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Poly­(decamethylene succinate-ran-decamethylene fumarate)­s (PDSFs) were synthesized, and their structure and crystallization behavior were systematically studied. All polyesters possess high crystallizability irrespective of composition and display the widest pseudoeutectic region that has been reported to date. Nevertheless, both melting point– and crystallization temperature–composition relationships demonstrate PDSFs display off-V-shaped behavior, indicating particular pseudoeutectic behavior. PDSF57 and PDSF37 located in the pseudoeutectic region are studied in detail. DF-rich crystals form first due to the assembly of fumarate-related preordered structure in the PDSF57 melt; then DS-rich crystals appear through plausible epitaxial crystallization. PDSF37 is proven to be located in the boundary of the pseudoeutectic region. DF-rich ordered structure forms first but cannot crystallize itself, which plays the role of the “soft epitaxial” substrate for DS-rich crystal formation; meanwhile, the DF-rich form crystallizes with the assistance of DS-rich crystals. The particular crystallization process of PDSF37 induces tightly attached structures between the two types of crystals and results in a fantastic and abrupt melting point jump.

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Biodegradable poly­(1,4-butylene adipate) (PBA) has been highlighted of distinct polymorphism and receives extensive research by adding nucleating agents. Usually, only a single nucleating mechanism is proposed for each composite system, which should not be sufficient. In present work, 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl)­methylene]-nonitol sorbitol (NX), a commercial sorbitol derivative, was incorporated into the PBA matrix to regulate the crystallization behavior. It is found that the non-isothermal crystallization temperature rises first and then declines, but the α-form crystal-forming ability becomes stronger with the increasing NX content, which is due to the limited solubility of NX in PBA. When NX is totally dissolved, the specific butterfly molecular shape of NX helps PBA chains form the gauche conformation, simultaneously promoting the crystallization rate and α crystal formation. However, once the NX content is ≥1%, crystallites of NX appear and play as physical cross-linking points for PBA chains; then, the mobility restriction becomes the major mechanism, which would slow down the crystallization rate but significantly enhance the formation of the α-form crystal-forming ability. PBA/NX composites possess a considerably lower degradation rate than neat PBA. Thus, this work demonstrates a novel additive of the dual mechanism to regulate the crystallization behavior and further the degradation rate of PBA.