Intrinsic viscosity molecular shape

However, dendrimeric and hyperbranched polyesters are more soluble than the linear ones (respectively 1.05, 0.70, and 0.02 g/mL in acetone). The solution behavior has been investigated, and in the case of aromatic hyperbranched polyesters,84 a very low a-value of the Mark-Houvink-Sakurada equation 0/ = KMa) and low intrinsic viscosity were observed. Frechet presented a description of the intrinsic viscosity as a function of the molar mass85 for different architectures The hyperbranched macromolecules show a nonlinear variation for low molecular weight and a bell-shaped curve is observed in the case of dendrimers (Fig. 5.18). 

There are two molecular contributions to the intrinsic viscosity one from shape, the other from size or volume, as summarized by the relation... 

Diffusion and sedimentation measurements on dilute solutions of flexible chain molecules could be used to determine the molecular extension or the expansion factor a. However, the same information may be obtained with greater precision and with far less labor from viscosity measurements alone. For anisometric particles such as are common among proteins, on the other hand, sedimentation velocity measurements used in conjunction with the intrinsic viscosity may yield important information on the effective particle size and shape. ... 

Staudinger showed that the intrinsic viscosity or LVN of a solution ([tj]) is related to the molecular weight of the polymer. The present form of this relationship was developed by Mark-Houwink (and is known as the Mark Houwink equation), in which the proportionality constant K is characteristic of the polymer and solvent, and the exponential a is a function of the shape of the polymer in a solution. For theta solvents, the value of a is 0.5. This value, which is actually a measure of the interaction of the solvent and polymer, increases as the coil expands, and the value is between 1.8 and 2.0 for rigid polymer chains extended to their full contour length and zero for spheres. When a is 1.0, the Mark Houwink equation (3.26) becomes the Staudinger viscosity equation. 

Since the intrinsic viscosity depends not only on the size of the macromolecule but also on its shape, on the solvent, and on the temperature, there is no simple relationship for the direct calculation of molecular weights from viscosity measurements. However, the Mark-Houwink-Kuhn equation gives a general description of how the molecular weight can be calculated from the intrinsic viscosity ... 

When the amount of catalyst was less than that necessary to give complete conversion, the general shape of the reaction curve remained the same—that is, an induction period followed by rapid propagation up to a plateau of limited conversion. Table III shows the results of experiments with various concentrations of catalyst in each of those experiments, a less than quantitative conversion was reached. Increases in the amount of catalyst produced proportionate increases in the yields. The fact that limiting conversions were reached is evidence for the presence of a termination process. That the intrinsic viscosities and, presumably, the molecular weights were relatively similar over a considerable variation of catalyst concentration suggests that the termination process was not caused by an impurity originally present in the monomer. 

C. Assume that the GPC curve of Polymer A is that of a polydisperse, tetrafunctionally star-branched polychloroprene (i.e. apolydis-perse polymer made up of different molecular weight star-shaped polymers having four arms of equal length). Calculate the weight and number molecular weight averages, the polydispersity and the intrinsic viscosity of this star-branched polychloroprene. 

Changes of the molecular weight distribution curve of as-polymerized poly-DSP crystals by heat treatment at 265 °C support the explanation of the behavior by means of the rigid rod-shaped molecular model. This implies that by thermal treatment the distribution curve becomes narrower than the original curve with a decrese of the intrinsic viscosity from 5.1 to 3.972. ... 

Compared to linear analogues of identical molecular weight and monomer composition, star-shaped polymers have smaller Ra, Rn, and lower intrinsic viscosity [i] [222, 226, 227] ... 

PROPERTIES OF SPECIAL INTEREST Unique dendrimer properties not found in traditional macromolecular architecture include (1) a distinct parabolic intrinsic viscosity curve with a maximum as a function of molecular weight (2) very monodispersed sizes and shapes (i.e., M /Mn routinely below 1.1 even at high molecular... 

The intrinsic viscosity, [77], represents a fundamental quantity. It is related to molecular mass, the structure and shape of dissolved macromolecules and reflects... 

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