TPeak, temperature at the maximum h

TPeak, temperature at the maximum heat release rate.According to previous reports [41,42], the mechanism of graphene in reducing the flammability of polymers was that it acted as “char reinforcers” in the condensed phase, so the char residues of pure PVA and its composite films were analyzed in detail. Fig. 12 shows the micromorphology of the char residues of pure PVA and PVA/DGNS-2.0 after combustion. As for the char layer of PVA, there are a lot of large holes on the surface. The incorporation of DGNS leads to a marked improvement in the compactness of char layer. This more compact char layer can impede the transfer of heat and gaseous products during combustion process and protect the underlying matrix from further burning.Fig. 12Download : Download high-res image (1MB)Download : Download full-size imageFig. 12. SEM micrographs of char residues for pure PVA (a, b) and PVA/DGNS-2.0 (c, d) at different magnifications.Furthermore, Raman spectra could be adopted to characterize graphitic degree of char residues and the corresponding results are shown in Fig. 13. The Raman spectra of the two samples exhibit two prominent peaks at about 1350 and 1590 cm−1. The former peak (called the D peak) is ascribed to the disordered or glassy carbon, while the latter (called the G peak) is assigned to the graphitized carbon. In general, the intensity ratio of D to G peaks (ID/IG) is employed to evaluate the graphitic degree of the char layer. A lower ratio of ID/IG corresponds to higher organized graphitic structure [43]. The ID/IG value of PVA/DGNS-2.0 (2.00) is significantly lower than that of pure PVA (3.18), suggesting that the presence of DGNS markedly improves the organized graphitic structure in the residual char. The char with higher organized graphitic structure performs a better barrier effect against the mass and heat transfer and enhances the fire resistance of the PVA composites during combustion process.Fig. 13Download : Download high-res image (301KB)Download : Download full-size imageFig. 13. Raman spectra of char residues for pure PVA (a) and PVA/DGNS-2.0 (b).EDX analysis was further conducted to obtain the detailed elemental information of char residues and the related results are presented in Fig. 14. The C/O atomic ratio of PVA/DGNS-2.0 (0.52) is dramatically higher than that of pure PVA (0.32), suggesting that DGNS can effectively facilitate the formation of high graphitized char. The high graphitized char layer has high thermal oxidative resistance, which can retard the heat and mass transfer between the fire and matrix, enhancing the flame-retardant properties of the materials.Fig. 14Download : Download high-res image (395KB)Download : Download full-size imageFig. 14. EDX results at the surface of the char residues of (a) pure PVA and (b) PVA/DGNS-2.0.4. ConclusionsIn this work, DNS86-assisted electrochemical exfoliation of graphite was used to synthesize the water-dispersible graphene nanosheets. XPS spectra revealed that DNS86 was successfully adsorbed on the surface of graphene nanosheets. HRTEM and AFM evinced that the exfoliated DGNS were mainly composed of 3 layers. Subsequently, the water-dispersible graphene nanosheets were well-dispersed within PVA matrix and formed strong interfacial adhesion due to H-bonding between fillers and polymer molecular chains. Incorporation of 0.3 wt% DGNS into PVA matrix led to a remarkable improvement in tensile strength by 50.1%. A 50.4% increase in tensile modulus was obtained with adding 2 wt% DGNS. In addition, DGNS had good flame retardance to PVA material. The PVA/DGNS-2.0 showed 60.9% reduction in PHRR and 7.7% reduction in THR, in contrast to pure PVA. By the analysis of the char residue of the PVA/DGNS composites, the presence of graphene could perform a compact barrier char layer, protecting the underlying matrix during combustion.