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Introducing intrinsic porosity to polymers
RESEARCH NEWS
Introducing intrinsic porosity to polymers POROUS MATERIALS
Solution processing of a soluble PIM (center) into a microporous powder (left) with a surface area of 850 m2g-1 or a robust, freestanding film (right) with a surface area of 690 m2g-1. (Courtesy of Neil B. McKeown, University of Manchester.)
The interconnected, molecular-sized pores of microporous materials are of great interest for absorption, separation, and catalytic applications because of their large and accessible surface area. The two most common classes of this type of material are zeolites and activated carbons, but there is much interest in microporous organic hybrid materials with similar structures. Neil B. McKeown and colleagues from the University of Manchester, UK have designed an
organic material that mimics the amorphous nanostructure of activated carbons [Budd et al., Chem. Commun., published online 5 December 2003]. The new class of porous material, called polymers of intrinsic microporosity (PIM), consist of networks of phthalocyanine, porphyrin, and hexaazatrinaphthylene. the rigid spirocyclic scaffold of the macromolecules prevents close packing in the solid state and leads to an open structure. The materials are intrinsically microporous – not as a result of the action of heating or processing. The structures have pores with an average diameter of 0.5 nm and remarkably high surface areas of 500-1100 m2g-1. The properties of PIMs can be tailored for a particular application. “The diversity of suitable monomers allows us to optimize the structure of PIMs,” McKeown told Materials Today. “The really exciting property of PIMs is their solubility,” he adds, “which allows solutionbased processing to give useful forms of the materials such as free-standing films. This is something that simply cannot be done with conventional microporous materials.”
Repellent on the surface POLYMERS Researchers from Nagoya University and Dai Nippon Printing Company in Japan have developed a novel method to render polymeric materials ultrawater-repellent [Teshima et al., Langmuir, (2003), ASAP Article 10.1021/la034265d, published online November 8]. Surface energy and morphology control the wettability of a solid surface. Incorporating F atoms, which have a small atomic radius and high electronegativity, can achieve a low surface energy. A smooth surface incorporating F atoms shows a water contact angle of 120°, indicating a very low surface energy. By roughening the surface of such a substrate, water contact angles greater than 150° have been achieved. If such treatments could be applied to transparent polymers it would be useful for windows, eyeglasses, and displays. But these polymers are not
Making memories with organic electronics
heat-resistant and low temperature treatments are needed. Katsuya Teshima and colleagues have
ELECTRONIC MATERIALS
developed just such a surface Organic materials promise inexpensive, lightweight, and easy-to-produce electronics, but have received little attention for possible memory devices. Stephen R. Forrest and colleagues at Princeton University and Hewlett-Packard Laboratories have developed a write-once, read-many-times (WORM) memory based on a hybrid organic/inorganic device [Möller et al., Nature (2003) 426, 166]. The device consists of a conductive electrochromic polymer, PEDOT (polyethylenedioxythiophene: polystyrene sulfonic acid), on a thin-film Si p-i-n diode deposited on a flexible stainless steel substrate. The memory ‘pixels’ are formed at the intersections of row and column electrodes. Reading row by row, a written pixel can be distinguished from an unwritten one by integrating a writable PEDOT fuse in series with the diode. The fuse can be closed or open, representing a ‘0’ or ‘1’. A significantly different voltage is required to open or close the fuse compared with that needed to read the device. The researchers predict that a 1 Mbit WORM memory is possible, which could be written in 1 s
12
January 2004
treatment for poly(ethylene terephthalate) or PET substrates. First, PET is treated with oxygen plasma to introduce surface roughness and hydrophilic functional groups. A hydrophobic functional (CFx or CHx) coating is applied using lowtemperature chemical vapor The architecture of the WORM memory and the materials used. (Courtesy of Stephen R. Forrest.)
deposition. The resulting surface is ultra-water-repellent, with a water contact angle >150°. The treated PET
and only 1 mm2 in size, assuming a 500 nm pixel size. WORM hybrid memory devices could be easier and cheaper to produce than conventional flash and erasable programmable read-only memory (EPROM) because it requires no critical alignment and is deposited on noncrystalline Si. The devices could also be stacked to form high-density, threedimensional memory. Although the devices cannot be overwritten, they could find application for rapid, archival storage, for video images, for example.
substrates also retain 90% transparency in the visible range. “Since our process can be conducted at a low temperature, below the glass transition temperature of many optical plastics, the technique will be suitable for the fabrication of ultra-waterrepellent optical devices made of polymeric materials,” says Teshima.
Introducing intrinsic porosity to polymers POROUS MATERIALS
Solution processing of a soluble PIM (center) into a microporous powder (left) with a surface area of 850 m2g-1 or a robust, freestanding film (right) with a surface area of 690 m2g-1. (Courtesy of Neil B. McKeown, University of Manchester.)
The interconnected, molecular-sized pores of microporous materials are of great interest for absorption, separation, and catalytic applications because of their large and accessible surface area. The two most common classes of this type of material are zeolites and activated carbons, but there is much interest in microporous organic hybrid materials with similar structures. Neil B. McKeown and colleagues from the University of Manchester, UK have designed an
organic material that mimics the amorphous nanostructure of activated carbons [Budd et al., Chem. Commun., published online 5 December 2003]. The new class of porous material, called polymers of intrinsic microporosity (PIM), consist of networks of phthalocyanine, porphyrin, and hexaazatrinaphthylene. the rigid spirocyclic scaffold of the macromolecules prevents close packing in the solid state and leads to an open structure. The materials are intrinsically microporous – not as a result of the action of heating or processing. The structures have pores with an average diameter of 0.5 nm and remarkably high surface areas of 500-1100 m2g-1. The properties of PIMs can be tailored for a particular application. “The diversity of suitable monomers allows us to optimize the structure of PIMs,” McKeown told Materials Today. “The really exciting property of PIMs is their solubility,” he adds, “which allows solutionbased processing to give useful forms of the materials such as free-standing films. This is something that simply cannot be done with conventional microporous materials.”
Repellent on the surface POLYMERS Researchers from Nagoya University and Dai Nippon Printing Company in Japan have developed a novel method to render polymeric materials ultrawater-repellent [Teshima et al., Langmuir, (2003), ASAP Article 10.1021/la034265d, published online November 8]. Surface energy and morphology control the wettability of a solid surface. Incorporating F atoms, which have a small atomic radius and high electronegativity, can achieve a low surface energy. A smooth surface incorporating F atoms shows a water contact angle of 120°, indicating a very low surface energy. By roughening the surface of such a substrate, water contact angles greater than 150° have been achieved. If such treatments could be applied to transparent polymers it would be useful for windows, eyeglasses, and displays. But these polymers are not
Making memories with organic electronics
heat-resistant and low temperature treatments are needed. Katsuya Teshima and colleagues have
ELECTRONIC MATERIALS
developed just such a surface Organic materials promise inexpensive, lightweight, and easy-to-produce electronics, but have received little attention for possible memory devices. Stephen R. Forrest and colleagues at Princeton University and Hewlett-Packard Laboratories have developed a write-once, read-many-times (WORM) memory based on a hybrid organic/inorganic device [Möller et al., Nature (2003) 426, 166]. The device consists of a conductive electrochromic polymer, PEDOT (polyethylenedioxythiophene: polystyrene sulfonic acid), on a thin-film Si p-i-n diode deposited on a flexible stainless steel substrate. The memory ‘pixels’ are formed at the intersections of row and column electrodes. Reading row by row, a written pixel can be distinguished from an unwritten one by integrating a writable PEDOT fuse in series with the diode. The fuse can be closed or open, representing a ‘0’ or ‘1’. A significantly different voltage is required to open or close the fuse compared with that needed to read the device. The researchers predict that a 1 Mbit WORM memory is possible, which could be written in 1 s
12
January 2004
treatment for poly(ethylene terephthalate) or PET substrates. First, PET is treated with oxygen plasma to introduce surface roughness and hydrophilic functional groups. A hydrophobic functional (CFx or CHx) coating is applied using lowtemperature chemical vapor The architecture of the WORM memory and the materials used. (Courtesy of Stephen R. Forrest.)
deposition. The resulting surface is ultra-water-repellent, with a water contact angle >150°. The treated PET
and only 1 mm2 in size, assuming a 500 nm pixel size. WORM hybrid memory devices could be easier and cheaper to produce than conventional flash and erasable programmable read-only memory (EPROM) because it requires no critical alignment and is deposited on noncrystalline Si. The devices could also be stacked to form high-density, threedimensional memory. Although the devices cannot be overwritten, they could find application for rapid, archival storage, for video images, for example.
substrates also retain 90% transparency in the visible range. “Since our process can be conducted at a low temperature, below the glass transition temperature of many optical plastics, the technique will be suitable for the fabrication of ultra-waterrepellent optical devices made of polymeric materials,” says Teshima.