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Synthesis and Self-Assembly of Organometallic Semiconducting PMMA-b-PFS-b-PS-b-PFS-b-PMMA Pentablock Copolymers

Authors
Publisher
Technische Universität
Publication Date
Disciplines
  • Physics

Abstract

It was the goal of the present work to synthesize a series of well-defined pentablock copolymers of styrene (S), [1]dimethylsilaferrocenophane (FS) and methyl methacrylate (MMA) by sequential addition of monomers. Lithium naphthalide was used as difunctional initiator and a 1,1-dimethylsilacyclobutane (DMSB) mediated 1,1-diphenylethylene (DPE) end-capping technique was applied to ensure high block efficiency. The PMMA-b-PFS-b-PS-b- PFS-b-PMMA systems have molecular weights of Mn = 50,000 g/mol, polydispersity indices of PDI ≤ 1.12 and PFS volume fractions of around φ PFS=0.15. First, the difunctional middle block of PS was formed, followed by triblock PFS-b-PS-b-PFS, which then - after appropriate end-capping - was employed as a macroinitiator to polymerize MMA, leading to the final pentablock copolymer. The block copolymers were characterized by SEC and NMR. Under optimum reaction conditions - i.e. when chain termination during DPE/DMSB end-capping is minimized - the pentablock copolymer was obtained in approximately 75 % purity: the crude product contained - in addition to the pentablock - some tetrablock (20%) and triblock (5%) copolymers. This raw material, however, could be purified by selective precipitation procedures which addressed the differences in polarity of the constituents. By that method, almost quantitative removal of the triblock was possible whereas a small amount of the tetrablock remained in the pentablock fraction. Fortunately the presence of such small amounts of tetrablock copolymers did not influence the pentablock copolymer micromorphologies in an irreproducible way. Thus, profound investigations were possible to analyze the bulk properties of the materials. The pentablock copolymers were examined with the help of TEM and SAXS for their microphase behavior. The self-assembly was studied starting from dichloromethane, tetrahydrofuran and toluene solutions, after taking into consideration the solubility parameters. The formation of thermodynamically stable micromorphologies was forced by storing the solvent cast films in the saturated atmosphere of the corresponding solvent for 3-4 weeks followed by annealing at elevated temperatures 180 DegC - 220 DegC. Prior to this, it was ensured by TGA and DSC analysis that the pentablock copolymers are of sufficient thermal stability for this treatment. Annealing under various conditions followed by TEM analysis revealed good phase separation and the formation of highly ordered microstructures, such as spheres-on/in-spheres, spheres on lamellar interface, biphasic lamellar, cylinders and spheres on cylinders micromorphologies. These morphologies were further verified and supported by SAXS scattering profiles. Conductivity measurements on original and doped samples (I2 and TCNE) of PFS containing homo and block copolymers with/without microphase separation were carried out by dielectric spectroscopy (DRS). The maximum conductivity observed for the doped PFS sample (10-7 S/cm) decreased to 10-10 S/cm on dilution by domains of PS and PMMA in the block copolymers of various architectures.

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