Sample swelling

Table II shows data obtained by sequential hexanes and ethanol extractions of the desilylated networks. Attempts to extract the networks prior to desilylation failed because the samples swelled to such a great degree that they lost their mechanical integrity and disintegrated. The amount of hexanes extractable material is less than 3% in all cases which indicates high copolymerization yields. Ethanol extractables were not determined because the presence of the byproducts of the desilylation, which are in the ethanol, would have given artificially high readings.

For comparison, a telechelic sulfonated polystyrene with a functionality f = 1.95 was prepared. In cyclohexane the material forms a gel independent of the concentration. At high concentrations the sample swells. When lower concentrations were prepared, separation to a gel and sol phase was observed. Thus, dilution in cyclohexane does not result in dissolution of the gel even at elevated temperatures. Given the high equilibrium constant determined for the association of the mono functional sample, the amount of polymer in the sol phase can be neglected. Hence, the volume fraction of polymer in the gel phase can be calculated from the volume ratio of the sol and gel phases and the total polymer concentration. The plot in Figure 9 shows that the polymer volume fraction in the gel is constant over a wide range of concentrations. 

In this section, we will describe the theory of swelling of polyelectrolyte networks. The simplest problem of this type concerns a network sample swelling freely in an infinite solvent. The solvent may contain some low-molecular-weight salt. This problem will be considered in Sect. 2.1. 

A. Add 2 g of sample to 50 mL of water. The sample swells to form a stiff, granular, slightly opalescent mucilage. 

DNA is fully hydrated with about 20 water molecules per nucleotide [522, 853-862], as indicated in Fig. 24.2. The hydration depends strongly on the water activity aw, which can be adjusted by addition of salt. At and above complete hydration, B-DNA prevails. It changes to some other conformation if the hydration is reduced, to an attainable minimum of 3.6 water molecules per nucleotide. If water is added in excess, the sample swells but remains in the B-DNA form [856, 857]. 

The bulk modulus fitted with the new method is equal to the reference value as well except for pressures too close to the critical point of CO.. We could also for the first time demonstrate a varying equilibrium size of an aerogel as a function of pressure. The sample swells as pressure is increased and shrinks again upon depressurization. 

Elastomers are cross-linked macromolecules above the glass transition temperature. The cross-link density is the fundamental engineering quantity which, for instance, determines the modulus of elasticity. Usually, it is measured during vulcanization of well-defined rubber samples in a vulcameter by the moment necessary to perform a given torsional shear of the test sample. Swelling experiments can be performed alternatively, but are problematic for filled elastomers. Such measurements are based on the assumption that the measured quantity does not vary over the sample volume. Inhomogeneous cross-link densities can be determined from the surface hardness, but volumetric resolution is achieved by conventional methods only after cutting the sample. 

Toluene Swelling. All samples swell substantially when immersed in toluene. It is assumed that toluene, a good solvent for polybutadiene, with a low dielectric constant (e—2.38 at 25°C) is absorbed preferentially in the hydrocarbon polymer phase and excluded from the ionic domains. Swelling is then equivalent to changing the distance between domains. 

Figure 6-8. Effect of swelling on the Mooney-Rivlin plot of natural rubber where Vr is the volume fraction of elastomer in the swollen sample, (swelling liquid, n-decane). [After Mullins, J. Appl. Polym. Sci., 2, 257 (1959), by the permission of John Wiley Sons, Inc.]...

Duval (3) suggested that since the walls of the crucible are heated more strongly than the center, the use of a plate and a thin layer of sample would be the best sample holder, whereas the high-walled crucible would be the worst. However, certain samples swell or spatter when heated, so that the use of crucibles with high walls is necessary. Duval does not recommend a covered crucible, however, since this would cause the horizontal mass plateaus to be longer. This was illustrated with the pyrolysis of magnesium ammonium phosphate. In an open crucible, there appeared to be a discontinuity between the loss of water and that of ammonia, while in ihe covered crucible there appeared to be a short horizontal or at least a break as soon as the ammonia stopped coming off. 

Figure 6-22. Influence of the solvent power on the swelling of weakly cross-linked samples of polystyrene (cross-linked with divinylbenzene). From left to right unswollen sample, swelling in the poor solvent cyclohexane (xo high), swelling in the good solvent benzene (xo low).

Since usually volume changes were measured. If the sample swells... 

Finally, oil resistance of the elastic part reached 80%. After 24 h exposure to an oil product, low-modulus samples swell and crack. Fire resistance tests carried out by the technique deseribed in [9, 10] showed their combustion time over 1/4 min. 

Millar" Suspension Each sample swelled to equilibrium. 

However, the water electrolysis is not excluded in this case. The two reactions determine the decay both of pH and ionic strength of solution at the PPy electrode. The change of local pH causes the sample contraction. The change of electrodes polarity causes the sense modification of the two above mentioned reactions, and the corresponding change of pH causes sample swelling. 

The physical nature of these blends does not appear to be the same as that of rubber-modified polystyrenes. When this type of ABS polymer is treated with a solvent such as methyl ethyl ketone the sample swells and only partially breaks up this indicates that rubber networks permeate the styrene-acrylonitrile copolymer matrix. When rubber-modified polystyrenes are treated with a solvent such as toluene, complete disintegration into fine particles occurs. 

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