Effect of molecular structure on porous structure and performance of star-branched poly(lactic acid) ultrafiltration membranes

Poly(lactic acid) (PLA) is a biomass-derived biodegradable polymer that shows promise as a circular resource for water-treatment membranes. The molecular structure of PLA affects its phase separation behavior, enabling control of pore formation to improve membrane performance. Changing the molecular chain density of PLA also affects its biodegradation behavior. However, conventional polymerization methods hardly provide PLA with both narrow molecular-weight distribution and high molecular weight, which limits research on the molecular structure of PLA. In this study, linear and star-branched PLA with four or eight arms were synthesized via organocatalyzed ring-opening polymerization, achieving a weight-average molecular weight ( M w ) of ∼95,000 and narrow dispersity (≤ 1.2). PLA membranes were prepared via nonsolvent-induced phase separation and the effects of branch number and M w on pore structure and hydrolysis behavior were investigated. The membrane surfaces exhibited fine pores with underlying finger-like macrovoids. Molecular weight cut-off increased with branch number (linear: 15,000 Da; four: 17,000 Da; eight: 52,000 Da). This trend was attributed to the decrease in solution viscosity with increasing branch number, which facilitated phase separation and pore growth. The decrease rate of molecular weight in accelerated hydrolysis tests (pH 9, 65 °C, 215 h) was 90% for four-arm PLA, 70% for linear, and 49% for eight-arm PLA. A moderate PLA branching structure promoted hydrolysis, whereas an excessive branch number increased the chain density near the branch points, inhibiting water penetration and hydrolysis. These findings indicate that branch number, M w , and viscosity control are important for improving membrane performance and degradability. • Star-branched PLA with low dispersity was synthesized via organocatalysis. • Star-branched PLA membranes were prepared via nonsolvent-induced phase separation. • Pore diameter and molecular weight cut-off increased with branch number of PLA. • Eight-arm PLA membrane exhibited the highest water permeance. • Four-arm PLA membrane was hydrolyzed faster than linear and eight-arm membranes.