Low‐Density Nanocellular Foams Produced by High‐Pressure Carbon Dioxide

A new type of nanocellular polymeric material based on PMMA and a MAM triblock copolymer is presented. The production avoids the use of physical additives and leads to completely homogeneous nanostructured polymers with a large number of CO₂ nucleation sites. The foamed materials show average cell sizes <200 nm and relative foam densities of 0.4, presenting a homogeneous cell structure. A physical effect not measured before in nanocellular materials is demonstrated, which leads to an increase of the glass transition temperature due to the confining effect of PMMA chains in the cell walls of the nanocellular foam. The effects of changing saturation pressure and MAM content in the cellular structure are described, together with three‐point bending Young's modulus measured using DMA.

     

Investigation of the nanocellular foaming of polystyrene in supercritical CO2 by adding a CO2‐philic perfluorinated block copolymer

Nanocellular foaming of polystyrene (PS) and a polystyrene copolymer (PS‐b‐PFDA) with fluorinated block (1,1,2,2‐tetrahydroperfluorodecyl acrylate block, PFDA) was studied in supercritical CO₂ (scCO₂) via a one‐step foaming batch process. Atom Transfer Radical Polymerization (ATRP) was used to synthesize all the polymers. Neat PS and PS‐b‐PFDA copolymer samples were produced by extrusion and solid thick plaques were shaped in a hot‐press, and then subsequently foamed in a single‐step foaming process using scCO₂ to analyze the effect of the addition of the fluorinated block copolymer in the foaming behaviour of neat PS. Samples were saturated under high pressures of CO₂ (30 MPa) at low temperatures (e.g., 0°C) followed by a depressurization at a rate of 5 MPa/min. Foamed materials of neat PS and PS‐b‐PFDA copolymer were produced in the same conditions showing that the presence of high CO₂‐philic perfluoro blocks, in the form of submicrometric separated domains in the PS matrix, acts as nucleating agents during the foaming process. The preponderance of the fluorinated blocks in the foaming behavior is evidenced, leading to PS‐b‐PFDA nanocellular foams with cell sizes in the order of 100 nm, and bulk densities about 0.7 g/cm³. The use of fluorinated blocks improve drastically the foam morphology, leading to ultramicro cellular and possibly nanocellular foams with a great homogeneity of the porous structure directly related to the dispersion of highly CO₂‐philic fluorinated blocks in the PS matrix. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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

Bimodal 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.     

Intercalated Polycarbonate/Clay Nanocomposites: Nanostructure Control and Foam Processing

Intercalated polycarbonate (PC)/clay nanocomposites (PCCN)s have been prepared successfully through the melt intercalation method in the presence of a compatibilizer. The internal structure and morphology of the PCCNs has been established by using wide‐angle X‐ray diffraction (WAXD) analyses and transmission electron microscopic (TEM) observations. The morphology of these nanocomposites and degradation of the PC matrix after nanocomposites preparation can be controlled by varying surfactants used for the modification of clay and compatibilizer. The intercalated PCCNs exhibited remarkable improvements of mechanical properties when compared with PC without clay. We also discuss foam processing of one representative PCCN using supercritical CO₂ as a foaming agent.

TEM bright field image of intercalated polycarbonate/synthetic fluorohectorite nanocomposite.     

超临界CO2中合成聚碳酸酯

在超临界CO₂介质中由双酚A（BPA）和碳酸二苯酯（DPC）合成了双酚A型聚碳酸酯（PC）。反应生成的苯酚能够溶解扩散到超临界流体中。用红外光谱（FT-IR）、核磁共振谱（¹H-NMR）表征了产物的结构。凝胶渗透色谱（GPC）测试表明，合成过程中存在线形缩聚和成环两种不同的反应机理，产物的重均分子量高达117740，分子量分布指数P_d=1.33;差示扫描量热法（DSC）测试表明，超临界CO₂能增塑PC致使其玻璃化转变温度（T_g）降低。考察了反应时间、搅拌转速、反应温度等因素对PC分子量的影响。在反应压力为10 MPa下较佳反应时间为50 h，较佳搅拌转速为800 r·min^-1，较佳反应温度为120℃。     

Polypropylene‐dispersed domain as potential nucleating agent in PS and PMMA solid‐state foaming

The potential of using dispersive domains in a polymer blend as a bubble nucleating agent was investigated by exploiting its high dispersibility in a matrix polymer in the molten state and its immiscibility in the solid state. In this experiments, polypropylene (PP) was used as the nucleating agent in polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at the weight fraction of 10, 20, and 30 wt %. PP creates highly dispersed domains in PS and PMMA matrices during the extrusion processing. The high diffusivity of the physical foaming agent, i.e., CO₂ in PP, and the high interfacial tension of PP with PS and PMMA could be beneficial for providing preferential bubble nucleation sites. The experimental results of the pressure quench solid‐state foaming of PS/PP and PMMA/PP blends verified that the dispersed PP could successfully increase the cell density over 10⁶ cells/cm³ for PS/PP and 10⁷ cells/cm³ for PMMA/PP blend and reduce the cell size to 24 μm for PS/PP and 9 μm for PMMA/PP blends foams. The higher interfacial tension between PP and the matrix polymer created a unique cell morphology where dispersed PP particles were trapped inside cells in the foam. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis of copper–polystyrene nanocomposite particles using water in supercritical carbon dioxide medium and its antimicrobial activity

Copper‐encapsulated polystyrene nanocomposite particles were prepared through ex situ dispersion of Cu nanoparticles into monomer droplets and subsequent polymerization using water in supercritical carbon dioxide (water‐in‐sc‐CO₂) at 70°C. First, colloidal dispersion of copper nanoparticles was synthesized by chemical reduction of copper chloride (CuCl₂) using sodium borohydrate (NaBH₄) as reducing agent. Colloidal dispersion of copper nanoparticles was added slowly during the polymerization of styrene using water‐in‐sc‐CO₂ medium at 70°C and 20.68 MPa. Cu nanoparticle encapsulated polymer particles were characterized by UV, X‐ray diffraction, thermogravimetric analysis, SEM, and TEM. Cu nanoparticles were uniformly distributed inside the polymer matrix during the polymerization process. This work represents a simple way to prepare a variety of metal nanoparticles encapsulated polymer particles using water‐in‐sc‐CO₂ medium. The Cu/polystyrene nanocomposite particles exhibit antimicrobial activity against a number of bacteria. The current work represents a simple, cheap and universal way to prepare a variety of metal–polymer nanocomposite materials. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Processing of polypropylene–clay nanocomposites: Single‐screw extrusion with in‐line supercritical carbon dioxide feed versus twin‐screw extrusion

This work investigates two different melt‐blending strategies for preparing compatibilized polypropylene‐clay nanocomposites, specifically: (1) conventional twin‐screw extrusion, and (2) single‐screw extrusion capable of direct supercritical carbon dioxide (scCO₂) feed to the extruder barrel. Proportional amounts (3 : 1) of maleic anhydride functionalized polypropylene compatibilizer and organically modified montmorillonite clay at clay loadings of 1, 3, and 5 wt % are melt‐blended with a polypropylene homopolymer using the two approaches. The basal spacing, degree of exfoliation, and dispersion of organoclay is assessed using X‐ray diffraction, transmission electron microscopy, and rheology. In terms of the latter, both steady shear and small‐amplitude oscillatory shear provide information about the apparent yield stress and solid‐like terminal behavior respectively. Finally, nanoindentation is performed to determine the room temperature modulus of each melt‐blended nanocomposite. The results reveal unequivocally that the high shear of the twin‐screw process is vastly superior to the single‐screw with in‐line scCO₂ addition in generating well‐exfoliated, percolated polypropylene‐clay nanocomposites. It is likely that increased contact time between clay and scCO₂ is necessary for scCO₂ to positively affect exfoliation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 884–892, 2007     

Crystal nucleation and growth in poly(ethylene terephthalate)/alumina‐nanoparticle composites

The effect of nanoparticles on nonisothermal polymer crystallization was investigated using poly(ethylene terephthalate) (PET) nanocomposites with alumina (Al₂O₃) nanoparticles of average size 38 nm. The filler content in the nanocomposites was varied from 0 to 10 wt %. The interparticle spacing was observed to decrease (as expected) with an increase in loading of the nanoparticles. Contrary to previous reports in the literature on semicrystalline polymer‐based composites with micron‐size and macroscale particles, our differential scanning calorimetry, transmission electron microscopy, and X‐ray studies showed that the addition of the nanoparticles did not cause heterogeneous nucleation of PET crystals in nanocomposites containing up to 3 wt % Al₂O₃. This is attributed to the nanoparticle curvature being comparable to the radius of gyration of the polymer. The addition of the nanoparticles was found to disrupt the spherulitic morphology of the PET because of their physical presence and their proximity to one another. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Nanoscale Cellular Foams from a Poly(propylene)‐Rubber Blend

Plastic foams with nano/micro‐scale cellular structures were prepared from poly(propylene)/thermoplastic polystyrene elastomer (PP/TPS) systems, specifically the copolymer blends PP/hydrogenated polystyrene‐block‐polybutadiene‐block‐polystyrene rubber and PP/hydrogenated polystyrene‐block‐polyisoprene‐block‐polystyrene. These PP/TPS systems have the unique characteristic that the elastomer domain can be highly dispersed and oriented in the machine direction by changing the draw‐down ratio in the extrusion process. A temperature‐quench batch physical foaming method was used to foam these two systems with CO₂. The cell size and location were highly controlled in the dispersed elastomer domains by exploiting the differences in CO₂ solubility, diffusivity, and viscoelasticity between the elastomer domains and the PP matrix. The average cell diameter of the PP/TPS blend foams was controlled to be 200–400 nm on the finest level by manipulating the PP/rubber ratio, the draw‐down ratio of extrusion and the foaming temperature. Furthermore, the cellular structure could be highly oriented in one direction by using the highly‐oriented elastomer domains in the polymer blend morphology as a template for foaming.

     

Control of Bubble Size and Location in Nano‐/Microscale Cellular Poly(propylene)/Rubber Blend Foams

Plastic foam with nano‐/micro‐scale cellular structures was prepared from a poly(propylene) (PP)/propylene‐ethylene copolymer (PER) blend by controlling bubble nucleation sites and bubble growth in disperse PER domains. Batch foaming experiments using a CO₂ pressure quench method were conducted at room temperature. The bubble size and location were highly controlled in disperse PER domains by exploiting the differences in CO₂ solubility and viscoelasticity between the PER domains and the PP matrix. The average cell diameter of PP/PER blend foams can be controlled within 0.5–2 µm by the PP/PER ratio, depressurization rate, and foaming temperature.

     

Nanocellular Foams of PS/PMMA Polymer Blends

A nanocellular PS/PMMA polymer blend foam was prepared, where bubble nucleation was localized in the PMMA domains. The blend, which contains dispersed nanoscale PMMA islands, was prepared by polymerizing MMA monomers in a PS matrix to form highly dispersed PMMA domains in the PS matrix by diffusion mixing. The resulting blend was foamed with CO₂ at room temperature. A higher depressurization rate at lower foaming temperature made the bubble diameter smaller and the bubble density larger, and a higher PS composition in the blend resulted in a larger bubble density. A void with 40–50 nm in average diameter and a pore density of 8.5 × 10¹⁴ cm^?3 was obtained as for the finest nanocellular foams.

     

Preparation of polyethylene‐octene elastomer/clay nanocomposite and microcellular foam processed in supercritical carbon dioxide

Polyethylene‐octene elastomer (POE)/organoclay nanocomposite was prepared by melt mixing of the POE with an organoclay (Cloisite 20A) in an internal mixer, using poly[ethylene‐co‐(methyl acrylate)‐co‐(glycidyl methacrylate)] copolymer (E‐MG‐GMA) as a compatibilizer. X‐ray diffraction and transmission electron microscopy analysis revealed that an intercalated nanocomposite was formed and the silicate layers of the clay were uniformly dispersed at a nanometre scale in the POE matrix. The nanocomposite exhibited greatly enhanced tensile and dynamic mechanical properties compared with the POE/clay composite without the compatibilizer. The POE/E‐MA‐GMA/clay nanocomposite was used to produce foams by a batch process in an autoclave, with supercritical carbon dioxide as a foaming agent. The nanocomposite produced a microcellular foam with average cell size as small as 3.4 µm and cell density as high as 2 × 10¹¹ cells cm^?3. Copyright © 2005 Society of Chemical Industry     

Synthesis of polycarbonate‐co‐poly(p‐ethylphenol) and CdS nanocomposites

The enzyme‐catalyzed synthesis of poly(p‐ethylphenol) (PEP) was modified by copolymerization with polycarbonates through triphosgene at low temperature to form polycarbonate‐co‐poly(p‐ethylphenol) (PC‐co‐PEP). FTIR, NMR, GPC, and thermal analysis verified the formation of PC‐co‐PEP. The copolymers have an optical absorption in the UV range. CdS semiconductor nanocrystallites were synthesized in reversed micelles with subsequent in situ enzymatic copolymerization of p‐ethylphenol and 4‐hydroxythiophenol in the same medium. TEM and ATR–FTIR showed that the polymer precipitated in spherical morphologies, incorporating CdS nanocrystals into the polymer matrix, with surface hydroxyl groups. The polymer/CdS core was then dispersed into polycarbonate. The polymer/CdS nanocomposites showed higher optical aborbance in the UV‐vis range when compared to the polymer matrix without CdS. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1851–1868, 1999     

Processing and Material Characteristics of a Carbon‐Nanotube‐Reinforced Natural Rubber

CNT/NR composites were fabricated based on a CNT treatment using an acid bath followed by ball‐milling with HRH bonding systems. Thermal properties, vulcanization characteristics and mechanical properties of the CNT/NR composites were characterized. Compared to CB, the incorporation of CNTs into NR was faster and the energy consumption was less. The over‐curing reversion of CNT/NR composites was alleviated. After acid treatment and ball‐milling, the dispersion of CNTs in the rubber matrix and the interaction between CNTs and the matrix was improved. The performance of the CNT/reinforced NR composites was enhanced by the incorporation of the treated CNTs as compared to neat NR and CB/NR composites.

     

Selectivity of Rhodium‐Catalyzed Hydroformylation of 1‐Octene during Batch and Semi‐Batch Reaction using Trifluoromethyl‐Substituted Ligands

The regioselectivity of catalysts generated in situ from dicarbonyl rhodium(I)(2,4‐pentanedione) and trifluoromethyl‐substituted triphenylphosphine ligands has been evaluated during the hydroformylation of 1‐octene. The influence of batch or semi‐batch operation, the solvent, and the number of trifluoromethyl substituents has been investigated. During batch operation in a supercritical carbon dioxide (CO₂)‐rich system the differential n:iso ratio increases from approximately 4 to a value of 12–16 at about 90–95 % conversion for the catalyst based on bis[3,5‐bis(trifluoromethyl)phenyl]phenylphosphine. For semi‐batch conditions using hexane a constant n:iso ratio is obtained over a broad conversion range. Batch hydroformylation in neat 1‐octene is faster than in a supercritical CO₂‐rich, one‐phase system, with a similar overall selectivity as observed in the supercritical case. The results provide further directions for the development of ligands that are especially designed for the separation of homogeneous catalysts in continuously operated hydroformylation in scCO₂.     

CO2‐Induced Mechanical Reinforcement of Polyolefin‐Based Nanocellular Foams

The effect of CO₂‐induced crystallization on the mechanical properties, in particular the yield and the ultimate stresses, of polyolefins is studied. PP and SEBS copolymer blends are used as examples and foamed after sorption of CO₂ at temperatures below T_m. CO₂ sorption thickens the crystalline lamellae and consequently increases T_m from 160 to 178 °C for both pure PP and PP/SEBS blend systems. Foams with an average cell size smaller than 250 nm retain the ultimate stress at the level of the polymer before foaming, even without the effect of CO₂‐induced crystallization. Including CO₂‐induced crystallization, the yield and the ultimate stresses of the foam can be improved by 30 and 50% over solid PP and by 22 and 40%, for solid PP/SEBS blends, respectively.

     

双酚A甲醛酚醛环氧树脂／纳米SiO2复合材料的形态和性能

采用溶液共混法将纳米SiO2粒子分散到双酚A甲醛酚醛环氧树脂（bis-ANER）与二氧化乙烯基环己烯（VCD）的混合物中，再与固化剂甲基六氢邻苯二甲酸酐（MHHPA）混合，制备了不同SiO2含量的bis-ANER／VCD／MHHPA／SiO2纳米复合材料。用扫描电镜、透射电镜、材料试验机、冲击试验机、热分析法对其固化产物的断面形态、力学性能和热性能进行了研究。结果表明，纳米SiO2粒子在环氧树脂基体中的分散是均匀的，粒径在30-50nm左右。1．5％（质量分数，下同）的SiO2粒子分散到bis-ANER／VCD/MHHPA中使材料的拉伸、弯曲和冲击强度分别提高了112％、66％和118％，断裂伸长率由1．85％提高到3．37％，玻璃化转变温度提高了5．4℃，热降解温度略有提高，降解起始阶段反应机理与未加SiO2粒子的材料一样，符合一级反应。     

Influence of supercritical CO2 on the interactions between maleated polypropylene and alkyl‐ammonium organoclay

Supercritical carbon dioxide (scCO₂) has been proposed as an effective exfoliating agent for the preparation of polymer‐layered silicate nanocomposites, though there is limited fundamental understanding of this mechanism. This study looks at the interactions of this unique green solvent with three maleated polypropylenes of varying anhydride content and molecular size with an alkyl‐ammonium organoclay. Mixtures of compatibilizers and organoclay were melt‐annealed in a high pressure batch vessel at 200°C and subjected to either a blanket of nitrogen or scCO₂ at a pressure of 9.7 MPa. The structures and properties of these melt‐annealed mixtures were characterized by X‐ray diffraction, transmission electron microscopy, Fourier Transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and contact angle measurement. The results indicate that the plasticizing influence of scCO₂ aided intercalation and exfoliation for intercalants of moderate molecular size and anhydride content which would otherwise have limited diffusion into the clay galleries. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011