Bimodal Cellular Structure Research Articles

Preparation of Porous UHMWPE Composites with Functional Bimodal Cellular Structure and Research on its Oil Absorption Performance

AbstractPorous polymers with bimodal cellular structure have excellent hydrophobicity and structure, while preparation of porous polymers with functional bimodal cellular structures is challengeable. In this work, porous ultrahigh‐molecular‐weight polyethylene (UHMWPE)/polyethylene glycol (PEG) composites with functional bimodal cellular structures are fabricated based on the simple and environmentally friendly supercritical carbon dioxide microfoaming technology. The thermal performance, the foaming performance of UHMWPE/PEG composites, and the oil absorption performance under various extreme environments of UHMWPE/PEG porous samples are investigated. Representative sea‐island structures are observed on the fracture surface of various UHMWPE/PEG composites by scanning electron microscopy (SEM). When the PEG content exceeds 3%, the porous UHMWPE shows a distinctive bimodal cellular structure. The results of the water contact angle test in various extreme environments indicates that the porous UHMWPE has stable hydrophobicity, exhibits good absorption performance and excellent reusability for different oils. The permeation pressure tests demonstrate that the permeation pressure of water is significantly higher than that of oil. Therefore, the functional porous UHMWPE/PEG has good application prospects in oil water separation and oil absorption.

Bimodal Cellular Structure Evolution in PBAT Foams Incorporated by Carbon Nanotubes and Graphene Nanosheets

Bimodal Cellular Structure Evolution in PBAT Foams Incorporated by Carbon Nanotubes and Graphene Nanosheets

Preparation of PLA with High Impact‐Toughness and Reduced Internal Stress via Formation of Laminated, Bimodal Structure with Micro/Nanocells

AbstractThe pressure‐induced‐flow (PIF) processing method is used to fabricate oriented, self‐toughening poly(lactic acid) (PIF‐PLA) with excellent mechanical properties. However, residual internal stress in PIF‐PLA may lead to their deformation and cracking. Hence, to eliminate internal stress, solid‐state supercritical carbon dioxide foaming (sc‐CO2 foaming) is applied to PIF‐PLA, with the resultant structure being called FOAM‐PLA. The microstructure and mechanical properties of the PLAs are characterized pre‐ and post‐foaming. Scanning electron microscopy results show PIF‐PLA to exhibit oriented texture structures composed of staggered microfibers. Following sc‐CO2 foaming, the PIF‐PLA is converted into a bimodal cellular structure consisting of micro and nano‐cells. According to the 2D wide‐angle X‐ray diffraction and Raman spectra results, this structure of the FOAM‐PLA can, through a marginal reduction of orientation, release significant amounts of internal stress. Moreover, the impact strength of FOAM‐PLA is found to be 32.7 kJ m−2, which is 10.2 times higher than that of crystalline PLA. This paper depicts the evolution of the PLA microstructure through the stages of PIF‐processing and the subsequent solid‐state sc‐CO2 foaming. Meanwhile, the mechanism of the internal stress reduction is promoted.

Controlling the structure and density of PMMA bimodal nanocellular foam by blending different molecular weights

The bimodal cellular structure of foam is known to provide unique physical properties. In addition, it may help reduce foam density. This study proposes a simple method of creating a bimodal microcellular/nanocellular structure by blending PMMAs of different molecular weights using CO2 as the blowing agent. Nanofoam, with its low relative density, is the right candidate for thermal insulation, but obtaining it is challenging. Introducing micron-sized bubbles into homogeneous nanocellular structures may be one way to reduce the foam density and potentially not affect some of the properties. Beyond the bimodal structure, a transition from an ultramicrocellular to a nanocellular structure and from a closed-cell to an open-cell structure was observed. These transitions may be related to the relaxation time and weight percentage of non-entangled PMMA content. Formation of the bimodal nanocellular or open-cell structure may be predicted by the viscoelastic properties such as the relaxation time.

An innovative strategy to regulate bimodal cellular structure in chain extended poly(butylene adipate‐co‐terephthalate) foams

AbstractA facile and effective approach for manufacturing poly(butylene adipate‐co‐terephthalate) (PBAT) foams with obvious bimodal cellular structure (BCS) was proposed utilizing chain extension and batch supercritical CO2 foaming technology. Ethylene‐glycidyl methacrylate copolymer (EGMA) as crystalline chain extender was selected to modify PBAT. The results of torque tests, differential scanning calorimetry, and rheological performances demonstrated that the melt strength, viscoelasticity, and crystallization properties of PBAT were improved after introducing EGMA. The influences of EGMA content and foaming temperature on the cellular structure of PBAT foams were investigated systematically. By decreasing the foaming temperature, the size of both large and small cells in the bimodal chain extended PBAT‐3 (CPBAT‐3) foams decreased, as well as their BCS became distinct gradually. The BCS in diverse CPBAT foams was successfully controlled via varying EGMA content and foaming temperature. Finally, the formation mechanism of BCS in diverse PBAT foams was presented.

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...

Tuning cell structure and expansion ratio of thick-walled biodegradable poly(lactic acid) foams prepared using supercritical CO2

Thick-walled poly(lactic acid) samples are foamed using supercritical carbon dioxide as physical foaming agent over a wide saturation time range using a constant-temperature mode and a wide foaming pressure range using the constant-temperature mode and a varying-temperature mode. Using the constant-temperature mode, three regions with no-celled core and two regions with cells of different diameters appear on the fractured surfaces of the foamed samples prepared at 5 and 10 min saturation times, respectively, whereas a relatively uniform cellular structure is obtained at 20–180 min saturation times. Raising the foaming pressure can improve the cellular structure uniformity. Moreover, prolonging saturation time or raising foaming pressure results in rupture of more cell walls and so formation of open-celled structure to a certain extent. Using the varying-temperature mode, a bimodal cellular structure with stamen-like cells and a trimodal cellular structure with an extraordinarily high expansion ratio (76.2) are successively achieved during raising the foaming pressure (18–22 MPa). The formation mechanisms for the bimodal and trimodal cellular structures are analyzed based on the result of the foaming pressure effect on the cellular structure in the foamed poly(lactic acid) samples prepared using the constant-temperature mode.

A novel poly (vinyl alcohol)/poly (ethylene glycol) scaffold for tissue engineering with a unique bimodal open-celled structure fabricated using supercritical fluid foaming

In this study, a novel poly (vinyl alcohol) (PVA)/poly (ethylene glycol) (PEG) scaffold was carefully designed via thermal processing and subsequent supercritical fluid (SCF) foaming. Interestingly, a bimodal open-celled structure with interconnected networks was successfully created in the plasticized PVA (WPVA)/PEG scaffold. Large cells were produced from the nucleation sites generated in the PVA phase during rapid depressurization, while plenty of small pores generate in the cell walls of the big cells. The formation mechanism of this cellular structure was studied by considering the various phase morphologies and the diffusion behaviour of the carbon dioxide (CO2) in individual phases. In addition, the intermolecular interactions of the WPVA/PEG blend were studied using X-ray diffraction and FTIR analysis. The results demonstrate that various types of hydrogen bonds among the hydroxyl groups on the PVA chains, PEG and water molecules are formed in the blend system. The realization of thermoplastic foaming of the PVA/PEG blend benefits from the interactions of complexation and plasticization between water and PEG molecules. The SEM images also revealed that L929 fibroblast cells were able to attach and spread on surfaces of the WPVA/PEG samples. Thus the WPVA/PEG scaffold with unique bimodal cellular structure is nontoxic and favours the attachment and proliferation of cells, making it promising for use as the candidate for tissue engineering applications.

Mechanical Properties of PMMA‐Sepiolite Nanocellular Materials with a Bimodal Cellular Structure

AbstractBimodal cellular poly(methyl methacrylate) with micro‐ and nano‐sized (300–500 nm) cells with up to 5 wt% of sepiolite nanoparticles and porosity from 50% to 75% are produced by solid‐state foaming. Uniaxial compression tests are performed to measure the effect of sepiolite concentration on the elastic modulus and the yield strength of the solid and cellular nanocomposites. Single edge notch bend tests are conducted to relate the fracture toughness of the solid and cellular nanocomposites to sepiolite concentration. The relative modulus is independent of sepiolite content to within material scatter when considering the complete porosity range. In contrast, a mild enhancement of the relative modulus is observed by the addition of sepiolite particles for the foamed nanocomposites with a porosity close to 50%. The relative compressive strength of the cellular nanocomposites mildly decreases as a function of sepiolite concentration. A strong enhancement of the relative fracture toughness by the addition of sepiolites is observed. The enhancement of the relative fracture toughness and the relative modulus (at 50% porosity) can be attributed to an improved dispersion of the particles due to foaming and the migration of micro‐sized aggregates from the solid phase to the microcellular pores during foaming.

Nanocomposites foams of poly(ethylene-co-vinyl acetate) with short and long nanocellulose fibers and foaming with supercritical CO2

Nanotechnology applied to polymer foams is an emerging field, because even at low filler contents, different cellular morphologies can be obtained in the development of foams. Based on these premises, this work addresses the development of poly(ethylene-co-vinyl acetate)—EVA foams reinforced with two types of nanocellulose (a) short fibers (NC), and (b) long fibers (NCF). The foams were expanded in an autoclave with supercritical carbon dioxide (CO2). Different expansion conditions were evaluated, with variations of temperature and pressure. The main results indicate that the presence of the fibers significantly reduces the size of the cells and increase the number of cells per unit area. However, by increasing the NC content, nanocellulose fibers agglomeration is observed, resulting in the formation of a bimodal cellular structure.

Low Density Non-crosslinked Closed/Open Cell Polypropylene Foams with High Mechanical Properties

Low density polypropylene based foams with different cellular structures have been produced by the improved compression molding route using a high melt strength polypropylene as polymer matrix. In addition, different types of nanoparticles have been introduced in the formulation (multi-wall carbon nanotubes, organomodified nanoclays and natural nanoclays) to modify the structure and properties. The results have showed a clear correlation between the open cell content of the foams and the mechanical properties in compression. In the unfilled polypropylene high specific mechanical properties are only achievable with low values of open cell content. In comparison, for an equal value of the interconnectivity between cells, the samples containing nanoclays present much higher specific properties. This result is attributed to the reinforcement of these nanoparticles in the solid matrix, due to an improved exfoliation during the foaming process and the presence of a bimodal cellular structure. The produced foams have interesting properties with stiffness similar to those of commercial polymer foams used for the core of sandwich panels.

Foaming of Poly(lactic acid) Using Supercritical Carbon Dioxide as Foaming Agent: Influence of Crystallinity and Spherulite Size on Cell Structure and Expansion Ratio

Using supercritical carbon dioxide (Sc-CO2) as physical foaming agent, foamed poly(lactic acid) (PLA) samples were prepared in a batch process via constant- and varying-temperature modes (CTM and VTM). Their crystallinity, cellular structure, and expansion ratio were investigated. In the CTM, the samples foamed at low saturation temperatures present three regions (skin, inner, and core regions). The uniformity of cellular structure is much improved with increasing saturation temperature. In the VTM, saturation temperature exerts a significant impact on the size of spherulites formed in gas saturation stage. Large spherulites evolve into entities surrounded by elongated cells or submicro-sized cells in interlamellar regions after foaming at 140 °C and into small cells (mean diameter of 0.6 μm) at 160 °C, whereas small spherulites generally evolve into stamen-like cell structure at 140 °C. Interestingly, uniform cellular structure with high expansion ratio (49.8) or bimodal cellular structure can be obtained by tuning saturation and foaming temperatures.

In situ preparation of cross‐linked polystyrene/poly(methyl methacrylate) blend foams with a bimodal cellular structure

In situ preparation of a cross‐linked poly(methyl methacrylate) (PMMA) and polystyrene (PS) blend and its foaming were investigated for creating a bimodal cellular structure in the foam. Methyl methacrylate (MMA) monomer was dissolved in PS under supercritical CO2 at a temperature of 60 °C and a pressure of 8 MPa, and the polymerization of MMA was conducted at 100 °C and 8 MPa CO2, with a cross‐linking agent in PS. The blend was successively foamed by depressurizing the CO2. CO2 played the roles of plasticizing the PS and enhancing the monomer dispersion in PS during the sorption process and as a physical blowing agent in the foaming process. The cross‐linking agent was used for controlling the elasticity of polymerized PMMA domains and differentiating their elasticity from that of the PS matrix. The difference in elasticity delayed the bubble nucleation in the PMMA domains from that in the PS and made the cell size bimodal distribution, in which the smaller cells ranging from 10 to 30 µm in diameter were located in the wall of large cells of 200–400 µm in diameter. The effects of the initial MMA content, the concentration of cross‐linking agent, and the depressurization rate on the bimodal cell structure and bulk foam density were investigated. Copyright © 2011 John Wiley & Sons, Ltd.

Deformation of Flexible Polyether Polyurethane Foams with Bimodal Cellular Structure

Structural and mechanical properties of light-weight flexible polyether polyurethane foams with bimodal cellular structures are discussed. The formation of a two level cellular structure (β- and α-, respectively from 0.1 to 0.6 and from 3 to 12 mm in diameter) were considered. Compressive stress-strain curves of flexible foams with monomodal and bimodal cellular structures were evaluated for a wide range of strain rates. The Dementjev-Tarakanov 14-sided model of polyhedron structure was used in calculations using the elastic parameters method. This paper discusses research and develops a method of calculation of heterogeneity and deformation of flexible foams reviewing the results of laboratory and calculation investigations.

Deformation of elastic polyurethanes with a bimodal cellular structure

This article examines the cellular structure and mechanical properties of elastic foamed polyurethanes with a bimodal cellular structure (BFPUs). It is shown that the sizes of the oval cells (3–12 mm) in the elastic foamed polyurethenes that are studied are a tenth of an order greater than the sizes of the polyhedral cells (0.1–0.6 mm). Conversely, for rigid foamed polyurethanes, the polyhedral cells are more than a tenth of an order larger than the oval cells (microcells). The equations of the compression curve of BFFUs are found, and it is established that the deformation of BFPUs is determined by the deformation function of the cellular structure and the viscoelastic properties of the polymer matrix. The deformation functions of the cellular structure and the relaxation properties of BFPUs are determined. It is shown that choosing BFPUs with a cellular structure improves the comfort properties of the foamed material: the softness coefficient increased by 8–19% in the investigated case, while the support coefficient increased by 15–35%.