Degassing polymer solutions

Column preparation is the most difficult task within the IGC-experiment. In the case of packed columns, the preparation technique developed by Munk and coworkers is preferred, where the solid support is continuously soaked wifli a predetermined concentration of a polymer solution. In the case of capillary IGC, columns arc made by filling a small silica capillary with a predetermined concentration of a degassed polymer solution. The one end is then sealed and vacuum is applied to the other end. As the solvent evaporates, a thin layer of the polymer is laid down on the walls. With carefully prepared capillary surfaces, the right solvent in terms of volatility and wetting characteristics, and an acceptable viscosity in the solution, a very uniform polymer film can be formed, typically 3 to 10 xm thick. Column preparation is the most time-consuming part of an IGC-experiment. In the case of packed columns, two, three or even more columns must be prepared to test the reproducibility of the experimental results and to check any dependence on polymer loading and sometimes to filter out effects caused by the solid support. Next to that, various tests regarding solvent sample size and carrier gas flow rate have to be done to find out correct experimental conditions. 

The role of cavitation in ultrasound degradation has been confirmed repeatably in most experiments where cavitation was prevented, either by applying an external hydrostatic pressure, by degassing the solution, by reducing the sound intensity or the temperature, polymer chain scission was also largely suppressed [117]. 

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

The polymer or the polymer solution cools down because the vaporization energy required for the vaporization process is taken from the in-fed polymer solution. The required ZSK size depends on the gas quantity to be removed, the gas velocity, the rheological behavior of the polymer melt, and the thermodynamic behavior of the degassed components, the screw speed, and the throughput. Mechanical energy has to be introduced in the processing zone... 

The use of a flash valve will further ensure a constant feed of the polymer solution into the ZSK to avoid uncontrolled degassing of the process material in the supply line to the ZSK. A pulsating intake would not permit a stable process in terms of safe operation of the degassing zones and stable pelletizing of the degassed polymer. 

Suitable screw configuration and screw speed ensure compensation of heat loss, particularly after flash and also after the initial forward venting. The vacuums in the individual degassing zones are designed to avoid excessive foaming of the polymer solution or even crumbs in the degassing opening. 

Compared to conventional processing tasks, the extruder drive motor is usually quite small for degassing processes, because the intake of polymer solutions or polymer melts does not require a high torque consumption. The use of a back vent depends on the amount of... 

Coperion Werner Pfleiderer have a wealth of test and production experience related to degassing polymer melts with twin-screw extruders. Based on the process features described, the ZSK offers optimum solutions and can also be easily adapted for new tasks. The ZSK degassing machines also meet increasing quality requirements for polymers regarding residual degassing of solvents and monomers and the drive to save energy and raw materials in continuous manufacturing processes. 

The spinning of asymmetric hollow fibers with the skin on the inside closely resembles the procedure used in casting flat-sheet membranes. Figure 3.1510 is a schematic diagram of a spinneret used to spin these fibers. The degassed and filtered polymer solution is forced under pressure into a coaxial tube spinneret. The liquid is extruded through an annular orifice and the hollow fiber (still liquid) is stabilized and precipitated by an internal coagulating fluid (usually water) which flows out the center tube. 

To further demonstrate the livingness of the process a chain extension of PNIPAAm was carried out. The initial block was obtained by using a ratio [M]o/[BIBA]o/[CuCl]o/[CuCl2]o/[Me6TREN]o of 120/1/1.6/0.4/2 with a NIPAAm concentration of 0.5 M. Then the block copolymer was synthesized by sequential addition after 38 min of a degassed aqueous solution of monomer (0.5 M) without purification of the macro-initiator. A CuCl-based catalyst was chosen to perform the reaction to avoid any termination. Indeed, in water, bromide-terminated polymers can be sensitive to halogen abstraction by nucleophilic substitution. Then with CuCl the resulting polymer-halide bound C-... 

In the previous chapter it was described how degassing aqueous solutions in contact with hydrophobic polymers open pathways for polymer surface patterning. In the absence of degassing, nanobubbles can nucleate on hydrophobic surfaces. In this chapter the structuring effect of nanobubbles on hydrophobic surfaces is discussed. 

The polymer sample is put, after weighing, into the sample flask and the apparatus is evacuated. Degassed solvent is distilled into the measuring burette and from there a desired amount of solvent is distilled into the sample flask. The Hg-manometer is filled from the storage bulb and separates the polymer solution from the burette. Care must be taken to avoid leaving any solvent in the manometer. The apparatus is kept at constant measuring temperature, completely immersed in a thermostat for several days. After reaching equilibrium, the pressure is read from the manometer difference and the concentration is calculated from the calibrated burette meniscus difference corrected by the amount of vaporized solvent in the unoccupied space of the equipment. The pure solvent vapor pressure is usually precisely known from independent experiments. 

Figure 4.4.8. Isopiestic vapor-sorption apparatus with built-in manometer using a quartz spring 1 - connection to the vacuum, 2-9 -stop corks, 10, 11, 12 - connections to nitrogen, 13 - degassing flask for the pure solvent, 14, 18 - buffers, 15 - cold trap, 16,19 - Hg-ma-nometers, 17,20 - mercury float valves, 21 -pure solvent reservoir at temperature Ti provided by 22 - thermostat, 23 - temperature controlled air box, 24 - measuring cell, 25 - quartz spring (four quartz springs can be inserted into the equilibrium cell, only one is shown), 26 - pan with the polymer solution, 27 - closing plug sealed with epoxy resin, 28 - heating to avoid solvent condensation.

Head-space gas chromatography is a modem tool for the measurement of vapor pressures in polymer solutions that is highly automated. Solutions need time to equilibrate, as is the case for all vapor pressure measurements. After equihbration of the solutions, quite a lot of data can be measured continuously with reliable precision. Solvent degassing is not necessary. Measurements require some experience with the equipment to obtain really thermodynamic equihbrium data. Calibration of the equipment with pure solvent vapor pressures may be necessary. HSGC can easily be extended to multi-component mixtures because it determines all components in the vapor phase separately. 

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