Injection molding of highly filled microcrystalline cellulose/polycaprolactone composites with the aid of reversible Diels-Alder reaction

To tackle the challenge of producing highly filled polymer composites using the traditional injection molding technique, which is characterized by the fairly high melt viscosity that makes mold filling difficult, the authors propose a solution based on dynamic covalent chemistry. As demonstrated by the proof-of-concept experiments, the 4-arm star-shaped polycaprolactone (PCL) oligomers and microcrystalline cellulose (MCC) are crosslinked by the reversible Diels-Alder (DA) bonds. The flowability of the compounds greatly decreases due to the dissociation of the intercomponent DA bonds at the retro-reaction temperature, and the networked architecture is reconstructed during cooling as a result of the forward DA reaction. Consequently, the high-loading MCC fillers are well distributed in the matrix and covalently bonded to the nearby PCL, forming a striking contrast to the control in which linear PCL acts as the matrix. The DA bonds crosslinked biodegradable PCL composites exhibit decent mechanical strength (20.7 MPa) even at the MCC fraction of 65 wt%, which is superior to those (5–12.2 MPa) of the highly filled PCL composites (with filler contents of 50–63.8 wt%) reported so far. The proposed approach has sufficient expansibility for the fabrication of the highly filled polymer composites constructed by other types of matrix and fillers.