Gel-fraction

Polymers in Solution. Polyacrylamide is soluble in water at all concentrations, temperatures, and pH values. An extrapolated theta temperature in water is approximately —40° C (17). Insoluble gel fractions are sometimes obtained owing to cross-link formation between chains or to the formation of imide groups along the polymer chains (18). In very dilute solution, polyacrylamide exists as unassociated coils which can have an eUipsoidal or beanlike stmcture (19). Large aggregates of polymer chains have been observed in hydrolyzed polyacrylamides (20) and in copolymers containing a small amount of hydrophobic groups (21). 

NR pressure-sensitive adhesives with a high tackifier content can be used as commercial tapes and surgical plasters. These PSA require the elimination of the gel fraction and a reduction in molecular weight to facilitate solution. 

Preparation of cholesta-5,7-diene-ia,3/3-diol a solution of 500 mg of the 1,4-cyclized adduct of cholesta-5,7-dien-3/3-ol-ia,2a-epoxideand 4-phenyl-1,2,4-triazoline-3,5-dione in 40 ml of tetrahydrofuran is added dropwise under agitation to a solution of 600 mg of lithium aluminum hydride in 30 ml of THF. Then, the reaction mixture liquid Is gently refluxed and boiled for 1 hour and cooled, and a saturated aqueous solution of sodium sulfate is added to the reaction mixture to decompose excessive lithium aluminum hydride. The organic solvent layer is separated and dried, and the solvent Is distilled. The residue Is purified by chromatography using a column packed with silica gel. Fractions eluted with ether-hexane (7 3 v/v) are collected, and recrystallization from the methanol gives 400 mg of cholesta-5,7-diene-la, 3/3-diol. 

A series of polyamine disulphides (polyaniline disulphide, polyamine disulphide, and polyparaphenylenedi-amine disulphide) represent effective thermostabilizers of cured polyethylene, and provide a decrease in gel fraction 2.5-3 times as large as that in case of inhibited thermal destruction. Stabilizers of normal polyethylene (Neozone D , Santonox R ) are inefficient as stabilizers of cured polyethylene, these substances decompose and even initiate thermal destruction of cured polyethylene. 

Polyamine disulphides are effective thermostabilizers of cured polyethylene up to 400°C. In presence of polyamine disulphides a decrease in gel fraction is one half as large as that of nonstabilized cured polyethylene over the temperature range from 350-380°C [46]. 

A drop in gel fraction of nonstabilized cured polyethylene amounts to 50% after 25-h exposure, 75% after 50 hours and after a 75-h exposure the complete fall is observed. At the same time, a decrease in gel fraction in presence of polyaniline disulphide is observed only after a 50-h exposure and comes to only 2%, whereas... 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

Number-Average Chain Length of the Primary Molecules which Belong to the Gel Fraction. 

Tg=—88 Amorphous, except for polymers prepared by free radical polymerization at 0°C or lower. Latter crystallize on cooling or stretching. Soft rubbery. Generally contains gel fraction insoluble even in hydrocarbon solvents... 

The relative abundances of cross-linkages and of various primary species in the sol and in the gel fractions may be obtained by extension of this procedure. Since 4>l equals the fraction of the cross-linkages (or of the cross-linked units) occurring in the sol fraction, we can write... 

The molecular distributions for polymers formed by condensations involving polyfunctional units of the type R—A/ resemble those for the branched polymers mentioned above, except for the important modification introduced by the incidence of gelation. The generation of an infinite network commences abruptly at the gel point, and the a-mount of this gel component increases progressively with further condensation. Meanwhile, the larger, more complex, species of the sol are selectively combined with the gel fraction, with the result that the sol fraction decreases in average molecular complexity as well as in amount. It is important to observe that the distinction between soluble finite species on the one hand and infinite network on the other invariably is sharp and by no means arbitrary. 

The TLC screening of the sihca gel fractions of B. carterii revealed in fraction 1, after derivatization with anisaldehyde reagent, a single violet zone (Rf 0.85). The purification steps of this zone are illustrated in Figure 16.5. The zones of the following fractions 2 and 3 were not separated from each other. Their separation and purification steps are shown in Figure 16.6, whereas the appropriate steps of fraction 4 are demonstrated in Figure 16.7. 

Fluorescence Radioactivity MS Complete Release Blank A nalyse supernatant or gel fraction ... 

In this contribution, we report equilibrium modulus and sol fraction measurements on diepoxidet-monoepoxide-diamine networks and polyoxypropylene triol-diisocyanate networks and a comparison with calculated values. A practically zero (epoxides) or low (polyurethanes) Mooney-Rivlin constant C and a low and accounted for wastage of bonds in elastically inactive cycles are the advantages of the systems. Plots of reduced modulus against the gel fraction have been used, because they have been found to minimize the effect of EIC, incompleteness of the reaction, or possible errors in analytical characteristics (16-20). A full account of the work on epoxy and polyurethane networks including the statistical derivation of various structural parameters will be published separately elsewhere. 

Figure 2. Theoretical curves for superimposed reduced moduli Gd /RT(mol/ cm 3) of epoxy-amine networks versus the gel fraction ws. Fraction of epoxy groups in monoepoxide is O, 0 , 0.2 A, 0.33 V, 0.5. Key O, , A, V, dry networks , A, , swollen networks value of front factor A indicated.

Figure 4. Theoretical curves for reduced moduli Gd/RT (mol/cm3) of polyurethane networks from polyoxypropylene triols versus the gel fraction w . Value of A indicated. Networks from LHT-240. Continued.

SOLUBLE/INSOLUBLE (GEL) FRACTION. If crosslinking predominates over scission (when G(crosslink) > 4 G(scission)), the decrease in soluble fraction above the gel dose, may be used to derive G values for both processes. An equation was derived by Charlesby and Pinner for the most probable molecular weight distribution and similar equations have been derived for other distributions. 

Polymer networks are formed from functional precursors by covalent bond formation [1], As a result, molecular weights and polydispersity increase and the system passes through a critical point, the gel point. At this point, an infinite structure (molecule) is formed for the first time. Beyond the gel point, the fraction of the infinite structure (the gel) increases at the expense of finite (soluble) molecules (the sol). The sol molecules become gradually bound to the gel and eventually all precursor molecules can become a part of the gel - the network. This is not always the case for different reasons sometimes sol is still present after all functional groups have reacted. In passing from the gel point to the final network not only the gel fraction increases, but also the network becomes denser containing increasing amounts of crosslinks and strands between them called elastically active network chains. 

Figure 5.10 Dependence of the gel fraction, wg, on molar ratio of [OH]/[NCO] groups, ah, for poly(oxypropylene)triol (Niax LG 56)4,4 -diisocyanatodiphenyl-methane system. The dependence has been reconstructed from data of ref. [78]...

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