The incorporation of the secondary co-units may not only change the kinetics and polymorphism of crystallization but also affect the melting behavior of polymers, leading to the memory effect even above the equilibrium melting temperature. In this work, a series of butene-1/norbornene random copolymers with 0−4.57 mol % steric norbornene (NBE) co-units was synthesized using rac-ethyl(1-indenyl) 2 zirconium dichloride as the catalyst. The melt crystallization, solid-phase transition, and memory effect associated with incomplete melting were studied with differential scanning calorimetry and in situ wide-angle Xray diffraction. The results show that unlike the polybutene-1 homopolymer, which always generates tetragonal crystals, the presence of steric NBE co-units can cause the copolymers to crystallize into trigonal form I′. The correlations of crystallization polymorphism with the NBE concentration and temperature were quantitatively established. For copolymer NBE2.36, the threshold temperature for generating pure form I′ crystals is 70 °C, and this decreases to 55 °C when the NBE concentration is increased to 4.57%. Moreover, for crystallized tetragonal form II, its spontaneous phase transition into thermodynamically more stable form I was significantly accelerated. On the other hand, the melting behaviors of the copolymers were influenced by the incorporation of NBE co-units, leading to the appearance of a memory effect. Interestingly, the memory effect not only accelerates the crystallization kinetics but also alters the crystallization polymorphism; the formation of form I′ was enhanced relative to form II. At a melt temperature of 120 °C, the initial form II crystallites dominate the memory effects because of their large lamellar thickness, while at a higher melt temperature (140 °C), form I′ is dominant due to its high thermal stability.

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Polymorphism is determined by the intrinsic molecular architecture designed via chemical synthesis and the external flow stimuli applied in practical processing. In this work, flow-induced crystallization of butene-1/1,5-hexadiene random copolymers with 1.38–2.81 mol % cyclic methylene-1,3-cyclopentane (MCP) co-units was studied with the combination of in situ synchrotron wide-angle X-ray diffraction (WAXD) and an extensional rheometer. The results show that the incorporation of unique MCP co-units facilitates the appearance of trigonal form I′ that was not kinetically favored in quiescent crystallization of polybutene-1 homopolymer, and the high MCP concentration of 2.81 mol % even leads to the generation of pure form I′ at 50 °C. Interestingly, applying a small flow of strain ε = 0.3 not only accelerates kinetics of form I′ but also rejuvenates the crystallization of the tetragonal form II in this copolymer. As flow strain was further increased, form II even appears earlier than form I′ for ε = 0.5 and mainly consists of the generated crystallites for ε = 3. Moreover, these effects of flow on crystallization exhibit a strong dependence on the concentration of MCP co-units, where flow of strain 0.3 is sufficient to induce the copolymer with 2.31 mol % MCP co-units to first crystallize into form II and to enhance the copolymer with 1.38 mol % MCP co-units to generate pure form II. Based on the systematic in situ WAXD results, a phase diagram of flow-induced crystallization was established with the variables of strain and MCP concentration, which reveals the mutual effects of molecular factor and external stimuli on crystallization.

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