Highly stretchable and self-healable polymer gels from physical entanglements of ultrahigh-molecular weight polymers

Abstract

Highly stretchable and self-healing polymer gels formed solely by physical entanglements of ultrahigh-molecular weight (UHMW) polymers were fabricated through a facile one-step process. Radical polymerization of vinyl monomers in ionic liquids under very low initiator concentration conditions produced UHMW polymers of more than 10⁶ g/mol with nearly 100% yield, resulting in the formation of physically entangled transparent polymer gels. The UHMW gels showed excellent properties, such as high stretchability, high ionic conductivity, and recyclability. Furthermore, the UHMW gel exhibited room temperature self-healing ability without any external stimuli. The tensile experiments and molecular dynamics simulations indicate that the nonequilibrium state of the fractured surfaces and microscopic interactions between the polymer chains and solvents play a vital role in the self-healing ability. This study provides a physical approach for fabricating stretchable and self-healing polymer gels based on UHMW polymers.

Fig. 1.. Conceptual illustration of UHMW gels.

(A) Schematic procedure for the synthesis of the UHMW gel. (B) Photograph and schematic illustration of the UHMW gel. (C) Transmittance spectrum of the 10-mm-thick UHMW gel.

Fig. 2.. Synthesis of UHMW gels.

(A) Dependence of M_n and monomer conversions on c_AIBN for the PMMA/[C₂mIm][TFSI] and PMMA/toluene systems. Polymerization temperature is 80°C. (B) Photographs of the PMMA/[C₂mIm][TFSI] solutions polymerized at 80°C with various c_AIBN, showing the corresponding M_n of PMMA. (C) The relationship between solvent viscosity and M_n for the PMMA/IL and PMMA/toluene systems. The viscosity of toluene is adjusted by dissolving commercial PMMA (M_w = 1.5 × 10⁴ g/mol) at different concentrations. (D) Monomer content versus M_n for the PMMA/[C₂mIm][TFSI] system. At a high MMA content, phase separation (denoted by the white circles) into a monomer-rich region and a polymer-rich region was observed (34).

Fig. 3.. Physicochemical characterization of UHMW gels.

(A) Temperature dependence of the storage (G′, closed symbols) and loss (G″, open symbols) moduli and (B) shape stability for the 30 wt % PMMA/[C₂mIm][TFSI] systems with various molecular weights. (C) Temporal variation of the scattered light intensity for the 41-kDa PMMA/[C₂mIm][TFSI] and 1740-kDa PMMA/[C₂mIm][TFSI] systems. cps, count per second. The 1740-kDa PMMA/[C₂mIm][TFSI] system is referred to as the UHMW gel in this study.

Fig. 4.. Mechanical properties and self-healing behavior of UHMW gels.

(A) Tensile stress–strain curves of the chemically cross-linked PMMA/[C₂mIm][TFSI] gel [red, reproduced with permission from (38), American Chemical Society 2017], UHMW PMMA/[C₂mIm][TFSI] gel (black), and recycled UHMW PMMA/[C₂mIm][TFSI] gel (blue). (B) Tensile stress–strain curves of the UHMW gels with various polymer/IL combinations. A logarithmic scale on the vertical axis is used. (C) Photographs of the UHMW PMMA/[C₂mIm][TFSI] gel after healing at room temperature for 6 hours. The red and yellow arrows indicate the contact point of two cut gel pieces and a breaking point of the healed gel, respectively. The fact that the breaking point of the gel does not start from the contact point implies that the healing process at the interface completed. (D) Tensile stress–strain curves of the pristine and healed UHMW gels with different healing times. (E) Nyquist plots of impedance spectra for the 300% stretched and unstretched UHMW gels before and after the self-healing process.

Fig. 5.. Detailed investigation of self-healing behavior of UHMW gels.

(A) Tensile stress–strain curves for UHMW PMMA/[C₂mIm][TFSI] gels after the self-healing process for 6 hours with different waiting times. (B) Schematic illustration for the healing process of UHMW gels with/without the waiting time. (C) Tensile stress–strain curves for the pristine and healed PMMA/[C₂mIm][TFSI] and PEMA/[C₂mIm][TFSI] gels. (D and E) Pair correlation functions between the oxygen atom of the methacrylate polymers and proton atoms of the C₂mIm cations for (D) PMMA/[C₂mIm][TFSI] and (E) PEMA/[C₂mIm][TFSI]. (F) Calculated R_g for PMMA (top) and PEMA (bottom) in [C₂mIm][TFSI].