Single Molecule Force Spectroscopy Study On The Nano-Mechanical Properties Of Polymer Fiber Materials

Because of its good mechanical properties and chemical stability,fiber materials have become an indispensable material in modern life and have been widely used in a variety of fields,such as textile,environmental protection,construction.And fiber materials have attracted significant attentions of many researchers due to its superior mechanical properties.The excellent properties of fibrous materials come from their unique structures.So it is pretty important to study the relationship between fiber structure and mechanical properties from the molecular level.Which will be useful for the design of high performance fiber materials?In this thesis,nanomechanical properties of nylon 6 fiber and natural tussah silk fiber were investigated by using atomic force microscopy(AFM)-based single-molecule force spectroscopy(SMFS).The polymer chain,either isolated single polymer or polymer that reside on the fiber surface,was unfolded by external force that is perpendicular to the long axis of polymer segments.First of all,we choose the folding structure formed by single nylon-6 molecule as a model system.The nylon-6 was covalent linked in between the substrate and AFM tip.Our results demonstrated that the nylon-6 single molecule in water could form a folding structure,which could be unfold and refolded reversibly during our experiment.However,we found that in the hexadecane nylon 6 single chain cannot form a folding structure.And we also investigated the effect of different substrates on the formation of single molecule folding structures of nylon-6,and found that the folded structure of nylon-6 can only form at certain type of substrate.In the process of unfolding,the intramolecular hydrogen bond and the interaction between the substrate and the molecular chain(van der Waals or intermolecular hydrogen bond interaction)are destroyed at the same time.Considering the fact that nylon-6 resembles the protein in its backbone structure,which makes the study of folding and unfolding of nylon-6 single molecule also very helpful in understanding the mechanism of protein fibrosis.Then,we further studied nanomechanical properties of single nylon-6 molecule in its condensed-states(edge-on crystal,fiber).The study of condensed matter system is helpful to understand the relationship between the microstructure of nylon fibers and the mechanical properties of fiber materials.Our results show that the unfoding process in the condensed system is similar to that of the isolated single molecule system,where regular sawtooth peaks were observed and the value in the value will not drop to zero the force curves are.However,due to the existence of defective and amorphous phase in the condensed system,the molecular chains are entangled with each other,resulting in a greater value of the unfolding force.Finally,we investigated both the regenerated tussah silk protein and the natural tussah silk fiber comparatively.We found that the folding of tussah silk protein is similar to that of nylon-6.Therefore,we can speculate that the fiber material with similar chemical composition and structure will exhibit similar nanomechanical properties.