Bimodal Cell Structure Research Articles

Lightweight, porous, stretchable nanocomposite foams with sandwich-like and bimodal cell structure by supercritical fluids-assisted processing for flexible strain sensor

Developing strain sensors with wide detection range and high sensitivity at a low conductive nanofillers content still faces challenges. Herein, thermoplastic polyurethane (TPU)/multi-walled carbon nanotubes (MWCNTs) nanocomposite foams with a multilayer alternating and bimodal cell structure is prepared by stacking hot pressing and supercritical carbon dioxide batch foaming process. Consequently, the synergistic effect of sandwich-like and bimodal cell structure endows the foam sensor of FTPU-C3-L10 T80P10 (TPU/3wt.%MWCNTs nanocomposite foams with ten layers alternating structure foamed at 80℃ and 10MPa) 0.34 vol.% MWCNTs loading, broad response range of 0~150%, high sensitivity with a gauge factor (GF) of 101, fast response speed of 180 ms, as well as cyclic reproducibility and durability of 1000 cycles. Furthermore, the fabricated foam sensor can effectively monitor basic human motion from tiny to large. This work provides a facile strategy for designing and producing strain sensors applied in various fields, including human motion detection, artificial intelligence devices, and medical diagnosis.

Effect of crystalline transformation on supercritical CO2 foaming and cell morphology of isotactic polybutene-1

As a polycrystalline polymer, isotactic polybutene-1 (iPB-1) will form different crystalline structures when it crystallizes under different conditions. In this work, we found a simple method to manipulate the cell structure of iPB-1 foams by controlling the annealing time of crystalline form II at room temperature and employing supercritical CO2 foaming. As the content of crystalline form II increases, the storage modulus (G′) and complex viscosity (|η * |) increase, and the solubility of CO2 in iPB-1 increases. Crystalline form II has a wider foaming temperature window compared to crystalline form I and exhibits a more uniform cell structure at temperatures between 95 °C and 125 °C. The average cell diameters of crystalline form II and I are 33.9 µm and 11.2 µm at 115 °C and 15 MPa CO2, respectively. A small amount of crystalline form I acts as a heterogeneous nucleation agent during the foaming process, resulting in the formation of a “petal-like” cell structure. During the foaming saturation process, the melted portion of iPB-1 with unstable crystalline form II transforms into crystalline form I′ under high-pressure CO2, while the unmelted portion transforms into crystalline form I. As the pressure increases, the cell structure of iPB-1 undergoes a transition from “petal-like” to bimodal cell structure, ultimately achieving a more uniform structure. Moreover, increasing foaming pressure can also change the cell structure of the foamed material from closed-cell to open-cell. The occurrence of bimodal melting peaks in the foamed samples is beneficial for the adhesion of beads during the molding process.

Microcell morphology evolution and mechanical performance of UHMWPE/PEG porous materials with bimodal cell structure

The bimodal cell structure with both large and small cells has been reported to have more excellent thermal insulation, vibration damping and mechanical properties. In this work, UHMWPE/PEG porous materials with bimodal cell structure were prepared by a simple one-step decompression micro-foaming technology using only supercritical carbon dioxide (sc-CO2). The effects of PEG content, molecular weight of PEG, saturation temperature, pressure and time on the evolution of cell morphology were investigated. The results show that when the content of PEG is 7%, the UHMWPE/PEG composite porous material exhibits stable bimodal cell morphologies in most process range, and a mechanism for the formation of bimodal cell is proposed. Moreover, UHMWPE/PEG porous materials with bimodal cell structure show higher mechanical properties. When the relative densities are similar, the anti-compression properties of bimodal porous materials are significantly better than those of unimodal porous materials.

Controllable cell structures of poly(ether-block-amide) foams via isothermal melt crystallization-foaming in supercritical CO2

A simple isothermal melt crystallization-foaming method was proposed to regulate the crystallinity (Xc) of PEBA to prepare poly(ether-block-amide) (PEBA) foams with uniform or bimodal cell structures by supercritical CO2. PEBA with different Xc values was obtained by controlling the isothermal crystallization temperature (Tiso) and time (tiso). The rheological and DSC results showed that the viscoelasticity and Xc of the samples increased with decreasing Tiso and increasing tiso. PEBA foams with Xc above 1.1% exhibited a distinct bimodal cell structure. The increased crystals acted as heterogeneous nucleation sites to increase the cell density from 6.8 × 106 cells/cm3 to 3.58 × 1010 cells/cm3. The size of large cells decreased from 206.4 µm to 48.0 µm, and the number ratio of small to large cells increased from 2.34 to 65 times. Finally, a mechanism to explain the cell structure variation in PEBA foams was proposed, providing a new strategy for regulating the cell structure of polymeric foams.

Improved thermal insulation and compressive property of bimodal poly (lactic acid)/cellulose nanocomposite foams

In this work, an innovative PLA/CNF nanocomposite foam with a bimodal cell structure is prepared by a simple one-step depressurization foaming process using only supercritical carbon dioxide (ScCO2) as the foaming agent. Only at a specific foaming temperature, PLA/CNF nanocomposites foam with a bimodal cell structure could be obtained. According to the different crystallization kinetics and nucleation efficiency of samples, it was inferred that the crystallization rate and phase interface would affect the cell structure. The prepared PLA/CNF nanocomposite foam with a bimodal cell structure had an expansion ratio as high as 20 times and thermal conductivity of 0.041 w m−1 k−1, which exhibited low density and excellent thermal-insulation property. Meanwhile, the PLA/CNF nanocomposite foam exhibited excellent compression performance due to the presence of CNFs, which showed promising application in packaging and construction materials.

Utilization of supercritical CO2 for controlling the crystal phase transition and cell morphology of isotactic polybutene-1 foams

Isotactic polybutene-1 (iPB-1) is a semi-crystalline polymer with multiple crystal forms. In this work, we found a novel method to obtain and adjust the expansion ratio and bimodal cellular structures of iPB-1 foams with annealing and foaming using supercritical CO2. The form I′ can be obtained via the supercritical CO2 annealing, and the ratio of form I and I′ can be adjusted by controlling simply the annealing time. The effects of crystal form ratio on the thermal behavior, rheological behavior and cell morphology of foams were analyzed. Energy storage modulus (G′) and complex viscosity (|η*|) decrease with the increased content of form I. Crystal form I′ has a lower foaming temperature window and bubble growth resistance than form I, resulting in larger cell diameter and expansion ratio. The bimodal cell structure can be observed in the samples with 1:1 form I′/form I, where the average large and small cell size are 122 and 40 µm at 109 °C and 10 MPa CO2, respectively. To regulate the cell distribution, various ratios of mixed crystal phases were applied to investigate their effect on the foaming behavior and bimodal cells structure at different temperatures. Even with a similar expansion ratio, significantly different cell distributions of bimodal cell structure can be easily prepared by simply adjusting the content of form I′, which can broaden the application range of iPB-1 foams.

Facile preparation of super lightweight and highly elastic thermoplastic polyurethane bead blend foam with microporous segregated network structure for good interfacial adhesion

The foamability of three different grades of TPU beads was studied, and two of them were selected to fabricate super lightweight and highly elastic TPU blend foams. Four different types of TPU combination blend foams with a microporous segregated network structure (MSNS) were prepared by solution-compression molding in combination with supercritical fluid (SCF) foaming. The results showed that the interface was well combined, and a bimodal cell structure was produced; the continuous phase constituted a MSNS. All TPU blend foams exhibited good springback performance, among which, TPUb70/TPUc70 blend foam had lowest hysteresis loss (3.8%) and lowest stress loss (2.6%). Thermal conductivity results showed they all have good thermal insulation, and the lowest thermal conductivity was 0.037 W/(m·K) for TPUb70/TPUc70 foam. This study offers a simple and cost-effective approach preparing high-performance foams with cushion, sound and shock absorption, and heat preservation for use in electronics, home appliances, sports, etc.

Formation mechanism of bi-modal cell structure polystyrene foams by synergistic effect of CO2-philic additive and co-blowing agent

A new method was developed for the formation of bi-modal cell structure polystyrene (PS) foams by synergistic effect of CO2-philic additive (PDMS) and co-blowing agent (alcohol). The results showed that PDMS and alcohol synergistically increased the solubility and diffusion coefficient of CO2 into PS and reduced the desorption rate from PS. The blowing agent formed a high-concentration area in the PDMS (6000 g/mol) phase to promote bubble nucleation and growth in advance. The moderate size and number of cells were pre-formed, slowing the CO2 desorption and increasing the duration of bubble growth, so that bi-modal cell structure PS foams with high cell density and volume expansion ratio were obtained. Moreover, the solution and diffusion behavior of CO2 and the cell morphology could be easily tuned by changing the composition of complex additives, which might be a promising strategy for preparing bi-modal cell structure in different foaming processes.

Preparing Polypropylene Auxetic Foam by a One‐Pot CO2Foaming Process

At present, the direct preparation of polymer foam with negative Poisson ratio (auxetic foam) from nonpolar polymer resins is still a challenge. Herein, a one‐pot CO2foaming method is established to directly prepare closed‐cell auxetic foams with different cellular structures (bimodal cell structure and homogeneous cell structure) from polypropylene (PP) resin by introducing hydrophilic additives (NaCl) into PP and regulating foaming conditions. The effects of foaming temperature, saturation time, and size of added NaCl on Poisson's ratio are studied. The comparison with PP/talc composites shows that the presence of NaCl plays a crucial role in the formation of the negative Poisson ratio of foam. Furthermore, the regulation of the size of negative Poisson ratio is achieved by controlling the amount of NaCl in PP/NaCl composites. This study has important reference significance for the preparation of nonpolar polymer auxetic foams.

Cell Structure Variation in Poly(ether-mb-amide) Copolymer Foams Induced by Chemi-Crystallization

The objective of this study was to clarify the cell structure variation of the poly(ether-mb-amide) (PEBA) copolymer with the chemi-crystallization induced via thermal treatment at the melt state. Thermal annealing was conducted using a compression molding machine at the melt state for various times (i.e., 3, 30, 60, 120, and 300 min) to promote chemi-crystallization. The chemical structure change was studied with Fourier transform infrared spectra, and the viscosity change was investigated using a rheometer. The crystallization behavior was studied with differential scanning calorimetry and X-ray diffraction. The PEBA samples were foamed using a two-step batch foaming process, and the cell structure was investigated with a scanning electron microscope. It was found that the chemical structure of PEBA was hardly changed during the thermal treatment, while the complex viscosity was decreased. It was speculated that degradation occurred probably via a random chain scission mode. Thus, the chemi-crystallization was promoted for the crystalline segments because of the less suppression from the noncrystalline segments ascribed to the shorter chains. Consequently, three types of cell structures were observed in PEBA: uniform cell structure, bimodal cell structure, and micro/nanosized cells formed within the spherulites. A scheme was presented to interpret the relationship between the cell structure variation and the crystalline states in PEBA samples. The amorphous, partially melted, and unmelted regions were classified during foaming to interpret the formation of the three types of cell structures.

Preparation of Polypropylene Foams with Bimodal Cell Structure Using a Microporous Molecular Sieve as a Nucleating Agent

A novel approach for fabricating polypropylene (PP) foam with bimodal cellular structure utilizing CO2 as a physical foaming agent was established by adding porous molecular sieve (PMS) as nucleati...

Adjusting cell structure of polypropylene composite foams by controlling the size and dispersed state of NaCl particles during CO2 batch foaming process

In this work, a novel method for regulating cellular structure of polypropylene (PP) foam under CO2 batch foaming with water as disperse medium was established by addition of hydrophilic fillers (Such as NaCl). The presence of NaCl increased adsorption amount of water in PP/NaCl composites. As a result, both CO2 and adsorbed water acts as blowing agents during CO2 batch foaming. Thus the dispersed state and size of NaCl particles directly affect the mixed state of two blowing agents (water and CO2) and the competition between two types of heterogeneous nucleation from NaCl particles and PP crystals during foaming, which are the fundamental reasons for the formation of the foamed materials with different cellular structures, such as isolated large cell surrounded by uniform small cell structure, bimodal cell structure and homogeneous cell structure. Another unexpected finding was that the water absorption of PP/NaCl composites increased with the decrease of NaCl particle size, which is beneficial to play the in-situ cooling effect and co-blowing agent of absorbed water to control the foaming behavior of PP, and avoid the damage of foam mechanical properties due to the high content of filler.

Comparative study on foaming process of thermoplastic polyester and polyether polyurethane with supercritical CO2 as foaming agent

ABSTRACTThe aim of this study was the comparison of beads foaming process and synergistic effect of foaming process parameters on melting crystalline characteristics between thermoplastic polyester and polyether polyurethane(TPU) via batch process using supercritical CO2(scCO2) as blowing agent. Method was based on the generation of expandable TPU foam beads (ETPU) prepared by self-designed autoclave foaming apparatus. The surface morphology, cell structure, and crystalline melting characteristics of the ETPU depending on the processing parameters, such as saturation pressure (Pf), saturation temperature (Tf), and saturation time (tf), were investigated. Results demonstrated both ETPU exhibited fine closed cell morphology with polygons structure and would induce the disappearance of multiple endothermic peaks during the foam process. As any of those three process parameters increased, the HS content, high crystalline melting and Er were improved, while with regard to cell structure, Pf demonstrated the opposite effect compared with Tf and tf. Comparing with polyether TPU, polyester TPU possess a larger cell density and a clear bimodal cell structure could be found under certain condition, while beads foaming process had a greater impact on polyether TPU, which had a low density and the appropriate tf is 2h.

Microcellular morphology evolution of polystyrene/thermoplastic polyurethane blends in the presence of supercritical CO2

In this article, a facile melt blending and solid batch foaming approach was proposed to prepare microcellular polystyrene/thermoplastic polyurethane (PS/TPU) blending foams with supercritical carbon dioxide (CO2). Compared with those of pure PS and pure TPU, an interesting phenomenon about the enhanced complex viscosity and storage modulus, as well as decreased loss factor of PS/TPU blends, was found. The solubility of CO2in the PS/TPU blends was enhanced, owing to the CO2solubilization effects of TPU. An interesting bimodal cell structure (BCS) was observed in the PS/TPU blending foams with the TPU content of 10, 15, and 20%. Consequently, a significant conclusion could be speculated that the generation of BCS in the PS/TPU blending system depended on not only the viscosity and morphology of the polymer blends but also the solubility and diffusivity of the CO2as well as the type of cell nucleation. The thermal insulation property of PS foam was improved by the introduction of TPU.

Cell nucleation in dominating formation of bimodal cell structure in polypropylene/polystyrene blend foams prepared via continuous extrusion with supercritical CO2

It is generally difficult to predict the cell structure foamed from a binary polymer blend compared to a single polymer, as many factors in polymer blends add to the complexity of the foaming process without easy control, such as blend morphology, blend viscosity, and solubility and diffusivity of the foaming agent, etc. In this study, polypropylene (PP)/polystyrene (PS) blend foams with a bimodal cell structure were prepared by using supercritical carbon dioxide (scCO2) in continuous extrusion. By comparison of the cell structure of blend foams before and after extraction, it was found that the large cells were mainly formed in PS phase, while the small ones were mostly formed in PP phase. A possible mechanism was proposed to explain the formation of the bimodal cell structure. It was believed that a prominent difference in cell nucleation in the two phases of the PP/PS blends was required for the formation of bimodal cell structure. The proposed mechanism was confirmed by further experimental results, which showed that the bimodal cell structure could be changed by altering the nucleation ability in PP phase and PS phase through changing experimental conditions, such as the content of scCO2 and the pressure drop, or through applying a heterogeneous nucleating agent.

Ultrasound-assisted-pressure-induced-flow leading to superior polymer/carbon nanotube composites and foams

In the presented work, nano-sized carbon particles were coated onto the surface of semicrystalline polymer pellets and molded into an intermediate preform, with a honeycomb-like structure, at elevated temperatures and pressures. The preforms were further processed through pressure induced flow (PIF) at higher pressures or ultrasound assisted pressure induced flow (UAPIF) at lower pressures to achieve polymer nanocomposites with a co-continuous and ‘brick and mud’ structure. The particle-coated polymer pellets, preforms, and composites can be saturated or partially saturated with a blowing agent, so foaming may occur during or after molding to fabricate polymer nanocomposite foams with a layer-by-layer bimodal cell structure. Using polypropylene and carbon nanotubes as an example, and carbon dioxide as the blowing agent, we show that these new materials can provide superior mechanical and physical properties with good electrical conductivity and foam expansion ratio, not achievable by conventional nanocomposites and foams with the same composition.

Formation mechanism and tuning for bi-modal cell structure in polystyrene foams by synergistic effect of temperature rising and depressurization with supercritical CO2

Polystyrene (PS) foams are fabricated using supercritical carbon dioxide (Sc-CO2) as a physical foaming agent in a batch foaming process. It is demonstrated that a bi-modal cell structure (BMCS) is obtained by the synergistic effect of temperature rising and depressurization using a varying-temperature mode (VTM). The formation mechanism for the BMCS is revealed. Cell nucleation occurs in the supercritical CO2-saturated polymer both in temperature rising and depressurizing stages. The nuclei formed in the former stage develop into large cells, whereas the ones formed in the latter stage evolve into small cells. The effects of foaming temperature, saturation temperature, second saturation time and foaming pressure in the VTM on the cellular structure of PS foams were investigated. The results showed that no BMCS is formed when the foaming temperature is only 10°C higher than saturation temperature. By manipulating the foaming parameters, the BMCS PS foams with a wide range of mean diameters (50.0–713.7μm) and densities (2.1×103–3.6×106cells/cm3) for the large cells, and a wide range of foam density (0.09–0.73g/cm3) are obtained.

Effect of Poly(butylenes succinate) on Poly(lactic acid) Foaming Behavior: Formation of Open Cell Structure

Biodegradable poly(lactic acid) (PLA) based poly(butylenes succinate) (PBS) foams with open cell structure were prepared via batch foaming method using supercritical carbon dioxide as blowing agent. It was found that PLA was immiscible with PBS, and PBS phase was dispersed as tiny spheres or large domains at various concentrations. The addition of PBS reduced the viscosity of the blends. During the foaming process, the PLA/PBS interfaces acted as cell nucleation sites and the low melt strength PBS contributed to the formation of cell connection channels, which resulted in open cell structure. The investigation of PBS content and foaming temperature found that PLA/PBS (80/20) foamed at 100 °C, obtaining the highest cell opening rate (96%). Two-step depressurization foaming experiments proved that bimodal cell structure PLA/PBS foams with high cell opening rate (97%) were able to be fabricated.

Fabrication of polystyrene/nano‐CaCO3 foams with unimodal or bimodal cell structure from extrusion foaming using supercritical carbon dioxide

The article surveyed the fabrication of polystyrene (PS)/nano‐CaCO3 foams with unimodal or bimodal cellular morphology from extrusion foaming using supercritical carbon dioxide (sc‐CO2). In order to discover the factors influenced the cell structure of PS/nano‐CaCO3 foams, the effects of die temperature, die pressure, and nano‐CaCO3 content on cell size, density, and morphology were investigated detailed. The results showed that the nano‐CaCO3 content affected the cell size and morphology of PS/nano‐CaCO3 foams significantly. When the die temperature and pressure was 150°C and 18 MPa, respectively, the foams with 5 wt% nano‐CaCO3 exhibited the unimodal cellular morphology. As the nano‐CaCO3 content increased to 20 wt%, a bimodal cell structure of the foams could be obtained. Moreover, it was found that the bimodal structure correlated more strongly with the pressure drop than the foaming temperature. The article revealed that unimodal or bimodal cellular morphology of PS/nano‐CaCO3 foams could be achieved by changing the extrusion foaming parameters and nano‐CaCO3 content. POLYM. COMPOS., 37:1864–1873, 2016. © 2015 Society of Plastics Engineers

High Melt Strength Polypropylene with Wide Molecular Weight Distribution Used as Basic Resin for Expanded Polypropylene Beads

The high melt strength polypropylene (HMSPP) is fabricated by direct polymerization through controlling the species and ratios of the external electron donors in the Ziegler–Natta catalyst system at different reaction stages. The polypropylene exhibits wide molecular weight distribution and higher melt strength. The expanded polypropylene (EPP) beads are prepared by the above HMSPP through an autoclave-based batch process. The effects of melt strength on the cellular morphology of EPP are investigated by scanning electron microscope and melt strength meter, respectively. Results indicate higher melt strength facilitates the control of the foaming process and the cellular structure compared with a commercial propylene homopolymer such as T03. The bimodal cell structure is obtained with the single foaming agent CO2 in both of the EPP beads from HMSPP and commercial polypropylene.