The Langmuir film technique allows control of polymer organization at the air-water interface and thus the elaboration of chemically nanostructured surfaces. Nanostructuration has been particularly studied in the case of block copolymers with different hydrophiliclipophilic balance. Using Langmuir films, it is possible to adjust the morphology by controlling thermodynamic parameters, such as surface density and pH or salinity of the subphase. Polymer blends at the air-water interface are another interesting alternative for developing micro or nano-structured surfaces. Both interface effects and 2D confinement sometimes lead to unexpected mixing behavior compared to analogous 3D blends . The transfer of these nanometer-thick polymer mixed films onto a solid substrate by the Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) technique then makes possible to obtain tailor-made polymer coatings that could be applied in various domains such as energy conversion, wetting and biotechnologies.
Regioregular poly(3-alkylthiophenes) (P3AT) are rigid conducting polymers widely used in different devices such as field effect transistors or photovoltaic cells, but also as surface coatings to tune wetting properties or for biological applications due to their biocompatibility [2, 3, 4, 5]. The device performances strongly depend on the polymer arrangement. Pi-Pi interactions between chains lead to the formation of semi-crystalline thin films at room temperature, in which the polymer tends to adopt a nanofiber organization.
Nevertheless, the organization degree significantly depends on the processing conditions (deposition method, solvent, concentration…). This emphasizes the importance of the film deposition technique to control the polymer organization. The Langmuir film technique can provide an alternative way to spin-coating or drop-casting generally used for device elaboration.The PhD project aims at studying the self-organization mechanisms of these systems at the air-water interface and then after monolayer transfer onto solid substrate using the LS technique.
The first objective is the study of the effect of the dipole moment of the pendent chains and thus of their hydrophobic character on the structure of P3AT Langmuir films. Thus, P3AT monolayers with different lateral chain length and various functionalities at the end-chain (CH3, COOH, COOK) will be elaborated. The second objective is to study the mixing properties of P3AT with polydimethylsiloxane (PDMS), a well-known biocompatible polymer forming low surface free-energy coatings, by varying the P3AT chemical structure, the length of PDMS chains, and the thermodynamic parameters. Indeed, it has been shown that mixing rigid P3AT with another compound improves monolayer organization . Di-methacrylate terminated PDMS will also be considered in order to study the effect of its free-radical cross-linking under UV irradiation on the polymer blend self-organization [7, 8]. The influence of the cross-linking density will also be investigated by varying the PDMS chain length.
Once the P3AT/PDMS mixed films characterized at the air-water interface by compression isotherm and Brewster angle microscopy observations at the mesoscopic-scale, their structure will be in situ analyzed on the SIRIUS beamline of SOLEIL by grazing incidence small and wide angle x-ray scattering (GISAXS and GIWAXS) but also by neutron reflectivity using a deuterated PDMS. The structure-property relationships of these monolayers will also be characterized at the air-water interface by specular and off-specular x-ray reflectivity (XRR). The evolution of the height fluctuation spectrum will be investigated as a function of the blend composition and for the selected P3AT derivatives, but also when PDMS is crosslinked. This will highlight the influence of the functional group of P3AT side chain on the rigidity of the monolayers and the effect of PDMS incorporation and cross-linking within these rigid conducting polymer films. At the same time, the chemical composition at the interface will be probed by x-ray fluorescence measurements.
Secondly, these monolayers will be transferred onto solid substrates using direct or reverse LS technique. Topography and nano-mechanical surface imaging (adhesion, Young's modulus) will then be carried out by atomic force microscopy (AFM). Different P3AT-PDMS LS mixed films will then be characterized by scanning x-ray spectromicroscopy (e.g NANOSCOPIUM beamline at SOLEIL) in order to analyze the structure and chemical composition of the films at spatial resolutions of a few tens of nanometers (via sulfur from to thiophene units and silicon from dimethylsiloxane units). These characterizations will be completed at spatial resolutions of less than 25 nm by x-ray photoemission electron microscopy (X-PEEM - HERMES beamline at Soleil).
The organization of polymers will thus be studied at different scales depending, on the one hand, on the structure of P3AT, the amount of PDMS, its molar mass, and whether if it is cross-linked or not, and on the other hand, on thermodynamic parameters. In order to highlight the application of these films in the field of surface treatment, a final objective of the PhD thesis is to study the wetting properties of water and oil on these composite coatings with a well-defined structure by dynamic contact angle measurements. The variation in the mixture composition and the choice of the functional group of the P3AT side chains should generate chemically controlled surfaces with tunable wettability.
 A. El Haitami, M. Goldmann, F. Cousin, G. Dosseh, S. Cantin; Langmuir 31 (23), 6395–6403 (2015).
 X. Ji , A. El Haitami, F. Sorba, S. Rosset, G. T.M. Nguyen, C. Plesse , F. Vidal , H. R. Shea, S. Cantin; Sensors Actuators B Chem 261, 135–143 (2018).
 S. Oh, M. Yang, J. Bouffard, S. Hong, S.-J. Park; ACS Appl. Mater. Interfaces 9, 12865–12871 (2017).
 K. S. Ahn, H. Jo, J. B. Kim, I. Seo, H. H. Lee, D. R. Lee; ACS Appl. Mater. Interfaces 12, 1142–1150 (2020).
 V. V. Korolkov, A. Summerfield, A. Murphy, D. B. Amabilino, K. Watanabe, T. Taniguchi, P. H. Beton; Nature Com. 10, 1537 (2019).
 E. A. Da Silva, L. Caseli, C. A. Olivatia; Colloids and Surfaces A: Physicochemical and Engineering Aspects 529, 628–633 (2017).
 A. El Haitami, E.H.G. Backus, S. Cantin; Langmuir 30 (40), 11919–11927 (2014).
 A.-S. Vaillard, A. El Haitami, L. B. Dreier, E. H. G. Backus, S. Cantin; Langmuir 36(4), 862–871 (2020).
Supervisors: Sophie CANTIN (LPPI, CY Cergy Paris Université) ; Philippe FONTAINE (Soleil Synchrotron) ; Alae EL HAITAMI (LPPI, CY Cergy Paris Université)
PhD position Funding: doctoral contract with co-funding from CY Cergy Paris University and SOLEIL Synchrotron
Candidate profile: Master in physics and/or chemistry (with at least a score > 12/20). Training or experience in the field of polymer materials will be appreciated.
Alae El Haitami
e-mail : firstname.lastname@example.org
Application to be sent before May 28 including CV, M1 and M2 transcript and a cover letter.
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