3.1 Characterization of GO, G-Ag, RGO and GNP
3.1.1 Structural characterization of graphene nanomaterial by FTIR, XRD and Raman analysis
Figure 2(a) depicts the FTIR spectra of synthesized graphene oxide (GO), reduced graphene oxide (RGO), Ag nanoparticle decorated reduced graphene oxide (G-Ag) and graphene nanoplatelets (GNP). The FTIR spectrum of GO exhibits a broad peak appeared at around 3398 cm− 1 which is attributed to the stretching vibration mode of –OH groups present on the adsorbed water on GO surface. The other peaks that appeared at 1720, 1618, 1365, 1248 and 1112 cm− 1 are accredited to the C = O (carboxylic acid) stretching vibration, the vibration of unoxidized sp2 graphitic domain, stretching vibration mode of C-O (epoxy) groups, stretching vibration mode of C-O-C and C-OH (alkoxy) bond stretching vibration, respectively [28]. The intensity of these above peaks of oxygen functional groups decreases, especially for C = O and –OH groups in the FTIR spectrum of G-Ag which strongly suggests the partial reduction of GO by DMF solvent. Besides this, a new peak appears at around 1575 cm− 1 owing to the vibration of C = C bonds in the reduced graphene oxide nanosheets. In the wavenumber range of 1400 − 1000 cm− 1, the peaks related to oxygen functional groups are still present although their intensities decrease [29]. IR spectrum of RGO exhibits two peaks that appear at around 1150 and 1560 cm− 1 owing to vibration of C-O-H and C = C bonds, respectively which indicate the effective reduction of GO to RGO by the chemical process. These two peaks also appear at around 1155 and 1535 cm− 1 in the FTIR spectrum of GNP [30, 31]. No other significant peak is observed in the IR spectrum of GNP.
The XRD patterns of GO, G-Ag, RGO and GNP are depicted in Fig. 2(b). Pristine graphite powder exhibits a strong peak at 2θ = 27.4° which is attributed to (002) plane with interlayer spacing (d) of 0.33 nm in the XRD pattern [32]. After strong oxidation and exfoliation into GO, this peak disappears in the XRD pattern and an intense peak is observed at 11.23° corresponding to the (001) plane with a d-spacing of 0.81 nm. This increase in d-spacing between consecutive basal planes after oxidation is mainly due to the incorporation of oxygen functional groups and water molecules into the carbon structure [33]. The as-synthesized G-Ag nanomaterial shows four intense peaks which are appeared at 2θ = 38.05°, 44.18°, 64.35° and 77.28° corresponding to (111), (200), (220) and (311) planes of face centered cubic (FCC) silver nanoparticles (JCPDS No. 04-0783). This confirms the presence of silver nanoparticles on the graphene surface. Instead of 11.23° peak for the (001) plane, a broad peak is observed in the range of 2θ = 20–30° assigned to the (002) plane, which is attributed to the restored graphitic moiety after the reduction of GO [29]. XRD pattern of RGO shows an intense and broad peak which appear at 2θ = 26.38° owing to the (002) plane with a d-pacing of 0.39 nm which is very consistent with the reported result [34]. The XRD pattern of received GNP exhibits two peaks at 26.68° and 54.7° corresponding to (002) and (004) planes. The sharp main peak observed at 26.68° with a d-spacing of 0.34 nm is assigned to the crystalline (002) carbon plane [35].
Figure 2(c) compares the Raman spectra of GO, G-Ag, RGO and GNP, all of which have two distinctive peaks. The D and G bands in the Raman spectrum of GO appear at 1360 cm− 1 and 1572 cm− 1 owing to the sp2 carbon domains and lattice disorder, respectively [29, 32]. First order scattering of E2g phonons causes this G band to appear near 1590 cm− 1 in the Raman spectra of G-Ag, whereas the breathing mode of k-point photons of A1g symmetry is responsible for the D band at 1330 cm− 1 region [29]. ID/IG ratio increases from 0.95 to 1.06 when GO was converted to G-Ag. This increment in the intensity ratio of two bands gives a strong indication of GO reduction and the emergence of new, isolated and smaller graphitic domains. The ID/IG ratio calculated for RGO is 1.46, which gives a strong indication of reduction of GO to RGO by the chemical process. These D and G bands are observed at around 1334 cm− 1 and 1573 cm− 1 in the Raman spectra of received GNP and the ID/IG ratio becomes very small (0.23) compared to GO and RGO [36].
3.1.2 XPS analysis
Graphitic materials' elemental composition and corresponding chemical state have been studied using X-ray photoelectron spectroscopy (XPS) and the results are also shown in Fig. 2. Figure 2(d) depicts the XPS survey spectra of all graphene materials in which the highest measured binding energies for C1s and O1s peaks are 284.5 eV and 531.0 eV, respectively for synthesized graphene oxide [37]. The wide scan XPS spectrum of G-Ag exhibits an extra peak associated with Ag3d. The C1s peak intensity becomes dominant in G-Ag, while the strength of the O1s peak is considerably reduced, indicating successful GO reduction [38]. Also, the presence of Ag 3d peaks in survey spectra demonstrates effective reduction and anchoring of Ag nanoparticles on RGO nanosheets. The C/O ratio of GO rises from 2.13 to 4.39 in G-Ag, suggesting that the oxygen functional groups of GO get reduced by DMF solvent [29]. This C/O ratio also becomes very higher when GO has been chemically reduced by NaBH4 to RGO whereas GNP has the highest C/O ratio among different graphene materials used here [39].
The high-resolution C1s spectra of GO (Fig. 2(b)) show four peaks centered at 284.6 eV (C = C/C-C), 286.2 eV (C-OH), 287.6 eV (C = O), and 288.8 eV (O = C-OH) which reveal the existence of several functionalities on the GO sheets such as hydroxyl, epoxide, carbonyl and carboxylic groups [37]. The intensity of the C-OH signal is significantly reduced in G-Ag as shown in Fig. 2(f), while the sp2 carbon peak increases, indicating that the epoxy group aids in the anchoring of silver nanoparticles on RGO sheets and the repair of the sp2 carbon networks. This observation points to GO deoxygenation upon reduction with DMF at 85°C. The presence of the Ag 3d doublet for Ag NPs is shown in Fig. 2(g), where two bands at 368.5 and 374.5 eV are attributed to the binding energies of the respective Ag 3d5/2 and Ag 3d3/2 electronic states. Again, there is a 6.0 eV difference between the doublets of the Ag3d electronic state, indicating that Ag0 is present in G-Ag nanomaterial [29]. Figure 2(h) depicts the high-resolution C1s spectra of chemically reduced RGO nanosheets in which the three peaks owing to C-OH, C = O and O = C-OH groups get reduced strongly after the chemical reduction of GO which indicates the successful reduction of oxygen functional groups onto RGO surface [39]. The C1s spectra of GNP are shown in Fig. 2(i) where the intensity of the peaks related to oxygen functionalities is very less compared to GO, G-Ag and also RGO [40]. The elemental compositions and their atomic percentage data of these graphene nanostructured materials obtained from XPS analysis are summarized in Table 1.
Table 1
Elemental composition of graphene nanomaterials obtained from XPS measurements
Graphene nanofiller | Elemental composition (atomic %) |
C1s | O1s | Ag3d |
GO | 67.31 | 32.69 | |
G-Ag | 69.57 | 18.36 | 12.07 |
RGO | 82.96 | 17.04 | |
GNP | 95.74 | 4.26 | |
3.1.3 Morphological analysis
FESEM and TEM techniques were employed to analyze the morphology of the synthesized GO, G-Ag and RGO nanostructured materials and as-received GNP and are shown in Fig. 3. From the FESEM image of GO, as shown in Fig. 3(a), it can be seen that the GO sheets have lengths in the micrometer range and are created by the unavoidable fracturing of graphite flakes during the oxidation process. It is also observed that the GO sheets overlap with each other while maintaining their spacing as oxygen functional groups are introduced in between them. The sheets have some wrinkles on them, which could be attributed to the strong oxidative treatment during preparation process [41]. Figure 3(b) shows the Ag nanoparticles decorated RGO sheet in which tiny nanoparticles are produced on the surface of reduced graphene sheets, and that their size and dispersion are nearly uniform throughout the nanosheets' surface. The FESEM image of chemically reduced graphene oxide (RGO) sheet of micrometer length is presented in Fig. 3(c), while the SEM image of graphene nanoplatelet (GNP) is shown in Fig. 3(d). Reduced graphene oxide sheet and platelet-like morphology of GNP are very clear from these figures.
Figure 3(e) and (f) display TEM micrographs of synthesized GO and G-Ag. The TEM image of GO displays plate-like nanosheets stacked on top of each other, whereas G-Ag show decorated Ag nanoparticles (NPs) on the surface of RGO. The size of the decorated AgNPs on the nanosheet surface was calculated using Image J software from TEM images, and it was observed that the size of the nanoparticles ranges in between 2.0 nm to 14.0 nm, with an average diameter of ~ 6.0 nm, as shown in Fig. 3(i). Figure 3(j) illustrates the lattice fringe space of a single Ag nanoparticle, which was likewise calculated to be 0.24 nm. Four distinct concentric circular rings, which correspond to the crystal planes ((111), (200), (220) and (311)) of FCC silver, can be seen in the SAED pattern of G-Ag (Fig. 3(k)) [38]. This outcome supports the formation of spherical crystalline AgNPs on the reduced graphene oxide surface. Figure 3(g) shows the TEM image of RGO where graphene oxide sheets are effectively reduced by sodium borohydride. TEM image of GNP is clearly depicted in Fig. 3(h) in which graphene sheets in the shape of platelets are layered on top of one another.
UV-visible spectra of prepared GO and G-Ag were recorded to study the creation of Ag nanoparticles on RGO surfaces and also the reduction process of GO by DMF. From Fig. 3(l), we can see that UV-visible absorption spectra of GO have two characteristic peaks are appeared at around 225 nm and 294 nm attributed to the π-π* transition of aromatic C = C bond and n-π* transition of C = O bond, respectively [42]. However, this peak at 225 nm disappears and a broad peak at around 268 nm is exhibited by AgNPs decorated RGO (G-Ag) in the UV-visible spectra. This gives a strong indication of the successful reduction of GO sheets by DMF at 85°C. Further, a new broad peak at 424 nm, corresponding to the synthesis of AgNPs, has developed in the UV-visible spectra of G-Ag nanomaterials. The generated AgNPs' surface plasma resonance (SPR) band is responsible for this peak [29]. Here, a single UV-visible peak corresponding to silver nanoparticles indicates the synthesis of entirely spherical Ag nanoparticles over the RGO surface.
3.1.4 Antibacterial activity of G-Ag
The antibacterial property of the prepared Ag nanoparticle decorated reduced graphene oxide (G-Ag) was determined by measuring zone of inhibition (ZOI) against both gram-positive strain (E. Coli) and gram-negative strain (B. Subtilis). Figure 4 depicts the digital images of auger plate containing Ag nanoparticle decorated reduced graphene oxide. The calculated ZOI are ~ 1.9 cm and ~ 2.3 cm against E. Coli and B. Subtilis, respectively. Because Ag nanoparticles are greatly attracted by the DNA (containing sulphur and phosphorous groups) of bacterial cells, the silver nanoparticles on the graphene surface interact with the bacterial cell wall and form channels or pores. The formation of Ag + from Ag° starts the creation of reactive oxygen species (ROS). The massive creation of ROS species permeated the cytoplasmic fluid of the bacterial cell wall, killing the respiratory system of the bacterial cell [43].
3.2 Characterization of LLDPE/graphene nanocomposites
3.2.1 Morphological analysis
An important consideration for investigating the mechanical and barrier properties of polyethylene/graphene nanocomposites is the level of dispersion of graphene nanoparticles inside the polymer matrix [44]. The FESEM images of the cryo-fractured surface of LLDPE composites loaded with G-Ag, RGO and GNP nanofillers are depicted in Fig. 5(a-f). Figure 5(a) and (d) show the FESEM images of LLDPE/G-Ag nanocomposites with G-Ag concentration of 3 and 5 wt% of LLDPE. It can be observed that G-Ag sheets are uniformly dispersed in the thermoplastic matrix for lower G-Ag loading (3 wt%), but for 5 wt% of G-Ag concentration some agglomerated structures can be found in the LLDPE matrix. The inset image in Fig. 5(a) clearly shows the Ag nanoparticles formed on reduced graphene oxide surface. The dispersion state of RGO in the thermoplastic matrix is depicted in Fig. 5(b) and (e). Here also, RGO nanosheets are uniformly dispersed in the thermoplastic matrix for both 3 and 5 wt% but filler distribution is better for the former case. Because graphene has a large surface area, it may create strong interfacial bonds with polymer macro-chains facilitating uniform filler distribution. Figure 5(c) and (f) show the GNP dispersion in the LLDPE matrix, where it can be seen that graphene nanoplatelets are uniformly dispersed throughout the whole polymer matrix. The alignment of the filler particles and the dispersion of the graphene platelets are strongly anisotropic, as shown by both of these figures. GNP's exceptional diameter to thickness ratio allows for more uniform dispersion of these nanofillers in the polymer matrix. Finally, FESEM analysis shows that GNP nanofillers are more consistently distributed in the LLDPE matrix than G-Ag and RGO. Figure 5(g-i) depicts the TEM micrographs of graphene-filled LLDPE composites contained with 3 wt% of graphene concentration. The TEM image of LLDPE/G-Ag nanocomposite is depicted in Fig. 5(g) in which the black spots represent the Ag nanoparticles that are formed on the surface of RGO sheets. Figure 5(h) shows the RGO dispersion in the thermoplastic matrix, whereas the uniform dispersion of GNPs in the thermoplastic matrix is clearly shown in Fig. 5(i).
3.2.2 Mechanical properties
It is anticipated that nanocomposites will have improved mechanical characteristics. Here, the criteria include effective filler particle dispersion and their adherence to the base polymer [12]. Figure 6(a) represents the variation of tensile strength of LLDPE/graphene composites as a function of graphene loading. It is noticed that the tensile strength of net LLDPE is 15.3 MPa. The incorporation of 1 to 5 wt% graphene enhances the tensile strength of LLDPE thermoplastic nanocomposites in all three cases. The maximum enhancements in tensile strength of thermoplastic composites are 22.34, 25.6 and 30.9% for 3 wt percent of G-Ag, RGO and GNP loading, respectively. RGO nanosheets generate composites with a better tensile strength than forG-Ag owing to the better distribution of RGO sheets than G-Ag sheets in the LLDPE matrix. However, tensile strength is always higher for GNP reinforced LLDPE thermoplastic nanocomposites than for other types of graphene nanocomposites. LGNP3 exhibits the highest tensile strength (19.9 MPa) among all nanocomposites. As morphological analysis reveals that due to their especial diameter to thickness ratio, GNPs are distributed and dispersed more uniformly in the LLDPE matrix phase compared to RGO and G-Ag. Therefore, tensile strength gets improved higher than in LRGO and LG-Ag nanocomposites. The variation in tensile modulus of LLDPE/graphene nanocomposites with an increase in graphene concentration is depicted in Fig. 6(b). The modulus of all nanocomposites increases as graphene content increases. With a modulus of 208 MPa for unloaded LLDPE nanocomposite, the modulus rises to 272 MPa, 276 MPa and 292 MPa when filled with 5 wt% of G-Ag or RGO or GNP, respectively. The reinforcing effects of graphene nanofillers with high aspect ratio, which hinder the movement of linear low density polyethylene macro-chains, induce this improvement in modulus [45]. LGNP nanocomposites exhibit higher modulus values compared to LRGO and LG-Ag nanocomposites due to better reinforcement effect and uniform dispersion of GNPs in the thermoplastic matrix.
Figure 6(c) and (d) show the findings from measurements of nanoindentation. It is evident from Fig. 6(c) that neat LLDPE displays a hardness of 0.055 GPa. The hardness is shown to have significantly improved with the addition of graphene nanomaterials. The maximum hardness values obtained for LG-Ag, LRGO and LGNP nanocomposites are 0.074, 0.075 and 0.078 MPa, respectively with 5 wt% of each graphene content. The interconnected network formed by the nanofiller particles and the strong interfacial interaction between graphene sheets and LLDPE macro-chains may be the causes of the increase in hardness with rise in graphene loading. As a result of improved polymer-filler interaction and significantly stronger interfacial bonding between graphene nanosheets and thermoplastic macro-chains, incorporation of GNPs into the thermoplastic matrix causes a more pronounced increment in the hardness of the composite films compared to RGO and G-Ag. Figure 6(d) depicts the change in the plasticity index of graphene-loaded thermoplastic nanocomposites as a function of graphene concentrations. The figure shows that when graphene concentration increases, the plasticity index steadily drops. With the loading of GNP, this reduction is more pronounced. Neat LLDPE shows a plasticity index of 0.35 which is lowered to 0.25, 0.23 and 0.22 for 5 wt% of G-Ag, RGO and GNP loading, respectively in the polymer matrix. Plasticity or flowability declines when graphene nanofiller loading increases because graphene nanostructured materials impede the movement of LLDPE chains [46].
3.2.3 Thermal properties
Thermo-gravimetric analysis (TGA)
TGA tests were performed to investigate the effects of GNP, RGO and G-Ag loading on the thermal stability of LLDPE. Figure 7(a-c) represents the TGA thermographs (residual weight% versus temperature) of neat LLDPE and LLDPE/graphene nanocomposites whereas their DTG plots (first derivative of TGA plots) are depicted by inset figures. Neat LLDPE degrades at the lowest temperature, but the degradation temperature of LLDPE/graphene nanocomposites rises as the graphene loading increases. The existence of graphene in thermoplastic nanocomposites confers thermal stability, as evidenced by the gradual shift of the TGA curves to higher temperatures as the graphene content in all cases increased. Table 2 provides a summary of the thermal data acquired from TGA analysis. Heat resistance index for LLDPE and their corresponding nanocomposites was calculated by employing Eq. 2 [27].
Heat resistant index, \(HRI=0.49 ({T}_{5}+0.6 \left[{T}_{30}-{T}_{5}\right])\) (2)
Here \({T}_{5}\), \({T}_{30}\) and \({T}_{50}\)stand for the decomposition temperatures related to 5, 30 and 50% weight loss, respectively. From Table 2 it can be noticed that the decomposition temperatures of the pure LLDPE increase with the addition of all types of carbonaceous graphene nanofillers and continue to rise as the graphene loading is increased. The interfacial contact between graphene and polymer chains and good dispersion of these fillers reduces the mobility of polymer chains near carbonaceous nanofillers, hence facilitating a preferential barrier path that reduces the decomposition rate [47]. Thus, the decomposition temperature rises as filler loading increases. Increased HRI is also indicative of enhanced polymer-graphene interactions. Residual char increases with an increase in graphene concentrations compared to the neat LLDPE. The maximum decomposition temperatures (\({T}_{max}\)) obtained from the DTG plots of all nanocomposite samples are also provided in Table 2. The table demonstrates that the maximum decomposition temperature of the LLDPE/graphene nanocomposite increases with graphene concentration.
Table 2
Thermal data obtained from TGA analysis
Sample designation | \({T}_{5}\) (°C) | \({T}_{30}\)(°C) | \({T}_{50}\)(°C) | HRI (°C) | \({T}_{max}\)(°C) |
Neat LLDPE | 395.2 | 427.2 | 437.9 | 203.0 | 441.2 |
LGNP1 | 398.0 | 430.5 | 441.4 | 204.5 | 444.8 |
LGNP3 | 401.1 | 434.5 | 445.9 | 206.3 | 447.9 |
LGNP5 | 407.8 | 436.4 | 447.1 | 208.2 | 448.0 |
LG-Ag1 | 397.6 | 430.5 | 441.4 | 204.5 | 444.4 |
LG-Ag3 | 399.3 | 433.2 | 444.1 | 205.6 | 446.4 |
LG-Ag5 | 401.2 | 434.5 | 445.6 | 206.4 | 446.8 |
LRGO1 | 397.1 | 429.9 | 440.9 | 204.2 | 444.4 |
LRGO3 | 402.6 | 433.8 | 444.6 | 206.4 | 447.2 |
LRGO5 | 403.5 | 434.1 | 443.2 | 206.7 | 446.5 |
Differential scanning calorimetry (DSC)
DSC was employed to investigate the effects of graphene loading on crystallization behavior, glass transition temperature (\({T}_{g}\)) and crystallization melting point (\({T}_{m}\)) of the LLDPE thermoplastic. Figure 7(d-f) depicts the DSC 2nd heating cycles for all types of nanocomposites whereas corresponding cooling cycles are represented in Fig. 7(g-i). Percentage crystallinity and glass transition temperatures and melting temperatures obtained from these measurements are provided in Table 3. The findings show that pure LLDPE film melts at 114.1°C. With the incorporation of graphene, Tm is about the same for lower graphene percentage but increases slightly in all cases for increased graphene loading. Nanocomposite samples LGNP5, LG-Ag, and LRGO exhibit melting point of 114.6, 115.4 and 114.9°C, respectively. Neat LLDPE shows a glass transition temperature of -119.3°C which shifts to higher temperature when graphene nanomaterials are incorporated into the LLDPE matrix. As graphene concentration increases from 1 to 5 wt%, \({T}_{g}\) rises continuously. This is ascribed to the interaction between graphene and LLDPE chains, which reduces segmental mobility and consequently increases \({T}_{g}\) with graphene insertion. The more uniform dispersion of GNPs in LLDPE matrix compared to G-Ag and RGO sheets helps to cause more restriction of segmental mobility of thermoplastic macro-chains making the \({T}_{g}\) higher than LRGO and LG-Ag nanocomposites. The percentage of crystallinity (%Xc) for LLDPE was evaluated by Eq. 3 using melting endotherm, Hm and corresponding % Xc data are presented in Table 3.
$$\%{X}_{C}=\frac{{H}_{m}}{w {H}_{m}^{0}}$$
3
Here, \(w\) denotes the mass fraction of the thermoplastic in the nanocomposite samples, \({H}_{m}\) and \({H}_{m}^{0}\) represent the melting enthalpy of the composite sample and melting enthalpy of LLDPE in 100% crystalline state and taken as 293 J/g for all nanocomposites [27].
From Table 3 it can be seen that the net LLDPE exhibits crystallinity of 38.4%. In nanocomposites containing graphene, the percentage of crystallinity decreases as the filler concentration increases for all varieties of graphene inclusions. LLDPE/graphene nanocomposites exhibit crystallinity percentages of 35.07, 36.23, 36.55% with 5 wt% of GNP, G-Ag and RGO loading, respectively. These findings are also consistent with the percentage crystallinity of polyethylene/graphene nanocomposites as reported previously in the literature11,12. Carbonaceous inclusions restrict the polymer chain movement and thus hinder the orderly arrangement of macromolecular chain segments [47]. Better GNP dispersion in LLDPE matrix compared to other graphene nanomaterials leads to more reduction in % crystallinity of the thermoplastic nanocomposites.
Table 3
Melting point, percentage crystallinity and glass transition temperatures for LLDPE and LLDPE/graphene nanocomposites
Sample designation | \({T}_{m}\)a (°C) | \({\% X}_{c}\)b | \({T}_{g}\)c (°C) |
Neat LLDPE | 114.1 | 38.4 | -119.3 |
LGNP1 | 112.2 | 37.31 | -116.3 |
LGNP3 | 113.5 | 36.56 | -114.5 |
LGNP5 | 114.6 | 35.07 | -114.1 |
LG-Ag1 | 111.9 | 37.42 | -117.3 |
LG-Ag3 | 115.1 | 37.19 | -116.4 |
LG-Ag5 | 115.4 | 36.23 | -115.9 |
LRGO1 | 112.0 | 37.17 | -116.8 |
LRGO3 | 115.0 | 36.98 | -115.1 |
LRGO5 | 114.9 | 36.55 | -114.5 |
\({T}_{m}\)a denotes the melting temperature, \({T}_{g}\)c represents the glass transition temperature and \({\% X}_{c}\)b represents the percentage crystallinity for the neat LLDPE and LLDPE nanocomposites. |
3.2.4 Rheological properties
The filler distribution and microstructure created by nanofiller particles in the matrix phase have a substantial impact on the rheological behaviour of polymer composites [48]. The rheological properties measured for neat LLDPE and graphene loaded LLDPE nanocomposites are depicted in Fig. 8. Figure 8(a-c) shows how filler concentration affects storage modulus for LG-Ag, LRGO and LGNP nanocomposites with varying angular frequencies. These figures show that the storage modulus increases gradually as the filler concentration increases in all the cases. This rise in storage modulus is greater at lower frequencies, but it grows slowly at higher frequencies. The reinforcing impact of graphene platelets is primarily responsible for this improvement. GNP-filled LLDPE composites exhibit higher storage modulus compared to LG-Ag and LRGO nanocomposites. As FESEM analysis confirms that GNPs are dispersed in the LLDPE matrix by generating a network topology, therefore storage modulus increases more for LGNP nanocomposites. With the incorporation of graphene in PE matrix, the resulting nanocomposites become stiffer. In all three situations, modulus increases consistently as graphene loading increases. However there is a dramatic increase in modulus value when graphene loading rises from 3 to 5 wt percent.
The variations of complex viscosity with change in angular frequency for neat LLDPE and LLDPE/graphene nanocomposites are depicted in Fig. 8(d-f). Complex viscosity values for all types of nanocomposites are found to be greater than pure LLDPE and increase with graphene loading. Graphene nanostructured materials form network structure through the thermoplastic matrix and act as reinforcements for the graphene loaded nanocomposites. Compared to LRGO and LG-Ag composites, LGNP composites exhibit higher complex viscosity values due to more uniform GNP dispersion in the matrix phase which facilitates a strong reinforcement effect than RGO or G-Ag. Viscosity values are larger at lower frequency region, but as frequency increases, the viscosity value diminishes for all the cases. The difference in complex viscosity values for nanocomposite samples with neat LLDPE is greater at low frequencies than at high frequencies.
3.2.5 Barrier properties
Polyethylene/graphene nanocomposites must have high barrier properties in order to be utilized as food packaging materials [46, 49]. By lowering both the oxygen transmission rate (OTR) and the water vapor transmission rate (WVTR), the barrier or permeability qualities of nanocomposite films can be enhanced. In this aspect, graphene nanomaterial is quite essential like nanoclay. These graphene materials slow the transport of oxygen and vapor molecules through the composite films by functioning as barriers throughout the polymer phase. As a result, the oxygen and vapor permeability are reduced by these nanofiller particles. Graphene, in essence, creates a tortuous path across the sample thickness for O2 and water vapor molecules [50, 51].
Figure 8(g) and (i) depict the OTR and WVTR values as a function of graphene content for all sets of LLDPE/graphene nanocomposites. The observed OTR and WVTR values for neat LLDPE are 3542 cc/m2/day and 7.54 g/m2/day, respectively. As graphene content in the LLDPE matrix increases, both the OTR and WVTR values of the corresponding nanocomposites films decrease continuously. When graphene concentration becomes 3 wt%, the corresponding LG-Ag, LRGO and LGNP nanocomposites show OTR values of 2585, 2547 and 2420 cc/m2/day and WVTR values of 3.41, 3.29 and 2.46 g/m2/day, respectively. This decrease in both OTR and WVTR values with graphene insertion is ascribed to the reinforcing impact of graphene nanosheets of nanoscale dimensions, as well as improved polymer-graphene interfacial interactions. The addition of GNPs in the LLDPE thermoplastic matrix facilitates more tortuous path for these permeants through the nanocomposite film thickness. The change in relative permeability (ratio of permeability of composite film to neat polymer film) for all sets of nanocomposites with the increase in graphene content is represented in Fig. 8(h) and (j). With increasing graphene concentration, relative permeability (both oxygen and vapor) diminishes continually, and this change is more obvious for GNP reinforced thermoplastic nanocomposite films. Figure 8(k) represents the tortuous pathway created by graphene nanostcutured materials for water and oxygen molecules through LLDPE nanocomposite film.
3.2.6 UV barrier
When using polymer nanocomposites as packaging materials, in some cases, light transmittance of these nanocomposite films needs to be considered. Table 4 compares the percentages of UV-vis light transmission of neat LLDPE film and LLDPE/graphene composite films, whereas UV-visible transmittance spectra of neat LLDPE film and nanocomposite films are represented in Fig. 8(l). The neat LLDPE film is exceptionally transparent, with a maximum transmittance of 90.01 percent at 800 nm [52]. Nanocomposites' transmittance value decreases when graphene is added to the LLDPE matrix. Graphene inclusions create a tortuous path for UV radiation, which prevents light from passing through or being scattered. For LGNP nanocomposite films, a greater drop in transmittance (percent) is noted. GNPs are dispersed more evenly in the thermoplastic matrix, which results in related nanocomposite films with lower transmittance values.
Table 4
UV-visible light transmittance percentage of neat LLDPE film and LLDPE/graphene composite films
Nanocomposite film | 220 nm | 400 nm | 600 nm | 800 nm |
Neat LLDPE | 6.83 ± 0.44 | 36.76 ± 0.47 | 68.33 ± 0.61 | 90.01 ± 0.59 |
LGNP1 | 4.83 ± 0.31 | 7.24 ± 0.56 | 8.64 ± 0.34 | 9.50 ± 0.45 |
LGNP3 | 0.80 ± 0.23 | 1.22 ± 0.41 | 2.89 ± 0.33 | 3.89 ± 0.37 |
LGNP5 | 0.52 ± 0.13 | 0.54 ± 0.17 | 0.56 ± 0.21 | 0.58 ± 0.29 |
LG-Ag1 | 5.7 ± 0.26 | 12.84 ± 0.38 | 17.47 ± 0.55 | 20.08 ± 0.49 |
LG-Ag3 | 3.49 ± 0.28 | 6.78 ± 0.36 | 11.61 ± 0.47 | 14.32 ± 0.41 |
LG-Ag5 | 1.3 ± 0.35 | 3.21 ± 0.33 | 4.89 ± 0.41 | 5.89 ± 0.33 |
LRGO1 | 0.74 ± 0.18 | 7.85 ± 0.57 | 12.47 ± 0.45 | 15.08 ± 0.68 |
LRGO3 | 4.01 ± 0.29 | 6.13 ± 0.59 | 9.32 ± 0.29 | 10.73 ± 0.37 |
LRGO5 | 0.51 ± 0.21 | 1.74 ± 0.51 | 3.13 ± 0.19 | 4.01 ± 0.28 |
3.2.7 Antibacterial activity of LLDPE/G-Ag nanocomposite films
The standard plate count method was used to assess the antibacterial efficacy of the prepared LG-Ag nanocomposite films with respect to neat LLDPE film as control. After 1 h of incubation, colony-forming units (CFU) per milliliter of solution were used to quantify bacterial growth. The decrease in bacterial growth reflects the polymer nanocomposite film's antibacterial effectiveness [53]. Figure 9 depicts the CFU/mL of the bacterial strains (both E. Coli and B. Subtilis) as determined by the conventional plate count method for the developed films as a function of G-Ag concentration in the nanocomposite films. The control LLDPE film exhibited no substantial antibacterial activity for both types of bacterial strains. When G-Ag in the nanocomposite films increases from 1 to 5 wt%, the calculated CFU per mL of solution decreases in both cases. But the bacteria reduction is more prominent for B. Subtilis strains compared to E. Coli strain. So, the LG-Ag nanocomposite films show higher antibacterial effectiveness against B. Subtilis than E. Coli bacterial strains.
The antibacterial activity of nanocomposite films was further testified by FESEM analysis of these incubated films and corresponding SEM micrographs are shown in Fig. 10. Figure 10(a) shows SEM image of neat LLDPE film as control with E. Coli whereas LLDPE film with B. Subtilis is depicted in Fig. 10(e). No significant bacterium damage is seen from these figures indicating no antibacterial activity of control thermoplastic films. But irreversible bacteria damage is clearly seen in the SEM images (Fig. 10(b-d)) of LG-Ag nanocomposite films with E. Coli bacterial strains. The intercellular leakage of bacterium content along with irreversible cell damage, as shown in Fig. 10(f-h) signifies a more pronounced effect of these nanocomposite films on B. Subtilis than E. Coli [26]. These leaky cells provide proof of the evasive behavior of the polymer nanocomposite films exhibited. The results are consistent with the plate count tests, which demonstrate the polymeric film materials' significant antibacterial capabilities. Figure 10(i) and (j) depict the high-resolution FESEM images of E. Coli and B. Subtilis bacteria, respectively. More clear bacteria damage and leakage of bacterium content are seen from these micrographs.
The LG-Ag nanocomposite film’s antibacterial activity is regarded as a widely-accepted "capturing and killing" process [20], which is graphically depicted in Fig. 11. The bacterium initially became stuck to the film surface. RGO's enormous specific surface area aids in making effective contact with the bacterial cell wall. The capturing phenomenon of bacteria is referred to as the main interaction of the silver nanoparticles in RGO nanosheets with the bacterial cell wall. The AgNPs can then release Ag+ ions, producing reactive oxygen species (ROS) as a result. ROS is a crucial component in the destruction of the bacterial respiratory system, which results in the death of the bacterium. RGO sheets can serve as stabilizing nanoparticle support in this composite material. This could make the Ag nanoparticles (AgNPs) more accessible to the bacterial cell wall. Here, AgNPs may pierce through the bacterial cell wall membrane, extending to the inactivation of the bacteria. Through a capturing-killing process, this nanocomposite activity can lead to irreversible cell damage in a specific area.
3.2.8 Electrical properties
Figure 12(a) depicts the electrical conductivities of LLDPE/graphene nanocomposites as a function of G-Ag, RGO and GNP concentration. The electrical conductivity of the thermoplastic composite increases as graphene content increases. But the conductivity value is higher for G-Ag loaded composites compared to LRGO and LGNP composites. The presence of Ag nanoparticles on the graphene surface enhances the electrical conductivity of the resultant thermoplastic nanocomposites. Neat LLDPE exhibits an electrical conductivity of 2.72 × 10− 12 S/cm which is increased to 4.37 × 10− 7, 6.7 × 10− 6 and 8.9 × 10− 8 S/cm for 5 wt% of GNP, G-Ag and RGO, respectively loaded in the thermoplastic matrix. This enhancement in electrical conductivity values of nanocomposites with the addition of highly conductive graphene is owing to the development of a continuous and interconnected networking pathway in the composite material. LGNP composites show higher conductivity values compared to LRGO composites as the dispersion of GNPs in the LLDPE matrix is much better than in RGO sheets. Uniform dispersion of GNPs in polymer matrix provides more contact points between graphene sheets compared to RGO which finally leads to higher conductivity value of LGNP composites than LRGO composites.
The AC electrical conductivity of LG-Ag, LRGO, and LGNP nanocomposites was examined as a function of nanofiller content throughout a frequency range of 1 to 10 MHz. Figure 12(b-d) shows the variation of AC conductivity of the LLDPE nanocomposites with the increase in GNP or G-Ag or RGO content. As expected, nanofiller loading results in a continuous improvement in AC electrical conductivity for all types of nanocomposites. Neat LLDPE displays linear frequency dependence in AC electrical conductivity, indicating that this polymer is insulating. At lower frequencies, the pure LLDPE exhibits no phase current flow; but, at higher frequencies, the interfacial polarization mechanism amplifies current flow. At lower concentrations of graphene, the AC conductivity increases with frequency shift, whereas at higher frequencies, polymer composites exhibit more or less frequency-independent characteristics. Graphene inclusions will be segregated from one another at lower graphene loading levels. As the frequency rises, electrons become excited and move from one conductive spot to another. As a result, a virtual conductive network is created, increasing conductivity at higher frequencies [15, 27]. For higher graphene concentration (say 5 wt%), the location of graphene creates a continuous, dense conductive network channel through the LLDPE matrix. Now that electrons can easily pass across this networking conduit, AC conductivity is nearly frequency independent. As a result, at higher frequencies, AC conductivity increases marginally. AC conductivity is higher for LG-Ag nanocomposites due to the more conductive nature of G-Ag nanomaterials. Also, LGNP composites exhibit higher AC conductivity than LRGO due to the construction of more interconnected conducting pathways for GNPs in the thermoplastic matrix. From both DC and AC electrical conductivity plots, it can be seen that conductivity value increases sharply when graphene content increases from 3 to 5 wt%. So, for all types of LLDPE/graphene nanocomposites, 3 wt% of graphene loading can be considered as an electrical percolation threshold.
3.2.9 Thermal conductivity
Because graphene nanomaterials exhibit high thermal conductivity, their usage as fillers has been recognized as a useful method for obtaining highly thermally conductive composites [17]. Figure 12(e) compares the thermal conductivity values for LLDPE/graphene nanocomposites as a function of graphene loading to study the effect of fillers on thermal conductivity of the thermoplastic nanocomposites. The results indicate that the thermal conductivity of the LLDPE nanocomposites rises continuously with an increase in G-Ag or RGO or GNP concentration because of the increased contact area. The maximum thermal conductivity obtained for LG-Ag, LRGO and LGNP nanocomposites are 0.56, 0.53 and 0.59 W/m.K, respectively for 5 wt% of each graphene loading. However, GNP-loaded thermoplastic composites show higher thermal conductivity compared to other composites. Platelet-shaped fillers have advantages over other morphologies due to their wide contact area, which allows for considerably tighter contact between neighboring platelets, hence reducing phonon scattering [9]. Better graphene dispersion in the LLDPE thermoplastic matrix caused furthermore reduction in phonon scattering and interfacial thermal resistance, which finally enhances the thermal conductivity of the thermoplastic nanocomposites. Figure 12(f) illustrates the thermal conductivity improvement percentage relative to that of pure LLDPE. The thermal conductivity of LG-Ag, LRGO, and LGNP nanocomposites increases by 82, 77, and 96.6 percent, respectively. Graphene's high aspect ratio allows for the establishment of a good interface between them and the thermoplastic matrix, which is essential for achieving excellent heat conductivity in composite materials.
3.2.10 EMI SE
The depletion of electromagnetic radiation by shielding materials is referred to as EMI SE. Effective electromagnetic shielding effectiveness is the sum of reflection, absorption, and multiple internal reflection losses at existing interfaces caused by external electronic sources. The total shielding effectiveness can be written as the logarithmic ratio of incident power (PI) of electric or magnetic field intensity to transmitted power (PT) (see Section 4 in Supplementary Information). The attenuation of electromagnetic radiation is highly reliant on the shielding material's electrical properties. The composite material must have good electrical conductivity for effective EMI shielding [27, 54].
The change of the total EMI SE (SET) of the LGNP, LG-Ag, and LRGO nanocomposites recorded in the X-band frequency with altering graphene concentration is depicted in Fig. 13(a), (d) and (g). Neat LLDPE displays extremely low shielding effectiveness of about − 2 dB due to its EM radiation transparency. The incorporation of conducting graphene nanomaterials in the LLDPE matrix increases shielding efficacy notably. SET increases to around − 19, -21 and − 17 dB for 5 wt% of GNP, G-Ag and RGO loading in the thermoplastic matrix which are greater than the commercial requirement (-15 dB) [28]. This increment in SE value is credited to the construction of conducting channels by graphene across the thermoplastic matrix. EMI SE of LG-Ag composites is higher than LGNP and LRGO composites as G-Ag sheets construct more conducting networks throughout the whole matrix compared to other types of graphene. Also, GNP-loaded nanocomposites exhibit higher shielding effectiveness than LRGO nanocomposites. As morphological analysis reveals that GNPs are more uniformly dispersed in the matrix than RGO, therefore, developing more interconnected conductive pathways throughout the thermoplastic matrix.
To better comprehend the shielding process in thermoplastic composites, SET, SER, SEA and absorption, reflection, and transmission coefficients (A, T, and R) are computed (see Section 4 in Supplementary Information) and displayed with different graphene contents evaluated at a frequency of 10 GHz for all three cases. Figure 13(b), (e) and (h) show the variation of shielding efficiency through absorption and reflection as a function of graphene content, whereas Fig. 13(c), (f), and (i) depict how the amount of graphene affects the A, T and R coefficients. In all cases, SER is higher than SEA for less graphene content. But for 3 and 5 wt% of graphene content, SEA becomes higher than shielding effectiveness by reflection mechanism. According to Fig. 13(c), (f) and (i), the R value is greater than the A and T values for LLDPE thermoplastic composite containing 1 wt percent graphene loading, indicating that at extreamly low graphene loading, reflection of incident EM waves predominates over absorption and transmission of these electromagnetic waves. With more conductive graphene loaded, transmission of EM waves is negligible and absorption takes precedence over reflection. Such absorption-dominant EMI shielding behavior of reduced graphene oxide filled polymer nanocomposite film was also observed by Bhawal et al [28]. The interconnected conducting network formed by graphene nanomaterials in the LLDPE matrix at higher filler loading helps to absorb more EM waves. In addition, graphene's surface oxygen functional groups interact with electromagnetic waves and absorb EM radiation.