- Synthesis, Characterization, and Properties of Tailored Functional Block Copolymers (2011)
- This thesis covers the design, synthesis, characterization, and application of functional block copolymers (BCP) based on a polymer analogous approach and includes three main subjects. The first subject is the implementation of a specially constructed reactor setup for sequential anionic polymerization that allows parallel block copolymer synthesis based on one identical A-block on a lab scale. For this reason, this setup facilitates the preparation of block copolymer series in a combinatorial fashion. It consists of one main reactor and three secondary reactors with individual temperature control. The addition of monomers or additives to each reactor can be handled separately. AB diblock copolymer and ABC triblock copolymer series were prepared with different lengths of the final block as well as different chemical structures of the last block. The second subject covers the synthesis, characterization, processing and application of new liquid crystalline azobenzene-containing block copolymers designed as materials for holographic data storage. Therefore, these polymers contained an amorphous, optical inert poly(methyl methacrylate) (PMMA) or polystyrene (PS) matrix and a functional segment based on polyhydroxystyrene (PHS). Different lengths of flexible spacers and/or mixtures of two spacer lengths were employed to connect the mesogenic chromophores to the polymer backbone. The structure-property relation of functionalized BCPs and the resulting mesophase was investigated. Holographic experiments were conducted on selected examples of the photo-addressable polymers. Smectic annealed samples or amorphous quenched samples were obtained by different sample preparation methods to investigate the influence of the liquid crystalline order. While the initial sensitivity to light induced orientation of the polymer systems remained unaffected, the writing times and level of postdevelopment were improved for quenched samples. Variation in spacer lengths resulted in decreasing smectic order with decreasing spacer length as well as for mixtures of two different spacer lengths promoting lower writing times in the holographic experiments. Additionally, the temperature dependence of the temporal evolution of the refractive index modulation in the smectic polymers was studied. A significant decrease of writing times and an enhancement of the postdevelopment were revealed at elevated temperatures. Stable holographic gratings could be obtained even at 100 °C. 1.1 mm thick samples, that are a prerequisite for volume holographic data storage with a high data storage density, were prepared by injection molding of blends of photoaddressable BCPs with PMMA or PS. Preliminary results confirmed the long-term stability of inscribed holographic gratings and demonstrated angular multiplexing of holographic volume gratings. The third subject covers the synthesis and characterization of new cyanobiphenyl-containing ABA triblock copolymers and their application as BCP gelators for the low molecular weight liquid crystal (LC) 4-cyano-4’-(pentyl)biphenyl (5CB). Based on the selective solubility of the A and B blocks in the nematic solvent, ABA triblock copolymers can be used for the thermoreversible gelation of 5CB. To this end, ABA and ABA’ triblock copolymers comprised of PS A-blocks and a cyanobiphenyl-functionalized PHS B-block with a high degree of polymerization were prepared by the combination of anionic polymerization, using two different synthetic routes, and polymer analogous attachment of the mesogens. Series of linear gelators were prepared with variations in B-block length, A-block lengths and star shaped BCPs by coupling linear ABA’ triblock copolymers. Structure-property relations of the cyanobiphenyl-functionalized polymers regarding the mesophase characterization revealed a dependence of solubility in the nematic 5CB on spacer length. A comprehensive study was conducted to investigate the influence of the BCP backbone and architecture on the gelation of 5CB. Oscillating rheology measurements and thermal characterization were employed to investigate the thermoreversible LC gels. Most of the BCP gelators achieved gelation of 5CB at a mass concentration of 5 wt%. The properties of the different gels where compared at this fixed concentration. The influence of the gelator backbone on the gel properties was investigated by comparing different sets of triblock copolymers. While a short functionalized B-block resulted in high network density and, thus, a high elasticity of the gel the length of the A-blocks influenced the node stability. The LC gel using a star-shaped gealtor exhibited a significantly higher elasticity than with the respective linear block copolymer gelator.