Camphor method, for molecular weights

The Determination of Molecular Weight by the Freezing-Point Method. The freezing-point method is a very useful way of determining the molecular weights of substances in solution Camphor, with its very large constant, is of particular value for the study of organic substances. 

Sometimes camphor is used as the solvent (Rast method) because of its high constant ( 38). However, it is very prone to form solid solutions which vitiate this method of determining molecular weights, since it is based on the assumption that the first solid to be deposited will be the pure solvent. The use of camphor and related solvents is therefore not recommended. The possibility of obtaining solid solutions should be recognized in all cases, for this is one of the main limitations to the use of this method. 

Parameters such as, synthesis medium (HCl, camphor sulfonic add (CSA) or dodecylbenzene sulfonic acid (DBSA)), molar ratio aniline/oxidant, method for oxidant addition, temperature, polymerization duration, dedoping conditions will have effects on polymer properties in terms of yield, chain length, effective conjugation, defects rate, and electrical properties. Temperature was kept to 0 C, -30 C or -40 C and reactions were stopped after various durations in order to control the molecular weight of samples. Polymerization durations were varied from 1 hour to 5 days and depended on the reaction temperature. The inherent viscosities of the polyanilines (Pani) were determined at 25 °C in 0.1%w solutions in concentrated sulfuric acid (95 %), using an Ubbelohde viscometer. For instance, high viscosity samples (1.4 dl/g) were obtained after 3 days at -40 °C vsMe low viscosity samples (0.6 dl/g) were obtained after 1 hour at 0 A relationship was found between polymerization duration and inherent viscosity for polymers synthesized at low temperature (- 40 °C). Inherent viscosity increases from 0.6 to 1.4 when duration of polymerization increases from one to five days. In the case of synthesis at 0°C no correlation was obtained between duration of polymerization and inherent viscosity. A careful control of other parameters (synthesis medium, molar ratio aniline/oxidant, method for oxidant addition) have permitted to get samples with inlierent viscosities ranging from 0.55 to 2.1 dl/g in a reproducible way. 

The simplest method to use is reversible salt formation polarimetric and spectropolari-metric studies of salts of optically inactive polyacids (such as polyacrylic, polyitaconic or poly-p-vinylbenzoic acids) with OA bases (like quinine or nicotine) as well as optically inactive bases (polyvinylpyridines) with OA acids (tartaric, mandelic, t/-camphor-/3-sulfonic or with L-menthylbromoacetate) have been reported by Schulz et al f 176, 177]. They found that the optical activity of the poly-salts affected by the microconfiguration of the polymer chain are quite different from the corresponding salts of OA low-molecular weight compounds the ORD curves of isotactic and atactic poly-2-vinylpyridine salts with D-tar-taric acid were found to be anomalous, with a higher value of rotation for the isotactic polymeric salt but in the case of methacryloylnicotine salts it seems that the observed differences can be explained by the assumption of increased specific rotation of the associated nicotine cations in the polymeric salt [178] and not by a conformational effect. 

For a solution of 1 g of polyethylene with M2 = 500 in 100 g of camphor Tf = 178.4°C, A//f = 2.58 kcal/mol, M = 152), Tj - r is 0.48°C. As the molecular weight of the solute increases, measurement of the temperature lowering demands more sensitive equipment and the method becomes impractical. In order to measure the number of molecules by colligative methods, it is necessary to use dilute solutions. In practice, we often extrapolate results to infinite dilution by some suitable linearizing techniques. 

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