Unexpected brittleness: Does the major component in binary polymer blends always make sense?

The structures and properties of melt-miscible poly(oxymethylene) (POM)/poly(butylene succinate) (PBSU) blends with low POM contents (<= 10 wt%) have been investigated. As a rubbery semicrystalline polymer, neat PBSU is tough and highly ductile. However, when blended with small amounts of rigid POM, such as only 3 wt% POM, PBSU becomes very brittle. Both the elongation at break and strength of the blends were significantly lower than those of neat PBSU and POM. This unexpected brittleness of blends with the rubbery polymer as the dominant component has not been reported previously, as the major component is generally thought to dominate blend performance. Detailed investigation indicated that the brittleness of POM/PBSU blends with low POM contents originated from their unique morphologies. The POM/PBSU blends were miscible in the melt state. Cooling the crystalline/crystalline polymer blends led to POM first crystallization in the PBSU melts, followed by PBSU crystallization in the POM crystal-filled PBSU melts. In the PBSU-dominant blends, POM crystallized into weak (loose/coarse) crystal networks dispersed in the PBSU melts. These POM crystals nucleated PBSU crystallization through an epitaxial mechanism upon further cooling. Therefore, PBSU almost simultaneously crystallized on the edge of POM crystals and formed less connected PBSU crystals, resulting in blends that were weak and brittle due to the weak POM crystal networks and less connected PBSU spherulites. This mechanism was further proven by varying the POM crystallization conditions. The blends with low POM contents were ductile (similar to the mechanical behavior of neat PBSU) if the POM crystallized into tiny less connected crystals. (C) 2018 Elsevier Ltd. All rights reserved.