Blending slow diffusion rates

The spin-diffusion rate kc can be used to calculate the domain size by adopting a proper morphology [86, 97, 98]. For example, the domain size of PPO//-PS is calculated to be 5 nm by putting kc = 135 s and D = 5 x 10 m s into Equation (10.6). From the relaxation experiments, the value of kc can be obtained only when the characteristic nonsingle exponential decay curves are observed, that is, only when kc is comparable to the ATa and Kq values. Unfortunately, in most miscible blends, kc is so fast that the decay curves are identical or too slow to average the decays. 

The spontaneous mixing of the two polymers will transpire at a rate which reflects the degree of miscibility of the system. As X approaches the critical value for phase separation, "thermodynamic slowing down" of the interdiffusion will occur [12]. The rate of increase of the scattering contrast reflects the proximity of the system to criticality, as well as the strong composition dependence of the glass transition temperature of the blend. Extraction of a value for either the self diffusion constants [13,14] or the interaction parameter is not feasible from the presently available data. 

The determination of phase separation in partially miscible polymer blends by means of thermal analysis is often difficult because of the small demixing enthalpy and the slow rate of the diffusion-controlled process. Dreezen et al. (2001) smdied the phase separation of PEO/PES and PEO/Aramid blends by optical microscopy, conventional DSC and MTDSC. The onset of phase separation from optical microscopy corresponds very well to the onset of a small stepwise increase in the MTDSC heat capacity (Fig. 10.31). 

Solid-state reactions generally require high temperatures. In particular, bulk diffusion, such as the diffusion of Ni ions into a Ti02 lattice, is a relatively slow process and will control the reaction rate for pigment formation. Therefore, ideal raw batches should consist of intimately blended nano-sized particles to minimize diffusion distances for the dopants and to generate homogeneous particles. Although manufacturers work towards this ideal, the perfect raw batch has yet to be achieved in a cost-effective manner. 

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