Evaluation of Cross-Linking and Scission Yields in Irradiated Polymers from the Dose Dependence of the Weight- and z-Average Molecular Weights

A procedure has been developed for the determination of scission and cross-linking fields, G(S) and G(X), in irradiated polymers by combination of the ordinate intercepts of plots of ([Mw(0)/ MW(D)] - 1)/D and ([Mz(0)/A/2(D)] - 1)/D versus dose (D). The required weight- and z-average molecular weight data for a given sample can be obtained by using the Rayleigh and schlieren optical systems, respectively, to record the solute distribution in a single sedimentation equilibrium experiment. The procedure has been tested by application to simulated data for a range of initial molecular weight distributions and values of G(S)/G(X) and applied to previous sedimentation equilibrium results (Nichol, J. M.; Donnell, J. H.; Rahman, NLP.; Winzor, D. J. J. Polym. Sci., Polym. Chem. Ed. 1977,15, 2919) for a polystyrene sample with Afw(0)/Mn(0) = 1.03 and G(S)/G(X) = 1

Quantitative characterization of radiation degradation in polymers by evaluation of scission and cross-linking yields

This article presents a survey of literature on the quantitative characterization of the radiation degradation of polymers in terms of scission and cross-linking. An outline of the theoretical expressions for such characterization is followed by a general discussion of ways in which those equations may be used to quantify radiation chemical yields of scission, G(S), and cross-linking, G(X). Specific considerations of their practical application begin with the classical Charlesby-Pinner treatment of solubility measurements on extensively degraded polymers, and then switch to characterization by means of solution studies of polymer samples subjected to much lower radiation doses. Included in those discussions is the evaluation of G(S) and G(X) by quantifying the effect of irradiation on molecular size distributions inferred from sedimentation velocity, gel permeation chromatography and dynamic light scattering measurements, and also their evaluation from the dose dependence of average molecular weights determined by osmometry, classical light scattering, sedimentation equilibrium and gel permeation chromatography. Copyrigh

Experimental tests of charge conservation in macromolecular interactions

A combination of equilibrium dialysis and ultrafiltration has been used to demonstrate the conservation of charge in the interaction between bovine serum albumin and methyl orange in Tris-HCl buffer, pH 7.4, I = 0.05 M; and also in the dimerization of α-chymotrypsin in acetate/chloride buffer, pH 3.9, I = 0.11 M, containing various concentrations of indole (0-10 mM) in order to displace the equilibrium position towards monomer. In the former study the magnitude of the negative charge on the albumin was shown to increase linearly with the number of molecules of methyl orange bound to the protein, the observed slope (0.96 ± 0.08) of this relationship being in excellent agreement with that predicted on the basis of charge conservation for attachment of the univalent, negatively charged methyl orange ligand. In the study of α-chymotrypsin, the net charge (expressed per monomeric enzyme unit) was + 10 in solutions in which the mole fraction of monomer varied between 0.47 and 0.88, the extent of this range having been established by means of constituent association equilibrium constant obtained from sedimentation equilibrium studies

Measurement of the net charge (valence) of a protein

A procedure is described which enables the net charge (valence) of a macroion to fbe determined without assumptions being made about the extent of any specific ion binding. Studies with ovalbumin, bovine serum albumin and lysozyme are used to illustrate the method, which entails a combination of dialysis and ultrafiltration

Effects of thermodynamic nonideality in ligand binding studies

Effects of thermodynamic nonideality are considered in relation to the quantitative characterization of the interaction between a small ligand. S, and a macromolecular acceptor. A, by two types of experimental procedure. The first involves determination of the concentration of ligand in dialysis equilibrium with the acceptor/ligand mixture, and the second, measurement of the concentration of unbound ligand in the reaction mixture by ultrafiltration or the rate of dialysis method. For each situation explicit expressions are formulated for the appropriate binding function with allowance for composition-dependent nonideality effects expressed in terms of molar volume, charge-charge interaction and covolume contributions. The magnitudes of these effects are explored with the aid of experimental studies on the binding of tryptophan and of methyl orange to bovine serum albumin. It is concluded for experiments conducted utilizing eiiher equilibrium dialysis or frontal gel chromatography that, provided a correction is made for any Donnan redistribution of ligand, theoretically predicted acceptor-concentration dependence is likely to be negligible and that use of the conventional binding equation written for an ideal system is appropriate to the analysis of the results. Use of ultrafiltration or the rate of dialysis method requires examination of the assumption that the activity coefficient ratio yy/y for the reaction mixture approximates unity; but again reassurance is provided that nonideality manifested as a dependence of the binding function on acceptor concentration is unlikely to be significant

Application of ESR spectroscopy to the kinetics of free radical polymerization of methyl methacrylate in bulk to high conversion

The concentration of propagating radicals during the free radical polymerization of methyl methacrylate in bulk to high conversion at 60°C was determined - (1) by "in situ" measurements in the spectrometer, and (2) using a cryogenic quenching procedure. The radical concentration showed a sigmoidal relationship with polymerization time, and the conformation of the radical was different below and above the gel point of ca. 30% conversion. A value of kt=2.5 (±0.3)×107 1 mol-1 s-1 was obtained below the gel point. The variation in kt, without allowance for a decreasing efficiency factor (f), was determined over the entire conversion range

Kinetic parameters for polymerization of methyl methacrylate at 60°C

Fourier transform infrared spectroscopy (FTi.r.) and electron spin resonance spectroscopy (e.s.r.) have been used to follow the kinetics of the polymerization of methyl methacrylate to high conversion at 60°C. The FTi.r. absorbance at 6152 cm-1 was used to monitor the time dependence of the concentration of double bonds, and the concentration of the polymer-chain propagation radicals was monitored using the e.s.r. absorption spectrum. These data were analysed to obtain instantaneous estimates of the kinetic rate parameters for propagation and termination across the range of conversion, and of the initiator efficiency at high conversion. The kinetic parameters were found to be consistent with values obtained by other methods and with the predictions of recent theories