Evaluation of Molecular Weight

Flexible polymer chains expand with increasing solvent power of the medium, leading to an increase in [77] with increasing polymer solvation. For chains of a similar kind, varying in length (homologous series), the relationship between [77] and molecular weight, M, may be represented by the Mark-Houwink relationship [19-22], 

If Equation (11) represents the relationship between [77] and M for a monodisperse polymer sample, then the intrinsic viscosity for a polydisperse sample containing weight fraction Wi with a molecular weight Mj will be 

As specific viscosities are additive in the limit of infinite dilution, the weight average intrinsic viscosity is obtained for a polydisperse polymer and My is defined as 

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Evaluation of molecular weights after ultrasonic scission of high molecular weight polymers (PMMA and PS) in the presence of a radical trap has been claimed to provide evidence of the termination mechanism.1,1 However, scission gives radicals as shown in Scheme 5.10. 

Aldehyde content. The most logical evaluation of molecular weight by chemical means would be the estimation of terminal aldehyde, either by an absolute reducing value or by derivatization (as osazone formation or characterization as a carbonyl derivative following oxidation). 

Section 7 introduces a recently developed on-line coupled gel permeation chromatography (GPC) and NMR system, in which a 500-MHz H NMR spectroscopy is used as a detector for GPC. The system facilitates evaluation of molecular weight dependence of polymer characteristics such as tacticity and comonomer composition and determination of molecular weight without using standard polymer samples. 

Matsumoto K, Talukdar B. Endo T. (2010). Methacrylate-based ionic liquid radical polymerization/ copolymerization with methyl methacrylate and evaluation of molecular weight of the obtained homopolymers, Polym. Bull, vol.66, n°2, pp.l99-210, Qanuary 2011), ISSN 0170-0839... 

The use of cryoscopic measurements for the determination of activity coefficients in aprotic solvents has been very limited, although much information is available on the evaluation of molecular weights and studies of solvent association. Relative to protic solvents more information is available and the investigations of Gillespie and coworkers in sulphuric acid and related compounds have been compiled to indicate the general scope of this technique in protic solvents. The information is presented in alphabetical order by solvent. 

Nobile, M. R., Cocchini, F. Evaluation of molecular weight distribution from dynamic moduli. 

Table 4.1 lists values of as well as AH and ASf per mole of repeat units for several polymers. A variety of experiments and methods of analysis have been used to evaluate these data, and because of an assortment of experimental and theoretical approximations, the values should be regarded as approximate. We assume s T . In general, both AH and ASf may be broken into contributions Ho and So which are independent of molecular weight and increments AHf and ASf for each repeat unit in the chain. Therefore AHf = Hq + n AHf j, where n is the degree of polymerization. In the limit of n AHf = n AHf j and ASf = n ASf j, so T = AHf j/ASf j. The values of AHf j and ASf j in Table 4.1 are expressed per mole of repeat units on this basis. Since no simple trends exist within these data, the entries in Table 4.1 appear in numbered sets, and some observations concerning these sets are listed here ... 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

Since viscometer drainage times are typically on the order of a few hundred seconds, intrinsic viscosity experiments provide a rapid method for evaluating the molecular weight of a polymer. A limitation of the method is that the Mark-Houwink coefficients must be established for the particular system under consideration by calibration with samples of known molecular weight. The speed with which intrinsic viscosity determinations can be made offsets the need for prior calibration, especially when a particular polymer is going to be characterized routinely by this method. 

Under 0 conditions occurring near room temperature, [r ] = 0.83 dl g for a polystyrene sample of molecular weight 10. f Use this information to evaluate tg and for polystyrene under these conditions. For polystyrene in ethylcyclohexane, 0 = 70°C and the corresponding calculation shows that (tQ /M) = 0.071 nm. Based on these two calculated results, criticize or defend the following proposition The discrepancy in calculated (rQ /M) values must arise from the uncertainty in T>, since this ratio should be a constant for polystyrene, independent of the nature of the solvent. 

SynChropak CATSEC columns are evaluated similarly using a polyvinyl-pyridine standard of molecular weight 600,000 and cytidine. The mobile phase is 0.1 % trifluoroacetic (TEA) acid containing 0.2 M sodium chloride. Minimum plate counts are listed in Table 10.4. 

Dissolution/reprecipitation processes were evaluated for the recycling of poly-epsilon-caprolactam (PA6) and polyhexamethyleneadipamide (PA66). The process involved solution of the polyamide in an appropriate solvent, precipitation by the addition of a non-solvent, and recovery of the polymer by washing and drying. Dimethylsulphoxide was used as the solvent for PA6, and formic acid for PA66, and methylethylketone was used as the non-solvent for both polymers. The recycled polymers were evaluated by determination of molecular weight, crystallinity and grain size. Excellent recoveries were achieved, with no deterioration in the polymer properties. 33 refs. 

Two improved methods of molecular weight calibration using broad MWD standards have been proposed and some evaluation has been done experimentally for aqueous and nonaqueous SEC. The experimental evaluations indicate that both methods appear very promising and justify further experimental investigation. It is recommended that these new calibration methods be evaluated for a wide range of polymers,packings and mobile phases. 

Few sedimentation or diffusion measurements have been made on the starch components, although methods of estimating polymolecularity of samples from sedimentation136 and diffusion136 measurements have been evaluated. The molecular-weight distribution can also be obtained from comparisons of Mn, Mw, and Mz, 137 and more directly by fractional pre-... 

The isotope-coded affinity tag approach utilizes chemical labeling that allows quantitation when combined with mass spectrometry. ICAT is desirable because the chemical labeling takes advantage of the mass defects of monoisotopic stable isotopes. ICAT uses an ICAT reagent to differentially label protein samples on their cysteine residues. ICAT is advantageous because it permits the evaluation of low-abundance proteins and proteins at both extremes of molecular weights and isoelectric points.60... 

Mention should be made of an expedient for evaluating correct molecular weights without resort to equilibrium dialysis or using (dn/dc2)x>M. It has been used by Vrij and Overbeeck146) for half neutralised polymethacrylic acid in 0.1 molar solutions of sodium halides. The relevant equation is... 

The "Mannich" adduct synthesized from the condensation of formaldehyde, 2-(methylamino)ethanol and poly-4-vinylphenol as shown in Structure I, has been evaluated as a function of molecular weight versus corrosion resistance as measured by salt spray and humidity tests on Bonderite 1000, an iron phosphate conversion coating. The molecular weight of the polymer was varied from approximately = 2,900 to 60,000. The corrosion resistance results were essentially equivalent over the molecular weight range evaluated. 

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