Molecular weight dimensions

Dimension of length Molecular weight Dimension of mass... 

The presence of glass particles modifies the in vitro degradation pattern of the polymer. In general, the degradation of PLA depends on several factors, which include its crystallinity, molecular weight, dimensions, composition. 

Light Scattering Detection for Molecular Weight, Dimensions, and Other Characteristics... 

The monolayer amount adsorbed on an aluminum oxide sample was determined using a small molecule adsorbate and then molecular-weight polystyrenes (much as shown in Ref. 169). The results are shown in the table. Calculate the fractal dimension of the oxide. 

One of the most important fiinctions in the application of light scattering is the ability to estimate the object dimensions. As we have discussed earlier for dilute solutions containing large molecules, equation (B 1.9.38) can be used to calculate tire radius of gyration , R, which is defined as the mean square distance from the centre of gravity [12]. The combined use of equation (B 1.9.3 8) equation (B 1.9.39) and equation (B 1.9.40) (tlie Zimm plot) will yield infonnation on R, A2 and molecular weight. 

In homopolymers all tire constituents (monomers) are identical, and hence tire interactions between tire monomers and between tire monomers and tire solvent have the same functional fonn. To describe tire shapes of a homopolymer (in the limit of large molecular weight) it is sufficient to model tire chain as a sequence of connected beads. Such a model can be used to describe tire shapes tliat a chain can adopt in various solvent conditions. A measure of shape is tire dimension of tire chain as a function of the degree of polymerization, N. If N is large tlien tire precise chemical details do not affect tire way tire size scales witli N [10]. In such a description a homopolymer is characterized in tenns of a single parameter tliat essentially characterizes tire effective interaction between tire beads, which is obtained by integrating over tire solvent coordinates. 

We began this section with an inquiry into how to define the size of a polymer molecule. In addition to the molecular weight or the degree of polymerization, some linear dimension which characterizes the molecule could also be used for this purpose. For purposes of orientation, let us again consider a hydrocarbon molecule stretched out to its full length but without any bond distortion. There are several features to note about this situation ... 

In summary, T j, gives a truer approximation to a valid equilibrium parameter, although it will be less than T owing to the finite dimensions of the crystal and the finite molecular weight of the polymer. We shall deal with these considerations in the next section. For now we assume that a value for T has been obtained and consider the simple thermodynamics of a phase transition. 

Mean dimension of particles in packing (pm) Molecular weight of polystyrene ... 

In Example 10.5 we extracted both the molecular weight and the radius of gyration from Ught-scattering data. There may be circumstances, however, when nothing more than the dimensions of the molecule are sought. In this case a simple alternative to the analysis discussed above can be followed. This technique is called the dissymmetry method and involves measuring the ratio of intensities scattered at 45° and 135°. The ratio of these intensities is called the dissymmetry ratio z ... 

If the solute size is approximately the (apparent) membrane-pore size, it interferes with the pore dimensions. The solute concentration in the permeate first increases, then decreases with time. The point of maximum interference is further characterized as a minimum flux. Figure 4 is a plot of retention and flux versus molecular weight. It shows the minimum flux at ca 60—90% retention. 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Individual pore size columns have variable pore volume, and because the column dimensions are fixed, the combination of different columns must result in variable slope of the overall calibration curve and hence variable degrees of resolution as a function of molecular weight. 

The optimum flow rate for most SEC separations using conventional PLgel column dimensions (internal diameter 7.5 mm) is 1.0 ml/min. It may be of some benefit to work with lower flow rates, particularly for the analysis of higher molecular weight polymers where the reduced flow rate improves resolution through enhanced mass transfer and further reduces the risk of shear... 

Molecular ma.s.s is expre.s.sed in units of daltons (D) or kilodaltons (kD) in diis book alternatively, the dimension-le.ss term molecular weight,. symbolized by Mr, and defined as the ratio of die ma.ss of a molecule to 1 dalton of mass, is itsed. 

More commonly, a fraction, based on chemical type, molecular weight or volatility, is heart-cut from the eluent of the primary column and introduced into a secondary column for more detailed analysis. If the same mobile phase is used in both dimensions, fractions may be diverted by means of pressure changes-an approach first used in 1968 in GC-GC by Deans (35), and applied by Davies et al. in SFC-SFC (36). If the mobile phases are different, valves are employed, and special... 

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