Liane Häußler

Investigation of liquid sensing mechanism of poly(lactic acid)/multi-walled carbon nanotube composite films

The liquid sensing mechanism of melt-processed poly(lactic acid) (PLA)/multi-walled carbon nanotube (MWNT) composite films was investigated for the influence of MWNT loading, solubility parameters of solvents used, solvent transport behaviours, resultant electrical resistance changes, as well as crystallization of the PLA matrix. The diffusion, sorption and permeation coefficients of neat PLA and the composites were estimated, indicating that MWNT network structures block solvent molecules from penetrating into the polymer matrix. Solvent-induced crystallization of the polymer matrix was observed. Isothermally crystallized composites showed reduced resistances, a significant decrease of sorbed solvent content and a reduction of the resulting resistance changes on the solvent contact. In the context with sensing results on MWNT mats, it was proposed that the liquid sensing mechanism of PLA/MWNT composites consists of the overall electrical resistance changes caused by the structural variation of the conductive MWNT network in the polymer matrix and additional interactions between the MWNT and solvent molecules.

Modification with alkyl chains and the influence on thermal and mechanical properties of aromatic hyperbranched polyesters

All-aromatic hyperbranched polyesters with hydroxy endgroups were functionalized with aliphatic n-alkyl carboxylic acids. The length of the n-alkyl chain as well as the degree of modification were varied and the resulting, partially amphiphilic polymers were characterized by differential scanning calorimetry (DSC). With both increasing degree of modification and increasing length of the alkyl chain the glass transition temperature decreases due to reduced intermolecular hydrogen bonding. When the alkyl chains start to crystallize Tg of the hyperbranched polymers increases again. The mechanical properties of the former brittle hyperbranched polyester were improved by modification with C12 chains and a stable free standing film was obtained by compression molding. The film was investigated by means of dynamic mechanic analysis (DMA) and microscopy, exhibiting a low temperature thermal transition and phase separation within the scale of a light microscope. Furthermore melt rheology measurements were performed on the starting polymer and on the C12 modified product. The complex viscosity is reduced strongly by the modification of the aromatic hyperbranched polyester.

Synthesis and properties of polyamidines

Polymers with amidine groups NHCRN in the main chain were prepared by acid catalysed melt polycondensation of 4,4′-diaminodiphenyl methane (DAPM) with various orthoesters. The resulting polyamidines were characterized by 1H-, 13C- and 15N-NMR spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TGA) and wide angle x-ray scattering (WAXS) measurements. The properties of the polyamidines are strongly determined by the substituent R on the amidine group. Some peculiarities of the polyformamidine (RH) could be attributed to the conformation of the amidine group. Glass transition temperatures varied from 106 to 161°C depending on the substituent R. All polyamidines possessed good thermal stability up to 370°C and, with exception of the polyformamidine, good solubility in common solvents.

Surface structure of fluorinated polymers and block copolymers

SummariesThe self-organisation of semifluorinated segments was used to create polymers and block copolymers with ultra-low surface free energy. The aromatic polyester poly(p-phenylene oxydecyl-perfluorodecyl-isophthalate) showed an extremely low value ofγsv of 9mN/m. It was proven that this low value was caused by an ordered surface structure. Introduction of these semifluorinated polyester segments into block copolymers with a basic structure (A–B)n resulted in a combination of good film-forming properties of polysulfone with the low surface free energy of the semifluorinated polyester. Thus, materials were obtained having both good mechanical performance as well as anti-adhesion properties. The block copolymers can be used to prepare blends with the respective homopolymers with different degrees of phase separation.The polymers and block copolymers can form a self-cleaning surface when coated on roughened aluminium substrates. It was shown that the adsorption of proteins, such as human serum albumin and fibrinogen from buffer solutions, significantly decreased on such polymer surfaces.RésuméL’autoorganisation des segments semi-fluorés a été utilisée pour créer des polymères et des bloc copolymères avec une énergie surfacique libre ultrabasse. Le polyester poly(p-phenylène oxydécylperfluorodécyl-isophthalate) aromatique a montré une très basse valeur deγsv de 9mN/m. Il a été prouvé que cette basse valeur était causée par une structure surfacique ordonnée. L’introduction de ces segments de polyester semi-fluorés dans des bloc copolymères ayant une structure fondamentale (A–B)n a ajouté les bonnes qualités du polysulphone vis à vis de la formation du feuil à la basse énergie surfacique libre du polyester semi-fluoré. Donc on a obtenu des matériaux qui démontraient une bonne performance mécanique aussi bien que des propriétés d’anti-adhésion. Les bloc polymères peuvent être utilisés avec les homopolymères respectifs pour préparer des mélanges ayant de différents degrés de séparation de phase.Les polymères et les bloc polymères peuvent former une surface autonettoyante quand ils revêtissent un substrat d’aluminium rendu rugeux. On a montré que l’adsorption des protéines telles que le sérum humain, albumine et le fibrinogène des solutions tampon diminue sur de telles surfaces polymériques.ZusammenfassungDie Selbstausrichtung von semifluorinierten Segmenten wurde verwendet, um Polymere und Block-Copolymere mit extrem niedriger freier Oberflächenenergie herzustellen. Das aromatische Polyester Poly(p-phenylen oxydecyl-perfluorodecyl-isophthalat) zeigte einen extrem niedrigen gsv-Wert von 9mN/m. Es wurde gezeigt, daß dieser niedrige Wert auf eine gut geordnete Oberflächenstruktur zurückgeführt werden konnte.Die Hinzufügung von semifluorinierten Polyestersegmenten zu Block-Copolymeren mit einer Struktur von (A–B)n ergab eine Kombination von den guten filmbildenden Eigenschaften von Polysulfon mit der niedrigen freien Oberflächenenergie von semifluoriniertem Polyester — in anderen Worten, ein Material das gute mechanische Eigenschaften mit einer niederen Adhäsionskraft vereinbarte. Durch die Verwendung von Block-Copolymeren lassen sich Mischungen mit den respektiven Homopolymeren mit verschiedenen Graded der Phasentrennung herstellen.Die Polymere und Block-Copolymere können auf aufgerauhte Aluminiumoberflächen aufgetragen werden, wo sie eine selbstreinigende Oberfläche bilden. Es wurde ausserdem gezeigt, daß die Adsorption von Proteinen wie Serum Albumin oder Fibrinogen von Pufferlösungen auf so beschichteten Oberflächen deutlich zurückging.

Synthesis and Phase Separation Behaviour of High Performance Multiblock Copolymers

The phase separation behaviour of block copolymers of the type (A–B) n (segmented block copolymers, multiblock copolymers) containing segments with different degrees of flexibility was investigated using a stepwise variation of the incorporated blocks A and B. Amorphous/rigid blocks (polysulfone segments) were systematically combined with amorphous/flexible blocks (poly(tetramethylene glycol), semiflexible/nematic blocks (different kinds of aromatic polyesters and poly(ester imides) as well as semicrystalline fluorinated polyester blocks). All multiblock copolymers were synthesized using a conventional transesterification polycondensation in the melt and the polycondensation results are discussed. The phase separation in the synthesized block copolymers was investigated both experimentally and theoretically. Flory–Huggins interaction parameters of the block copolymers were calculated and used together with mean field calculations in order to understand the phase separation behaviour of all block copolymers correlated to their chemical structure. On the other hand, the onset of phase separation in a series of block copolymers with a systematic variation of block molecular weights was examined experimentally, mainly by DSC and SAXS. Finally, the question is discussed whether or not the degree of phase separation in the investigated systems determines the properties of the polymers or not. This is for example outlined for the mechanical properties and, in particular, the surface properties of fluorinated block copolymers.

Thermal stability of electron beam‐irradiated polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) was subjected to 1 MeV electron beam irradiation in air. The thermal stability and the degradation fragments of the irradiated polymer were studied in dependence on the radiation dose up to 4 MGy by thermogravimetric analysis coupled with mass spectrometry. The TGA results confirm the known decrease in the thermal stability of irradiated PTFE with increasing radiation dose. At the thermal degradation, CO2, HF, and fluorocarbon fragments are evolved from the irradiated samples. CO2 and HF are formed by decomposition of peroxy radicals up to 250°C. In addition, low molecular weight fluorocarbons are desorbed from the irradiated PTFE. At temperatures above 300°C, CO2 is formed by the decarboxylation of radiation-induced COOH groups inside the PTFE.

The effect of specific nucleation on molecular and supermolecular orientation in isotactic polypropylene

The molecular and supermolecular orientation, morphology and structural changes observed during cold drawing of injection moulded isotactic polypropylene modified by specific a, and β nucleating agents were studied by polarised photoacoustic FTIR spectroscopy, wide-angle X-ray diffraction and differential scanning calorimetry. Significantly lower molecular orientation was found in the core of the β-nucleated injection moulded specimens as compared to unmodified and α-nucleated materials. This has been ascribed to the fast growth of the β-crystallites which inevitably dislocates the flow-induced orientation within the crystalline regions and in their vicinity. Moreover, it was found that the presence of the developed (3-crystallites distinctly diminishes the efficiency of the orientational solid-state drawing assessed on both levels of the hierarchical structure (molecular and crystalline). This structural observation is directly connected with macroscopic softening effect of the β-phase: lowering the yield stress and flattening the neck shoulder. Thus, the interrelation between the microstructural and macroscopic effects of the β-phase could be described as a feedback process.

Thermal deblocking of masked low molecular isocyanates I. Aliphatic isocyanates

The thermal deblocking of masked isocyanate model substances was investigated by means of simultaneous thermal analysis and quadrupole mass spectrometry system (STA/QMS). The results were compared with additional measurements carried out by differential scanning calorimetry (DSC) and temperature dependent Fourier transform infrared spectroscopy (FTIR). The measured values which were determined as the deblocking temperature depend on the method of investigation. Thermogravimetric and mass spectrometric data depend on the volatility of the components and could differ from those observed by DSC and/or FTIR. Furthermore, the mechanism of deblocking is very complex. Secondary reactions obviously play an important role.

Influence of Semiconductor Thickness and Molecular Weight on the Charge Transport of a Naphthalenediimide-Based Copolymer in Thin-Film Transistors

The N-type semiconducting polymer, P(NDI2OD-T2), with different molecular weights (MW=23, 72, and 250 kg/mol) was used for the fabrication of field-effect transistors (FETs) with different semiconductor layer thicknesses. FETs with semiconductor layer thicknesses from ∼15 to 50 nm exhibit similar electron mobilities (μs) of 0.2-0.45 cm2 V(-1) s(-1). Reduction of the active film thickness led to decreased μ values; however, FETs with ∼2 and ∼5 nm thick P(NDI2OD-T2) films still exhibit substantial μs of 0.01-0.02 and ∼10(-4) cm2 V(-1) s(-1), respectively. Interestingly, the lowest molecular weight sample (P-23, MW≈23 kg/mol, polydispersity index (PDI)=1.9) exhibited higher μ than the highest molecular weight sample (P-250, MW≈250 kg/mol, PDI=2.3) measured for thicker devices (15-50 nm). This is rather unusual behavior because typically charge carrier mobility increases with MW where improved grain-to-grain connectivity usually enhances transport events. We attribute this result to the high crystallinity of the lowest MW sample, as confirmed by differential scanning calorimetry and X-ray diffraction studies, which may (over)compensate for other effects.

Self-assembly and viscoelastic properties of semifluorinated polyesters

The structure and dynamics of polyesters composed from a rigid aromatic backbone and oxyalkylperfluoroalkyl side chains have been studied using differential scanning calorimetry, small- and wide-angle X-ray scattering, pressure-volume-temperature measurements and rheology. The system was found to undergo three phase transitions. Starting from lower temperatures they reflect the melting of the fluorinated side chains, the unfreezing of the backbone mobility and the melting of the smectic layered structure. In addition, rheology has shown that these compounds are thermorheologically complex at all temperatures investigated and exhibit non-Newtonian low-frequency response.