Flow-Induced Crystallization in Butene-1/1,5-Hexadiene Copolymers: Mutual Effects of Molecular Factor and Flow Stimuli

Abstract

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.

Regulating Two Distinct Formation Pathways of the Thermodynamically Stable Phase to Tune Crystal Polymorphism: The Case of Butene/Pentene Copolymers

“…Liu et al found that in butene-1/1,5-hexadiene copolymers, both tetragonal and trigonal phases were generated under quiescent crystallization, while the flow enhances the crystallization of form II more than that of form I′. 13 This is consistent with the flow-induced crystallization of form II in the butene/pentene copolymers studied in the present work. However, it is interesting to observe the flow-triggered occurrence of the II-I phase transition at the same temperature of form II formation in this work.…”

“…43,44 The molecular weight and distribution were measured with gel permeation chromatography (PL-GPC 220) at 120 °C with trichlorobenzene as the eluent. The concentrations of pentene co-units in copolymers were determined using the hightemperature 13 C nuclear magnetic resonance. The molecular characteristics of polymers studied in this work are given in Table 1.…”

“…Flow-Induced Crystallization and Phase Transition. Recently, it was reported that in the polymorphic butene-1 copolymers, the flow is able to change the crystallization competition between the tetragonal and trigonal phases (form I′ vs form II) 13 and accelerate the kinetics of phase transition (from form II to form I). 37,38,57 Then, the effect of flow on the formation of the thermodynamically stable trigonal phase (including forms I′ and I) has a 2-fold meaning, i.e., the flow dependences of the melt crystallization (into form I′) and the solid phase transition (into form I).…”

“…19 Liu et al found that the acceleration effect of flow on the crystallization kinetics of form II is much more pronounced than that of form I′. 13 Recently, it has been found that the flow also affects the phase transition of the form II crystallites generated. Chen et al studied the phase transition of the flowinduced form II crystallites and found that the induction time of II-I phase transition decreases with increasing orientation degree of form II lamellae.…”

“…Polybutene (PB) is one novel polyolefin with excellent mechanical properties including high creep resistance and high toughness. − For this polymorphic polymer, those excellent properties of PB mainly result from its thermodynamically stable modification of the trigonal phase. − In practice, the customary melt processes of PB often generate the tetragonal phase with the kinetic advantage, which has the growth rate two orders of magnitude larger than that of the trigonal phase. , However, this form II subsequently transforms into more stable form I at room temperature spontaneously. Therefore, there are two available pathways to obtain the thermodynamically stable trigonal phase: one is the direct melt crystallization of the trigonal phase from the amorphous state (referred to as form I′), , and the other is the indirect solid phase transition of the initially generated tetragonal form II (referred to as form I). − Because of the complex polymorphic behavior, the facile strategy of obtaining the thermodynamically stable trigonal phase is the key to extending the application of PB materials, especially when the quiescent phase transition takes weeks and leads to a long processing time.…”