Properties Of Linear Aromatic Polyamides Predicted Upon Molecular Simulation

As special high performance materials,aromatic polyamide fibers were concerned by all the society,and they also play an important role in the family of polyamides.But the computer simulation report about the performance of aromatic polyamide was rare so far.In this paper,molecular simulation methods were used to predict the thermal and mechanical properties of poly(p-phenylenediamine-alt-2,6-diformyl multiphenyl)and two sets of new linear aromatic amide polymers were designed.Group Contribution(GC)method and Molecular Dynamics(MD)simulation were used to simulate the glass transition temperatures of poly(p-phenylenediamine-alt-2,6-diformyl multiphenyl).The change of the density,specific volume,radius of gyration and energy interactions along with temperature were analyzed in the MD simulation.Results show that the free volume theory can explain the glass transition phenomenon of poly(p-phenylenediamine-alt-2,6-diformyl multiphenyl),and the change of the non-bond energy interactions with temperature is the essential reason.The temperature values obtained by the two methods are close to each other,and both of them are higher than 480 K,which indicate that these polymers are expected to be used as high temperature resistant polymeric materials.From poly(p-phenylenediamine-alt-2,6-diformyl tetraphenyl)to poly(p-phenylenediamine-alt-2,6-diformyl heptaphenyl),with increase of the number of benzene rings in the vertical direction on the side chain,the glass transition temperature was reduced 10 K,the main reason is with increase of the number of benzene rings,these polymers' space steric hindrance increase,furthermore,the intermolecular distances also increase,which maybe produce "internal plasticization" effect,thus leading to the decrease trend of these polymers' glass transition temperatures.Similarly,the molecular dynamics method model was used to predict the mechanical properties of poly(p-phenylenediamine-alt-2,6-diformyl multiphenyl).And the effect of side chain length on mechanical properties of linear polyamides was investigated.Simulation results show that with the increase of the number of benzene rings on the side chain,these polymers' bulk modulus,shear modulus and young's modulus exhibit an unconspicuously downward trend.But their density decrease with the increase of the number of benzene rings obviously,and the probable reason may be that with the increase of the number of benzene rings,these polymers' molecular space steric hindrance is enhanced,which affects the rigidity of these polymers.At the same time,molecular dynamics model was used to simulate the poisson's ratios of poly(p-phenylenediamine-alt-2,6-diformyl multiphenyl).Results show that if multiphenyl changes from tetraphenyl to hexaphenyl,the poisson's ratios are all positive,while poly(p-phenylenediamine-alt-2,6-diformyl heptaphenyl)has negative poisson's ratio in xoy plane.These result are inconsistent with results obtained by molecular mechanics model simulation in the past,indicating that if polymer molecules are arranged in different order in space,they can exhibit different structural properties.Furthermore,two sets of new linear aromatic amide polymers,i.e.poly(4,4'-diaminobiphenyl-alt-2,6-diformyl terophenyl)and poly(p-phenylenediamine-alt-2,6-diformyl hexatriyne),were designed.The glass transition temperatures of two polymers were predicted by group contribution(GC)method,both are higher than 540 K,which indicate that both polymers are expected to be used as high temperature resistant material.The two sets of aromatic amide polymer's bulk modulus,shear modulus,young's modulus and other mechanical parameters also were predicted.Results show that both linear aromatic amide polymers have an inverted-honeycomb like structure,which makes the polymers exhibit negative poisson's ratio in xoy plane.