Molecular Dynamics Simulation On Mechanical Properties Equivalence And Self-assembled Morphology Of Polymeric Materials

Based on exploring the relationship between macro-properties and micro-structures of polymers and their nanocomposites,the equivalent law of macro-mechanical response of polymer system under a set of mechanical properties is studied by molecular dynamics simulation method in this work,in which the validity mechanism is explored from the micro-scale.Meanwhile,we systematically study the ordered self-assembly morphology of various multi-blocks copolymers and its ordered-phase transition behaviors.The main contents and innovations are as follows:(1)The investigation on the superposition principle and validity mechanism of polymers and their nanocomposites under various mechanical properties.By adopting the classic coarse-grained model,we focus on the three critical mechanical properties such as the stress relaxation,the storage modulus versus the frequency obtained from the dynamic periodic shear deformation,and the uniaxial tensile stress-strain,and emphasize that the Arrhenius types Time-temperature superposition principle(TTSP)should be guaranteed for different mechanical properties when the temperature is higher than the glass transition temperature(Tg).Importantly,the Williams-Landel-Ferry and Vogel-Fulcher-Tammann equations are both adopted to quantitatively analyze non-linear TTSP behavior when the temperature approaches Tg.On the other hand,we verify that the TTSP seems to be valid for the IPNs-DC system,while it does not hold for both PNCs and IPNs-SF systems.Furthermore,for PNCs filled with NPs grafted with polymer chains,the TTSP recovers back to be valid because of the enhanced compatibility between polymer and NPs attributed to the grafted polymer chains.This work confirms that TTSP can be applied to two-phase compatible polymer systems,and also provides the necessary theoretical basis for the preparation of high performance composites applied for wide temperature and time domain.(2)The investigation on the self-assembly morphology and its ordered-phase transition behavior of multi-blocks copolymers(MBCPs).Herein,by employing molecular dynamics simulation,we establish the phase diagram for the linear tetrablock copolymers(ABAB),exhibiting six typical ordered phases confirmed by snapshots,MSDs and RDFs.Second,we examine the branch-like,star-like and linear alternating copolymers and find that only the increasing branch density significantly leads to the occurrence of phase transition.Particularly,we illustrate that the configurations of the islands chains arrangement of MBCPs can be described in terms of tail,loop and bridge conformations,and present a unified theoretical framework to rationalize the topological state of the sphere phase and infer that the entanglements within the internal reaction layer between different A-blocks result in the inhomogeneous distribution of the island size even with the controlled molecular weight and composition ratio between each block.Finally,we find that the ABAB tetrablock copolymers filled with moderate spherical NPs exhibit a clear OOT from spheres to a complex morphology similar to the classic double gyroid or cylinder phases.The RDFs results illustrate that the maximum amount of the B-block within the second andior third layers for the filled spherical NPs connects different NPs effectively,leading to the complicated OOT behavior.Generally,this fundamental study could provide some guidelines for designing and fabricating high performance MBCPs by manipulating the formation of the ordered phases.