scCO2 Foaming Research Articles

Fabrication of skinless cellular poly (vinylidene fluoride) films by surface-constrained supercritical CO2 foaming using elastic gas barrier layers

Removing the solid skin of polymer foams is a challenging approach for supercritical carbon dioxide (scCO2) foaming. Herein, a surface-constrained foaming method was used to fabricate skinless cellular poly (vinylidene fluoride) (PVDF) films. The use of organosilicone and nature latex as gas barrier layers resulted in low CO2 diffusion rates of 2.16 × 10−11 m2/s and 4.73 × 10−13 m2/s, and trigger heterogeneous nucleation of cells on the PVDF/barrier layer interface. The skin layer and the cell size gradient in the region close to the surface were successfully eliminated, and the surface cell density and cell size could be controlled in the range of 7.2 × 106 cells/cm3, and 13–41 µm, respectively. Moreover, the fabricated skinless cellular PVDF films exhibited an enhanced melting temperature and crystallinity due to the plasticizing effect of scCO2. The present study provides new thoughts for the fabrication of skinless polymer films using surface-constrained scCO2 foaming.

Supercritical Foaming and Impregnation of Polycaprolactone and Polycaprolactone-Hydroxyapatite Composites with Carvacrol

Polycaprolactone (PCL) and polycaprolactone-hydroxyapatite (PCL-HA) scaffolds were produced by foaming in supercritical carbon dioxide (scCO2) at 20 MPa, as well as in one-step foaming and impregnation process using carvacrol as an antibacterial agent with proven activity against Gram-positive and Gram-negative bacteria. The experimental design was developed to study the influence of temperature (40 °C and 50 °C), HA content (10 and 20 wt.%), and depressurization rate (one and two-step decompression) on the foams’ morphology, porosity, pore size distribution, and carvacrol impregnation yield. The characterization of the foams was carried out using scanning electron microscopy (SEM, SEM-FIB), Gay-Lussac density bottle measurements, and Fourier–transform infrared (FTIR) analyses. The obtained results demonstrate that processing PCL and PCL-HA scaffolds by means of scCO2 foaming enables preparing foams with porosity in the range of 65.55–74.39% and 61.98–67.13%, at 40 °C and 50 °C, respectively. The presence of carvacrol led to a lower porosity. At 40 °C and one-step decompression at a slow rate, the porosity of impregnated scaffolds was higher than at 50 °C and two- step fast decompression. However, a narrower pore size distribution was obtained at the last processing conditions. PCL scaffolds with HA resulted in higher carvacrol impregnation yields than neat PCL foams. The highest carvacrol loading (10.57%) was observed in the scaffold with 10 wt.% HA obtained at 50 °C.

Fluorescence assisted visualization and destruction of particles embedded thin cell walls in polymeric foams via supercritical foaming

Supercritical CO2 foaming is widely used to prepare microcellular foams and particles are frequently added to regulate cellular structure. Nevertheless, microcellular foams present a complicated structure (three-dimension cells and two-dimension cell walls embedded with particles), which is a huge challenge to enlighten scCO2 foaming behavior and corresponding properties. Therefore, a strategy was proposed to take fluorescence’s advantage of real-time and field visualization in scCO2 foaming: (1) cell wall rupture process was studied via in-situ visualization; (2) dual fluorescence design was used to construct particle dispersive state in a cell; (3) polystyrene particles were chemical fluorescence modified to resist strong extraction of scCO2 fluid. This work reveals that effect of particles on bubble nucleation is two folds: interface promoted bubble nucleation, but elastic strain energy inhibited bubble nucleation. By in-situ visualization, irregular particle dominates in initiating cracks in cell walls due to the much larger stress concentration compared with spherical particles.

Boosted Mechanical Piezoelectric Energy Harvesting of Polyvinylidene Fluoride/Barium Titanate Composite Porous Foam Based on Three-Dimensional Printing and Foaming Technology.

The popularity of intelligent and green electronic devices means that the use of renewable mechanical energy has gradually become an inevitable choice for social development. However, it is difficult for the existing energy harvesters to meet the requirement for efficient collection of discrete mechanical energy due to the limitation of traditional two-dimensional (2D) film deformation. In this research, a green and convenient supercritical carbon dioxide foaming (Sc-CO2)-assisted selective laser sintering method was developed, and piezoelectric energy harvesters with a 3D porous structure of polyvinylidene fluoride (PVDF)/barium titanate (BaTiO3) were successfully constructed. The 3D structure combined with the porous structure made full use of the normal space, amplified the stress–strain effect, and improved the piezoelectric output capability. Under the synergistic effect of BaTiO3, the foams exhibited high output with an output voltage of 20.9 V and a current density of 0.371 nA/mm2, which exceeded most of the known PVDF/BaTiO3 energy harvesters, and the prepared piezoelectric energy harvester could directly light up 11 green light-emitting diodes and charge a 1 μF commercial capacitor to 4.98 V within 180 s. This work emphasizes the key role of 3D printing and Sc-CO2 foaming in fabricating 3D piezoelectric energy harvesters.

Fabrication of Thermoplastic Polyurethane Foams with Wrinkled Pores and Superior Energy Absorption Properties by CO2 Foaming and Fast Chilling

AbstractPrecise control and delicate manipulation of pore morphology are challenges for supercritical CO2 (scCO2) foaming of polymer foams to boost their performances and extend their applications. Herein, a simple two‐step scCO2 foaming and fast chilling method is proposed to fabricate thermoplastic polyurethane (TPU) foams with unique wrinkly structures inside the pores by introducing a fast cooling procedure in an ice water bath right after the foaming process. The combined effects of the increase of TPU surface tension and the decrease of CO2 pressure inside the pores induce intense intermolecular stress on the pore walls, which trigger the formation of micro‐sized wrinkles on the pore walls. The wavelength of the wrinkly structure can be reduced from 2.8 to 1.1 µm by raising the foaming temperature, which is associated with the wrinkled structure changing from hexagon to labyrinth. Compared with conventional foams, wrinkled foams possess superior compressive modulus, strength, and energy absorption performance, and wrinkled foams with wrinkles display better energy absorption. This study provides a facile approach to produce wrinkled foams with controllable morphology and gives insights into the remarkable compressive performance of the wrinkled foams, which make them promising materials for energy absorption applications.

High sound insulation property of prepared polypropylene/polyolefin elastomer blends by combining pressure-induced-flow processing and supercritical CO2 foaming

Light weight, high sound insulation property polymers have vast potential application in aviation, auto industry and the military application. Pressure induced flow (PIF) process form multi-layer-oriented structure in polymer matrix, and supercritical CO2 foaming (ScCO2) introduce gas into the matrix and increase the impedance mismatch between the phase, both of them enhance the polymer sound insulation property. In this paper, the sound insulation property of polypropylene (PP)/polyolefin elastomer (POE) blended composites prepared by the PIF process and the ScCO2 foaming were measured using the impedance tube. The sound insulation property is up to 103.56 dB in the frequency band of 1000–6000 Hz for 5 mm PIF PP/POE foam material, which it is 67.15 dB higher than PP/POE foam. By employing the multi-scale ffinite element analysis (FEA) modeling method, one two-phase FEA model of the monolith of 0.1 mm size and another 29 mm diameter bulk sample were developed based on SEM image of PIF PP/POE foamed materials at different scales. The monolith FEA model provided the effective material property for bulk sample. The finite element calculation results were consistent with the experimental result, verified the validity of the FEA modelling approach. The FEA simulation results reveal that the multi-layer-oriented structure of PIF material introduce the multi-layer impedance mismatch inside each monolith, and incident sound wave is reflected by those impedance mismatch, and this mechanism produce the high sound insulation property of PIF PP/POE foam materials.

Fabrication of wrinkled thermoplastic polyurethane foams by dynamic supercritical carbon dioxide foaming

Precise control and regulation of the cell structure in supercritical CO2 (scCO2) foaming is the key to boost the performance and extend the application of polymer foams. In this work, dynamic scCO2 foaming method is proposed to fabricate thermoplastic polyurethane (TPU) foams with unique wrinkly structures by introducing a continuous scCO2 flow field during scCO2 saturation. The scCO2 flow field applied on the TPU matrix generated intramolecular stress and resulted in the formation of wrinkles on the surface of cells. The structure of wrinkles was controlled by pivotal processing factors including foaming temperature, pressure, scCO2 flow rate, and scCO2 flow time. Attributing to the wrinkly structure, the wrinkled foams possessed superior compressive modulus, strength, recoverability, and energy absorption performance than conventional TPU foams, which make them a promising material for buffering and energy absorption applications. The dynamic scCO2 foaming method opens avenue for scCO2 foaming process involves dynamic field.

Efficient electrical conductivity and electromagnetic interference shielding performance of double percolated polymer composite foams by phase coarsening in supercritical CO2

There is a growing interest in porous conductive polymer composites (CPCs) in view of lower density and more sufficient material utilization than solid counterparts, but they are still limited by poor performance. Few measures have been taken to deal with the problem by now. Herein, double percolated structure, supercritical carbon dioxide (scCO2) annealing treatment and scCO2 foaming were first combined to fabricate polystyrene (PS)/poly (methyl methacrylate) (PMMA)/multi-wall carbon nanotube (MWCNT) composite foams with porous double percolated structure. Due to the effective phase coarsening in scCO2 before foaming, the composite foams with density of 0.49 g/cm3 exhibited excellent electrical conductivity of 2.69 S/m and electromagnetic interference shielding effectiveness of 25.30 dB at thin thickness of 2 mm and low MWCNT loading of 2 vol%. In addition, absorption was the dominant shielding mechanism. For composite foams, the percolation threshold decreased from 0.14 vol% to 0.07 vol% after phase coarsening in 180 °C scCO2. This work provides a green and versatile method for preparing porous CPCs with double percolated structure and improved electrical conductivity.

Controlling the foam morphology of supercritical CO2‐processed poly(methyl methacrylate) with CO2‐philic hybrid nanoparticles

AbstractHighly CO2‐philic nanoparticles, octatrimethylsiloxy polyhedral oligomeric silsesquioxanes (POSS) are used to increase the affinity of poly(methyl methacrylate) (PMMA) to CO2 in supercritical carbon dioxide (scCO2) foaming, thus to improve its foaming performance and the foam morphology. PMMA and PMMA‐POSS composite foams were produced based on the two‐factorial design, at the upper and lower experimental conditions of pressure, temperature, processing time, and venting rate. The foams of PMMA‐5% POSS composites exhibited smaller average pore sizes and higher pore densities than neat PMMA and PMMA‐0.5% POSS composites. The smallest average pore diameter (0.3 μm) and the highest pore density (6.33 × 1012 cm−3) were obtained with this composite processed at 35°C, 32 MPa, for 24 h and depressurized with fast‐venting rate (0.4 MPa/s). ScCO2 processing decreased the density of the polymer by more than 50%.

Heterogeneous silicon rubber composite foam with gradient porous structure for highly absorbed ultra-efficient electromagnetic interference shielding

Heterogeneous silicone rubber (VMQ)/silver plated glass fiber (Ag@GF)/multiwalled carbon nanotubes (MWCNT)/ferroferric oxide (Fe3O4) composite foams with gradient porous structure were fabricated through layer by layer combination and supercritical carbon dioxide (scCO2) foaming. The average electromagnetic interference shielding effectiveness (EMI SE) and absorption coefficient (A) of the composite foam are as high as 78.6 dB and 0.82 respectively, because of the reasonable progressive dissipation mechanism. The resonance cancellation caused by the encounter of incident and reflected electromagnetic (EM) waves further improved the microwave absorbing performance of the foam, and an absorption ratio of up to 94% can be obtained at 7.68 GHz. Moreover, the composite foam exhibits excellent stability of EMI shielding performance under multiple bending. This composite foam has great application prospects in the field of EM protection of wearable flexible electronics, high-power instruments and the forthcoming 5th-Generation communication products.

Conductive thermoplastic polyurethane nanocomposite foams derived from a cellulose/MWCNTs aerogel framework: simultaneous enhancement of piezoresistance, strength, and endurance

Cellulose/MWCNTs/TPU nanocomposite foams were successfully fabricated via a freeze-drying technique and ScCO2 foaming. The microcellular structure of the composite foams showed potential application for polymer composite foams in wearable sensor devices.

Striking effect of carbon nanotubes on adjusting sc-CO2 foaming performance of PS/LLDPE blends and forming semi-open cellular structure

The effect of carbon nanotubes (CNTs) on the phase structure and foaming performance of PS/LLDPE blends during batch foaming of sc-CO2 was studied. It was found that the phase structure of PS/LLDPE = 50/50 blends with CNTs could be transformed from bi-continuous to sea-island morphology, when a sufficient quantity of CNTs nanofillers was added. As a result, the foaming ability of PS/LLDPE = 50/50 blends was improved effectively. Meanwhile, the critical value of CNTs for significantly improving the foaming ability of blends decreased slightly with the increase of foaming temperature. In contrast, for the PS/LLDPE blends with sea-island phase structure (such as 20/80), the addition of CNTs did not significantly change the morphology and foaming capacity of the blends. Very interestingly, the foam with semi-open cellular structure (meaning that there were small pores within the large cell wall) was obtained in the foamed PS/LLDPE blends after tetrahydrofuran (THF) etching PS phase. Benefiting from the interfacial channel effect formed by the presence of CNTs, the etching speed and efficiency of PS were significantly improved, and the size for the small pore on the cell wall could be adjusted by the content of PS dispersed phase in the PS/LLDPE/CNTs nanocomposites.

A promising strategy for efficient electromagnetic interference shielding by designing a porous double-percolated structure in MWCNT/polymer-based composites

Porous conductive polymer composites (CPCs) are supposed to be the most prospective substitutes for metal-based materials in the field of electromagnetic interference (EMI) shielding but are still confronted with the challenge of poor performance. To solve the dilemma, we firstly design a porous double-percolated structure in co-continuous polystyrene (PS)/polymethyl methacrylate (PMMA)/ MWCNT system with a judicious combination of melt blending and supercritical carbon dioxide (scCO2) foaming. At thickness of 2 mm and MWCNT content of 1.61 vol%, the porous composite possessed low-density (0.4 g/cm3), high electrical conductivity (1.64 S/m) and efficient EMI shielding performance (23.08 dB). Moreover, the percolation threshold decreased from 0.18 to 0.14 vol%, and the specific EMI SE increased from 37.79 to 57.7 dB·cm3/g compared to the solid counterparts. This work offers a novel idea for the design of lightweight EMI shielding materials with high performance, exerting tremendous potential in electronics and aerospace fields.

Microcellular structure assisted phase transformation of polyvinylidene fluoride/titanium dioxide nanocomposites

A novel nanocomposite foam with microcellular structures based on poly(vinylidene fluoride) (PVDF) and titanium dioxide (TiO2) was fabricated by the combination of melt compounding and supercritical carbon dioxide (scCO2) foaming. To improve its dielectric performance, silane modified and unmodified titanium dioxide nanoparticles were added as reinforcing fillers at low weight percentages (0.5, 1, and 5 wt%) during the melt blending process. It was found that the incorporation of nanoparticles had a strong influence on cell morphology. As a result, the foaming process significantly altered the dielectric, and mechanical properties of the composite foams. The dielectric constants of the composite foams were no longer frequency dependent while tan delta was lowered at least by a factor of 10. Furthermore, the porous structure generated by foaming also assisted the α-to-β phase transformation of the PVDF matrix in a way similar to mechanical stretching. Such method is superior to other phase transformation techniques since β-phase PVDF can be produced in bulk geometries instead of a thin film configuration.

Influence of molecular structure on the rheological properties and foamability of long chain branched polypropylene by “one-pot” reactive extrusion

In this work, reactive twin screw extrusion was conducted to synthesize long chain branched polypropylenes (LCB-PPs) in a “one-pot” process in which dicumyl peroxide (DCP) initiated maleic anhydride (MAH) grafting onto the linear PP, and the concomitant coupling reaction between ethylene diamine (EDA) and MAH grafted polypropylene (PP-g-MAH) proceeded in series. Fourier transfer infrared spectroscopy (FTIR) on the prepared materials confirmed the occurrence of both reactions. A series of LCB-PPs were prepared using different amounts of EDA, MAH and DCP to study their effects and determine the optimal synthesis conditions. The prepared materials were characterized by size exclusion chromatography (SEC) and rheological analysis to ascertain the polymer microstructure. The foamability of the LCB-PPs by supercritical carbon dioxide (scCO2) foaming and foam morphology were investigated. The LCB-PPs were found to have vastly improved foamability and cellular morphology. Under optimal conditions, a foam expansion ratio of over 20 was achieved.

A two-step process for the preparation of thermoplastic polyurethane/graphene aerogel composite foams with multi-stage networks for electromagnetic shielding

A two-step method as follows of preparing thermoplastic polyurethane/graphene aerogel (TPU/GA) composite foams with multi-stage network were used for the first time. TPU/GA composites with 3D network were obtained through the vacuum assisted impregnation using the GA as template firstly, and the cellular structure was then introduced by scCO2 foaming. It was revealed that the electrical conductivity of the TPU/GA composite foams reached 50 S/m at the GA content of 2 wt%, and the EMI shielding value of which was 34.3 dB, resulting from the interconnected graphene networks in TPU/GA composite foams. In addition, EMI shielding absorption coefficient A was improved because of the increased paths for EMI waves in TPU/GA composite foams. It demonstrated an alternative method for preparing flexible polymer/graphene composite foams with favorable EMI shielding absorption performance.

Cellular structure design by controlling rheological property of silicone rubber in supercritical CO2

Rheological properties, determined by cross-linked structure of silicone rubber and plasticization of CO2, would affect the cell nucleation and the cell growth during supercritical CO2 (scCO2) foaming. In this study, the effects of the crosslinking density and CO2 saturation condition on rheological properties have been comprehensively analyzed. The silicone rubber foams with different cellular structures were designed by adjusting scCO2 foaming conditions. The results show that the suitable crosslinking density (1.47 × 10−4 mol cm-3) would be beneficial to prepare silicone rubber foams with high porosity (80.6 %). The foaming behavior of silicone rubber would be greatly affected by the saturation temperature and the saturation pressure, resulting in several orders of magnitude changes in cell density, but the saturation time has little influence. This research helps to reveal the connection between rheological property and foaming behavior, and provide an eco-friendly way to design the cellular structure by adjusting the rheological property of silicone rubber.

Fabrication of PCL Scaffolds by Supercritical CO2 Foaming Based on the Combined Effects of Rheological and Crystallization Properties.

Polycaprolactone (PCL) scaffolds have recently been developed via efficient and green supercritical carbon dioxide (scCO2) melt-state foaming. However, previously reported gas-foamed scaffolds sometimes showed insufficient interconnectivity or pore size for tissue engineering. In this study, we have correlated the thermal and rheological properties of PCL scaffolds with their porous morphology by studying four foamed samples with varied molecular weight (MW), and particularly aimed to clarify the required properties for the fabrication of scaffolds with favorable interconnected macropores. DSC and rheological tests indicate that samples show a delayed crystallization and enhanced complex viscosity with the increasing of MW. After foaming, scaffolds (27 kDa in weight-average molecular weight) show a favorable morphology (pore size = 70–180 μm, porosity = 90% and interconnectivity = 96%), where the lowest melt strength favors the generation of interconnected macropore, and the most rapid crystallization provides proper foamability. The scaffolds (27 kDa) also possess the highest Young’s modulus. More importantly, owing to the sufficient room and favorable material transportation provided by highly interconnected macropores, cells onto the optimized scaffolds (27 kDa) perform vigorous proliferation and superior adhesion and ingrowth, indicating its potential for regeneration applications. Furthermore, our findings provide new insights into the morphological control of porous scaffolds fabricated by scCO2 foaming, and are highly relevant to a broader community that is focusing on polymer foaming.

Dynamic self generation of hydrogen bonding and relaxation of polymer chain segment in stabilizing thermoplastic polyurethane microcellular foams

Cell stabilization of TPU foams made from supercritical CO2 (scCO2) foaming remains a huge challenge and significantly limits TPU foams’ applications. One key point is the absence of molecular mechanism during TPU foam shrinkage and not to mention the solution. In this work, the interactions within/among TPU macromolecular chain(s) are investigated in-depth via several in-situ characterization methods, hence clarified the molecular mechanism during TPU foam shrinkage and uncovers the direction to achieve cell stabilization. It is revealed that dynamical self-generation of hydrogen bonding and polymer chain segments’ relaxation after foaming play significant roles in cell stabilization. Base on this molecular mechanism, strategies of strengthening and accelerating hydrogen bonding, inducing another primary bonding besides hydrogen bonding among molecular chains, fixing hard segments while relaxing soft segments are proposed. Consequently, this work guides the direction to produce thermoplastic elastomer foams of large expansion ratio with less shrinkage via scCO2 foaming.

Preparation of high-strength and lightweight microcellular polysulfone foam with a segregated CNT network for excellent electromagnetic shielding.

Fabrication of microcellular polymer composite foam based on high-performance plastic is a promising strategy for preparing the lightweight, high-strength and multifunctional materials. Herein, we proposed a facile and green method to prepare microcellular polysulfone/carbon nanotube (PSU/CNTs) composite foams with segregated structure by combining solid-phase milling and supercritical carbon dioxide (scCO2) foaming. The segregated PSU/CNTs foam with as low as 5.0 wt% CNT was provided with a good electrical conductivity of 5.2 S m−1 and an acceptable electromagnetic interference shielding effectiveness (EMI SE) of 23.7 dB, respectively. Moreover, the segregated PSU/CNT foam exhibited an ultralow percolation threshold of 0.06 vol%. An absorption-dominant shielding feature was observed for segregated PSU/CNT foam, which could be attributed to the synergistic effect of the perfect CNT networks and the microcellular structure in PSU domains. In addition, benefitting from the inherent properties of the PSU matrix, foam density dropped to 0.69 g cm−3, and the material still possessed a high specific compression strength of 38.8 MPa cm3 g−1. Therefore, our work provided an insight into the preparation of lightweight, high-strength and multifunctional materials that might have great potential applications in aerospace and military areas.