Effect of epichlorohydrin-modified Kevlar nanofibres on thermodynamic properties of different rubbers: Experiment and molecular simulation

Abstract

"Poly(p-phenylene terephthalamide) was a high-performance polymer known by its trade name Kevlar［1］. Kevlar nanofibers (KNFs) had reinforcing effect on rubber due to its high strength, high modulus, and excellent thermal and chemical stability［2］. The poor interfacial adhesion between the Kevlar fibres and the matrix limited the overall performance of the composite to a large extent. Thus, epichlorohydrin was used to modify KNFs and investigate the reinforcing effect of epichlorohydrin-modified KNFs(m-KNFs)with different rubber-rubber matrices. With the development of computer technology and nanotechnology, molecular dynamics (MD) simulation was considered to be an effective method to obtain detailed information of interfacial interactions, and could explain the macroscopic performance differences of polymers through simulation calculations at the molecular level. Herewith we built 3 systems of m-KNFs/carboxy terminated nitrile rubber (XNBR), m-KNFs/styrene-butadiene rubber (SBR), and m-KNFs/SBR/XNBR and studied the enhancement mechanism of mechanical properties of m-KNFs on XNBR, SBR, and SBR/XNBR by combining experiment with MD simulation. The typical formulation of nanocomposites was rubber 100 phr (mass, similarly hereinafter), m-KNFs 5 phr, ZnO 2 phr, stearic acid 2.4 phr, accelerator CZ 2.2 phr, and sulfur 1.5 phr. In MD simulation, mean square displacement (MSD) was calculated through equation (1)［3］: ■(1) where ri(0) and ri(t) were the positions of particle i at the initial and t moment, respectively; N was the total number of atoms in the selected molecules of the system; was the average of squared atomic displacements. MSD of m-KNFs/XNBR, m-KNFs/SBR, and m-KNFs/SBR/XNBR nanocomposites were calculated using MD simulation and the results were shown in Fig 1. According to the definition, the decreasing MSD indica-ted that interfacial interactions limited the molecular mobility of the different structures in the interfacial layer, which indicated stronger interfacial interactions. It could be found that the MSD values of m-KNFs/XNBR system were much lower than those of m-KNFs/SBR and m-KNFs/SBR/XNBR systems. This phenomenon indicated that the stronger interaction between m-KNFs and XNBR matrix made the rubber chains of the composite system much more bounded. It could also be found from Fig 1 that compared with the pure rubber, m-KNFs/rubber nanocomposite had an increasing storage modulus (E′) and a decreasing loss factor (tan δ). For the m-KNFs/XNBR/SBR system, the XNBR/SBR blended rubber had two glass transition temperatures(Tg), located at -50.1 ℃ and -2.6 ℃, corresponding to the Tg of the SBR phase and the XNBR phase, which indicated that SBR and XNBR were incompatible. After the addition of m-KNFs, the Tg belonging to the SBR and XNBR phases were close to each other, which indicated that m-KNFs improved effectively the compatibility of SBR with XNBR."

Keywords

• epichlorohydrin， kevlar nanofibers， butadiene styrene rubber，carboxybutyronitrile rubber， molecular simulation，thermodynamic performance， mean square displacement，