Dynamically asymmetric blends

Fig. 12.25 Result for the dynamic asymmetric blend PS/PVME at a composition 80 %/20 %. Lower panel solid spheres, dielectric data dashed line, temperature dependence of the relaxation times expected for PVME in the blend solid lines indicate the temperature dependence of the relaxation times for the pure components. Upper panel temperature dependence of the heat capacity measured by DSC (All data were taken from reference Cendoya et al. (1999))...

Besides of the segmental dynamics also the chain dynamics in blends is affected by the d5mamical asymmetry (Brodeck et al. 2010). Recently a generalized Langevin approach was presented to calculate the chain (Rouse dynamics) for dynamically asymmetric blends (Colmenero 2013). A further discussion is beyond the scope of this chapter. 

Colmenero, J., 2010. Chain dynamics in dynamically asymmetric polymo"blends. In Multi-Scale Dynamics of Structured Polymeric Materials, The Michelin Materials Science Chair at ESPCI ParisTech Lectures, December 6-7, pdf of lecture available on the web. 

Brodeck, M., Alvarez, R, Moreno, A. J., Colmenero, J., and Richter, D. 2010. Chain motion in nonentangled dynamically asymmetric polymer blends Comparison between atomistic simulations of PEO/PMMA and a generic beadspring model. Macromolecules 43 3036-3051. 

The hterature about dielectric relaxation is rich. For instance, there are several recent reviews available for that field (Simon and Schonhals 2003 Runt 1997 Floudas et al. 2011 Colmenero and Arbe 2007). For instance, by application of dielectric spectroscopy to polymeric blends, the phase behavior of a system can be probed or the degree of miscibihty of the blend components in the different phases can be discussed and estimated. This concerns also the question of the dynamic heterogeneity in miscible blend systems or confinement effects in dynamically asymmetric polymer blends (Colmenero and Arbe 2007). 

Besides the dynamic heterogeneity discussed above, binary miscible polymer blends can be considered as dynamically asymmetric if the two components have a large difference in the glass transition temperatures. Usually the dynamic asymmetry is defined by A = where x " is the relaxation time of the... 

Similar results have been also reported by Adachi et al. (Urakawa et al. 2002) for PVME/PS, for the blend PMMA/PEO (Maranas 2007), or for blends of PVDF/ PMMA (Sy and Mijovic 2000). Besides dielectric spectroscopy quasielastic neutron scattering is found to be quite useful to investigate the molecular dynamics of dynamic asymmetric polymer blends because spatial information is provided by this technique (see, for instance, Tyagi et al. 2006, 2007). For a more detailed discussion, the reader is referred to the literature (Colmenero and Arbe 2007). 

The SPH method provides an efficient way for the numerical simulations of the phase-separation phenomena in polymer blends. For instance, Okuzono used this approach to simulate a specific type of phase separation - the so-called viscoelastic phase separation - experimentally found in polymer solutions and dynamically asymmetric mixtures. Examining the effect of stress relaxation time on morphology of domains, it was shown that the more viscous phase forms network-like domains when the stress relaxation time is large. 

The same group has also observed enhanced impact and toughness behaviour of the blends due to the secondary phase separation effect on the same rubber-modified system. The effect is originated due to the combined effect of hydrodynamics, viscoelastic effects of the rubber phase, diffusion, surface tension, polymerisation reaction and phase separation. In a dynamic asymmetric system, the diffusion of the fast dynamic phase is prevented by the slow dynamic phase, and hence the growth of the fast dynamic phase is retarded by the slow dynamic phase. In the case of low viscosity blends the... 

Wang and Cakmak (52) studied the development of structure hierarchy in tubular film blown dynamically vulcanized PP/EPDM blend. The blown films were found to exhibit an unusual asymmetric structure. The PP phase was found to fibriUate at all the outside surface, while the inner surface remained relatively featureless. This was attributed to disproportionally rapid cooling of the outside surface by the air steam blown externally onto the film being extruded. This, in turn, resulted in sohdification of very thin PP surface layers that caused their fibrillation under the heavy stress they had to endure. Increase in the blow-up ratio was found to expand this web-like surface texture. As a result of this fibrillation mechanism, the increase of both the blow-up ratio and draw-down ratios was found to reduce the mechanical properties. 

COMPONENT DYNAMICS OF HIGHLY ASYMMETRIC POLYMER BLENDS... 

Interchain Coupled Chain Dynamics in Highly Asymmetric Polymer Blends... 

The Coupling Model is consistent with all the properties and has fundamental support from quasielastic neutron scattering and simulations. However, the emphasis of the entire section is on the many properties of component dynamics in HAPB that deserve attention and alternative explanation by researchers in glass transition and polymer chain dynamics and viscoelasticity. This is because the new physics found in the segmental and chain dynamics of components in highly asymmetric polymer blends could possibly revolutionize the current understanding of polymer dynamics and viscoelasticity. 

Other aspects of the dynamic behavior of polymer blends that are not observed in the pure components are a breakdown of time-temperature superposition [156,158] and a pronounced asymmetry of the relaxation spectrum. This is evident in the dielectric spectra of PS/PVME as well as other miscible blends that are strongly temperature dependent and display an asymmetric shape towards lower frequencies that becomes more pronounced with decreasing temperature [156,157]. This differs... 

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