https://doi.org/10.3390/chemosensors11010028
“A comprehensive Raman scattering-based characterization of a full bio-based polyamide loaded with graphene nanoplatelets or layered double hydroxides (LDH) was assessed. The potential of the Raman spectroscopy was used to reveal several particularities of the nanocomposite structures induced by thermal treatment. Thus, a complete morpho-structural picture was obtained in combination with scanning electron microscopy (SEM) analysis of the neat polyamide and polyamide nanocomposites exposed at different thermal conditions (room temperature, 80 °C, and 145 °C). The analysis of G, D and 2D Raman peaks and their relative intensity ratio ID/IG, revealed the fact that the presence of graphene in polyamide is suitable for improving the essential physical properties and is also responsible for the decrease in the defects’ occurrence in the graphene layers. The surface of nanocomposites based on full bio-based polyamide, with different 2D fillers (graphenic and non-graphenic structures), was carefully evaluated before and after the thermal treatment by employing SEM and Raman analyses. The two thermal treatments allowed different chain mobility of the polymer (first temperature being over the polymer Tg and second one close to the melting phase in the viscoelastic stage). The spectroscopic and microscopic investigation was used to determine the conformational changes in filler aggregates and polymer surface, respectively.”
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2.4. Scanning Electron Microscopy
The topographical examination of the bio-PA1010s and nanocomposites was performed using scanning electron microscope (SEM) Hitachi SU 8230 Cold Field Emission, coupled with EDS analysis (Oxford Instruments, AZtec Software, version 3.3, Oxford, UK) used for elemental detection, operating at an acceleration voltage of 15 kV. Millimeter-sized block samples (between 5 to 10 mm in diameter) were examined first by EDS and the morphology was examined after the samples were coated with a 9 nm layer of gold.
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3.1. SEM Characterization
3.1.1. SEM Investigation of the Bio-PA1010 and Nanocomposites
The interface between the polymeric material and injection mould, as well as the contact surfaces showed a relative uniform morphology (Figure 1a,b). These micrographs also highlight the exact imprint mark of the demoulded area (the effect of the mould), while the edges appear as a result of the processing surface. The bio-PA1010 material (reference sample) crystallization occurred fast on the mould sample interface on extended planar areas. At the same time, small technological cavities were seen in the SEM images. This is most likely due to the escape of entrapped gases, during the hot-melt process. The material is carbon based with low levels of oxygen and nitrogen content, according to the EDS analysis (Figure 1c,d). This is in good agreement with amide repetitive groups found in the polyamide macromolecular chain.
Figure 1. Representative SEM micrographs of bio-PA1010, reference sample: low magnification image showing the smooth surface and the demoulded area of the sample (a); detailed image of the technological cavities (marked in b) with yellow arrows); the area selected for the EDS analysis (c); and the EDS spectrum and elemental composition of the selected area (d).
Compared to the reference sample, the surface of the bio-PA1010 nanocomposites filled with LDH exhibits a higher roughness (Figure 2a–c). This might be due to the small LDH particle aggregates that migrated at the bio-PA1010 surface. However, the planarity of the demoulding was similar to that of the bio-PA1010 sample.
Figure 2. Representative SEM micrographs of bio-PA1010 nanocomposites filled with LDH 5%: low magnification image showing the rough surface of the sample, with LDH aggregates (marked in (a) with yellow arrows); detailed view of the LDH aggregates (b); the area selected for the EDS analysis (c); and the EDS spectrum and elemental composition of the selected area (d).
The graphene–bio-PA1010 samples had increased roughness, in comparison with the reference sample and bio-PA1010 nanocomposites (Figure 3). Moreover, the surface had rough edges that gave it a particular skin-like morphology. These morphological details were associated with both agglomerated graphene particles and a preferential phase of crystallization skin-like type. In higher resolution SEM images, some micrometric-sized aggregates based on graphenic structures and polyamide can be found (flakes). These flakes have a relatively homogeneous distribution which suggests a potential contribution to the crystallization process which took place on the injection mould surface. It is clearly noticed that at higher resolutions, the aggregates do not contain pure graphene, but a composite phase of graphene in combination with polyamide (in contrast with LDH polyamide composites).
Figure 3. Representative SEM micrographs of bio-PA1010 filled with 4% C500 GNP: low magnification image showing the rough surface of the sample (a); detailed view of the rough surface, with flakes and graphenic aggregates marked with yellow arrows (b,c); and high magnification image of graphenic aggregates surrounded by flakes (d); the area selected for the EDS analysis (e); and the EDS spectrum and elemental composition of the selected area (f).
The LDH particles have a more uniform distribution on the surface of the bio-PA1010 nanocomposites sample, as compared to the flakes observed in the graphene–bio-PA1010 samples. A lower roughness for LDH in comparison with graphene composites indicates a crystallization process in mild conditions (Figure 2 and Figure 3). These aspects can be correlated with several molecular interactions: one LDH can participate in the crystallization of bio-PA1010 with more polar groups than C500. The second one with a lower polarity difference could improve the dispersion degree in the matrix. Aggregates seen in the SEM images are relatively uniformly dispersed on the bio-PA1010 demoulded surface (after injection). At high magnifications, the LDH aggregates can be clearly observed. They were randomly covered with polyamide, while some others were found uncovered. These later aspects clarify true compatibility on the unmodified LDH with polar polymers such as bio-PA1010. The absence of hydrophobic domains in the LDH molecular structure can be considered as a real limitation for a high dispersion degree in the bio-PA1010.
3.1.2. SEM Investigation of the Bio-PA1010 Nanocomposites Filled with Graphene Nanoplatelets C500-4% and LDH-5% after Thermal Treatment at 80 °C
The thermal treatment at 80 °C allows the rearrangement of the polyamidic phases in the presence of LDH. This allows the LDH to “sink” into the polymer matrix which makes it unobservable (Figure 4b). The surface morphology changes since the composite phase remodels. This leads to the disappearance of the potential tensions occurred during injection moulding. The resulted surface morphology (“tree bark”) had small protuberances indicating the rearrangement of the polymer composite phase (based on amide units–LDH interaction), different from the previously evidenced LDH aggregates (Figure 2b,c).
Figure 4. Representative SEM micrographs of bio-PA1010 filled with 5% LDH nanocomposites after thermal treatment at 80 °C (a) and low magnification images showing the relatively smooth surface of the sample and detailed views of the small protuberances (b–d).
The thermal treatment at 80 °C of the C500-4% graphene–bio-PA1010, showed a pronounced debonding effect of the GNP aggregates decorated with polyamide (Figure 5). Large aggregates, similar to the pre-thermal treatment ones (Figure 3b–d) were seen on most of the analyzed areas. At a closer look of the sample, flake-like particles with dimensions closer to the elementary particles can be seen. Since they appear with a smooth edge at the margins of the layered structures, the polyamide–GNP composite phase occurrence is very probable. This correlates with the details seen in Figure 4d, where very well defined, solitary flake-like particles can be observed. Solitary aggregates are scarce, while the sharp edges disappear, due to the polyamide coverage, which confirms a better interaction for bio-PA1010 with C500, than for LDH.
Figure 5. Representative SEM micrographs of bio-PA1010 filled with 4% C500 graphene nanoplatelets after thermal treatment at 80 °C (a); low magnification images of the sample with progressive magnification showing the smooth surface and embedded graphenic aggregates (b–f).
3.1.3. The SEM Analysis/Investigation of the Bio-PA1010 Nanocomposites filled with Graphene Nanoplatelets C500-4% and LDH-5% Thermal Treated at 145 °C
The LDH-5% sample treated at 145 °C (Figure 6) had a highly different morphology compared to the C500-4% sample, treated at 145 °C (Figure 7). Plastic deformations can be observed in the demoulded areas (Figure 6b), probably due to the mechanical properties considerably changed by LDH in the polyamide. It seems that the embedding process of the LDH into the polymer phase was not as effective as it was at the 80 °C. In this scenario, these particles can act as effort concentrators for future materials development.
Figure 6. Representative SEM micrographs of bio-PA1010 filled with LDH-5% nanocomposites after thermal treatment at 145 °C (a) and low magnification image showing the smooth surface of the sample detailed view of the plastic deformation that occurred in the sample (b).
Figure 7. Representative SEM micrographs of bio-PA1010 filled with C500-4% nanocomposites after thermal treatment at 145 °C (a,b).
In the case of bio-PA1010–GNP, the favorable interaction between partners allows the composite phase to act as a whole (Figure 7) indicating a promising way for future materials development. The same aspects were confirmed later in the Section 3.2, with more details on the specific interactions between polymer and filler molecular units.
In the SEM images presented in Figure 7, the polymorphic phases generated by different forms of GNP aggregates (GNP-elementary particles, GNP-aggregated particles) can be seen. In a comparative view, it seems that GNP are more compatible with the bio-PA1010 matrix than LDH. The assembling on hydrophobic interactions in the bio-PA1010 nanocomposites appeared as more favorable then polar–polar ones. The advantage of the hydrophobic one is that it does not interrupt the amido–amido interaction between bio-PA1010 chain molecules, which are the driving force for the crystallizing. Moreover, one can notice that GNP are able to act also in the bio-PA1010 amorphous region.
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