Mechanically Strong and Highly Stiff Supramolecular Polymer Composites Repairable at Ambient Conditions

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2020Mechanically Strong and Highly Stiff Supramolecular Polymer Composites Repairable at Ambient Conditions Jingjing Zhu, George Y. Chen, Li Yu, Haolan Xu, Xiaokong Liu and Junqi Sun Jingjing Zhu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , George Y. Chen Laser Physics and Photonic Devices Laboratories, School of Engineering, University of South Australia, Mawson Lakes, South Australia 5095 Google Scholar More articles by this author , Li Yu College of Health Science and Environmental Engineering, Shenzhen Technology University, Shenzhen 518118 Google Scholar More articles by this author , Haolan Xu Future Industries Institute, University of South Australia, Mawson Lakes, South Australia 5095 Google Scholar More articles by this author , Xiaokong Liu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Junqi Sun State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.201900118 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It is a formidable challenge to fabricate healable polymeric materials with high mechanical strength and stiffness due to the highly suppressed diffusion of their polymer chains. Herein, a high-strength, highly stiff, and repairable/healable supramolecular polymer composite was fabricated by complexing poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) in aqueous solutions, followed by molding into desired shapes. Exquisitely tuning the electrostatic and H-bonding interactions between PAA and PAH led to associative phase-separation and in situ formation of nanostructures in the resultant PAA–PAH composites. The H-bonded assembly of PAA–PAH complexes existed as nanospheres were dispersed homogeneously in the continuous phase as an electrostatic assembly of PAA–PAH complexes. Such a structural feature endowed the PAA–PAH copolymer with a double-cross-linked structure, enabling significant reinforcement of the material.a The PAA–PAH composites exhibited a tensile strength and an elastic modulus as high as ∼ 67 MPa and ∼ 2.0 GPa, respectively. Due to the benefits from the reconstruction of the complexes, such as reversible electrostatic interactions and H-bonds between PAA and PAH, the PAA–PAH composite could be repaired/healed readily under ambient conditions (25 °C, 40% humidity) by using the liquid-like form of the PAA–PAH complexes (i.e., coacervate). The healing strategy reported here provides a supplementary method for easy repair or healing of high-strength and stiff supramolecular polymer materials. Download figure Download PowerPoint Introduction The development of high-strength and stiff polymeric materials is always a crucial goal in polymer research, intended to replace traditional metal and ceramic materials with lightweight and cost-effective plastics for engineering applications. Meanwhile, the exploration of simple strategies for repairing/healing high-strength polymeric materials is essential for the restoration of their mechanical performance and function when the materials undergo inevitable damages. In the past two decades, healable polymeric materials that could self-repair damages have been developed extensively based on the discovery of pre-embedded reactive healing agents1–5 (i.e., extrinsic healing) or reversible covalent/noncovalent bonds6–14 (i.e., intrinsic healing). Extrinsic healing materials exhibit advantages, in that they are capable of autonomously repairing damages. However, such materials rely on the healing agents pre-embedded into the deliberately designed capsule-based or vascular structures, and thus, only allow a limited degree of damage–repair cycles at a given position.15–18 Comparatively, the intrinsic healing materials do not require the incorporation of healing agents, thereby, exhibit superior processability and the ability to repair local damages several times.15–19 There are three prerequisites for the intrinsic healing process: (1) intimate contact between the fractured interfaces, (2) diffusion of the polymer chains across the fractured interfaces, and (3) reconstruction of the reversible covalent/noncovalent bonds to repair the fracture. Therefore, manual/external interventions are always required to meet these three intrinsic healing prerequisites, such as elevating temperature,7–9,20–22 irradiating light,10–12,23,24swelling/softening the material by solvents,22,25–30 applying pressure,30–32 and others. Even so, intrinsic healing materials, capable of healing under ambient conditions are generally soft and deformable and mostly in the form of hydrogels or elastomers that feature high polymer chain mobility/flexibility.6,23,25,33–42 Unfortunately, it is a formidable challenge to fabricate high-strength and stiff polymeric materials that could heal damages intrinsically under ambient conditions. This is because the diffusion of the polymer chains in rigid materials is hindered significantly, and the rigidity is not favorable for the intimate contact between fractured interfaces. Notably, Aida and co-workers32 recently developed a high-strength and intrinsically healable material by cross-linking short polymer chains via a high density of nonlinear H-bonds. This material, with a glass transition temperature (Tg) of 27 °C, exhibited a tensile strength of ∼ 45 MPa and an elastic modulus of 1.4 GPa at a temperature of 21 °C, which demonstrated the ever-highest mechanical performance among the previously reported room-temperature healable materials. A fractured sample from this fabrication could effectively repair at 24 °C by application of constant external pressure of 1.0 MPa (i.e., ∼ 10 atm) for over 6 h. In principle, it is unattainable to implement the intrinsic healing function for highly rigid materials without manual/external interventions, even if the material comprises high-density, reversible, covalent/noncovalent bonds. Accordingly, it is of high significance to develop simple healing strategies that could take place under ambient conditions to repair highly rigid materials, based on reversible covalent/noncovalent bonds. Herein, we report a new strategy to simply repair/heal highly rigid PAA–PAH composite material under ambient conditions by employing an intermediary healing agent with the same composition as the target material based on their reversible noncovalent interactions. By exquisitely tuning the intermolecular interaction modes, we complexed the commercially available PAA and PAH, which resulted in supramolecular composites with the same composition but in two distinct states. One state was the rigid and glass-like PAA–PAH composite with a high Tg of 147 °C, reinforced via in situ assemblies of phase-separated nanostructures (Scheme 1a–i), which exhibited a tensile strength (σ) and an elastic modulus (E) as high as ∼ 67 MPa and ∼ 2.0 GPa, respectively. The other state was the viscoelastic, liquid-like PAA–PAH complex (i.e., coacervate) (Scheme 1a–iii), which functioned as an intermediary healing agent able to repair the fractured glass-like PAA–PAH composite via electrostatic and H-bonding interactions (Scheme 1b–i, ii, and iii). Accordingly, the rigid PAA–PAH composite could be repaired completely with full recovery of its high mechanical performance after solidification of the PAA–PAH coacervate in ambient conditions without the assistance of external stimuli (Scheme 1b–i, ii, and iii). Complexation of polymers in solutions43–51 or at interfaces52–59 (e.g., layer-by-layer [LbL] assembly) based on noncovalent interactions has been studied extensively for decades. Recently, polymer complexation has become a simple emerging method to fabricate bulk supramolecular composite materials with tailored compositions and functions.25,26,60–65 Due to the existence of high-density reversible noncovalent interactions, materials derived from polymer complexes are potentially healable, while implementation of the healing function has to meet the three prerequisites described earlier. Previous reports from our lab and labs of other groups demonstrated that materials of polymer complexes, in the hydrated (i.e., hydrogel)25,26,65 or elastomeric63 state, exhibited intrinsic healing behaviors by the assistance of water or salt swelling. Notably, these healable hydrated or elastic polymer complex–based materials are all soft and deformable with tensile strengths lower than 10 MPa.25,26,63,65 In our present work, we demonstrate that the highly rigid PAA–PAH supramolecular composite (σ = 67 MPa and E = 2.0 GPa) could be repaired by simply employing the PAA–PAH coacervate as the healing agent under ambient conditions (25 oC; 40% relative humidity). The healing strategy reported here provides a supplementary method for a simple repair of highly rigid materials that comprise noncovalently bonded components. Scheme 1 | Preparation and healing of the high-strength and stiff PAA–PAH composite. (a) Demonstration of the consistency of a PAA–PAH composite and a PAA–PAH coacervate. (b) Schematic illustration of healing of the PAA–PAH composite via the PAA–PAH coacervate. Download figure Download PowerPoint Experimental Methods Materials PAA, PAH, poly(ethylene oxide (PEO), deuterium chloride (DCl), and deuterated water (D2O) were purchased from Sigma-Aldrich, Shanghai, China. HCl (37%), NaOH, and NaCl were purchased from Beijing Chemical Reagents Co., China. The pH values of the solutions were adjusted with either 1 M HCl or 1 M NaOH. Deionized water was used for all experiments. Preparation of the PAA−PAH composites The PAA–PAH composites with different compositions were prepared via the procedure illustrated in Scheme 1a and Supporting Information Figure S1. Equal volumes of PAA (Mw of 450,000, pH of 3.0) and PAH (Mw of 17,500, pH of 10.0) were prepared as aqueous solutions. Predesigned concentrations were mixed under stirring, using the following PAA:PAH feed monomer molar ratios: (1.0∶1.5), (1.0∶1.0), (1.5∶1.0), and (2.2∶1.0), corresponding to the following PAA:PAH concentrations: (0.04 M and 0.06 M), (0.06 M and 0.06 M), (0.06 M and 0.04 M), (0.09 M and 0.04 M), respectively. The precipitates of PAA-PAH complexes formed instantaneously under ambient conditions (25 oC; 40% relative humidity), and resultant precipitates were collected by centrifugation, followed by compression molding via two pieces of glass slides under a pressure of ∼ 30 kPa. Subsequently, the samples were allowed to dry under the same conditions until their appearances transitioned from white precipitates to homogeneous materials to yield the final PAA–PAH composites. Preparation of the PAA–PAH coacervate Equal volumes of PAA (Mw of 250,000, 0.06 M, pH of 7.4) and PAH (Mw of 17,500, 0.04 M, pH of 7.4) aqueous solutions that contain 2.5 M NaCl were mixed under stirring at the PAA:PAH feed monomer molar ratio of 1.5∶1.0. The coacervate formed instantly under our established ambient conditions. After that, the PAA–PAH coacervate was precipitated from the mixed aqueous solution and then collected by centrifugation. Preparation of the PAA–PEO elastomer The PAA–PEO elastomer was prepared according to our previous report.63 Briefly, aqueous solutions of PAA (Mw of 450,000, 4 mg mL−1, pH 2.5) and PEO (Mw of 600,000, 4 mg mL−1, pH 2.5) were mixed under stirring at the PAA:PEO volume ratio of 1.6∶1.0, the feed monomer molar ratio between PAA and PEO is 1.0∶1.0. The precipitates of PAA–PEO complexes was formed instantaneously under our established ambient conditions at room temperature, and the resultant precipitate was collected by centrifugation, followed by compression molding for 2 days via two pieces of glass slides at a pressure of ∼ 15 kPa. Subsequently, the sample was dried under ambient conditions, and finally, a transparent rubber-like sheet of PAA–PEO elastomer was obtained. Polymer characterization 1H NMR spectra were recorded on a Bruker 500 MHz spectrometer (Shanghai, China). X-Ray photoelectron spectroscopy (XPS) measurements on the cross-sections of the samples were performed on a Thermo ESCALAB 250 X-ray photoelectron spectrometer (Thermo Scientific, Waltham, MA, USA) using Al/Kα (1486.6 eV) monochromatic X-ray source. The take-off angle of the photoelectron was fixed at 90° with respect to the sample plane. Transmission electron microscopy (TEM) observations were made on a JEM-2100F microscope (JEOL Ltd., Beijing, China), operating at an acceleration voltage of 200 kV. To prepare the samples for TEM measurements, the PAA–PAH composites were embedded in an epoxy resin and sliced into ultrathin sections. A drop of an aqueous solution of uranyl acetate (1 wt%) was added onto the prepared samples and left to reach equilibrium for 1 min, followed by removal of the excess solution via filter paper. Scanning electron microscopy (SEM) images were recorded under vacuum using a Hitachi SU8020 SEM (Tokyo, Japan). The tensile tests were implemented on a universal testing machine (Shimadzu AG-I 1 kN; Chenhua, Shanghai, China) at the stretching speed of 10 mm min−1. The compression tests were conducted on a universal testing machine (Instron 5944 2 kN; PA, USA) at the compressing speed of 5 mm min−1. Dynamic mechanical analysis (DMA) was carried out on an RSA-G2 dynamic mechanical analyzer (TA Instruments, New Castle, DE, USA) at a frequency of 1 Hz in the three-point bending mode by heating the samples from 30 to 240 °C with a heating rate of 3 °C min−1. The thermal gravimetric analysis (TGA) measurements were conducted on a Q500 thermogravimetric analyzer (TA Instruments) under a nitrogen atmosphere at a heating rate of 10 °C min−1. Digital photographs were captured using a Canon SX40 HS camera (Canon China Co. Ltd, Hong Kong). Rheological measurements were carried out on an AR2000 controlled-stress rheometer (TA Instrument) equipped with 25 mm stainless-steel parallel plate geometry at 25 °C. Oscillatory frequency sweep data were collected in the range of 0.1–100 rad s−1, fixing the strain amplitude at 0.1%. Results and Discussion Preparation and composition of the PAA–PAH composites A scheme of the PAA–PAH composite preparation process is shown in Supporting Information Figure S1. After mixing aqueous solutions of the PAA and PAH at varying predesigned concentrations under stirring, precipitates of the complexes formed immediately due to electrostatic and H-bonding interactions between the polymers.66,67 Then, the precipitates were collected via centrifugation and further processed via compression molding between two glass slides under the pressure of ∼ 30 kPa. The PAA–PAH composite material was finally obtained after the sample was dried/dehydrated under the ambient condition (Scheme 1a–i). Four PAA–PAH composites with different compositions were fabricated based on the PAA–PAH complexes prepared from different PAA:PAH feed monomer molar ratios, as described in Materials and Methods section in Supporting Information Figure S2, and characterized by 1H NMR analysis ( Supporting Information Figure S3). The monomer molar ratios between PAA and PAH in the resultant PAA–PAH composites were finally measured to be 1.0:1.0, 1.3:1.0, 1.6:1.0 and 2.5:1.0, respectively. Accordingly, we denoted the as-fabricated PAA–PAH composites as PAA1.0–PAH1.0, PAA1.3–PAH1.0, PAA1.6–PAH1.0, and PAA2.5–PAH1.0, respectively. These four PAA–PAH composites were prepared from the PAA-PAH mixture solutions that possessed different pH values, viz, 7.6, 3.7, 3.1 and 2.8, respectively. Notably, the pH of the PAA–PAH mixtures determined their degrees of ionization of the carboxylic acid groups of PAA and the amino groups of PAH, which dictated further the interaction modality between the polymers of the PAA–PAH complexes and the composites. Figure 1a–1d shows photographs of the as-fabricated PAA–PAH composites and the pH values from which the composites were prepared. The PAA1.0–PAH1.0, PAA1.3–PAH1.0, and PAA1.6–PAH1.0 composites were glass-like materials (Figure 1a–1c), whereas the PAA2.5–PAH1.0 composite with the highest PAA fraction appeared as a white and opaque material (Figure 1d). Note that the pressure applied for the compression molding was crucial for the acquisition of the homogeneous PAA1.0–PAH1.0, PAA1.3–PAH1.0, and PAA1.6–PAH1.0 composites, because pressure facilitated the interdiffusion and reorganization of polymer chains; meanwhile, without applied pressure, dry white precipitates were obtained instead of the transparent or semitransparent PAA–PAH composite achieved under pressure. We carried out a proper investigation of the intermolecular interactions of the four PAA–PAH composites samples by XPS measurements. Figure 1e–h shows the high-resolution C 1s XPS spectra, and Figure 1i–l shows the high-resolution N 1s XPS spectra of the four samples. The displayed C 1s peaks are deconvoluted into four components assigned to the C–C/C–H (284.6 eV), C–O/C–N (285.8 eV), –COO− (287.8 eV), and –COOH (288.5 eV) groups,68,69 and the N 1s peaks are deconvoluted into two components assigned to the –NH3+ (401.1 eV) and –NH2 (399.3 eV) groups.70,71 Accordingly, the molar ratios between the –COO− and –COOH groups and that between the –NH3+ and –NH2 groups in each PAA–PAH composite, indicated in Figure 1e–l, were calculated using the area ratios between the corresponding subpeaks. We found that the –COOH: –COO¯ and –NH3+: –NH2 molar ratios increased as the proportion of PAA increased in the PAA–PAH composites. This result is because the PAA–PAH composites with higher PAA fractions originate from the PAA–PAH mixture solutions with lower pH, where carboxylate groups tend to be protonated to the carboxyl groups (–COOH) and the amino groups tend to be ionized to form polyatomic ions (NH3+). Subsequently, the molar proportions of the –COO−, –COOH, and–NH3+,–NH2 groups in each PAA–PAH composite could be calculated based on the determined –COOH:–COO− and –NH3+:–NH2 molar ratios, and the measured monomer molar ratios between PAA and PAH. As shown in Figure 2, the total molar proportions of charged (i.e., –COO− and –NH3+) groups in the PAA1.0–PAH1.0 and PAA1.3–PAH1.0 composites are 83.3% and 75.0%, respectively, indicating that the charged groups were the majority, compared with the neutral (i.e., –COOH and –NH2) groups in these two materials. Relatively, the PAA1.6–PAH1.0 and PAA2.5–PAH1.0 composites comprised comparable amounts of charged (i.e., –COO− and –NH3+) and neutral (i.e., –COOH and –NH2) groups. Thus, the molar proportions of the charged versus neutral groups were 59.8% and 40.2% in the PAA1.6–PAH1.0 composite and 49.6% and 50.4% in the PAA2.5–PAH1.0 composite, respectively. Therefore, the related contents of the four types of functional groups exhibited notable differences in the different PAA–PAH composites, which was likely to result in different interaction modes between the polymers. Three main types of supramolecular interactions are likely to have occurred when complexing PAA and PAH, as follows: (1) The electrostatic interactions between the charged –COO− and –NH3+ groups. (2) H-Bonding interactions between the neutral –COOH and –NH2 groups. (3) H-bonding interactions between the neutral –COOH groups. Hence, we envisioned that the electrostatically charged components of the cross-link and the neutral components of the H-bonds could be incompatible in the PAA–PAH composites, resembling the incompatibility between the charged and neutral segments observed in block copolymers.72 Accordingly, we predicted that the phase separation might have occurred in the PAA–PAH composite when an appropriate ratio between the charged and the neutral components was reached. Figure 1 | Photos and XPS measurements of the PAA–PAH composites. (a–d) Photos of the different PAA–PAH composites. The sizes of the samples shown in (a–c) are 1.5 × 1.5 cm2, and the size of the sample shown in (d) is 1.5 × 0.8 cm2. (e–l) High-resolution C 1s and N 1s XPS spectra measured from the cross-sections of the PAA1.0–PAH1.0 (e and i), PAA1.3–PAH1.0 (f and j), PAA1.6–PAH1.0 (g and k), and PAA2.5–PAH1.0 (h and l) composites. Download figure Download PowerPoint Microstructures of the PAA–PAH composites The microstructures of the as-fabricated four PAA–PAH composites were investigated further by TEM and SEM. Figure 3a–c shows the TEM images of the sliced PAA1.0–PAH1.0, PAA1.3–PAH1.0, and PAA1.6–PAH1.0 samples stained by uranyl acetate. The PAA1.0–PAH1.0 and PAA1.3–PAH1.0 composites exhibit typical morphologies of amorphous polymers with featureless structures (Figure 3a and 3b) indicating homogeneous constitutions of the composites. Intriguingly, the PAA1.6–PAH1.0 composite exhibited a distinct, well-defined phase-separated structure composed of brighter, discrete nanospheres and a continuous phase of darker, amorphous polymers (Figure 3c and Supporting Information Figure S4). The nanospheres were quite uniform with narrow size distribution and displayed an average diameter of 19.6 ± 4.2 nm ( Supporting Information Figure S5). It was apparent that the uranyl ions preferentially bind to the charged –COO− groups,73,74 and the content of the charged groups (59.8%) was higher than that of the neutral groups (40.2%) in the PAA1.6–PAH1.0 composite. Accordingly, we deduced that the continuous phase shown in Figure 3c was composed mainly of the electrostatically cross-linked charged (i.e., –COO− and –NH3+) components, whereas the phase of the nanospheres was composed mainly of H-bonded neutral (i.e., –COOH and –NH2) components (Figure 3d). Since the amount of –COOH groups was much higher than the –NH2 groups (32.9% vs 7.3%), we deduced that the phase of the nanospheres consisted predominantly of PAA, cross-linked via the H-bonded dimers of the –COOH groups, as shown in Figure 3d, which also reveals that the PAA1.6–PAH1.0 composite exhibits a double-cross-linked polymer network in which the electrostatically cross-linked continuous phase provided further cross-linking via the nanospheres. Figure 3e shows the SEM image of the cross-section of the PAA2.5–PAH1.0 composite. Instead of being a compact and continuous material, the PAA2.5–PAH1.0 composite was formed by the accretion of polymer particles. Due to such a particulate structure, it scattered light significantly; hence, the PAA2.5–PAH1.0 composite appeared opaque and white (Figure 1d). The fact that PAA is highly excess in this latter composite, it resulted in insufficient cross-linking between the PAA and PAH polymer chains. Thus, the complexation between PAA and PAH resulted in precipitates with the morphology of powders ( Supporting Information Figure S2h), which induced further the ultimate particulate structure of the PAA2.5–PAH1.0 composite. Figure 3 | Microstructures of the PAA–PAH composites. (a–c) TEM images of the sliced PAA1.0–PAH1.0 (a), PAA1.3–PAH1.0 (b), and PAA1.6–PAH1.0 (c) composites stained via uranyl acetate. (d) Schematic illustration of the phase-separated nanostructures and the interaction modes between PAA and PAH in the PAA1.6–PAH1.0 composite. (e) SEM image of the cross-section of the PAA2.5–PAH1.0 composite. Download figure Download PowerPoint Mechanical properties of the PAA–PAH composites The mechanical properties of the four PAA–PAH composites were measured based on tensile tests; the results obtained are summarized in Table 1. Figure 4a shows the typical stress–strain curves of the four different compositions of the PAA–PAH composites. We found that all the PAA–PAH composites were quite stiff with elastic modulus > 1.2 GPa. In particular, the PAA1.6–PAH1.0 composite exhibited optimum mechanical performance, its tensile strength, elastic modulus, and toughness were 67.2 ± 2.4 MPa, 2.0 ± 0.40 GPa, and 1.21 ± 0.18 MJ m−3, respectively. In contrast, the PAA2.5–PAH1.0 composite exhibited the lowest mechanical strength (32.6 ± 4.2 MPa) and toughness (0.47 ± 0.17 MJ m−3), due to the noncontinuous particulate structure. The exceptionally superior mechanical performance of the PAA1.6–PAH1.0 composite benefitted from the in situ formed phase-separated nanostructure. Such a structure featured a double-cross-linked network (Figure 3d), which, significantly, reinforced the PAA1.6–PAH1.0 composite with tensile strength, elastic modulus, and toughness. Importantly, the in situ formed phase-separated structure gave rise to superior interfacial compatibility between the nanospheres and the continuous phase of the PAA1.6–PAH1.0 composite, compared with conventional strategies used to reinforce polymer composites via application of inorganic nanofillers.75–77 Figure 4 | Mechanical properties of the PAA–PAH composites. (a) Typical stress–strain curves of the PAA–PAH composites with different compositions. (b) Comparison of the mechanical performances, in terms of tensile stress and strain at break, between the PAA1.6–PAH1.0 composite and some commonly used plastics.78 PS, ABS, PE, PF, PMMA, and CAB represent polystyrene, acrylonitrile butadiene styrene, polyester, phenol-formaldehyde, poly(methyl methacrylate), and cellulose acetate butyrate acrylics, respectively. Download figure Download PowerPoint Table 1 | Summary of the Mechanical Properties of the PAA–PAH Composites at Varying Compositions PAA–PAH Composites Tensile Strength (MPa) Strain at Break (%) Elastic Modulus (GPa) Toughness (MJ m−3) PAA1.0–PAH1.0 47.2 ± 3.4 2.4 ± 0.24 1.2 ± 0.10 0.48 ± 0.08 PAA1.3–PAH1.0 53.8 ± 5.0 2.5 ± 0.36 1.3 ± 0.38 0.58 ± 0.11 PAA1.6–PAH1.0 67.2 ± 2.4 3.9 ± 0.81 2.0 ± 0.40 1.21 ± 0.18 PAA2.5–PAH1.0 32.6 ± 4.2 2.8 ± 0.67 1.5 ± 0.13 0.47 ± 0.17 Figure 2 | Relative molar proportions of the –COO−, –NH3+, –COOH, and –NH2 groups in the PAA–PAH composites to yield different compositions. Download figure Download PowerPoint Figure 4b shows the comparison between the mechanical performance, in terms of tensile strength and strain at break, of the PAA1.6–PAH1.0 composite and some widely used engineering plastics.78 For instance, the mechanical performance of the PAA1.6–PAH1.0 composite is comparable to that of poly(methylmethacrylate) (PMMA).78 The as-fabricated PAA–PAH composites were quite stable under the ambient conditions, and almost the same mechanical performances were attained after the samples were stored for different lengths of times. Even after incubation at relatively high humidity of 85% for 5 days, the mechanical performance of the PAA1.6–PAH1.0 composite was still well maintained ( Supporting Information Figure S6). The DMA indicated that the PAA1.6–PAH1.0 composite exhibited a Tg of ∼ 147 °C, and a TGA revealed that the decomposition of the PAA1.6–PAH1.0 composite occurred at temperature > 400 °C ( Supporting Information Figure S7). Repair of the PAA–PAH composite via the PAA–PAH coacervate Based on the reversible electrostatic and H-bonding interactions between PAA and PAH, the high-strength and stiff PAA1.6–PAH1.0 composite could be repaired by simply using the liquid-like form of the PAA–PAH complexes as the healing agent (Scheme 1b). The electrostatic interactions between PAA and PAH were suppressed significantly using salts (e.g., NaCl) due to the charge-screening effect,25,46,51 which is an effective way to tailor the properties of the PAA–PAH complexes. When aqueous solutions of PAA and PAH were mixed with 2.5 M NaCl at the feed PAA:PAH monomer molar ratio of 1.5∶1, PAA–PAH complexes formed existed in a viscoelastic liquid-like state (i.e., PAA–PAH coacervate) and precipitated from solution (Scheme 1a–iii). The mass content of NaCl in the dried PAA–PAH coacervate was measured to be ∼ 6.5% ( Supporting Information Figure S8). The rheological measurement revealed that the PAA–PAH coacervate exhibited a transition from a liquid-like to a solid-like state at the oscillation frequency of 2.5 rad s−1 ( Supporting Information Figure S9), corresponding to a characteristic chain relaxation time of 0.4 s. Since this timescale is close to our observation time, we presumed that the as-prepared PAA–PAH coacervate existed in a viscoelastic liquid-like st