Heterogeneity of the blend

The phase behavior of polymer blends that is mixtures of two chemically different polymers is experimentally well accessible in a window which is bounded at high temperatures by thermal decomposition temperature, of the polymer components and at low temperatures by the glass transition temperature, r, of the system. This is shown schematically, in Figure 16. When LCST and UCST merge, we have immiscibility or heterogeneity of the blend (see Figure 17). 

These blends, similarly to those consisting of NBR and PVC, are intended to combine the ozone resistance of EPDM with the oil resistance of NBR. NBR/EPDM blends lack a homogeneous phase because the difference in the polarity of the two materials is excessive. The heterogeneity of the blends can be seen in Fig. 4. With a view to obtaining a finer dispersion of the EPDM particles in the NBR two manufacturers produce NBR/EPDM blends by mixing latices. It is not yet possible to assess the suitability of the resulting materials for their intended purposes. 

In general, the multiphasic heterogenous nature of the impact grade styrene-based polymers is the root cause of their opaque-turbid nature. In determining the transparency of the blends, size and the size-distribution pattern of the dispersed phase along with the refractive index difference between the continuous and the dispersed phases are two very important criterion [133]. 

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

Figure 8.15 (A-F) hiistogram representations of the PLS score images showing the statistical distributions of the API class. Heterogeneity In the blend Is Indicated by deviations from a normal distribution and can be expressed as percent standard deviation (%SD), calculated by dividing the standard deviation by the mean.

Diblock copolymers, especially those containing a block chemically identical to one of the blend components, are more effective than triblocks or graft copolymers. Thermodynamic calculations indicate that efficient compat-ibilisation can be achieved with multiblock copolymers [47], potentially for heterogeneous mixed blends. Miscibility of particular segments of the copolymer in one of the phases of the bend is required. Compatibilisers for blends consisting of mixtures of polyolefins are of major interest for recyclates. Random poly(ethylene-co-propylene) is an effective compatibiliser for LDPE-PP, HDPE-PP or LLDPE-PP blends. The impact performance of PE-PP was improved by the addition of very low density PE or elastomeric poly(styrene-block-(ethylene-co-butylene-l)-block styrene) triblock copolymers (SEBS) [52]. 

One can also make a distinction based on the nature of the blend of the components. This may be (1) homogeneous on a molecular or a microscopic scale or (2) heterogeneous on a macroscopic and/or microscopic scale. 

Under such chromatographic conditions it is possible to determine the heterogeneities of the polymer chain selectively and without any influence of the polymer chain length. LC-CC has been successfully used for the determination of the functionality type distribution of telechelics and macromonomers [104-109], for the analysis of block copolymers [111-114], macrocyclic polymers [115], and polymer blends [116-118]. 

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