Molecular mass, distribution

An interesting feature of these polymers is that they have a tetramodal molecular mass distribution which has been deliberately built in and which is claimed to improve processability. This is achieved by the following procedure ... 

When the styrene has been consumed, to give living polymers of narrow molecular mass distribution, more styrene and more catalyst is added. The styrene adds to the existing chains and also forms new polymer molecules initiated by the additional sec-butyl-lithium. 

FIGURE 2.4 Calibration curve of dextran on Sephacryi S-300 SF. Calibration curves were calculated from one chromatogram of a broad MWD reference sample using data for the molecular mass distribution as obtained by a calibrated gel filtration column ( , upper curve) and on-line MALLS ( ). The calibration curve was found useful for estimating the size of globular proteins. [Reproduced from Hagel et al. (1993), with permission.]... 

The broad pore size distribution of Sepharose makes it well apt for the analysis of broad molecular mass distributions of large molecules. One example is given by the method for determination of MWD of clinical dextran suggested in the Nordic Pharmacopea (Nilsson and Nilsson, 1974). Because Superose 6 has the same type of pore size distributions as Sepharose 6, many analytical applications performed earlier on Sepharose have been transformed to Superose in order to decrease analysis time. However, Sepharose is suitable as a first try out when no information about the composition of the sample, in terms of size, is available. 

E. F. Vainstein, A. A. Sokolovskii, and A. S. Kuzminskii, Kinetics of the Changing Products Molecular-Mass Distribution in Thermodegradation of Associated Polymers, Polymer Yearbook, (R. A. Pethrick and G. E. Zaikov, eds.) Gordon and Breach, London, vol. 9, pp. 79-101 (1993). 

Fig. 4. Molecular mass distribution of Fraction A purified on Concanavalin A -cellulose on Superose 12 column. Buffer - 0.05 M phosphate, pH 7.0, 0.15 M NaCl, fraction size 0.5 ml/min. Exopolygalacturonase activity determined with penta(D-galactosiduronic) acid pH 5.0 and pH 3.8 (0—0)-...

Mesocosms placed in shallow Finnish lakes were used to evaluate changes brought about by extended incubation of biologically treated bleachery effluent from mills that used chloride dioxide. The mesocosms had a volume of ca. 2 m and were constructed of translucent polyethere or black polyethene to simulate dark reactions. The experiments were carried out at ambient temperatures throughout the year, and sum parameters were used to trace the fate of the organically bound chlorine. In view of previous studies on the molecular mass distribution of effluents (Jokela and Salkinoja-Salonen 1992), this was measured as an additional marker. Important featmes were that (a) sedimentation occurred exclusively within the water mass within the mesocosm, (b) the atmospheric input could be estimated... 

Jackson, C., Computer simulation study of multi-detector size-exclusion chromatography of branched molecular mass distributions, /. Chromatogr. A, 662,... 

Currently, the most widely used chromatographic technique for the molecular mass distribution analysis of a polymer is SEC. The widespread use of SEC is because of its... 

A measure of the breadth of the molecular mass distribution is given by the ratios of molecular mass averages. The most commonly used ratio Mw/Mn — H, is called the polydispersity index. Wiegand and Kohler discuss the determination of molecular masses (weights) and their distributions in Chapter 6. 

Ionic chain polymerisations refer to chain mechanisms in the course of which the propagation step consists of the insertion of a monomer into an ionic bond. The strength of this ionic bond can vary, depending on the nature of the species, the temperature and the polarity of the solvent, between a closed ionic pair in contact up to free ions (see Figure 23). Final polymer microstructure (configuration,...) and molecular mass distribution depend on the actual nature of the active ionic species. 

A characteristic of polyolefins synthesized with metallocene catalysts is their significantly lower polydispersity compared to one obtained by using heterogeneous Ziegler-Natta catalysts. Such narrower molecular mass distributions can lead to different mechanical properties of the resulting material. 

Equilibrium molecular mass distribution in step polymers. 

The longer the reaction time, the lower the molecular mass Distribution of all species can be calculated all along the reaction... 

Except for biopolymers, most polymer materials are polydisperse and heterogeneous. This is already the case for the length distribution of the chain molecules (molecular mass distribution). It is continued in the polydispersity of crystalline domains (crystal size distribution), and in the heterogeneity of structural entities made from such domains (lamellar stacks, microfibrils). Although this fact is known for long time, its implications on the interpretation and analysis of scattering data are, in general, not adequately considered. 

A general principle is governing the relation between physical parameters and underlying distribution functions. Its paramount importance in the field of soft condensed matter originates from the importance of polydispersity in this field. Let us recall the principle by resorting to a very basic example molecular mass distributions of polymers and the related characteristic parameters. 

In the basic molecular mass distribution, N(M), the number N of molecules in a sample is plotted vs. their molecular mass, M. Figure 1.2 presents a sketch of a... 

Figure 1.2. A number molecular mass distribution N (M) of an ideal chain polymer. N (M) is defined for integer multiples of Mm, the monomer mass. The integer factor, P, is called the degree of polymerization... 

With lower-molecular-weight polymers unit cell parameters may also vary with the molecular mass distribution. For poly(ethylene terephthalate) the history of reported unit cell parameters reflects the progress of chemical processing technology [105]. 

The concentration of the synthase or the number of enzyme copies has been assumed to have an influence on the molecular mass and molecular mass distribution of the synthesized polymer [33],but this has not been confirmed. The only variables found so far to control the molecular mass of the polymer are the initial ratio of substrate to enzyme levels, and the concentration of inducing factors in the culture medium [34-36] cf. also Chap. 9 of this book. 

Since it may be assumed that part of poly(3HB) synthesized is degraded during accumulation, that the equilibrium determines the content of poly(3HB), the molecular mass, and molecular mass distribution, a detailed analysis of the regulation of the poly(3HB) cycle will be useful for a better understanding as well as optimization of industrial production of poly(3HB). 

Relative molecular mass distributions for components of biochemical and polymer systems can be determined with a 10% accuracy using standards. With biochemical materials, where both simple and macro-molecules may be present in an electrolyte solution, desalting is commonly employed to isolate the macromolecules. Inorganic salts and small molecules are eluted well after such materials as peptides, proteins, enzymes and viruses. Desalting is most efficient if gels with relatively small pores are used, the process being more rapid than dialysis. Dilute solutions of macro-molecules can be concentrated and isolated by adding dry gel beads to absorb the solvent and low RMM solutes. 

FIGURE S.8 Changes in the molecular mass distribution of peptides and the radical-scavenging capacity of miso extracts prepared from unrinsed squid meat at different points in the fermentation period measured with an online HPLC-DPPH system. Source Giri et at. (2011a). Permission has been obtained for the use of copyrighted material from Elsevier B.V. 

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