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«DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Fach Chemie der Fakultät für Biologie, ...»

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Functional Cylindrical Polymer Brushes and

Their Hybrids with Inorganic Nanoparticles

DISSERTATION

zur Erlangung des akademischen Grades eines

Doktors der Naturwissenschaften (Dr. rer. nat.)

im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften

der Universität Bayreuth

vorgelegt von

Youyong Xu

Geboren in Jiangsu/China

Bayreuth, 2008

Die vorliegende Arbeit wurde in der Zeit von April 2004 bis Spetember 2008 in Bayreuth am Lehrstuhl Makromolekulare Chemie II unter Betreuung von Herrn Prof. Dr. Axel H.

E. Müller angefertigt.

Vollständiger Abdruck der von Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. Nat.).

Dissertation eingereicht am: 06.10.2008 Zulassung durch die Promotionskommission: 15.10.2008 Wissenschaftliches Kolloquium: 16.12.2008 Amtierender Dekan: Prof. Dr. Axel H. E. Müller

Prüfungsausschuß:

Prof. Dr. A. H. E. Müller (Erstgutachter) Prof. Dr. A. Böker (Zweitgutachter) Prof. Dr. K. Seifert (Vorsitzender) Prof. Dr. H. G. Alt To my parents and Jiazhen Table of Contents

TABLE OF CONTENTS

Summary/Zusammenfassung

1. Introduction I-1

2. Overview of this Thesis II-1 Individual Contributions to Joint Publications II-15

3. Double-grafted Cylindrical Brushes: Synthesis and Characterization of Poly(lauryl methacrylate) Brushes III-1

4. pH and Salt Responsive Poly(N,N-dimethylaminoethyl methacrylate) Cylindrical Brushes and Their Quaternized Derivatives IV-1

5. Manipulating Cylindrical Polyelectrolyte Brushes on the Nanoscale by Counterions: Collapse Transition to Helical Structures V-1

6. Switching the Morphologies of Cylindrical Polycation Brushes by Ionic and Supramolecular Inclusion Complexes VI-1

7. Manipulating the Morphologies of Cylindrical Polyelectrolyte Brushes by Forming Inter-Polyelectrolyte Complexes with Oppositely Charged Linear Polyelectrolytes: An AFM Study VII-1

8. Direct Synthesis of Poly(potassium 3-sulfopropyl methacrylate) Cylindrical Polymer Brushes via ATRP using Supramolecular Complex with Crown Ether VIII-1

9. Hybrids of Magnetic Nanoparticles with Double-hydrophilic Core-shell Cylindrical Polymer Brushes and Their Alignment in Magnetic Field IX-1

10. Sin

–  –  –

Summary 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 Summary/Zusammenfassung spheres when SDS was added. Introducing α- or β-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 18crown-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 thiolfunctionalized polyhedral oligomeric silsequioxane (POSS) to poly(glycidyl methacrylate) brushes. Their pyrolysis in air resulted in porous silica materials.

Summary/Zusammenfassung Zusammenfassung Verschiedene zylindrische Polymerbürsten wurden mit der “grafting-from” Strategie synthetisiert. Zu diesem Zweck wurde ein sehr langes Poly(2-hydroxylethyl methacrylat) Polymerrückgrat (DPn = 1500), welches über anionische Polymerisation hergestellt wurde, mit einem ATRP (Atom Transfer Radical Polymerisation) Initiator verestert und anschließend wurden unterschiedliche Monomer mittels ATRP polymerisiert.

Verschiedene Polymerbürstenarchitekturen, wie z.B. homogene oder Kern-Schale Bürsten, wurden entsprechend dem Verwendungszweck hergestellt. Mehrere funktionelle Monomere wurden für die Synthese eingesetzt um die Möglichkeit für weiterführende Funktionalisierungen und Anwendungen zu ermöglichen. Nanoskopische Hybridmaterialien wurden aus organischen Bürsten und anorganischen Nanopartikeln, wie Magnetit oder polyhedralen Silsesquioxanen (POSS), über nicht-kovalente und kovalente Anknüpfungen dargestellt.

Doppelt gepfropfte Poly(laurylmethacrylat)bürsten mit Seitenketten, die über zusätzliche kurze Alkylseitenketten verfügen, wurde synthetisiert. Die Alkylseitenketten sorgten für eine exzellente Löslichkeit in unpolaren Lösungsmitteln (n-Hexan, Paraffin) und die Kristallisierbarkeit der Seitenketten wurde mittels DSC gezeigt.

Die ATRP vermittelte Pfropfung von N,N-Dimethylaminoethylmethacrylat (DMAEMA) führte zu zylindrischen, schwachen Polyelektrolytbürsten. Diese zeigten pH-Schaltbarkeit in wässriger Lösung. Starke Polyelektrolytbürsten konnten durch eine Quarternisierungsreaktion hergestellt werden. Deren Verhalten in Gegenwart verschieden geladener Gegenionen wurde untersucht. Bei ausreichender Zugabe monovalenten Salzes zeigte sich ein Kollaps der zylindrischen in eine globuläre Struktur. Bei Zugabe von zwei- und dreiwertigen Gegenionen konnten helikale Übergangszustände vor dem Kollaps gefunden werden. Spezielle photoschaltbare Gegenionen, welche Photoaquation zeigen, erlaubten ein Schalten der Struktur von wurmartigen Partikeln zu sphärischen und zurück zu wurmartigen Bürsten.

Die Morphologien der kationischen Polymerbürsten konnten auch über eine ionische Komplexbildung mit anionischen Tensiden (Natriumdodecylsufat, SDS) und über supramolekulare Einschlussverbindungen von Cyclodextrinen und SDS verändert werden.

Summary/Zusammenfassung Die Bürsten zeigten einen Übergang von wurmartigen zu sphärischen Partikeln mit einer perlkettenartigen Übergangsstruktur bei Zugabe von SDS. Anschließendes Zugeben von α- oder β-Cyclodextrinen erlaubte eine Erholung der Struktur von kollabierten Sphären zu wurmartigen Molekülen. Adamantylammoniumchlorid, welches eine stärkere Einschlussverbindung mit SDS bildet, führte zur erneuten Freisetzung von SDS und zu einem wiederholten Kollaps der Struktur.

Die Interpolyelektrolytkomplexbildung (IPEC) mit linearem Poly(Natriumstyrolsulfonat) (PSS) erlaubte eine Regulierung der Polymerbürstenstruktur in ähnlicher Weise. Übergänge von wurmartigen zu sphärischen Strukturen, mit helixartigen Zwischenzuständen, wurden ebenfalls gefunden.

Eine neue Strategie zur direkten Herstellung von starken anionischen Polyelektrolytbürsten mittels ATRP von Kaliumsulfopropylmethacrylat in DMSO, in dem das Kaliumion durch Kronenether komplexiert ist, wurde entwickelt. Wohldefinierte wurmartige Morphologien wurden mittels Rasterkraftmikroskopie und kryogener Transmissionselektronenmikroskopie gezeigt.

Zudem wurden bis-hydrophile Kern-Schale Polymerbürsten hergestellt, die pHSchaltbarkeit zeigten. Magnetische Hybridzylinder wurden durch Einlagerung von Magnetitnanopartikeln hergestellt. Diese konnten großflächig durch externe Magnetfelder orientiert werden.

In einem letzten Teil wurden anorganisch-organisch Hybridzylinder über die kovalente Anbindung von thiolfunktionalisierten POSS Molekülen an Glycidylmethacrylatbürsten realisiert. Deren Pyrolyse an Luft resultierte in porösem Silica Material.

1. Introduction I-1

1. Introduction

With the rapid development of nanotechnology, tremendous research works have been focused on the preparation of various nanostructures1, 2. For different purposes of applications, different nanostructures are required3, 4. Nanoparticles (NPs)5, nanowires (NWs)6, nanotubes (NTs)7 and other nanostructures have been successfully prepared. Due to their various architectures and functionalities, soft materials like polymers have been playing extremely important roles for the templated synthesis, surface protection, surface functionalizations of nanostructures8. Recent progress in living/controlled polymerization techniques has enabled the preparation of polymers with various well-defined topologies.

Scheme 1-1 summarizes the general polymer architectures.

–  –  –

Polymer brushes When the grafting density of polymer chains tethered to other polymer chains, spherical, or planar substrates is high enough, polymer brushes are formed9. According to the substrates to which the polymers are grafted, polymer brushes can be classified as planar polymer brushes (PPBs)10, spherical polymer brushes (SPBs)11 and cylindrical polymer brushes (CPBs)12, 13. Scheme 1-2 shows these three different topologies. Due to the high grafting densities and the great steric repulsion, these grafted polymer chains

1. Introduction I-2

generally adopt quite stretched conformations, leading to special properties and related applications, either in solutions or on surfaces.

–  –  –

Scheme 1-2. Possible architectures of polymer brushes.

Cylindrical polymer brushes When a linear polymer is grafted with a large number of relatively short side-chains, cylindrical polymer brushes are formed9, 12, 13. Due to their anisotropic nature in topology, they have attracted more and more research interest in their synthesis, bulk, or solution properties, as well as the applications of such polymers. They are also denoted as “bottle brushes” or “molecular brushes”.

Three main strategies have been successfully developed for the synthesis of CPBs:

grafting through, grafting to, and grafting from. Scheme 1-3 shows the processes for each preparation strategy. In the following, the advantages and disadvantages of each strategy are described in detail.

Grafting through. CPBs can be prepared by the polymerization of the preformed macromonomers14-16. This was the first pioneering method in the synthesis of CPBs. For the first time, Tsukahara et al successfully obtained CPBs by the radical polymerization of macromonomers14. Short polymeric macromonomers with polymerizable vinyl chain end were first made by anionic polymerization and sequential end-functionalizations.

Further radical polymerizations of the pre-made macromonomers yielded CPBs with uniform side chains. This method was then further employed by Schmidt et al15, 16 and Ishizu et al17, 18 to prepare CPBs with different functionalities and architectures. The most obvious merit of this method lies in the high grafting density of the side-chains.

Unfortunately, inevitable disadvantages are also associated with this method. Generally,

1. Introduction I-3

due to the great steric hindrance caused by the macromonomers, it is difficult to get very long CPBs. Conventional radical polymerizations can only afford CPBs with a rather broad distribution of the backbone length. More importantly, it is very difficult to obtain complete conversions of the macromonomers, which results in a more difficult purification process. In order to remove the residual macromonomers from the reaction mixtures, fractionations are generally required. These, in combination, have limited the use of this strategy for the preparation of CPBs with well-defined structures.

Scheme 1-3. Three different strategies for the preparation of CPBs13.



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