Interfacial Properties And Mechanical Enhancement Between Functionalized Graphene And Polymer

Graphene has increasingly attracted attention as a promising candidate to take the position of carbon nanotubes in the reinforcement for polymers mechanical, electrical and thermal properties. Due to the difficulties to devise all kinds of experiments to study the interfacial characteristics of graphene-polymer composites, molecular dynamics?MD? simulations are becoming useful methods to investigate the interfacial reinforcement mechanisms of graphene-polymer composites.In this paper, the pull-out processes of the functionalized graphene sheet?FGS? from polyethylene?PE?, poly?methyl methacrylate??PMMA? and polyvinyl alcohol?PVA? matrix using MD simulation with ReaxFF reactive force field. The effect of functionalization on the interfacial properties for different polymer matrix systems and their interfacial enhancement mechanisms have been studied. The main contents show as follows:?1? The graphene/polymer composites with PE, PMMA and PVA matrix were prepared using the full atom model, where various functional groups, namely hydrogen?-H?, epoxy?-O-?, hydroxyl?-OH?, amino?-NH₂? and methyl?-CH₃? groups, were randomly attached to both sides of pristine graphene sheets through covalent bonding respectively. The 2.5, 5.0, 7.5, or 10%?called coverage degree below? of the carbon atoms on pristine graphene have been end-grafted by functional groups. A total of 55 sets of samples has been prepared in MD simulation. Meanwhile, the simulation method of pull-out process from the polymer matrix was determined.?2? The effect of functionalization on interfacial structure properties was investigated for PE, PMMA and PVA matrix systems and its mechanism was analyzed. The study shows that-CH₃ and-OH functional groups insert obviously into the polymer matrix, while-H and-O- functional groups basically stay outside the polymer matrix. The extent of insertion may be slightly affected by the distance of covalent bond grafted on graphene. The results also suggest that the aggregation of polymer exists on the graphene surface in all systems and the insertion of functional groups into polymer matrix impede the aggregation. More importantly, the effect of insertion is mainly dominated by the volume of functional groups.?3? The interfacial hydrogen bonds and interaction energy were investigated in equilibrated system. Meanwhile, the conditions of formation of hydrogen bond in the interface are discussed. The study shows that some functional polar groups on the FGS form hydrogen bonds with the PMMA and PVA polar polymer matrix. However, the hydrogen bonds were not found in non-polar PE matrix systems. The polar groups?-Oand-OH? on the FGS lead to an increase of interaction energy indicating a stronger bonding strength at the interface, while the non-polar groups?-CH₃ and-H groups? lead to a decrease of interaction energy indicating a weaker interfacial bonding strength. Beside, we found that the interaction energy of PVA matrix systems was much larger than that of PE and PMMA matrix systems under the same conditions.?4? The effect of functionalization on interfacial structure evolution and mechanical properties was investigated for different matrix systems and its mechanism was analyzed. The study shows that the movement of pulled-out chains is impeded by the functional groups on the graphene sheet. Both of the number of pull-out chains and interfacial shear strength increase with the increase of coverage degree. The real shearing surface will be transferred from the interface between the FGS and polymer to the matrix. The results suggest that the geometric constrains at the interface may be the principal contributor to the enhancement of interfacial shear strength. The study also shows that-OH and-CH₃ groups have an outstanding reinforcing effect on the interfacial shear strength no matter what the matrix system is. Beside, we find that the interfacial shear strength of PVA matrix systems is much larger than that of PE and PMMA matrix systems under the same conditions.This paper will help to understand the intrinsic microscopic mechanism of functionalized graphene reinforced polymer materials, and has a certain theoretical guiding significance for the preparation of new functionalized graphene/polymer materials.