Poly rheological experiments

Thus for undiluted polymers the relaxation behaviour can be examined over a wider range of apparent frequencies. Similar functions can be constructed for other regions of the phase diagram and other rheological experiments. The method of reduced variables has not been widely tested for aqueous crosslinked polymers. Typically these are polyelectrolytes crosslinked by ionic species. Some of these give rise to very simple relaxation behaviour. For example 98% hydrolysed poly(vinyl acetate) can be crosslinked by sodium tetraborate. The crosslink that forms is shown in Figure 5.31. 

The non-aqueous HIPEs showed similar properties to their water-containing counterparts. Examination by optical microscopy revealed a polyhedral, poly-disperse microstructure. Rheological experiments indicated typical shear rate vs. shear stress behaviour for a pseudo-plastic material, with a yield stress in evidence. The yield value was seen to increase sharply with increasing dispersed phase volume fraction, above about 96%. Finally, addition of water to the continuous phase was studied. This caused a decrease in the rate of decay of the emulsion yield stress over a period of time, and an increase in stability. The added water increased the strength of the interfacial film, providing a more efficient barrier to coalescence. 

Poly(N,N-dimethylacrylamide) (PDMA) star polymers with 2, 3, and 4 arms and with dodecyl chains as hydrophobic end-caps were obtained in high yield with narrow polydis-persities by means of the RAFT technique. At sufficiently high concentration they form interconnected hydrophobic domains, i.e., a transient network. SANS experiments show that these hydrophobic domains contain about 20 dodecyl chains and they interact repulsively due to the PDMA chains that separate them. While the static structure is only very little affected by the number of arms this applies not at all to the dynamic properties as observed by DLS and rheology. Here one observes in DLS a complex, trimodal relaxation process, where the slower modes become more pronounced with increasing munber of arms. The fast mode corresponds to the diffusion of the hydrophobic domains while the second mode shows no q-dependence and corresponds in its values to the time deduced from the cross-over of G and G" in the oscillatory rheological experiments. Finally the slowest motion shows a rather pronounced q-dependence and is pre-stunably linked to a more complex relaxation mechanism of... 

The dilatational rheology of the poly(vinylacetate) monolayer onto an aqueous subphase has been studied between 1°C and 25°C by Monroy et al. [59], These authors have used the combination of several techniques. By this way, the exploration of a broad frequency range was possible. The relaxation experiments have shown multiexponential decay curves, whose complexity increases with decreasing the temperature. A regularization technique has been used to obtain the relaxation spectra from the relaxation curves and the dilatational viscoelastic parameters have been calculated from the spectra. The shapes of the relaxation spectra agree with the predictions of the theoretical model proposed by Noskov [100],... 

This standard covers measurement of the rheological properties of polymers with both stable and unstable melt viscosity parameters at various temperatures and shear rates. The test procedure lists typical test temperature conditions for polyethylene 190°C, for polypropylene 230°C, for poly(vinyl chloride) 170-205°C, however, this indicates that the most useful data are generally obtained at temperatures consistent with processing experience. The test method also prescribes using the Rabinowitsch shear rate correction (see above) and indicates that the basic rheology equations (17.10), (17.15) and (17.16) yield true shear rate and true viscosity for Newtonian fluids only for non-Newtonian fluids only the apparent shear rate and viscosity are obtained. 

The WSL theory developed by Leibler has been shown to be incorrect because of deviations from the fundamental underlying mean-field assumption. Figure 13.14 shows experimental results for a poly(ethylene-propylene/ethylethylene) (PEP-PEE) diblock copolymer that has been fit to the predictions of the Leibler theory without any adjustable parameters, since the ODT and / were calculated from rheological measurements (Bates et al., 1990). This mean-field theory does not qualitatively describe the behavior of this material. Other experiments have indicated that the RPA approximation (Sttihn and Stickel, 1992) and the Gaussian coil assumption (Bates and Hartney, 1985 Holzer et al., 1991) are inaccurate near the ODT. 

Yu et al. (2011) studied rheology and phase separation of polymer blends with weak dynamic asymmetry ((poly(Me methacrylate)/poly(styrene-co-maleic anhydride)). They showed that the failure of methods, such as the time-temperature superposition principle in isothermal experiments or the deviation of the storage modulus from the apparent extrapolation of modulus in the miscible regime in non-isothermal tests, to predict the binodal temperature is not always applicable in systems with weak dynamic asymmetry. Therefore, they proposed a rheological model, which is an integration of the double reptation model and the selfconcentration model to describe the linear viscoelasticity of miscible blends. Then, the deviatirMi of experimental data from the model predictions for miscible... 

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