The Study Of The Force-Induced Unfolding Of Polymer Chain Within The Single Crystal By Single Molecule Force Spectroscopy

The study of the deformation process and corresponding mechanism of crystalline polymer materials has become an important area of polymer physics.The corresponding results in this area would be beneficial to rationally design and develop high performance crystalline polymer materials.Researchers have known that the stability of polymer lamellae has a significant impact on the mechanical properties of polymer materials.However,due to the limitation of research methods,people used to study the structure and morphology changes of crystalline polymer materials at macroscopic or mesoscopic level.It is still unknown that how the molecular structure,the crystal thickness,the chain conformation and the external micro-environment affect the mechanical response of polymer lamellae.The single molecule force spectroscopy(SMFS)method makes it possible to directly study the force-induced unfolding of polymer lamellae at the single molecule level.In this PhD thesis,the AFM imaging and SMFS was combined to study the unfolding process and corresponding mechanism of different single polymer chains within single crystals.The PhD thesis mainly contains the following four parts:1.We reveals the nature of terminal group effect on the unfolding process of single chain within PEO single crystal.We used AFM-based SMFS method to study the force-induced unfolding of PEO chains with different terminal groups from the PEO single crystal.The results show that the terminal groups affect the unfolding process,especially the unfolding of a certain number of PEO stems.Through the loop in the amorphous region,the non-adjacent stems would be influenced by the terminal group during the unfolding.The location of terminal groups on the top or bottom surface would influence the force to unfold the chain within the single crystal.2.We study the effect of coupling groups on the force-induced unfolding of PEO chain within the single crystal,and finally tune the nano-mechanical response of PEO single crystals.We used several diisocyanates to separately react with PEO to prepare polymer chains with different coupling groups,and grow single crystals by dilute solution self-seeding method.Then we use SMFS method to study the unfolding process of polymer chain within the single crystal.The results show that the coupling groups affect a certain number of PEO stems during the unfolding process.Changing the spacer between neighboring coupling groups can effectively tune the proportion of PEO stem that is affected by the coupling groups during the unfolding process.Changing the size of coupling groups can effectively tune the force and energy to unfold the chain.A long amorphous PS block can influence the force to unfold the polymer chain within the single crystal.3.The relationship between chain conformation and single molecule mechanical properties of polymer single crystals has been established.We used SMFS method to study the force-induced unfolding of zigzag-conformation PCL in single crystals and helical-conformation PLLA in single crystals.Combining cell parameters and intra-chain group arrangement,we analyze the unfolding process of polymer chain within the single crystal.The zigzag-conformation PCL chain had strong intermolecular interactions which leads to stick-slip motion of the PCL chain during the unfolding,whereas the helical-conformation PLLA chain had weak intermolecular interactions which lead to smooth motion of PLLA chain during the unfolding.4.The air phase AFM-based single molecule force spectroscopy method has been established.The air phase SMFS method is used to study the force-induced unfolding of PEO chain from the single crystals.The results obtained in air is compared with that obtained in liquid phase SMFS method.We find that the external micro-environment has a significant effect on the chain unfolding process.The mechanical stability and the energy barrier of the PEO single crystal increase in air.Moreover,it is easy to extend the range of stretching rate and improve the force precision of SMFS method in air.