Rheology provides insight into flow induced nano-structural breakdown and its recovery effect on crystallization of single and hybrid carbon nanofiller filled poly(lactic acid)

Abstract

Despite the indispensable contribution of crystallization to the final properties of poly(lactic acid) (PLA), the combined effect of steady shear and nanofiller(s) remains mostly unsettled. This work employed a combination of steady shear and oscillatory rheological measurements to study shear induced nano-structural breakdown and subsequent nanoparticle orientation and overall structural recovery in PLA nanocomposites containing functionalized, single and hybrid carbon nanofillers (fCNTs & fGnPs) with different dimensionalities. Similar rheological measurements were used to study the effect of the flow induced nanoparticle orientation on nonisothermal as well as isothermal crystallization of PLA under both applied pre-shear and quiescent conditions. It was found that addition of the carbon nanofillers into PLA had a significant intensifying effect on the flow induced contribution to the acceleration of the crystallization kinetics, particularly after a pre-shear. These findings could be explained in the light of the simultaneous increase in nucleation density and amplification of flow induced molecular orientation driven by the carbon nanofillers. In the case of the nanocomposites containing hybrids of the nanofillers showed greater enhancing effect on crystallization of PLA compared to mixture law predictions. This was attributed to the favorable effect of 2D fGnPs on the dispersion of 1D fCNTs which could, in turn, lead to the availability of a larger surface area of nanofillers and therefore stronger nanoparticle induced molecular orientation. Finally, the effect of increasing pre-shear rate on the flow sensitivity parameter defined as kss/kq was found to be synergistic for all nanocomposite samples.