C.M. Gapstur, H. Mahfuz
Florida Atlantic University,
Keywords: CNT, GNP, fracture toughness, hybrid nanocomposite, Triton X-100, vinyl-ester
Summary:In this paper, we compared the fracture toughness (KIC) and strain energy release rate (GIC) of three systems of hybrid nanocomposites by modifying surface characteristics of the carbon nanotubes (CNT) and graphene nanoplatelets (GNP) reinforcements. The polymer matrix was vinyl-ester (VE) resin. The surfactant used was Triton X-100 (TX-100). Two variations of each nanoparticle were implemented as follows: (1) as-received carboxyl (COOH) functionalized CNT and GNP, designated as f-CNT and f-GNP; (2) TX-100 surface treated f-CNT and f-GNP, designated as sf-CNT and sf-GNP. After several iterations with these reinforcements, an optimized concentration of 0.25wt% f-GNP and 0.5wt% f-CNT, that produced the highest KIC and GIC values, was finalized. Coupons for fracture toughness tests were then fabricated using the traditional synthesis procedures for nanocomposites. The following three systems were studied with the optimum concentration of GNP and CNT: System-1 with f-GNP and f-CNT; System-2 with sf-GNP and f-CNT; System-3 with sf-GNP and sf-CNT. KIC and GIC values were determined according to ASTM D5045-14. The single-edge-notch-bending testing configuration was employed with specimen dimensions as 112mm x 24mm x 12mm. Each specimen batch furnished two specimens, and a sharp notch, with an average length of 12mm, was induced in the center of one of the two specimens from each batch. The specimen with the notch was deployed in the fracture test, while the un-notched specimen was deployed in the indentation test. Specimens were then loaded in flexure mode utilizing a Zwick-Roell testing machine, and the Standard recommended ambient temperature of 23°C and crosshead rate of 10mm/min were applied. Load-displacement curves were produced for both the fracture and indentation tests, and a typical load-displacement curve is shown in Figure 1. The size criteria were satisfied, fracture test compliance was verified and indentation corrections were made according to ASTM D5045-14. KIC and GIC were calculated and validated per the Standard and are displayed in Table 1. Remarkable improvement in fracture toughness parameters were observed in both K1C and G1C with nanoparticles inclusion. KIC increased by as much as 46% from 1.14 (neat-VE) to 1.66 MPa*m½ for System-1. It was also determined that GIC increased by up to 97%, from 370 (neat-VE) to 730 J/m2 for System-3. This suggests that surface treatment was helpful for improving G values, while K values increased without any treatment. In an effort to investigate the source of improvement, differential-scanning-calorimetry (DSC), dynamic-mechanical-analysis (DMA) and thermogravimetric-analysis (TGA) were performed. Both DSC and DMA curves confirmed an increase of 6°C in glass transition temperature (Tg). Increased Tg implies restriction in chain mobility and enhancement in cross-linking density due to presence of nanoparticles. Networked nanoparticles nucleated with polymer chains and served as pinning points to restrict crack propagation, thus enhancing the fracture toughness. Thermogravimetric analysis (TGA) showed a slight increase in the maximum thermal decomposition temperature (Tp) from 410°C (neat-VE) to 414°C (Systems-1,2,3). This is also indicative of networked nanoparticles embedded within the VE matrix, which delayed complete evaporation. Fourier-transform-infrared-spectroscopy (FTIR) and field-emission-scanning-electron-microscopy (FESEM) studies will be performed and included in the paper.