Static Mechanical Properties And Dynamic Viscoelastic Responses Of High-performance Elastomers Via Molecular Dynamics Simulation

As strategic materials,elastomers are widely used in all aspects of people's daily life.With the continuous progress of society,it is very urgent to fabricate high-performance elastomers with high mechanical properties and long service life.The key to fabricating high-performance elastomers is to understand the molecular mechanisms behind the macroscopic properties or regulate the microstructure of the materials to further establish the relationship between the microstructure and the macroscopic properties of the elastomers and their nanocomposites.However,it is challenging to quantitatively characterize the microstructure of the materials at the molecular level by traditional experimental characterization methods due to the structural complexity of elastomers.Therefore,it makes the construction of the microstructuremacroscopic performance relationship meet the bottleneck.Computer simulation technology can accurately track the microstructural evolution in situ within materials and easily realize the control of single factor variables,which provides a bridge for the microstructure and macroscopic performance.In view of the above problems,we mainly use molecular dynamics(MD)simulation method to study the static mechanical properties,dynamic viscoelastic response and their molecular mechanism of elastomers.The main research contents are as follows:(1)The interpenetrating polymer networks(IPNs)are constructed by the integration of flexible polymer network and stiff polymer network.It is found that the tensile stress of the IPNs significantly exceeds the sum of that of the corresponding single flexible and stiff network.This indicates that the mechanical properties of the IPNs are not linear combination of the two components,but the remarkably synergistic effect.In addition,with increasing the rigidity of the stiff network,the mechanical strength of the IPNs shows a single peak curve,corresponding to around k=100.We attribute this to a much larger contribution of the non-bonded interaction energy and the expansion effect of the stiff chains.Furthermore,if the stiff polymer network is the first network,increasing the content of flexible chain can effectively improve the mechanical properties of the IPNs.While,when the first network is flexible network,an optimized concentration(around 60%molar ratio)of the stiff network occurs to achieve the best perfomance,which mainly affected by the total non-bond interaction energy and the total-bond energy.It implies that enthalpy plays the dominant role rather than entropy in affacting the mechanical properties of the IPNs.(2)The molecular mechanisms behind the Mullins effect and fatigue of elastomers under a long-term cyclic loading are studied.Firstly,the validation of our MD simulation method is confirmed by capturing the key characteristics of the Mullins effect,such as the occurrence of residual strain,stress softening after the first loading,the stability of stress-strain behavior after several cycles,and the recovery of stress-strain response at high temperature.Then the influence of the cross-linking density on the unfilled system is studied.For low cross-linking system,the Mullins effect results from the chain slippage and chain extension along the tensile direction,but with the increase of cross-linking density,the Mullins effect is mainly caused by covalent bond rupture.In addition,the bonds on the chain backbone undergo much greater damage and overcome more external forces compared to the cross-linking bonds between the chains.There is an optimal cross-linking density for the unfilled system to optimize both the mechanical strength and fatigue resistance,because the polymer chain with high mobility can dissipate more energy.Finally,the mechanical response of these corresponding polymer nanocomposites(PNCs)is discussed.The incorporation of nanoparticles(NPs)can prevent the fracture and fatigue to improve the service life of the materials.However,with the increase of the cycle,NPs become more and more aggregated that however can be increasingly improved by introducing the physical attractive interaction or interfacial chemical cross-linking between NPs and polymer chains.In particular,the system with physical attractive interaction can not cause bond rupture within 100-cycle loading,the Mullins effect of which is mainly attributed to the slippage of polymer chains on the surface of NPs.Covalent bond scission,however,occurs in the system with interfacial chemical crosslinking,which contributes significantly to the Mullins effect.Furthermore,most of these bond breakages occur on the chain backbone,followed by cross-linked bonds in the interface and then cross-linked bonds between chains.(3)A new kind of all-polymer nanocomposites is constructed by introducing SCNPs into the matrix chains.The uniaxial tensile tests reveal that the PNCs filled with SCNPs have a better mechanical strength than the unfilled system.In the all-polymer nanocomposites,the mechanical strength initially increases and then decreases as the intramolecular cross-linking ratio of SCNPs increases,and an optimal cross-linking ratio(around 30%)occurs to achieve the best mechanical strength.The underlying reasons are that at a higher crosslinking ratio of SCNPs,the extent of interpenetration between SCNPs and matrix chains decreases,and the internal restriction of the cross-linking bonds in the SCNPs increases,accompanied by the weaker interfacial bonding and the lower bond orientation of the SCNPs.The results of the triaxial tensile deformation indicate that the mechanical toughness of the all-polymer nanocomposites is superior to the unfilled system and the conventional PNCs filled with rigid NPs,especially under the stronger interfacial interaction,owing to the soft characteristic of the SCNPs and the internal bond rupture of the crosslinked SCNPs to resist the crack extension.Cyclic tensile tests reveal that the all-polymer nanocomposites have the lowest fractional hysteresis loss(FHL)than those of the unfilled system and conventional PNCs.Furthermore,by implementing the dynamic shear tests,we find that the all-polymer nanocomposites show unremarkable non-linear behavior(Payne effect)and lower energy loss.At the same time,the all-polymer nanocomposites have lower FHL than the traditional PNCs at higher interface strength.Therefore,the all-polymer nanocomposites exhibit their unique advantages in balancing the mechanical strength,mechanical toughness and dynamic hysteresis loss simultaneously,which becomes more prominent when the interfacial adhesion is stronger.Our work provides a novel route to reconcile the conflicts of the toughness-strength as well as the toughness-hysteresis to fulfill the versatility of materials such as elastomers and gels with their great potential applications in flexible robots,actuators and sensors.