Morphological analysis
AFM analysis was carried out to get more insights on the morphology of developed polymer composites, i.e., showing the miscibility/immiscibility of PBS with PLA, and the dispersion/distribution of CNC within PLA-PBS blend as shown in Fig.1(a-e). Moreover, the topography of the surface was determined by three dimensional (3D) topographies in Fig.1(f-j). Fig.1(a) displaying the PLA-PBS sample with the smoothest surface as compared to other composites. In addition, the AFM images of PLA-PBS composite obtained from either adhesion (Fig.1(a)) imaging do not exhibit a two-phase system, i.e., least dark regions are observed, implying that PBS is miscible with PLA. However, the surface topography becomes rougher with the addition of CNC and roughness increases with increase in CNC content as evident from Fig.1(b-e) and 3D topography in Fig.1(g-j).
In case of CNC reinforced PBS-PLA blends, a phase separation is observed due to presence of the interface between the flat surfaces and peaks. When the CNC content is higher, more sharper peaks are observed, as indicated by darker areas on the surface Fig.2(g–j), thereby exhibiting the better dispersion of the CNC’s [18]. The depth histogram presented by Fig.2, reveals the maximum depths of the of PLA-PBS-CNC (0, 0.5,0.75,1 and 1.5wt.%) composites in the range of 35-45, 38-47, 45-50 and 80-90 mV respectively. However, with increase in CNC loading the threshold depths of the dispersed phase increases significantly and reveals pronounced phase separated morphology as compared to PLA-PBS blend. Hence, the AFM analysis confirms a better dispersion of CNC’s were attained throughout the blend composites.
The SEM micrography of the fractured surfaces of PLA-PBS-CNC with CNC (0, 0.5, 0.75, 1 and 1.5wt.%). The fracture surface of PLA-PBS is rougher, indicating a ductile behaviour, this may be due to presence of PBS reducing brittle nature of PLA[19]. Moreover, there was no debonding observed in the composites with CNC’s upto 1wt.%, implying good matrix−CNC interactions. This interaction results into improved tensile strength of composites as evident from Fig.4. However, the fracture surface of composite with 1.5wt.% of CNC exhibited lot of cavities, showing the existence of agglomeration of the CNC’s resulting into reduced mechanical properties as clearly evident from Fig.4 and Fig.5.
Mechanical Behavior
It is evident from Fig.4 that tensile strength of composites initially decreases on addition of 0.5wt.% of CNC followed by increased values on addition of CNC upto1 wt.%. The incorporation of CNC increases the strength due to presence of well dispersed CNC particles causing transfer of load uniformly when applied axially to the composite material. Above this percentage tensile strength decreases due to agglomeration of higher CNC content act as stress concentrators, facilitating propagation of defects at the interface growing larger on application of load resulting in reduction of strength [20]. In contrast, the tensile modulus of the composites decreased with addition of CNC upto 1wt.% resulting into decreased stiffness of composite material and beyond this it increases insignificantly.
From Fig.5, the elongation at break of composites increases on addition of CNC upto 1wt% thus improving their flexibility. The tenacity was also found to decline initially then increases upto 1wt% of CNC, beyond this it decreases. From Table 1, tensile stress and strains are found to be enhanced by the addition of CNC upto 1wt%. The improvement in the elongation at break and strain at break upto 1wt% of CNC reveals enhanced tensile toughness [21]. This can be due to uniform distribution of CNC that resist deformations in the composites thereby improving their mechanical properties however on further addition agglomeration and insufficient wetting of the higher content of CNC’s by polymer matrix reduces their strength as well as their mechanical properties.
Table 1. Tensile properties of PLA-20PBS and PLA-20PBS-(0,0.5,0.75,1, and1.5wt.%) CNC bio-composites
CNC
(wt.%)
|
Tensile strain at break
(mm/mm)
|
Tensile stress at break
(MPa)
|
Strain at break
(mm/mm)
|
Tensile strain at max. load
(mm/mm)
|
0
|
1.24×10-2
|
0.193
|
0.8×10-4
|
8.61×10-3
|
0.5
|
1.38×10-2
|
0.194
|
2×10-4
|
10×10-3
|
0.75
|
1.63×10-2
|
0.346
|
1.6×10-4
|
13×10-3
|
1
|
1.75×10-2
|
0.383
|
2.3×10-4
|
13.8×10-3
|
1.5
|
1.19×10-2
|
0.341
|
1.1×10-4
|
9.9×10-3
|