Modification of Poly(Lactic Acid) Rheological Properties Using Ethylene–Vinyl Acetate Copolymer

The effect of ethylene–vinyl acetate copolymer (EVA) addition on the rheological properties of poly(lactic acid) (PLA) was studied. EVA exhibited strain-hardening behavior in the transient elongational viscosity due to its long-chain branch structure, whereas PLA did not show any strain-hardening. The blends showed sea-island structure, in which the size of EVA droplets decreased with the vinyl acetate content in EVA. It should be noted that the blends showed strain-hardening behavior even though EVA is not in the continuous phase. During elongational flow, EVA droplets deform to the fibrous shape owing to hydrodynamic force applied by the matrix PLA, and eventually their deformation is greatly reduced as a result of the strain-hardening. Consequently, the blend system behaved like a rigid-fiber dispersion, which are known to show enhanced elongational viscosity. Finally, the processability of a tubular-blown film was improved by the EVA addition because of the strain-hardening.