*3.1. Mechanical Properties*

The mechanical properties of hybrid film were evaluated by uniaxial tensile testing. The specimens were cut into a rectangle strip with 30 mm in length and 10 mm in width. Tests were conducted using a universal testing machine with 200 N load cell at a constant cross-head speed of 0.5 mm/min. To reveal the reproducibility of the results, three samples were fabricated and tested for each hybrid film. The experimental results reported in this work are the averaged values.

Figure 3 plots the typical stress–strain curves of the hybrid films with different GNP contents ranging from 0 wt.% to 50 wt.%. The mechanical properties including the Young's modulus, tensile strength and fracture strain can be extracted from the stress–strain curve. Table 1 lists the tensile strength and fracture strain of the hybrid films. Based on published literature, the mechanical properties of CNT buckypaper, tensile strength of 2–94 MPa, Young's modulus of 2.1 MPa to 3.84 GPa, and fracture strain of 0.3–2% have been reported [18]. Present results are within the range of typical MWCNT buckypaper. It can be observed that both the tensile strength and fracture strain are decreasing with the increase of the GNP content as shown in the inset of Figure 3. Tensile strength and fracture strain of GNP-0 (0 wt.% GNP and 100 wt.% MWCNT) are 17 MPa and 8.2%, respectively, which are 105% and 86% higher than that of GNP-50 (50 wt.% GNP and 50 wt.% MWCNT). These results can be

inferred from the SEM images that the MWCNT bundles exhibit not only strong Van der Waals and π–π interactions but also mechanical interlocking through entanglements and form a strong robust network [33]. However, the graphene sheets are mainly assembled by an in-plane contacting via Van der Waals forces without being strongly inter-connected. Thus, the tensile strength of the hybrid film is decreasing as the MWCNT content decreases.

**Figure 3.** Stress-strain curves of the hybrid films with different weight percentage of graphene nanoplatelets (GNP).


GNP-50 8.3 ± 0.9 4.4 ± 0.7

**Table 1.** Mechanical properties of the hybrid film with different weight percentage of graphene nanoplatelets (GNP).

The enlarged stress–strain curves as shown in Figure 4 can be divided into three stages. In stage I (strain ranging from 0 to 0.2%), the wavy MWCNTs are first straightened upon tensile loading, causing little change in stress with linear stress-strain relationship [17]. In stage II (strain ranging from 0.2 to 1.0%), the deformation happens under fairly law stress and the joints between MWCNTs and GNPs inside the film are stretched resulting in a higher elastic modulus which is analogized to the disentanglement of polymer chain [18]. In stage III (strain >1.0%), with further stretching of the hybrid film, interlock between GNPs and MWCNTs gradually fails and the network becomes loosely, a non-linear stress-strain relationship is observed. At the initial stage of loading, significant straightening took place both in the GNPs [34] and MWCNTs leading to alignment along the tensile direction, after which the curves became almost linear at higher strains. Elastic moduli of the hybrid films in stages I and II of the tensile testing are shown in the inset of Figure 4. The elastic modulus of the hybrid film in stage II is higher than that of stage I by approximately 200 MPa. It appears that GNPs were easier to be straightened due to the slippage of the overlapped GNPs; i.e., it is more flexible than MWCNTs which were interlocked with each other. The tensile strength

and Young's modulus of the hybrid film measured as a function of GNP content are shown in Figures 3 and 4, respectively. Both properties consistently increased with decreasing GNP content, indicating the dominance of MWCNT on mechanical properties of the hybrid film.

**Figure 4.** Enlarged stress –strain curve of MWCNT/GNP hybrid films.
