Computer Simulation Study and Theoretical Design of the Structure and Mechanical Properties of Polymer/Grafted Nanoparticle Composites(PNCs)

Author:Shi Zuo

Supervisor:lv zhong yuan


Degree Year:2019





Polymer nanoparticle composites(PNCs)are materials consist of polymer and nanoparticles(NPs).In contrast with pure polymer materials,PNCs have much more advanced thermal,optical,electrical,mechanical properties etc.,hence PNCs have much more wider application areas nowadays.It is now well-known that the size,shape,interaction with matrix polymer and especially self-assembly morphologies of NPs play vital role in macro properties of PNCs.Therefore,understanding the mechanism of NP self-assembly and how the self-assemblies influence the macro properties of PNCs at molecular level has been become recurrence challenge in material science.In recent decades,as the computation force has been dramatically developed,molecular simulation methods play more and more important role in studying various phenomena at molecular level.Here in our present work,we investigate the self-assembly processes and dispersion states of NPs in PNCs and mechanical properties of PNCs consist of well-dispersed bi-modal tethered NPs in glassy state.Our work includes:1.Tethering polymer chains chemically identical to the polymer matrix onto NPs’ surfaces is a widely used method to acquire PNCs with desired dispersion state of NPs.In our study,we investigate the NPs self-assembly processes under conditions of various matrix polymer/tethered chain length ratios and grafting densities.By defining a self-assembly morphology measurement,we construct a detailed phase diagram in parameter space consists of matrix polymer/tethered chain length ratios and grafting densities.On the other hand,we studied self-assembly processes of bi-modal tethered NPs at given matrix polymer/short grafted chain length ratio and relatively low grafting densities of long grafted chains,we construct detailed phase diagrams in parameter space of matrix polymer/long grafted chains verses grafting density of short grafted chains.Afterwards,the mechanisms of miscibility of NPs are studied by the phase diagrams we constructed.2.It is widely accepted that adding nanoparticles(NPs)into polymer matrices can dramatically alter the mechanical properties of the material,and that the properties at the NP/polymer interface play a vital role.By performing coarse-grained molecular dynamics simulations,we study the stress–strain behavior of polymer/NP composites(PNCs)in a glassy state under a triaxial tensile deformation,in which the NPs are well dispersed in the system via bimodal grafting.In this study,by tuning the nanoparticle/polymer interface via changing miscibility of short grafted chains in the polymer matrix,we find that if the short grafted chains are akin to the polymer matrix,the NPs are well shielded and the corresponding PNCs act similarly to the pure polymer in their stress-strain behavior.However,with decreasing of the miscibility of short grafted chains in polymer matrix,the NPs act as initiators of cavitation during tensile deformation,at strains pre-yielding,cavitation emerge through out whole box as the NPs are well-dispersed.This phenomenon helps to make the PNCs more ductile and enhance its toughness.3.We take a further step in tuning interfaces around NPs,we change the short grafted chains into short di-block copolymers(s DBC)with the outer block miscible in the matrix.By tuning the interactions between NPs’ surfaces and inner block of s DBC,we find that,the enthalpic effects of s DBC-matrix polymer are hardly effected.All our designed systems have identical mechanical response at yielding.However,in post-yielding regime,s DBC with weaker inner block-NP interactions are more loosely packed on the NPs’ surfaces and have higher mobility.The response of the s DBC corona are more intense with decreasing inner block-NP interactions.The highly mobile,less restricted s DBC coronas suppressed the cavitation at strain-softening regime at post-yielding,leads to higher toughness of materials.