Gel fraction determinations

Gel Fraction Determinations. The irradiated polyethylene samples were measured for gel content by extraction with boiling xylene for 72 h. Only one sample, NBS 1475 irradiated to 8.0 Mrad in vacuum at 300 K, showed a measurable gel content. 

Various techniques have been used depending on the polymer and the nature of the chemical transformations infrared, ultraviolet and electron spin resonance spectroscopy, vapour phase chromatography, mass spectrometry, molecular weight and gel fraction determination, luminescence measurements, etc. These techniques have recently been discussed in a well-documented review on analytical methods applied to the study of the photodegradation of polymers [19]. 

Infrared spectroscopy (5.131. By following the disapearance of the IR absoipdon of the acrylic double tend one can quantitatively evaluate both the maximum rate of polymerization and the amount of residual unsaturation in the laser-cured polymer. Polymerization profiles are shown on Figure 8 for the curing of a polyester-aciylate photoresist under pulsed laser irradiation at 337.1 nm in the presence of air or pure nitrogen. The main disadvantage of IR spectroscopy is that, like gel fraction determination, it requires discrete measurements at various exposures and thus becomes a tedious, time consuming technique. 

Some properties of Penicillium fellutanum pectinesterase were studied. The optimum of pectinesterase action was detected at pH 5 and 45 °C. The enzyme was stable at pH 4 — 5 and 40 °C (pH 5) for 240 min. and was specific towards lemon pectin. An enzyme preparation composed mainly of pectinesterase was partially purified by gel filtration. Pectinesterase activity was accumulated in one of the obtained fractions. Molecular weights of fraction determined were found to be 46,000 and 1,200. Disk electrophoresis in polyacrilamide gel of the purified preparation revealed two protein bonds with one active component. The partially purified enzyme had the kinetic characteristics =... 

Cotmnercial cellulose triacetate samples were fractionated by both fractional precipitation and preparative gel permeation chromatography (GPC). The triacetate fractions were characterized by vlsco-metry, high speed membrane osmometry (HSMO) and GPC. A fair agreement has been found between the molecular weights of various triacetate fractions determined by the three procedures. 

The positive intercepts in Figure 7 show that post-gel(inelastic) loop formation is influenced by the same factors as pre-gel intramolecular reaction but is not determined solely by them. The important conclusion is that imperfections still occur in the limit of infinite reactant molar masses or very stiff chains (vb - ). They are a demonstration of a law-of-mass-action effect. Because they are intercepts in the limit vb - >, spatial correlations between reacting groups are absent and random reaction occurs. Intramolecular reaction occurs post-gel simply because of the unlimited number of groups per molecule in the gel fraction. The present values of p , (0.06 for f=3 and 0.03 for f=4 are derived from modulus measure- ments, assuming two junction points per lost per inelastic loop in f=3 networks and one junction point lost per loop in f=4 networks. 

Rather than measuring the amount of material in the gel fraction, it might be easier to determine the time at which gelation of a network takes place. (For example, one could focus a laser beam on a sample and note when the speckle interference pattern freezes. ) Gelation occurs in a developing network if the crosslink fraction a exceeds a critical value a, given as(23)... 

Figure 3. Dependence of the intrinsic viscosity O ([v])> nmr signal 0 (I), and gel fraction + (wt. percent W%) as a function of equivalent percentage of Ca2+ added. ([77] was determined by isoionic dilution of a 6 g/1 solution of sodium polygalacturonate.)...

Soxhlet Extraction Analysis. Powdered samples of GMC were degassed and sealed under vacuum into pyrex tubes, and irradiated at room temperature (25-30 °C) in a Cobalt-60 Gammacell. The dose rate determined by Fricke dosimetry was 0.24 Mrad/hr. Doses up to 20 Mrad were utilized. After irradiation, the tubes were opened and the samples placed in cellulose thimbles. A Soxhlet extraction was performed with methylethyl ketone at 76 1°C for 18-20 hrs. The insoluble material was dried to constant weight and the gel fraction calculated from initial weight. 

Although chemical constitution obviously plays the major role in determining the thermodynamic quality of a solvent, the influence of steric properties on solvent quality remains an open question. In order to answer this question we have plotted our dissolution-measurement-indicator for thermodynamic quality—the gel fraction molecular weight which stands for the size of the molecule (Figure... 

Determination of the sol fraction together with the primary distribution thus provides a method for calculation of the crosslinking density q. The gel fraction increases usually rather sharply as soon as the gel point is passed. For example, in a network composed of monodisperse primary chains with 2.5 crosslinks per primary molecule, 90% of the polymer is included in the gel fraction. 

Network structures have been quantitatively determined by means of real-time XH NMR T2 relaxation experiments for several polymers [174-178]. The effect of the curing conditions on sol and gel fractions and the spatial heterogeneity of the network structure has been studied for polyethylene [174], polyacrylamide [175], PDMS [176], BR [177], epoxy resins [178] and EPDM [179]. 

Recently, a novel method for determining the microstructure of crosslinked polybutadiene in latex using solution 13C-NMR technique was reported [133]. The surfactant and polymer concentrations in the latex were adjusted to give a good signal resolution of the latex sample, as indicated by half-width of the resonance peak at 32.7 ppm. Under these conditions, the S/N ratio was almost identical to that of sample in solution, as shown in Figure 11.31. The microstructure of sol and gel fractions in a radical initiated polybutadiene, determined by this technique, was similar to that of solution measurements. 

The measure of the cross-linking is determined by the gel fraction—the fraction of the polymer which, after irradiation, is not soluble any more in methyl ethyl ketone (MEK) due to the presence of two- and three-dimensional net. The gel fraction is denoted usually by g, and as a function of the absorbed dose is given usually by the Charlesby-Pinner equation17, relating the sol fraction, s9 with the dose ... 

An attempt to develop an HPLC/UV method for detecting tetryl in plant stems was unsuccessful because of binding of the compound to macromolecules in the tissue (Army 1981a). A methodology based on solvent extraction of plant tissues followed by silica gel fractionation and HPLC/UV detection was used for the determination of tetryl in plant tissues (Harvey et al. 1993). This method allowed 82.7% recovery of tetryl from fortified bush bean leaves. 

Lee S (1993) Gel-content determination of polymers using thermal field-flow fractionation. In Provder T (ed) Chromatography of polymers characterization by SEC and FFF. American Chemical Society, Washington, DC, pp 77-88... 

Sol-gel analysis is an important tool for distinguishing between polymers that can be cross-linked by irradiation and polymers that are not cross-linkable by radiation techniques. These experiments are typically done for wall-to-wall gels and offer the possibility to determine the irradiation conditions for microgel formation. Charlesby and Pinner first obtained a simple expression relating the soluble part of a polymer sample, the sol fraction (5) or sol content (in contrast to the insoluble part, the gel fraction (g) or gel content) to the absorbed dose D of radiation [10, 12],... 

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