«Molecular Engineering of the Block Copolymer Junction to Control Chemistry of the Pore in Nanoporous Thin Films A dissertation submitted to the ETH ...»
DISS.-NO. ETH 21899
Molecular Engineering of the Block
Copolymer Junction to Control Chemistry
of the Pore in Nanoporous Thin Films
A dissertation submitted to the
for the degree of
Doctor of Sciences
M. Sc., University of Science and Technology of China
born November 17th, 1982
citizen of China
accepted on the recommendation of
Prof. Dr. Dieter A. Schlüter, examiner
Prof. Dr. Chuanbing Tang, co-examiner Prof. Dr. Peter Walde, co-examiner Dr. Anzar Khan, co-examiner Prof. Dr. Walter Steurer, chairman Acknowledgements Undoubtedly, my PhD study has been the most enjoyable time so far in my life, because I have gained so much support and kindness throughout these years. I would like to take this opportunity to express my heartfelt gratitude to all the people who helped me, this thesis would not be accomplished without them.
First of all, sincere thanks to Prof. Dr. Dieter A. Schlüter, for providing me a precious opportunity to be a part of the polymer chemistry group. His insightful thoughts, dedication and rigorous attitude towards science, and incredible ability to balance academic research and cooperation always orient and inspire me.
Without Prof. Dr. Afang Zhang’s encouragement and support, my first year in ETH Zürich would have been a hard time. Even under his supervision for just one year, the scientific knowledge, synthetic skills, and many other things I learnt from him have continued to be valuable.
My biggest debt of gratitude must go to my supervisor, Dr. Anzar Khan. He is always there whenever I need guidance and discussion, full of support, patience, motivation, and immense knowledge. He is enthusiastic, energetic, and is one of the smartest people I have known. He always gives me the freedom and independence to pursue various ideas without objection. His mentorship is paramount in providing a well-rounded experience consistent with my long-term career goals. He was and remains my best role model for a scientist and mentor.
I would like to send my great appreciation to all the wonderful people I had the privilege to work with, Ma Huan, Dr. Chaoxu Li, Dr. Emilia Paunescu, Mohammad Mirmohades, Abbas Khaydarov, Dr. Swati De, Dr. Nathalie Merlet-Lacroix, Nergiz Cengiz, Peter Tittmann (EMEZ), Stephan Handschin (EMEZ), Ikhlas Gadwal, and Christine Hottinger, the thesis would not be possible without their hard work. It goes without saying that I am also greatly indebted to Julia Ann Bättig, for her scientific discussion, her friendship, and she was the one who was always there to help me quickly proofread and give suggestions during the thesis writing period.
To Prof. Dr. Joona Bang and his students at Korea University, I am grateful for the chance to visit and be a part of the lab for one month. Thanks for welcoming me as a friend and teaching me so much about the polymer thin film processing technique.
Thanks to members of LSST group, especially Dr. Shivaprakash Narve Ramakrishna, for being the ultimate lab-neighbor, providing a great AFM work environment, and for their help and chats.
My special thanks to Dr. Thomas Schweizer, he is always offering me his kind help in the lab without any hesitation, and deep appreciation for the amazingly quick construction of my glove box.
I want to thank all the present and past members of our group, for sharing their knowledge and providing a warm and fertile environment to study. In particularly, my labmates, Samuel Jakob and Simon Timothy Cerqua, thanks for their skill in spreading happiness on those scientifically dark days. Simon Timothy Cerqua and Bernd Deffner are further thanked for translating the thesis summary into German language. I should also express my gratitude to Daniela Zehnder, our great secretary, for all her kind help and assistance during my stay in Zürich.
Special thanks go to Prof. Dr. Chuanbing Tang, Prof. Dr. Peter Walde, Prof. Dr.
Walter Steurer, for taking time out of their busy schedule to read my thesis and be in my committee.
To my friends, Dr. Xing Chen, Dr. Yufei Li, Dr. Xiang Li, and Dr. Xiaojun Wang, who are always the great support in my life.
Finally, but most importantly, to my family, thanks for providing unconditional love and care.
Chapter 1: Introduction
Aim and Organization of this Thesis
Chapter 2: Supramolecular Mimics of Phase Separating Covalent Diblock Copolymers 2.1 Introduction
2.2 Results and Discussion
2.4 Experimental Details
Chapter 3: Synthesis and Thin Film Assembly of Dynamic Covalent Diblock Copolymers 3.1 Introduction
3.2 Results and Discussion
3.4 Experimental Details
Chapter 4: Hydrazone-Linked Diblock Copolymers: Synthesis, Self-assembly, and Porosity Generation in Thin Films 4.1 Introduction
4.2 Results and Discussion
4.4 Experimental Details
Chapter 5: Assembly of Interacting Binary Blends of Diblock Copolymers in Thin Films 5.1 Introduction
5.2 Results and Discussion
5.4 Experimental Details
A nanoporous membrane contains enclosed channels spanning the thickness of the membrane material. The diameter of these channels can vary from several to a few hundred nanometers. Due to these pores, a nanoporous thin film can be used as a size-selective filter as molecules/objects with lower dimensions than the pore diameter can pass through them whereas the larger ones cannot. An interesting possibility occurs when the surface chemistry of a nanopore is modified. This allows for additional parameters, such as charge and hydrophobicity, to come into play in deciding the efficacy of a membrane. If the surface chemistry of the pore-wall could indeed be controlled, then many other interesting avenues come to the fore. For instance, a catalyst, such as an enzyme, can be immobilized onto the pore wall. In this manner, molecular discrimination by the pores, based upon size, charge, and hydrophobicity, can be combined with catalytic activity of the enzymes. It is for these reasons that development of methods by which the surface chemistry of a nanopore can be tailored are of considerable significance. Towards this end, the present thesis explores control strategies over surface chemistry of the pores in nanoporous thin films derived from self-assembling block copolymer precursors. In general, the block copolymer building blocks are carefully designed to have incompatible blocks, asymmetric block lengths, and a reversible copolymer junction. Self-assembly of such copolymers results in nanostructured thin films exhibiting highly ordered cylindrical morphology. Removal of the nanosized cylinders by reversing the reversible copolymer linkage then affords ordered nanoporous membranes that contain chemically reactive functionalities within the nanopores. These functional groups can be subjected to chemical manipulation (such as change in chemical charge), covalent functionalization through reforming the reversible linkage, or non-covalent functionalization through establishing supramolecular contacts.
E ine nanoporöse Membrane ist über ihrer gesamte Fläche von Kanälen durchzogen welche durch die gesamte Membran hindurch gehen. Der Durchmesser dieser Kanäle variiert von wenigen bis mehreren hundert Nanometern. Aufgrund dieser Poren können solche Membrane als größenselektive Filter verwendet werden. Moleküle oder Objekte welche kleinere Ausmaße als der Porendurchmesser haben können die Membran passieren während größere Moleküle oder Objekte nicht hindurch können. Eine interessante Anwendung ergibt sich, wenn die Innenseite der Poren chemisch modifiziert wird. In diesem Fall kann die Durchlässigkeit auf verschiedene Parameter, wie Ladung und Hydrophobie hin eingestellt werden. Eine Möglichkeit die Poreninnenwände gezielte chemisch zu verändern wäre zum Beispiel das Anbringen eines Enzyms als Katalysator. In diesem Fall könnte die Selektivität einer Membran hinsichtlich Größe, Ladung und Hydrophobie mit der katalysierenden Eigenschaft eines Enzyms kombiniert werden.
Aus den oben genannten Gründen ist es von enormer Wichtigkeit Methoden zu entwickeln, die es ermöglichen, die chemische Beschaffenheit der Poren zu verändern.
Mit diesem Ziel vor Augen, beschäftigt sich die vorliegende Arbeit mit der Erforschung von Strategien zur Kontrolle der Oberflächenchemie von Poren in nanoporösen dünnen Filmen welche durch Selbstorganisation von Blockcopolymeren hergestellt werden. Die einzelnen Blöcke der Blockcopolymere sind auf Inkompatibilität, asymmetrische Blocklänge und eine reversible Blockverknüpfung hin ausgelegt. Selbstorganisation von solchen Blockcopolymeren führt zu nanostrukturierten dünnen Filmen welche eine hochgradig geordnete zylindrische Morphologie aufweisen. Die Zylinder mit einer Größe im Nanometerbereich können II durch den Bruch der reversiblen Copolymer Bindung entfernt werden. Dies führt zu nanoporösen Membranen, welche chemisch reaktive Gruppen an den Poreninnenwänden besitzen. Diese funktionellen Gruppen können nun chemisch weiter modifiziert werden z.B. durch die Veränderung der Ladung, kovalente Funktionalisierung mittels der reversiblen Verknüpfungsmöglichkeit oder durch eine nicht kovalente Funktionalisierung mittels supramolekularer Wechselwirkungen.
III Chapter 1: Introduction
S elf-assembly of block copolymers is a promising route to creating nanoporous materials of controlled pore dimensions. This chapter briefly describes the two major pathways adopted for transforming a self-assembled diblock copolymer thin film into a nanoporous membrane and relates these studies to the working hypothesis and ideological evolution of the present thesis.
Block copolymers are composed of two or more chemically different polymer chains, which are covalently bound together. In the case of block copolymers featuring two immiscible blocks, phase separation of the two segments on the macro scale is not possible as both blocks cannot detach from one another and hence microphase separation into ordered microstructures with length scales of the order of ten to a hundred nanometer occurs.1-2 Depending on the Flory-Huggins interaction parameter between the monomer units, the length of the block copolymers and the composition, different structures are formed due to the balancing act of the enthalpic interfacial energy between the blocks and the entropic chain stretching energy of the individual blocks. Body centered cubic packed spheres, hexagonally packed cylinders, and alternating lamellae are the most common microstructures found for conformationally symmetric diblock copolymers. These nanostructured thin films can be transformed into nanoporous materials upon selective removal of the minor polymer domains (Figure 1.1).2-3 In this way, highly ordered nanoporous thin films of
defined pore sizes and narrow pore size distribution can be obtained. Initially, removal of the minor domain was achieved by degradation of the polymer backbone.
For instance, poly(methyl methacrylate) (PMMA) domains were removed via UV degradation.4 Ozonolysis was the method of choice to degrade poly(isoprene) (PI) and poly(butadiene) (PBD) segments.5 Poly(lactide) (PLA) blocks were removed by chemical etching methods.6 Finally, strong acidic conditions could be employed for the removal of poly(ethylene glycol) (PEG) and poly(dimethylsiloxane) (PDMS) polymers.7 These processes are sufficient for converting the polymer thin film into a nanoporous structure. However, there are issues associated with these systems. For example, harsh etching conditions may broaden the pore size distribution. More importantly, the nature of the functional group at the end of an etching process remains ill-defined and cannot be selected and controlled precisely. For example, in the widely utilized case of PMMA, UV degradation can lead to the formation of, among others, terminal epoxide, internal epoxide 1,2,3-trisubstituted and 1,2disubstituted internal olefin, 1,2-disubstituted terminal olefin, aldehyde, and geminaldimethylester chain-end functionalities.8 Figure 1.1. Assembly of a diblock copolymer into cylindrical morphology and removal of the cylindrical domains to yield a nanoporous thin film.
In an alternative approach, cleavage of the copolymer junction – rather than polymer backbone – is shown to efficiently generate nanoporous materials from the self-assembled block copolymer thin film. In this approach, only one reaction – breakage of the copolymer junction – is required for selective removal of the minor
polymer block. Penelle and coworkers first demonstrated this strategy by employing a poly(styrene)-block-poly(methyl methacrylate) copolymer connected through an anthracene dimer (Scheme 1.1).9 The synthesis of this dimer required 150 hours of photoirradiation conditions. Clearly, PS and PMMA homodimers were also produced during the course of the reaction. Nonetheless, films were prepared and annealed at 170 °C. At this temperature, however, the anthracene photodimer thermally reverted back to the homopolymers. Hence, the temperature was decreased to 135 °C. This lowering of annealing temperature limited the extent of homopolymer formation (to about 30%) and allowed for polymer self-assembly into a cylindrical morphology.
The minor domains were then removed under thermal conditions. However, no chemistry was performed within the pores.
Scission of PS-PMMA diblock copolymer connected through an anthracene dimer into constituent homopolymers.