Crosslinking between Phases

Another method of compatibihzation involves crossUnkingbetween the phases of a phase separated system. This method is employed in one of the most common of commercial polymer blends, i.e., elastomer blends utilized in tire construction. In order to achieve the proper balance of properties for tire applications, crosshnked blends of phase separated elastomers are often employed. Sulfur (or peroxide) crosslinking will lead to covalent bridges between the phases, thus assuring proper translation of mechanical stress from one phase to the other. 

Basically all conventional sulfur (or peroxide) crosslinked unsaturated elastomer blends could be classified as co-crosslinked compatibilized blend examples. Various ercamples of these combinations are discussed in Section 4.2. Other crosslinking chemistry has been noted in several references with examples discussed in the following. 

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In the interphase between domains and the crosslinked continuous phase there may be regions that are immobilized by the network and are liquid crystalline at room temperature. 

Heat-resistant [218] soft foams were prepared from the blends of hdPE with E-P random copolymers. The azodicarbanamide acts as a thermal antioxidant and the crosslinking of the blend was increased by electron beam radiations and foamed at 225 °C with 2320% expansion. A blend of 35 wt.% PE-PP (8 92), 15 wt.% E-P block copolymers, and 50 wt.% EPDM showed accelerated weathering resitance [219] 1000 h probably due to crosslinking between constituents of the block copolymer, polyblend and EPDM. The effect of filler and thermodynamic compatibility on kaolin-filled PE-PP blend was studied by Lipatov and coworkers [220]. The thermodynamic interaction parameter (%) decreased and thermodynamic stability increased by filler addition, the degree of crystallinity decreased with increasing thermodynamic compatibility of the components due to sharp decrease in the phase separation rate during cooling. 

Polyurethane elastomers derive their elastomeric properties from phase separation of the hard and soft copolymer segments, such that the hard (urethane) segment domains serve as crosslinks between the amorphous soft segment domains, which are usually polyesters or polyethers. We are interested in the systems in which the hard segments are prepared from diphenyl-methane 4,4 -diisocyanate (MDI) with a linear diol as the chain extender ... 

The disperse crosslinked elastomer phase shows behavior similar to that of a polymeric filler. Viscosity is highly dependent on shear rate [7], but hardly dependent on temperature [7]. The flow exponent is between 3 and 5. 

Figure 8.5. Schematic showing fine structure within domains for an IPN. The solid black circles represent crosslinks. The distance between phase domains is less than half the chain contour length between crosslinks in the network, thus permitting phase separation to occur. At the same time, some network segments provide the requisite interconnection between the phase domains.

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