Non-crosslinked thermoplastic reticulated polymer foams from crystallization-induced structural heterogeneities

Reticulated polymer foams are commercially relevant materials suitable for a broad range of applications including impact absorption, environmental protection, biological scaffolds and sacrificial templates for the fabrication of functional materials. However, to the best of our knowledge, scalable fabrication of reticulated polymer foams is limited to crosslinked, thermoset materials which are difficult to process and recycle. We report a facile method to prepare reticulated foams of noncrosslinked, thermoplastic polypropylene (PP) containing a small amount of polytetrafluoroethylene (PTFE) fibers, in a fully-scalable continuous extrusion process. Under the optimum conditions, foams with a mass density of 0.07 g/cm(3) and an open-cell content of 97.7% are achieved. The formation of a reticulated foam structure is rarely observed in thermoplastic materials, and is attributed to four effects of the PTFE fibers taking place concomitantly during the foaming process: 1) localized plasticization of the matrix due to the CO2-philicity of the PTFE fibers which creates weak sites for pore formation, 2) generation of crystalline heterogeneities during extrusion which forms hard segments in a soft matrix, 3) decrease in cell wall thickness, and 4) reinforcement of the cell strut which preserves the cell trace. The method proposed herein may be extended to other thermoplastic materials for fabricating reticulated foams. (C) 2017 Elsevier Ltd. All rights reserved.