Studies On The Structures And Properties Of Inorganic Polymers Based On Solution Adsorption

Polymers include organic and inorganic polymers according to their components.Common natural or synthetic polymers such as nylon,cellulose,rubber,protein,DNA and other carbon-containing polymers are organic polymers.Inorganic polymers are those molecules that the backbone is covalently bonded by atoms except carbon.Typical products of inorganic polymers include boron nitride,polysilane,polysulfide,elemental boron,polyphosphazene and red phosphorus,etc.It is a fact that the structure of polymers can largely influence the usability and processing performance.At present,the structure of organic polymers,such as the double helix structure of DNA,have been extensively studied.However,for some inorganic polymers,due to their complicated valence models and amorphous nature,some key properties including chemical structure remain largely unknown.This situation has limited the development of inorganic polymer chemistry for a long time.Therefore,how to systematically study the structure and properties of inorganic polymers at the single-molecule level is a great challenge.By utilizing the solubility of substances,a series of studies can be carried out.For example,single-molecule samples can be prepared then single-molecule imaging and single-molecule mechanical manipulation can be conducted.In recent decades,atomic force spectroscopy（AFM）based single-molecule force spectroscopy（SMFS）has shown its powerful capability in studying the elasticity of single-chain polymers and the interactions between single-chain polymers and the environment.The high resolution of AFM-based SMFS ensures that various covalent and non-covalent effects can be keenly identified.In addition,AFM-based SMFS can be carried out in complex environments such as liquid phases,which provides valuable conditions for exploring the potential properties of molecules.In this study,we attempt to combine solution adsorption methods,such as AFM-based SMFS,scanning tunneling microscopy（STM）imaging,AFM imaging,spherical aberration corrected transmission electron microscopy（AC-TEM）imaging with quantum mechanics（QM）calculations to study the structure and other key properties of several typical inorganic polymers.At first,the molecular structure of amorphous red phosphorus（a-red P）is determined by single molecule methods based on solution adsorption.Furthermore,the assembly between a-red P andβ-CD in dilute solutions is discussed.In addition,the structure of am-B is determined and the self-assembly behavior of am-B chains in dilute solution is studied through topological analysis,single-molecule elasticity studies and other methods.Finally,in this study,we use an amphiphilic inorganic polymer（polyphosphazene,PPZ in brief）to explore the Hofmeister effect.In this way,the Hofmeister effect is quantified at the single-molecule level.Moreover,the traditional Hofmeister sequence is expanded.The main conclusions of this study are as following:（1）This study confirms that a-red P is a“zigzag”ladder-like linear inorganic polymer.Force measurements have proved that a single-molecule a-red P has considerable contour length.The normalized force curves can be superposed well.This result indicates that a-red P should be a linear molecule.Additionally,results from molecular weight test and STM imaging experiment further confirm that a-red P is a linear inorganic polymer.The molecular chain lengths obtained by different methods are consistent well with each other.Through quantum mechanics（QM）calculations,the theoretical single-chain elasticities of several possible linear structures of a-red P are obtained then compared with the experimental single-chain elasticity of a-red P.It is found that only the theoretical elasticity of the zigzag ladder structure is consistent with the experimental elasticity.These results indicate that a-red P exists as a long chain rather than a two-dimensional or three-dimensional structure.The zigzag ladder structure is the exact structure of a-red P.（2）It is confirmed that am-B adopts a rigid ladder-like linear inorganic polymer.Dissolution experiments show that am-B can be completely dissolved in absolute ethanol,which indicates that am-B is more likely to be a linear inorganic polymer instead of an insoluble 2D or 3D structure.Results from AFM-based SMFS and molecular weight measurements show that the molecular chain of am-B can approach to 1μm and the average molecular weight is about 110 k Da,indicating that am-B is a typical linear polymer.AFM imaging shows that am-B possesses a single-atom thickness,which can be used to exclude the possible structure with a larger thickness.In the rest of the structures,the theoretical elasticity of the linear ladder structure is consistent with the inherent single-chain elasticity of am-B measured experimentally.Through AC-TEM imaging,the structural information of an am-B chain in the horizontal direction is obtained.The results show that the width of an am-B chain and the length of the repeating unit in the chain are both consistent with the linear ladder structure.Therefore,both topological and molecular elastic studies strongly indicate that the linear ladder structure is the exact structure of am-B.（3）According to the covalent characteristics and electronic properties of a-red P and am-B,self-assembly behaviors of the two inorganic polymers in dilute solution are studied.It is found that the rigid structure of a-red P is helpful to maintain its conformational stability in solution,prompting a-red P to assembly withβ-CD in aqueous solution to form a rotaxane.Rotaxane can be further self-assembled into regular supramolecular structure.The driving force for assembly of rotaxanes is intermolecular hydrogen bonds.Without the participate of guest compounds,am-B chains can be self-assembled into ordered nanosheets in dilute solution after long time placement.The driving force for the self-assembly of am-B may be the strong inter-chain vd W forces determined by the electron deficiency of B atoms.（4）In this thesis,the influence of concentration and species of monovalent ion in aqueous solution on the single-chain elasticity of PPZ is studied.It is found that the Hofmeister effect is mainly determined by the surface charge density of anions.For common ions（ions that do not contain electron-deficient atoms）,the larger ion size will make the stronger chaotropic effect in aqueous solution.If the ion contains electron-deficient central atoms（such as Al and B atoms）,the chaotropic ability will more determined by the electron-deficient central atom.The more obvious the electron-deficient effect of the central atom will make the ions the stronger chaotropic ability.The chaotropic capacity of different anions is:Cl^-<SCN^-<<PF₆^-<Al F₄^-<BF₄^-.The latter three anions have not yet been included in the traditional Hofmeister sequence.Moreover,it is found that the maximum binding density of anions on an amphiphilic polymer chain is mainly determined by their size（the smaller the ion size,the tighter the arrangement,resulting in the more rigid molecular chain.In contrast,the larger the ion size,the sparser the arrangement on the chain,resulting in lower rigidity）.In addition,the study found that the classic covalent bonding model is not suitable for PPZ.