Fabrication of Highly Interconnected Poly(ε-caprolactone)/cellulose Nanofiber Composite Foams by Microcellular Foaming and Leaching Processes

In this study, microcellular polycaprolactone (PCL)/sodium bicarbonate (NaHCO3)/cellulose nanofiber (CNF) composite foams with highly interconnected porous structures were successfully fabricated by microcellular foaming and particle leaching processes. Supercritical CO2 (scCO2) served as a physical foaming agent, NaHCO3 was chosen as a chemical foaming agent and porogen, and CNF acted as a heterogeneous nucleating agent. The effect of scCO2, NaHCO3, and CNF on pore structures and the cofoaming mechanism were investigated. The results indicated that the addition of NaHCO3 and CNF increased the melt strength of the PCL matrix significantly. During the foaming process, the presence of CNF can form a rigid network due to the hydrogen bonding or mechanical entanglement between individual nanofibers, improving the nucleating efficiency but slowing down the cell growth rate. Additionally, due to the interaction of "soft" PCL matrix and "hard" domains in a PCL-based composite during the foaming process, together with the NaHCO3 leaching process, highly interconnected cell structures appeared. The obtained PCL/NaHCO3/CNF composite foams had a cell size of 15.8 μm and cell density of 6.3 × 107 cells/cm3, as well as an open-cell content of 82%. The reported strategy in this paper may provide the guidelines and data supports for the fabrication of a PCL-based porous scaffold.