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Article Dans Une Revue ChemPhysChem Année : 2018

2D Spherulites of a Semi-Fluorinated Alkane: Controlled Access to Either Radial Or Ring-Banded Morphologies

Résumé

Thin films of as emi-fluorinated alkane cast onto solid sub-strates consist of well-formed two-dimensional non-birefrin-gent ring-banded and/or radial spherulites. Controlling the experimental conditions allows orientation of the crystallization toward either radial-only or ring-banded-only morphologies. Intermediate states were also captured in which both radial and ring-banded spherulites coexist. Monitoring of the formation of these intermediate states broughte videncef or af irst crystallization modet hat sweeps radially outwards from ac en-tral nucleusu ntil the propagating fronte dge experiences a second crystallization mode that proceeds through ad iffusion-controlled rhythmic crystallization mechanism that leads to high (% 2 mm) concentricr idges. These 2D spherulites were investigated by optical and atomicf orce microscopies,i nterfero-metric profilometry,and off-specularn eutron scattering. Polycrystalline aggregatesw ith an essentially spherical outer boundary,c alled spherulites, are observed to form from al arge variety of compounds, includingo rganic compounds andm in-erals. [1] Two-dimensional spherulites have been characterized in films resulting, for example, from solvent evaporation during casting of liquid drops of solutions of such materials on solid substrates. Concentric ring-banded spherulites constitute an intenselyi nvestigated sub-class of spherulites that usually possess chiroptical properties. Such ring-banded spherulites are formed by polymers (e.g. polyethylene and poly(ethylene adi-pate)), [2] but also by various small organic molecules (e.g. phthalic acid, [3] aspirin, [4] hippuric acid, [5] mannitol, [6] testoster-one propionate [7]), and by inorganic compounds(e.g. potassium dichromatea nd boric acid [8]). Am ajor distinction needs to be made between spherulites that have at wisted crystal morphology , which is responsible for the "banded" optical texture, and spherulites that have ar eal wave-like topography with actual valleys separated by ridges. The former do not necessarily have a" real" relief, while the latter can exhibit extinction bands or not. Lately,s ubstantial interest has focusedo nn on-birefringent ring-banded spherulites. [9] The latter have only been observed with af ew polymers and never with small molecules. Improving control over the morphology of 2D spheru-lites and understanding their mechanisms of formation,w hich can involve at wisting of the crystal or ar hythmic crystalliza-tion process, both being able to act in interplay,a re neededi n order to tailor new materialc haracteristics. Here we report first that as imple semi-fluorinated alkane, C 10 F 21 C 16 H 33 (F10H16), when deposited as at hin film on solid surfaces (glass or silicon wafers), forms non-birefringent 2D spherulites. To the best of our knowledge,t his is one of the rare examples of non-birefringent 2D spherulites obtained with small organic molecules. Remarkably,depending on the experimental conditions, we could obtain radial-only, or ring-banded-only spherulites. We attained control of the morphologyo ft he spherulites by adjusting key experimental parameters that govern crystallization. We provide evidencef or two crystalliza-tion eventst hat include an initial outwards crystallization starting from an ucleus that generates radial fibrous spherulites, followed by ar hythmical precipitation that produces concentric rings. C n F 2n + 1 C m H 2m + 1 (FnHm)d iblocks are highly hydrophobic am-phiphilic,a mphisteric anda mphidynamic molecules that display am arked propensity fors elf-assembly,n anocompartmen-tation and nanostructuration. [10] In their bulk solid state they form fibers, liquid crystals and gels. [11] FnHm diblock monolay-ers actually consist of highly monodisperse, disk-like domains (20-40 nm in diameter) that form ordered hexagonal lattices. [12] Such diblocks have also been used as film and shell components for modifyinga nd controlling film phase behavior,e mul-sion stability, liposomes tability and permeability,a nd fiber formation. [13] No spherulite wase ver reported to form from FnHm diblocks. Films of C 10 F 21 C 16 H 33 (F10H16) were preparedb yc asting chloroform/ethanol solutions of the compound on glass plates or silicon wafers.T wo different film thicknesses, 1.5 and 3.5 AE 0.2 mm, as determined by interferometric profilometry,w ere obtained by setting the concentrationso ft he solutions at 1 and 4mgmL À1 ,r espectively.W hen the films were heated above the isotropic temperature of F10H16 (74 AE 2 8C, as determined by differentials canningc alorimetry,s ee Supporting In-[a] X.

Domaines

Matériaux
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Dates et versions

hal-02408139 , version 1 (12-12-2019)

Identifiants

Citer

Xianhe Liu, Salomø Mielke, Christophe Contal, Damien Favier, Akihisa Yamamoto, et al.. 2D Spherulites of a Semi-Fluorinated Alkane: Controlled Access to Either Radial Or Ring-Banded Morphologies. ChemPhysChem, 2018, 19 (1), pp.29-33. ⟨10.1002/cphc.201701064⟩. ⟨hal-02408139⟩
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