2 search hits
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Synthesis, Characterization, and Properties of Tailored Functional Block Copolymers
(2011)
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Robin Pettau
- 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.
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Polymeric Nanoparticles for the Modification of Polyurethane Coatings
(2011)
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Sandrine Tea
- Rubber-based nanomodifiers were successfully synthesized following two different strategies and were used as impact modifiers in polyurethane (PU) automotive clearcoats to improve chip resistance. Various narrowly distributed polybutadiene-b-poly(methyl methacrylate) (B-M) block copolymers differing in composition and molecular weights were synthesized and studied with respect to their self-assembly in organic selective solvents. Dynamic light scattering and transmission electron microscopy measurements revealed that spherical micelles were obtained in acetonitrile for all block copolymers, independently of the polymer concentration. Their radii varied from 11 to 69 nm depending on the molecular weight of the initial linear block copolymer and their aggregation behavior in acetonitrile followed the model established by Förster and Antonietti for strongly segregated block copolymers. In DMF and acetone, block copolymers with 85 %wt PMMA were dissolved as unimers. For lower methacrylate contents, the sizes of the obtained spherical micelles were decreasing from DMF to acetone independently of the polymer concentration. The calculated interaction parameters confirmed acetonitrile as the best solvent for PMMA followed by DMF and acetone as the poorest one. The size of the spherical aggregates could be tuned by the molecular weight and/or by the nature of the selective solvent. Polybutadiene-b-poly(n-butly acrylate) (B-nBA), polybutadiene-b-poly(n-butyl methacrylate) (B-nBMA) and polybutadiene-b-poly(t-butyl methacrylate) (B-tBMA) did not show such a large choice in selective solvents and spherical micelles were obtained in DMF, DMAc and acetone respectively. Cross-linking of the polybutadiene core of the obtained micelles was performed in solution using two different methods: cold vulcanization and radical reaction upon the decomposition of a photo-initiator under UV radiation. Both methods allow retaining the spherical shape of the micelles leading to narrowly distributed non fusible nanospheres. In the case of B-M nanoparticles, the degree of cross-linking seemed independent of the amount of cross-linker used. Unlikely, B-nBMA and B-nBA nanoparticles exhibited increasing degrees of cross-linking with the amount of photo-initiator introduced. Their degrees of cross-linking were particularly lower than those of B-M nanoparticles. The hydrolysis of the t-BMA corona of the nanoparticles obtained from B-tBMA linear block copolymers self-assembly in selective solvent resulted in water soluble nanoparticles carrying acid functions and thus potentially exhibiting pH-responsive behavior. Various hyperstars consisting of a hyperbranched PB core and (meth)acrylate arms were synthesized by anionic self-condensing vinyl copolymerization (SCVCP) of divinylbenzene and butadiene followed by the anionic polymerization of the linear (meth)acrylate arms. The amount of hyperbranched products resulting from SCVCP could be enhanced by introducing additional DVB to the reaction while polymerizing. The topology of the hyperbranched PB cores was confirmed by viscosity measurements. All Mark-Houwink-Sakurada exponents were significantly below the value for linear PB. The initiation of (meth)acrylate arms was confirmed by NMR spectroscopy. Upon the arm-growth reaction, the branched topology was retained as witnessed by further viscosity measurements. The introduction of cross-linked nanoparticles based on linear block copolymers did not disturb the transparency of PU coatings. Even after curing reaction, the nanoparticles were well-dispersed into the coating. TEM observations confirmed this last result where neither aggregation nor flocculation of the cross-linked nanoparticles was observed. Hyperstar polymers were found to undergo self-assembly upon the curing reaction leading to “onion-like” structured aggregates, in the case of PMMA hyperstars, with sizes as large as 200 nm. Aggregates of the same size order were observed for the other hyperstars but no defined structures were found. For all hyperstar modified coatings, the transparency of the films was altered. In both cases, cross-linked nanoparticles and hyperstar modified coatings, improvements of chip resistance were observed. The improvements were even better with increasing amount of cross-linked nanoparticles but no effect was noticed on the hardness of the coatings. Similar trends were observed for the hyperstar modified coatings.
