Studies on the Threading-Unthreading Process of Polymer Chains/Molecular Tubes and Its Single Molecular Force Spectra
|Course||Polymer Chemistry and Physics|
|Keywords||Atomic force microscopy Single molecular force spectra Flory-Huggins lattice model Self-consistent field theory Cyclodextrins Tobacco mosaic virus|
Recently,self-assembly of polymer chains has been widely studied and the threading-unthreading behaviors of the chains and the ring-like（or tube-like） molecules show extraordinary importance among these assembling behaviors.It is because studying this kind of behaviors could not only improve our understanding of the basic theories but also show widely applications in nano-materials.More importantly,since many basic processes of life actions belong to this kind of behaviors,e.g.the assembling of the tobacco mosaic virus protein and its RNA chain, DNA chains drilling through the pores on the karyon membrane,learning this kind of behaviors would help US to discover some secrets of the lives. The theoretical analysis,the computer simulation and the experiments have been presented in this paper to study the behaviors of the macromolecular chains threading into the ring-like or tube-like molecules.Moreover,attempt of using physical-chemical methods to explain the assembling of TMV proteins and RNA chains has also been tried.Otherwise,dynamic research of pattern formation in two phase system of nematic liquid crystal and hard particles has also been presented.The main contents of this paper are presented as following:（1）In section 2,a statistical thermodynamics theory based on the Fiery-Huggins lattice model about the threading-unthreading transition and the stacking of ring-like molecules has been presented,for example,the cyclodextrins（CDs）threaded by linear polymer chains.The volume of the ring-like molecules and the chains,the orientational entropy of the ring-like molecules and the configurational entropy of the chains are considered in the model.It is found that the ring-like molecules,such as CDs,cooperatively threaded by the linear polymer chains when the inclusion energy is strong enough.One important result is that the threading-unthreading equilibrium is independent of lengths of molecular nanotubes and linear polymer chains.By investigating the cases of with and without lateral stacking energy between the ring-like molecules,we found that the threading-unthreading equilibrium is also independent of the stacking energy.It is also found that the stacking energy between the ring-like molecules could drive them to stack together even without polymer chain. The distribution of the stacked tube length is also calculated in this section.（2）Polymer chains are required to join onto the tips in many of the AFM force spectrum experiments,such as estimating the interaction between antibody and antigen,studying the driving force of self-assembly,etc.The knowledge of the chains on the tip is very important for further researches,but generally it is hard to obtain the necessary information by normal methods because of the fewness of the chains.In section 3,a model based on the self-consistent mean field theory（SCMFT）has been developed to inspect the approaching process of the AFM tip grafted with a polymer chain and a method has been developed to inspect the state of the grafted chain on the atomic force microscopy（AFM）tip from the simple force spectrum experiment.In section 3,force curves and the chain distributions have been calculated on the condition of different grafting positions,different chain numbers and different chain lengths.The influences of solvents and the chain-substrate interaction have also been investigated by simulations.Real force spectroscopy of poly（ethylene glycol）（PEG） chains on the fresh mica jointed on AFM tip has been recorded,and several force curve types which correspond to the different states of the grafted chains were assorted.The simulations fit the experimental results very well,providing a strong support to the model.（3）In section 4,the unthreading process ofα-CDs molecular tube and poly（ethylene glycol）（PEG）has been investigated by single molecular force spectroscopy.The experiment was designed as follow:A few PEG chains have been jointed onto the AFM tip andα-CDs molecular tubes（α-CDMTs）have been physical adsorbed on the mica surface.When the tip approaches to allα-CDMT physisorbed on the substrate, the PEG chain with one end bonded on the tip tends to be threaded into the MT because the polymer chain is more like to stay in the hydrophobic environment than in the water.Then,as the tip moves away from the substrate,the PEG chain will be pulled out from the MT.The force on the tip exerted by the polymer chain is measured from the distortion of the cantilever,which gives the energetic information of the unthreading process.Theories based on Gaussian model and freely jointed chain（FJC）model are used to analyze the unthreading process and predicted a plateau in the force curve,which was observed in the experiment.Detailed analysis indicates that the inclusion energyεinclcould be calculated from the height of the plateau and the value is about 44.9±7.0KJ/mol·CD,which agrees very well with the value obtained from the thermo-chemical measurement.（4）In section 5,the TMV has been imaged both in air and solution by AFM.The hydrochloric acid and DMSO have been used to peel a part of the TMV protein shell and the peeled TMV morphologies have been presented by AFM in air.It is found that the TMV protein could be peeled by hysrochloric acid and DMSO and the adsorption force in acid（PH＜2.8）on blank mica is stronger than that in neutral condition on Mg2+modified mica.Otherwise,the computer simulation based on Langevin dynamics has been performed to study the pulling process of polyelectrolyte（PE） chain spirally twisted on the cylinder surface with charges.It is found that the helix PE chain on the charged cylinder surface will cooperatively uncoil first and then be pulled out of the tube during the pulling process.Although the rough model could not fit the TMV,it is valuable to discuss the complex of the charged chain and the cylinder because many systems in life could be reduced to this model.（5）In section 6,numerical results from the modeling and computer simulation（oy using Time-Dependent-Ginzburg-Landau[TDGL]equation）of the nematic liquid crystal/hard particles composites pattern formation have been presented.The polarizing light pattern from the computer simulation fit the experiment results well, and this gives supports to the model.Real space correlation function and structure factor have been calculated which obey dynamical scaling with an increasing by time t.The defects number also has been checked and it obeys dynamical scaling with a decreasing by time t but an increasing by particle number.The interaction of hard particles and liquid crystal will decrease the ordering rate of the order parameter,the grow rate of the domain size and the elimination rate of the defects.