Copolymerization solution

The solution-processed doped silicon films described above (baked at 500 °C for 2 hr) exhibited high electrical resistivity (greater than 300 Qcm), which is the measurement limit of the instrument we used. To lower the resistivity, we tried an additional rapid thermal annealing (RTA) of the film prepared from the copolymerized solution with 1 wt% phosphorus concentration. In this RTA, the SiC plate on which the sample was placed was irradiated with infrared (IR) light from a 1-kW IR lamp. The RTA conditions were 600 °C for 2 hr, 650 °C for 20 min, 700 °C for 5 min, and 750 °C for 5 min these temperatures were that of the SiC plate, and the temperature of the Si film is estimated to be several dozens of degrees lower than that. 

Figure 5.17. Electrical resistivity and Raman scattering spectroscopy of doped-Si films formed from the copolymerized solution. [Reproduced with permission from Ref. 25. Copyright 2007 The Japan Society of Applied Physics.]...

Figure 5.18 shows the relationship between the resistivity and phosphorus concentration of the initial solution for the film formed from various solutions and heated under the same polycrystallizing RTA conditions (750 °C for 5min). As the initial phosphorus concentration increases, the resistivity decreases down to 2.1mQcm. The film formed from a l-wt% postpolymerization addition solution and the film formed from a 0.01-wt% copolymerized solution exhibit almost the same resistivity, which is reasonable since the two films have almost the same amount of phosphorus atoms, as shown in Fig. 5.16. To apply these doped-silicon films to the source and drain regions of poly-Si TFTs, the initial concentration of 0.1-1 wt% will be sufficient in the case of the copolymerized solution for this heating condition.

Finally, we tried to activate dopant atoms using pulsed laser irradiation, which is effective in lowering the process temperature. The light source was a 308-nm XeCl excimer laser, which is a standard source for crystallizing a-Si films used in the LTPS process.19 A test sample of 76 nm thickness, prepared from the copolymerized solution (l-wt% phosphorus, 30-min UV irradiation, 500 °C 2hr annealing), was irradiated using a XeCl laser at various intensities to activate the dopant atoms. Figure 5.19 shows the relationship between the... 

Solubilization oftropicamide, a poorly water-soluble mydriatic/cycloplegicdrug, by poloxamers or Pluronics was studied (Saettone et al., 1988). The polymers evaluated as solubilizers for the drug included L-64, P-65, F-68, P-75, F-77, P-84, P-85, F-87, F-88, and F-127. The authors measured a range of physicochemical properties, such as solubility oftropicamide in polymer solutions, partition coefLcient of the drug between isopropyl myristate and copolymer solutions, critical micelle concentration of the copolymers, and viscosity of the copolymeric solutions containing tropicamide. 

Generalized first-order kinetics have been extensively reviewed in relation to teclmical chemical applications [59] and have been discussed in the context of copolymerization [53]. From a theoretical point of view, the general class of coupled kinetic equation (A3.4.138) and equation (A3.4.139) is important, because it allows for a general closed-fomi solution (in matrix fomi) [49]. Important applications include the Pauli master equation for statistical mechanical systems (in particular gas-phase statistical mechanical kinetics) [48] and the investigation of certain simple reaction systems [49, ]. It is the basis of the many-level treatment of... 

Homogeneous GopolymeriZation. Nearly all acryhc fibers are made from acrylonitrile copolymers containing one or more additional monomers that modify the properties of the fiber. Thus copolymerization kinetics is a key technical area in the acryhc fiber industry. When carried out in a homogeneous solution, the copolymerization of acrylonitrile foUows the normal kinetic rate laws of copolymerization. Comprehensive treatments of this general subject have been pubhshed (35—39). The more specific subject of acrylonitrile copolymerization has been reviewed (40). The general subject of the reactivity of polymer radicals has been treated in depth (41). 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

There are two main advantages of acrylamide—acryUc-based flocculants which have allowed them to dominate the market for polymeric flocculants in many appHcation areas. The first is that these polymers can be made on a commercial scale with molecular weights up to 10—15 million which is much higher than any natural product. The second is that their electrical charge in solution and the charge density can be varied over a wide range by copolymerizing acrylamide with a variety of functional monomers or by chemical modification. 

The monomer can also be copolymerized with acrylamide. Because of the high chain-transfer rate of aUyflc radicals, the molecular weights tend to be lower than for acryflc polymers. These polymers are sold either as a viscous solution or a dry powder made by suspension polymeriza tion (see Allyl monomers AND POLYPffiRS). 

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Polymerization and Spinning Solvent. Dimethyl sulfoxide is used as a solvent for the polymerization of acrylonitrile and other vinyl monomers, eg, methyl methacrylate and styrene (82,83). The low incidence of transfer from the growing chain to DMSO leads to high molecular weights. Copolymerization reactions of acrylonitrile with other vinyl monomers are also mn in DMSO. Monomer mixtures of acrylonitrile, styrene, vinyUdene chloride, methallylsulfonic acid, styrenesulfonic acid, etc, are polymerized in DMSO—water (84). In some cases, the fibers are spun from the reaction solutions into DMSO—water baths. 

The principal solution to fabrication difficulties is copolymerization. Three types of comonomers are commercially important vinyl chloride acrylates, including alkyl acrylates and alkyimethacrylates and acrylonitrile. When extmsion is the method of fabrication, other solutions include formulation with plasticizers, stabilizers, and extmsion aids plus applying improved extmsion techniques. The Hterature on vinyHdene chloride copolymers through 1972 has been reviewed (1). 

Studies of the copolymerization of VDC with methyl acrylate (MA) over a composition range of 0—16 wt % showed that near the intermediate composition (8 wt %), the polymerization rates nearly followed normal solution polymerization kinetics (49). However, at the two extremes (0 and 16 wt % MA), copolymerization showed significant auto acceleration. The observations are important because they show the significant complexities in these copolymerizations. The auto acceleration for the homopolymerization, ie, 0 wt % MA, is probably the result of a surface polymerization phenomenon. On the other hand, the auto acceleration for the 16 wt % MA copolymerization could be the result of Trommsdorff and Norrish-Smith effects. 

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

The glass-transition temperature in amorphous polymers is also sensitive to copolymerization. Generally, T of a random copolymer falls between the glass-transition temperatures of the respective homopolymers. For example, T for solution-polymerized polybutadiene is —that for solution-polymerized polystyrene is -HlOO°C. A commercial solution random copolymer of butadiene and styrene (Firestone s Stereon) shows an intermediate T of —(48). The glass-transition temperature of the random copolymer can sometimes be related simply as follows ... 

Once a metal surface has been conditioned by one of the above methods, a coupling agent composed of a bifimctional acid—methacrylate similar to a dentin adhesive is appHed. This coupling material is usually suppHed as a solvent solution that is painted over the conditioned metal surface. The acidic functional group of the coupling molecule interacts with the metal oxide surface while the methacrylate functional group of the molecule copolymerizes with the resin cement or restorative material placed over it (266,267). 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

SBR is produced by addition copolymerization of styrene and butadiene monomers in either emulsion or solution process. The styrene/butadiene ratio controls the glass transition temperature (To) of the copolymer and thus its stiffness. T ... 

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