Fabrication and Characterization

In order to release the fibroin fibers and have them react directly with the polymer matrix [53], the sericin layer was removed (although not completely) and the silk fibers were in the form of a reeling. Then [52] the fibers were cut into segments of 5 |rm and placed them in an oven in order for them to dry and minimize their moisture. Finally, the fibers were mixed with PLA and the composites were fabricated using a Hakke MiniLab twin-screw micro-extruder. The melting temperature used during fabrication was kept constant at 180 °C, which is below the degradation temperature for the silk fibers. The content of the fibers in all samples was 5 wt%.

In Fig. 4.11a, b it is seen that the fibers in the PLA were aligned in the same direction, while in Fig. 4.11c,d it is seen that during molding some of the 1 ^m fibers were separated into the 15 nm fibrils which comprise them, and dispersed into the PLA.

The tensile and flexural properties of the samples were tested with an MTS machine (Alliance RT/50). The elastic modulus of pure PLA samples was ~3.2 GPA, while for the PLA/fiber composites it increased by 27% to ~4.1 GPa [52]. The flexural modulus of pure PLA (~4 GPa) remained practically unaffected by the addition of the silk fibers. Furthermore, the tensile strength was ~70.7 GPa with and without the silk fibers, while the strain at fracture was 31% lower for the PLA/fiber composite than the pure PLA [52]. This decrease in fracture strain could be due to various factors, since the mechanical properties of fiber composites depend on the fiber concentration, length, diameter, and fiber-matrix interface strength. The morphology of a cross section of the deformed composite structures upon fracture is shown in Fig. 4.12. A gap was present between the fiber-PLA interface, indicating that the interface bonding was not strong, and therefore the stress from the PLA was not transmitted efficiently to the fibers. This could be due to residual sericin left on the fiber surface, which inhibits good chemical bonding with the PLA [53]. The large gaps between the fiber and the matrix act as flaws (voids) in the material, which reduce its strength. It should be noted that the apparent increase in elastic modulus is due to the fact that the modulus is measured when the material undergoes small loads, at which the stress transfer from the matrix to the fiber can occur,

(a) & (b)

Fig. 4.11 (a) & (b): Optical micrographs of cut-off view (along the longitudinal direction of the sample) of the 5% silk fiber/PLA composite; and scanning electron microscopy of the micro fibrils inside the PLA (c) and (d) (Reproduced with permission from Ref. [52])

Scanning electron microscopy images showing the fractured surface of (a) silk/PLA composites, in which the fiber pull out is seen and (b) pure PLA (Reproduced with permission from Ref. [52])

Fig. 4.12 Scanning electron microscopy images showing the fractured surface of (a) silk/PLA composites, in which the fiber pull out is seen and (b) pure PLA (Reproduced with permission from Ref. [52])

even when the interface strength is weak. With increased loading, however, the stress transfer cannot occur through weak interfaces and de-bonding takes place, creating voids, which reduce the tensile strength and fracture strain. In addition to poor interface contact, the low content of the silk fibers can also affect the lack of increase in tensile strength, over pure PLA.

 
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