Nanocellular Polymers Research Articles

A sustainable methodology to produce open-cell porous membranes with control on the dense layer thickness

A new approach to produce porous membranes with dense or porous top layer is proposed in this work by employing a solvent-free method. PMMA/MAM formulations were selected as a base material in order to create open-cell or close-cell structures by gas dissolution foaming employing CO2 as a blowing agent. Furthermore, by introducing the gas diffusion barrier approach to CO2 dissolution foaming, it is possible to control the thickness of the dense layer in both edges, obtaining defect-free membranes (i.e., completely dense without pin-holes). The effectiveness of nanocellular polymers as gas separation membranes was evaluated. In this way, the permeability, selectivity, and permeance were correlated to the cellular structure (open or close-cell) as well as to the dense layer thickness. Furthermore, the effective thickness of the selective layer has been calculated from gas permeability measurements, obtaining an accurate control of that parameter from the tunable cellular structure. Therefore, membranes composed of desired selective layer and a porous structure as a mechanical support are produced by a solvent-free methodology.

Evaluation of methods to accurately characterize the thermal conductivity of micro-and nanocellular polymers based on poly(methyl-methacrylate) (PMMA) produced at lab-scale

The characterization of the thermal conductivity of new and enhanced thermal insulators developed at lab-scale is a challenge. The small dimensions of the prototypes make impossible the use of the conventional techniques because steady-state methods require large samples. Furthermore, the accuracy of transient methods to measure the thermal conductivity is not clear. In this work, we compare four different approaches to measure the thermal conductivity of small prototypes of nanocellular poly(methyl-methacrylate) (PMMA). Both steady-state and transient techniques are used. Results show that the transient plane source method is not suitable for the characterization of these materials (the deviation from the steady-state methods is on average higher than 15%). In addition, two different approaches for measuring the thermal conductivity of small samples via a steady-state technique are proposed and validated.

Micronization as a solution for enhancing the thermal insulation of nanocellular poly(methyl-methacrylate) (PMMA)

This work shows a route to reduce the thermal conductivity of nanocellular poly(methyl-methacrylate) (PMMA). This approach is based on micronizing to replace the continuous solid phase by a discontinuous one. PMMA powders with densities of 147–195 kg/m3, formed by particles of 100 μm with nanometric cells inside them, are produced by milling. Micronization allows increasing the overall porosity maintaining the cell size. Results prove that after milling it is possible to obtain open cell nanoporous PMMA powders with thermal conductivity below that of the bulk materials (15% reduction). The reduction is not only due to a density decrease, but a result of the new structure of the powder material. The discontinuity of the solid phase and the increase in radiation extinction are the key factors allowing this improvement. This route is confirmed as a promising alternative to enhance the performance of nanocellular polymers.

Thermal conductivity of low-density micro-and nanocellular poly(methyl-methacrylate) (PMMA): Experimental and modeling

Nowadays, finding new materials with enhanced thermal insulation properties has become a mandatory task to reduce energy consumption and CO2 emissions. In recent years, nanocellular polymers have aroused great attention due to their very interesting combination of properties, which include reduced conduction through the gas phase thanks to the Knudsen effect.There are plenty of theoretical works hypothesizing the thermal insulation performance of nanocellular polymers. However, there is a lack of experimental results, especially at low densities. In the present work, the thermal conductivity of low-density microcellular and nanocellular poly(methyl-methacrylate) (PMMA) was measured to evaluate the different heat transfer mechanisms acting on these structures. PMMA foamed sheets with relative densities ranging from 0.09 to 0.18 and cell sizes between 400–4000 nm were produced by gas dissolution foaming using CO2 as a physical blowing agent. Samples were measured at various temperatures, resulting in thermal conductivities between 37.4 and 46.6 mW/(m·K) at 10 °C. Experimental results have been analyzed to build a semi-empirical model able to predict the thermal conductivity and each heat transfer mechanism contribution. To do this, a novel method to determine the solid structure factor from the slope of the thermal conductivity versus the temperature curve is introduced.

Thermal Conductivity of Nanoporous Materials: Where Is the Limit?

Nowadays, our society is facing problems related to energy availability. Owing to the energy savings that insulators provide, the search for effective insulating materials is a focus of interest. Since the current insulators do not meet the increasingly strict requirements, developing materials with a greater insulating capacity is needed. Until now, several nanoporous materials have been considered as superinsulators achieving thermal conductivities below that of the air 26 mW/(m K), like nanocellular PMMA/TPU, silica aerogels, and polyurethane aerogels reaching 24.8, 10, and 12 mW/(m K), respectively. In the search for the minimum thermal conductivity, still undiscovered, the first step is understanding heat transfer in nanoporous materials. The main features leading to superinsulation are low density, nanopores, and solid interruptions hindering the phonon transfer. The second crucial condition is obtaining reliable thermal conductivity measurement techniques. This review summarizes these techniques, and data in the literature regarding the structure and thermal conductivity of two nanoporous materials, nanocellular polymers and aerogels. The key conclusion of this analysis specifies that only steady-state methods provide a reliable value for thermal conductivity of superinsulators. Finally, a theoretical discussion is performed providing a detailed background to further explore the lower limit of superinsulation to develop more efficient materials.

Easy-way production of highly transparent nanocellular polymers films

The key parameters and production route for the easy production of highly transparent nanocellular films based on polymethylmethacrylate (PMMA) with thickness below 1 mm are herein described.The use of the gas dissolution foaming process (CO2 as physical blowing agent) with saturation pressures of 40 and 50 MPa, a saturation temperature of 0 °C and a saturation time of 2.5 h together with an adequate approach to foam the samples leads to the production of directly transparent nanocellular PMMA with relative densities in the range of 0.47–0.80 and cell sizes in the internal part of the film ranging 23–29 nm.The produced materials present maximum transparency of 61% for a thickness of 0.5 mm without any post-processing, a value comparable to the one previously obtained in the literature for nanocellular PMMA produced using a long-lasting process that included additional time-consuming post-processing of the samples.The produced cellular materials have been in-deep characterized, and the influence of several key parameters such as the solid skin thickness, transition region, and cellular structure on the final optical transmittance has been studied.

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.

Cyclic Gas Dissolution Foaming as an Approach for Simultaneously Reducing Cell Size and Relative Density in Nanocellular PMMA.

A new approach to produce nanocellular polymers combining small cell sizes with low relative densities is presented herein. This production method, based on gas dissolution foaming, consists of performing a double saturation and foaming cycle. Thus, nanocellular polymethylmethacrylate (PMMA) has been produced through a first saturation at different saturation conditions (6, 10, and 20 MPa and −32 °C), at constant foaming conditions (60 °C for 1 min). Then, the nanocellular PMMAs obtained from the previous step were again saturated at different saturation conditions, 10 MPa 24 °C, 31 MPa 24 °C, 35 MPa 22 °C, and 6 MPa −15 °C and foamed at different temperatures (40, 80 and 100 °C) for 1 min. This new approach allows the cells created in the first saturation and foaming cycle to further grow in the second cycle. This fact permits producing nanocellular polymethylmethacrylate sheets combining, for the first time in the literature, cell sizes of 24 nm with relative densities of 0.3.

Amorphous Polymers' Foaming and Blends with Organic Foaming-Aid Structured Additives in Supercritical CO2, a Way to Fabricate Porous Polymers from Macro to Nano Porosities in Batch or Continuous Processes.

Organic polymers can be made porous via continuous or discontinuous expansion processes in scCO2. The resulting foams properties are controlled by the interplay of three groups of parameters: (i) Chemical, (ii) physico-chemical, and (iii) technological/process that are explained in this paper. The advantages and drawbacks of continuous (extrusion, injection foaming) or discontinuous (batch foaming) foaming processes in scCO2, will be discussed in this article; especially for micro or nano cellular polymers. Indeed, a challenge is to reduce both specific mass (e.g., ρ < 100 kg·m−3) and cell size (e.g., average pore diameter ϕaveragepores < 100 nm). Then a particular system where small “objects” (coreshells CS, block copolymer MAM) are perfectly dispersed at a micrometric to nanometric scale in poly(methyl methacrylate) (PMMA) will be presented. Such “additives”, considered as foaming aids, are aimed at “regulating” the foaming and lowering the pore size and/or density of PMMA based foams. Differences between these additives will be shown. Finally, in a PMMA/20 wt% MAM blend, via a quasi one-step batch foaming, a “porous to nonporous” transition is observed in thick samples. A lower limit of pore size (around 50 nm) seems to arise.

On the interaction of infrared radiation and nanocellular polymers: First experimental determination of the extinction coefficient

Among the various mechanisms playing a role in the heat transfer through nanocellular polymers, the radiation contribution remains the most unknown, since there is a lack of experimental data about how infrared light interacts with such structures. In this work, we present the first experimental measurements of the transmittance in the infrared region of nanocellular polymers. Infrared transmittance spectra of a collection of polymethylmethacrylate (PMMA)-based micro- and nanocellular polymers with a constant density and a wide range of cell sizes (from 14 nm to 20 μm) were obtained and evaluated to calculate the extinction coefficient. Results show that, as expected from theoretical considerations, a reduction of the cell size increases the amount of infrared radiation transmitted, that is, the scattering is reduced as cell size reduces. Nanocellular polymers were proved to act as Rayleigh-like scattering points, showing the transmittance both an intense wavelength and cell size dependence. As a consequence, the extinction coefficient reduces in the nanoscale. From these data, it is possible to conclude that the scattering due to the cellular structure can be neglected for very small cell sizes (smaller than 200 nm), but it must be considered for larger cell sizes. The obtained results were used to model the thermal conductivity including the radiation contribution, showing that at low relative densities and small cell sizes this heat transfer term becomes significant in nanocellular polymers.

Nanocellular Polymers with a Gradient Cellular Structure Based on Poly(methyl methacrylate)/Thermoplastic Polyurethane Blends Produced by Gas Dissolution Foaming

AbstractGraded structures and nanocellular polymers are two examples of advanced cellular morphologies. In this work, a methodology to obtain low‐density graded nanocellular polymers based on poly(methyl methacrylate) (PMMA)/thermoplastic polyurethane (TPU) blends produced by gas dissolution foaming is reported. A systematic study of the effect of the processing condition is presented. Results show that the melt‐blending results in a solid nanostructured material formed by nanometric TPU domains. The PMMA/TPU foamed samples show a gradient cellular structure, with a homogeneous nanocellular core. In the core, the TPU domains act as nucleating sites, enhancing nucleation compared to pure PMMA and allowing the change from a microcellular to a nanocellular structure. Nonetheless, the outer region shows a gradient of cell sizes from nano‐ to micron‐sized cells. This gradient structure is attributed to a non‐constant pressure profile in the samples due to gas desorption before foaming. The nucleation in the PMMA/TPU increases as the saturation pressure increases. Regarding the effect of the foaming conditions, it is proved that it is necessary to have a fine control to avoid degeneration of the cellular materials. Graded nanocellular polymers with relative densities of 0.16–0.30 and cell sizes ranging 310–480 nm (in the nanocellular core) are obtained.

Fabrication of microporous PMMA using ionic liquids: An improved route to classical ScCO2 foaming process

The use of supercritical CO2 (ScCO2) has been established as a standard technique to produce micro- and nanocellular polymers with controlled morphologies and cell sizes between 500 nm and several microns. However, overcoming the gap between micro and nanocellular polymers implies the use of long saturation times (up to several days) and low temperatures (below 0 °C) combined with high saturation pressures (above 30 MPa). This work presents a different approach to obtain microporous poly (methyl methacrylate) (PMMA) structures with controlled morphologies using a physical procedure employing ionic liquids that are mixed in solution with PMMA. The ionic liquid can produce interesting fine microporous materials simply by being removed with distilled water. The work compares the two fabrication methods previously mentioned to critically analyze the benefits and drawbacks of each preparation route. The method in this paper opens the possibility to obtain microporous polymers by employing ionic liquids, thus reducing the time of processing, avoiding specific foaming equipment and broadening the applicability of crosslinked and semi crystalline polymers. Low velocity tensile test results are also presented to point out the improvement in the mechanical parameters of microporous films obtained from ionic liquids compared to those of films foamed using ScCO2.

Highly anisotropic nanocellular polymers based on tri-phasic blends of PMMA with two nucleating agents

Abstract One strategy to produce nanocellular polymers is the use of nucleating species to promote nucleation. Whereas two-phase systems are widely studied, tri-phasic blends with two nucleating agents are uncommonly investigated. In this work, nanocellular polymers are obtained using tri-phasic blends of polymethylmethacrylate (PMMA) with two nucleating agents: needle-like sepiolites and a polymethylmethacrylate-polybutylacrylate-polymethylmethacrylate (MAM) block copolymer. Blends of PMMA with different concentrations of MAM and a fixed amount of sepiolites are produced by extrusion. Results show that at low MAM contents (1 wt%), the nucleation is a combination of the action of the two additives, but the addition of sepiolites induces the appearance of anisotropic structures. Meanwhile, at high MAM concentrations (10 wt%), MAM nanostructuration controls the nucleation and sepiolites increase the anisotropy. The alignment of the MAM micelles and the sepiolites in the extrusion direction promotes coalescence in this direction, leading to highly anisotropic nanocellular structures. Mean cell sizes of 100–300 nm and an average anisotropy ratio of 2.77 are obtained thanks to the combined effect of MAM and sepiolites.

Production of PMMA-based nanocellular polymers using low demanding saturation conditions

Abstract Nanocellular polymers are novel materials characterized by nanometric cell sizes with huge potential in many industrial applications. However, their production is challenging and usually requires demanding processing conditions that make difficult the production at industrial scale. In this work, we prove that nanocellular polymers based on PMMA can be obtained, thanks to the addition of nucleating species, using low saturation pressure (6 MPa) and room saturation temperature in a gas dissolution foaming process. Nanoparticles and block copolymers at different concentrations are used to obtain a collection of nanocellular polymers with densities ranging 0.16–0.58 and cell sizes from 120 to 900 nm. Comparison with the literature results shows that the processing parameters used in this work are the less demanding ever used for producing nanocellular PMMA.

Cover Image, Volume 68, Issue 6

The cover image is based on the Research Article Anisotropy in nanocellular polymers promoted by the addition of needle‐like sepiolites by Victoria Bernardo et al. DOI: 10.1002/pi.5813. image

Overcoming the Challenge of Producing Large and Flat Nanocellular Polymers: A Study with PMMA

Although nanocellular polymers are interesting materials with improved properties in comparison with conventional or microcellular polymers, the production of large and flat parts of those materials is still challenging. Herein, gas dissolution foaming process is used to produce large and flat nanocellular polymethylmethacrylate samples. In order to do that, the foaming step is performed in a hot press. The methodology is optimized to produce flat samples with dimensions of 100 × 100 × 6 mm3, relative densities in the range 0.25–0.55 and cell sizes around 250 nm. Additionally, foaming parameters are modified to study their influence on the final cellular structure, and the materials produced in this paper are compared with samples produced by using a most conventional approach in which foaming step is conducted in a thermal bath. Results obtained show that an increment in the foaming temperature leads to a reduction in relative density and an increase of cell nucleation density. Moreover, differences in the final cellular structure for materials produced by both foaming routes are studied, proving that although there exist some differences, the mechanisms governing the nucleation and growing are the same in both processes, leading to the production of homogeneous materials with very similar cellular structures.

Anisotropy in nanocellular polymers promoted by the addition of needle‐like sepiolites

AbstractThis work presents a new strategy for obtaining nanocellular materials with high anisotropy ratios by means of the addition of needle‐like nanoparticles. Nanocellular polymers are of great interest due to their outstanding properties, whereas anisotropic structures allow the realization of improved thermal and mechanical properties in certain directions. Nanocomposites based on poly(methyl methacrylate) (PMMA) with nanometric sepiolites are generated by extrusion. From the extruded filaments, cellular materials are produced using a two‐step gas dissolution foaming method. The effect of adding various types and contents of sepiolites is investigated. As a result of the extrusion process, the needle‐like sepiolites are aligned in the machine direction in the solid nanocomposites. Regarding the cellular materials, the addition of sepiolites allows one to obtain anisotropic nanocellular polymers with cell sizes of 150 to 420 nm and cell nucleation densities of 1013–1014 nuclei cm−3 and presenting anisotropy ratios ranging from 1.38 to 2.15, the extrusion direction being the direction of the anisotropy. To explain the appearance of anisotropy, a mechanism based on cell coalescence is proposed and discussed. In addition, it is shown that it is possible to control the anisotropy ratio of the PMMA/sepiolite nanocellular polymers by changing the amount of well‐dispersed sepiolites in the solid nanocomposites. © 2019 Society of Chemical Industry

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.

Low-density PMMA/MAM nanocellular polymers using low MAM contents: Production and characterization

Low-density nanocellular polymers are required to take advantage of the full potential of these materials as high efficient thermal insulators. However, their production is still a challenging task. One promising approach is the use of nanostructured polymer blends of poly(methyl methacrylate) (PMMA) and a block copolymer poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) (MAM), which are useful for promoting nucleation but seem to present a severe drawback, as apparently avoid low relative densities. In this work, new strategies to overcome this limitation and produce low-density nanocellular materials based on these blends are investigated. First, the effect of very low amounts of the MAM copolymer is analysed. It is detected that nanostructuration can be prevented using low copolymer contents, but nucleation is still enhanced as a result of the copolymer molecules with high CO2 affinity dispersed in the matrix, so nanocellular polymers are obtained using very low percentages of the copolymer. Second, the influence of the foaming temperature is studied. Results show that for systems in which there is not a clear nanostructuration, cells can grow more freely and smaller relative densities can be achieved.For these studies, blends of PMMA with MAM with copolymer contents from 10 wt% and as low as 0.1 wt% are used. For the first time, the production strategies proposed in this work have allowed obtaining low density (relative density 0.23) nanocellular polymers based on PMMA/MAM blends.

Modeling the heat transfer by conduction of nanocellular polymers with bimodal cellular structures

Nanocellular polymers are a new generation of materials with the potential of being used as very efficient thermal insulators. It has been proved experimentally that these materials present the Knudsen effect, which strongly reduces the conductivity of the gas phase. There are theoretical equations to predict the thermal conductivity due to this Knudsen effect, but all the models consider an average cell size. In this work, we propose a model to predict the thermal conductivity due to the conduction mechanisms of nanocellular materials with bimodal cellular structures, that is, with two populations of cells, micro and nanocellular. The novelty of our work is to consider not only the average cell size, but the cell size distribution. The predictions of the model are compared with the experimental conductivity of two real bimodal systems based on poly(methyl methacrylate) (PMMA), and it is proved that this new model provides more accurate estimations of the conductivity than the models that do not consider the bimodality. Furthermore, this model could be applied to monomodal nanocellular polymers. In particular, for monomodal materials presenting a wide cell size distribution and at low densities, the model predicts important variations in comparison with the current models in the literature. This result indicates that the cell size distribution must be included in the estimations of the thermal conductivity of nanocellular polymers.