Supercritical CO2 foaming of PCL covalent networks : taking benefit from the thermo-reversible Diels-Alder cycloaddition

Foams are versatile materials encountered in our daily life for a wide variety of uses such as cushioning, thermal and acoustic insulation or medical applications. The combination of the mixed properties between a continuous matrix and gas cells and the diversity of pore structures represent a powerful tool for the design of new materials. Among the different polymer foam fabrication processes, the use of supercritical CO2 has been one of the most investigated in the past decade. Nevertheless, the design of crosslinked polymer foams with high foaming ratio still remains a challenge. Various crosslinking processes mainly based on heating, irradiation with the addition of an external agent have been applied after foaming but remain difficult to perform due to mass transfer issues of the crosslinking agent. When crosslinking occurs before foaming, it dramatically limits the material expansion.

In order to overcome these drawbacks, the present work aims taking advantage of the thermoreversible Diels-Alder cycloaddition to elaborate foams of poly(ε-caprolactone) (PCL) covalent networks. Based on this reaction, we considered to induce cross-linking after the foam expansion by playing on the thermal equilibrium of the thermoreversible Diels-Alder cycloaddition. Therefore, low molar mass star-shaped PCL end-capped by furan or maleimide were impregnated with CO2 under supercritical conditions and then foamed under appropriate control of the pressure and temperature. Annealing about 2h at 40°C allows Diels-Alder adducts to be formed and a stable PCL network keeping at least 80% of the expanded volume is obtained. The resulting foam possesses a much higher volume expansion than a pre-crosslinked sample foamed in the same conditions, thanks to the low crosslinking ratio during foaming. These foams exhibit also improved thermal stability thanks to its chemical crosslinking as compared to non-crosslinked PCL foams. Interestingly, these foams possess shape memory properties due to the semi-crystallinity of the PCL. Thermal stability and shape memory properties were evaluated by dynamic mechanical analysis in both tensile and compression testing with controlled force mode, stress and temperature ramps.