| The physical and chemical properties of polymer materials depend not only on their chemical composition,but also on the arrangement of molecular chains on hierarchical spatial scales.For crystalline polymers,scientists have developed a series of mature theories in terms of thermodynamics,crystallization kinetics,and crystal structure.As polymer science developed,traditional polymeric materials are not sufficient to meet the ever-growing needs of our modern society,and thus some new concepts,components and structures are emerging.As a new type of hybrid polymer,chain-type giant molecules generally refer to macromolecules with precisedefined structures obtained by precise synthesis of various rigid molecular nanoparticles as monomers.Owing to the different chemical composition of chain-type giant molecules from that of traditional polymers,its molecular arrangement in three-dimensional space would have many new features.This dissertation is focused on the crystallization behavior of side-chain giant molecules.Based on previous work,we developed a more efficient and precise synthesis strategy to prepare a series of homopolymer and block side-chain giant molecules using polyhedral oligomeric silsesquioxane（POSS）and polyoxometalate（POM）as building blocks.Utilizing this set of efficient synthesis strategy,we can accurately control the number,sequential order and hydrophilic functional groups of a series of side-chain giant molecules.Thus,the effects of these molecular structure parameters on the crystal structure and surface properties of the 2D assemblies are systematically studied.The specific research work is as follows:（1）Explore the crystallization behavior difference homopolymeric and block-type side-chain giant molecules,and the effect of chain length on the two-dimensional assembly behavior of sidechained giant molecules.Using isobutyl POSS（BPOSS）and vinyl POSS（VPOSS）as building blocks,by means of "deprotection-addition" strategy,homogeneous and block side-chain giant molecules were prepared to explore the difference in the crystallization behavior.The assembled structure was characterized by electron microscopy（TEM）and atomic force microscopy（AFM）.Subsequently,a series of block-type side-chain giant molecules Bn A（n = 1,2,3,4,5）（B stands for isobutyl POSS,A stands for carboxylic acid POSS）,Bn A2（n = 2,3,4）and（BA）n（n = 1,2,3）were designed and synthesized to explore the effect of molecular chain length on the crystallization behavior of giant molecules.By controlling the growth conditions of solvent volatilization,we have grown 2D nanocrystals specific cases,and their morphologies were characterized by TEM and AFM.AFM thickness measurements and small-angle X-ray scattering（SAXS）data show that the thickness of2 D nanosheets is strongly related to the number ratio of the two building blocks.The 2D nanocrystals thickness decreased with the increase in the number ratio of BPOSS/APOSS.In addition,selected area electron diffraction（SAED）and wide-angle X-ray diffraction（WAXD）data indicate,the structure of the 2D nanocrystals is sandwich-like,that is,the middle BPOSS bicrystalline layer is covered by the upper and lower amorphous APOSS/molecular backbone layers.Finally,a simple formula is deduced to calculate the overall thickness of the nanocrystals based on the geometrical model of the 2D nanocrystals,which proves the accuracy of the experimental conclusions.（2）Effect of chain sequence on 2D assembly behavior of side-chain giant molecules.Two groups of side chain giant molecules B2AB2,B3 AB,B4A and BBAA,BABA,BAAB,ABBA with controllable sequences were synthesized in a modular manner.The effects of structural primitive sequences on the 2D assembly behavior of giant molecules were systematically studied.TEM and AFM test results show that,the change of APOSS’s position along the chain couldn’t affect the assembly behavior of giant molecules and the thickness of 2D nanosheets.In addition,the structures of B2AB2,B3 AB and B4 A nanosheets are relatively regular.However,the structural regularity of BBAA,BABA,BAAB and ABBA nanosheets is low.Because the distribution density of APOSS in the hydrophilic layer affects the arrangement of BPOSS primitives.If the distribution density is large,the force between APOSS elements is also large,which will affect the arrangement of BPOSS in the crystalline layer to a certain extent,and then affect the shape of the 2D lamellar structure.Finally,the sandwich-like structure of 2D nano-sheet is proved again by SAED and WAXD measurement.（3）Effect of hydrophilic functional groups on 2D assembly behavior of side-chain giant molecules.POSS and POM molecular nanoparticles were used as building blocks.side-chain giant molecules B4 A,B4D（D stands for diol POSS）,B4N（N stands for amino POSS）and B4 POM with different hydrophilic functional groups were precisely synthesized.TEM data show that,whether the hydrophilic functional groups of side-chain giant molecules are cationic,anionic or non-ionic,they can successfully assemble into 2D nanosheets with regular shape.AFM thickness measurement and SAXS test data show that,the thickness of 2D nanosheets depends on the molecular weight and density of hydrophilic components.SAED and WAXD test results further prove that the two BPOSS crystal layers in the two-dimensional nanosheet structure are arranged in parallel on the plane of the crystal and covered by the upper and lower APOSS/backbone layers to form a sandwich-like structure.Regardless of the type of hydrophilic functional group,as long as the solvation effect between the hydrophilic component and the solvent is strong enough to overcome the interaction between the hydrophilic components,it can effectively prevent the accumulation of giant molecules along the normal direction of the crystal,thereby forming discrete two-dimensional nanocrystals.（4）Lateral multicomponent regulation of two-dimensional assembly of side-chain giant molecules.Applying the method of active crystallization-driven self-assembly,the side-chain giant molecules B4 A and B2 POM were induced to self-assemble into two-dimensional nanocrystals surrounded by B4A+B2POM in two different solvent systems.First,under appropriate conditions,B4 A molecules were induced to assemble into single component 2D nanosheet in THF/acetonitrile and acetone/water mixed solvents.Then,the high concentration B2 POM solution were carefully dropped into the 2D nanosheet seed solution of B4 A.B2POM molecules will take B4 A nanosheet as the crystal seed.Lateral epitaxial growth was carried out along the 2D nanosheet,and the peripheral structure of the nanosheet composed of B2 POM molecules was grown.We found that the surrounding structures formed in the two solvent systems were slightly different.The lateral dimension of B2 POM surrounding layer in THF acetonitrile system is larger than that in acetone water system,which is due to the different solvation effects. |
