Steered Molecular Dynamics Simulations Study On The Nano-Mechanical Properties Of Polymer Single Crystals

The study on the relationship between the polymer chain conformation in the single crystal and its mechanical properties is of crucial importance for the design/preparation of high performance polymer materials.In this thesis,by choosing several polymer single crystals as model systems,we investigated systematically the effect of polymer conformation within its single crystal on the force-induced melting process of a single polymer chain by pulling the corresponding polymer chain out of its crystal phase using steered molecular dynamics simulations.In the first chapter,some basic knowledge/introduction on polymer crystallization,molecular interactions and AFM-based single molecule force spectroscopy(SMFS)were introduced.In addition,some basic knowledge on steered molecular dynamic simulations as well as recent progress on the application of SMD/SMFS in the investigation of molecular interactions were reviewed briefly.From chapter two to chapter six,we performed SMD simulations on nano-mechanical properties of several typical polymer single crystals(PEO,PE,PA 66,PA 6 and PCL)in which polymer chains exist as different conformations(namely,helical,planar zigzag,hydrogen-bond-containing planar zigzag and non-planar zigzag)by pulling individual polymer chains out of their single crystals.In each chapter,the following constant-velocity-pulling SMD simulations were performed: 1)polymer chain of different lengths(with three folds(six segments)or single segment);2)stretching speed effect;3)the polymer loop effect;4)adjacent and non-adjacent folding.In PEO and PA 66 single crystal system,the effect of crystal thickness on the melting force was also investigated.In addition,the effect of the length of amorphous chain on the apparent melting force was investigated in PE,PA66,PA6 and PCL system.The main findings are as following:1.For helical PEO single crystal:(1)The melting force-extension curve of the polymer chain with three folds shows three main plateau-like peaks,each peak corresponds to the unfolding of one of the three folded structures.There are several small sawtooth peaks on each of the force plateau,and those small sawtooth peaks come from the movement of the PEO loop along the helical groove within the crystal phase.(2)The melting force increases with the increase of crystal thickness.(3)The size of polymer loop formed between adjacent PEO fragments does not affect the melting force,since during stretching the loop size gets decreased first before it is unfolded.(4)The peak value of the melting force during the stretching of a linear PEO chain(without folds)out of the crystal is half that of folded ones,which show the great contribution of the polymer loop to the melting force.(5)The non-adjacent folding mode can cause the increase of force fluctuation.2.For planar zigzag PE single crystal:(1)The unfolding of the PE chain with three folds produces three main force peaks,each peak corresponds to the unfolding of one folded structures.In addition there are many small sawtooth peaks on each of the three major force peaks.Such sawtooth peaks come from the change of free energy,which originates from the steric hindrance change during the relative movement of zigzag PE chains.(2)The size increment of the polymer loop does not cause the increase of melting force,and the size of the loops on the top surface remains unchanged before they are unfolded,which is different from PEO system.(3)The stretching curves obtained on polymer chain with three folds is similar to that in single linear chain systems,which indicates that the contribution of the polymer loop to the melting force of PE single crystal is very small.(4)The non-adjacent folding mode does not cause the increase of melting force.3.For PA66 and PA6 single crystals with planar zigzag conformation and multiple intermolecular hydrogen bonds:(1)The trends of the force-extension curves in the unfolding of PA66 and PA6 chains with three folds are similar to that in the PE system.However the rupture forces are much higher than that in PE system,which can be ascribed to the rupture and reformation of multiple hydrogen bonds.Due to the difference in chain composition(which affects the hydrogen bond formation)the fine structure of sawtooth pattern of PA66 is different from that of PA6.(2)Similar to PE system,the size of the polymer loops on the top surface remains unchanged before they are unfolded.And the size of polymer loop does not show obvious effect on the melting force.(3)In PA66 system,the melting force increases with the increase of the thickness of the single crystal.(4)The size of the polymer loop on the top surface remains unchanged before they are unfolded both in PA66 and PA6 systems.(5)In PA66 system,the spring constant used in SMD simulations can affect dramatically the chain movements and the apparent nanomechanical properties.4.For non-planar zigzag PCL single crystal:(1)The unfolding of the PCL chain with three folds produced three major peaks that are similar to PE.There are small sawtooth peaks on each major peak,but the fluctuations of these sawtooth peaks are smaller than that in PA system and bigger than that in PE system.The distance between adjacent small sawtooth peaks is equal to the length of one(or half)PCL repeat unit.(2)The peak value of melting force obtained on PCL chain with three folds are nearly two times of that obtained in single linear chain systems.This result indicates that the polymer loop contributes a lot to the mechanical properties in non-planar zigzag PCL single crystal.In the last chapter,we compare all types of force-extension curves obtained in all of the crystal systems we have studied in order to understanding the difference of the melting process in each system.Namely,(1)results obtained in PEO(helical),PE(planar zigzag)and PA66(planar zigzag plus hydrogen bonds)systems;(2)results obtained in PA66 and PA6 systems(different chain composition);(3)results obtained in PE and PCL systems(planar zigzag versus non-planar zigzag);(4)all the curves in PEO,PE,PA66,PA6 and PCL.Our results show that the force-extension curves of helical PEO chain show the smallest fluctuations compared with that of zigzag polymer chain in PE,PA66,PA6 and PCL systems.The amorphous polymer loop contribute a lot to the melting force in PEO and PCL system,while contribute very little to the mechanical properties of PE,PA66 and PA6.The inconsistency between the melting temperature and the melting force in several crystal systems indicates that the energy pathways for thermal and force-induced melting may be different.At last,the future research directions are suggested at the end of this chapter.