nm) at temperatures greater than 80 °C and

Characteristics of DBS/PPG Organogels

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Figure 12. Activation energy (Ea) values extracted by fitting eq 1 in the text to the data in Figure 11 and provided as a function of λ for PPG425 organogels with 4% (O) and 6% (b) DBS. Solid lines serve as guides for the eye, whereas the dashed line is a linear extrapolation of the curve through the high-λ data of the organogel with 6% DBS.

Figure 11. Variation of absorbance with reciprocal temperature at the same four λ listed in the caption of Figure 10 for PPG425 organogels with (a) 4% DBS and (b) 6% DBS. The solid lines are least-squares fits of an Arrhenius expression (eq 1 in the text) to the data.

then become increasingly more dependent on λ as the temperature is reduced. On the basis of these data, we conclude that the signature of an organogel with a developed DBS microstructure detectable by spectrophotometry (over the λ range shown in Figure 9) at any given temperature is an absorbance curve that varies with λ. According to this criterion, as well as the data and discussion presented earlier regarding organogels with more than 2% DBS, the PPG425 organogels with 4 and 6% DBS are both expected to possess DBS microstructures, and yield λ-dependent absorbance curves, at all temperatures investigated (since the network formation temperature is expected to exceed 87 °C). This expectation is confirmed in Figure 11. In this case, absorbance levels are observed to increase monotonically with increasing reciprocal temperature (or, conversely, with decreasing temperature). Whereas the lines shown in Figure 10 connect the data points, those in Figure 11 are linear regressions to an Arrhenius expression of the general form

A ) Ao exp(Ea/RT)

(1)

where A represents the normalized absorbance, Ao is a systemspecific constant, Ea is an activation energy associated with microstructural refinement, R is the universal gas constant, and T denotes absolute temperature. Regressions of eq 1 to the data in parts a (4% DBS) and b (6% DBS) of Figure 11 yield excellent fits to the four data sets shown in each part (as well as three additional data sets not shown in each part for the sake of clarity), suggesting that the mechanism responsible for DBS microstructural refinement is thermally activated. This observation is consistent with recent rheological findings18 indicating that the mechanical properties of DBS/PPG organogels follow time-temperature superpositioning through the use of an Arrhenius-based shift factor. Values of Ea derived from the slopes of the lines in Figure 11 are presented as a function of λ in Figure 12 for comparison. Both sets of Ea(λ) curves increase with increasing λ, which, in accord with intuitive expectation, implies that it becomes increasingly more difficult for the DBS molecules and nanofibrils to form microstructures at large length scales. In the organogel

with 4% DBS, Ea increases by just over 50% from 400 to 700 nm. The dependence of Ea on λ in this organogel is surprisingly linear over the range of λ examined, with a slope of about 13 J/mol‚nm and an ordinate intercept of 2.0 kJ/mol. If Figure 12 can be used to estimate the activation energy that must be overcome for microstructure of a specific size scale to form and/or coarsen and if Ea(λ) remains linear outside the range of λ investigated, then the nanofibrils measuring 10 nm in diameter would have to overcome an energy barrier of about 2.1 kJ/mol (which, for reference, is comparable to the value of thermal energy at ambient temperature, 2.5 kJ/mol). The Ea(λ) curve for the organogel with 6% DBS is everywhere less than that for the organogel with 4% DBS, implying that the energy penalty associated with DBS microstructural refinement decreases as more DBS becomes available at higher DBS concentrations. Moreover, the curve for the organogel with 6% DBS remains nearly parallel to that for the organogel with 4% DBS only at large λ (>600 nm). This observation suggests that, at least at these two DBS concentrations, the largest energy penalty is commonly associated with the development/refinement of large-scale (hierarchical) microstructure. In contrast, the reduction in Ea with decreasing λ for the organogel with 6% DBS becomes more pronounced (relative to the extrapolated linear relationship depicted by the dashed line in Figure 12) at small λ. According to these measurements, the 2% increase in DBS concentration (from 4 to 6%) serves to facilitate the formation of DBS microstructure, particularly at small length scales. Conclusions In this work, the microstructural features and stability of selforganized DBS in PPG are investigated through the use of polarized light microscopy, transmission electron microscopy, X-ray diffractometry, and spectrophotometry. Birefringent spherulite-like features generated by the DBS network at sufficiently high DBS concentrations are found to dissolve (and reform upon subsequently cooling) at temperatures far below Tm of DBS. The network responsible for these features consists of interconnected DBS nanofibrils measuring about 10 nm in diameter and stabilized by hydrogen bonds. According to XRD analysis, dry DBS networks produced by removing PPG from the organogels exhibit similar, but not identical, crystal structures as pure DBS. Thus, the nanofibrils observed by TEM are most likely crystalline, in which case the self-organization of DBS molecules by hydrogen bonding must occur in nonrandom fashion. At larger length scales, spectrophotometric results reveal that absorbance is most pronounced at small λ in all the organogels, irrespective of DBS concentration or PPG molecular weight. At temperatures above the dissolution temperature,

2098 J. Phys. Chem. B, Vol. 105, No. 11, 2001 absorbance curves are, for the most part, independent of λ. As the DBS microstructure forms and is detectable by spectrophotometry, the absorbance curves become λ-dependent, thereby permitting accurate estimation of the temperature range over which large-scale DBS microstructure forms. If the DBS microstructure is already developed at temperatures of interest, then the absorbance obeys Arrhenius-type behavior with respect to temperature. Extracted activation energies for microstructural refinement from such absorbance data are found to increase with increasing λ and decrease with increasing DBS content. These results indicate that DBS/PPG organogels possess very complex structural elements, ranging from crystalline nanofibrils to largescale microstructures (i.e., spherulite-like features), that initially form by nucleation and subsequently grow at different length scales by a thermally activated mechanism. Acknowledgment. We gratefully thank Prof. P. K. Kilpatrick (NC State University) for the generous use of his spectrophotometer and Prof. G. R. Mitchell (University of Reading) for his assistance in performing the X-ray diffractometry. References and Notes (1) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (2) Fuchs, K.; Bauer, T.; Thomann, R.; Wang, C.; Friedrich, C.; Mu¨lhaupt, R. Macromolecules 1999, 32, 8404. (3) Schamper, T.; Jablon, M.; Randhawa, M. H.; Senatore, A.; Warren, J. D. J. Soc. Cosmet. Chem. 1986, 37, 225. (4) Nahir, T. M.; Qiu, Y. J.; Williams, J. L. Electroanalysis 1994, 6, 972. (5) Silva, F.; Sousa, M. J.; Pereira, C. M. Electrochim. Acta 1997, 42, 3095. (6) Yamasaki, S.; Ohashi, Y.; Tsutsumi, H.; Tsujii, K. Bull. Chem. Soc. Jpn. 1994, 68, 146. (7) Kobayashi, T.; Takenaka, M.; Hashimoto, T. Kobunshi Ronbunshu 1998, 55, 613. (8) Wilder, E. A. Ph.D. Dissertation, North Carolina State University (in progress). (9) Thierry, A.; Fillon, B.; Straupe´, C.; Lotz, B.; Wittmann, J. C. Prog. Colloid Polym. Sci. 1992, 87, 28.

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