The Study Of The Nano-Mechanical Properties Of Polyethylene Oxide Single Crystal

The mechanism, thermodynamics, kinetics of polymercrystallization and melting are parts of the important contents of polymerphysics. At the same time, relevant research will be very useful indesigning/engineering new polymer-crystal-based materials. In the past,the molecular mechanism of polymer crystallization and melting can onlybe derived on the basis of the change of crystal structure andconformation during crystallization/melting. Nevertheless, themechanism is still not completely understood due to the relatively smallamount of clear-cut experimental verification at microscopic level. Theappearance of atomic force microscopy (AFM)-based single-moleculeforce spectroscopy (SMFS) makes it possible to investigate directly thepolymer folding/unfolding at single molecule level. In this treatise, thesingle chain folding/unfolding and inter-polymer interactions in the singlecrystal of polyethylene oxide have been studied by a good combination ofAFM imaging and AFM-based SMFS. In doing these, a new method thatcan be used to investigate the chain folding mode as well as themechanism of crystallization/melting of polymer single crystal has been developed. Our study extends the AFM-based SMFS to the investigationof polymer interactions in their condensed states. The main findings areas follows:1) By a combination of AFM imaging and SMFS, we have successfullyextracted a single polyethylene oxide chain from its single crystal anddirectly measured the interaction strength between folded polymerfragments to be around40pN (at a pulling speed of2μm/s). This resulthas been further confirmed by the steered molecular dynamics (SMD)simulations, repeatedly stretching-relaxation experiment, heightdependence and speed dependence of the plateau force. Our current studyextends the usefulness of SMFS to the investigation of polymerinteractions in their condensed states.2) The combined techniques of AFM-based SMFS and SMD have beenused to study the folding pattern of a polyethylene oxide (PEO) chain inits single crystal. Our results show that folding pattern of a PEO chain inthe crystal formed in dilute solution is highly regular and loops on thecrystal surface is relatively small, supporting the adjacent re-entry foldingmodel. While in the crystal obtained from the melt, the non-adjacent folding with large and irregular loops contributes to big force fluctuationsin the force-extension curves. In the spiral crystal, PEO stem incrystalline region tilted as a result of surface stresses and the lamellarbecame thicker in (spiral) twisting region. The established method can beused to study the folding mode of the polymer chain in other polymercrystal or even more complicated condensed states.3) The nature of force induced unfolding (melting) of a single PEOchain from its single crystal has been disclosed by systematicinvestigation of the effects of solvent, force loading rate, unfoldingpathway and the thickness of crystal on the nanomechanical properties ofindividual PEO chains. The results show that solvent-polymer interfacialenergy and the lattice energy of the crystal determine the mechanicalstability of the polymer crystal. The loading rate dependence of unfolding(melting) force indicates that the process happened in non-equilibriumstate. The combination of dynamic force spectroscopy and SMDsimulation revealed two unfolding energy landscapes, which correspondto the unbinding of van der waals bonds under uniform shear loading andone-by-one tear loading. To get a more detailed insight into the unbinding force, an analytical model that relates the solvophobic effect, entropicelasticity and lattice interaction to measurable force is introduced. Inparticular, the experimental results show a linear correlation between theinterfacial free energy and unbinding forces, in agreement with thepredictions of the analytical model.4) The in situ monitoring of the dynamic recrystallization process of asingle polymer chain has been realized successfully. The recrystallizationhas been investigated by using both the constant speed mode and constantforce mode (i.e., the force clamp mode) of SMFS as well as the singlemolecule analytical model. Two steps during the recrystallization wereidentified at single molecule level:1) the solvent-polymer interfacialenergy induced adsorption;2) the adjustment of the adsorbed polymerfragments and gradual crystallizaiton supporting the crystallizationmodel proposed by Strobl.5) Established the relationship between single molecule mechanicalproperties and mesoscopic mechanical properties. At mesoscopic level,PEO molecules are squeezed out of their single crystal by AFM tip. Thebreaking force strongly depends on the approaching speed, tip radius and polymer-solvent interfacial tension. In order to bridge mechanicalproperties of single-molecule with mesoscopic level, we quantify the freeenergy that PEO molecules were unfolded from its single crystal bydifferent unfolding modes (i.e., pulling and squeezing). The resultsshowed that the energy for pulling the identical number of PEO chainsfrom the crystal phase to the amorphous phase is about23.4%higherthan that by squeezing. The difference is mainly due to contributions fromthe partially crystallized states of the molecules that have been squeezedout.