Advances in Carbon Nanomaterial- Based Green Nanocomposites
7.4 Unique Properties of Carbon-Based Green Nanocomposites
Carbon-nanomaterials possess unique chemical and physical proper- ties. The blending of these CNs with a suitable matrix (e.g., bio-polymer) can result in the carbon-based green NCs. Surface area, size, and shape, molecular relationships, sorption properties, optical, electronic, and ther- mal properties are some of the unique properties of above NCs that have been utilized in environmental applications. Few of the unique properties of these green NCs have been discussed.
7.4.1 Size and Structure
The structural properties of different NCs can be modified by the incorpo- ration of carbon nanomaterials as fillers into polymer matrix. For example, in the epoxidized soybean oil/citric acid/c-MWCNTs bionanocomposite the diameter of the c-MWCNTs increases on attachment of the polymer, layers on the surface of the c-MWCNTs [53]. In another example, the BET surface area of the lignin-based carbon/ZnO NC displayed higher BET
surface area as compared to the pure ZnO [54]. Multifunctional super para-magnetic NC has been developed using unripened fruit of Cassia fis- tula and Aloe vera. The surface area for Aloe vera based NC developed micropores having a honeycomb shape. However, the octanol based NC possess larger pores with excellent staircase type formation [55]. A novel green NC (GnZVI loaded on PAC) demonstrated a reduced surface area after loading of iron nanoparticles [56].
In the thermoplastic polyurethane/graphene oxide (GO) composite scaffolds, as the amount of GO increases the average pore diameter of the composite scaffolds decreases. Also, due to the embedded GO, the sur- face of the biocomposites becomes rough [57]. Sodium alginate/graphene oxide (SA/GO) fibers displayed a rough surface and belt-like structure [58]. Chitosan/reduced graphene oxide (CS/RGO) mesoporous NC demonstrated large specific surface area [59]. Magnetic CS/GO demon- strated irregular shape having layered structure with fine dispersion of Fe3O4 nanoparticles on the surface of GO [60].
7.4.2 Thermal and Mechanical Properties
Carbon nanomaterials (CNs) have some distinct properties such as high strength, stiffness, exceptional electrical and thermal properties.
Carbon nanomaterials were employed as ideal reinforcing fillers for polymer matrices to achieve high performance and additional features, for the development of bionanocomposites. In PLA/CNT bionanocom- posites, the Young’s modulus, thermal stability increases whereas ten- sile strength and ultimate elongation decrease as compared to pure PLA [61]. In another study, on increasing the loading on the multiwalled CNT (MWCNT) in PLLA/MWCNT bionanocomposites, the DC conductivity increases [62]. The PLA/MWCNT-g-PLA bionanocomposites demon- strated a strong impact of PLA chain length on the morphological, elec- trical, and mechanical properties. As the length of PLA chain increases, the tensile properties, dispersability electrical resistivity also increases [63]. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/MWCNTs (PHBV- g-MWCNTs) bionanocomposite have good transparency in the visible wavelength range, improved thermal stability and mechanical barrier, high tensile strength of PHBV, and reduced water uptake and water vapor permeability [64]. PEI/MWCNT bionanocomposite displayed enhanced thermal stability as compared to pure PEI [65]. Several studies on starch- CNT composites for the dispersal of fillers into matrix have been reported [66–71]. Starch-based NC reinforced with MWCNTs previously wrapped with a starch-iodine complex exhibited significantly enhanced mechanical
properties such as Young’s modulus and tensile strength [72, 73]. Starch/
MWCNT NCs demonstrated excellent thermal stability, mechanical properties, and water resistance capacity as compared to untreated wood composites. Addition of 0.50 phr f-MWCNT can reduce the flammability of wood composites upto 30% [74]. Soy protein/MWNTs NC displayed enhanced mechanical and water resistance properties which depends on the size and contents of MWNTs [75]. Nacre-like SPI-based NCs (SPI/G/
CNT/NFC) demonstrated enhanced thermal stability, tensile strength, UV-visible barrier performance and hydrophobicity in comparison to neat SPI film which may be due to the different types of interactions [76].
The epoxidized soybean oil/citric acid/c-MWCNTs bionanocomposites displayed enhanced thermal stability and tensile strength. This enhance- ment in different properties could be attribute to the intense interactions between c-MWCNTs and epoxidized soybean oil-citric acid polymer net- works [53].
Nanocomposites of poly-L-lactide (PLA), bio-polyamide (PA), and poly(butylene adipate-co-terephthalate) (PBAT) based NC with different quantity of RGO demonstrated enhanced properties [77]. Multifunctional (MFC) NCs have been prepared using graphene GO, and RGO. The dif- ferent properties such as thermal stability and electrical conductivity of MFC NCs depend on the type of filler used [78]. The incorporation of graphene materials with aqueous-based silk fibroin proteins can enhance its elastic modulus, thermal, and other physical properties [79]. Sodium carboxymethyl cellulose/silk fibroin/RGO displayed enhanced thermal stability, glass transition temperature and surface roughness [80]. The structure, thermal, mechanical, and electrical properties of unplasticized and glycerol plasticized chitosan/graphene (CS/GS) NCs can be modified by changing the content of filler and plasticizer [81]. Similarly, GO and RGO have been employed as nanofillers for the fabrication of CS/GO and CS/RGO NCs. The incorporation of different nanofillers (GO, RGO) can affect structural, thermal, microstructural, mechanical, and surface prop- erties of the above NCs. Similarly, GO and RGO have been employed as nanofillers for the fabrication of CS/GO and CS/RGO NCs. The incorpo- ration of different nanofillers (GO, RGO) can affect structural, thermal, microstructural, mechanical, and surface properties of the above NCs [82].
GO/CS NC films exhibited enhanced mechanical properties without the loss of the optical transparency [83]. The inclusion of filler networks in the polymeric matrices of multifunctional biodegradable NC materials composed of natural rubber latex and graphene or GO can significantly enhance the electrical, chemical, and mechanical properties, as com- pared to the unfilled polymer [84]. In Jatropha curcas oil-based alkyd/
epoxy/GO bionanocomposites the homogeneous dispersion of GO into the polymer matrix can significantly enhance thermal stability, tensile strength and elastic modulus, whereas reduces the curing time [85]. GO and xanthan gum (XG) [GO/XG-Fe(III)] NC film exhibited high mechan- ical strength, specific thermal conductivity, and stability against vari- ous organic solvents and acidic solution [86]. Chitosan-carbon dots NC hydrogel film displayed superior UV-visible blocking, swelling, thermal, mechanical and hydrophobic properties as compared to CS hydrogel film [87]. A ternary, bioinspired NC film (WH-MMT-rGO) exhibited good thermal stability and a high strength as compared to other hemicellulose- based films or wood auto-hydrolysate-based films [88]. The compressive modulus of thermoplastic polyurethane/GO composite scaffolds was sig- nificantly enhanced with the addition of GO, as compared to the pure matrix [57]. Small amount of ND can significantly improve the mechani- cal properties of epoxy/ND and poly(vinyl alcohol)/ND NC [89]. Young’s modulus of ND may increase several times compared with the neat poly- mer. ND dispersion can be improved due to the interfacial interaction between matrix/nanofiller.
7.4.3 Electrical Properties
CNTs possess high electrical and thermal conductivity, which improve the charge and heat transportation of green NCs [90, 91]. Thermoelectric materials based on cellulose/CNT NCs were developed. SWCNT-based composites exhibits high electrical conductivity and Seebeck coefficient as compared to MWCNT-based composites. The electrical conductivity for both types were decreased by lyophilization, but did not affect the Seebeck coefficient of MWCNT-based NCs. Higher Seebeck coefficients were mea- sured at 3 and 4 wt% in SWCNT containing aerogels than for films but significant lower values at higher loadings. The CNT addition results in increased thermal conductivity in the films, whereas the lyophilization sig- nificantly reduced it for the aerogels [92].