Synthesis and characterization of functional diblock copolymers
In search of materials for high-density optical data storage by irradiation with polarized laser light liquid crystalline polymers containing azobenzene functionalities in the side chains have attracted great attention. These azobenzene groups generate different photoisomerization phenomena which can be exploited for optical data storage by using holographic techniques. In this PhD study a new strategy for the synthesis of side chain liquid crystalline block copolymers is employed. SC-LC block copolymers with different composition of the mesogenic block and with different side chain spacer lengths were synthesized. The first approach used was the recent and most accepted radical polymerization technique Atom Transfer Radical Polymerization (ATRP). A side chain azobenzene containing styrene monomer was synthesized and successfully polymerized by ATRP technique. The second method used was the synthesis of template polystyrene block polybutoxystyrene (PS-b-PBS) copolymers by living anionic polymerization technique and successive deprotection to give PS-b-PHS. These polymers were then functionalized with azobenzene containing bromo precursors attached through a flexible alkyl spacer. The mesogenic monomers and block copolymers were characterized by 1H-, 13C-NMR spectroscopy and also by DSC. The molecular weight of the polymers was determined by SEC. Furthermore, the stability of the induced anisotropy in the mesogenic block copolymers was studied by optical analysis. In order to synthesize thermally stable SC-LC polymers highly fluorinated styrene polymers were used as templates. The highly fluorinated new monomer 2,3,5,6-tetrafluoro-4-methoxystyrene (TFMS) was synthesized by a nucleophilic substitution reaction between 2,3,4,5,6-pentafluorostyrene (FS) and sodium methoxide. This monomer was then successfully polymerized in high yield and with low PDI by the ATRP method. The homopolymers were functionalized by demethylation and successive alkylations under phase transfer catalysis reactions with bromo precursors bearing different substituents. The ATRP of TFMS was comparatively faster than that of styrene and its highly fluorinated analogue FS. These results and the potential nucleophilic substitution in the para position of FS prompted us to attempt the preparation of novel FS monomers with 4-fluoroalkoxy side chains and subsequently investigate their polymerization potential with a view to the self assembly process of these materials at surfaces. In this context materials with a fluorine content varying in a large range have been synthesized and characterized by different techniques. The ATRP technique was found as a suitable tool to control the Mn and also the PDI of the polymers. Materials with low fluorine content were synthesized by use of fluorine containing initiators leaving the fluorine cluster at one end. Highly fluorinated materials were prepared by use of fluorine rich monomers TFMS, 2,3,5,6-tetrafluoro-4-(2,2,3,3,3-pentafluoropropoxy)styrene [TF(F5)S] and 2,3,5,6-tetrafluoro-4-(2,2,3,3,4,4,5,5,6,6,7,7,8, 8,8-pentadecafluorooctaoxy)styrene [TF(F15)S], resulting in both homopolymers as well as block copolymers. Materials with an intermediate fluorine content were achieved by copolymerization of these monomers with styrene. The TF(F5)S and TF(F15)S are another two new monomers subjected for ATRP. These monomers were synthesized by a nucleophilic substitution reaction between FS and its corresponding alcohol. The structures of the monomers were confirmed by 1H-, 13C-, 19F-NMR and also by FT-IR spectroscopy. The ATRP of these monomers results in fast polymerization and high yields. Polymers with relatively low PDI and in a broad range of Mn were synthesized. The block copolymers phase separate into PS/PFS phases as evidenced by two Tgs when the shortest block constitutes more than 10 mol %. A preliminary surface characterization of these polymers was performed by XPS and by measurement of the contact angle with water. Both analyses provide evidence for the segregation of fluorinated blocks resulting in low energy surfaces.
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