Solution polymerization spinning

Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

PLEDs contain polymeric emissive materials that are almost exclusively processed by solution coating (spin coating or inkjetting). This has been discussed in Chapter 2. While most polymer work uses fluorescent emissive materials, there are a few examples of phosphorescent materials being incorporated into a polymer chain and being used as phosphorescent emitters. This part of the materials discussion will be covered in Chapter 4. 

Although there are many variations on how carbon fibers are made, the typical process starts with the formation of PAN fibers from a conventional suspension or solution polymerization process between a mixture of acrylonitrile plastic powder with another plastic, such as methyl acrylate or methyl methacrylate, and a catalyst. The product is then spun into fibers, with the use of different methods, in order to be able to achieve the internal atomic structure of the fiber. After this, the fibers are washed and stretched to the desired fiber diameter. This step is sometimes called "spinning" and is also vital in order to align the molecules inside the fiber and thus provide a good basis for the formation of firmly bonded carbon crystals after carbonization [7]. 

Figure 1. Examples of EPR-spectra of solutions of spin-marked poly-4-vinylpyridine (PVP-1, polymerization...

Polymeric thin films in this thickness range can easily be prepared from solution by spin coating or doctor blading techniques. Drying the films at elevated temperatures under high vacuum or under inert atmosphere is necessary to remove residual solvent traces, which cause softening of the material and contribute to fast relaxation of the oriented chromophores to an isotropic state. 

FIGURE 2.52 Typical PAN precursor manufacturing steps based on solution polymerization and wet spinning. 

Copper 4-bromobenzenethiolate can be polymerized at 200°C in quinoline solution. Electron spin resonance spectroscopy shows the existence of organic free radicals throughout the polymerization. In addition, Cu + are observed. These phenomena suggest a radical mechanism of polymerization. It was early suspected that the reaction mechanism should not be a... 

The manner in which the spinning dope is prepared depends primarily on the polymerization method and form of the polymer. In solution polymerization, the same solvent that is used in the polymerization can be carried into spinning therefore, it is not necessary to recover the polymer as a separate intermediate. The end result of the polymerization is a solution of the polymer in the spinning solvent, and to prepare the dope it is only necessary to adjust the polymer concentration. In heterogeneous aqueous polymerization, the polymer is usually recovered by filtration in the form of a wet-polymer cake. If the spinning dope solvent is an aqueous salt solution, then the wet polymer may be dissolved directly without drying... 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

There are several techniques that allow to produce polymeric nanofibers, such as drawing, template synthesis, phase separation, self-assembly, solution blow spinning, and electrospinning. The drawing process requires a polymer with appropriate viscoelastic properties that is able to be deformed and kept connected by cohesive forces. Besides being simple and inexpensive, this technique is very limited for conjugate polymers, since most of them have lower solubility and form solutions with a small viscous modulus. 

The above examples are all heterogeneous solution polymerizations. Heterogeneous solution polymerization occurs in most cases when the Hildebrand solubility parameter values of the solvent and polymer differ by at least 1.8h(122). The presence of macroradicals in these precipitated polymers has been demonstrated by electron spin resonance (esr). 

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