Strain recovery and stress relaxation behaviour of multiblock copolymer blends physically cross-linked with PLA stereocomplexation

Abstract

Polylactide (PLA) stereocomplexes have attracted attention due to their ability to improve the thermal stability of bioplastics. Here, we evaluate whether PLA stereocomplexes can form stable physical cross-links in blends of a multiblock copolymer with poly(l-lactide) and poly(ε-caprolactone) segments (PLLA-PCL) and a poly(d-lactide) oligomer (PDLA). Through the investigation of the strain recovery in step-cycle experiments and compliance of stress relaxation behaviour with a three-component model for the deformation of semi-crystalline polymers, PLA stereocomplexes were found to possess sufficient stability in the true strain range εH < 2.25 to be described as firm physical netpoints at 70 °C in the studied blends with PLA stereocomplex content ϕc SC ≥ 1.1 wt%, when the PCL domains are melted. Limiting ϕc SC ≤ 6 wt% broadens the behaviour inherent to elastic cross-linked networks to the strain values up until breakage of the samples, while the increase of ϕc SC triggers plastic deformations typical for semi-crystalline polymers. Redistributing of internal stresses from the amorphous to crystalline domains at increase of ϕc calculated with the adopted model was identified as reason of PLA stereocomplexes failure as stable physical network junctions at higher ϕc SC. Within the experimentally determined strain and composition ranges, in which PLA stereocomplexes possess structural stability, they can form robust cross-links in a polymer network. The knowledge gained here provides valuable design criteria for multifunctional thermoplastic elastomers.