Effect of LCB Density on Strain Hardening Behavior and Processing Stability of PHA/LCB-PBAT Blends

Polyhydroxyalkanoate (PHA), a microbial derived biodegradable polymer, offers excellent biodegradability but suffers from severe thermal degradation during melt processing because its melting temperature lies close to its degradation temperature. In this study, long-chain branched poly(butylene adipate co terephthalate) (LCBPBAT) was melt-blended with PHA to improve melt processing stability and impart strain hardening under extensional flow.

Blends were prepared over the entire composition range (100/0 to 0/100) at 170 °C, 50 rpm, for 10 min. SAOS, time sweep, extensional viscosity fixture (EVF), tensile testing, and SEM revealed a morphological transition from droplet matrix to co continuous (50/50) and phase inversion structures with increasing LCBPBAT content, accompanied by progressive suppression of melt property deterioration. However, strain hardening emerged only from 50/50, where LCB-PBAT first forms a continuous phase, and was absent in PHA rich compositions. This is attributed to the relatively low LCB density of LCBPBAT (SHCmax ≈ 2.4 for the neat resin), which falls below the critical entanglement density upon dilution with PHA.

To examine this hypothesis, the epoxy based chain extender Joncryl ADR 4468 was used to increase the LCB density of LCB PBAT. A two-step reactive blending route was adopted, in which Joncryl was pre reacted with LCB PBAT prior to blending with PHA at a fixed 80/20 composition. Six samples were prepared by varying Joncryl content (0.1, 0.25, 0.5 phr) and pre reaction time (10, 30, 60 min). FTIR was used to quantify epoxy conversion, while SAOS, time sweep, SEM, and EVF evaluated viscoelasticity, morphology, and strain hardening behavior.

This work identifies LCB density as the key parameter governing strain hardening in PHA rich blends and proposes control of chain extender reaction conditions as a strategy for tuning the processability of biodegradable polymer blends.