2.1

Preparation of microcapsules

Poly(urea-formaldehyde) (PUF) microcapsules containing an acrylic healing liquid were synthesized using an oil-in-water method . Urea (Sigma, UK) and formaldehyde solution (37 wt%, Sigma, Switzerland) were used to form the microcapsule shell. The healing liquid was composed of triethylene glycol dimethacrylate (TEGDMA, Sigma–Aldrich, Switzerland) and N,N-Dimethyl-p-toluidine (DEPT, Merck KGaA, India) (“oil phase”). 2.5 g ethylene-maleic anhydride (EMA, Sigma, UK) was fully dissolved into Milli-Q water (97.5 g) to prepare the aqueous solution. The “water phase” was prepared by adding 26 mL EMA solution to 100 ml Milli-Q water at room temperature. Subsequently, a lab mixer (IKA Works (Asia), RW20 DZM. N, Sdn. Bhd. Malaysia) with a three-bladed propeller (radius: 20 mm) was used to stir the solution at 300 rpm . Subsequently, 2.5 g urea, 0.25 g ammonium chloride (Merck KGaA, Germany) and 0.25 g resorcinol (Sigma–Aldrich, India) were added to this aqueous solution. Ammonium chloride and resorcinol were added with the aim to improve the strength and toughness of the microcapsules . In order to achieve microcapsules of large, medium and small size, the stirring speed was varied from 400 rpm (large size) to 800 rpm (medium size) and 1200 rpm (small size), respectively . Lower stirring speeds resulted in excessively large microcapsules, stable emulsion droplets could not be formed at higher stirring speeds. The pH was adjusted to 3.5 by addition of 1 M sodium hydroxide solution to the solution. Subsequently, 40 mL self-healing liquid (mixture of TEGDMA 99 wt% and DEPT 1 wt%) was added dropwise to the solution using a single syringe infusion pump (KDS, Infusion), after which 6.3 g formaldehyde solution was added to this emulsion in order to initiate the formation of the microcapsule shells. Thereafter, the emulsion was placed in a water-bath at 55 °C for 4 h. After polymerization of the microcapsule shells, the emulsion was cooled down and the microcapsules were filtered using a vacuum filter. Finally, the microcapsules were washed with water and acetone and were air-dried at room temperature for 24 h.

2.2

Characterization of microcapsules

Scanning electronic microscopy (SEM, Zeiss, Gemini) was used to characterize the morphology of the microcapsules. Based on the SEM images, the microcapsule size and shell thickness were measured (Imaging software Fiji, Image J 1.47v). Microcapsule size was measured by counting 150-200 microcapsules, while measurement of shell thickness was based on counting 10 shell images.

Fourier-transform infrared spectroscopy (FTIR, PerkinElmer, Spectrum Two) was applied to confirm successful encapsulation of the healing liquid in the microcapsules. The amount of encapsulated healing liquid was tested by thermogravimetric analysis (TGA, Mettler Toledo, TAG/DSC2) and an acetone washing-based method . For the TGA test, microcapsule samples were heated from 20 °C to 500 °C with a heating rate of 10 °C min ⁻¹ . For the acetone washing method, a known amount of microcapsules was squashed and washed using acetone to remove the self-healing liquid. The remaining microcapsule shells were weighted to calculate the amount of healing liquid in the samples.

2.3

Preparation of self-healing composites

To prepare the microcapsule-enriched self-healing resin-based composites, we manually mixed the microcapsules, initiator (benzoyl peroxide, BPO, ACROS, New Jersey, USA) and the commercially available flowable composite (Clearfil Majesty™ ES Flow, Kuraray, Japan) and applied the mixture into rectangular molds (polydimethylsiloxane, PDMS) in size 12.5 mm × 5 mm × 2.5 mm. Various groups with different concentrations of initiator (0.5 wt%, 1.0 wt%, 2.0 wt%), microcapsules (5 wt%, 10 wt% and 15 wt%) and microcapsule size (small, medium and large) were prepared (see Table 1 for composition of various experimental groups). Composites were cured for 20 s (both sides) with a LED polymerization unit (Bluephase 16i, Ivoclar Vivadent, output >1300 mW/cm ² ). SEM analysis was performed on the healed fracture surfaces, which were fractured by a universal testing machine (AMETEK, LLOYD, LS5) and tightly put back together with rubber bands (Ormco Corporation, Mexico) for 72 h for healing at room temperature (∼21 °C).

2.4

Fracture toughness and self-healing efficiency

Fracture toughness was tested by the single-edge V-notched method. In each sample, a notch with a depth of 1/3 thickness and width 0.7 mm, was prepared at the center of one side with a customized cutting machine. Then, a pre-crack was produced by sliding a sharp razor blade (10 times) in the bottom of the notch. All samples ( n = 8 per experimental group) were stored at room temperature in deionized water for 24 h before testing and were subjected to three-point bending tests using a universal testing machine (AMETEK, LLOYD, LS5). Fracture toughness K _IC was calculated with the formulas below .

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