Effect Of Topology On Nanomechanical Properties Of Polymers

The topology of polymers have attracted extensive attention because it can endow polymer materials with unique properties and functions.As one of the key properties of polymer,mechanical properties can influence or even determine the application of polymer materials.Therefore,it is important to establish the relationship between the topological structure and the mechanical properties of the polymer.Relevant research is of great significance for the design and preparation of functional polymer materials.Traditional characterization methods such as bulk stretching can only provide the average information on the mechanical properties of polymers,including both intramolecular and intermolecular contributions,which makes it difficult to extract the influence of one factor.Exploring the relationship between the topology and the mechanical properties of the polymer at the single molecular level will help to establish the relationship between the topology and the nanomechanical properties of the polymer.The advent of atomic force microscopy（AFM）-based single-molecule force spectroscopy（SMFS）has made it possible to manipulate and mechanical probing single polymer chain.In this thesis,the nanomechanical properties of several polymers with different linear and cyclic topological structures were investigated by AFM-based SMFS,the results are as follows:1.Explore the higher order structure and nanomechanical properties of linear polyphenylacetylene block copolymers with different conformations.Circular dichroism（CD）experiments showed that poly（3,5-disubstituted phenylacetylene）with cis-cis and cis-trans conformations formed two different helical structures,contracted and extended,respectively.When the block helical structure was stretched by external force,two force plateaus with different heights appear on the force curve at about～55 p N and～100 p N,respectively,which show different mechanical stability.By stretching two corresponding polyphenylacetylene homopolymers,it was found that the contracted helix is more stable than the extended helix,and the elongation ratio of the contracted helix is larger than that of the extended helix.The experiment results provide an important reference for the regulation of topological structure and mechanical properties of linear polymers.2.The single-molecule mechanical properties of covalent nanogrid polymer（PG-Cz）based on fluorene and carbazole molecules were investigated.Two different types of stretching curves were obtained,one with sawtooth plateau and the other with single-peak shape（i.e.,the force monotonically increases with stretching）.By using linear polyfluorene as a contrast,only a single-peak force curve was obtained,indicating that the sawtooth plateau force signal in PG-Cz system comes from the grid topology.Combined with computer simulation,we attributed this difference in mechanical properties to the spatial orientation of four long-chain substituents in the grid structure and the different connection modes between adjacent grids.The sawtooth in the force curve originates from the length release due to the change of the grid shape induced by the force.Cyclic stretching–relaxation experiments showed that the polymer chain can still return to the original structure after multiple stretching-relaxation cycles,and the transition process is reversible.Dynamic force spectrum experiments showed that the force of this conformational transition increases with the increase of the pulling rate.By Bell-Evans model fitting and calculating,the rate constant k_offof the conformational transition process of the nanogrid was 11.68 s^-1,and the distance X_βfrom the ground state to the transition state was about 0.115 nm.As a comparison,the nanomechanical properties of polymer chains with smaller cyclic structures（polyspirothiopyran）composed of dynamic covalent bonds were further explored.Unlike the larger grid systems described above,we observed the opening of cyclic structures induced by external forces.A short“shoulder”plateau instead of a sawtooth plateau appeared in the force curve at 400 p N.This difference was attributed to the smaller cyclic structure and hidden length in the polyspirothiopyran system,resulting in a lower ring-opening elongation.The dynamic parameters of the ring-opening process were obtained by fitting the force curves with Bell-Evans model.The k_offwas 1.0×10^-6s^-1,and the distance X_βfrom the ground state to the transition state was approximately 0.21 nm.The very small k_offvalue(much smaller than 11.68 s^-1of the grid system)indicates that ring-opening reaction are more difficult to occur than grid conformational transition in the natural state.3.The nanomechanical properties of supramolecular polymer systems with cyclic structure which driven by coordination interaction have been systematically investigated.When stretched by external force,the coordination bond in the cyclic structure will rupture successively,accompanied by the release of hidden length in the cyclic structure,resulting in a sawtooth plateau force curve.Quantum chemical calculations result also confirmed the changing process of cyclic topology under external force.Through the cyclic stretching-relaxation experiments,we observed the re-formation of ruptured coordination bonds（i.e.,cyclic topology）in situ.Based on the fingerprint force signals of this polycyclic topology,we have accurately measured the binding strength of terpyridine with iron(Fe²⁺),which is about three times than that obtained by old methods.Through dynamic force spectroscopy experiments at different rates,the dissociation rate constant k_offof terpyridine with Fe²⁺was obtained to be 8.09×10^-3s^-1,and the ground state to transition state distance X_βwas approximately 0.066 nm.This dissociation rate constant is about four orders of magnitude higher than that of the dynamic covalent bond system above,indicating that the stability of the coordination bond based cyclic topology is weaker than that of the dynamic covalent bond system in the absence of external forces.In addition,it was found that the types of coordinated ions had a great influence on the stability of the cyclic structure,and the removal of coordinated metal ions could prevent the formation of the cyclic topology,and the force curve is monotonically rising with a single peak.The results will help to design and prepare high performance metallo-supramolecular polymer functional materials.