Molecular Simulation Study On Mechanical Behavior And Properties Of Pillared Graphene/Polylactic Acid Composites

The use of fully biodegradable plastics to replace traditional refractory plastics such as polystyrene and polypropylene has become one of the important ways to deal with the increasingly serious "white pollution" caused by the abuse of plastic products.Polylactic acid(PLA),as a fully biodegradable plastic in line with the carbon neutral concept,is an ideal material to replace common plastics,and has been vigorously developed and widely used in various fields.However,PLA has defects such as slow crystallization rate,poor mechanical properties,low heat resistance,and poor electrical properties,which limit its wide application.Researchers at home and abroad have found that the introduction of nanomaterials into PLA can solve these problems through a large number of studies.In order to make up for the defects of PLA in terms of electrical and thermodynamic properties,this paper takes PLA and pillared graphene(PG)nanostructures as the research objects,and uses PG as the reinforcement of PLA to construct PG/PLA composites.The mechanical properties,solubility parameters and cohesive energy density of PLA under different degrees of polymerization were analyzed by molecular dynamics simulation;the effects of different temperatures on the mechanical properties and system energy of pure PLA and PG/PLA composites;The glass transition temperature(Tg)of PLA and PG/PLA;the effects of different functional groups on the mechanical properties,Tg,etc.of PG/PLA composites.The main research contents are as follows:(1)Molecular models of PLA material systems were established with PLA single chains of different degrees of polymerization,and then the structural optimization and energy optimization of these models were carried out successively,and the dynamic equilibrium conditions were reached through molecular dynamics simulation,and then their mechanical properties were calculated,as well as solubility parameters,etc.are tested and calculated.Finally,the simulation results were compared with the experimental values,and it was found that the optimal degree of polymerization for simulating the properties of PLA was 30.(2)Design and build a reasonable PG structure,and then pure PLA system model and PG/PLA composite model are constructed according to the selected optimal polymerization degree of PLA.Finally,the mechanical properties of pure PLA material and PG/PLA composite material at different temperatures are calculated,and Tg of these two materials is predicted.The research results show that after the introduction of PG into PLA,the strength,stiffness and Tg of the composites at different temperatures are improved.At normal temperature and pressure,the Young’s modulus,shear modulus and the bulk elastic modulus was increased by 25.2%,10.6%and 4.4% compared with pure PLA material,respectively,while the Tg of PG/PLA composite was increased by 1.6% compared with PLA.(3)In order to prevent the PG structure from agglomerating in the PLA matrix,hydroxyl,amino and carboxyl groups were introduced into the PG model to functionalize it,and then the PG/PLA composite material models with different functional groups were established.The results show that compared with PG/PLA composite,the strength,stiffness and Tg of functionalized PG/PLA composite are significantly improved,and the carboxyl group has the best enhancement effect.The young’s modulus,shear modulus,bulk elastic modulus and Tg of carboxyl functionalized PG/PLA composites were increased by 26.2%,11.7%,10.7% and3.6%,respectively,compared with PG/PLA composites.In addition,the binding energy of functionalized PG/PLA composites is higher,and the PG structure with functional groups can further reduce the movement space of the PLA molecular chain and limit the movement of the PLA molecular chain,which has a stronger adsorption function to the PLA matrix.