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In this paper, a supercritical CO2 (sc-CO2) foaming-based method is reported for fabricating controllable macroporous carbon monoliths with uniform cellular structures. In the proposed method, we obtain carbon monoliths through sequential steps of hot-pressing, sc-CO2 foaming, and carbonization. X-ray diffraction, Fourier-transform infrared, and Raman results demonstrate that the PAN foams transform into semi-graphitic carbons following carbonization, and the unchanged peak positions of different carbons indicate that changing foaming condition does not affect carbonization degree of the carbon monoliths. The carbon monolith fabricated from a foaming condition of 10.34 MPa and 100 °C exhibits an electrical conductivity of 132 S m-1, which is greater than that of many typical porous carbon materials. Therefore, the proposed method provides a scalable and flexible strategy for fabricating macroporous carbon monoliths.

Poly (lactic acid) (PLA) is a biodegradable and biocompatible polymer derived from renewable resources that demonstrates an immense potential in replacing fossil-based polymers. Nevertheless, its inherent brittleness and poor thermal resistance significantly restrict its further applications. Developing an environmentally friendly and cost-effective method for strengthening and toughening PLA without sacrificing its other functional properties remains a challenge. In this study, we report a novel method to reinforce PLA by carbon dioxide (CO2) treatment at room temperature. Remarkably, CO2 treatment led to simultaneously enhanced strength, toughness, ductility and thermal resistance. Under 3.5 MPa CO2 treatment, PLA demonstrated the best mechanical performance with tensile strength, ductility, and impact toughness enhanced by 25%, 1214%, and 134%, respectively. Moreover, the Vicat softening temperature was significantly enhanced from 55.8 °C to 155.3 °C. Wide angle X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry elucidate the strengthening and toughening mechanism to be CO2-induced crystallization. The small-angle laser scattering and scanning electron microscope demonstrate the formation of numerous nanorod crystallites, which are responsible for coincidentally enhancing the strength, toughness, and thermal resistance. Thus, CO2 treatment at room temperature provides a green and effective method to provide high-performance PLA products that have immense potential in biomedical, automotive, and other structural applications.

Poly(ether-block-amide) (PEBA) is an advanced thermoplastic elastomer that has very low material density and excellent resilience over a wide temperature range. High-performance PEBA foams enabled by microcellular foaming show great prospects in sports, safety protection, and medical rehabilitation, yet their preparation is still greatly limited by the unclear processing-structure-property relationships. Herein, microcellular foaming with CO2 as the blowing agent was developed to fabricate high-performance PEBA foams. Foaming temperatures and CO2 pressures were varied to investigate their effects on foam structure, and hence PEBA foams with a tailored cellular structure were achieved. With the structure-tunable PEBA foams, the processing-structure-performance relationships were clarified. It was demonstrated that a higher expansion ratio and larger cell size lead to significantly enhanced resilience and reduced energy loss coefficient. In particular, PEBA foams with an expansion ratio of up to 24.5-fold were prepared, which possess a drop-ball resilience of up to 80 %, obviously superior to current foams. Moreover, the PEBA foams with larger cells exhibit significantly improved cyclic compression stability, and the minimum energy loss coefficient is lower than 15 %. The super-elastic PEBA foams with tailored structure exhibit promising prospects in advanced resilience applications.

Microcellular foaming is an advanced eco-friendly foaming technology with carbon dioxide as blowing agent. Generally, high-pressure supercritical blowing agents is needed to achieve microcellular structure with cell size less than 10 μm, which places high requirements on the foaming equipment and brings safety issues, thereby resulting in high production and maintenance costs. Herein, the stretch-assisted microcellular foaming technology was developed to prepare TPU, TPU/nanographite, and TPU/PMMA foams with significantly refined cellular structure. The results demonstrate that stretch can dramatically promote cell nucleation. For the neat TPU, the cell density can be increased by more than 8 orders of magnitude by applying a linear strain of 50 % in microcellular foaming. Increasing the amount of stretching can continue to refine the cell morphology. Compared with the neat TPU foam, the TPU/nanographite composite foam and the TPU/PMMA composite foam show further refined cellular morphology under the same foaming condition. Surprisingly, with low-pressure (300 psi) gaseous carbon dioxide as blowing agents, microcellular TPU and TPU/nanographite foams, and nanocellular TPU/PMMA foam were achieved by the stretch-assisted microcellular foaming. Furthermore, the modified classical cell nucleation theory by considering the role of elastic distortion energy in cell nucleation was proposed to clarify the cell nucleation behavior in the stretch-assisted microcellular foaming process. According to the theory, the synergistic effect of stretch-induced cell nucleation and heterogeneous phase-induced cell nucleation is the underlying mechanism of the significantly refined cellular morphology.

Porous shape memory polymers (SMPs) are attracting people's attention due to their extensive application prospect. Herein, a novel and flexible method to produce bio-friendly polylactic acid (PLA)/polymethyl methacrylate (PMMA) shape memory foam by a two-step CO2 microcellular foaming is proposed. The achieved foams exhibit outstanding shape memory performance, such as a maximum shape fixity ratio of 98 %, a maximum shape recovery ratio of 86 %, and further the recovery ratio remaining more than 75 % even after 10 compression-recovery cycles. The additive PMMA phases uniformly disperse with spherical morphology in nanoscale, and they result in the greatly decreased crystallinity and crystal size of PLA. Meanwhile, the extensive interfaces lead to increased crystal density. In this context, more amorphous molecular chains left in the cell wall act as reversible segments, and more numerous denser crystals act as net points, which together can decrease the resistance in the shape memory process, thus, facilitating the significant promotion of shape memory capability of the as-achieved PLA/PMMA foams. Therefore, it exhibits a promising future to produce environmentally friendly SMPs by a simple, cost-efficient, and clean microcellular foaming technology.