Design of Nanohybrid Systems from a Partially Fluorinated

Jun 21, 2015 - Nanohybrid systems are prepared from organogels of a partially fluorinated molecule and from thermoreversible gels of syndiotactic poly...
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Design of Nanohybrid Systems from a Partially Fluorinated Organogelator and Syndiotactic Polystyrene Thermoreversible Gel Ahmad Nawaz Khan, Marc Schmutz, Johann Lacava, Abdelaziz Al Ouahabi, Thi-Thanh-Tam Nguyen, Philippe J. Mesini, and Jean-Michel Guenet* Institut Charles Sadron, CNRS UPR22-Universite de Strasbourg, 23 rue du Loess, BP84047 F-67034 Strasbourg Cedex, France ABSTRACT: Nanohybrid systems are prepared from organogels of a partially fluorinated molecule and from thermoreversible gels of syndiotactic polystyrene. The thermodynamic behavior, morphology, and structure are investigated by using differential scanning calorimetry, atomic force microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The outcomes of these investigations suggest that the fibrils of the organogel coil around the sPS fibrils, probably through a heterogeneous nucleation process. These systems therefore differ from previously investigated sPS/OPV systems (oligo vinylene phenylene) where OPV fibrils pervade the sPS network.

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INTRODUCTION Hybrid nanomaterials prepared from organic and inorganic compounds have been shown to be a viable and interesting field of research.1 The challenging issue is to design hierarchical nanosized morphologies with specific properties and functionalities for the hybrid system. The recent developments in the making and study of organogels have opened up new horizons for preparing nanofunctional materials.2−6 Their fibrillar morphology is of special interest as specific properties are obtained, such as scaffolds for excitation energy transfer and light harvesting.7,8 However, in many cases low-concentration organogels, which are of interest, are not tractable as such for fabricating materials mainly because of their poor mechanical properties.9,10 Recently, we have explored another way of making hybrid materials through the use of thermoreversible gels from covalent polymer and organogels, or more generally selfassembling systems.11−13 Such an approach is of particular interest for preparing functional materials with improved physical and mechanical properties, which are not found for the individual systems. So far we have prepared three types of such hybrid systems: (a) encapsulation of self-assembled filaments in polymer gel fibrils11 (b) making of intermingled networks between a polymer thermoreversible fibrillar gel and an organogel,12 (c) sheathing covalent polymer fibrils by self-assembled nanotubes.13 These hybrid nanomaterials possess the superior mechanical properties of the polymer gels while exhibiting the functional properties of the self-assembled molecules or organogels. In this article we report on an investigation into the making of a hybrid system between thermoreversible gels from syndiotactic polystyrene and organogels. Syndiotactic polystyrene produces fibrillar thermoreversible gels of high modulus whose open porous structure can be retained by supercritical solvent extraction. It possesses unique properties both as a scaffold and as an absorbant medium for a large variety of organic solvents.14−16 The organogelator is derived from a 3,5© 2015 American Chemical Society

bis(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10)17 by replacing the hydrogens from the decyl moiety by fluorine atoms (named BHPBF in what follows). BHPB-10 can form hollow nanotubes in a specific solvent,17 which is not the case for its fluorinated counterpart.18 However, the solvents where nanotubes are produced, chiefly alkanes or cyclic alkanes, are not appropriate for the formation of fibrillar sPS thermoreversible gels. The latter are rather formed in benzene derivatives14−16,19 where BHPB-10 organogels consist of solid fibrillar structures. Conversely, fluorinated counterpart BHPBF produces twisted structures where the fluorinated moiety is located in the core.18 We have estimated that this fluorinated molecule could be a good candidate for forming hybrid system with the original aim of preparing superhydrophobic20 materials through its sheathing of the polymer fibril. While the making of a hybrid material through a sheathing process was successful, as shown in what follows by means of various techniques, superhydrophobicity was not achieved.

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EXPERIMENTAL SECTION

Materials. (3,5-Bis(5-hexylcarbamoylpentyloxy)-benzoic acid 1,1,2,2-tetrahydroperfluorodecyl ester) was used as a gelator (Scheme 1) and was synthesized through the same procedure to synthesize the nonfluorinated parent compound.17 It is designated as BHPBF in what follows. The syndiotactic polystyrene (sPS) used in this study was from Dow Chemicals under the trademark Questra 101 (a gift from C. Daniel). 13C NMR shows that the content of syndiotactic triads is over 98%. The molecular weight distribution by GPC in trichlorobenzene at 135 °C was found to be Mw = 3.2 × 105 g/mol, with a polydispersity of 3.9. o-Xylene of high-purity grade (99%) was purchased from Aldrich and used without further purification. The systems were prepared by heating and gently stirring mixtures of sPS and/or BHPBF in o-xylene until homogeneous solutions were obtained. Gelation was achieved by cooling the mixture to the desired Received: April 15, 2015 Revised: June 21, 2015 Published: June 21, 2015 7666

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Langmuir Scheme 1Chemical Structure of the BHPBF Molecule

where Zs and Zo are the number of electrons in the scattering structure and in the solvent, e and μ are the charge and the mass of the electron, ms is the molar mass of the solute, NA is Avogadro’s number, and c is the speed of light in vacuum. Neutron Scattering. Small-angle neutron scattering experiments (SANS) were performed on D11, a camera located at Institute LaueLangevin (Grenoble, France). A wavelength of λm = 0.6 nm was used, with Δλ/λm = 9%. A built-in two-dimensional sensitive detector was used (further details available on the ILL Web site http://www.ill.fr). The following q range was accessed: 0.03 < q (nm−1) < 2.5, where q = (4π/λ) sin(θ/2) and θ is the scattering angle by using three sample− detector distances (34, 10, and 4 m). The cell efficiency correction and absolute calibration were achieved by normalizing all spectra with the spectrum of light water. The light water cross section dΣ/dΩ determined experimentally for D11 at the used wavelength was taken as dΣ/dΩ = 0.985 cm−1 for λ = 0.6 nm. Corrections for transmission as well as subtraction from the intensity scattered by the empty cell were systematically made prior to solvent scattering and incoherent background subtraction. The incoherent scattering from the hydrogenous species was computed by means of an experimentally derived relation21

temperature. We have purposely restricted our study to the following concentrations: CsPS = 0.15 g/cm3 (abbeviated as 15%) and CBHPBF = 0.01 g/cm3 (abbreviated as 1%) for the making of the hybrid materials. Detailed studies of BHPB/o-xylene organogels and sPS/o-xylene systems can be found in refs 18 and 19, respectively. Techniques. Differential Scanning Calorimetry. The gel melting and gel formation were investigated using a Diamond DSC from PerkinElmer. Heating and cooling rates ranging from 2.5 to 20 °C/ min were used. Gels (∼30 mg) were placed into volatile stainless steel sample pans which were hermetically sealed with an O-ring to prevent solvent evaporation. To erase the sample’s thermal history, a first melting process was systematically applied, and only the cooling and second heating were taken into consideration. Atomic Force Microscopy. AFM experiments were carried out at room temperature using a Nanoscope III instrument (Digital Instruments, Santa Barbara, CA). A silicon nitride cantilever (Scientec, France) with a spring constant of 25−50 N/m and a rotating frequency of 280−365 kHz was used. Organogel films were prepared by the deposition of a drop of hot, homogeneous solution onto a mica substrate, and the solvent was evaporated in a vacuum oven. For sPS gels and hybrid gels, supercritical carbon dioxide extraction was done using SFT-110 (Supercritical Fluid Technologies, Inc.) at T = 40 °C, P = 200 bar, and extraction time t = 60 min. The surface topography of the samples was examined in tapping mode at scanning rate of 1 Hz. Transmission Electron Microscopy. For freeze fracturing of the samples, a piece of gel was placed between two copper sample holders so as to form a sandwich and then frozen rapidly by plunging it into liquid nitrogen and finally transferred to a homemade cryo-fracturing apparatus (developed by Dr. J.-C. Homo). The sandwich was cleaved and immediately shadowed with Pt/C at 45°(2 nm thickness), and finally a thick carbon layer (20 nm) was deposited 90° to the surface. These samples were rinsed with chloroform so as to keep only the replicas that were subsequently deposited onto a 400 mesh copper grid. TEM observations were performed on a TECNAI G2 (FEI) microscope operating at 200 kV, and the images were taken using an Eagle 2k (FEI) ssCCD camera. X-ray Diffraction. SAXS was performed using a Nanostar diffractometer (Bruker-Anton Paar) that operates with a pinhole collimator and a wire proportional gas detector. A monochromatic (λ = 1.54 Å, Cu Kα1) and almost parallel beam (divergence = 0.03°) was obtained through a confocal mirror with an advanced W/Si multilayer coating (XENOCS, SA). The size of the incident beam on the sample was close to 300 μm. The sample-to-detector distance was set at 22 cm for the scattering vectors ranging from q = 0.5 to 3 nm−1, with q = 4π sin(θ/2)/λ, where λ and θ are the wavelength of the incident beam and the scattering angle, respectively. Cells of 1 mm thickness made up with calibrated mica windows were used as sample holders. Data reduction was carried out according to a standard procedure for isotropic X-ray scattering. Specifically, an iron source was used for detector cell efficiency and solid angle corrections as well as a silver behenate sample for calibrating the transfer momentum scale. Absolute calibration was obtained using Lupolen as a standard. After all corrections and normalizations, the intensities were corrected from the scattering of the solvent. Whenever necessary, the contrast factors were calculated by using the following relation

KX =

2 vs ⎞ NAe 4 ⎛ − Z Z ⎜ ⎟ s o vo ⎠ ms 2μ2c 4 ⎝

Iinc = ϕH × 8.65

NH VH

(2)

in which ϕH is the hydrogenous species volume fraction, NH is the number of protons per hydrogenous species, and VH is their molar volume. The data processing yields absolute intensities in cm−1. Whenever necessary the contrast factor was calculated by using the following relation

KN =

(as − aovs/vo)2 NA ms 2

(3)

where a represents the scattering amplitudes of the molecules and the solvent, vs/vo is the ratio of the molar volumes of the same, ms is the molecular weight of the molecules, and NA is Avogadro’s number.

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RESULTS AND DISCUSSION This study is an attempt at “sheathing” polymer fibrils with helices of BHPBF molecules as was successfully achieved with iPS/BHPB-10 systems.13 As a result, the w/w ratio sPS/ BHPBF has to be large enough and was estimated to be at least 15, hence the use of CsPS = 0.15 g/cm3 (abbeviated as 15%) and CBHPBF = 0.01 g/cm3 (abbreviated as 1%). CBHPBF = 0.01 g/cm3 is the minimum concentration required for obtaining relevant results from techniques such as SAXS and SANS. The sPS concentrations above CsPS = 0.15 g/cm3 are rather difficult to handle. It is worth emphasizing that homogeneous solutions are easily obtained with this ternary system. For these concentrations, DSC experiments performed on cooling reported in Figure 1 show that sPS gelation takes place at T = 63 ± 1 °C (on the basis of the minimum in the exotherm) while BHPBF gel formation occurs at T = 20 ± 1 °C. The gelation enthalpies per gram of gel for the binary systems are ΔHsPS = 5 ± 0.7 J/g and ΔHBHPBF = 1.1 ± 0.2 J/g. For the ternary systems, an exotherm is still seen for the gelation of sPS yet located at a slightly lower temperature of T

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Figure 1. Thermograms obtained on cooling at 5 °C/min. Systems as indicated.

= 58 ± 1 °C while the gelation enthalpy is hardly modified (ΔHsPS = 5 ± 0.7 J/g). Conversely, the exotherm related to BHPBF gel formation stands some 7 °C lower, and the gelation enthalpy has decreased significantly (ΔHBHPBF = 0.25 ± 0.05 J/ g). The slight decrease in the sPS gelation temperature may suggest that the addition of BHPBF entails an increase in solvent quality toward the polymer. As to the BHPBF component, the strong decrease in the gelation enthalpy together with the lowering of the gel threshold may suggest a vanishing of the BHPBF fibril−fibril interaction and correspondingly a privileged interaction with the polymer fibrils. An investigation of the molecular structures in what follows may throw some light on the phenomena at play. In Figure 2, AFM pictures of supercritical carbon dioxide extracted samples and electron microscopy micrographs of freeze−fracture samples reveal the morphology of the binary systems and of the ternary gel. For the BHPBF/o-xylene system, TEM and AFM do reveal the same morphology (Figure 2a,b) consisting of regularly twisted helical structures with quite a large pitch (about 6 nm) that form large bundles through regular alignment over large distances. Note that in a previous paper we came to the conclusion that the fluorinated moiety lies in the center of the helix.18 Gels from sPS/o-xylene exhibit the classical randomly dispersed network structure with a micrometric mesh size (Figure 2c).14−19 The ternary system (Figure 2d) strongly resembles the sPS network structure and in no case displays the features observed in the BHPBF/o-xylene binary system. This would support the interpretation derived from DSC experiments, namely, a large loss of BHPBF helix alignment. If the helical features of the BHPBF are present, they should most probably show up. For instance, 1% BHPB-10 (the hydrogenous counterpart of the present polymer) features were clearly observed in 15% iPS gels.13 Also, image analysis of the AFM pictures (software CT-Fire, http://loci.wisc.edu/software/ctfire) for determining the average cross-sectional radii reveals a Gaussian distribution peaking at 16−17 nm for both systems with σ = 3 nm (Figure 2c,d). Admittedly, this analysis is to be taken as a rough estimate because an accurate determination would require investigating thinner samples so as to measure the height of the fibrils. However, it shows no significant change between the samples prepared from binary and ternary systems. Scattering experiments on these systems have been further performed to unveil the molecular structures.

Figure 2. Top left (a): AFM micrographs of BHBPF/o-xylene organogel at CBHPBF = 0.1 × 10−2 g/cm3. Top right (b): TEM picture obtained on a freeze−fracture sample of BHPBF gels.14 CBHPBF = 0.01 g/cm3. Lower left (c): AFM picture of an sPS xerogel, initial concentration CsPS = 0.15 g/cm3. Lower right (d): AFM picture of an sPS/BHPBF xerogel, with initial concentrations CsPS = 0.15 g/cm3 and CBHPBF = 0.01 g/cm3. All AFM pictures are 2 μm × 2 μm squares.

SAXS experiments are reported in Figure 3. The binary systems and the ternary systems have been investigated. For 1%

Figure 3. SAXS patterns in the range of 0.5 ≤ q ≤ 3 nm−1 plotted by means of a Kratky plot (q2I(q)vs q) of BHPBF/o-xylene CBHPBF = 0.01 g/cm3 (◊); sPS/o-xylene gels CsPS = 0.15 g/cm3 (○); sPS/o-xylene/ BHPBF/o-xylene CBHPBF = 0.01 g/cm3, CsPS = 0.15 (●). The solid line stands for the sum of (◊) and (○). Intensities are in cm−1, and transfer momenta are in nm−1.

BHPBF/o-xylene gels, oscillations are seen that correspond to the superhelical structure of the twisted fibrils. The maximum at q = 2.016 nm−1 is related to the packing of the molecules in this helix. For the 15% sPS/o-xylene gel no special features are observed, unlike for the BHPBF gels. This behavior reaches a 1/q4 plateau (Figure 4) which is expected from a solid fibril as one is dealing with a two-density system (fibril + solvent). A departure from this plateau arises from the complex formed between sPS and o-xylene so that chains are separated by intercalated solvent molecules.15 7668

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Figure 4. The same SAXS data as used for Figure 3 plotted here by means of a Porod plot (q4I(q)vs q) in the range 0.43 ≤ q ≤ 2 nm−1. sPS/o-xylene gels CsPS = 0.15 g/cm3 (○), sPS/o-xylene/BHPBF/oxylene CBHPBF = 0.01 g/cm3, and CsPS = 0.15 (●). The dotted line is calculated as in Figure 3. Intensities are in cm−1, and transfer momenta are in nm−1.

Figure 5. SANS curve plotted by means of a Kratky plot (q2I(q) vs q) for sPSD/BHPBFH/o-xyleneD (CBHPBFH = 0.01 g/cm3, CsPSD = 0.15 g/ cm3). Intensities are in cm−1. Full line = hollow tube with rout = 16 nm and γ = 0.65; (···) fit with helices of the type reported previously plus bundles of helices (two types of bundles of cross-sectional radii r = 18 nm and r2 = 5 nm); (· · ·) fit with only bundles of helices of crosssectional radii r1 = 18 nm and r2 = 5 nm. Inset = scattering curve previously obtained14 for binary system BHPBFH/o-xyleneD (CBHPBF = 0.02 g/cm3.

If the gels were intermingled in the ternary system, then the scattering intensity should simply be the sum of the intensities recorded in the binary systems as has been observed for OPV/ iPS intermingled gels.12 As can be seen this is not the case as the maximum at q = 2.016 nm−1 is hardly visible in the ternary system. Conversely, the intensity in the low-q range is much higher for the ternary system. As is shown in Figure 4, the 1/q4 plateau intensity is about 3 times that observed in the sPS/oxylene gel. In a two-density model, the intensity reads for SAXS22 2 ⎛ vs ⎞ S I(q) ≈ ⎜Zs − Zo⎟ vo ⎠ Vq 4 ⎝

The scattering intensity exhibits oscillations due to the crosssectional scattering of the cylindrical objects that can be either polymer fibrils plus organogel fibrils or sheathed polymer fibrils. It is worth noting that the scattering intensity differs significantly in the low-angle region from that obtained for binary system BHPFB/o-xyleneD. (For a comparison, see the inset of Figure 5.) This outcome agrees with the AFM findings, namely, that the structure of BHPBF in the binary system is not seen in the ternary system. Fits were performed by attempting to match the theoretical scattering with the maxima. The following models were therefore considered on the basis of cylinder-like objects23−26 because helices scatter like cylinders26 in the explored q range (namely at low resolution): (i) Helices such as those reported in a previous paper plus bundles of helices.14 The best fit needs to consider two types of bundles of cross-sectional radii r1 = 18 nm and r2 = 5 nm, respectively. The theoretical intensity then reads

(4)

where S/V is the surface/volume ratio of the particle. Fibrils can be approximated to cylinders, in which case this ratio varies simply as 1/r where r is the cylinder cross-sectional radius.23 That the intensity increases by about a factor of 3 may originate in two effects: either the cross-sectional radius of the fibrils has decreased by a factor of 3 or the contrast factor has increased 3-fold. Because there is no effect of a drastic decrease of the fibril cross-sectional radius as seen from the AFM experiments, the second effect is rather to be considered. It may arise from a sheathing process of the sPS fibrils by BHPBF helices. Indeed, the contrast factor is to increase under these conditions as the fluorine atoms augment significantly the number of electrons per unit volume. A rough estimate shows that the term (Zs − (vs/voZo))2 is about 102 for sPS/o-xylene whereas it amounts to about 5 × 104 for BHPBF/o-xylene. As a result, just one layer of BHPBF helices could be enough to increase by about 3 times the contrast factor. One may, however, argue that BHPBF fibrils also scatter like 1/q4 so that the increase in the scattering intensity may arise from the coexistence of sPS fibrils and BHPBF fibrils. An additional study by small-angle neutron scattering (SANS) may allow one to settle this issue by toying with the contrast of each component with respect to the solvent’s. Results obtained on the system sPSD/BHPBF/o-xyleneD are reported in Figure 5. Using deuterated o-xyleneD allows one to tremendously increase the contrast of BHPF as this molecule contains 49 hydrogen atoms. As to the contrast of sPSD, although not being strictly zero in deuterated o-xyleneD, it is quite negligible with respect to the contrast of BHPBF (note that strictly matching the sPSD contrast by using mixture oxyleneD/o-xyleneH is not possible). As a result, the scattering intensity is chiefly due to the BHPBF molecular structure.

⎡ 0.46γ ⎤2 2.13 (J1(qrout) − γJ1(qγrout))⎥ q2I(q) ∝ Aq⎢ J1(qγrout) + qrout ⎣ qrout ⎦ ⎡ 4πJ 2 (qr ) ⎤ ⎡ 4πJ 2 (qr ) ⎤ 2 1 + Bq⎢ 1 2 ⎥ + Cq⎢ 1 2 ⎥ ⎢⎣ (qr1) ⎥⎦ ⎣⎢ (qr2) ⎥⎦

(5)

with rout being the outer cross-sectional radius of the twisted helix, γ= rin/rout being the ratio between the outer and inner radii of the twisted helix, and A, B, and C being adjustable constants for taking into account the fractions of the different cylinders. (ii) Only bundles of helices of cross-sectional radius 5 and 18 nm. ⎡ J 2 (q × 18) ⎤ ⎡ J 2 (q × 5) ⎤ ⎥ ⎥ + Cq⎢4π 1 q2I(q) ∝ Bq⎢4π 1 ⎢⎣ (q × 18)2 ⎥⎦ ⎢⎣ (q × 5)2 ⎥⎦

(6)

(iii) A hollow cylinder. As a matter of fact, if the BHPBF helices wind around the polymer fibrils, then their structure is equivalent to a hollow cylinder at this level of spatial resolution. The equation used reads21 q2I(q) = KN πqC BHPBFμL ⎤2 ⎡ 2 ⎢ [J (qrout) − γJ1(qγrout)]⎥ ⎦ ⎣ (q(1 − γ 2)rout) 1 7669

(7)

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Langmuir where rout is the external radius of the hollow cylinder, γrout is the internal radius, with the latter corresponding to the outer radius of the polymer fibrils under the present conditions, μL is the mass per unit length, and CBHPBF is the concentration of BHPBF. The best fit gives rout = 16 nm, rin = γrout = 10.4 nm, and μL = 1.71 × 105 g/mol nm (the latter value obtained with KN = 1.9 × 10−3). We consider the latter fit to be the best fit because it yields the correct positions of the maxima and the correct envelope of the scattering curve. This fit could be clearly improved by introducing some cross-sectional polydispersity together with the detector resolution. The way BHPBF possibly interact with the polymer fibrils is depicted in Figure 6. This is reminiscent of

the BHPBF helices do not form bundles. Indeed, intermolecular terms exist only for cylinder-like objects when these objects are parallel.25 This was not the case for the helices in the binary system BHPBF/o-xylene18 (inset of Figure 5). Replacing deuterated o-xylene by its hydrogenous counterpart (sPSD/BHPBF/o-xyleneH) reveals the molecular structure of the polymer fibrils because the scattering of BHPBF is negligible with respect to the polymer’s (Figure 7).

Figure 7. SANS curves plotted by means of a Porod plot (q4I(q) vs q) for sPSD/BHPBFH/o-xyleneH (CBHPBFH = 0.01 g/cm3, CsPSD = 0.15 g/ cm3). Intensities are in cm−1, and transfer momenta are in nm−1. Inset: only those data selected from the zone defined by the dotted lines are plotted, and the number of data points has also been reduced so as to highlight the fit by means of eq 8 for q < q* (solid line). Transfer momenta qo and q* are indicated.

The results are identical to the SAXS scattering curves, in particular, the observation of 1/q4 behavior via both techniques. Interestingly, lower q values are available in these SANS data that provide one with additional pieces of information. The scattering curve can be examined in light of a structural model developed for fibrillar thermoreversible gels27 where gels are characterized by an array of fibrils of cross-sectional distribution w(r) ≈ r−λ bound by two cutoff radii rmax and rmin. Two regimes can then be identified: The transitional regime is for rmax > q−1 > rmin where the intensity is written

Figure 6. Schematic representation of the coiling of BHPBF fibrils along the polymer fibrils. Close-up view: a possible way in which BHPB molecules may interact with the polymer fibrils and subsequently pile up to form the helical structure observed in their organogels from o-xylene.

the growth of bindweed along a post, namely, the coiling is not necessarily regular. The thickness δ of the superhelical structure winding up along the polymer fibrils (δ = rout − rin = 5.6 nm) suggests that it consists of or is close to the parameters of the helix described in a previous paper18 for which the estimated diameter is 5.8 nm (Figure 6). In this helix the fluorinated moiety lies in the core of the helix so that the interaction with the polymer fibrils would occur through the hydrogenous moiety. The pitch of the superhelical structure, Psh, cannot be presently derived with data at hand. Indications may be tentatively obtained from the value of the mass per unit length. This value is rather high, which may suggest a rather compacted superhelix because μL for a single helix is about 2 × 103 g/mol nm. As a matter of fact, if the superhelix would be totally compacted, resembling a nanotube, then a rough estimate of the mass per unit length yields 3.4 × 105 g/mol nm3. Also, the pitch may not be regular so that diffraction experiments may not introduce any additional information. The process at play is probably a partial heterogeneous nucleation effect as was the case for the sheathing of iPS fibrils by BHPB-10 nanotubes:13 sPS fibrils form first on cooling and act as heterogeneous nuclei to trigger the growth of BHPBF fibrils, meanwhile preventing them from further aggregating as they would do in the organogel. That the scattering curve can be simply reproduced without taking into account the intermolecular term further means that

2

q I(q) ∝

⎡ 4π 2ρ⎢⎣A(λ)q λ −

1

⎤

λ ⎥ ⎦ λrmax

r

∫r max w(r ) dr min

(8)

where A(λ) is derived from the following expression containing the gamma (Γ) function28

( 3 −2 λ ) λ+1 λ+3 λ+1 2λΓ( 2 )Γ( 2 )Γ( 2 ) Γ(λ)Γ

A(λ) =

(9)

The intercept of the fitting curve with the q axis, namely, for q4I(q) = 0, yields rmax through 1 = [λπA(λ)]1/ λ qo rmax (10) A fit in the transitional range gives λ = 1.66 and qo = 0.054 nm−1, which entails A(λ) = 0.43 and rmin = 1.23/qo = 15 nm. The crossover scattering vector q* obtained by the intersection of the transitional regime and the Porod regime (1/q4 for qrmin > 1) reads (Figure 7, inset) 1 = [λπA(λ)]1/ λ q∗ rmin (11) 7670

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which eventually gives rmin = 0.617/q* = 1.4 nm. The value of rmax is in good agreement with the estimated value from AFM but also with the value derived from neutron scattering (10.4 nm). This further suggest, as was already mentioned in a previous paper,13 that only those sPS fibrils with a given cross-sectional radius are sheathed. Indeed, if all the sPS fibrils were sheathed, then the coiling of the BHPBF superhelix around these fibrils would produce structures with a large distribution of cross sections (rout and rin), resulting in the vanishing of the oscillations seen in the scattering intensity. As was mentioned in the Introduction, no superhydrophobicity was observed as the contact angle remains virtually identical for the dried sPS and the dried hybrid system (120 ± 4°).29 This is consistent with the picture derived from the structural experiments showing the fluorinated moiety not exposed to the fibril environment.

REFERENCES

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CONCLUDING REMARKS In this article we further show that it is possible to prepare hybrid materials from a ternary system of polymer thermoreversible gel/organogel/solvent. A dispersion of one component into the other occurs with no macroscopic phase separation. The body of experimental techniques used here suggests that the organogel superhelical structure coils around the polymer fibrils, possibly through a heterogeneous nucleation process. The interaction of the polymer fibrils and the organogel superhelical structure takes place through the hydrogenous moieties as the fluorinated moieties lie in the center of the helical structures. As a result, an increase in the hydrophobicity of this material is not expected and indeed is not observed. This system therefore differs from OPV/sPS intermingled gels recently investigated by our team, where one component, the organogel, pervades through the network of the other component, the sPS thermoreversible gel.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +33 388414099. Present Addresses

(A.N.K.) School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12 Islamabad, Pakistan. (T.-T.-T.N.) Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, 69007 Lyon, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been carried out as part of a grant from the French National Agency (ANR, project MATISSE, grant no. 11-BS08-001). A.N.K. also gratefully acknowledges a grant from the joint venture of the French government and the Higher Education Commission (HEC) of Pakistan (2-2(7) PDFPFrance/HEC/2012/01) together with a CNRS grant (370478) for a postdoctoral fellowship. We greatly appreciate the technical assistance of C. Contal, C. Blanck, G. Fleith, and C. Saettel for carrying out AFM, TEM, and DSC. We are also indebted to Dr. R. Schweins, senior scientist at ILL, for his experimental assistance with the D11 SANS camera. 7671

DOI: 10.1021/acs.langmuir.5b01339 Langmuir 2015, 31, 7666−7672

Article

Langmuir (24) Mittelbach, P.; Porod, G. Zur Röntgenkleinwinkelstreuung verdünnter kolloiden Systeme. Acta Phys. Austriaca 1961, 14, 185− 211. (25) Oster, G.; Riley, D. P. Scattering from Cylindrically Symmetric Systems. Acta Crystallogr. 1952, 5, 272. (26) Pringle, O. A.; Schmidt, P. W. Scattering From Helical Filaments. J. Appl. Crystallogr. 1971, 4, 290. (27) Guenet, J. M. Scattering by prolate, cross-section polydispersed cylinders applicable to fibrillar thermoreversible gels. J. Phys. II 1994, 4, 1077. (28) Gradshteyn, I. S.; Ryshik, I. M. Table of Integrals, Series and Products; Academic Press: London, 1979. (29) The contact angle was measured in water and glycerol by the classical drop method. No difference was observed between dried sPS gels and dried sPS/BHPBF hybrid systems.

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DOI: 10.1021/acs.langmuir.5b01339 Langmuir 2015, 31, 7666−7672