Fiber retraction

In the fiber mode, the sorbent coated fiber is housed in a microsyringelike protective holder. With the fiber retracted inside the syringe needle, the needle is used to pierce the septum of the sample vial. The plunger is depressed to expose the sorbent-coated fiber to the sample. After equilibrium is reached or at a specified time prior to reaching equilibrium, the fiber is retracted into the protection of the microsyringe needle and the needle is withdrawn from the sample. The sorbent is then interfaced with an analytical instrument where the analyte is desorbed thermally for GC or by solvents for HPLC or capillary electrophoresis. For the in-tube mode, a sample aliquot is repeatedly aspirated and dispensed into an internally coated capillary. An organic solvent desorbs the analyte and sweeps it into the injector [68,130,133]. An SPME autosampler has been introduced by Varian, Inc., that automates the entire process for GC analyses. 

Flow of the blend at melt temperatures generally stretches the discontinuous phase from its initial shape. Interfacial tension between the immiscible components will oppose this process and attempt to drive the system to a low-energy spherical-morphology state. Studies of these phenomena in a rheometer permit the estimation of the time required for both processes and the interfacial tension [90-93]. Alternatively, one can estimate the time required for the latter process, and the interfacial tension, from the evolving shape of the discontinuous phase using either a fiber break-up [94,95] or fiber-retraction [33,96] experiment. Interfacial tension depends on the molecular weight of each component [96,97] and on temperature, so it is preferable to measure interfacial tension for the materials of interest at their fabrication temperature. 

Ellingson PC, Strand DA, Cohen A, Sammler RL and Carriere C (1994) Molecular weight dependence of polystyrene/poly(methyl methacrylate) interfacial tension probed by imbedded-fiber retraction. Macromolecules 27 1643-7. 

Xing et al. (2000) compared five different techniques for the measurement of interfacial tension in a model polystyrene (PS)/polyamide-6 (PA-6) system at a constant temperature. The techniques include three dynamic methods (the breaking thread, the imbedded fiber retraction, and the retraction of deformed drop), one equilibrium method (the pendant drop), and a rheological method based on linear viscoelastic measurements. The advantages, the limitations, and the difficulties of each technique were discussed and compared. 

Optim max utilizes a key benefit of the manufacturing process that allows the extent of set in the fiber to be manipulated in a controlled manner. Optim max fiber is wool that has been temporarily set while stretched, so that the fiber retracts when released in hot moist conditions by about 25% in length. This fiber offers scope for innovation in yarn and fabric design. For example, when Optim max is blended with normal wool, spun into yam, and relaxed in hank form in hot water, the Optim max fibers retract and force the normal wool fibers to buckle. 

Carriere CJ, Cohen A (1991) Evaluation of the interfacial tension between high molecular weight polycarbonate and PMMA resins with the imbedded fiber retraction technique. 

The interfacial tension at the interface between two polymers is an expression of different energetics of bulk materials. It reflects differences in thermodynamics, which are related to the x parameter, as shown by Eq. (3.4). The experimental evaluation of the interfacial tension with polymeric melts is extremely difficult due to problems associated with sample preparation and equilibration [77, 78]. Several techniques have been proposed for the measurement. The most commonly used techniques include the pendant drop method, the embedded fiber retraction technique, and the breaking thread method. Classical equilibrium interfacial tension experiments like the pendant drop technique are very difficult to apply to high polymers because of their high melt viscosities (10 -10 Pas). There are many practical problems associated with the pendant drop technique ... 

A hexagonal phase appears at atmospheric pressure in ultra-drawn PE fibers containing highly extended chains when the fiber is heated above the normal temperature while the ends are held fixed to prevent retraction [89-92]. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

PET is not strictly Newtonian, or else it could not be fiber-forming. Polymers with the latter property develop increasing tension due to retraction forces as they become oriented, so that localized necks do not grow and become discontinuities. At high shear rates, molecular orientation will also reduce the resistance to shearing. 

Earlier attempts to use the AFM for mechanically stretching chromatin fibers have run into a rather unexpected artifact. Long native chromatin fibers isolated from chicken erythrocytes, or fibers assembled in vitro from purified histones and relatively short, tandemly repeated DNA sequences were deposited on mica or glass surfaces and pulled with the AFM tip [69,70]. In such stretching experiments the scanning of the sample in the x- and y-direction used for imaging was disabled, and the cantilever-mounted tip was allowed to move only in the z-direction, i.e., upwards and downwards, away and towards the surface. When the AFM tip is pushed into the sample, it may attach to the sample by non-specific adsorption upon retraction it stretches the sample and force-extension curves are recorded (see Fig. lb for an explanation of a typical force curve). 

In the worse case, where either sample temperature, pressure or reactor integrity issues make it impossible to do otherwise, it may be necessary to consider a direct in situ fiber-optic transmission or diffuse reflectance probe. However, this should be considered the position of last resort. Probe retraction devices are expensive, and an in situ probe is both vulnerable to fouling and allows for no effective sample temperature control. Having said that, the process chemical applications that normally require this configuration often have rather simple chemometric modeling development requirements, and the configuration has been used with success. 

SPME uses a polymer-coated fused-silica fiber, typically 1 cm 100 m, that is fastened into the end of a fine stainless steel tube contained in a syringelike device and protected by an outer stainless steel needle. In use, the plunger of the device is depressed to expose the fiber to the sample matrix so that the organic compounds to be sorbed onto the fiber. The plunger is retracted at the end of the sampling time, and then it is depressed again to expose the fiber to a desorption interface for analysis typically by GC or LC. In a recent variation of this technique, the so-called in-tube SPME, the polymer is not coated on a fiber but on the inside of a fused-silica capillary before analysis by LC. 

Here is a student procedure to measure nicotine in urine. A 1.00-mL sample of biological fluid was placed in a 12-mL vial containing 0.7 g Na2CO , powder. After 5.00 pig of the internal standard 5-aminoquinoline were injected, the vial was capped with a Teflon-coated silicone rubber septum. The vial was heated to 80°C for 20 min and then a solid-phase microextraction needle was passed through the septum and left in the headspace for 5.00 min. The fiber was retracted and inserted into a gas chromatograph. Volatile substances were desorbed from the fiber at 250°C for 9.5 min in the injection port while the column was at 60°C. The column temperature was then raised to 260°C at 25°C/min and eluate was monitored by electron ionization mass spectrometry with selected ion monitoring at m/z 84 for nicotine and m/z 144 for internal standard. Calibration data from replicate... 

NOTE Figure G 1.6.1 demonstrates the sequence of injection. It is important to follow this sequence carefully to avoid destruction of the fiber. Removing the fiber through a septum without retracting the fiber will strip the coating from the fiber and thus destroy it. 

Prior to each use, a SPME fiber should be cleaned by exposing it for 2 to 10 min to a temperature that is within the range recommended by the manufacturer for desorption. Since the fiber will have been cleaned during sample desorption, subsequent sampling within a few hours does not require cleaning, provided the fiber is kept retracted and away from high levels of volatiles. 

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