Ultrasonic Irradiation Enhanced Cell Nucleation in Microcellular Poly(lactic Acid): A Novel Approach to Reduce Cell Size Distribution and Increase Foam Expansion

Abstract

The preparation of poly(lactic acid) (PLA) foam with a well-defined cell structure, high crystallinity, a high expansion ratio, and good mechanical properties is critical to its broader applications. However, achieving these properties in PLA foam simultaneously is challenging, because high crystallinity generally results in nonuniform cell nucleation and suppresses cell growth in the case of solid-state foaming. This study presents a novel approach using ultrasonic irradiation (UI) to achieve the desired properties in PLA simultaneously. As expected, CO2-saturated PLA samples at 5 MPa have a high crystallinity (23.4%), and foamed PLA samples at various foaming temperatures exhibit low foam expansion and nonuniform cell structure. By introducing UI at the very start of the foaming, however, the resultant PLA foams presented a significant and concurrent increase in cell structure uniformity and cell density: cell density increased about 2 orders of magnitude, the expansion ratio increased 1–2 times, the elongation at break increased 2 times, and the specific tensile strength increased 1.1 times, compared to samples without UI. Further investigation indicated that the enhanced cell nucleation induced by UI was the main reason for this unique phenomenon. Our study provides a simple but efficient and cost-effective method to fabricate PLA foams that possess excellent mechanical properties.