Synthesis and Self-assembly of ABX-type Giant Molecules Based on Polyhedral Oligomeric Silsesquioxanes

Author:Shao Yu

Supervisor:yang shu guang zhang wen bin


Degree Year:2019





The investigation on structure-property relationship is an eternal topic in the field of material science and engineering.Materials with unique properties always have sophisticated,hierarchical and ordered structures,however,accurate analiysis and effective regulation of these complex structres are still a great challenge.For instense,organisms in nature,from the entire tissue to the small protein,all have synergistic interaction of complex multi-level structures,enabling them to perform unique functions.Whether in the field of life sciences or materials science,with the deepening of research,scientists experience paradigm shifts from the empirical rules of understanding phenomenons to the trial and error strategies which rely on serendipities,and finally to the rational design by comprehensive understanding of the ture nature.In recent years,due to the rapid development of research methods,such as synchrotron radiation technology,cryo-electron microscopy technology,etc.,researchers have a deeper understanding of the structure,laying a solid foundation for further rational design of functional materials.As a consequence,materials science is extremely active,new materials are emerging,and material compositions are becoming more complex.Therefore,the study of complex phase structures has important scientific significance,not only can promote new basic discoveries,but also necessary to solve obstinate problems and ultimately serve the society.Besides the traditional“top-down”lithographic technics,the“bottom-up”approach of self-assembly is probably the most feasible and universal method for the construction of complex three-dimensional nanostructures.Self-assembly is ubiquitous,such as crystals,colloids,bilayers,globular proteins,and micro/nanoscale phase separations,they are all molecular self-assembly.In this thesis,we focused on the design,synthesis and bulk self-assembly of giant molecules.Through a novel molecular design combined with efficient synthetic chemistry,a series of ABX-type triblock giant molecules are constructed and their self-assembly rules are further investigated.Broadly speaking,giant molecules are subordinate to macromolecules and both are members of soft matters.In a narrow sense,giant molecules are different from traditional polymers based on small monomers,they are constructed from a class of bulding blokcs called molecular nanoparticles(MNPs).In this thesis,the T8-polyhedral oligomeric silsesquioxanes(POSS)were widely used.These giants are unique primitives which have both the sensitivity of small molecules and the assembly properties of liquid crystals or block polymers with compatible feature sizes.We believe that molecular nanoparticles to giant molecules are just like protein domains to proteins.And we envision that molecular nanoparticles can be used as“artificial domains”(or“structural sythons”and“functional sythons”)to achieve rational modular design and functionalization of macromolecules.In this thesis,we try to expend the library of giant molecules and to understand their assembly mechanisms,the correlation to material designs or material properties is only mentioned on the final perspective.The main content including design,synthesis,characterization and bulk self-assembly study of regio-isomeric ABA-type shape amphiphiles;design,synthesis and bulk self-assembly of ABC-type bola form giant surfactants.The nomenclature of ABX is helped to distinguish blocks with different chemical compositions.Detailed information includ:(1)In the molecular design of ABA-type shape amphiphiles,building blocks of special regio-isomeric properties were introduced as the core skeleton.Then,MNPs including rod-like FPOSS,cubic BPOSS,spherical tBPOSS,and hydroxyl bonded DPOSS were tethered in para-,meta-,and ortho-configuration of T8 core to construct six groups of giant regio-isomers.These isomers pocess exactly the same molecular composition and,the molecular weight is monodispersed.The chemical identities of these giants were thoroughly characterized using combined techniques such as Nuclear Magnetic Resonance Spectroscopy(NMR),Fourier Transform Infrared Spectroscopy(FT-IR),Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry(MALDI-TOF MS).The regio-configuration is discussed in detail especially by the analysis of 29Si NMR spectra and Size Exclusion Chromatography(SEC).(2)After the successful construction of ABA-type regio-isomers,we studied their bulk self-assembly behaviors using Differential Scanning Calorimeter(DSC),lab-scale and synchrotron X-ray scattering/diffraction facilities(SAXS/WAXD)and Transmission Electron Microscope(TEM).We found that FPOSS based shape amphiphiles tend to self-assemble to layered structures due to the flat interface.However,different molecular configerations will gives distince phase behaviors in these layered strucutres.For instence,the phase stablility of OH-FPOSS-OH isomers follow the rule of para->meta->ortho-,which dictated by molecular symmetry.The layer d-spacings are also related to the regio-proerties which effect packing models and gives a bigger size of ortho-than meta-than para-.In particular,a Frank-Kaper A15 supramolecular lattice is obtained in ortho-2BPOSS-DPOSS-2BPOSS while para-isomer shows a hexagonal lacttice with the space group of P63mmc and meta-isomer may adapte the symmetry of Immm.Besides,the following discoveries may have general interests:1)regio-isomers have exactly the same chemical compositions and molecular weights will go through distinct self-assembly routes.2)the hydrodynamic volume of these isomers has the general rule of para->meta->ortho-revealed by SEC,indicating para-isomer has the most extended configuration.3)the enthropy gian of giant molecules can be neglected due to the rigid skeleton and mono-dispersity,thus,the main contributions of phase stability rely on enthalpy which is guided by molecular symmetry,interfacial energy,and interactions.4)in order to drive the self-assembly towards ordered structures,collective second order interactions are necessary such as hydrogen bonding or crystallization.5)the geometry factors(shape and size)of MNP motifs are also crucial for self-assembly,such as the conjunction point of rod-like FPOSS is flat and wide which doesn’t favor a curved interface,thus they have a broad LAM region.The above understandings expound the unique sensitivety of giant moelcules and the special regio-isomeric influence on the phase behaviors of macromolecules,which opens new dimensions on the study of molecular assembly.(3)There are not many phase structures found in the soft matter field.On the contrary,numerous structures of hard materials(metal alloys and mineral salts),such as common faced face-centered cubic(FCC)phase,hexagonal close packed(HCP)phase and rare Frank-Kasper topological close-packed A15 phase,Sigma phase or quasicrystal(QC)structures,often lead to very different material properties.How to rationally design and systematically discover non-traditional phases in soft matters is an important scientific issue.In the third part,we envision the molecular self-assembly using giant molecules to mimic the metallurgy process of above-mentioned hard materials.In order to make those phases we designed the bola-form ABC-type giant molecules inspired by AnBm binary crystals.The tri-block is constructed using a soft polymer chain tethered with two MNPs on both ends.By tuning the volume fraction of the middle part polymer,we tried to drive two ends to form spherical domains like two atoms in binary crystals.Coordination number is controlled by manipulating the sizes of these spheres and the topology of the matrix polymer.In this way,a systematic approach can be established which will largely broaden the meaning of soft matter assembly.In the preliminary trials,we synthesized several bola-form model compounds such as:FPOSS-PSn-DPOSS,FPOSS-PSn-3PDMS,FPOSS-x(PS)m-PSn-y(PS)m-DPOSS.The complexity in both chemical compositions and structure formations makes it a great challenge to fulfill the ultimate goal.However,some progress has been made as follows:1)chemical conjugation methods are explored to evaluate the bonding stability,which is important for the construction of complex molecular achitectures.2)ABC-type model compounds with low molecular weight PS have been successful synthesized and thoroughly characterized,especially by MALDI-TOF MS spectrometry.(4)In the last part,the bulk self-assembly of synthesized ABC bola-form giant moelcules were studied.In order to systematic screen the unconventional phases using above mentioned method,the matrix PS must have significant high effective volume fraction in order to drive the phase separation towards spherical phases,which further can minic the AnBm binary crystals.However,low molecular weight polymers in model compounds are unlikely to drive the phase separation to form spherical structures.Nevertheless,these model giants did show fruitful self-assemblied structures such as tetragonal pack core-shell cylinders,hexagonal packed core-shell cylinders,hexagonal perforated lamellas,and other complex structures.These assemblies have both similar parts like traditional tri-block copolymers and also process differences.Especially,the electron desity contrast is very high due to the introduction of silicon atoms and flurine atoms which enable the comprehensive investigation by X-ray experiments.This study is fundamental for soft matter metallurgy and,we hope using this strategy,newly discovers can be established in the molecular self-assembly.