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Unusual II–I Phase Transition Behavior of Polybutene-1 Ionomers in the Presence of Long-Chain Branch and Ionic Functional Groups

An, Chuanbin, Lou, Yahui, Li, Yulian, Wang, Bin, Pan, Li, Ma, Zhe, Li, Yuesheng
Macromolecules 2019 v.52 no.12 pp. 4634-4645
active sites, annealing, composite polymers, cooling, crystallization, differential scanning calorimetry, glass transition temperature, iodine, melting, moieties
The mutual effects of long-chain branch and ionic functional groups on polybutene-1 (PB-1) phase transition from tetragonal form II into hexagonal form I of polybutene-1 were investigated using differential scanning calorimetry and various thermal protocols. The novel butene-1/11-iodo-1-undecene (PB-IUD) copolymer was synthesized to incorporate the long-chain branches, and its iodine groups were reacted as the active sites to introduce ionic functional groups with BF₄–, Tf₂N–, and PF₆– counterions. To the best of our knowledge, this is the first work to introduce physical ionic bonding into polybutene-1 (PB-1) ionomers and explore the affected phase transition. The results show that compared with the linear homopolymer, the long-chain branch largely retards the II–I phase transition of the PB-IUD copolymer. Unexpectedly, after introducing the ionic functional groups, ionomers have significantly accelerated transition with respect to reference PB-IUD, although they have exactly the same branching densities. This II–I phase transition of the ionomer can even happen at the crystallization temperature, where there is actually no cooling step to provide internal thermal stress. This indicates that additional crystallization-associated internal stress may be generated in ionomers for triggering form I nucleation. Moreover, the correlations of transition kinetics with annealing and crystallization temperatures were explored in depth. Ionomer phase transition can happen in a broad temperature range, which covers from the glass-transition temperature to high temperatures close to the melting region. Utilizing a stepwise annealing protocol, it was found that this broad transition temperature window originates from the persistent nucleation ability at elevated temperatures. On the other hand, ionomer transition kinetics increases with decreasing crystallization temperature, which, however, is opposite to that of the homopolymer. Based on this, a continuous cooling protocol was proposed and verified capable of endowing the branched ionomers with transition faster than the homopolymer.