Rubbery elastomers

The properties of the polysilanes, like those of the polyphosphazenes, depend greatly on the nature of the substituent groups. Polysilanes cover the entire range of properties from highly crystalline and insoluble, through partially crystalline, flexible solids, to glassy amorphous materials and rubbery elastomers. 

A variety of polymers contain the element boron.42,44,52 60 One of the simplest consists of chains of boron fluoride, of repeat unit -BF-, and can be prepared by the reaction of elemental boron with boron trifluoride at high temperatures. The polymer is a rubbery elastomer, but it has been little studied because of its hydrolytic instability and tendency to ignite spontaneously in air.42 However, a variety of other structures are formed with a number of metals, for example chains with Fe, ladders with Ta, sheets with Ti, tetragonal structures with U, and cubic structures with Ca and Ar. Boron also forms chains that are analogues of poly(dimethylsiloxane), with repeat unit -BCH-O-42... 

The Backbone. The linear inorganic backbone imparts an unusual combination of properties. First, perhaps unexpectedly in view of the unsaturated structure, the skeletal bonds have a low barrier to torsion (perhaps as low as 0.1-0.5KcaF repeating unit), which becomes translated into one of the most flexible backbones known throughout polymer chemistry. This means that some polyphosphazenes have glass-transition temperatures (Tg) as low as -100 °C. It also means that, in the absence of microcrystallinity, numerous polymers of this type are rubbery elastomers. This is a key property for... 

Metabohx, located in Cambridge, Mass., is producing sample quantities of PHB and of other PHA plastics. One member of this family, polyhydroxyoctanoate (PHO), is a rubbery elastomer, with an extension at break of 380%, compared to only 5% for PHB. It also has lower tensile strength than PHB.i ... 

The chain extender structure strongly influences the PUs mechanical performance. Modifying the ratio between the polyol and chain extender, PUs may result in a change from a hard, brittle material to a rubbery elastomer, as a result of the variation of the HS concentration (defined as the ratio of the mass of the non-polyol components to the total mass of the polymer) [2-4]. 

Tasdemir and Yildirim [22] showed that rigid plastics such as epoxies, PS, PVC, polymethyl methacrylate (PMMA), PP, PC, unsaturated polyester resins and PA can be toughened and their impact properties improved by the incorporation in the formulation of 5-20% of a rubbery elastomer such as ABS terpolymer. 

It was later noticed that the hexachlorocyclotriphosphazene can be polymerized by heating in the melt to yield an uncrosslinked linear high polymer, poly[(dichloro)-phosphazene] (Allcock et al, 1965-1966 Rose, 1968). Further heating results in the formation of an insoluble crosslinked material. This leads in both cases to a transparent, rubbery elastomer which hydrolyzes slowly, when exposed to moisture, forming phosphate, ammonia and hydrochloric acid. At temperatures above 350°C, the polymer depolymerizes to cyclic oligomers. The uncrosslinked species serve as highly reactive polymeric precursor. 

Substances that have been used in this context include glass fiber (occasionally glass beads), carbon fiber, carbon nanotubes, carbon black, graphite, fuUerenes, graphite chemically modified clays and montmorillonites, silica, and mineral alumina. Other additions have been included in polymer formulations, including calcium carbonate, barium sulfate, and various miscellaneous agents, such as aluminum metal, oak husks, cocoa shells, basalt fiber, silicone, rubbery elastomers, and polyamide powders. The effects of such additions of polymer properties are discussed next. 

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