Strand -cutting method

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

By having a surge tank attached to the strand burner, only a very slight pressure rise occurred as a result of the burning of the strand. Consequently, burning occurred at essentially constant pressure. This method has also been adopted for use with strands which have been cut from sheet proplnt and even strands sliced from sections of cast grains. Fig 1 shows a typical strand burner assembly... 

To incorporate fragments of foreign DNA into a plasmid vector, methods for cutting and rejoining of double-stranded DNA are required. Identification and manipulation of restriction endonucleases in the 1960s constituted a key discovery for cloning. Today, commonly used restriction enzymes are produced recombinantly and distributed widely. 

Both linear and supercoiled double-stranded DNA can be used as substrate for site-selective scission. Typical methods for the scission of linear double-stranded DNA are given in Sections 7.6.2.1 and 7.6.2.2. The substrate is obtained by treating supercoiled PBR322 plasmid DNA by a restriction enzyme EcoRI. Here, the plasmid DNA is cut at one-site, providing a linear DNA (this DNA is known as form III). Section 7.6.2.3 describes the method for site-selective hydrolysis of supercoiled DNA. Figure 7.9(a)... 

Polyurethane fibers (elastanes) can be made via either of the following two ways. In the first technique, the PU is first synthesized using the prepolymer method, and then the PU is spun from solution (approximately 30% PU) to form a filament or yarn. The yarn can either be collected as one long strand or be cut as it is collected. The second method, called reactive spinning, involves synthesizing the polyurethane and forming the filament all at the same time using a reactive bath to spin the fibers. ... 

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