Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

Injectable hyaluronan/MnO2 nanocomposite hydrogel constructed by metal-hydrazide coordinated crosslink mineralization for relieving tumor hypoxia and combined phototherapy

Hydrogel-based drug delivery holds great promise in topical tumor treatment. However, the simple construction of multifunctional therapeutic hydrogels under physiological conditions is still a huge challenge. Herein, for the first time, a multifunctional hyaluronan/MnO₂ nanocomposite (HHM) hydrogel with injectable and self-healing capabilities was constructed under physiological conditions through innovative in situ mineralization-triggered Mn-hydrazide coordination crosslinking. The hydrogel formed from Mn²⁺ and hydrazided hyaluronan under optimized conditions exhibited a high elastic modulus >1 kPa, injectability, self-healing function, stimuli-responsiveness and catalase-like activity. In vitro and in vivo biological experiments demonstrated that our HHM hydrogel could not only efficiently relieve hypoxia by in situ catalytic decomposition of endogenous H₂O₂ into O₂ but also achieve synergistic photodynamic/photothermal therapy of 4T1 breast cancer in a mouse tumor model. This study presented a novel mineralization-driven metal-hydrazide coordination crosslinking approach and developed a multifunctional therapeutic platform for O₂-enhanced efficient topical dual-phototherapy of breast cancer.

The effect of polytetrafluoroethylene particle size on the properties of biodegradable poly(butylene succinate)-based composites

Poly(butylene succinate) (PBS)/polytetrafluoroethylene (PTFE) composites, including three types of PTFE powders, were prepared by melt blending using a HAAKE torque rheometer. Microcellular foams were successfully fabricated by batch foaming with supercritical fluids (scCO₂). The effects of PTFE powder type on crystallization, rheological properties and foaming behavior were studied. PTFE L-5 and PTFE JH-220 powders showed good dispersion in the PBS matrix, and PTFE FA-500 powder underwent fibrillation during the melt blending process. All three PTFE powders gradually increased the crystallization temperature of PBS from 78.2 to 91.8 ℃ and the crystallinity from 45.6 to 61.7% without apparent changes in the crystal structure. Rheological results revealed that PBS/PTFE composites had a higher storage modulus, loss modulus, and complex viscosity than those of pure PBS. In particular, the complex viscosity of the PBS/P500 composite increased by an order of magnitude in the low-frequency region. The foamed structure of PBS was obviously improved by adding PTFE powder, and the effect of fibrillated PTFE FA-500 was the most remarkable, with a pore mean diameter of 5.46 μm and a pore density of 1.86 × 10⁹ cells/cm³ (neat PBS foam: 32.49 μm and 1.95 × 10⁷ cells/cm³). Moreover, PBS/P500 foam always guarantees hydrophobicity.

Preparation and properties of melamine-formaldehyde rigid closed-cell foam toughened by ethylene glycol/carbon fiber

Toughing melamine-formaldehyde (MF) rigid closed-cell foams were prepared by using ethylene glycol (EG) and carbon fiber (CF) as composite toughening agents. The pulverization rate, compressive strength, bending strength, cellular structure, closed-cell ratio, water absorption ratio, thermal conductivity, thermal stability, limiting oxygen index (LOI), and char yield were characterized to study the morphology, mechanical, thermal, and fire-retardant properties of as-prepared toughing MF rigid foams. The pulverization rate result showed that introduction of composite modifier can obviously improve the toughness of MF rigid foams. The cellular structure, closed-cell ratio, and water absorption results showed that the addition of EG/CF can increase the closed-cell ratio and control the cell size of MF rigid foams. The compressive strength and bending strength results showed that the incorporation of composite modifier of MF rigid foams dramatically improved the mechanical properties. The LOI, char yield, and thermal stability results showed that the toughing MF rigid foams remained more intact char skeleton with flame-retardant effect, thus reducing the fire hazards. The as-prepared toughing MF rigid foams showed the best comprehensive performance with pulverization rate of 5.21%, compressive strength of 355.3 kPa, bending strength of 0.44 MPa, closed-cell ratio of 79.1%, water absorption of 9%, thermal conductivity of 0.031 W m−1 K−1, and LOI of 39.6%. Compared with unmodified MF rigid foams, toughing rigid closed-cell MF foams possess excellent pulverization rate, compressive strength, bending strength, cellular structure, thermal insulation, and flame retardancy.

Fabrication of Poly(butylene succinate)/Carbon Black Nanocomposite Foams with Good Electrical Conductivity and High Strength by a Supercritical CO2 Foaming Process

Lightweight, high-strength and electrically conductive poly(butylene succinate) (PBS)/ carbon black (CB) nanocomposite foams with a density of 0.107–0.344 g/cm³ were successfully fabricated by a solid-state supercritical CO₂ (ScCO₂) foaming process. The morphology, thermal and dynamic mechanical properties, and rheological behavior of the PBS/CB nanocomposites were studied. The results indicate that the CB nanofiller was well dispersed in the PBS matrix and the presence of a proper CB nanofiller can accelerate the rate of crystallization, improve the thermal stability, enhance the stiffness, and increase the complex viscosity of PBS/CB nanocomposites. These improved properties were found to play an important role in the foaming process. The results from foaming experiments showed that the PBS/CB nanocomposite foams had a much smaller cell size, a higher cell density, and a more uniform cell morphology as compared to neat PBS foams. Furthermore, the PBS/CB nanocomposite foams also possessed low density (0.107–0.344 g/cm³), good electrical conductivity (~0.45 S/cm at 1.87 vol % CB loading), and improved compressive strength (108% increase), which enables them to be used as lightweight and high-strength functional materials.

Effect of PBAT on physical, morphological, and mechanical properties of PBS/PBAT foam

This study examines the effect of poly(butylene adipate- co-terephthalate) (PBAT) content on the physical, morphological, and mechanical properties of poly(butylene succinate) (PBS)/PBAT foam. A compression molding technique was used to prepare the PBS/PBAT foam using the chemical blowing agent azodicarbonamide and the cross-linking agent dicumyl peroxide. The chemical structure and morphological properties of PBS/PBAT foam were examined via Fourier transform infrared and scanning electron microscopy techniques, respectively, whereas tensile and flexural properties were investigated using a universal testing machine. The results reveal that the incorporation of PBAT barely enhances the viscosity of the PBS/PBAT blend, producing only minor changes in the average cell size of PBS/PBAT foam. However, increasing the PBAT content contributes to a relatively significant improvement in the flexibility and toughness of PBS/PBAT foam, where a decrease in Young’s modulus and tensile strength of the PBS/PBAT foam is observed compared with those of the PBS foam. Similar behavior to the tensile results is noticed for the flexural properties of the neat and PBS/PBAT foams.