Polyimide/silica systems

The chemical structures of the materials used to prepare the hybrids are shown in Figure 24.13. In both cases, the hybrid material is a polyimide/silica system derived by the sol-gel processing of a polyamic acid and an appropriate alkoxysilane solution... 

Polydimethylsiloxane and Zr oxo species have been reacted to give hybrid materials which were characterized by solid-state NMR.149 TEOS/PEG (polyethylene glycol) materials are biphasic systems. The materials were studied by 13C NMR, EPR, and thermal analysis.150 Other systems studied include titania/polyvinylacetate,151 titania/PEG,152 silica/polyacrylates,153 polyimide/ silica,154"156 linseed oil alkyds/titania,157 and PVC/titania and vanadia/sulfonated polyaniline... 

Relevant systems are mbber-toughened epoxy resins, high-impact acrylic (PMMA particles in mbber matrix, obtained by radical polymerization of 80/20 MMA/EVAc mixture), polyimide/silica hybrid materials (obtained using the sol-gel method), and a very high strength ( 60 MPa) mbbers (obtained by peroxide cure of a hydrogenated nitrile rubber/ zinc dimethacrylate system) [Inoue, 1995]. 

Figure 24.14 SEM micrographs of fractured surfaces of polyimide/silica hybrids (25% w/w Si02) (a) systems based on low MW polyamic acid (Skybond 703) and (b) systems based on high MW polyamic acid (Pyre ML).

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

The concept of using decoupled coils has been developed further by Li et al. [19]. In this case, an RF switch was used for time-domain multiplexing of the signals into a single receiver channel. A four-coil system was constructed for operation at 6 T in a wide-bore (89 mm) magnet. Each coil was fabricated by using 17 turns of 50 pm diameter copper wire wrapped around a 180 pm i.d., 355 pm o.d., polyimide-coated fused silica capillary, giving an observe volume... 

A CE system is a very simple analytical separation instrumentation consisting of only a high-voltage power supply, detector, and capillary. Separations are performed in fused-silica capillaries which are supplied with a thin outer coating of polyimide to make them strong and flexible. Coated capillaries with an internal diameter of 50-100 pm were usually hlled with gel or polymer solution for DNA separation. 

Reports of new materials and formulations and resin properties are prolific. Articles of a topical or applied interest include probes for in-situ hardness measurements on adhesives, photobase generators for image recording devices, oxygen inhibition in packaging applications, resins for sign boards, potentiometric sensors, new photodefinable polyimides, visible curable resists, " clay composites, putties, silica fillers, curable paints, soluble photocurable systems, fluorinated coatings and in-... 

Since the pioneering work of Tehennepe et al. [152] in 1987, many efforts have been made filling the polymeric matrix with zeolites in order to improve their stability. There are several companies that offer pervaporation organic membranes and composite membranes such as Sulzer Chemtech [153]. Commercial pervaporation and vapor permeation installations utilize polymeric membranes, like PVA (Sulzer Chemtech), polyimide (Vaperma), per-fiuoropolymers (MTR and Compact Membrane Systems), and polyelectrolytes (GKSS) or ceramic membranes, like zeolite A (Mitsui, Mitsubishi, Inocermic) and silica... 

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