Reduced Shear Degradation of Poly(Lactic Acid) in Hybrid Composites Containing Carbon Nanotubes and Carbon Microfibers

In this study, we investigated carbon nanotube reinforced nanocomposites and hybrid composites containing both carbon microfibers and carbon nanotubes of poly(lactic acid) (PLA) and the effect of the fillers on the molecular weight of the PLA matrix. Dynamic mechanical analysis and rheometric measurements revealed that incorporating either carbon nanotubes or carbon microfibers into pure PLA reduces the linear viscoelastic (LVE) range. This reduction arises from early disruption of interfacial bonds between the matrix and the reinforcements, as well as the breakdown of contacts among aggregated nanotubes under small strains, both of which trigger nonlinearity. In contrast, adding carbon nanotubes to carbon microfiber‐reinforced PLA to form a hybrid composite shifted the LVE limit to higher strain values. This improvement is attributed to the nanotubes effectively bridging the matrix–fiber interfaces, enhancing interfacial stress transfer and delaying the onset of microstructural rearrangements responsible for nonlinear behavior. Size Exclusion Chromatography (SEC) showed that the molecular weight of the matrix decreased significantly in the composites containing only carbon microfibers, while the presence of the carbon nanotubes prevented the molecular chains from weight reduction. This supported the hypothesis that the molecular chains and the well‐dispersed carbon nanotubes form an entangled network, which is a much more stable system that can protect from chain breakage during processing.