Overcoming the entropy of polymer chains by making a plane with terminal groups: a thermoplastic PDMS with a long-range 1D structural order

Yugen Chen; Fumitaka Ishiwari; Tomoya Fukui; Takashi Kajitani; Haonan Liu; Xiaobin Liang; Ken Nakajima; Masatoshi Tokita; Takanori Fukushima

doi:10.1039/d2sc05491d

Overcoming the entropy of polymer chains by making a plane with terminal groups: a thermoplastic PDMS with a long-range 1D structural order

Chem Sci. 2023 Jan 4;14(9):2431-2440. doi: 10.1039/d2sc05491d. eCollection 2023 Mar 1.

Authors

Yugen Chen^{1

2}, Fumitaka Ishiwari^{1

2}, Tomoya Fukui^{1

2}, Takashi Kajitani³, Haonan Liu⁴, Xiaobin Liang⁴, Ken Nakajima⁴, Masatoshi Tokita⁴, Takanori Fukushima^{1

2

5}

Affiliations

¹ Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku Yokohama 226-8503 Japan.
² Department of Chemical Science and Engineering, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku Yokohama 226-8503 Japan ishiwari@chem.eng.osaka-u.ac.jp.
³ Open Facility Development Office, Open Facility Center, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku Yokohama 226-8503 Japan.
⁴ Department of Chemical Science and Engineering, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku Tokyo 152-8550 Japan.
⁵ Living Systems Materialogy (LiSM) Research Group, International Research Frontiers Initiative (IRFI), Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku Yokohama 226-8503 Japan.

Abstract

Due to its unique physical and chemical properties, polydimethylsiloxane (PDMS) is widely used in many applications, in which covalent cross-linking is commonly used to cure the fluidic polymer. The formation of a non-covalent network achieved through the incorporation of terminal groups that exhibit strong intermolecular interactions has also been reported to improve the mechanical properties of PDMS. Through the design of a terminal group capable of two-dimensional (2D) assembly, rather than the generally used multiple hydrogen bonding motifs, we have recently demonstrated an approach for inducing long-range structural ordering of PDMS, resulting in a dramatic change in the polymer from a fluid to a viscous solid. Here we present an even more surprising terminal-group effect: simply replacing a hydrogen with a methoxy group leads to extraordinary enhancement of the mechanical properties, giving rise to a thermoplastic PDMS material without covalent cross-linking. This finding would update the general notion that less polar and smaller terminal groups barely affect polymer properties. Based on a detailed study of the thermal, structural, morphological and rheological properties of the terminal-functionalized PDMS, we revealed that 2D assembly of the terminal groups results in networks of PDMS chains, which are arranged as domains with long-range one-dimensional (1D) periodic order, thereby increasing the storage modulus of the PDMS to exceed its loss modulus. Upon heating, the 1D periodic order is lost at around 120 °C, while the 2D assembly is maintained up to ∼160 °C. The 2D and 1D structures are recovered in sequence upon cooling. Due to the thermally reversible, stepwise structural disruption/formation as well as the lack of covalent cross-linking, the terminal-functionalized PDMS shows thermoplastic behavior and self-healing properties. The terminal group presented herein, which can form a 'plane', might also drive other polymers to assemble into a periodically ordered network structure, thereby allowing for significant modulation of their mechanical properties.