Single Molecular Force Spectroscopy Study On The Nano-Mechanical Properties Of Polymer Single Crystals Of Zigzag Conformation

For rational design of semi-crystalline polymer-based materials and nano-structure,it is crucial to establish the relationship between the chain conformation in the crystal structure of a semi-crystalline polymer and the nanomechanical properties.Here we choose typical polymer single crystals of polyamide 66(PA66)and polyethylene(PE),which are the most popular engineering plastics and fibers containing zigzag chain conformation in their crystal phase,as the model system to investigate the singlemolecule mechanical properties by a combination of atomic force microscopy(AFM)imaging and single molecule force spectroscopy(SMFS).The chain movement during the force-induced melting was monitored at single molecule level and factors that affect such a movement has been revealed.In chapter 1,the polymer crystal model was firstly reviewed briefly and the main research methods for nano-mechanical properties of polymer crystals were introduced.In addition,the principle of AFM and AFM-based SMFS techniques were introduced systematically.At the end of this chapter,we reviewed the progress of the application of SMFS in polymer fields.In chapter 2,polyamide 66(PA66)and polyamide 6(PA6)single crystals were firstly prepared and characterized,and the unfolding process of PA66(or PA 6)single molecule from its single crystal was investigated.By using self-seeding method we prepared the PA66 and PA6 single crystals successfully.The long and narrow lamella crystals of PA66 and PA6 single crystals with flat surface were characterized by AFM imaging.The α crystalline phase was confirmed by X-ray diffraction(XRD).Typical sawtooth unfolding force-extension curve and profiles with multiple length-increment steps were obtained on PA single crystals by constant speed and constant force mode,respectively.In addition,the effects of the solvent polarity,loading rate,the spring constant of loading device and the length of amorphous chain between single crystals and the tip on the melting force have been investigated systematically.Our results show that:(1)The solvent with higher polarity greatly reduces the unfolding force of the single PA66 chain.(2)The unfolding force is independent on the loading rate.(3)Stiffer loading device can increase the unfolding force.(4)The shorter amorphous chain exhibit bigger effective spring constant Keffective,eventually leading to greater unfolding force.In chapter 3,we investigated the stick-slip motion at the single-molecule level in the hydrogen bonding network of polyamide(PA)single crystals through atomic force microscopy imaging and single-molecule force spectroscopy.Stick-slip is a ubiquitous motion in hydrogen bonding network,which confers the corresponding materials with excellent toughness and strength.The experimental study of the stickslip mechanism remains challenging because of the complexity of stress accumulation and release.Comprising a defined molecular structure and chain arrangement and strong intermolecular interactions,PA single crystal is an ideal system for the study of the stick-slip motion.Our results show that a stiffer force-loading device can enhance the possibility of hydrogen bonds rebinding by increasing the fracture force and facilitating stress release,as a result,more small sawtooth peaks appear.By comparing the sawtooth peak of PA66 and PA6,we confirm that the chain rotates while slipping,and the slip distance is dependent on the unit structure of the hydrogen bonding network.In chapter 4,we focused on the single molecule study of polyethylene(PE)single crystal,which has the zigzag chain conformation but without hydrogen bonding by using combined techniques of atomic force microscopy(AFM)imaging and AFMbased single-molecule force spectroscopy(SMFS).PE single crystals of four different thickness were prepared and investigated systematically.Our results show that the apparent mechanical stability of single PE chain significantly increased with increase of the thickness of PE single crystal.The stiffer loading device and shorter amorphous chain greatly enhanced the unfolding force of the single PE chain and promoted the formation of intermediates during unfolding process resulting from the increase of the number of small sawtooth peaks.In chapter 5,we made a detailed comparison of the single-molecule mechanical properties of PA66 and PE and explored the origin of the difference of their mechanical properties.The results show that due to the contribution of multiple hydrogen bonds the unfolding force of PA66 is significantly higher than PE.The stick-slip motion was observed in both samples during melting from their single crystals,and the stick-slip motion was promoted by stiffer loading device.These results deepen our understanding of the origin of mechanical properties of polymer single crystal of zigzag conformation and may be useful for tuning the mechanical response of corresponding polymer materials.