The influence of carbon nanotube aspect ratio on the foam morphology of MWNT/PMMA nanocomposite foams

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.

Micro foaming performance of scCO2-aid glutaraldehyde/hexametaphosphate/thermoplastic starch foams modified by alkali treatment and montmorillonite nano-platelets

The micro foaming performance, moisture resistance and dynamic viscosity of scCO2-aid glutaraldehyde/hexametaphosphate/thermoplastic tapioca starch (GA/SHMP/TOS) foams were considerably improved by proper NaOH treatment. The expansion ratio, resilience rate, dynamic viscosity values of these NaOH modified foams improved to a maximum, as the time for NaOH treatment approached a proper value. The dynamic viscosity, expansion ratio and resilience rate of the scCO2-aid GA/SHMP/TOS foams modified using 110 atm scCO2-pressure, the proper alkali treatment time, SHMP loading and varying montmorillonite (MMT) loadings improved further, as their MMT loadings approached a proper value of 2.5 part per hundred parts of tapioca starch (PHTOS). Relatively large dynamic viscosity (7.1x104 Pa·s), extremely large expansion ratio (∼75), cell density (1.1x109 cells/cm3) and/or resilience rate (∼80%) were acquired for the scCO2-aid GA/SHMP/TOS/MMT foam modified using the proper alkali treatment time and MMT loading. Thermal analyses results showed that crystallization onset temperatures and crystallization rates of scCO2-aid GA/SHMP/TOS/MMT foams modified using the proper alkali treatment time and varying MMT loadings improved to a highest value by adding 2.5 PHTOS of MMT nano-platelets. Possible reasons accounting for the considerably improved micro foaming performance of scCO2-aid GA/SHMP/TOS/MMT foams modified using the proper alkali treatment time and MMT loading are proposed in this study.

Effect of low aspect ratio one-dimensional nanoparticles on properties of photocrosslinked alginate nanocomposite hydrogels

One-dimensional nanoparticles such as fibers, wires, represent an exciting type of filler for obtaining nanocomposite hydrogels due to their high aspect ratio. Unlike continuous fibers, in composites with short fibers, stress transfer occurs not only at the cylindrical surface of the fiber but also at the fiber ends. This work aimed to study the influence of the length of low aspect ratio one-dimensional nanoparticles on the rheological, swelling and thermal properties of nanocomposite hydrogels. We synthesized nanocomposite hydrogels via photopolymerization of methacrylated alginate macromonomers and aluminum oxide nanoparticles, nanowires and nanofibers as fillers. The main difference between the nanoparticles lies in their length. Longer nanoparticles (nanofibers) provided a glassier structure and a more significant reinforcement to the hydrogel, for example, at the concentration of 0.5 wt% the complex modulus increased approximately 2-fold and 8-fold for nanowires and nanofibers, respectively. Similarly, the incorporation of nanofibers produced hydrogels with a lower swelling capacity (2.5-fold decrease), which was attributed to a more connected network structure due to the higher aspect ratio of the nanofibers. Finally, both nanoparticle types produced an increase in the activation energy of thermal degradation.

Thickness of a three-sided coin: A molecular dynamics study

This Letter presents a numerical study across parameter space to calculate the aspect ratio (ratio of length to diameter) of a fair "three-sided coin": a cylinder that when tossed, has equal probabilities of landing heads, tails, or sideways. The results are cast in the context of previous analytical studies, and the various mechanisms that govern the dynamics of coin tossing are compared and contrasted. After more than 7×10^{8} tosses of coins of various aspect ratios, this study finds the critical aspect ratio to be slightly less than (but not exactly equal to) sqrt[3]/2≈0.866.

Superiority of graphite coated metallic-nanoparticles over graphite coated insulating-nanoparticles for enhancing EMI shielding

A superior EMI shielding effectiveness (SE) for composites with a metallic(Ni)@graphite core and lower SE for a dielectric(MnO)@graphitic core of carbonaceous materials.

Polyolefinic nanocomposite foams: Review of microstructure-property relationships, applications, and processing considerations

In this review, we survey the state of the art on polymeric foams incorporating nano-scale fillers. Particular focus of the review is on foams from polyolefinic nanocomposite formulations incorporating a wide variety of fillers. The nano-scale additives can influence the foam structure and properties in two ways: Firstly, they can act as composite reinforcement to enhance the mechanical properties and functionality of the matrix polymer; and secondly, they can act as foaming-processing aids through modification of the rheological, thermal and crystallization properties of the matrix as well as serving as heterogeneous nucleation sites. Through a combination of these influences, and using advanced processing techniques it is possible to achieve nanocomposite foams that have higher cell density, and more uniform cell size or controlled cell-size distribution. Such controlled foam morphologies, in turn, can yield better specific mechanical properties resulting in more effective light-weighting solutions. Further, the nano-scale additives can impart additional desired functionality resulting in multi-functional foams. In this article, we provide an overview of the mechanical, thermal and a few other relevant functional properties – such as piezoelectric sensitivity, acoustics, and filtration efficiency – of foams prepared using nanocomposite formulations, along with the processing considerations for achieving high quality foams using such materials.

Carbon particulate and controlled-hydrolysis assisted extrusion foaming of semi-crystalline polyethylene terephthalate for the enhanced thermal insulation property

This work presents a facile method to produce low-density PET foams using pristine semi-crystalline resin by moisture-induced controlled-hydrolysis in a tight processing window (moisture content ∼ 0.12 wt.%). We investigated the effect of moisture and moisture containing activated carbon (AC) on the foam expansion ratio, cell morphology, and PET resin degradation and crystallization properties. Controlled-hydrolysis increased the melt-flow rate of PET resin (intrinsic viscosity: 0.52 to 0.54 dL/g) without losing crystallinity, and thus the PET foams possess better tensile properties (∼2 MPa stress and ∼100% strain) and higher thermal stability (>200°C) than chemically modified PET foams. The foam density could be made as low as ∼ 0.15 g/cm3 using a lab scale twin-screw extruder. A strand array die was also designed to produce plate-shaped foam samples. AC allowed easier control of the moisture content and delayed resin degradation in extrusion. Both AC and micrographite (mGr) could stabilize the PET foam morphology in extrusion and serve as good infrared attenuation agents (IAAs) in a simulated housing thermal insulation experiment.

Nitrogen-Doped Carbon Nanotube/Polypropylene Composites with Negative Seebeck Coefficient

This study describes the application of multi-walled carbon nanotubes that were nitrogen-doped during their synthesis (N-MWCNTs) in melt-mixed polypropylene (PP) composites. Different types of N-MWCNTs, synthesized using different methods, were used and compared. Four of the five MWCNT grades showed negative Seebeck coefficients (S), indicating n-type charge carrier behavior. All prepared composites (with a concentration between 2 and 7.5 wt% N-MWCNTs) also showed negative S values, which in most cases had a higher negative value than the corresponding nanotubes. The S values achieved were between 1.0 μV/K and −13.8 μV/K for the N-MWCNT buckypapers or powders and between −4.7 μV/K and −22.8 μV/K for the corresponding composites. With a higher content of N-MWCNTs, the increase in electrical conductivity led to increasing values of the power factor (PF) despite the unstable behavior of the Seebeck coefficient. The highest power factor was achieved with 4 wt% N-MWCNT, where a suitable combination of high electrical conductivity and acceptable Seebeck coefficient led to a PF value of 6.1 × 10−3 µW/(m·K2). First experiments have shown that transient absorption spectroscopy (TAS) is a useful tool to study the carrier transfer process in CNTs in composites and to correlate it with the Seebeck coefficient.

Nanoscopic and Macro-Porous Carbon Nano-foam Electrodes with Improved Mass Transport for Vanadium Redox Flow Batteries

Although free-standing sheets of multiwalled carbon nanotubes (MWCNT) can provide interesting electrochemical and physical properties as electrodes for redox flow batteries, the full potential of this class of materials has not been accessible as of yet. The conventional fabrication methods produce sheets with micro-porous and meso-porous structures, which significantly resist mass transport of the electrolyte during high-current flow-cell operation. Herein, we developed a method to fabricate high performance macro-porous carbon nano-foam free standing sheets (Puffy Fibers, PF), by implementing a freeze-drying step into our low cost and scalable surface-engineered tape-casting (SETC) fabrication method, and we show the improvement in the performance attained as compared with a MWCNT sheet lacking any macro pores (Tape-cast, TC). We attribute the higher performance attained by our in-lab fabricated PF papers to the presence of macro pores which provided channels that acted as pathways for electrolytic transport within the bulk of the electrode. Moreover, we propose an electrolytic transport mechanism to relate ion diffusivity to different pore sizes to explain the different modes of charge transfer in the negative and the positive electrolytes. Overall, the PF papers had a high wettability, high porosity, and a large surface area, resulting in improved electrochemical and flow-cell performances.

Thermal Conductivity and Electrical Resistivity of Melt-Mixed Polypropylene Composites Containing Mixtures of Carbon-Based Fillers

Melt-mixed composites based on polypropylene (PP) with various carbon-based fillers were investigated with regard to their thermal conductivity and electrical resistivity. The composites were filled with up to three fillers by selecting combinations of graphite nanoplatelets (GNP), carbon fibers (CF), carbon nanotubes (CNT), carbon black (CB), and graphite (G) at a constant filler content of 7.5 vol%. The thermal conductivity of PP (0.26 W/(m·K)) improved most using graphite nanoplatelets, whereas electrical resistivity was the lowest when using multiwalled CNT. Synergistic effects could be observed for different filler combinations. The PP composite, which contains a mixture of GNP, CNT, and highly structured CB, simultaneously had high thermal conductivity (0.5 W/(m·K)) and the lowest electrical volume resistivity (4 Ohm·cm).

Fabrication and characterization of polymeric cellular foams for low-density computed tomography phantom applications

Computed tomography imaging phantom devices have proven to be beneficial in improving computed tomography diagnostic techniques. Though commercial phantoms are available with tissue mimicking properties, there is a lack of low-density tissue specificity and variety. This study proposes a method for the fabrication of various low-density tissue mimicking computed tomography imaging phantoms. By illustrating the fabrication technique, material properties can be shown to be controlled and assessed against characteristic computed tomography imaging properties, most particularly, the computed tomography number in Hounsfield Units. A batch cellular foaming technique was utilized on thermoplastic polyurethane with ranging heated water bath foaming times from 0.5 to 10 min to fabricate polymeric computed tomography phantoms of controlled foam material properties. Computed tomography number values were experimentally measured. Additionally, separate experimental measurements were made on the foam characteristic properties of fabricated thermoplastic polyurethane foams. A relative decreasing trend was exhibited between the foam characteristic properties of cell density, average cell size, and material density to computed tomography number.

Ultralow-Threshold and Lightweight Biodegradable Porous PLA/MWCNT with Segregated Conductive Networks for High-Performance Thermal Insulation and Electromagnetic Interference Shielding Applications

Lightweight, biodegradable, thermally insulating, and electrically conductive materials play a vital role in achieving the sustainable development of our society. The fabrication of such multifunctional materials is currently very challenging. Here, we report a general, facile, and eco-friendly way for the large-scale fabrication of ultralow-threshold and biodegradable porous polylactic acid (PLA)/multiwalled carbon nanotube (MWCNT) for high-performance thermal insulation and electromagnetic interference (EMI) shielding applications. Thanks to the unique structure of the microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm³ possesses a percolation threshold of 0.00094 vol %, which, to our knowledge, is the minimum value reported so far. Furthermore, the material exhibits excellent thermal insulation performance with a thermal conductivity of 27.5 mW·m^-1·K^-1, which is much lower than the best value of common thermal insulation materials. Moreover, it also shows outstanding EMI shielding performance characterized by its high shielding effectiveness (SE) values and absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm³·g^-1, which is superior to those of other shielding materials reported so far. Thus, this novel multifunctional material and its general fabrication methodology provide a promising way to meet the growing demand for high-performance multifunctional materials in sustainable development.

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.

Effect of Different Types of Nano-particles on the Morphology and Mechanical Properties of EPDM Foam

Foams are attractive materials because of their unique properties and applications in aerospace, transportation, isolation and engineering fields. Nanocomposites can be employed to achieve unique characteristics. It is convenient to use nano-particles for controlling the cell structure of foams and also to modify their mechanical properties. To further assess this effect, a variety geometrical shapes of nano-filler were employed in nanocomposites in order to investigate the effect of final foam micro structure and mechanical properties. Both Box-Behenken and factorial methods were chosen in this study for designing the samples. The performance of Box-Behenken experiment designs were evaluated. It was revealed that foam structure and presence of nano-particles in the structure is more effective than reinforcement of foam constitutive material.

Reasons and remedies for the agglomeration of multilayered graphene and carbon nanotubes in polymers

One of the main issues in the production of polymer nanocomposites is the dispersion state of filler as multilayered graphene (MLG) and carbon nanotubes (CNTs) tend to agglomerate due to van der Waals forces. The agglomeration can be avoided by using organic solvents, selecting suitable dispersion and production methods, and functionalizing the fillers. Another proposed method is the use of hybrid fillers as synergistic effects can cause an improvement in the dispersion state of the fillers. In this review article, various aspects of each process that can help avoid filler agglomeration and improve dispersion state are discussed in detail. This review article would be helpful for both current and prospective researchers in the field of MLG- and CNT-based polymer nanocomposites to achieve maximum enhancement in mechanical, thermal, and electrical properties of produced polymer nanocomposites.

Emerging trends in eco-compliant, synergistic, and hybrid assembling of multifunctional polymeric bionanocomposites

The quest to develop eco-benign polymeric hybrid materials arose out of the need to protect the environment from the harmful effects of synthetic petroleum polymeric waste and meet the specific needs of industries such as oil and gas, aerospace, automotives, packaging, electronics biomedicals, pharmaceuticals, agricultural, and construction. This has resulted in synergistic hybrid assembling of natural fibers, polymers, biopolymers, and nanoparticles. Bionanocomposites based on inorganic nanoparticle reinforced biofiber, polymers and biopolymers, and polysaccharides such as chitosan, alginate, and cellulose derivatives, and so on, exhibiting at least a dimension at the nanometer scale, are an emerging group of nanostructured hybrid materials. These hybrid bionanocomposites exhibit structural and multifunctional properties suitable for versatile applications similar to polymer nanocomposites. Their biocompatibility and biodegradability provide opportunities for applications as eco-benign green nanocomposites. This review presents state-of-the-art progress in synergistic nanotechnological assembling of bionanocomposites relative to processing technologies, product development, and applications.