Ultrasonic investigation of brittle-ductile transition in high-density polyethylene and nitrile-butadiene rubber blends

It was reported by Hourston and Hughes [1] and Kuleznev et al. [2] that compatibility of polymer blends could be examined via ultrasonic velocity measurement. Singh et al. [3] and Singh and Singh [4] measured the ultrasonic velocity of compatible, semicompatible and incompatible polymeric blends in dilute solutions. They found that the plot of the ultrasonic velocity versus composition showed linearly for a compatible polymer blend, while it exhibited an inverted s-type curve for an incompatible polymer blend. Their experimental results also indicated that the plot of the ultrasonic velocity versus composition lay in between the two extremes for a semicompatible polymer blend, i.e., between stype and a straight line. Furthermore, Kuleznev et al. [2] and Singh and Singh [5] extended the ultrasonic velocity measurement to polymeric blends in the solid state. They demonstrated that the ultrasonic technique could indicate the extent of the compatibility between polymers. In the past decade, investigations [6, 7] of polymer compatibility by ultrasonic measurement have again attracted attention, and the method is regarded as suitable for determining compatibility. Recent investigation [8] of the blends of highdensity polyethylene (HDPE) and nitrile±butadiene rubber (NBR), the latter containing 24% by weight of acrylonitrile, revealed that introducing a small amount of NBR into HDPE could greatly elevate the matrix toughness. The NBRs are derived from the copolymerization of butadiene and acrylonitrile, and the strong polarity of acrylonitrile is able to result in incompatibility of HDPE and NBR. To determine why NBR can still toughen HDPE, we prepared a series of HDPE=NBR blends in which the NBRs contained different acrylonitrile contents. We examined the compatibility of various NBRs with HDPE by the ultrasonic velocity measurement. From our experimental data, we found that the ultrasonic velocity measurement was also useful for characterizing the ductile±brittle transition in polymer blends. HDPE is a commercial product; its melting index is 0.5 and the density is 0.955. All NBRs are commercial products that contain 18, 20, 24, 26, 30, 35 and 40% by weight of acrylonitrile and are respectively labeled as NBR18, NBR20, NBR24, NBR26, NBR30, NBR35 and NBR40. The NBRs were plasticated in a double roller mixer at ambient temperature for 10 min, then were blended with HDPE at 155±158 8C for 10 min. Prepared blends were molded into plates 4 mm thick at 180 8C under hot-pressure, then were processed into specimens for the Izod impact test and ultrasonic velocity measurement. The compressional ultrasonic velocities of the blends were performed by the same manner of Singh and Singh's [4]. The frequency was 2.6 MHz. The fracture morphology was observed using scanning electron microscopy (SEM; Hitachi X-650). Fig. 1 shows the relationship plots of the Izod impact strength versus acrylonitrile content of NBRs at a certain constant blend composition. It is obvious that a ductile±brittle transition appears as the acrylonitrile content is in the range of 26% to 30% by weight. This transition can be observed in the same variation range of acrylonitrile content despite the variation of the blend composition. That the ductile±brittle transition is not related to the blend composition may be because of the dominant role of the compatible extent of every kind of NBR with HDPE. Because NBRs consist of butadiene and acrylonitrile in different compositions, compatibility of HDPE and various NBRs should respond to the variation of acrylonitrile content in NBRs due to its strong polarity and incompatibility with HDPE. According to the experimental data of Jorgensen et al. [9], by investigating the phase structure of various NBRs, the NBRs were found to have two transitions if the acrylonitrile content in NBR was less than about 30% by weight. The one in the lowtemperature region was assigned to butadiene-rich NBR phase, while the other one in the hightemperature region was dedicated to acrylonitrilerich phase. Both phases could be separated by the