Shear-Induced Evolution of Cellular Structure and Thermo-Mechanical Properties in PLA/HNC and In-situ Fibrillated PLA/PTFE Composite Foams.

High-performance cellular biopolymeric foams require precise control of processing and filler-induced microstructure. In this study, poly(lactic acid) (PLA) composite foams with halloysite nanoclay (HNC) and polytetrafluoroethylene (PTFE) were produced via twin-screw extrusion foaming using azodicarbonamide. The effect of screw speed (20-120 rpm) on morphology, crystallization, and compressive properties was investigated. In PLA/HNC, increasing screw speed improved filler dispersion and heterogeneous nucleation, reducing cell size by ∼50-60% and yielding a maximum void fraction of ∼20.5% at 60 rpm. In contrast, PLA/PTFE exhibited complete in situ fibrillation into a three-dimensional nanofibrillar network (∼100-500 nm), whose density increased continuously with screw speed without saturation. PLA/PTFE showed over a 20-fold reduction in crystallization half-time of PLA at 130 °C (vs ∼4-fold for HNC) due to its high nucleation surface area. It also achieved superior compressive performance (∼68-70 MPa·cm3/g strength and ∼11 J/g toughness at 120 rpm), exceeding PLA/HNC by ∼50% and ∼80%, respectively, through fibril-driven reinforcement mechanisms of cell walls.