Rheological and mechanical behaviour of poly(lactic acid)/polyethylene glycol blends

Three different chain architectures of PLA including; linear PLA (L-PLA), branched PLA (B-PLA) and linear/branched PLA (L,B-PLA) were blended with polyethylene glycol (PEG) having Mw of 1,000 g/mol, as a plasticizer, in various PEG concentrations (0, 5, 10, 15 and 20 wt%). A range of characterization techniques were used to investigate the molecular weight characteristics, thermal properties, mechanical properties, morphological properties and rheological behaviour of these blends. L,B-PLA showed evidence of a synergistic behaviour in both their melt rheological and impact properties. This was attributed to the miscibility and chain entanglement of B-PLA and L-PLA as is evident in the region of 30-70 wt % of B-PLA content. The addition of PEG plasticizer lowered the glass transition temperature (Tg) and increased the crystallinity and crystallization rate of the blends as expected. The addition of PEG increased the impact strength and % elongation at break while the tensile strength and Young’s modulus were decreased. PLA with higher PEG loading produced lower shear viscosity and elastic properties compared to pure PLA. In terms of extensional rheology, strain-hardening behaviour was observed in both unprocessed B-PLA and unprocessed L,B-PLA. The plasticized blends (B-PLA/PEG and L,B-PLA/PEG) also exhibited the strain hardening behaviour at higher strain rates from 1.0 s-1. However, they exhibited lower extensional viscosity values. On the other hands, strain hardening behaviour of L-PLA/PEG blends was not observed at any strain rates tested. From the Rheotens experiment for B-PLA and L,B-PLA it was observed that the increasing PEG content led to a decrease of the melt strength values and increased drawability of the blends. <br><br>The most beneficial composition was produced to be the film and its properties were explored.The L,B-PLA blend (with 10 wt% PEG) was chosen to produce a film by blown film extrusion. The films exhibited improved % elongation at break. Significant correlations were observed including phase separation between thermal and morphological properties, the values of crystallinity (% Xc) and impact strength. Finally, the “three phase” model of semicrystalline polymer can be used to explain the effect of the addition of PEG into PLA on the impact strength properties. The blends exhibited more rigid amorphous and less mobile amorphous regions at higher PEG concentration, which led to the improvement of impact strength. This has been described as the “pillow effect” phenomenon.