Dietrich Prescher

SYNTHESIS OF LIQUID-CRYSTALLINE POLY(METH)ACRYLATES WITH 4-TRIFLUOROMETHOXY-AZOBENZENE MESOGENIC SIDE-GROUPS

Abstract Side-group liquid-crystalline polymers with fluorine-containing mesogens were synthesized and characterized. 4-Trifluoromethoxy-aniline was diazotized and coupled with phenol to give 4-hydroxy-4′-trifluoromethoxy-azobenzene. Alkylation of this compound with α,ω-dibro-moalkanes, followed by reaction with the potassium salts of acrylic or methacrylic acid, yielded the monomers. Their radical polymerization gave the corresponding polyacrylates or polymethacrylates, respectively. The polymers were characterized by size exclusion chromatography (SEC) and by differential scanning calorimetry (DSC).

VARIOUS SYNTHETIC APPROACHES TO FLUOROALKYL P-NITROPHENYL ETHERS

Abstract Homologous 1H,1H-perfluoroalkyl p-nitrophenyl ethers (alkyl = C 2 -C 8 ) were synthesized using different methods. The results are discussed in context with contradictory comments of the literature. The fluoroalkoxylation of p-chloronitrobenzene occurs in only one step, but it is limited to small fluoroalkyl groups. The fluoroalkylation of p-nitrophenol via sulphonic acid esters is a better synthetic route. Differences in reactivity and yield between tosylates, mesylates and triflates are found and discussed. The preferred synthesis includes the use of trifluoromethane sulphonic acid esters.

Synthetic utility of 3-(perfluoro-1,1-dimethylbutyl)-1-propene. Part III. Synthesis and properties of (perfluoro-1,1-dimethylbutyl) acetic and propionic acids and their salts

Abstract The new highly fluorinated acids, CF3CF2CF2C(CF3)2CH2COOH and CF3CF2CF2C(CF3)2CH2CH2COOH, were synthesised by oxidation of the title alkene, CF3CF2CF2C(CF3)2CH2CHCH2, and of its alcohol derivative, CF3CF2CF2C(CF3)2CH2CH2CH2OH, respectively. Sodium salts of the new fluorinated acids were prepared and some of their properties as surfactants have been determined and compared with those of totally fluorinated sodium alkanoates with the same number of carbon atoms.

Synthesis of 4-(1H,1H-perfluoroalkoxy)-4′-(6-methacryloyloxy-hexyloxy)-azobenzenes and their liquid crystalline intermediates

Abstract The synthesis and characterization of 4-(1H,1H-perfluoroalkoxy)-4′-(6-methacryloyloxy-hexyloxy)-azobenzenes with different lengths of the fluorinated tail group (C2-C8) and their liquid crystalline intermediates, starting from 1H,1H-perfluoroalkyl-p-nitrophenyl-ethers, is described. The liquid crystalline properties of the compounds were studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods. In contrast with their non-fluorinated analogues, all the liquid crystalline products display smectic mesophases caused by the fluorination of the tail group.

Zum mechanismus des strahlenchemischen abbaus von polytetrafluorethylen

Zusammenfassung Es wird uber die Anderung der Polymerstruktur und den Reaktionsmechanismus beim Abbau von Polytetrafluorethylen (PTFE) durch Einwirkung energiereicher Strahlung (Elektronenstrahlung) berichtet. Fur die Deutung und Beurteilung der Vorgange beim strahlenchemisch initiierten PTFE-Abbau werden spektroskopische Untersuchungen herangezogen und kinetische Betrachtungen angestellt. Erstmals in praparativem Masstab durchgefuhrte Abbauversuche mit PTFE liefern uber die qualitative und quantitative Zusammensetzung der Endprodukte neue Erkenntnisse fur die mechanistische Interpretation des Abbauprozesses. Unter definierten Bestrahlungsparametern der anregenden Elektronenstrahlung wird der rein strahlenchemische Abbau von einem thermischen Abbau uberlagert. Verantwortlich dafur ist der durch Abbremsung der beschleunigten Elektronen im Polymeren erzeugte Temperaturanstieg (Warmestaueffekt). Es werden kinetische Gesetzmasigkeiten der intermediar entstehenden Abbauprodukte und der Rekombination der durch Bestrahlung gebildeten Radikale in der Polymerschmelze und an der Polymeroberflache diskutiert und der Abbaugeschwindigkeit des makroskopischen Gesamtprozesses gegenubergestellt. Der strahlenchemische Abbau in inerter Atmosphare (N2, Ar) fuhrt uber Perfluoralkylradikale zu perfluorierten Alkanen und Alkenen. Beim Abbau in Gegenwart reaktionsfahiger Gase bilden sich funktionalisierte Verbindungen, z. B. entstehen bei Anwesenheit von Sauerstoff in der reaktiven Zone uber perflourierte Peroxyl- und Oxylradikale schlieslich unter CC-Spaltung die Fluoride von Perfluoralkansauren und Carbonyldiflouorid.

Linear and nonlinear optical properties of Langmuir–Blodgett multilayers from chromophore-containing maleic acid anhydride polymers

Abstract Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive or active photonic devices. The polymers were synthesized by means of polymer analogous reaction of azo chromophores with a maleic acid anhydride polymer which bears saturated hydrophobic side chains. The resulting polymer contains, besides hydrophilic groups and hydrophobic side chains, azo chromophores with high second-order hyperpolarizability which are attached to the main chain via a spacer. In order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore. Langmuir–Blodgett (LB) technique was applied for preparation of multilayer structures with distinct supramolecular architecture from these polymers. They are characterized by high anisotropy and a special combination of linear and nonlinear optical (NLO) properties. The polymers were deposited both in Y-type and Z-type LB layers. The Y-type layers of the azopolymer show high birefringence and dichroism but small nonlinear response which originates from the first deposited layer only. Multilayers of the azopolymer deposited in Y-type configuration are characterized by high internal order which can be concluded from a pronounced Bragg-peak and Kiessig-fringe patterns as obtained from gracing incidence X-ray scattering. Experiments to deposit the active polymer directly in Z-type configuration by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the result that well-ordered, anisotropic films are obtained only if the number of deposited monolayers is limited to a value of about 30. The layers deposited in Z-type manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and Kiessig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation (SHG) experiments. A second kind of Z-type multilayers was prepared by alternate deposition of the active azopolymer and of another maleic acid anhydride polymer without chromophores. They show similar linear optical properties like Y-type layers but due to the noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, thick layers of the azopolymer prepared by alternate deposition LB-technique were prepared, which can be used as active waveguide for second-order NLO application.

New chromophores and polymers for second-order nonlinear optics

Linear conjugated, fluorine-containing chromophores are differing from conventional chromophores like disperse red one by a reduced linear refractive index. Second order nonlinear optical susceptibilities of asymmetrically substituted fluorine-containing dyes reach, however, values as high as those of DR I or of similar chromophores. This can be concluded from measurements of solvatochromy of absorption and fluorescence spectra, electroabsorption measurements, and from quantum chemical calculations carried out for a series of azomethine and azo dyes. The chemical constitution of the conjugated part of investigate chromophores has an important influence both on linear and non nonlinear optical properties. So azomethine dyes show a blue shifted absorption in comparison to the absorption of azo chromophores. This goes along with lower off-resonant second order susceptibility. Introduction of fluorine into the acceptor part of the molecule by attaching a CF3- group does not lead to a significantly lower second order susceptibly in comparison to non-fluorinated chromophores. Perfluorination of the acceptor part result in case of the investigated molecules in an increased second order susceptibility as compared to the non-fluorinated chromophore. Polymers which contain these chromophores are either prepared by physical doping or by covalent bonding of the dyes to the polymeric main chain.In case of physically doped polymers the classes of thermally stable allylic polymers and of polyphenyl quinoxalines (PPQs) were used as matrix material. These polymers show miscibility with the investigated chromophores up to high concentrations and are haracterized by glass transition temperatures between 140 degrees C and 180 degrees C or 180 degrees C and 350 degrees C (PPQs) dependent on chemical structure and on dye concentration. Polymers which contain the active chromophores in side chains were prepared on basis of maleic acid anhydride or as novolak epoxy amine aduct polymer. The former can be utilized for preparation of waveguides by means of Langmuir-Blodgett technique while the later provide films of excellent quality by spin-coating and subsequent corona poling. Second order susceptibilities of the polymer films as determined by second harmonic generation reach values between 5 and 27 pm/V dependent on chromophore concentration and preparation conditions.

Proton affinities of some polyfluoroalkanes in comparison to the unsubstituted alkanes. The estimation of the proton affinities of polytetrafluoroethylene and polyethylene by applying theoretical methods

Abstract Applying ab initio and semiempirical quantum chemical methods, the proton affinities ( PA ) for some simple polytetrafluoroethylene ( PTFE ) models, and also for propane as a polyethylene ( PE ) model, have been estimated. Using AM1, a systematic (more than 20–30 kcal mol −1 ) overestimation of the proton affinities was found. In contrast, a much better agreement with ab initio data was obtained by using MNDO and PM3. While the former values are too high, an underestimation by PM3 was established. Reliable results are obtained, however, when the mean values from MNDO and PM3 for PA s are used. Further investigations were carried out, applying MNDO and PM3 to larger perfluorinated alkanes and all three semiempirical methods to alkanes. Using the mean PA s from MNDO and PM3, convergence values of 116 kcal mol −1 were obtained for PTFE and 154 kcal mol −1 for PE . According to the theoretical results, protonation of alkanes leads to a C=H bond cleavage in agreement with experiment. Ab initio MPn/6-31G*//3–21G ( n = 2,3) geometries of protonated propane, where all bonds are kept after protonation, correspond to a transition state. The calculated PA of approximately 135 kcal mol −1 falls between the respective values for perfluorinated alkanes and alkanes. Neither by ab initio nor by semiempirical procedures was it possible to locate an alkane/proton complex corresponding to a minimum. Consequently, one may conclude that protonation of PTFE is accompanied by moderate PA without bond cleavage, while PE may be protonated only by destruction of the C=H bond. The energy difference of these two processes is high (38 kcal mol −1 ).

Molecular structure investigations on fluorine containing polyazomethines by means of the MALDI–TOF–MS technique

Abstract Three new aromatic polyazomethines have been synthesized by solution polycondensation reactions of terephthalic dialdehyde with fluorine containing aromatic diamines. The polymers were obtained in high yields. The investigation of these polymers by means of matrix-assisted laser desorption ionization–time-of-flight–mass spectrometry (MALDI–TOF–MS) proved the initial members of the four theoretical possible series of the corresponding oligomers with 2–10 repeating units per molecule.

Liquid crystalline side group polymers with azo-chromophores and fluorinated tails of varying length

Abstract A series of side group liquid crystalline polymethacrylates with 4′-(1H,1H-perfluoroalkoxy)-azobenzene mesogenic units was synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods depending on the tail length. The phase behaviour is discussed as a function of the length of the tail groups. Very high clearing temperatures up to 290°C were observed for the polymers with long tail groups.