Influence of crystallization kinetics on the cellular structure of biodegradable PLA foams during supercritical foaming

Abstract

Poly (lactic acid) (PLA) foams prepared by supercritical CO₂ foaming offer a sustainable route toward lightweight biodegradable materials, yet their cellular uniformity and mechanical stability are strongly limited by the poorly controlled coupling between crystallization and gas expansion. This study establishes a thermodynamic–kinetic framework to clarify how CO₂-induced plasticization regulates crystallization kinetics, bubble nucleation, and structural fixation in PLA foams. DSC, SAXS, Avrami kinetic analysis, SEM morphology statistics, and compression testing reveal that dissolved CO₂ lowers the crystallization activation energy from $95.0\pm 2.1$ to $82.0\pm 1.6\hspace{0.17em}\text{kJ}\hspace{0.17em}\text{mo}{\mathrm{l}}^{-1}$ , accelerates heterogeneous nucleation, and promotes the formation of finer cellular structures. An optimized crystallinity range of 30–45% was identified, within which the foams exhibited an average cell size below $100\hspace{0.17em}\mu \mathrm{m}$ , a cell density of approximately $\left(1.84–2.24\right)\times {10}^7\hspace{0.17em}\text{cells}\hspace{0.17em}\mathrm{c}{\mathrm{m}}^{-3}$ , and a foam density of $0.23–0.26\hspace{0.17em}\mathrm{g}\hspace{0.17em}\mathrm{c}{\mathrm{m}}^{-3}$ . Within this structural window, crystalline lamellae reinforce cell walls while maintaining sufficient gas expansion, resulting in a compressive modulus of 28–36 MPa and an energy absorption capacity of 2.1–2.6 MJ m⁻³. Excessive crystallization, however, causes premature structural freezing and pore inhomogeneity. These results provide quantitative guidance for designing mechanically robust and structurally uniform biodegradable foams through controlled crystallization–foaming synergy.