We found that the blends of nitrile butadiene rubber (NBR) and polyvinyl chloride (PVC) exhibited lower critical solution temperature (LCST)-type phase behavior in which a single-phase blend tends to phase separate at elevated temperatures when the acrylonitrile content of NBR was 29.0%. The tan δ peaks, which originated from the glass transitions of the component polymers measured by dynamic mechanical analysis (DMA), were largely shifted and broader in the blends when the blends were melted in the two-phase region of the LCST-type phase diagram, suggesting that NBR and PVC are partially miscible in the two-phase structure. The TEM-EDS elemental mapping analysis using a dual silicon drift detector revealed that each component polymer existed in the partner polymer-rich phase, and the PVC-rich domains consisted of aggregated small PVC particles the size of several ten nanometers. The partial miscibility of the blends was explained by the lever rule for the concentration distribution in the two-phase region of the LCST-type phase diagram.

We investigated the phase-separated structure of nitrile butadiene rubber (NBR)/polyvinyl chloride (PVC) blends with different acrylonitrile (AN) contents in the NBR, using dynamic mechanical analysis measurements and scanning-transmission-electron-microscopy (STEM)–energy-dispersive-X-ray-spectroscopy (EDS) elemental analysis. Two separate sharp tan δ peaks were observed in the blend at the lower AN content of 18.0%, whereas a broad peak was observed in the blends with the higher AN contents of 29.0 and 33.5%, due to the increase in miscibility, as expected from the decrease in the solubility parameter difference with the increasing AN content. The STEM–EDS elemental analysis for the concentration distribution showed that the NBR was mixed in the large PVC domains with a diameter of several micrometers, and the excluded PVC existed around the interface of the domain–matrix phases in the blend with the lower AN content, whereas small domains with a diameter of several tens of nanometers were dispersed in the blend with the higher AN content. The concentration difference in PVC between the PVC domain and the NBR matrix became smaller with increasing miscibility as the AN content increased although the blends contained the same PVC content of 40 wt%.