Isothermal Crystallization of Triple Crystalline Biodegradable PEO-b-PCL-b-PLLA Triblock Terpolymers

J. K. Palacios1, A. Tercjaks2, J. Zhao3, N. Hadjichristidis3, A. J. Müller*1,4

1POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Donostia-San Sebastián, Spain
2Chemical and Environmental Engineering Department, Polytechnic School, University of the Basque Country UPV/EHU, Donostia-San Sebastián, Spain
3King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, KAUST Catalysis Center, Thuwal, Saudi Arabia
4IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

 PEO-b-PCL-b-PLLA triblock terpolymers are fascinating materials, as they can develop three different crystalline phases1. Adjusting the block composition and crystallization conditions, each block is able to crystallize sequentially and separately from a homogeneous melt. Thus, crystallization drives superstructure formation into mixed spherulites that include lamellae of PLLA, PCL and PEO. The PLLA block crystallizes first at higher temperatures upon cooling from the melt (and templates the morphology), then the PCL block, and finally the PEO block2. In this work, a three-stage crystallization protocol was applied to study the isothermal crystallization kinetics of each block. The crystallization was studied by WAXS, SAXS, DSC and AFM. At the PLLA crystallization temperatures, both PCL and PEO molten chains caused a strong diluent effect on PLLA crystallization. The overall crystallization rate of PLLA block in the terpolymers was faster than that of analogous diblock copolymers. In the second stage, the PCL crystallization was followed after PLLA was fully crystallized, and a nucleating effect (increased crystallization rate and decreased supercooling) induced by the previously formed PLLA crystals was observed. In the third stage, the PEO crystallization rate greatly decreased due confinement effects of PLLA and PCL crystals. Understanding the complexity of the crystallization behaviour of triple crystalline triblock terpolymers is at the very early stage of current research in crystallization of block copolymers3.

[1] J. Zhao, D. Pahovnik, Y. Gnanou and N. Hadjichristidis, Polym. Chem. 5, 3750 (2014). (link)
[2] J. K. Palacios, A. Mugica, M. Zubitur, A. Iturrospe, A. Arbe, G. Liu, D. Wang, J. Zhao, N. Hadjichristidis and A. J. Müller, RSC Adv. 6, 4739 (2016). (link)
[3] R. M. Michell and A. J. Müller, Prog. Polym. Sci. 54-55, 183 (2016). (link)

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