Molecular weight asymmetry

Fig. la-c. Asymmetric stars with a molecular weight asymmetry b chemical asymmetry c topological asymmetry... 

Molecular Weight Asymmetry. All the arms of the star are identical in chemical nature, but they have different molecular weights. 

The excellent correlation between hydrodynamic volume and [ri]M) has become the basis of the universal calibration curve (UCC) for polymers. Although other size parameters, namely those dependent on Rg, have been suggested, the applicability of UCC has recently been verified for a variety of polymers including those with a high chemical and molecular weight asymmetry (e.g., miktoarm stars where arms are of different composition). ... 

Molecular weight asymmetry. In this case, all the arms are chemically identical, hut have different molecular weights. 

Symmetric polymer blends do not exist in reality. A host erf asymmetries are present in real chemical alloys of interest These include attractive potential asymmetries (present even for isotopic blends) and specific interactions, molecular weight asymmetries and polydispersity, and single chain structural differences between the blend components (e.g., monomer shape and volume, backbone stiffness, and tacticity). Realistic accounting for most of these effects would seem to require an off-lattice description which includes local interchain density and concentration correlations, and compressibility effects [1, 2, 63, 66, 67, 80]. 

This term gives some information about the asymmetry of the molecular weight distribution and is important in analyzing sedimentation behavior in ultracentrifugation. 

Figure 7 shows the results of measurements of adsorption density by Parsonage, etal. [77] on a series of eighteen block copolymers, with poly(2-vinylpyridine) [PVP] anchors and polystyrene [PS] buoys, adsorbed from toluene (selective for PS) of variable molecular weight in each block. The results are presented as the reciprocal square of Eq. 28, that is, as a dimensionless number density of chains ct (d/Rg A)-2. For all but the copolymers of highest asymmetry, Eq. 28 is in good agreement with the data of Fig. 7. The high asymmetry copolymers are in the regime of the data of curves (a) and (c) of Fig. 3 where the large relative size...

In general, the less the asymmetry in the composition of the low molecular weight PS-b-PI, the larger molecular weight of the other component is required to allow formation of cylinders. This trend leads, however, to a certain ratio in which the system macrophase separates before cylinders are formed. Therefore, the formation of cylinders tends to occur in a range in which the miscibility between the chains abruptly changes. 

The same base material has been used by Rhone-Poulenc Industries to develop Ion-exchange membranes for desalination (21-25). Their research has concentrated on polymers of moderate D.S. and low molecular weight (a restriction imposed by their technique of sulfonatlon which may cause polymer degradation). While their method of membrane preparation Is not entirely clear, it Is evident that the Rhone-Poulenc membranes possess the desired structural asymmetry. In this form the SPSF membranes have proven to be equal to, and In some ways superior to, CA membranes. 

As for low-molecular-weight aliphatic compounds, also for polymers made up by aliphatic macromolecules the existence must be excluded of optical activity in solution connected with the molecular asymmetry arising from hindrances to the free rotation around >C—C< single bonds. 

The rotational diffusion constant in water at 25° and neutral pH as measured by electric birefringence (258) is 230 X 105 sec-1 or 0.73 X HT8 sec as a relaxation time. For a hydrodynamic ellipsoid of dimensions 66 X 22 A and a molecular weight of 14,000, the calculated relaxation tilde is 0.72 X 10-8 sec. However, the apparent asymmetry of the molecule from the X-ray structure corresponds to an axial ratio of no more than 2 1 rather than 3 1. 

The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies. 

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