Low molecular mass fractions

Elforts have been made to characterize the nature and content of individual components that are present in the low-molecular-mass fraction of the total mill effluents, which include the spent chlorination and alkali extraction stage liquors [2,4]. Approximately 456 types of compounds have been detected in the conventional bleach effluents, of which 330 are chlorinated organic compounds [22]. The compounds may be lumped into three main groups, namely, acidic, phenolic, and neutral (Table 2). Acidic compounds are further divided into the five categories of acids fatty, resin, hydroxy, dibasic, and aromatic acids. The most important fatty acids are formic and acetic acids. The dominant resin acids are abietic and dehydroabietic acids. Among the hydroxy acids identified, glyceric acid predominates. Dibasic acids such as oxalic, malonic, succinic, and mafic acids are derived from the lignin and carbohydrate fraction... 

Figure 7b. Gel electrophoresis of (B) low molecular mass fraction from Figure 3. Reproduced with permission from Ref. 65. Copyright 1982.

A9.3.5.9.2 Polymers are typieally not available in aquatic systems. Dispersible polymers and other high molecular mass materials can perturb the test system and interfere with uptake of oxygen, and give rise to mechanical or secondary effects. These factors need to be taken into account when considering data from these substances. Many polymers behave like complex substances, however, having a significant low molecular mass fraction which can leach from the bulk pol)mier. This is considered further below. 

The results of studies on the influence of molecular mass and molecular mass distribution of PIB on the kinetics of its thermal degradation are of interest because of the effect of chemical structure on the thermal stability of the polymer. Several high and low molecular mass fractions and non-fractionated samples of PIB with high and low molecular masses have been used in these studies. It has been found that the molecular mass of PIB sharply decreases from about two million to about 25,000 in the initial period (10% of weight loss) of polymer degradation under vacuum at 300 °C. Thereafter the decrease in molecular mass of the polymer decelerates. 

From fig. 1 it can be seen that in addition to the labile protein-bound fraction, metal-transferrin complexes in the case of actinides, there is also a low molecular mass fraction in which metal ions are bound to, and transported with, ligands such as the anions of amino and carboxylic acids. For the hard cations of the actinide elements, these complexing agents are predominantly carboxylic acids (from the citric acid cycle) (Duffield and Taylor 1986, Popplewell et al. 1975, Duffield et al. 1984, Metivier 1973) or inorganic anions such as carbonate, e.g., for uranium(VI) (Stevens et al. 1980). 

Unfortunately, because of the lability of the equilibrium in the interactions of the proteins and low molecular mass complexes indicated in fig. 1, it is difficult to determine the exact chemical speciation in such systems experimentally. Separation of fractions by mass, using, e.g., size-exclusion chromatography, upsets the equilibria under study so that results must be treated with great caution. As a result of this experimental uncertainty, computer-assisted methods have been developed which allow prediction of the chemical speciation pertaining to the low molecular mass fraction (May 1980, May et al. 1977, May and Williams 1977, Duffield and Williams 1989b). 

It is said that a pressure-sensitive adhesive always must be composed of a high-polymer base resin, which determines cohesion and specific adhesion, and of tackifying resins (tackifiers). In some systems, the tackifier may be replaced by low molecular mass fractions of the base polymer. To increase cohesion, the base resin in some systems is cross-linked or, in the case of rubber-based pressure-sensitive adhesives, vulcanized after application. 

Graph B illustrates the melting behavior of the low-molecular-mass fraction FRl with an average molecular mass of only 1,640. The measured crystallinity of this material is matched by the computed curve for the whole melting range. For the low-molecular-mass, multicomponent system, the separation can be described assuming eutectic behavior. 

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