11 search hits
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Polyelectrolyte Coatings with Internal Hierarchy
(2013)
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Julia Gensel
- The results presented in this thesis are focused on the surface modification by polyelectrolytes and polyelectrolyte copolymers. The internal structural hierarchy originate thereby from the self-assembly processes at different length scales. To generate different levels of hierarchy, the coatings were constructed by using either the layer-by-layer (LbL) deposition method (lateral chemical structure), the adsorption of supramolecular aggregates (lateral topographycal structure), or the combination of both. Using these techniques, one can control the properties of the coatings by varying the chemical structure of the polyelectrolytes, for instance, their charge density, thus providing a convenient way for their functionalization and the ability to tune properties of the surface. Therefore, we were working with systems which have variable charge densities. With this approach, we were able to produce thin and ultrathin nanostructured films with tunable properties and functionality.
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Amphiphilic Diblock Copolymers: Study of Interpolyelectrolyte Complexation in Organic Media and Nanoencapsulation of Melatonin
(2011)
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Evis Karina Penott-Chang
- Two oppositely charged homopolyelectrolytes poly(2-(methacryloyloxy)ethyl¬dimethyl-ethyl¬ammonium bromide) (PDMAEMAQ) and poly(acrylic acid) (PAA), and amphiphilic diblock copolymers based on polystyrene and the ionizable block poly(acrylic acid) were synthesized via Atom Transfer Radical Polymerization (ATRP). All polymers were characterized using 1H NMR and gel permeation chromatography to confirm their structure, molecular weight distribution and to follow the conversion. Poly(2-(dimethylamino)ethyl methacrylate), PDMAEMA, was quaternized with ethyl bromide to produce PDMAEMAQ with a quaternization degree of 98%. Furthermore, poly(acrylic acid) segments were obtained after hydrolysis of the poly(t-butyl acrylate) block. After characterization of all polymers, interpolyelectrolyte complexation in chloroform was carried out. A novel method was developed to transfer the insoluble polyelectrolytes into the organic solvent and subsequently form polymer/polymer interpolyelectrolyte (IPECs) in organic media. Therein, the polyelectrolyte were first reacted with oppositely charged low molecular weight surfactants (sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB) to form polyelectrolyte-surfactant complexes (PESCs). In organic solvents, analogously to the formation of IPECs in aqueous media, interpolyelectrolyte complexation takes place upon the direct mixing of organic solutions of two complementary PESCs. This process is accompanied by an entropically favorable release of the surfactant counterions (in the form of ion pairs or their aggregates in low polarity organic solvents), which were previously associated with the ionic groups of the polyelectrolytes in solution. These reactions are fast and lead to frozen and non-equilibrium macromolecular co-assemblies. The size and the morphologies of the IPECs in chloroform were extensively investigated using transmission electronic microscopy (TEM), scanning force microscopy (SFM), dynamic/static light scattering techniques, 1HMR and turbidimetric titrations, for two different systems: (i) homopolyelectrolyte/homopolyelectrolyte and (ii) homopolycation/negatively charged amphiphilic diblock copolymer. For the first system, the possible particle structures consist either of particles with a core formed by IPECs stabilized by fragments of the excess polymeric component or of vesicles (polymersomes). In system (ii), particles of micellar type with a core assembled from electrostatically coupled segments of the polymeric components can be found, surrounded by a corona built up either from a mixture of polystyrene blocks and excess segments of PDMAEMAQ+DS- chains or from a mixture of polystyrene blocks and excess parts of PA-CTA+ blocks, depending on which polymeric component was present in surplus during the interpolyelectrolyte complexation. Finally, nanocapsules loaded with melatonin were fabricated using a simple nanoprecipitation route employing a mixture of a diblock copolymer based on poly(methyl methacrylate) and PDMAEMA (PMMA-b-PDMAEMA) in combination with poly(ε-caprolactone), PCL. The diblock copolymers were synthesized via ATRP using PMMA-macroinitiators for the DMAEMA polymerization. Shape and size of the nanocarriers were visualized by TEM, cryogenic TEM and scanning electron microscopy (SEM). Standard TEM for nanocapsules showed an oily core surrounded by a thin layer composed of PCL/PMMA-b-PDMAEMA. Cryo-TEM also indicated the presence of spherical nanoobjects with a diffuse polymer corona. Encapsulation efficiencies were determined assaying the nanoparticles by HPLC and values of ca. 30-35% are shown by the nanocapsules. DLS measurements further confirmed well-defined unimodal particle size distributions for all formulations. It was also possible to successfully incorporate platinum nanoparticles into the nanocarrier, as evidenced by TEM, which opens up possibilities for promising applications like monitoring the circulation of the drug carrier within the body.
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Smart organic-inorganic nanohybrids of functionalized silsesquioxane nanoparticles
(2008)
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Manuela Schumacher
- The formation and characterization of smart organic-inorganic nanohybrids was studied. The inorganic part was formed by N,N-di(2,3-di¬hydroxy¬propyl)¬3-amino¬propyl¬functional silsesquioxane nanoparticles being highly functionalized with ca. 14 tertiary amino groups per particles, each amino group bearing four hydroxyl groups. Two different polymer systems were used for the organic side: amphiphilic block copolymer micelles of poly(n-butyl acrylate)-block-poly(acrylic acid) and star-shaped poly(acrylic acid)s, the latter serving as a model system for frozen micelles. In all cases the mixing of aqueous solutions of anionic block copolymer micelles or the anionic stars with solutions of the silsesquioxane nanoparticles led to the easy and straightforward formation of organic-inorganic nanohybrids. The structure of the complex nanohybrids depends on pH and salinity. The amount of incorporated silsesquioxane nanoparticles within the micelles or the stars under varying external stimuli was determined using a large number of methods. Complexation preserved the original size of the micelles - consisting of a PnBA core and a PAA corona - according to dynamic light scattering and static light scattering as well as light scattering titration measurements and asymmetric flow field-flow fractionation experiments. Fourier-transform infrared spectroscopy and dialysis measurements with fluorescently labelled silsesquioxane nanoparticles confirmed the nanohybrid formation over a relatively wide range in pH. Cryogenic transmission electron microscopy micrographs indicated a core-shell structure of the nanohybrids with gradual decreasing density of silsesquioxane nanoparticles. LS titrations gave an insight in the postulated interaction mechanism. Complexation in acidic media is driven by hydrogen-bonding and ionic interaction; in alkaline media nanohybrids are mainly formed due to ionic interaction. Depending on ionic strength, attractive Coulomb interactions may be either sufficient to promote complexation even at high pH, where hydrogen-bonding is absent (low ionic strength), or are screened (high ionic strength), resulting in less favourable interactions between micelles and silsesquioxane nanoparticles. The reason for the size conservation is most probably due to the kinetically frozen micellar core and the compensation of increased steric repulsion due to complexation and attractive interactions between the silsesquioxane nanoparticle and the charged PAA. The maximum of the interaction at 0.1 M NaCl could be deduced to be in the range 3.5 < pH < 7.5 NaCl. At low salinity (0.01 M NaCl) more nanoparticles were incorporated within the micelles. Nanohybrids exist even up to very basic conditions (pH < 9.5). The responsiveness of the system on pH and salinity as external stimuli was demonstrated by LS titration, dialysis and FT-IR measurements, thermogravimetric analysis (TGA) and AFFFF measurements. Quantifying the amount of nanoparticles incorporated in the micelles turns out to be a arduous task. SLS of dialysed and undialysed samples and AFFFF of undialysed samples clearly showed increased molecular weights of the formed nanohybrids. TGA - requiring an exhaustive dialysis procedure prior to the measurements - provided information about the amount of incorporated silsesquioxane nanoparticles within the micelles. Isothermal titration calorimetry (ITC) provided the possibility to investigate the complexation mechanism in greater detail. Small angle neutron scattering (SANS) experiments, conducted at basic conditions, provided information on the inner structure of the nanohybrids. A newly developed fitting model enabled the quantification of the amount of interacting nanoparticles under these conditions. All methods to determine the amount of nanoparticles incorporated within the micelles sustained the formation of the organic-inorganic nanohybrids. The absolute number of nanoparticles per micelle is quite high (in the range from 160 to 4300, depending on the used method and conditions), however, the calculated numbers of nanoparticles per acrylic acid unit are quite low (in the range from 0.002 to 0.053). The stars showed behaviour comparable to that of the micelles. According to DLS and SANS experiments their size was preserved during complexation. SANS and LS titration measurements demonstrated the increased mass of the nanohybrid stars compared to the net stars. Cryo-TEM micrographs confirmed the formation of organic-inorganic nanohybrid stars, indicating a morphology with gradually decreasing density of nanoparticles. An appropriate fitting model for the SANS data for this challenging system was developed that proved the interaction between the silsesquioxane nanoparticles and the PAA and enabled the calculation of the amount of entrapped silsesquioxane nanoparticles within one star. The determined values were comparable to the ones calculated for the micellar nanohybrids.
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Functional Cylindrical Polymer Brushes and Their Hybrids with Inorganic Nanoparticles
(2008)
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Youyong Xu
- Various cylindrical polymer brushes were synthesized via a grafting-from strategy. Very long poly(2-hydroxylethyl methacrylate) backbones of the brushes were prepared by anionic polymerization (DPn=1500), esterified with an ATRP initiator, and subsequently the side-chains were grafted by atom transfer radical polymerizations (ATRP). Cylindrical brushes with different architectures, such as brushes of single component, double-grafted brushes and core-shell brushes, were built according to the need of applications. A number of functional monomers were involved in the preparations of the brushes, providing possibilities for further functionalizations and uses. Nano-hybrids comprising organic cylindrical brushes and inorganic nanoparticles such as magnetite and polyhedral oligomeric silsesquioxane (POSS) were fabricated through non-covalent inclusion and covalent attachment respectively. Double-grafted poly(lauryl methacrylate) brushes carry side-chains containing dodecyl short grafts. The long alkyl chains provided good solubility in hydro-carbon solvents like n-hexane and paraffin oil. DSC measurements revealed that they undergo side-chain crystallizations. Grafting of N,N-dimethylaminoethyl methacrylate (DMAEMA) to the macro-initiator by ATRP yielded weak polyelectrolyte cylindrical brushes. They showed responsiveness to pH and salinity in solution. Strong cationic polyelectrolyte brushes were obtained by further quaternization of the PDMAEMA brushes. Their responses to counterions of different valencies were investigated. The addition of a sufficient amount of mono-valent salt induced the collapse of these brushes. When di- and tri-valent counterions were added, helical transition morphologies were recorded before the brushes collapsed into sphere-like structures. Special trivalent counterions, which can change valency through photo-aquation reactions, allowed switching the morphologies of the cationic brushes from worms to globules and back to worms. The morphologies of the cationic brushes could also be tuned by forming ionic complexes with the anionic surfactant sodium dedecyl sulfonate (SDS) and supramolecular inclusion complexes between cyclodextrins (CDs) and SDS. The brushes underwent transitions from worms, over pearl-necklace structures to totally collapsed spheres when SDS was added. Introducing alpha- or beta-cyclodextrins could bring the collapsed spheres back to worms. Adamantyl ammonium chloride, a more competitive inclusion agent, deprived SDS of CDs, and re-induced the spherical collapse of the brushes. The morphologies of the cationic brushes could be regulated in a similar way by forming inter-polyelectrolyte complexes (IPECs) with anionic linear poly(sodium styrene sulfonate) (PSS) in highly diluted solutions. Worm-to-sphere switching with helix-like transition states was also observed. A new strategy for the direct preparation of strong anionic polyelectrolyte cylindrical brushes without protection was introduced by forming supramolecular complexes between the monomer potassium sulfopropyl methacrylate (SPMA) and crown ether 18-crown-6 in DMSO using ATRP for the grafting-from processes. Well-defined worm-like morphologies were proven by atomic force microscopy (AFM) and cryogenic transmission microscopy (cryo-TEM). Water soluble double-hydrophilic core-shell cylindrical brushes were prepared and showed pH responsiveness. Magnetic hybrid cylinders were formed by introducing magnetite nanoparticles into the core. They could be aligned on a large scale on the substrates by applying magnetic fields. Finally, single-molecular hybrid cylinders were created by covalently attaching thiol-functionalized polyhedral oligomeric silsequioxane (POSS) to poly(glycidyl methacrylate) brushes. Their pyrolysis in air resulted in porous silica materials.
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Star-shaped Polyelectrolytes
(2007)
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Felix Plamper
- Star-shaped polyelectrolytes were prepared by means of atom transfer radical polymerization (ATRP) utilizing the core-first approach. Star-shaped poly(acrylic acid) (PAA) with 5, 8 and 21 arms and different arm lengths was prepared via the corresponding poly(tert-butyl acrylate) (PtBA) precursors having a glucose, saccharose or beta-cyclodextrin core. Adopting the attempt for preparation of PAA we used the same scaffolds for the preparation of star-shaped poly(dimethylaminoethyl methacrylate) (PDMAEMA). It is a weak cationic polyelectrolyte and it can be easily transformed to a strong one by quantitative quaternization (with methyl iodide) leading to poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide}, PMETAI). In order to reach high arm number a novel, hybrid silsesquioxane initiator with 58 initiation sites was introduced. The solution behavior of the obtained PAA stars was analyzed. Potentiometric titrations indicate a decrease of PAA’s acidity when increasing the arm number whereas a slight increase of the acidity was observed when increasing the molecular weight by increasing the length of the arms at constant arm number. The results are explained by the higher segment density of samples with short arms and high arm numbers, leading to a pronounced osmotic pressure inside the stars due to the presence of counterions. The osmotic pressure opposes further deprotonation, resulting in a decreased acidity. The osmotic coefficient decreases with increasing arm number, indicating higher counterion confinement within structures with higher branching. The use of strong polyelectrolytes facilitates the determination of the osmotic coefficient. It was seen directly that increasing arm numbers and decreasing arm lengths lead to a decrease of the osmotic coefficient. The osmotic coefficients of the investigated stars are in the range from 0.03 to 0.13, indicating the strong counterion confinement. Theory and experiment meet in the same order of magnitude. However the concentration dependence predicted by theory is not rendered by the experiment. The size of PMETAI stars in solution was investigated by dynamic light scattering (DLS), showing the expected collapse of the stars with increasing ionic strength. Electrostatic and osmotic screening leads to a retraction of the originally stretched arms, when no additional salt is present. However ion-specific effects lead to a more pronounced shrinkage when sodium chloride was exchanged with sodium iodide. The considerable osmotic pressure inside the star helps to incorporate multivalent counterions. The ion exchange reduces the number of counterions within the star, simultaneously increasing the translatory entropy of all counterions, since a multiple number of monovalent counterions is released into bulk for one multivalent counterion, which has been incorporated. The ion exchange leads to a decrease in osmotic pressure inside the star, reducing the strong stretching of the polymer’s arms, as seen by DLS. The collapse is more pronounced for counterions of higher valency. The switching of the counterion’s charge can therefore lead to smart polyelectrolytes. This was seen for the trivalent, light-sensitive hexacyanocobaltate(III), which by UV illumination transforms to a divalent counterion. Simultanously the hydrodynamic radius increases upon irradiation. Finally the thermoresponsive properties of aqueous solutions of star-shaped PDMAEMA were investigated. PDMAEMA is both pH-sensitive as temperature-sensitive, showing a miscibility gap at higher temperatures (LCST behavior). PDMAEMA shows a typical Flory-Huggins behavior irrespective to polymers architecture at high pH (in buffer), where it is virtually uncharged. Charge density starts to account for the deviations from ideal Flory-Huggins behavior at intermediate pH. The presence of multivalent ions leads in buffered solutions of PDMAEMA to the appearance of a miscibility gap at low temperatures (UCST behavior). In salt-free solutions the electrostatic stabilization is especially pronounced for polymers with high arm numbers (having higher charge density). No macroscopic demixing was observed for polymers with more than 9 arms.
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Micelles and Interpolyelectrolyte Complexes formed by Polyisobutylene-block-Poly([meth]acrylic acid) - Synthesis of Polymers and Characterization in Aqueous Solutions
(2007)
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Markus Burkhardt
- In this work PIB-b-PMAA copolymers with low PDI were studied, which self-assemble in aqueous solutions. A wide range of hydrophobic and hydrophilic block lengths were synthesized via combination of cationic an anionic polymerization. The data we have obtained by means of SANS and DLS point to an interesting dynamic behaviour of such micelles reacting on external stimulus of changes in pH from 10 to 7 and 5 respectively. The response is not only related to a change of the degree of neutralization of the PMAA block in the corona. Quantitative evaluation of SANS curves also shows a change of the size of the hydrophobic core formed by the PIB blocks, due to a change of the aggregation numbers. From cryo-TEM, a spherical shape of the micelles is clearly seen. This allows us to evaluate the SANS data using a model of a spherical particle with protruding arms. Evaluation of the SANS curves evidences about changes in Nagg with pH and with ionic strength. The higher the pH is, the more the arms of the micelle repel their neighbours and the higher the area at the core-corona interface of the micelle is. This leads to decreasing values of Nagg with rising pH. An increase in ionic strength has an opposite effect, resulting in higher Nagg upon improving screening of the charges of the PMAA. DLS measurements also show the response of the corona of the micelle on external stimuli. In principle, the PMAA block is more stretched the higher the number of charges on the arms of the micelles are. This also leads to an increasing Rh. Here the hydrophilic block dominates the response of the aggregate. For DLS, the influence of the PIB core and therefore the changes in Nagg can be neglected due to the longer PMAA block compared to the PIB block of the copolymers used in this work. Potentiometric titrations also show an effect of the ionic strength on the apparent pKa value, shifting it to lower values with increasing cNaCl, while the length of the hydrophilic block seems to play a minor role. For the evaluation of the cmc for different diblock copolymers the PIB block determines the properties of the micellar assemblies as well. The cmc clearly depends on the length of the hydrophobic PIB block. The longer the block is, the lower the cmc is found to be. Additionally a detailed investigation of IPECs formed by PIB-b-PMAA with P4VPQ is presented. The PIB-b-PMAA described above, are used to form water-soluble complexes with core-shell-corona structure. From cryo-TEM images, a spherical shape of the IPECs can be concluded. Slight differences in the overall shape of the complexed micelle give a hint on the proposed structure. The process of formation of complexes can be subdivided in a kinetically driven and a thermodynamically driven process. Upon addition of the polycation to the micellar PIB-b-PMAA solution, first an increase in turbidity of the solution can be observed. In this kinetically driven regime, large assemblies of micelles are formed. With time, these aggregates are equilibrating toward the thermodynamically more stable species of a single micelle with a complex species formed around the hydrophobic PIB core. The formation process can also be seen by means of SANS, leading to higher scattering intensity with increasing Z. SANS was used to follow the salt-induced dissociation of the complex as well. Increasing ionic strength of the IPEC solution leads to a release of the polycation, starting from about 0.2 M NaCl. Beyond 0.6 M NaCl, almost no difference in scattering behaviour of the IPEC solution compared to pure micelles can be stated. This suggests a total dissociation of the IPEC. By means of titration with a sodium selective electrode, the decrease of the activity of the Na ions can be explained by substitution of the polycation due to Manning condensation. Additionally the influence of point of time of addition of salt to an aqueous solution of a new diblock copolymer, PIB-b-PAA, is presented. By means of cryo-TEM, DLS and SANS an effect on the shape of the particles formed in solution could be obtained, whether the salt was added before dissolution (BD) or after dissolution of the polymer (PD). For the BD samples, a high PD of the particles can be seen from in cryo-TEM. Additionally sedimentation of a certain part of the polymer is another hint on larger aggregates. For the PD samples, spherical micelles with a core-corona structure are visible. According to cryo-TEM, their PDI is quite low. This suggests that interparticle exchange of unimers between the micelles is possible, at least before addition of the NaCl. Furthermore, it was shown, that changing the counterion to the "softer" Cs still allows the formation of equilibrium structures for BD samples, as seen from the spherical structure in cryo-TEM images. The influence of solvent for the SANS samples leads to a similar scattering behaviour for all measured samples.
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New Double-Responsive Micelles of Block Copolymers Based on N,N-Diethylacrylamide: Synthesis, Kinetics, Micellization, and Application as Emulsion Stabilizers
(2005)
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Xavier André
- Thermo- and pH-responsive block copolymers based on (meth)acrylic acid and N,N-diethylacrylamide were synthesized and their aqueous solution behavior was studied. Such bishydrophilic block copolymers represent an interesting class of stimuli-responsive water-soluble materials whose macroscopic properties can be triggered at the molecular level by tuning the temperature, the pH and the ionic strength of the solution. A new method was introduced for the synthesis of well-defined poly(N,N-diethylacrylamide) (PDEAAm) via living anionic polymerization using ethyl alpha-lithioisobutyrate (EiBLi) in the presence of triethylaluminium (Et3Al) as Lewis acid in tetrahydrofuran (THF) at -78 °C. Kinetic investigations were performed using in-situ Fourier-transform Near-Infrared (FT-NIR) fiber-optic spectroscopy. This is the first mechanistic study of the anionic polymerization of a dialkylacrylamide. The polymerization follows first order kinetics with respect to the effective concentration of active chains, [P*]0, but shows complex kinetics with respect to the actual monomer and initial aluminum concentrations. Upon addition of Et3Al, the polymerization rate constant, kp decreases, which is explained by the formation of an amidoenolate chain end/Et3Al complex of lower reactivity. It involves two equilibria: between noncoordinated and Et3Al-coordinated chain ends (deactivation of chain ends) as well as between free and Et3Al-activated monomer (activated monomer mechanism). These two effects are in a delicate balance that depends on the ratio of the concentrations of Et3Al, monomer, and chain ends. Thus, the polymerization rate of this system is governed simultaneously by the complex interplay between the activation of monomer (dependent on monomer and Et3Al concentrations) and the deactivation of chain ends (dependent on the ratio of concentrations of Et3Al to initiator). Polymers with narrow molecular weight distribution are obtained, indicating that the rate of interconversion between the different chain end species is greater than the polymerization rate. In contrast, such well-defined polymers are not found in the absence of Et3Al. PDEAAm polymers, synthesized using organolithium initiator in the presence of Et3Al, are rich in heterotactic (mr) triads and exhibit Lower Critical Solution Temperatures (LCST) in water with a cloud point at Tc = ca. 31 °C. By extending this synthetic concept and using poly(tert-butyl acrylate)-Li, and poly(tert-butyl methacrylate)-Li as macroinitiators, well-defined poly(tert-butyl acrylate)-block-PDEAAm, and poly(tert-butyl methacrylate)-block-PDEAAm block copolymers were obtained. Although the blocking efficiencies remained below 70 % a separation of block and homopolymers was easily possible. The narrowly distributed (AA)45-b-(DEAAm)360 block copolymer obtained after hydrolysis of the protecting tert-butyl groups exhibits interesting ‘schizophrenic’ micellization behavior in response to temperature, to pH, and to ionic strength of the aqueous media. Due to its asymmetric composition, two opposite micellar structures are expected. Indeed, the existence of different micellar aggregates, i.e., ‘crew-cut’ micelles with a PDEAAm core and inverse star-like micelles with PAA core, was proven by several analytical techniques, like Small-Angle Neutron Scattering (SANS), Dynamic and Static Light Scattering (DLS, SLS) and Cryo Transmission Electron Microscopy (cryo-TEM). Furthermore, all the transitions were found to be reversible. Finally, the synthesized bishydrophilic block copolymers were used for batch emulsion polymerizations of styrene, methyl methacrylate and n-butyl acrylate. In all cases, latexes with remarkable long-term stabilities were obtained, which is a very interesting feature from the colloidal point of view. The stabilization efficiency was found to be essentially adjustable by the pH due to the loss of the PDEAAm segment inside the latex particle. A detailed analysis of the particle size and particle size distribution was carried out using a variety of methods, including DLS, TEM and Asymmetric Flow Field-Flow Fractionation (AF-FFF).
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Synthesis and Characterization of Cationic Spherical Polyelectrolyte Brushes
(2005)
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Yu Mei
- In this thesis, the preparation of cationic spherical polyelectrolyte brushes by photo-emulsion polymerization has been presented for the first time. The structure and properties of the cationic SPB, have been comprehensively investigated by dynamic light scattering and atomic force microscopy, and compared with anionic SPB. Moreover, both of them were tested as dual retention-aid in conjunction with cationically modified polyacrylamide in papermaking. The interaction of SPB with negatively charged surfaces was investigated by AFM operated in Tapping Mode. It was demonstrated that the negative SPB forms two-dimensional aggregates of densely packed polymer particles, which can be explained by a particle-particle interaction dominating the repulsive interaction of the particles with the mica substrate. The positively charged SPB exhibits a completely different particle-surface interaction behaviour from that of negatively charged SPB. Here network-like structure films of dried particles without long-range 2D order are formed, which is due to the strong attractive particle-surface interaction of positively charged polyelectrolyte chains in the shell of the cationic SPB particles. These chains spread over the negatively charged mica surface and anchor the particles. The swelling behaviors of cationic and anionic SPB as a function of ionic strength in the system were studied by DLS. Adding more and more salt leads to a strong shrinkage of the surface layer as expected for polyelectrolyte brushes. For some ions, however, high salt concentrations may lead to a re-swelling of the brush layer in case of the cationic systems. This points to specific interactions of the counterions with the PATAC chains. This strong specific interaction between the counterions and the attached polyelectrolyte may even lead to flocculation of the particles at intermediate salt concentration. Surprisingly, for sodium iodide and magnesium sulfate the solubility increases again if the salt concentration is raised to 1 mol/l. At lowest ionic strength electrostatic interaction prevails and the brush layer is swollen in all cases by the osmotic pressure of the counterions. Intermediate salt concentrations lead to a partial screening of the electrostatic interaction and to a shrinkage of the brush layer. This effect can be well captured by the theory of Hariharan et al.. In case of cationic brushes, however, the shrinkage becomes very pronounced around salt concentrations of 0.1 mol/l. In some cases there is even a collapse of the surface layer due to specific interactions between the polyion and the counterions. Cationic systems re-swell if immersed in concentrated salt solutions. This is observed for monovalent as well as for divalent counterions. The analysis of the reduced excluded-volume parameter v/(lKl2) suggests that there is an adsorption of the counterions at high salt concentrations. The salting-in behavior thus finds an explanation in the increase of v due to the adsorption of salt ions. All data demonstrate that specific effects of different counterions lead to a behavior of the brush layer not expected from a purely electrostatic model. All specific effects seen at high concentrations of added salt can be explained by the increase of the reduced excluded-volume parameter which is due to the adsorption of salt ions. Cationic and anionic SPB were tested as dual-component retention system in combination with cationically modified polyacrylamide, and compared with the traditional "microparticle" system in which bentonite acts as secondary flocculant. The anionic SPB-based system shows high flocculation efficiency as tested under sheared conditions using a dynamic drainage jar. The high retention level of the SPB dual-component retention system can be explained by the enlarged specific surface area and cation exchange capacity of SPB, which are caused by grafted flexible polyelectrolyte chains. Furthermore, chemically grafted polyelectrolyte brushes show more advantages than bentonite, which may further delaminate upon dilution and cause CaCO3 fillers partially detach from the fiber. Images from field emission scanning electron microscopy of difference stages of retention testes suggest that the flocculation mechanism of anionic SPB and CPAM as dual-component retention system can be summarized in three steps: at first, excessive CPAM were added to bind cellulose fibers and CaCO3 fillers and formed loose macroflocs with positive charges; then the agglomerations were broken into microflocs by strong shear strength; at last, anionic SPB with negative charges were added and caused much finer and denser flocs, thus creating paper sheets with better optical appearance due to higher homogeneity. Images from FESEM images and AFM support the model of anionic SPB’s acting as a particle bridge between fibers and CaCO3 fillers.
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Counterion Distribution around a Macroion in Polyelectrolytes Probed by Anomalous Small-Angle X-ray Scattering
(2005)
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Mushtaq Patel
- A systematic and comprehensive study of polyelectrolytes in solution has been carried out using Anomalous Small Angle X-ray Scattering (ASAXS) in order to probe the counterion correlation around the macroion. In the course of this study, different polyelectrolyte systems such as, rod-like polyelectrolytes, spherical polyelectrolyte brushes and star shaped polyelectrolytes have been studied using ASAXS. ASAXS is the method of choice for the study of aqueous polyelectrolytes because it obviates the need of using labelled compounds or counterion exchange, as in the case of SANS or conventional SAXS. Scattering experiments performed near the absorption edge of the element under investigation, allow us to obtain the three partial scattering terms predicted by theory. All the three partial scattering terms are obtained for the first time experimentally and compared with the theoretical predictions of rod-like polyelectrolytes and spherical polyelectrolyte brushes. Rod-like polyelectrolytes bearing a rigid poly(p-phenylene) and comprising of two counterions per monomer unit with a persistence length of 21 nm were studied using ASAXS and osmometry. The results of the ASAXS experiments carried out on rod-like polyelectrolytes are compared with the predictions of the Poisson-Boltzmann Cell Model. The results suggest that the counterions are strongly correlated with the macroion. Osmotic coefficient measurements on rod-like polyelectrolytes show that approximately 20 percent of the counterions are osmotically active. The comparison of the experimentally obtained osmotic coefficient with the theory proceeds without adjusting the charge parameter. The results are in semi-quantitative agreement with the predictions of the Poisson-Boltzmann Cell Model. Spherical polyelectrolyte brushes consisting of a poly(styrene) core of 68 nm with densely grafted linear poly(acrylic acid) chains with Rubidium counterions were studied using ASAXS. In this thesis the first complete analysis of a colloidal polyelectrolyte brush by ASAXS has been presented. It has been demonstrated that there is a strong correlation of the counterions to the grafted polyelectrolyte chains of the spherical polyelectrolyte brushes. The correlation between ions and polymer chains in spherical polyelectrolyte brushes is much stronger than linear polyelectrolytes. The experiments therefore validate the theoretical predictions that most of the counterions are fully trapped within the brush and this leads to a strong stretching of the polyelectrolyte chains. Star-shaped polyelectrolytes bearing 21 arms of poly(acrylic acid) with Rubidium counterions were studied using SAXS and ASAXS. In SAXS experiments, the dependence of the maxima in the scattering experiments was observed to be concentration dependent. The ordering in star-shaped polyelectrolytes was maximum in the vicinity of the critical concentration. The maxima observed in the scattering experiments obey the scaling theory for polyelectrolytes. This is an indication of some order between the star-shaped polyelectrolytes. Evidence for the presence of any crystalline structure was not found. Scattering experiments with varying number of arms may be helpful to determine their role in the formation of crystalline structure. The three scattering terms predicted by ASAXS theory were also obtained for the star-shaped polyelectrolytes. It has been demonstrated that ASAXS is indeed a very robust method to study the polyelectrolytes in solution and to obtain the information regarding the distribution of counterions in solution. ASAXS studies of polyelectrolytes have demonstrated that the counterions are strongly correlated to the macroion.
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Generation of metal nanoparticles in spherical polyelectrolyte brushes and their application in heterogeneous catalysis
(2005)
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Geeta Sharma
- This thesis describes synthesis of spherical cationic polyelectrolyte brushes by “grafting from” technique. The spherical polyelectrolyte brushes have been used as “nanoreactors” for the synthesis of metal nanoparticles (Gold, Platinum and Silver). The catalytic activity of the resulting metal/polymer nanocomposites has been tested by heterogeneous hydrogenation of carbonyl groups. The synthesis of the cationic spherical polyelectrolyte brushes is achieved by a three-step procedure. Firstly, cationic polystyrene core particles are synthesized by emulsion polymerization by using a cationic surfactant and a cationic thermal initiator. In the second step, a thin layer of photoinitiator is generated around the particles by addition of the photoinitiator under “starved conditions” . The photoinitiator is a monomer, which is added when the formation of core particles is in the last stage. The photoinitiator polymerizes with the styrene to give a covalently bound photoinitiator. In the third and last Step, the brushes are grafted on the core particles by photoemulsion polymerization, where the initiation is triggered by UV/VIS radiation. The important parameters of the brushes- contour length and grafting density are determined by the cleavage of the chains from the surface by alkaline hydrolysis. The ester functionality within the photoinitiator is hydrolyzed under the harsh conditions of hydrolysis. The cleaved chains are analyzed to determine the molecular weight. The grafting of the charged polymeric chains stabilizes the colloids even under unfavorable conditions like high ionic strength and high pH. The behavior of brushes is investigated at different ionic strength and pH. At different ionic strengths brushes show three regimes- osmotic, salted and neutral regime. At increasing ionic strength, a shrinking in brush thickness is observed due to the screening of ionic charges. The cationic brushes (polyamino ethylmethacrylate hydrochloride and poly vinylbenzylamine hydrochloride) bear protonated amine functionality. The brushes are classified as annealed brushes as they are sensitive towards pH. The brushes lose the protons at high pH, to result in the uncharged brushes, causing them to shrink, which is followed by dynamic light scattering. Polyaminoethylmethyacrylate brushes are used as nanoreactors to synthesize the metal nanoparticles of gold and platinum. The water-soluble metal salts are to introduce metal ions in the brushes. The negatively charged metal ions (AuCl4- and PtCl6-2) interact with the cationic chains of the brushes. Dynamic Light Scattering is used to study the influence of the metal ions on the brushes. It is observed that metal ions induce much more pronounced shrinking as compared to the monovalent ions. The shrinking can be compared with the shrinking caused multivalent ions such as MgSO4. The metal ions once introduced are localized within the brushes due to strong correlation of counterions with the polyelectrolyte chains. The excess ions are cleaned by ultrafiltration. The trapped counterions render high contrast to the brushes and hence visualize brushes in Cryo-TEM. The metal ions can be reduced chemically by NaBH4 to generate nanoparticles. The particles are studied by high-resolution Transmission microscopy and cryogenic TEM. The metal nanoparticles formed are well spaced and crystalline in nature. The particles formed are found to be stable against aggregation. The silver nanoparticles are formed by using the anionic brushes (polyacrylic acid) and AgNO3 as the precursor salt. The catalytic activity of Platinum is tested by the heterogeneous hydrogenation of the carbonyl functionalities with dihydrogen under optimum conditions. The platinum nanoparticles give a 90% conversion of butyraldehyde to 1-butanol. The catalyst is found to be recyclable for a number of runs without losing the efficiency. The time dependent studies are carried out to gain insight in the mechanism and the kinetics of the reaction. The particles are found to be stable after the catalytic cycles. The gold nanoparticles supported on the polystyrene are found to be catalytically active for the same reaction. The gold particles are found to be recyclable for a number of runs without losing the efficiency. The bulk gold is completely inactive, hence the origin of catalytic activity is attributed to the quantum size effects. As the polystyrene particles are inert, the role of support particles in the origin of catalytic activity can be ruled out. The nanoparticles are found to become inactive for catalysis after 6 months of standing with a manifold increase in the particle size as revealed by microscopy.
