Filament-stretching techniques

In this method, the sample is held between two discs, the lower of which is attached to a shaft whose movement is controlled by a computer capable of generating an exponentially varying voltage, the shaft velocity being proportional to the applied voltage. The upper disc is attached to a load measuring 

A reverse-flow near the plates causes a delay in the development of the uniform cylindrical column, and a difference between the local and imposed extension rates at early times [Shipman et al., 1991], Problems may also be encoimtered due to adhesion of the flmd to the plates [Spiegelberg and McKinley, 1996] but this may be compensated for [Tirtaatmadja and Sridhar, 1993], Notwithstanding these difficulties, the technique can provide meaningful extensional viscosity data for polymer solutions, as the deformation is uniform and independently imposed, and the total strain is measurable. 

When a pendant drop forms slowly at the lower end of a capillary tube it ultimately falls and stretches the filament (which remains attached to the drop). For a Newtoiuan flmd the filament qmckly thins and breaks but long filaments can be formed from visco-elastic liqmds [Jones et al., 1990]. The forces acting on the falling drop are determined using a force balance, and the extensional stress determined as a function of time [Jones and Rees, 1982], The falling pendent drop technique is simple to set up and analyse, and provides consistent values of an apparent elongational viscosity. 

Another simple technique involves a hquid which is slowly extruded vertically downward through a capillary into another immiscible fluid of lower density (the submerged pendant drop technique). The heavier ejeeted fluid forms a sphere at the nozzle tip, and grows until the drop is no longer supported by surface tension. The drop then falls and stretches the ligament which connects the drop to the nozzle [Matta, 1984]. 

It is important to note that the Trouton ratio, Tr, defined as the ratio of the extensional viscosity to the shear viscosity, involves the shear viseosity evaluated at the same magnitude of the second invariant of the rate of deformation tensor where s is the rate of extension, i.e.  

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Another simple filament-stretching technique which has the advantage that it does not require measurements of forces acting on the filament, involves... 

There are other variations of the filament stretching technique. For example, filaments are clamped at one end and taken up on a rotating roll [86,87]. This reduces the amoimt of filament stretching to a more uniform level and produces a more constant extensional rate. In fact, when the following filament is taken up on a cold roll [87] a better constancy in the extensional rate is obtained. 

The techniques used in the Ml project were those of fibre spinning, open siphon, filament stretching, contraction flows, converging channel, opposing jets, climbing constants and falling drop methods. The results led to two major findings ... 

One major consequence of the Ml project was the development of a modified filament stretching instrument by Sridhar. In this device, the test sample is held horizontally between two Teflon discs and pulled equally at both ends at a programmable exponential rate such that a constant strain rate is achieved and the stress growth at a constant stretch rate is obtained (40). It appears though that the test sample has to adhere to the plates as the technique does not use aids to clamp samples. Consequently, it is not clear if the technique can be applied to products that are non-sticky or exhibit slip, which could be limiting factors for testing food products. 

Texturing procedures comprise mechanical distortions of the filaments to improve fiber characteristics such as the apparent volume (bulk), stretching properties (elasticity), shape (appearance), and the perceived feel (touch, handle) of the fiber material. The textured yarns are either fine tex (1.7-2.2 tex) or heavy tex (110 00 tex) materials. The former are used in apparel applications, mainly for woven and knitted stretch fabrics the latter are used primarily for carpets. A considerable number of processes and techniques have been developed for the production of textured filaments. A compilation of relevant processes has been presented by Wagner [223]. 

The explicit evidence of the macroradicals presence, under the conditions of cyclical fatigue by stretching, was demonstrated on the example of monofilaments based on poly(acrylonitrile-co-vinyl acetate-co-a-methylstyrene) copolymer. For this purpose, a special technique was applied, namely the filaments were impregnated with a solution of a stable radical l,l-diphenyl-2-picrylhydrazyl (DPPH) and supposed to cyclic fatigue. The generated macroradicals during the solicitation period reacted with a part of DPPH. Using ESR method, the difference between the initial concentration of the stable radicals and their final concentration allowed the calculation of... 

Rotational viscometers are also sometimes adapted to act as stretching devices, with the rotor of the apparatus serving as a roll onto which one end of the specimen that is being stretched is wound. The other end of the sample is attached to a slotted vertical spindle, and the force exerted by the polymer on this spindle is measured. The winder is usually raised during the course of the experiment to ensure that the filament does not wrap on itself. This technique has been used by Macosko and Lomtson, ) Everage and Ballman,(39) and Connelly et among others. 

Stretching or drawing is categorized as a physical pretreatment and usually involves the hollow fiber precursor only. This technique is adapted from the fabrication of carbon fiber and is sometimes referred to as post-spinning treatment. An ideal postspinning modification scheme would allow the removal of surface defects, attenuation of variations in filament diameter, and an enhanced retention of molecular orientation prior to heat treatment in order to obtain fibers with a good balance of stiffness and strength [20]. 

Electrospinning is an effective technique to produce continuous polymer fibres with diameters on submicrometre or nanometre scales (Reneker and Chun, 1996, Jayaraman et al 2004, Li and Xia, 2004b). It involves a process in which a polymer solution or melt is stretched into fine filaments under the action of a high electrical voltage. The basic electrospinning system, illustrated... 

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