Impact of citrate plasticizers and crystalline content on the stress relaxation behaviour of poly(lactic acid)

his paper investigates poly(lactic acid) (PLA) plasticized with citrate-based molecules, focusing on how different crystalline contents and the addition of 5 wt% acetyl tributyl citrate (PLA_ATBC) and acetyl triethyl citrate (PLA_ATEC) influence the stress relaxation behaviour of the biopolymer. Neat and plasticized PLA samples were injection moulded under controlled crystallization conditions, to obtain different crystalline contents. Fourier transform infrared analysis revealed polymer–plasticizer interactions through characteristic red shifts in the PLA spectrum. Plasticizers reduced the glass transition temperature and increased crystallinity from 33% in neat PLA to 43% for PLA_ATBC and 39% for PLA_ATEC while Gibbs–Thomson analysis evidenced thinner crystalline lamellae for the plasticized samples, due to the formation of less-perfect crystals. Elastic moduli decrease from 3.3–4.0 GPa for neat PLA to 3.2 GPa for PLA_ATBC and 3.4 GPa for PLA_ATEC, reducing stiffness. However, plasticizers improved elongation at break from 1.2–1.7% for neat PLA to 1.8–2.8% for PLA_ATBC and 1.4–2.6% for PLA_ATEC, although fracture remains brittle in all cases. Prony fitting on stress relaxation experimental curves showed that the residual stress decreased from 6.9 MPa for neat PLA to 3.7 MPa for PLA_ATBC and 4.2 MPa for PLA_ATEC, confirming that plasticization accelerates stress relaxation, with ATBC providing the fastest relaxation kinetics. Bleaching on the samples’ lateral surface is observed after the stress relaxation tests, evidenced by the formation of approximately 2–5 ∼m spherulites, evidencing strain-induced crystallization and highlighting the dynamic structural evolution of PLA under deformation, further enhanced in the presence of citrate-based plasticizers.