2 search hits
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New block copolymers of Isobutylene by combination of cationic and anionic polymerizations
(2004)
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Nemesio Martínez-Castro
- The studies presented in this thesis deal with the new block copolymers of isobutylene by combination of cationic and anionic polymerizations, which are new materials with numerous promising potential applications. A new method for the synthesis of tailored polyisobutylene(PIB)-based block copolymer by combination of controlled / living cationic and anionic polymerizations has been developed. In addition and parallel to these subjects, new synthetic routes for preparation of telechelic PIBs and conductive polymers have been investigated. The PIB precursors used for subsequent anionic polymerization and other processes were prepared by controlled / living cationic polymerization of isobutylene followed by quenching with thiophene under selected conditions. Quantitative functionalization of living PIB with thiophene (T) has been achieved. The process is complicated by coupling between living PIB and PIB-T formed by in situ deprotonation. By lithiation of PIB-T a new, convenient method has been demonstrated for the synthesis of PIB-based block copolymers, involving anionic initiation of tert-butyl methacrylate. A major improvement is that for an industrial process, lithiation by n-BuLi is much more convenient than metalation by Na/K alloy, which had to be used in former processes. Block copolymers with narrow and unimodal molecular weight distribution (MWD) were synthesized under well-controlled conditions. The lithiated thiophene-capped PIB was also used to prepare four-armed star PIB via chlorosilane coupling. Using the same method, new amphiphilic block copolymers, namely PIB-b-poly(N,N-dimethylacrylamide) and PIB-b-poly(ethylene oxide) were synthesized. The anionic polymerization of N,N-dimethylacrylamide (DMAAm) was carried out with a binary initiator system prepared from thienyllithium and the Lewis acids triethylaluminium (Et3Al), diethylzinc (Et2Zn) or triethylborane (Et3B) in THF. Polymerizations proceeded in a homogeneous manner and gave polymers having controlled molecular weights. However, in presence of alkoxides the polymerization results in polymers insoluble in THF, even using Et3Al. Finally, lithiated PIB-T, in conjunction with Lewis acids, was used to initiate the living anionic polymerization of DMAAm, resulting in the new amphiphilic block copolymer PIB-b-PDMAAm. The anionic polymerization of ethylene oxide (EO) was carried out with a binary initiator system prepared from thienyllithium and the polyiminophosphazene base tBu-P4 in THF. Lithilated PIB-T- in conjunction with tBu-P4 was used to initiate the living anionic polymerization of ethylene oxide. In a second method, PIB-b-PEO was synthesized using hydroxyl end-capped PIB as macroinitiator in conjunction with tBu-P4. The aggregation behavior in dilute aqueous solutions of a PIB-b-PEO copolymer, produced from the combination of cationic and anionic polymerizations, was investigated by dynamic light scattering in aqueous solution. The monoaddition of ethylene oxide to lithiated PIB-T is demonstrated as a new pathway for the synthesis of hydroxy-functional PIB (PIB-OH). This is a useful alternative to the tedious procedures used in the literature so far. PIB-OH was successfully used as a macroinitiator for the ring-opening polymerizations of L-lactide and epsilon-caprolactone catalyzed by stannous octoate. Combination of chromatographic methods and MALDI-TOF ass spectrometry gave information about the side reactions during polymerization. Quantitative functionalization of living PIB with 2-bromothiophene has been achieved. Using various coupling methods, this polymer was convertet to a macromonomer with a pendant thiophene group, which is accessible in the 1- and 5-position. Oxidative copolymerization with thiophene led to graft copolymers consisting of polythiophene-graft-PIB.
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Nanostructure formation in thin polymer films
(2004)
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Nicolaus Rehse
- In the first part of this thesis an improved process is presented to prepare laterally structured substrates via hierarchical self organization. A miscut silicon surface annealed at 1400 K under ultra high vacuum conditions is used. The resulting facets are stable against oxidation and form a topographic pattern which can be further modified to a chemical pattern via evaporation of gold on every other facet. By controlling the time of annealing, we create structures with a reproducible mean width ranging from 40 to 400 nm. Despite the rather complex ultra high vacuum treatment and an additional evaporation step, we are able to produce substrates in a relatively short time (36 h). These substrates show a nanometer sized structure over an area of 0.5 cm². The automation of the cleaning process and a controlled heating during the annealing increases the yield of high-quality, stepped substrates. These structures allowed us to study the behavior of ultra-thin polystyrene films on topographically structured substrates. The film thickness of some nanometers is comparable to the radius of gyration of the polymers. The substrate corrugation causes a regular variation of the film thickness. We start with a homogeneous film, which is annealed above the glass transition temperature. During annealing the films are stable or form long polymer nanochannels, which lie in the grooves of the substrate structure. The balance of the radius of gyration and the film thickness controls the stability of the polymer film, while the corrugation only triggers the dewetting. The same behavior is found for films on flat substrates. Here small contaminations nucleate the formation of holes. Evaporation of gold stripes and their modification with self assembled monolayers leads to chemical patterned substrates. This expands the possibilities to manipulate the substrate wettability on the nanometer scale. The second part of the thesis describes the formation of ordered structures in block copolymer films. ABC triblock copolymers show a large variety of morphologies in thin films. We have shown that surface reconstructions play an important role in the structure formation process of these structures. In very thin films, where the film thickness is smaller than the long period of the polymer's micro domains, confinement effects overlap with the surface effects. The component with the lowest surface energy is accumulated at the free surface. It needs a subtle balance between the different surface energies (external fields) and the interaction of the three polymer blocks (internal fields) to create a surface reconstruction. This was shown by variation of the chemistry of the end block and by changing the sequence of blocks in the experiment. To analyze the surface reconstruction we used selective staining along with scanning electron microscopy, selective etching in oxygen plasma in combination with scanning probe microscopy, as well as quantitative TappingMode atomic force microscopy. Surface reconstructions of block copolymers show remarkable similarities with reconstructions of single crystal surfaces. In both cases the driving force for a rearrangement is the decrease in surface free energy of the ideal surface. A second analogy between the lamella forming SBM triblock copolymer and Si(100) is the fact that two non-equivalent layers of matter aligned parallel to the free surface lead to two different terminations at the surface. This shows that the phenomenon of surface reconstructions is not limited to classic crystals. The results of this thesis give new insights in the behavior of polymers at surfaces and in thin films. This gives the opportunity to create or manipulate nanometer sized structures accurately via self assembly, external stimuli, or a combination of both.
