Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites

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Cite This: Langmuir XXXX, XXX, XXX−XXX

Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites of Semifluorinated Alkanes Coexistent with ClosePacked Self-Assembled Surface Nanodomains Xianhe Liu, Christophe Contal, Marc Schmutz, and Marie Pierre Krafft* University of Strasbourg, Institut Charles Sadron (ICS CNRS), 23 rue du Loess, 67034 Strasbourg, France

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S Supporting Information *

ABSTRACT: A series of semifluorinated alkanes (CnF2n+1CmH2m+1 diblocks, FnHm, n = 6, 8, 10; m = 16, 18, 20), when cast as films onto solid substrates, were found to form ring-banded or radial spherulites when heated above their isotropic temperature and subsequently cooled down to room temperature, demonstrating that the formation of twodimensional (2D) spherulites is a general feature of molecular fluorocarbon−hydrocarbon diblocks. These spherulites are not birefringent, a seldom encountered feature for such structures (never, so far, for spherulites made of small molecules). They also provide examples of fluorinated 2D spherulites. Film morphology was analyzed by optical microscopy, interferometric profilometry, atomic force microscopy (AFM), and scanning electron microscopy. Increasing the length of the Fn segment favors the formation of ring-banded spherulites, whereas short Fn segments tend to favor extended radial stripes. Variation of the cooling rate provides control over the size and morphology of the spherulites: slow cooling promotes fibers and radial spherulites, whereas fast cooling fosters ring-banded spherulites. The AFM studies of F10H16 films revealed that the latter consist of stacks of regularly spaced lamellae. We also observed that, remarkably, stacked lamellae (repeating distance ∼6 nm) can coexist with a layer of close-packed monodisperse circular self-assembled surface nanodomains of FnHm diblocks (∼30 nm in diameter); the latter are known to form from such diblocks at interfaces at room temperature. Substrates partially covered with F10H16 contain incomplete ring-banded spherulites and smaller objects in which the lamellae and circular nanodomains coexist.

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birefringence and nonbirefringence when film thickness increases.16 The formation of spherulites from polymers was usually ascribed to the rhythmical precipitation mechanism. Highly fluorinated spherulites are also uncommon and concern polymers,17 and there does not appear to be any mention yet of fluorinated two-dimensional (2D) spherulites. The modification of surface properties (e.g., wettability) might be achieved using such fluorinated spherulites. Examples of nanohybrids consisting of nanoparticle-loaded fluorinated polymeric spherulites have been proposed.18 Semifluorinated alkanes (CnF2n+1CmH2m+1, FnHm diblocks) are being extensively studied because of their unique combination of hydrophobic, amphiphilic, amphisteric, and amphidynamic properties,19 and their aptitude to act as cosurfactants with phospholipids.20,21 Earlier this year, we reported the unexpected first examples of spherulite formation from a diblock, F10H16.22 It therefore became mandatory to determine whether this was a common feature for FnHm diblocks and how the diblock structure affected the spherulite

INTRODUCTION Two-dimensional spherulites are generated on solid surfaces by solvent evaporation or melting of a variety of compounds.1 Many polymers [e.g., polyethylene, polypropylene, and poly(ethylene adipate)] form ring-banded patterns that exhibit optical birefringence under polarized light.2−5 Several small inorganic or organic molecules also produce ring-banded spherulites (e.g., phthalic acid,6,7 aspirin,8 hippuric acid,9 mannitol,10 and testosterone propionate11), sometimes in the presence of additives. Despite sustained investigation, the mechanism of formation of spherulites is still being debated. Concentric ring bands were proposed to be due to either a twisting of crystals, which results in periodic and continuous changes in crystallite orientation from edge-on to flat-on lamellae visible under polarized light, or to a rhythmic crystallization, the latter being essentially encountered for small molecules that do not have a helical structure. For some blends of polymers, for example, poly(aryl ether ketone) and poly(aryl ether ether ketone),12 phase separation occurred and induced the rhythmical crystallization growth. Reports of nonbirefringent spherulites are scarce,13−15 and we found no examples of nonbirefringent spherulites of small molecules. Poly(L-lactide) undergoes an alternation between © XXXX American Chemical Society

Received: June 6, 2018 Revised: November 5, 2018 Published: November 7, 2018 A

DOI: 10.1021/acs.langmuir.8b01893 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. DSC cooling curves of FnHm diblocks. (transmission mode) and electron microscopy observation, and 50 μL of solutions were cast onto 1 cm2 silicon wafers for AFM and for optical microscopy (reflection mode), by allowing the solvent to evaporate in air at room temperature. The substrates were previously treated using a plasma cleaner (Harrick Plasma, Ithaca NY) for 2 min. After casting, the samples were heated on a hot stage (Linkam Scientific PE 94) for 30 s unless mentioned otherwise, depending on the experiments, at a temperature 5−10 °C higher than the diblock’s isotropic temperature. The samples were then allowed to cool down to room temperature at various rates. The hot stage allowed the control of the cooling rate from 1 to 20 °C min−1. Faster cooling rates were obtained by exposing the film to room temperature or by contacting the substrate with an ice cube. Under these conditions the cooling rates were estimated to be ≥60 and ≥100 °C min−1, respectively. Differential Scanning Calorimetry. Thermal analysis was performed with a PerkinElmer DSC8500 differential scanning calorimeter. The diblocks (FnHm, n = 6, 8, 10; m = 16, 18, 20; 6− 10 mg of powder) were placed in aluminum capsules. The samples were preheated by ramping the temperature to 100 °C and then cooled to −40 °C at a rate of 20 °C min−1. The differential scanning calorimetry (DSC) thermograms were then obtained at a heating rate of 5 °C min−1 in the temperature range of −40 to 120 °C under nitrogen. Optical Microscopy. The 2D spherulites were observed with polarized and nonpolarized light (10× objective) using a Nikon Eclipse 90i microscope working in transmission mode and a Leica DMRX microscope working in reflection mode for the samples cast on glass plates and silicon wafers, respectively. Rapid image acquisition was achieved with an Infinity 2 CCD camera (Lumenera, Ottawa, Canada). At least three different samples were prepared and different positions were assessed to determine reproducibility. Atomic Force Microscopy. The images of the ring-banded spherulites were recorded in tapping mode and PeakForce tapping mode (AFM multimode 8, Bruker, Santa Barbara, USA). For the tapping mode, the cantilever (Budget Sensors) was fitted with a 3−10 nm silicon tip. The typical resonance frequency was 300 kHz and the spring constant was 40 N m−1. For the PeakForce tapping mode, the tip (Bruker, ScanAsyst) spring constant was 0.4 N m−1. The PeakForce tapping mode enables reduction of the applied force by 3 orders of magnitude with respect to the minimum force applied in the standard tapping mode. In our study, at least three different samples were analyzed and several positions were scanned on the silicon wafer for each sample. The error on the measurements along the z axis was estimated at ±0.5 nm. Scanning Electron Microscopy. The samples were prepared on glass slides as described above. The samples were observed without metallization with a Hitachi SU-8010 FEG scanning electron microscope (Hitachi, Tokyo, Japan) operating at 1 kV. Optical Interferometric Profilometry. The surface topology of the samples was analyzed with an interferometric microscope (Contour Elite 3D Optical Microscope, Bruker). The samples were prepared on silicon wafers. The film thicknesses (1.5 and 3.5 ± 0.2 μm, corresponding to the concentrations of 1 and 4 mg mL−1,

formation and morphology. On the other hand, it had been well-established that FnHm diblocks are able to generate highly monodisperse, self-assembled disk-like mesoscopic surface domains, 30−50 nm in diameter. These nanodomains organize in hexagonal lattices and form single diblock-thick films when deposited on water or spin-cast or transferred on solid surfaces.23−30 Their formation is immediate and occurs at room temperature. These nanodomains are quasi-crystalline31 and are stabilized by intermolecular dipole−dipole interactions.32 Langmuir monolayers of diblocks, when subjected to periodical shear stress, revealed a predominantly elastic behavior that was assigned to the formation of physical 2D gels that occurs even at zero surface pressure.33 FnHm diblocks also form fibers, liquid crystals, and gels.19,34−36 Diblocks uniquely form gels in both hydrocarbons and fluorocarbons,37,38 as well as in mixtures of fluorocarbons and hydrocarbons.39 These gels display fibrous crystalline structures that grow radially from the nucleation points. The semifluorinated alkane F10H16 forms both nonbirefringent radial and ring-banded spherulites after the melting and cooling of micrometric (1−4 μm) films cast on glass or silicon substrates.22 The morphology of these spherulites could be effectively controlled by adjusting the heating time and cooling rate during crystallization. In the present paper, we extend our study to a whole series of diblocks (CnF2n+1CmH2m+1, FnHm, n = 6, 8, 10; m = 16, 18, 20) to ascertain the extent of the phenomenon and determine the influence of the length of the Fn and Hm segments and cooling rates on film morphology. Atomic force microscopy (AFM), optical interferometric profilometry, and scanning electron microscopy (SEM) were used for a closer investigation of the ring-banded and radial spherulite structures. We also found, unexpectedly, that the lamellar spherulite structures formed during thermal treatment (melt-crystallization) can coexist with layers of the close-packed self-assembled discrete nanodomains that are observed to form at room temperature.

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MATERIALS AND METHODS

CnF2n+1CmH2m+1 diblocks (FnHm, n = 6, 8, 10; m = 16, 18, 20) were synthesized by the free-radical addition of perfluoroalkyl iodides to the appropriate terminal alkene19 according to refs 40 and 41 and purified by repeated crystallizations from methanol and ethanol. Chemical purity (>99%) was determined by thin-layer chromatography, nuclear magnetic resonance, elemental analysis, and matrixassisted laser desorption ionization time-of-flight mass spectrometry. Chloroform (>99.9%) and ethanol (>99.9%) came from Carlo Erba. Sample Preparation. The solutions of FnHm (1 or 4 mg mL−1) were prepared in chloroform/ethanol (1:1 in volume). Volumes of 200 μL of these solutions were cast onto 4 cm2 glass slides for optical B


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RESULTS AND DISCUSSION Thermal Behavior of FnHm Diblocks. The FnHm diblocks were analyzed by DSC for their thermal properties.

Figure 2. Optical images (transmission) of F10H16 forming (A) radial spherulites and (B) ring-banded spherulites, under normal light (left) and polarized light (right). The films were cast, heated, and crystallized at a cooling rate of 60 °C min−1. Thickness was ∼3.5 μm.

During cooling, all the diblocks, apart from F6H16, F8H20, and F12H16, display a single sharp peak that corresponds to the melting temperature Tm (Figure 1). The Tm values are in agreement with earlier reports.19 F6H16, F8H20, and F12H16 present additional peaks at −0.5, 37, and 85 °C, respectively, which suggest a solid−solid transition prior to isotropization. The cooling and reheating curves are essentially identical for all the diblocks investigated, meaning that the phase transition is reversible (Supporting Information, Figure S1). The isotropic transition peaks shift to higher temperatures as the length of either the Fn or the Hm segment increases. Nonbirefringent Spherulites from Small Fluorinated Molecules. An unusual feature is that the surface spherulites formed by the (F-alkyl)alkane diblocks investigated are not birefringent, as assessed by the optical microscopy observation under polarized light of numerous samples (no Maltese cross extinction pattern) (Figure 2). The samples prepared on glass substrates were observed in the transmission mode, and those cast on silicon wafers and destined for AFM and profilometry studies were observed in the reflection mode (Supporting Information, Figure S2). Only a few examples of nonbirefringent 2D spherulites have been reported so far.4,42−44 To the best of our knowledge, no nonbirefringent spherulites had been obtained from small (nonpolymeric) molecules. 2D Spherulite Formation: A General Feature of FnHm DiblocksMorphology-Controlling Parameters. Cast films of a series of FnH16 (n = 6, 8, 10, 12) diblocks were prepared using the same melting procedure and fast cooling rate (on an ice cube; ≥100 °C min−1). Optical microscopy showed that the F6H16 films consist of radial fibrous spherulites that develop from nucleation centers (Figure 3a),

Figure 3. Optical micrographs (×10, nonpolarized light) of (a) F6H16, (b) F8H16, and (c) F10H16 films cast, heated, and crystallized by fast cooling (≥100 °C min−1).

Figure 4. Optical images (×10, nonpolarized light) of (a) F8H14 and (b) F8H18 films cast, heated, and crystallized by fast cooling (≥100 °C min−1).

whereas F8H16 forms radial stripes that coexist with several ring bands within the spherulites (Figure 3b). F10H16 forms well-defined ring-banded spherulites with unequivocal ridges and valleys (Figure 3c). These images indicate that increasing the degree of fluorination of the molecules tends to favor the formation of ring bands, whereas decreasing it facilitates the formation of radial stripes. One possible reason for this is that, although the longer Fn blocks would increase the van der Waals interactions,19 the increased Fn/Hm length ratio could increase the mobility (slipperiness) among chains, which is expected to lead to looser crystal packing, thus hindering the growth of radial crystals and facilitating the formation of short-range, discontinuous crystals that stack up into banded structures (see below). The effect of the Hm block length is less pronounced. C


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Figure 5. Optical micrographs (×10, nonpolarized light) of F6H16 films cast, heated, and crystallized at various cooling rates: (a) 10, (b) 20 °C min−1, (c) at ambient temperature (≥60 °C min−1), and (d) on ice (≥100 °C min−1); AFM height image (e) and phase image (f) of the region framed in yellow in (d); (g) height profile of the central region (white line in (e).

Figure 7. (a) Interferometric profilometry image of a F10H16 ringbanded spherulite melt-crystallized on a silicon wafer; (b) interferometric height profile along the white line drawn on (a); (c) AFM image in tapping mode; (d) zoomed-in AFM height image, and (f) phase image of a ridge region (blue frame in c); (e) height image of a valley region (yellow frame in c) in tapping mode: (g) height image of the same ridge region as (d) in PeakForce mode. The samples for AFM were cast, melted, and cooled on silicon wafers.

In the F8Hm series, F8H14 films display radial spherulites with a few ring bands (Figure 4a), whereas only sketchy frontiers of ill-defined spherulites are seen for F8H18 (Figure 4b). The films of FnH20 (n = 8 and 10) and F12H16 show few regular patterns under optical microscopy (Supporting Information, Figure S3), suggesting that the longest molecules investigated (n + m ≥ 28) are less prone to formation of organized crystallized structures. The length of the diblock chains has thus a clear influence on film morphology. The rate of cooling of the cast films was identified as another critical parameter for controlling the morphology of FnHm spherulites. In the case of F6H16, low cooling rates (10 and 20 °C min−1) favor the formation of elongated, bent, and ramified radial fibers that grow from the nucleation centers with no clear-cut boundaries between the crystallites (Figure 5a,b). When the cooling rate is increased to 60 °C min−1, radial spherulites extending from distinct nuclei and definite boundaries are seen to form (Figure 5c). Contacting the film with ice provided even faster cooling, which favors the formation of smaller-sized radial spherulites (Figure 5d). The AFM studies of spherulites shown in the yellow frame of Figure 5d confirmed the radial structure expanding from their center (Figure 5e,f). The AFM profile shows that the depth of the central pit is ∼1.2 μm (Figure 5g). Slow cooling favors radial spreading and branching of extended fibers, whereas fast cooling increases the number of nucleation points, thus

Figure 6. Optical microscopies (×10, nonpolarized light) of films of F8H16 cast, heated, and crystallized at a cooling rate of (a) 20 and (b) 100 °C min−1; F10H16 cast, heated, and crystallized at (c) 10 and (d) 60 °C min−1; and films of F10H18 crystallized at (e) 60 and (f) 100 °C min−1. D


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(4 mg mL−1; thickness 3.5 ± 0.2 μm) cast, heated, and then cooled on silicon wafers. Profilometry clearly shows the ringbanded morphology and the concentric bands with ridges and valleys that have comparable width (20−25 μm; Figure 7a,b). The AFM images were captured using the tapping mode in which the atomic force microscope tip slightly penetrates beneath the surface of the films and the PeakForce tapping mode, which better preserves the surface. Two different regions were investigated: the top of a ridge (blue frame in Figure 7c) and the bottom of a valley (yellow frame in Figure 7c). Figure 7d,e indicates that in both cases the bands consist of lamellae stacked predominately parallel to the surface (flaton crystals). These stacks of lamellae adopt diverse orientations, in agreement with the neutron reflectivity results.22 The lamellar structure is clearly seen in the phase image of the ridge region (Figure 7f). The AFM investigations did not show significant changes in lamellar morphology when crossing from the ridges to valleys. The height of the crystal stacks was significantly larger on the ridges than in the valleys (∼207 nm vs ∼90 nm, Figure 7d,e). We also used the softer AFM PeakForce mode to examine the ridge of the bands (Figure 7g). It revealed that multiple nodules (diameter ∼100 nm) form a fragile ultrathin, likely amorphous top layer that covers the lamellae. The consistent observation of stacked-up lamellar structures (flat-on crystals) in both the ridges and valleys strongly indicates the absence of a driving force based on twisting or bending for the ring-banded spherulite formation. Instead, the formation of stacks of lamellae forming successive ridges and valleys is likely controlled by rhythmic precipitation during recrystallization. This is reminiscent of the formation of ringbanded structures during the crystallization of isotactic polystyrene and poly(ε-caprolactone),15,42 for which the bands are formed as a way of accommodating the decrease in specific volume that occurs upon crystallization. Stacked-up lamellae were also observed by AFM in the radial spherulites formed by F8H16 (Supporting Information, Figure S4). Coexistence of the Micron-Sized Stacked Lamellae with Organized Nanometer-Sized Discrete Surface Domains. FnHm diblocks are known to spontaneously selfassemble at room temperature into monodisperse surface domains of nanometric size when spread on the surface of water or on solids.23,24,26,45 This unique type of nanostructure has so far only been observed in monolayers and ultrathin f ilms. In this section, we report AFM studies performed on thinner films than those described above (i.e. 1.5 ± 0.2 μm, instead of 3.5 ± 0.2 μm) (Figure 8). These films were, however, considerably thicker than a monolayer (3−4 nm). Yet, these AFM studies revealed the coexistence of surface nanodomains

Figure 8. (a) Optical microscopy of a 1.5 μm thick F10H16 film cast, heated (at 76 °C for 30 s), and cooled down at 60° min−1, (b) AFM height image of the region shown in (a), (c) zoomed-in height image showing profiles (red, blue, and green lines) across adjacent lamellae for spacing determination using Nanoscope software, and (d) zoomed-in phase image. Concentration: 1 mg mL−1.

limiting the radial development of the crystals, and, hence, producing smaller spherulites. A similar trend in the influence of the cooling rate was found for all diblocks. For example, F8H16 forms branched, bent fibers at 10−60 °C min−1. Faster cooling (≥100 °C min−1) is required to generate radial spherulites (Figure 6a,b). F10H16 forms elongated fibers without boundaries when cooled at 10 °C min−1 and perfectly developed ring-banded spherulites when cooled at 60 °C min−1 (Figure 6c,d). F10H18 develops spherulites with clear boundaries when cooled at an ambient temperature (60 °C min−1) and smaller-sized spherulites when the cooling rate increases to 100 °C min−1 (Figure 6e,f). Altogether, well-developed ring-banded spherulites can be obtained from essentially all FnHm diblocks, provided the cooling rate is appropriate. Ring-Banded Spherulites of F10H16: Evenly Spaced Stacked Lamellae. Interferometric profilometry and AFM were used to advance our understanding of the band structure at the nanoscopic scale. Therefore, we used the F10H16 films

Figure 9. Optical micrographs of a 1.5 μm thick film of F10H16 crystallized on (a) a glass plate and (b) a silicon wafer heated at 76 °C during 3 min and cooled at an ambient temperature; (c) zoomed-in SEM image of the sample visualized in (a). Two types of differently sized objects, labeled L and S, are seen. E


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form without the need for heating, are present in spite of the growth and crystallization of the spherulites. Structures Found on Incompletely Covered Substrates. Heating a 1.5 μm thick film of F10H16 at 76 °C for a longer period of time (3 min instead of 30 s) induces the dewetting of the diblock in its liquid state, leading to only a partial coverage of the substrate’s surface. Optical microscopy then shows relatively large, partially developed ring-banded structures together with smaller structures (labeled L and S, respectively, in Figure 9), both on glass plates (Figure 9a) and on silicon wafers (Figure 9b) that are also seen by SEM (Figure 9c). The optical micrographs show that the structures L consist of fragments of ring-banded spherulites. AFM investigations were conducted on the smaller structures S, as the size of the L structures prevented their investigations by AFM. The AFM images of the structures S (Figure 10a) again reveal the presence of regular, well-formed, and close-packed surface nanodomains (30 ± 4 nm) that coexist with the 6 nm thick stacked lamellae (Figure 10b,c). It is likely that the surface nanodomains form faster than the spherulite lamellae during cooling, as they were shown to already self-assemble at room temperature.19,23,26 These nanodomains appear robust enough to withstand the spherulite crystallization process.

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CONCLUSIONS Melt-crystallization of cast films of a series of molecular FnHm diblocks (n = 6, 8, 10; m = 16, 18, 20) provides radial and/or ring-banded 2D spherulites, reminiscent of those formed by polymers, thus establishing that spherulite formation is a generic property of fluorocarbon−hydrocarbon diblocks. These structures constitute unique examples of nonbirefringent spherulites, as well as of highly fluorinated surface spherulites. The formation and morphologies of these spherulites and their sizes depend critically on the length of the fluorinated block and on the cooling rate. F-block lengthening favors the formation of ring-banded spherulites, whereas increasing the length of the hydrocarbon block has lesser effects. Fast cooling promotes the formation of small ring-banded spherulites, whereas slow cooling promotes the development of isolated fibers or fibrous radial spherulites. The AFM studies of the ring-banded spherulites of F10H16 reveal that their ridges and valleys consist of stacks of evenly spaced (∼6.1 nm) crystallized lamellae (flat-on crystals). Remarkably, these lamellar structures coexist with a layer of organized, closepacked discrete surface nanodomains of diblocks likely lying on the wafer’s surface. These surface nanodomains, which are known to self-assemble at room temperature, appear to withstand the crystallization process that leads to spherulite formation. When the substrate is incompletely covered by diblocks, fragments (L) of ring-banded spherulites are seen to form, which are accompanied by smaller objects (S) that reveal the simultaneous presence of nanodomains and lamellae.

Figure 10. (a) AFM image of a small structure S found on an incompletely covered silicon wafer imaged in Figure 9b in 1.5 μm thick films of F10H16 heated at 76 °C for 3 min and then cooled at 60° min−1; (b,c) zoomed-in AFM images of the red square drawn in (a), revealing the simultaneous presence of surface nanodomains and lamellae.

together with the melt-crystallized lamellae. A 1 mg mL−1 concentrated solution of F10H16 was deposited on a Si wafer and heated for 30 s at 76 °C and then cooled to ambient temperature (Figure 8a). AFM of the radial morphology displayed stacks of lamellar crystals (Figure 8b) along with a layer of close-packed nanodomains, both of which are clearly visible on the phase image (Figure 8d). The spacing between two lamellae was 6.1 ± 0.5 nm, as determined on the AFM image (Figure 8c), in agreement with the value determined by off-specular neutron scattering (∼5.7 nm).22 It is remarkable that the surface nanodomains (30 ± 4 nm in diameter), which

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01893. FnHm thermograms during heating; optical micrographs (reflection mode) under normal and polarized light of various diblock spherulites, showing the absence of F


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birefringence; optical micrographs of crystallized films of F8H20, F10H20, and F12H16; and optical micrographs and AFM images of radial spherulites of F8H16 (PDF)

(11) Shtukenberg, A.; Freundenthal, J.; Gunn, E.; Yu, L.; Kahr, B. Glass-crystal growth mode for testosterone propionate. Cryst. Growth Des. 2011, 11, 4458−4462. (12) Chen, J.; Yang, D. Phase behavior and rhythmically grown ringbanded spherulites in blends of liquid crystalline poly(aryl ether ketone) and poly(aryl ether ether ketone). Macromolecules 2005, 38, 3371−3379. (13) Wang, Z.; Alfonso, G. C.; Hu, Z.; Zhang, J.; He, T. Rhythmic Growth-Induced Ring-Banded Spherulites with Radial Periodic Variation of Thicknesses Grown from Poly(ε-caprolactone) Solution with Constant Concentration. Macromolecules 2008, 41, 7584−7595. (14) Li, Y.; Huang, H.; He, T.; Wang, Z. Band-to-Nonband Transition into Unique Poly(ε-caprolactone) Crystals by Modulating the Interplay of Diffusion and Growth. ACS Macro Lett. 2012, 1, 718−722. (15) Duan, Y.; Zhang, Y.; Yan, S.; Schultz, J. M. In situ AFM study of the growth of banded hedritic structures in thin films of isotactic polystyrene. Polymer 2005, 46, 9015−9021. (16) Huang, S.; Li, H.; Wen, H.; Yu, D.; Jiang, S.; Li, G.; Chen, X.; An, L. Solvent micro-evaporation and concentration gradient synergistically induced crystallization of poly(L-lactide) and ring banded supra-structures with radial periodic variation of thickness. CrystEngComm 2014, 16, 94−101. (17) Fujimori, A.; Hasegawa, M.; Masuko, T. Spherulitic structures of poly [tetrafluoroethylene-co-(perfluoroalkyl vinyl ether)]. Polym. Int. 2007, 56, 1281−1287. (18) Mamun, M. A. A.; Kasahara, Y.; Tasaki, T.; Fujimori, A. Spherulitic formation and characterization of partially fluorinated copolymers and their nanohybrids with functional fillers. Polym. Eng. Sci. 2017, 57, 161−171. (19) Krafft, M. P.; Riess, J. G. Chemistry, Physical Chemistry, and Uses of Molecular Fluorocarbon−Hydrocarbon Diblocks, Triblocks, and Related Compounds-Unique “Apolar” Components for SelfAssembled Colloid and Interface Engineering. Chem. Rev. 2009, 109, 1714−1792. (20) Riess, J. G.; Cornelus, C.; Follana, R.; Krafft, M. P.; Mahé, A. M.; Postelf, M.; Zarif, L. Novel fluorocarbon-based injectable oxygencarrying formulations with long-term room-temperature storage stability. Adv. Exp. Med. Biol. 1994, 345, 227−234. (21) Marie Bertilla, S.; Thomas, J.-L.; Marie, P.; Krafft, M. P. Cosurfactant Effect of a Semifluorinated Alkane at a Fluorocarbon/ Water Interface: Impact on the Stabilization of Fluorocarbon-inWater Emulsions. Langmuir 2004, 20, 3920−3924. (22) Liu, X.; Mielke, S.; Contal, C.; Favier, D.; Yamamoto, A.; Tanaka, M.; Krafft, M. P. 2D Spherulites of a semi-fluorinated alkane: Controlled access to either radial or ring-banded morphologies. ChemPhysChem 2018, 19, 29−33. (23) Maaloum, M.; Muller, P.; Krafft, M. P. Monodisperse Surface Micelles of Nonpolar Amphiphiles in Langmuir Monolayers. Angew. Chem., Int. Ed. 2002, 41, 4331−4334. (24) Fontaine, P.; Goldmann, M.; Muller, P.; Fauré, M.-C.; Konovalov, O.; Krafft, M. P. Direct Evidence for Highly Organized Networks of Circular Surface Micelles of Surfactant at the Air−Water Interface. J. Am. Chem. Soc. 2005, 127, 512−513. (25) de Viguerie, L.; Keller, R.; Jonas, U.; Berger, R.; Clark, C. G.; Klein, C. O.; Geue, T.; Müllen, K.; Butt, H.-J.; Vlassopoulos, D. Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface. Langmuir 2011, 27, 8776−8786. (26) Krafft, M. P. Large Organized Surface Domains Self-Assembled from Nonpolar Amphiphiles. Acc. Chem. Res. 2012, 45, 514−524. (27) de Gracia Lux, C.; Gallani, J.-L.; Waton, G.; Krafft, M. P. Stacking of self-assembled surface micelles in ultrathin films. ChemPhysChem 2012, 13, 1454−1462. (28) Feng, X.; Mourran, A.; Möller, M.; Bahr, C. AFM study of Gibbs films of semifluorinated alkanes at liquid crystal/air interfaces. ChemPhysChem 2013, 14, 1801−1805. (29) Theodoratou, A.; Jonas, U.; Loppinet, B.; Geue, T.; Stangenberg, R.; Keller, R.; Li, D.; Berger, R.; Vermant, J.;

AUTHOR INFORMATION

Corresponding Author

*E-mail: kraff[email protected]. ORCID

Marie Pierre Krafft: 0000-0002-3379-2783 Funding

French Research Agency (ANR-14-CE35-0028-01) and the NANOTRANSMED project. The "NANOTRANSMED" project is co-funded by the European Regional Development Fund (ERDF) in the framework of the INTERREG V Upper Rhine program ("Trenscending borders with every project") and by the Swiss Confederation and the Swiss cantons of Aargau, Basel-Landschaft and Basel-Stadt. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the French Research Agency (ANR-14-CE35-002801; PhD grant for X.L.), NanoTransMed, a project co-funded by the European Regional Development Fund in the framework of the INTERREG V Upper Rhine program (“Transcending borders with every project”) and by the Swiss Confederation and the Swiss cantons of Aargau, BaselLandschaft and Basel-Sdadt, as well as the GIS Fluor (CNRS) for a travel grant. The characterization and microscopy platforms are acknowledged for the use of their instruments.

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REFERENCES

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Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites

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