3.2. Raman Analysis
Raman spectroscopy has become a standard characterization tool for carbon nanostructures [36
]. Tensile, compressive and shear stress applied to carbon nanotubes induce strain, leading to changes in the C–C bond vibrations, which strongly affect the Raman spectrum [40
]. Stress induced changes in peak positions and intensities have been observed for individual carbon nanotubes [41
], SWCNT bundles [42
] and nanotube–polymer composites [43
]. Normalized Raman spectra of the nanocomposites with increasing MWCNTs and GNPs concentration including pristine nanofiller as a reference are depicted in Figure 3
and Figure 4
. In all single spectra, the main Raman bands—G, D and 2D, or also denoted as G′, band have been registered. The 1582 cm−1
band of graphene is known as the G band or tangential band and originates from in-plane vibration of sp2
carbon atoms. The band at about 2700 cm−1
, which is recognized as the G′ or 2D band, is much more intense than the G band in graphene compared to graphite or to multilayer graphene. This band is a second-order overtone mode and does not represent a disorder-induced mode. The 2D band has frequently been used to study MWCNT-polymer interfacial stress transfer. The 2D peak is an overtone of the D band, resulting from a two-phonon double-resonant Raman process [44
]. The D band at around 1350 cm−1
is often referred as disorder band or defect band and its intensity relative to the G band intensity is often used as a measure of carbon nanotubes quality. All Raman results concerning monofiller PLA-based nanocomposites are summarized in Figure 3
and Figure 4
, and Table 2
The presence of MWCNTs in the composites is supported by the characteristic D, G, and 2D modes of MWCNTs (Figure 3
). D, G, and 2D modes of the MWCNT/PLA composites exhibit positional upshift, more pronounced in the 2D band, as compared to the pure MWCNTs itself. Shifts to higher-frequencies, especially for the G band, have been reported in MWCNTs composites [45
]. This blue shift has been attributed to a disentanglement and dispersion of the CNT bundles in the polymer matrix, decreasing nanotube-nanotube interactions. The application of a deformation to CNTs results in a change in the C–C bond vibrations, leading to a change in the vibrational frequencies of the normal modes and thus to Raman band shifts. Strain-induced frequency changes have been already reported for MWCNTs composites [43
]. In terms of the relative Raman intensity, evaluated against the G mode intensity, and the full width at half maximum intensity (FWHM), we can observe different distinct behaviors: the D intensity decreased substantially upon rising carbon nanotube content in the composite, whereas the 2D mode exhibits a cutback of the relative intensity as well as a significant reduction of FWHM. These changes can be due to the interaction between PLA and carbon nanotubes. It is worth noticing, that the resonances of pure PLA, due to C–CH3
stretching at about 1042 cm−1
rocking mode at about 1127.4 cm−1
, were red-shifted by 2.1, 1.3, and 1.8 cm−1
in the Raman spectrum of 3 wt% MWCNT/PLA composite regarding the spectrum of 1.5 wt% MWCNT/PLA composite, indicating that the interaction of MWCNTs with the polymer occurs through C–CH3 groups [46
], see Figure 3
As already mentioned, the 2D band of the nanocomposites in Figure 3
is barely shifted to higher Raman frequencies, compared with the 2D band of pure carbon nanotubes. A possible reason for this could be as well the relatively low filler concentration in the polymer matrix compared to the neat MWCNTs [23
]. When the carbon nanotubes are multi walled, their diameter becomes larger, so the lattice strain is reduced, leading to a decline in the number of sp3
hybridized carbon, which means the tubular structure is not so stable [28
]. The weak blueshift of the G band at 6 wt% and 9 wt% MWCNT loadings could be understood in terms of the strain effect caused by the interaction between carbon nanotubes and the PLA matrix.
As the number of graphitic layers increases, the interaction between these layers becomes significant, reflecting in the appearance of a shoulder over the 2D band in the Raman spectra of GNP/PLA nanocomposites (Figure 4
). This shoulder shifts the position of the 2D band to the higher frequencies of the spectrum. Concerning graphene samples, the small differences in the G band position up to 3 cm−1
) are suggested to be within the range of fluctuations caused by unintentional electron or hole doping effects [48
]. It was found that the influence of the PLA structure on the Raman spectra of the nanocomposites decreases with rising carbon filler loading.
ratio shows the graphene quality. Regarding high quality defect-free graphene, this ratio will be equal to 2 (single or few layers graphene). The defined I2D
ratios of TNGNP and GNP/PLA nanocomposites are rather a hint for the presence of multilayer graphene in the composite structure (Table 2
). Mapping Raman analysis was carried out on each sample, acquiring spectra at least on three surface sample positions, in order to have a better picture of nanocomposite homogeneity.
Peaks ratio calculations disclosed that even the pristine GNP and MWCNT are not fully homogeneous. However, a high level of homogeneity occurred in the nanocomposites. That was confirmed by the low standard deviations in the ID
ratios for each sample, showing a high level of homogeneity in the nanocomposites structure (see Table 2
). The fewer lattice defects led to a lower ID
value. An indication of the low amount of structural defects in MWCNT/PLA nanocomposites is the narrow ID
ratio, mostly concerning the higher MWCNTs loadings that could signify good dispersion of carbon nanotubes in the polymer matrix. The peak intensity ratios ID
of graphene nanocomposites are slightly higher than that of TNGNP, suggesting the formation of more basal plane and edge defects in the carbon filler present in GNP/PLA that could enhance the interface interactions in the composite. Nevertheless the low ID
values of GNP/PLA nanocomposites, really close to the ID
ratio of pure graphene nanoplatelets, can be an indication that the structure of GNP has been well preserved during melt extrusion processing. The standard deviations for all ID
ratios have been taken into consideration when presenting the data in Table 2
3.3. XRD Analysis
Pristine graphene exhibits a basal reflection (002) sharp peak at 2θ = 27.0° corresponding to a d spacing of 3.370 Å in graphite layer structure (Figure 5
). The graphene showed a more intensive peak than the carbon nanotubes pattern at 2θ = 26.26°, indicating a more crystalline structure for GNP. Considering the spectral profile of MWCNT, the weak broad XRD peak at 2θ = 43.54° is assigned to graphitic (100) crystalline lattice.
A broad amorphous peak was observed as result of neat PLA XRD analysis at around 20.0° combined with a weak sharp peak appearing at 17.2° (Figure 6
). This is an indication of scattering in the polymer matrix and confirms that the neat PLA has a partially pseudo orthorhombic α crystalline structure with a left handed 10/3 helices chain conformation but predominantly it is a biodegradable polymer possessing an amorphous microstructure. The analysis of the patterns in Figure 6
is evidence that the intensity of the GNPs (002) peak increases with increasing GNPs loading in PLA, as expected. Regarding the XRD analysis of the 9 wt% GNP/PLA composite, a distinct diffraction is observed at 27.2°, relating to an interlayer spacing of 0.33 nm, based on the Bragg’s law, which is associated with the graphitic (002) plane. On the other hand, the position change indicates that the spacing between graphene nanoplatelets in the composites shortened (from an interlayer spacing of 0.34 nm of 6 wt% GNP/PLA to 0.33 nm for 9 wt% GNP/PLA based on Bragg’s law). The addition of GNPs affects the semi-crystalline structure of the polymer: the crystalline PLA peak at 2θ~17.2° in PLA/GNP nanocomposites is absent.
The high intensity recorded at 9 wt% GNP loading could be attributed to a relatively intense number of graphene layers organized in stacks. The X-ray diffraction of 6 wt% GNP/PLA nanocomposites revealed a less intensive graphite (002) pattern and weak feature of the broad PLA peak. The disappearance of PLA crystallinity with growing graphene content can be explained as a result of interfacial effects influencing the PLA semi-crystalline structural order. The XRD technique could provide information about the stacking thickness of the GNP crystallites. The average pristine nanoplatelet thickness as well as the size of the incorporated graphene was calculated by using the Scherrer equation. It was found that pure GNP has a thickness of about ~80 nm. This dimension remains roughly the same in the composite having 9 wt% GNP (~82 nm).
The influence of the carbon nanotubes loading on the polymer crystallinity is prominent in Figure 7
. The improvement of the sharp PLA peak at about 17.0° is more significant for the compound loaded with 9 wt% MWCNT and less intense for the composite possessing 3 wt% MWCNT. Both composite diffraction patterns are an indication of more crystalline structure compared to the neat PLA. Graphitic (002) and (100) patterns are visible mostly for the 9 wt% MWCNT/PLA composite, which could be a sign of good carbon nanotube dispersion in the polymer matrix. This behavior, which shows a different trend compared to that of GNP, can be attributed to the dimensionality of the nanoparticles and probably also to the different degree of functionalization of the nanocarbons, where carboxyl groups can act as crystallinity centers in the polymer matrix. Indeed, nanofillers are reported to act in nucleating in nanocomposites [49
], with crystalline layers covering the filler [50
]. On the other hand, functional groups can improve crystallization [51
], indeed only one nucleating site is needed, for the polymer, to start crystallization. Moreover, polymer chains crystalline formation continues more easily on different nanoparticle dimensionalities, which can influence the crystallization behavior of the polymer, (i.e. the polymer chains align along the nanofiller surface) [52
3.4. Tensile Test Analysis
There are three possible types of interactions for composites that have nanofiller in their structure (polymer–nanofiller, nanofiller–nanofiller, and interface–polymer interactions), but the mechanism and the magnitude of the load transfer between polymer matrices and the nanofiller are still unclear concerning mechanical properties.
Considering the influence of carbon nanofillers on the macromechanical properties of nanocomposites, the main and most important tensile mechanical characteristics; ultimate (tensile) strength, elongation at ultimate strength and Young’s modulus, have been compared. The dependence of ultimate strength as a function of filler content in MWCNT/PLA and GNP/PLA nanocomposites is shown in Figure 8
The nanocomposites possessing GNP in the PLA matrix exhibit a trend of a slight decrease in ultimate strength as well as in elongation at a break when increasing the content filler. A possible reason for this could be a poor dispersion of graphene in the polymer volume and/or changes in polymer crystallinity due to the nanoparticles addition. The character of the curve describing nanocomposites with MWCNT/PLA is quite different. The incorporation of MWCNT significantly raises the tensile strength and elongation at ultimate strength parameters up to 6 wt% carbon filler content, see Figure 8
. Above a 20% increase in the tensile strength of the MWCNT/PLA composite is observed at 6 wt% carbon nanotubes content compared to the neat PLA. The rising of carbon nanotubes loading from 1.5 wt% to 6 wt% enhances the elongation at ultimate strength, as can be seen in Figure 8
b. This is due to the interfacial polymer-filler interaction related to the distinct hybrid structure between MWCNT and polymer chains. The lower values of ultimate strength and elongation at ultimate strength for 9 wt% MWCNT/PLA nanocomposites could be explained by the high carbon nanotube concentration making the composite too crumbly and resulting in worst macromechanical properties.
Young’s modulus as a function of carbon filler content for both studied monofiller composite materials is presented in Figure 9
. An insignificant impact has been observed concerning the nanocomposite elasticity by adding GNP in a PLA matrix. The Young’s modulus is increasing in the case of carbon nanotubes loading up to 9 wt% filler content. The higher elasticity that could be seen for 9 wt% MWCNT/PLA nanocomposites is probably a result of good filler dispersion. The loadings above 3 wt% of graphene lead to a slightly lower Young’s modulus of the nanocomposites. This Young’s modulus reduction is insignificant and might be related to some negligible inhomogeneous distribution of carbon filler in the polymer, especially at 9 wt% GNP concentration.