Crystallization-Driven Design of Multilayered Poly(l-lactide) Foams

Production of semicrystalline polymer foams is a technological challenge, because crystal formation before or during foaming can hinder expansion. We succeeded to turn this challenge into innovation, by exploiting tailored crystal forma- tion in only a part of the material before foaming, to produce multilayered monomaterial foams. A layered crystalline structure was developed in an initially amorphous poly(l-lactide) disk, by precisely regulating the process parameters that control mass diffusion of the foaming agent. This allowed to fill the polymer with a tailored gradient of foaming agent, able to induce crystal formation in a part of the sample that became not foamable, and bubble formation in the parts exposed to lower amounts of foaming agent. Foams with multilayered structured morphology, made of alternating layers of either crystalline or amorphous materials, and of either foamed or unfoamed parts, were produced with a single polymer, which made them easily recyclable. This surpasses the up to date state of art of multilayered foams, which are currently made with alternating layers of different materials, hardly and costly to recycle.