Single Molecule Force Spectroscopy Study On The Nanomechanical Properties Of Syndiotactic Polypropylene Single Crystals

Semicrystalline polymer materials,such as polypropylene(PP),are widely used in various fields of our daily life due to its high performance.The crystal phase of PP dominates many aspects of the mechanical properties of PP materials.With the increasing demand on further improving the performance of PP,it is very important to disclose and tune the structure and function relationship at single molecule level.Such researches are helpful for the understanding the nanoscale mechanism about the change of material performance,and can eventually guide the performance tunning,design and preparation of high performance PP materials.Using traditional method,it is pretty difficult to measure the single molecule mechanical properties.With the appearance of atomic force microscopy(AFM)-based single molecule force spectroscopy(SMFS),make it possible to correlate the structure and mechanical peroperties at single molecule level.In the first chapter,the basic principle of AFM imaging and AFM-based single-molecule force spectroscopy are introduced in details,including the analysis of a force curve,the criteria for single chain stretching and its important application in the field of materials science.The target of this thesis is mainly about the study of mechanical properties of syndiotactic polypropylene crystals with different microstructure by using AFM-based single-molecule force spectroscopy.In chapter 2,our work mainly focused on the preparation and characterization of syndiotactic polypropylene crystals with different microstructures.Experimental conditions have been optimized for the preparation of solution and melt phase crystals,respectively.Based on the results of atomic force microscopy(AFM)imaging,transmission electron microscopy(TEM),X-ray powder diffraction(XRD),electrondiffraction(ED),we know that the morphology and crystal structure of syndiotactic polypropylene single crystals.The results show that we got elongated rectangular lamellar single crystals of syndiotactic polypropylene(s PP)grown from the melt of thin films and narrow and elongated crystals from solution crystallization.The work in this chapter laid the basis for the study of the microscale mechanical properties.In chapter 3,we carried out systematic single molecule force spectroscopy study on s PP crystals.First,we optimized the coupling conditions for the attachment of s PP chain to the AFM tip by modifying AFM tip with-NH2 and the surface of syndiotactic polypropylene single crystal with-OH group,respectively.Then,we stretch individual s PP molecules out of the crystal phase and measure the mechanical properties quantitatively.Similar study was performed on different crystal phase,such as spherulite structure and mesomorphic form.The results show that different crystalline structures of syndiotactic polypropylene have different mechanical properties,and form(I)crystal shows higher nanomechanical stability compared with other crystalline forms.At the same time,the crystal thickness can affect the mechanical properties of syndiotactic polypropylene single crystals.The mechanical stabilities of s PP chains in different crystal planes are quite similar.The results above deepen our understanding on the relationship between s PP crystal structure and mechanical property,which lay the foundation for performance optimization/tunning of syndiotactic polypropylene-based polymer materials.