Toughening Self-Healing Elastomers with Chain Mobility

Abstract

Enhancing fracture toughness and self-healing within soft elastomers is crucial to prolonging the operational lifetimes of soft devices. Herein, it is revealed that tuning the polymer chain mobilities of carboxylated-functionalized polyurethane through incorporating plasticizers or thermal treatment can enhance these properties. Self-healing is promoted as polymer chains gain greater mobility toward the broken interface to reassociate their bonds. Raising the temperature from 80 to 120 °C, the recovered work of fracture is increased from 2.86 to 123.7 MJ m^-3. Improved fracture toughness is realized through two effects. First, strong carboxyl hydrogen bonds dissipate large energies when broken. Second, chain mobilities enable the redistribution of localized stress concentrations to allow crack blunting, enlarging the size of dissipation zones. At optimal conditions of plasticizers (3 wt.%) or temperature (40 °C) to promote chain mobilities, fracture toughness improves from 16.3 to 19.9 and 25.6 kJ m^-2, respectively. Insights of fracture properties at healed soft interfaces are revealed through double cantilever beam tests. These measurements indicate that fracture mechanics play a critical role in delaying complete failure at partial self-healing. By imparting optimal polymer chain mobilities within tough and self-healing elastomers, effective prevention against damage and better recovery are realized.

Keywords: chain mobility; elastomers; fracture toughness; hydrogen bonding; self‐healing.

Figure 1

Molecular structure and material characterization. A) Chemical structure of CPU. B) Tensile stress–strain curves of CPU and control PU. C) Schematic illustration of PEG plasticizer disrupting the hydrogen bonds formed between carboxyl functional groups. D) DSC curves of CPU and CPU‐PEG blends (3, 5, and 8 wt.%) with at heating rate of 10 °C min⁻¹ from −15 to 60 °C. E) Schematic of micro‐phase separated CPU with hard and soft phases. F) SAXS profile of CPU and CPU‐PEG blends (3, 5, and 8 wt.%).

Figure 2

Mechanical properties of CPU and CPU‐PEG blends. A) Representative tensile stress–strain curves at a strain rate of 100 mm min⁻¹. B) Beginning three cycles of cyclic tensile tests with a strain limit of 1000%. C) Hysteresis areas of the first cycle of cyclic tensile tests at various limiting strains between 100 to 1000%.

Figure 3

Self‐healing of CPU and CPU‐PEG blends. A) Optical image of damaged films (top) and after self‐healing (bottom) for 192 h at room temperature (r.t.p). Scale bar represents 10 µm. B) Recovered work to fracture after healing dumbbell‐shaped samples for 192 h at room temperature (r.t.p). C) Recovered work to fracture after healing dumbbell‐shaped samples for various durations (6, 12, and 24 h) at elevated temperatures (80, 100, and 120 °C). All error bars are the standard deviation of three independent samples.

Figure 4

Fracture toughness behaviors of CPU and CPU‐PEG blends. A) Stages of crack evolution in CPU‐PEG elastomers. B) Stress–strain curves of notched samples under pure shear test. The cross indicates the point where crack advancement was initiated, and the corresponding strain represents the critical strain. C) Fracture toughness of CPU and CPU‐PEG blends (3, 5, and 8 wt.%). D) Digital images of notched CPU and CPU‐PEG samples at maximum strains at which crack blunting occurs. E) Change in crack length of notched CPU and CPU‐PEG films against time. F) Fracture toughness of CPU at various temperatures (40, 60, and 80 °C). G) Digital images of notched CPU samples at the maximum strains at which crack blunting occurs when subjected to various temperatures (40, 60, and 80 °C). All error bars are the standard deviation of three independent samples.

Figure 5

Double cantilever beam (DCB) measurements of CPU and CPU‐PEG blends. A) i) Schematic of DCB specimen, with P and Δ representing the load and displacement. ii) Schematic of DCB specimen cross‐section. iii) Digital image of DCB measurements. B) Adhesion energy of CPU and CPU‐PEG blends (3, 5, and 8 wt.%) for two self‐adhesion cycles for 192 h at room temperature (r.t.p). No adhesion was achieved for the second self‐adhesion cycle for CPU and CPU‐PEG3 due to the low chain mobility. C) Adhesion energy of CPU and CPU‐PEG blends (3, 5, and 8 wt.%) for two self‐adhesion cycles for 24 h at 80 °C.