The phase transition from the kinetically favored tetragonal form II into the thermodynamically stable hexagonal form I is the general phenomenon and core issue in application of polybutene-1-based materials. It is known that the variation of molecular structure by copolymerizing counits and the imposition of external stretching both greatly affect the phase transition. In this work, a series of butene-1/4-methyl-1-pentene (4M1P) random copolymers were synthesized with the dimethylpyridyl­amidohafnium/organoboron catalyst, where the 4M1P incorporated is the counit type of depressing II–I phase transition. Mechanical tests were combined with the in-situ wide-angle X-ray diffraction (WAXD) method to study the competing effects of the presence of 4M1P counits and stretching on the II–I phase transition. First of all, the quiescent experiments reveal that addition of 4M1P counits not only slows down transition kinetics but also decreases the ultimate form I fraction in the transition plateau. The 4M1P concentration ≥3.40 mol % is high enough to completely impede the II–I phase transition even when the aging time is as long as 4 months. Second, the stretching-induced phase transition was explored with the combined structural and mechanical information from WAXD and mechanical characterizations, respectively. The influence of stretching stimuli in the phase transition varies with 4M1P concentration. For low 4M1P concentration ≤1.00 mol %, stretching significantly accelerates the transition kinetics and induces the complete transition of form II. For intermediate 4M1P concentration 3.40 mol %, stretching effectively triggers the occurrence of the II–I phase transition, which does not start under quiescent conditions but only induces partial transition until fracture. For high 4M1P concentration ranging from 7.80 to 30.1 mol %, stretching just orientates the form II crystallites without starting any phase transition to form I. Third, as the concentration of 4M1P counits is increased, the phase transition is accomplished with different orientations, which determines the microscopic stress applied to lamellae. Then, detailed kinetics of the II–I phase transition was correlated to the stretching stimuli of the total true stress, component stresses parallel and perpendicular to the c-axis in the crystal lattice. It was interesting to find that transition kinetics is dominated by the component stress perpendicular to the c-axis for the off-axis orientation pathway. For the molecular mechanism of the phase transition, this indicates that the activated chain lateral slip is the dominant process for nucleation of form I within original form II.

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.

Flow-induced crystallization of poly­(vinylidene fluoride) (PVDF) was investigated for a broad temperature range from 160 to 220 °C by in situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Unexpectedly, the electroactive β-phase is obtained directly from the melt with an extensional flow at 160–200 °C, which is in contrary to the quiescent crystallization of generating the pure α-phase. For 220 °C, the observation of an equatorial SAXS streak without WAXD signals indicates the generation of fibrillar shish. Second, within the isothermal process after flow, the evolution of the flow-induced structure exhibits a strong temperature dependence. The generated β-phase triggers subsequent crystallite growth at 160–180 °C. However, at 190–220 °C, flow-induced fibrillar shish relaxes partially. Third, cooling triggers the crystallization of the α-phase, which competes with the β-phase to determine the final phase constitute. This work reveals the detailed formation and evolution processes of the flow-induced β-phase, which provides an effective approach to obtain the electroactive PVDF materials.

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.

The intrinsic coupling effect between the multiscale microstructure and macroscopic performance is a fundamental issue in polymer engineering and polymer physics. Combing the tensile testing and in situ wide-angle X-ray diffraction, the stretching-induced polymorphic transformation from tetragonal form II into hexagonal form I in the butene-1/1,5-hexadiene random copolymers was studied in this work. The mechanical response and crystallite evolution of the designed copolymers with various co-unit concentrations were combined to reveal the interplay between macroscopic stretching and microscopic phase transition for the whole deformation process. The results show that the elastic deformation does not change the structure significantly and it is the yield that triggers the II–I phase transition, which happens at a comparable strain of around 0.06 for all polymers studied. It was indicated that yield destroys the crystallite skeleton that bears the external stretching in the elastic deformation region and the generation of new tie chains enhances the stress transfer into lamellae, triggering the II–I phase transition. The generation of more transformed form I improves the stretching strength, and the increased stress is also required to ensure the proceeding of phase transition. A direct correspondence was established between the transition kinetics and true stress for the course of the II–I phase transition. Furthermore, after phase transition, the transformed form I increases the modulus of the molecular network by 2 orders of magnitude with respect to that of the original form II. This indicates that the crystallite acts as the physical cross-links for the molecular network and the enhancement of strain hardening is strongly dependent on the crystal modification.