Towards a new generation of polymeric foams: PMMA nanocellular foams with enhanced physical properties

Recent Advances in Ferroelectret Fabrication, Performance Optimization, and Applications

The growing demand for wearable devices has sparked a significant interest in ferroelectret films. They possess flexibility and exceptional piezoelectric properties due to strong macroscopic dipoles formed by charges trapped at the interface of their internal cavities. This review of ferroelectrets focuses on the latest progress in fabrication techniques for high temperature resistant ferroelectrets with regular and engineered cavities, strategies for optimizing their piezoelectric performance, and novel applications. The charging mechanisms of bipolar and unipolar ferroelectrets with closed and open-cavity structures are explained first. Next, the preparation and piezoelectric behavior of ferroelectret films with closed, open, and regular cavity structures using various materials are discussed. Three widely used models for predicting the piezoelectric coefficients (d33) are outlined. Methods for enhancing the piezoelectric performance such as optimized cavity design, utilization of fabric electrodes, injection of additional ions, application of DC bias voltage, and synergy of foam structure and ferroelectric effect are illustrated. A variety of applications of ferroelectret films in acoustic devices, wearable monitors, pressure sensors, and energy harvesters are presented. Finally, the future development trends of ferroelectrets toward fabrication and performance optimization are summarized along with its potential for integration with intelligent systems and large-scale preparation.

Production and Application of Polymer Foams Employing Supercritical Carbon Dioxide

Polymeric foams have characteristics that make them attractive for different applications. However, some foaming methods rely on chemicals that are not environmentally friendly. One of the possibilities to tackle the environmental issue is to utilize supercritical carbon dioxide ScCO2 since it is a “green” solvent, thus facilitating a sustainable method of producing foams. ScCO2 is nontoxic, chemically inert, and soluble in molten plastic. It can act as a plasticizer, decreasing the viscosity of polymers according to temperature and pressure. Most foam processes can benefit from ScCO2 since the methods rely on nucleation, growth, and expansion mechanisms. Process considerations such as pretreatment, temperature, pressure, pressure drop, and diffusion time are relevant parameters for foaming. Other variables such as additives, fillers, and chain extenders also play a role in the foaming process. This review highlights the morphology, performance, and features of the foam produced with ScCO2, considering relevant aspects of replacing or introducing a novel foam. Recent findings related to foaming assisted by ScCO2 and how processing parameters influence the foam product are addressed. In addition, we discuss possible applications where foams have significant benefits. This review shows the recent progress and possibilities of ScCO2 in processing polymer foams.

Progress in the Preparation, Properties, and Applications of PLA and Its Composite Microporous Materials by Supercritical CO2: A Review from 2020 to 2022

The development of degradable plastic foams is in line with the current development concept of being pollution free and sustainable. Poly(lactic acid) (PLA) microporous foam with biodegradability, good heat resistance, biocompatibility, and mechanical properties can be successfully applied in cushioning packaging, heat insulation, noise reduction, filtration and adsorption, tissue engineering, and other fields. This paper summarizes and critically evaluates the latest research on preparing PLA microporous materials by supercritical carbon dioxide (scCO₂) physical foaming since 2020. This paper first introduces the scCO₂ foaming technologies for PLA and its composite foams, discusses the CO₂-assisted foaming processes, and analyzes the effects of process parameters on PLA foaming. After that, the paper reviews the effects of modification methods such as chemical modification, filler filling, and mixing on the rheological and crystallization behaviors of PLA and provides an in-depth analysis of the mechanism of PLA foaming behavior to provide theoretical guidance for future research on PLA foaming. Lastly, the development and applications of PLA microporous materials based on scCO₂ foaming technologies are prospected.

Microcellular foams production from nanocomposites based on PS using MOF nanoparticles with enhanced CO2 properties as nucleating agent

The use of metal-organic frameworks (MOF) nanoparticles as nucleating agents in gas dissolution foaming processes is presented. In this work, MOF nanoparticles with three different particle sizes were synthetized and introduced in film composites based on polystyrene at 1 wt.%. The addition of nanoparticles with high affinity to CO2, which is the gas used as a physical blowing agent, can contribute to increase the nucleation efficiency in comparison with the classical heterogeneous route using non CO2-philic particles. Nanoparticles dispersion in solids and cellular structure in foams were studied as a function of the particle size and foaming parameters, studying for first time the impact of MOF nanoparticles on the nucleation by gas dissolution foaming. Nucleation efficiencies in the order of 10−2 were achieved for PS/MOF composites. In addition, the thermal stability of the cellular structure in the composites was enhanced regarding to PS matrix, preserving the cellular structure regardless the foaming temperature. Therefore, MOF nanoparticles have emerged as promising nucleating agents in foaming procedures.

In situ controlled and conformal coating of polydimethylsiloxane foams with silver nanoparticle networks with tunable piezo-resistive properties

Nanoparticle-polymer composites hold promise in enabling material functionalities that are difficult to achieve otherwise, yet are hampered to date by the scarce control and tunability of the nanoparticle collective properties on the polymer surface, especially for polymer foams featuring a complex three-dimensional pore network. Here we report on the controlled and conformal in situ coating of polydimethylsiloxane (PDMS) foams with silver nanoparticles (AgNPs) with surface coverage finely tunable over a large range, from 0 to 75%, via the one-step room temperature reduction of AgF directly on the PDMS surface. This enables the design of AgNP electrical networks on the PDMS foam surface with piezo-resistive properties tunable up to a factor of 1000. We leveraged the control of the piezoresistive properties of the AgNP electrical network formed on PDMS foams to fabricate flexible and wearable pressure sensors with sensitivity of 0.41 kPa^-1, an operation range >120 kPa, and a detection limit of 25 Pa. As a proof-of-concept application in wearable biomedical electronics, we successfully used the sensors to monitor the real-time radial artery pulse wave on the human wrist of a young male with high resolution.

A Review on Mechanical Models for Cellular Media: Investigation on Material Characterization and Numerical Simulation

Cellular media materials are used for automobiles, aircrafts, energy-efficient buildings, transportation, and other fields due to their light weight, designability, and good impact resistance. To devise a buffer structure reasonably and avoid resource and economic loss, it is necessary to completely comprehend the constitutive relationship of the buffer structure. This paper introduces the progress on research of the mechanical properties characterization, constitutive equations, and numerical simulation of porous structures. Currently, various methods can be used to construct cellular media mechanical models including simplified phenomenological constitutive models, homogenization algorithm models, single cell models, and multi-cell models. This paper reviews current key mechanical models for cellular media, attempting to track their evolution from their inception to their latest development. These models are categorized in terms of their mechanical modeling methods. This paper focuses on the importance of constitutive relationships and microstructure models in studying mechanical properties and optimizing structural design. The key issues concerning this topic and future directions for research are also discussed.

Effect of the Molecular Structure of TPU on the Cellular Structure of Nanocellular Polymers Based on PMMA/TPU Blends

In this work, the effects of thermoplastic polyurethane (TPU) chemistry and concentration on the cellular structure of nanocellular polymers based on poly(methyl-methacrylate) (PMMA) are presented. Three grades of TPU with different fractions of hard segments (HS) (60%, 70%, and 80%) have been synthesized by the prepolymer method. Nanocellular polymers based on PMMA have been produced by gas dissolution foaming using TPU as a nucleating agent in different contents (0.5 wt%, 2 wt%, and 5 wt%). TPU characterization shows that as the content of HS increases, the density, hardness, and molecular weight of the TPU are higher. PMMA/TPU cellular materials show a gradient cell size distribution from the edge of the sample towards the nanocellular core. In the core region, the addition of TPU has a strong nucleating effect in PMMA. Core structure depends on the HS content and the TPU content. As the HS or TPU content increases, the cell nucleation density increases, and the cell size is reduced. Then, the use of TPUs with different characteristics allows controlling the cellular structure. Nanocellular polymers have been obtained with a core relative density between 0.15 and 0.20 and cell sizes between 220 and 640 nm.

The Relationship between the Morphology and Elasticity of Natural Rubber Foam Based on the Concentration of the Chemical Blowing Agent

Concentrated natural latex was used to produce a rubber foam that is porous, elastic and well ventilated. The mechanical properties can be either soft or firm, depending on the formulation of the latex used. Briefly, concentrated natural latex was mixed with chemical agents to make the rubber foam on a laboratory scale using the Dunlop process. In this work, we changed the concentration of the chemical blowing agent in the latex. The morphological properties of the rubber foam were characterised using scanning electron microscopy, and the mechanical properties, or elasticity, were studied using compression experiments and the Mooney-Rivlin calculation. The results show that the concentration of the chemical blowing agent affects the morphological properties of the rubber foam but not the mechanical properties, indicating the heterogeneous structure of the rubber foam. The thermodynamic parameters (∆G and ∆S) and the internal energy force per compression force (F_u/F) of the rubber foam with various amounts of chemical blowing agent were also investigated. This study could be applied in the foam industry, particularly for pillow, mattress and insulation materials, as the present work shows the possible novel control of the morphological structure of the rubber foam without changing its mechanical properties. The difference in cell sizes could affect the airflow in rubber foam.

Bioinspired material architectures from bighorn sheep horncore velar bone for impact loading applications

Rocky Mountain bighorn sheep rams (Ovis canadensis canadensis) routinely conduct intraspecific combat where high energy cranial impacts are experienced. Previous studies have estimated cranial impact forces to be up to 3400 N during ramming, and prior finite element modeling studies showed the bony horncore stores 3 × more strain energy than the horn during impact. In the current study, the architecture of the porous bone within the horncore was quantified, mimicked, analyzed by finite element modeling, fabricated via additive manufacturing, and mechanically tested to determine the suitability of the novel bioinspired material architecture for use in running shoe midsoles. The iterative biomimicking design approach was able to tailor the mechanical behavior of the porous bone mimics. The approach produced 3D printed mimics that performed similarly to ethylene–vinyl acetate shoe materials in quasi-static loading. Furthermore, a quadratic relationship was discovered between impact force and stiffness in the porous bone mimics, which indicates a range of stiffness values that prevents impact force from becoming excessively high. These findings have implications for the design of novel bioinspired material architectures for minimizing impact force.

Microwave Foaming of Materials: An Emerging Field

In the last two decades, the application of microwave heating to the processing of materials has to become increasingly widespread. Microwave-assisted foaming processes show promise for industrial commercialization due to the potential advantages that microwaves have shown compared to conventional methods. These include reducing process time, improved energy efficiency, solvent-free foaming, reduced processing steps, and improved product quality. However, the interaction of microwave energy with foaming materials, the effects of critical processing factors on microwave foaming behavior, and the foamed product's final properties are still not well-explored. This article reviews the mechanism and principles of microwave foaming of different materials. The article critically evaluates the impact of influential foaming parameters such as blowing agent, viscosity, precursor properties, microwave conditions, additives, and filler on the interaction of microwave, foaming material, physical (expansion, cellular structure, and density), mechanical, and thermal properties of the resultant foamed product. Finally, the key challenges and opportunities for developing industrial microwave foaming processes are identified, and areas for potential future research works are highlighted.

Highly Ordered Nanocellular Polymeric Foams Generated by UV-Induced Chemical Foaming

Nanocellular polymer foams have shown significant potential for industrial applications because of their superior thermal, mechanical, and optical properties. Some of these properties may be further improved by enhancing the ordering of cell structures. However, it is challenging for conventional foaming methods to control both the cell size and ordering at the nanoscale. Here, we show an innovative method to produce highly ordered nanocellular polymer foams by incorporating the self-assembly of an asymmetric diblock copolymer with the UV-induced chemical foaming technique. The minor domains are designed to generate a gaseous compound from the partial cleavage of the functional group. It is demonstrated that the gas-producing reaction can be accelerated at a temperature low enough to prevent melting of the whole self-assembled template, by mixing a small amount of photoacid generator into the copolymer, followed by UV irradiation. The result is the production of polymer foams with the nanoscale cells highly aligned to the self-assembled domains.

Wrong expectation of superinsulation behavior from largely-expanded nanocellular foams

This work aims to predict the thermal conductivity of microcellular and nanocellular thermal insulation foams to explore the correlation between the cellular structure and the thermal insulating properties. Closed-cell foam consisting of cell walls and struts was used as the base geometry for modeling. The mathematical correlations to calculate the thickness of cell walls and the diameter of struts for a given cell size, the void fraction and the volume fraction of polymer located in struts were investigated. Then, a mathematical model for the conductive thermal conductivity including the dependency on the void fraction, the strut fraction and the Knudsen effect for gas was introduced. The radiative thermal conductivity was determined by analyzing the attenuation of radiative energy by absorption and scattering based on Mie's theory together with electromagnetic wave interference, as well as interference of propagating waves and tunneling of the radiative energy by evanescent waves in the cells. The thermal conductivity model was validated by experimental data and used to predict the thermal conductivity of polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at various cell sizes and volume expansion ratios. It was found that the radiative thermal conductivity plays a crucial role in nanocellular foam. The trade-off between the cell size and cell wall thickness when cell walls become thinner and highly transparent to thermal radiation was demonstrated, leading to the optimal volume expansion ratio at which the thermal conductivities were minimized. Perspectives for the manufacture of high-performance thermal insulation foams are also discussed.

Bubble Seeding Nanocavities: Multiple Polymer Foam Cell Nucleation by Polydimethylsiloxane-Grafted Designer Silica Nanoparticles

We describe a successful strategy to substantially enhance cell nucleation efficiency in polymer foams by using designer nanoparticles as nucleating agents. Bare and poly(dimethylsilane) (PDMS)-grafted raspberry-like silica nanoparticles with diameters ranging from ∼80 nm to ∼200 nm were synthesized and utilized as highly efficient cell nucleators in CO₂-blown nanocellular polymethyl methacrylate (PMMA) foams. The successful synthesis of core-shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer-Emmett-Teller measurements, and transmission electron microscopy. The cell size and cell density of the obtained PMMA micro- and nanocellular foams were determined by scanning electron microscopy. The results show that increased surface roughness enhances the nucleation efficiency of the designer silica particles. This effect is ascribed to a decreased nucleation free energy for foam cell nucleation in the nanocavities at the melt-nucleator interface. For PDMS grafted raspberry-like silica nanoparticles with diameters of 155 and 200 nm, multiple cell nucleation events were observed. These hybrid particles had nucleation efficiencies of 3.7 and 6.2, respectively. The surprising increase in nucleation efficiency to above unity is ascribed to the significant increase in CO₂ absorption and capillary condensation in the corresponding PMMA during saturation. This increase results in the presence of large amounts of the physical blowing agent close to energetically favorable nucleation points. Additionally, it is shown that as a consequence of cell coalescence, the increased number of foam cells is rapidly reduced during the first seconds of foaming. Hence, the design of highly efficient nucleating particles, as well as careful selection of foam matrix materials, seems to be of pivotal importance for obtaining polymer cellular materials with cell dimensions at the nanoscale. These findings contribute to the fabrication of polymer foams with high thermal insulation capacity and have relevance in general to the area of cellular materials.

The mechanics of solid-state nanofoaming

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO₂ as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gas-filled voids during the solid-state foaming process. Diffusion of CO₂ within the PMMA matrix is sufficiently rapid for the concentration of CO₂ to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO₂ is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall tearing and comparisons are made between the predictions of the model and nanofoaming measurements; it is deduced that the failure strain of the cell walls is sensitive to cell wall thickness.

Effect of Mold Temperature on the Impact Behavior and Morphology of Injection Molded Foams Based on Polypropylene Polyethylene⁻Octene Copolymer Blends

In this work, an isotactic polypropylene (PP) and a polyethylene–octene copolymer (POE) have been blended and injection-molded, obtaining solids and foamed samples with a relative density of 0.76. Different mold temperature and injection temperature were used. The Izod impact strength was measured. For solids, higher mold temperature increased the impact resistance, whereas in foams, the opposite trend was observed. In order to understand the reasons of this behavior, the morphology of the elastomeric phase, the crystalline morphology and the cellular structure have been studied. The presence of the elastomer near the skin in the case of high mold temperature can explain the improvement produced with a high mold temperature in solids. For foams, aspects as the elastomer coarsening in the core of the sample or the presence of a thicker solid skin are the critical parameters that justify the improved behavior of the materials produced with a lower mold temperature.

Nanocellular Polymers: The Challenge of Creating Cells in the Nanoscale

The evolution of technology means that increasingly better materials are needed. It is well known that as a result of their interesting properties, nanocellular polymers perform better than microcellular ones. For this reason, the investigation on nanocellular materials is nowadays a very topical issue. In this paper, the different approaches for the production of these materials in our laboratory are explained, and results obtained by using polymethylmethacrylate (PMMA) are shown. Homogeneous nucleation has been studied by using raw PMMA, while two different systems were used for heterogeneous nucleation; adding nanoparticles to the system and using nanostructured polymers as solid precursors for foaming. The effects of the different parameters of the production process (gas dissolution foaming process) have been evaluated for all systems being possible to establish a comparison between the materials produced by different approaches. Moreover, the limitations and future work to optimise the materials produced are also discussed.

Foaming Behavior and Microcellular Morphologies of Incompatible SAN/CPE Blends with Supercritical Carbon Dioxide as a Physical Blowing Agent

The foaming process and cellular morphologies of poly(styrene-co-acrylonitrile) (SAN)/chlorinated polyethylene (CPE) blends with supercritical carbon dioxide (scCO₂) as a blowing agent were investigated in this study. As compared to pure SAN foam in the same batch, the foamed blends with various CPE elastomer content had smaller average pore size and larger cell density. This is probably related to the inhibition of bubble growth by elastomer, resulting in poor melt flowability and strong viscoelasticity, and the efficient bubble heterogeneous nucleation caused by numerous phase interfaces inside the incompletely compatible blend system. In addition, many tiny interconnected holes through the pore walls were formed to connect adjacent micropores in foamed blend samples. The formation mechanism of such interconnected pores is probably due to the fracture of stretched melt around the bubble from phase interfaces with weak interactions. These facts suggest an effective path to control pore size, cell density and even interconnected pores of blend foams depends on the compatibility of the blend system and difference in foamability of individual components in supercritical CO₂.

Study on Foaming Quality and Impact Property of Foamed Polypropylene Composites

In the present work, foamed polypropylene (PP) composites were prepared by chemical foaming technology, and the foaming quality and impact property of the foamed PP composites were studied. The results showed that the foaming quality was significantly improved after the introduction of thermoplastic rubber (TPR) and polyolefin elastomer (POE). Meanwhile, it was found that the impact property depended on the intrinsic toughness and contribution of foams (cells) to the PP composites. Furthermore, the data regarding impact property in low temperature showed that when the temperature was between -80 and -20 °C, the impact properties of the foamed PP composites were higher than that of the unfoamed sample, which was due to the impact property being completely contributed by cells under this condition. Meanwhile, when the temperature ranged from -20 to 20 °C, the impact property of the unfoamed sample was higher, which was due to the PP matrix contributing more to the impact property under this temperature. This work significantly improved the foaming quality of foamed PP composites and provided reliable evidence for the improvement of impact property.

Silica-Assisted Nucleation of Polymer Foam Cells with Nanoscopic Dimensions: Impact of Particle Size, Line Tension, and Surface Functionality

Core-shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO₂-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO₂-philicity. The successful synthesis of core-shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core-shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO₂ saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO₂ at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.