Partial Miscibility and Concentration Distribution of Two-Phase Blends of Crosslinked NBR and PVC

Abstract

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.

Phase-Separated Structure of NBR/PVC Blends with Different Acrylonitrile Contents Investigated Using STEM–EDS Mapping Analysis

“…Thus, the large domain consisting of aggregates of small domains in the NBR-M/PVC blend shown in Figure 3 b is assigned to the PVC phase. Note here that there were some domains of the vulcanization accelerator, ZnO, with a size of several hundred nanometers that were seen as dense dark domains in Figure 2 b and Figure 3 b, and a black matrix in Figure 5 b [ 13 ]. By contrast, no structure was seen in the NBR-H/PVC blend ( Figure 5 c), although small domains with a diameter of approximately several tens of nanometers dispersed in the matrix are seen in Figure 3 c. No structure in Figure 5 c is attributed to the small size of domains, and the small difference in the PVC component between the PVC-rich phase and NBR-rich one, as discussed in the next section.…”

“…A single glass transition is seen at intermediate temperatures between those of the component polymers in miscible blends, whereas two separate glass transitions are seen in phase-separated blends. A shift in the glass transition temperature is observed in the partially miscible blends, in which the component polymers are mixed at the interface of the two phases [ 8 , 9 , 10 , 11 , 12 ], or each component polymer exists in the phase of the partner polymers, due to the liquid–liquid phase separation in the two-phase region of the phase diagram [ 13 ]. Such a structure difference is crucial for tailoring the properties of polymer blends to practical applications.…”

“…On the other hand, a two-phase structure has also been observed in the blends in the same AN content range of 23–45% [ 19 , 33 , 34 ]. Recently, we found that the NBR/PVC blends with an AN content of 29.0% exhibited a lower critical solution temperature-type phase behavior, in which a miscible blend tends to phase-separate with elevating temperature [ 13 ]. Hence, the miscibility and phase-separated structure are considered to be changed depending on the AN content of the NBR at the same blend composition.…”