Blends, miscible elastomer crystallinity

To reduce the cost, these elastomers have been diluted with some PBT homopolymer. Because of the chemical similarity between the hard segment of the copoly(ether-ester) elastomers and the PBT, they form fairly compatible blends. When the hard segment content in the copoly(ether-ester) is > 80 wt%, it was found to be completely miscible with PBT, showing a single T, amorphous phase and co-crystallization of the PBT segments of the elastomer with PBT homopolymer. As the hard segment content was lowered to < 60%, the blend exhibited incomplete miscibility, with two Tg s for two amorphous phases and also two separate crystalline phases. [Runt et al., 1989]. Nevertheless, a partial miscibility was indicated due to changes in the T observed in DSC and dielectric relaxation spectra. The partial miscibility and low interfacial tension between the phases makes the blend very compatible. 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

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