OPUS 4 | Search

Development and Application of a Classification System for Undisturbed and Disturbed Tropical Montane Forests Based on Vegetation Structure (2002)

Axel Paulsch

Classification of the vegetation was a central part of a DFG-financed analysis of undisturbed and disturbed montane rain forest ecosystems in Southern Ecuador. The aim of the study presented here was the development and application of a classification system based on structural characters for both, undisturbed and disturbed forests. In an altitude between 1570 m a.s.l. and 3100 m a.s.l. 139 plots of 20 m x 20 m were established and the abundance of 151 structural parameters was quantified per tree stratum. Cluster analysis allowed grouping of the 139 plots into 14 groups, called structural forest types. These types were: Secondary growth after road construction, Secondary growth after landslide, Plantation of Pinus patula, Forest on edges of natural gaps, Secondary ravine forest, Ravine forest under human influence, Primary ravine forest at lower altitude, Primary ravine forest at higher altitude, Microphyll ridge forest, Mesophyll ridge forest, Macrophyll ridge forest, and megaphyll ridge forest. Two forest types were investigated outside the closer investigation area and were classified as Secondary forest in Mindo and Primary forest in Cajanuma. Primary forest types were only found south of the Rio San Francisco, whereas on the northern side in a matrix of pastures and farmland only plan-tations and secondary vegetation along ravines could be detected. In the area of primary forest, a primary ravine forest type of lower altitudes turned into a type of higher altitudes around 2100 m a.s.l. in ravines. Both types were characterised by comparably great tree heights and wide stem diameters. On ridges, three types of ridge forest coexisted in the lower part until microphyll ridge forest dominated in altitudes above 2200 m a.s.l.. Timberline was reached at 2650 m a.s.l. on the eastern side of the cordillera, at approximately 3000 m a.s.l. on the western side in Cajanuma. Decreasing canopy height on ridges could be explained by water stress in wind exposed sites and unfavourable N-mineralisation conditions (due to decreasing pH, profile depth, raising C/N-values, and water stagnation with increasing altitude). Secondary forest types were characterised by human influence (felling of emergent trees), pioneer vegetation and high abundance of connecting elements like lianas and bamboo. The structural forest types could be integrated into accepted schemes of altitudinal zonation of the Andean vegetation without contradiction, but allowing a further distinction of the zone under investigation. Frequency analysis and calculation of correlation between variables allowed the reduction of the catalogue from 151 to 104 structural parameters by excluding variables that did not contribute to the clustering. This exclusion did not alter the result of the cluster analysis. On the basis of the reduced catalogue a key to the 14 structural forest types was developed. This key was applied to 100 plots of 10 m x 10 m in a parallel study of other project participants investigating the distribution of hummingbird-visited plants. Distribution patterns of this functional group could be related to the structural forest types. Recent investigations of bird and moth communities will also be tested for functional relationships.

Behavior and Ecology of Wild Slow Lorises (Nycticebus coucang): Social Organization, Infant Care System, and Diet (2002)

Frank Wiens

In this thesis I describe the social organization, infant care system, and diet of the slow loris Nycticebus coucang, a nocturnal arboreal prosimian primate, in the Malaysian rainforest. Data collected are locational data obtained during 1,000 h of radio-tracking slow lorises, behavioral data from visually observing radio-collared individuals, morphometric data, and records on food remains from fecal analysis. Though close-range encounters were very rare, I observed slow lorises to form stable social units ('social groups') characterized by home range overlap and friendly interactions among members and nonoverlap between groups. Four groups were observed, each consisting of a single adult female, a single adult male and a varying number of younger individuals. Group composition, together with relatively small testis volume and natal dispersal occuring in both sexes, hints towards a monogamous mating system. In one case an extended family group formed by delayed dispersal of a primary pair's offspring. Friendly interactions among group members included allogrooming, following, alternate click-calls, and sleeping in contact. Yet, members did not engage in any co-operative behaviour of the types usually thought to be responsible for group formation in gregarious mammals. One important factor contributing to the sharing of space between slow lorises is probably that chances of successful dispersal are low. However, subtle benefits arising from the presence of conspecifics (allogrooming, transfer of information on food resources) may also be crucial for the formation or maintenance of slow loris spatial groups. The infant care system was also notable for the very low frequency of direct encounters between animals. Active maternal care seemed to be limited to carrying the young to the sleeping place and regular suckling, and grooming of the infant. Non-maternal infant care appeared to be restricted to occasional infant grooming and infant carrying. It has been suggested that young lorises depend on their mothers for information on diet and that they obtain this information by watching their mothers feeding or interacting directly with their mothers over food. I tested this hypothesis on a group including one infant and four older individuals. The infant of the focus group only took food items to the mouth which were also part of its social group's diet and showed concordance in the frequency of use of feeding sites with other members of its group. These results speak against diet learning by trial and error. They indicate that diet learning by infants probably does depend on information obtained from older conspecifics. However, the infant of the focus group was not involved in direct interactions with conspecifics over food and fed mostly alone. It was not within a distance where it could see older conspecifics feeding more often than expected from the configuration and utilization of the individuals` home ranges. When feeding in vicinity of other slow lorises the infant never looked at them. This suggests that, contrary to expectations, visual observation or direct interaction over food are not the mechanisms by which information on food resources is passed from older individuals to young, but that other ways of obtaining such information are used by immature wild slow lorises. The slow loris is a slow-moving animal known to have a very low basal metabolic rate relative to other eutherian species of its body mass. A slow pace of life has been causally linked to a low intake rate of usable energy due to a diet that (1) is generally low in energy, (2) is unpredictably periodically scarce, and (3) contains high amounts of toxins or digestion inhibitors. In order to assess whether the slow loris faces any of these three kinds of limitation in energy supply I studied its dietary habits by direct observations of feeding behavior and by fecal analysis. The diet was composed of five distinct types of food: floral nectar and nectar-producing parts, phloem sap, fruits, gums (another group of plant exudates), and arthropods. The largest proportion of feeding time was spent on phloem sap (34.9%), floral nectar and nectar-producing parts (31.7%), and fruits (22.5%). These food types should provide the slow lorises with high amounts of easily digestible sugars indicating that slow lorises did not face an energy-poor diet. Furthermore, I found no evidence for seasonal food shortages: dietary habits were indistinguishable between rainy- and dry seasons, even though most dry season data were collected during periods of extreme drought induced by the 1997-1998 El Nino Southern-Oscillation event. However, many genera of food plants are known to contain secondary compounds that are toxic or reduce digestibility. I suggest that low metabolism in slow lorises is at least partly related to the need to detoxify secondary compounds in high-energy plant diet.

Self-Assembly of Block Copolymers in External Fields (2002)

Alexander Böker

The influence of external fields on the microdomain structure of block copolymers has been studied. Both surface fields and electric fields have been considered. External electric fields are used to create macroscopically oriented bulk samples. In order to circumvent limitations associated with the application of external fields to melts of high molecular weight block copolymers and multiblock copolymers of complex architecture, a new solvent-based procedure is introduced, i.e. the block copolymer microdomains are aligned by application of an electric field (E ~ 1 - 2 kV/mm) during solvent casting of bulk samples. In order to elucidate the dominating parameters and governing mechanisms, the microdomain orientation kinetics of concentrated block copolymer solutions exposed to a DC electric field is investigated by time-resolved synchrotron small-angle X-ray scattering (SAXS) at the ID02 beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. As a first model system, a lamellar polystyrene-b-polyisoprene block copolymer dissolved in toluene is used. The orientation kinetics follows a single exponential behavior with characteristic time constants varying from a few seconds to some minutes depending on polymer concentration, temperature, electric field strength, and system size. Furthermore, two mechanisms governing the electric field alignment of a lamellar block copolymer from concentrated solutions are identified. It is shown that depending on the segregation power (c µ fP, c µ 1/T) a single mechanism dominates the orientation process, i.e. in a weakly segregated system (low concentration or high temperature) the migration of boundaries prevails, whereas a stronger phase separated system (high concentration or low temperature) predominantly exhibits rotation of grains. In addition, the orientation kinetics slows down with increasing polymer concentration, which can be correlated to the respective solution viscosity and the mechanism of orientation. Moreover, the influence of the electric field strength on the orientation kinetics is determined, including a threshold value below which no electric field induced orientation could be achieved on the time scale of the experiment. The time constants of the fastest processes were in the range of 0.5 sec, reaching a final orientation described by order parameters of up to P2 = -0.35. Finally, the variation of temperature yields control of the governing mechanisms at a fixed polymer concentration. In additional studies, the dielectric contrast of the block copolymer components was varied systematically (PS-b-PI, PS-b-PMMA, PS-b-PtBMA, PS-b-PHEMA-b-PMMA, PS-b-P2VP). It is found that a high dielectric contrast leads to faster alignment kinetics (e.g. the time constants of the fastest processes for a PS-b-P2VP diblock system in THF are in the range of 0.3 sec) and reduces the threshold field strength (around 200 V/mm for PS-b-P2VP). Furthermore, it could be shown that the interplay between degree of phase-separation, solution viscosity and dielectric contrast is crucial to decide if a given polymer/solvent system can be used for electric field-induced microdomain alignment. For example, it was found that PS-b-PtBMA shows electric field-induced orientation of the microdomains while PS-b-PMMA does not. This can be explained by the larger interaction parameter cST compared to cSM leading to a phase-separated solution at lower viscosities. In a similar way, the introduction of a high dielectric constant middle block (PHEMA) into a PS-b-PMMA, which additionally enhances phase separation, is shown to be the key to creating a well-performing methacrylate-based block copolymer system for electric field induced alignment from solution. Finally, we could show that the even more complex lamellar and core-shell cylindrical PS-b-P2VP-b-PtBMA high molecular weight triblock copolymer systems could be oriented by virtue of an electric field from solution. In summary, it was demonstrated that electric field alignment of block copolymer domains from solution is a powerful tool to generate highly anisotropic bulk block copolymer samples. The large variety of parameters which we can control allows us to further improve the preparation of macroscopically aligned melt samples via solvent casting in the presence of an electric field.

Diversity of geometrid moths in a montane rainforest in Ecuador (2002)

Gunnar Brehm

The diversity of the very species-rich family of geometrid moths was investigated in a montane forest at the border of the Podocarpus National Park in southern Ecuador along an altitudinal gradient ranging from 1,040 m to 2,677 m above sea level. This study is part of a larger interdisciplinary project on diversity and functioning of a montane forest ecosystem. A total of 13,938 moths representing 1,010 species were sampled in light-traps at eleven elevational levels (two replicate sites each). Most species belonged to the subfamily Ennominae (500 sp.), followed by Larentiinae (391 sp.), Sterrhinae (58 sp.), Geometrinae (57 sp.), Oenochrominae (3 sp.), and Desmobatrinae (1 sp.). The study covers aspects of host-plant relationships, community structure, endemism, alpha-, beta-, and gamma-diversity. The role of different environmental factors as mechanisms for the diversity of the moths is discussed. Furthermore, methodological aspects of light-trap sampling, and selection of appropriate diversity measures and analytical tools are considered in this study.

Characterisation of Schwertmannite - Geochemical Interactions with Arsenate and Chromate and Significance in Sediments of Lignite Opencast Lakes (2002)

Simona Regenspurg

Fe(III) oxyhydroxysulfate schwertmannite [idealised formula: Fe8O8(OH)6SO4], frequently precipitates as a product of sulphide weathering in acidic, sulfate-rich waters. In order to investigate the environmental importance of schwertmannite, geochemical and mineralogical methods were applied in field- and laboratory-experiments. Thereby this thesis focussed on two consequential questions: The first (1) was if schwertmannite could enrich toxic compounds and therefore act as a sink in natural systems. By means of the anions arsenate and chromate, the uptake capacity by adsorption or substitution and consequential changes in crystal structure were characterised. Additionally, a possible re-mobilisation of these compounds was investigated because the metastable schwertmannite easily dissolves or transforms into other minerals. Predominantly, experiments with synthetically produced schwertmannite took place. The other main focus (2) comprised examinations of schwertmannite formation in the chemical environment of acidic lignite mining opencast lakes. Thereby its importance as pH-buffer, its formation probability and its variation by ageing were to be characterised. (1) Schwertmannite, as contained in precipitates of former ore mines, featured high amounts of As (up to 6700 ppm) and Cr (up to 800 ppm). Since there was hardly any detection of these elements in the corresponding drainage waters, the hypothesis arose that schwertmannite acts as a scavenger for these compounds. Assuming that in the redox environment of schwertmannite formation, As and Cr are mostly present in their highest oxidation-level (as arsenate and chromate) these anions were used for coprecipitation, adsorption- and stability-investigations with schwertmannite, to characterise its geochemical interactions. Synthesis (coprecipitation) experiments proved that schwertmannite, normally containing 12 to 14 wt.-% sulfate (both, structurally and adsorptively bound), incorporates up to 10.4 wt.- % arsenate and up to 15.4 wt.-% chromate. While the complete substitution of sulfate by chromate was possible without substantial variation of the crystal structure, the incorporation of arsenate only took place in presence of sulfate or chromate. Oxyanion uptake resulted in an advanced stability of schwertmannite as confirmed in long-term experiments at constant pH. This means that the transformation (due to ageing) to the mineral goethite, as well as mineral dissolution as a consequence of acid addition, was decelerated and toxic compounds were released in lower concentrations compared to sulfate. (2) The chemical environment in opencast lakes of lignite mining frequently shows optimal requirements for schwertmannite formation. Hydrochemistry of surface waters, as well as the sediment composition and partly the colloids were examined in 18 acidic mining lakes (AML) located in Germany. To predict the formation of solid phases in the lakes, chemical processes were modelled by equilibrium calculations. Surface-water composition served as input for calculations of chemical reactions (“PhreeqC”). It was shown that this mineral is the Fe(III)-controlling phase which is in a redox equilibrium with most surface waters. Geochemical analysis of sediment proved that schwertmannite is the primary mineral forming in the AML. It was supposed that a steady supply of Fe(II) into the O2-rich lake provokes schwertmannite precipitation as a consequence of Fe(II) oxidation. The associated release of protons debases the pH of lake water to a value of ~ 3. Further acidification results in mineral dissolution, a reaction which is associated with a release of hydroxide. Therefore, the cycle of precipitation and mineral dissolution adjusts the pH in the AML to a constant value and schwertmannite acts as pH-buffer. The orange-coloured layer at the sediment-water interface of AML, mostly consists of schwertmannite and goethite. It was shown (in ML 77) that with increasing depth the proportion of schwertmannite to goethite decreases. Due to the thermodynamic instability of schwertmannite with respect to goethite, schwertmannite transforms by time (or sediment-depth). Enhanced ageing can be achieved by increasing pH as demonstrated in a stability experiment (1 year) with schwertmannite between pH 3 and 7.

Development of Thermoplastic Elastomers with Improved Elastic Properties Based on Semicrystalline Block Copolymers (2002)

Holger Schmalz

In this work the synthesis and characterization of two novel types of thermoplastic elastomers (TPEs), exhibiting improved elastic properties compared to commercially available systems, is described. The first type comprises multiblock copoly(ether ester)s with semicrystalline hard segments and triblock copolymer soft segments. The second class of TPEs are systems based on ABA and ABC triblock copolymers with two glassy and one or two semicrystalline end blocks, respectively. The used strategy for increasing the elasticity of conventional copoly(ether ester)s based on poly(butylene terephthalate) (PBT) hard segments and polyether soft segments is the replacement of the continuous PBT hard phase in these systems by a disperse PBT hard phase. The incorporation of nonpolar segments is possible by using poly(ethylene oxide)-block-poly(ethylene-stat-butylene)-block-poly(ethylene oxide) (PEO-b-PEB-b-PEO) triblock copolymers, where the polar PEO blocks act as compatibilizer between the nonpolar PEB segments and the polar PBT segments during the melt polycondensation. The incorporated nonpolar PEB segments induce an enhanced microphase separation in the melt, which in turn results in the formation of a disperse PBT hard phase. Mechanical testing reveals a significantly improved elastic recovery compared to that of conventional copoly(ether ester)s exhibiting a continuous PBT hard phase. Morphological investigations reveal that this novel copoly(ether ester)s consist of a semicrystalline PBT hard phase and an amorphous soft segment phase, which is divided into a pure PEB phase, a PEO-rich phase besides a mixed PBT/PEO phase, and a pure amorphous PBT phase. The second part of this thesis deals with ABC triblock copolymers with one or two semicrystalline end blocks. Two main aspects were addressed: i) the interplay between morphology and crystallization, and ii) the comparison of ABA and ABC triblock copolymers with glassy (A), elastomeric (B) and crystalline (C) blocks. Several polyethylene-block-poly(ethylene-alt-propylene)-block-poly(ethylene oxide) (PE-b-PEP-b-PEO) triblock copolymers have been synthesized by sequential anionic polymerization of butadiene, isoprene, and ethylene oxide (PB-b-PI-b-PEO) followed by homogeneous catalytic hydrogenation. Anionic polymerization in a one-pot procedure was accomplished by using the phosphazene base t-BuP4, which enables the polymerization of ethylene oxide (EO) in the presence of Li+ counterions. Kinetic investigations on the EO polymerization reveal an unexpected induction period. It is concluded, that different factors contribute to the induction period, like break up of lithium alkoxide aggregates by t-BuP4, and chain length effects. Characterization of the PB-b-PI-b-PEO and PE-b-PEP-b-PEO triblock copolymers utilizing differential scanning calorimetry and special self-nucleation (SN) techniques reveals a strong influence of the confinement active during crystallization on the crystallization and SN behavior of the semicrystalline PEO and PE blocks. For low PEO contents large supercoolings are necessary to induce crystallization of PEO. Furthermore, the SN behavior of the PEO blocks is different compared to that of semicrystalline homopolymers, i. e. domain II (SN domain) is absent. This is a direct result of the confined crystallization of PEO within small isolated microdomains. In contrast, for the PE blocks a heterogeneous nucleation mechanism and the presence of all three SN domains, usually present in crystallizable homopolymers, is observed. In this case, PE crystallizes without any confinement from a homogeneous mixture of PE and PEP segments, which can be ascribed to their small segmental interaction parameter. In TPEs based on ABA triblock copolymers with two glassy end blocks, e. g. polystyrene-block-poly(ethylene-alt-propylene)-block-polystyrene (PS-b-PEP-b-PS), the middle block chains can either loop back into the same PS domain or form bridges between two different PS domains. However, only the bridges contribute to the elastic properties, which limits the elastic recovery of theses systems. The influence of a semicrystalline end block on the elastic properties has been investigated by comparison of PS-b-PEP-b-PE and the corresponding PS-b-PEP-b-PS triblock copolymers. For small elongations (< 300%) the PE containing triblock copolymers exhibit a significantly improved elastic recovery. This can be attributed to the increased bridge fraction induced by the immiscibility of the two different end blocks. In contrast, for high elongations (> 300%) the situation is reversed and the PS-b-PEP-b-PS triblock copolymers reveal better elastic properties. Obviously, glassy PS domains show a higher resistance against distortion compared to that of semicrystalline PE domains, especially at high strains.

Social life-styles in caterpillars: Behavioral mechanisms and ecological consequences (2002)

Claudia Ruf

The present thesis investigates ecological advantages, ethological adaptations, and behavioral mechanisms connected with sociality in larval Lepidoptera. Data on social thermoregulation, larval communication, developmental benefits, and advantages of survival are presented to contribute to a better understanding of the evolution and maintenance of social systems in caterpillars. I put the main emphasis of this work on the highly social, tent building European lasiocampid moth Eriogaster lanestris. In a comparative study aspects of the social biology of E. lanestris were compared to its congener E. catax and the confamilial species Malacosoma neustria. In addition to these tent building, central-place foraging species a fourth, non-tent building nomadic butterfly species, Araschnia levana (Nymphalidae) was also studied. Caterpillars of E. lanestris are behavioral thermoregulators. In the laboratory (i.e. in the absence of solar radiation) tightly aggregated caterpillars are able to rise their body temperatures compared to ambient temperature for 2.5-3K. Temperature gains are based on metabolic heat production (Chapter 4). Under field conditions grouped caterpillars are able to stabilize their body temperatures between 30-35°C independent of ambient temperature as long as solar radiation is sufficient. Optimal body temperatures are achieved by changing positions in and on the tent frequently. The functionality of the tent is based on the reduction of convective heat exchange (Chapter 5). Foraging bouts of E. lanestris caterpillars are highly synchronized under field conditions. The number of foraging bouts is temperature dependent (influence on speed of digestion and walking). Plasticity in foraging patterns optimizes nutrient uptake and therefore minimize developmental time (Chapter 6). Caterpillars of E. lanestris use trail pheromones for communication. The pheromone is applied by dragging the ventral median sternite of the last abdominal segment over the substrate. Trails contain information about age and suitability of foraging sites, which is encoded by quantitative graduation of the trail marker (opposing processes of aging and (over)marking) (Chapters 7+8). Vibrations of the tent caused by the increasing restlessness of caterpillars at the end of the digestion phase turned out to be tightly linked to the emergence of larvae from the tent and are supposed to play a role for synchronizing the foraging bouts (Chapter 9). Costs of social behavior in E. lanestris are high under field conditions. Total colony mortality (i.e. the complete loss of a female’s offspring) of 100 exposed colonies was 48%. The majority of colony losses was connected to the previous loss of the tent, mostly initiated by heavy rainfalls (Chapter 10). Comparing E. lanestris to E. catax and M. neustria revealed that despite similar life-history traits and overlapping habitat requirements very different ecological strategies evolved with regard to egg size, tent building behavior, foraging strategies, and activity patterns (Chapter 11). Females of A. levana produce several egg clutches of different size over their life time. Caterpillar group size strongly influences development and survival. Single individuals develop more slowly and achieve smaller weights compared to groups of 10, 40 (=mean natural clutch size), or 80 individuals during the social phase of the caterpillars’ life (i.e. first to third instar). Mortality is highest in singly bred individuals and lowest in groups of 40. The advantage of grouping for survival is supposed to result from the strong collective regurgitation behavior which enables the caterpillars to ward off natural enemies (Chapters 12+13). The thesis shows with the example of E. lanestris that highly evolved social systems in larval Lepidoptera afford high parental and larval investment which may in turn lead to high advantages, e.g. with regard to large reductions in developmental time. The extended social phase requires highly sophisticated communication processes. In A. levana flexible egg deposition leads to higher plasticity with regard to the costs of sociality. Nevertheless, physiological and ecological advantages of group living are evident as well.

Analysis of alkali-inducible genes of Bacillus subtilis (2003)

Akram Atalla

Using the DNA macroarray technique, it could be shown that more than 80 genes induced after alkali shock (Wiegert et al., 2001). While most of them are under the control of the alternative sigma factor (sigmaW), the remaining genes are under the control of one or more unknown regulator(s). By their signature, two of them kipR and yvdT code for regulatory proteins, while pspA, member of the sigmaW regulon, encodes another potential regulator. In this doctoral work, the genes kipR, yvd and pspA were analyzed. The kipR and yvdT genes code for a transcriptional regulator of the IcIR and TetR/AcrR family while the pspA encode a transcriptional anti-activator in E. coli . In Northern blot analyses, it could be shown that all three genes are induced after alkali shock. The transcription start points of the kipR and yvdT genes were identified by primer extension experiments, and it appeared that the transcription is dependent on a vegetative sigma A-like promoter. To identify genes which are under the negative control of the transcriptional anti-activator PspA, a DNA macroarray experiment was carried out. It turned out that several genes are repressed by a factor of at least three under conditions of PspA overproduction. By using the Far-western blot technique, a protein which might interact with the PspA protein was identified. This protein has a molecular weight approximately 50 kDa. In addition, expression of the pst operon ( pst stay for phosphate-specific transport) was analyzed which is induced after phosphate starvation and after alkaline shock. The genes of this operon are involved in the phosphate transport into the cytoplasma. By Northern-blot experiments, it could be shown that all genes of this operon are alkali-inducible. When the transcriptional start point was determined by primer extension, it turned out to be identical to the one determined under phosphate limitation. This transcription start point is preceded by a typical sigmaA-type promoter. Furthermore, alkali-induction is dependent on the PhoP-PhoR two-component signal transduction system. Phosphate-uptake experiments revealed that the uptake of inorganic phosphate was completely abolished after increasing the external pH value.

Novel Precursors for Polymer-Protein-Conjugate Synthesis via Reversible Addition-Fragmentation Chain Transfer Polymerization (2003)

Christine Maria Schilli

The RAFT polymerization of N-isopropylacrylamide with two different chain transfer agents, namely benzyl 1-pyrrolecarbodithioate and cumyl 1-pyrrolecarbodithioate, yielded polymers with narrow molecular weight distributions as well as Mn values that were in good agreement with the calculated ones. A comparison between the Mn values determined from gel permeation chromatography, GPC, and the values from MALDI-TOF mass spectrometry showed that the molecular weights obtained from GPC using polystyrene standards were considerably higher. A relation between log Mn,MALDI and log Mn,GPC was established, which permitted construction of a calibration curve for PNIPAAm polymers. In-situ Fourier-transform near-infrared spectroscopy was applied for the reliable determination of monomer conversions and it indicated living characteristics. Both polymerization processes showed an induction period that seems to be correlated with a retardation in rate, where the induction time is higher for the cumyl chain transfer agent as compared to the benzyl chain transfer agent of the same concentration. The induction periods decrease with decreasing transfer agent concentration and were explained in terms of the different stabilities of the respective radicals that add to monomer in the reinitiation step. The more stable cumyl radical adds slower than the benzyl radical. Both UV spectroscopy and MALDI-TOF mass spectrometry confirm the presence of the expected dithiocarbamate endgroups. MALDI-TOF characterization of the polymer samples showed the transfer agent endgroups together with some initiator-derived polymers. Endgroups that seemed to originate from disproportionation or transfer were the result of fragmentation under MALDI conditions as was shown by a post source decay analysis and MALDI-TOF characterization of the hydrolyzed polymer. With amine-reactive diacetone acrylamide, 2-vinyl-4,4-dimethyl-5-oxazolone and N-hydroxysuccinimide methacrylate, new monomers were polymerized via RAFT in a controlled manner. Poly(diacetone acrylamide) and poly(2-vinyl-4,4-dimethyl-5-oxazolone) showed low polydispersities and good control over molecular weight, where poly(N-hydroxysuccinimide methacrylate) displayed relatively high polydispersities despite the controlled polymerization evident from the monomodal GPC traces. These amine-reactive polymers were subsequently used for successful conjugation to the primary amino group of the model peptide glycine-leucine. For poly(N-isopropylacrylamide)-block-poly(acrylic acid), PNIPAAm-b-PAA, it was demonstrated that hydrogen bonding between N-isopropylacrylamide and acrylic acid units influences strongly its behavior in both the solid state and in solution. The block copolymers form micelles in aqueous solutions in dependence of pH and temperature. Cloud point measurements indicated the formation of larger aggregates at pH 4.5 and temperatures above LCST, whereas micelles formed at pH 5-7 and temperatures above LCST. At pH 5.6 and 50 °C, only micelles were found, whereas, at lower temperatures, larger aggregates and micelles coexist. Formation of larger aggregates by hydrogen bonding interactions was revealed by IR and Raman spectroscopy as well as by cryogenic transmission electron microscopy and dynamic light scattering. Differential scanning calorimetry yielded glass transition temperatures of PNIPAAm-b-PAA that were well above the transition temperatures of the homopolymers, demonstrating molecular interactions between the acrylic acid and the N-isopropylacrylamide blocks. Conjugation of sulfhydryl-terminated PNIPAAm to thiol disulfide exchange reagents and maleimides was probed for later conjugation to proteins. Evaluation of the different cross-linking systems resulted in the choice of maleimides as cross-linkers for subsequent conjugation to the protein streptavidin. Sulfhydryl-terminated PNIPAAm-b-PAA was conjugated to the streptavidin mutant S139C using a bismaleimide cross-linker and also direct conjugation via disulfide linkage. Both conjugations were successful and proceeded with more than 50 % conversion. Conjugation of PNIPAAm and PNIPAAm-b-PAA was also achieved by non-covalent attachment of the biotinylated polymers to wild-type streptavidin. Conjugates of wild-type streptavidin with biotinylated PNIPAAm-b-PAA were found to remain dissolved at temperatures above LCST even at very low pH values, which was in contrast to the observed precipitation of the unconjugated block copolymer at pH <= 4.5. Conjugates of wild-type streptavidin with biotinylated PNIPAAm of different molecular weights formed aggregates in aqueous solutions above LCST and a dependence of aggregate size on the size of the polymer was found

Phase Behavior and Structural Transitions in The Mixtures of Cationic Surfactants and Hydrophobic Counterions (2003)

Rami Abdel-Rahem

Anionic hydrophobic counterions with certain geometry adsorb onto the surface of cationic surfactant micelles and they minimize the repulsion between the headgroups, so the charge density on the surface is reduced. As a result of this, the micelle spontaneously changes its morphology due to a new packing for the head groups. The adsorption of 2-hydroxy-1-naphthoic acid 2,1 HNC and 6-hydroxy-2-naphthoic acid 6,2 HNC onto the surface of the cationic surfactant cetyltrimethylammonium hydroxide was studied. The results were compared to the published system 3-hydroxy-2-naphthoic acid 3,2 HNC/CTAOH. When an increasing amount of 2,1 HNC is introduced into a micellar solution of 100 mM CTAOH, one finds low viscous micellar solution, viscoelastic gel (consisting of rod like micelles), turbid region (two phase region), and viscoelastic liquid crystalline gel (consisting of multilamellar vesicles MLV with yield value). The complex viscosity (0.01 Hz) of 100 mM CTAOH rises by six orders of magnitude as the rodlike micelles form.It decreases then to the turbid region, and then rises again approximately six orders of magnitude. The second rising of the complex viscosity is accompained by the formation of a liquid crystalline phase which consists of multilamellar vesicles. This has been proven by DICM, FF-TEM and Cryo-TEM. The vesicles were polydisperse and ranged from 100 to 1000 nm in diameter. SANS detected the transition in the microstructure which was caused by changing the concentration of 2,1 HNC in the system. SANS calculations show results similar to that obtained by microscopic methods. Surprising rheological behavior was measured in the rodlike micelle region, at which storage modulus was about one order of magnitude higher than loss modulus and both were parallel in the frequency range 0.001-10 Hz. Such behavior usually indicates the presence of vesicles in the liquid crystal phases. It was proved that other rheological measurements can be used to distinguish the tow types, namely, amplitude sweep measurements, first normal stress difference N1 (Weissenbeg effect), the effect of adding electrolyte, and stress relaxation curves. When 6,2 HNC (new substitution of HNC) is added with an increasing amount to 100 mM CTAOH, a new phase behavior is observed. Here the structure changes from small micelle aggregates into rodlike micelles, and then a two phase region consisting of L1-phase and un-reacted 6,2 HNC is formed. No transition into MLV has been detected. In the case of 3,2 HNC and 2,1 HNC, the hydroxyl and the carboxyl group are neighboring, so they can effectively share in reducing the repulsion between the headgroups while the rings are in interaction with hydrocarbon tails. For 6,2 HNC the hydroxyl group is in position number 6 on the aromatic rings, which means that hydroxyl group is distant from the carboxyl group, thereby, less screening for the cationic charge in the micelle surface is obtained. Substitution of HNC plays a main role in controlling the microstructure and other physical properties such viscosity, Krafft point, ..etc. In the second part of this work, the hydrophobic counterion is fixed (2,1 HNC), and the length of the cationic surfactant‘s chain is changed from C16 into C14, C12, C10 and C8. For the system 2,1 HNC/ tetradecyltrimethylammonium hydroxide TTAOH similar phase behavior as 2,1 HNC/CTAOH is observed. At 2,1 HNC/TTAOH ratio r aproximately 1, formation of MLV is observed. After the neutralization addition of excess amount of 2,1 HNC is possible since the insoluble molecular form of 2,1 HNC becomes solublized in the formed MLV. Conductivity measurements prove that 2,1 HNC stays in the molecular form after the neutralization. A difference in the rheological behavior of the system 2,1 HNC/TTAOH compared to 2,1 HNC/CTAOH is seen. In the rodlike micelles region of 2,1 HNC/TTAOH, the solutions exhibit a short relaxation time compared to 2,1 HNC/CTAOH system. FF-TEM and SANS proved the formation of polydisperse MLV in this system with a maximum diameter of about 2000 nm and wall thickens of about 28 nm. As a result of this work, it is concluded that the role of the hydrophobic counterions with certain geometry could be looked upon as a co-surfactant with a shorter chain length which changes the bending rigidity, of the bilayer. They are surface active species that bind strongly on the micelle surface and change the packing parameter of the headgroups. It is suggested that the hydrophobicity of the counterion plays an important role in deciding the structure of the supramolecular assemblies such as vesicles, or micelles. As a consequence one can change the morphology of micelle species by changing the ratio of counterion /surfactant ion. These studies also suggest that by mixing cationic surfactant and hydrophobic counterion with varying cationic surfactants chain lengths, one can have a control over the supramolecular structures formed.

Doctoral Thesis

Refine

Author

Year of publication

Document Type

Language

Keywords

Institute

341 search hits