Self-Healing Polymer Composites Based on Hydrogen Bond Reinforced with Graphene Oxide

Yue Chen,¹ Ying Wang,¹ Tai Su,¹ Jiayi Chen,¹ Chao Zhang,¹ Xiaoxing Lai,¹ Dawei Jiang,^1,2,* Zijian Wu,³ Caiying Sun,¹ Bin Li^1,2,* and Zhanhu Guo^4,*

¹ College of Science, Northeast Forestry University, Harbin 150040, China

² Post-doctoral Mobile Research Station of Forestry Engineering, Northeast Forestry University, Harbin 150040, China

³ Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150040, China

⁴ Integrated Composites Laboratory (ICL), Department of Chemical Engineering, University of Tennessee, Knoxville TN 37996, USA

*E-mail: daweijiang@nefu.edu.cn (D. Jiang); libinzh62@163.com (B. Li); zguo10@utk.edu (Z. Guo)

Abstract

A self-healing supramolecular polymer composite (LP-GO) is designed and prepared via incorporation of graphene oxide (GO) to hyperbranched polymer by hydrogen-bonding interactions. The polymer matrix based on amino-terminated hyperbranched polymer is synthesized by dimer acid and diethylene triamine, while GO is prepared by the modified Hummers method. Infrared spectroscopy (FTIR), thermo gravimetric testing (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) is applied to characterize GO. Stressstrain test is utilized to characterize the obtained self-healing property of LP-GO. It is found that just a small amount of GO (up to 3 wt %) is needed to achieve a dramatic improvement in the mechanical properties, and self-healing efficiency of the polymer composites. After healing at 60 ^oC for 1 h, the addition of GO even restores the self-healing efficiency to 100% of its original tensile strength. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when being broken or cut, can be simply healed by contacting fractured areas again to self-heal at suitable temperature. Building on the unique self-healing properties, the simplicity of synthesis and the availability from renewable resources, etc., LP-GO bodes well for broader applications in the near future.

Table of Content

Modified graphene oxide was prepared for improving self-healing polymer composites with excellent self-healing properties and mechanical properties.

 

 

Keywords: Self-healing; Polymer composite; Graphene oxide; Hydrogen bond

Introduction

The organic polymers have found a wide utilization in diverse fields, such as textiles, packaging, automobiles, etc., owing to the characteristics of low cost and easy processing making them to be the basic materials in modern life.^1-3 In recent years, as significant efforts have been emphasized on the exploitation of novel polymers with enhanced functionality, including biodegradability,^4,5semiconductivity,electrical conductivity and self-healing capacity,^6-8 the usage of organic polymers has been developed rapidly, in which cross-linked polymers, such as epoxy resin and phenolic resin,have strong mechanical properties but relatively fragileleading to irreparablefracture or breakage under a certain amount of external force during processing and employing, so that applications of this kind of polymers are often subjected to limitation.^9,10  In view of thatself-heal can effectively extend the life of materials, realize recycling and reusing, and reduce a string of security issues and environmental problems invited by material failure, the research on self-healing polymers is of great significance.^11-15  The self-healing polymers can be divided into two categories, according to the different preparation methods. One is extraneous self-healing polymers, that is, the materials can realize the restoration of mechanical properties and integrity without external heating and other stimulation through embedding healing reagents into substrate.^16,17   White et al.¹⁸ first proposed microcapsule self-healing technology which has been widely utilized and developed. Thecyclopentinediocene was wrapped in a micro-capsule made of the urea formaldehyde resinbefore it was dispersed in diethylenetriamine cured epoxy resin oligomer till in touch with the Grubbs catalyst. When cracks were generated in the material, the microcapsules were broken, and the cyclopentadiene dimer, in the meantime, rapidly infiltrated into the cracks due to capillary siphonage. When it came into contact with Grubbs catalyst, the active ring-opening polymerization reaction occurred, and a highly cross-linked polymer network was createdin large quantity to achieve the purpose of healing. Only in this way does this kind of healing technology need additional healing agents, realize anything but single self-healing, and the release of healing agents forms new voids which will turn into new defects of the materials. The other is intrinsic self-healing polymersbased on reversible bonds including dynamic covalent or non-covalent bonds. Unlike the extraneous self-healing polymers, numerous dynamic chemical bonds or weak interactions are required to self-heal instead of additional healing reagents.^19,20 Theoretically, it is the main characteristic that intrinsic self-healing polymers have the ability tobring about multiple self-heal of materials, although its application is frequently restricted by thedemand of external conditions such as heating, ^21,22lighting, ^23,24 andelectromagnetic effects²⁵ to self-heal.At present, the supramolecularpolymers based on hydrogen bonding networks provide a promising access towards spontaneous self-healing polymers for the swift self-heal can be accomplished at low temperature.²⁶Hence, Leibler’s²⁷group reported a self-healing polymer composed of multifunctional fatty acids and urea compounds, as well as associated by molecular hydrogen bonding.If the material suffers damage, the hydrogen bondstend to break first owing to the weak strength so that the non-associated groups will appear in the fracturedarea, unless new hydrogen bond association can be producedagain at room temperature after bringing the two broken pieces gently back into contact. However, the addition of an 11%w/w plasticizer, necessary to lower the high glass transition temperature ofobtained self-healing polymers, restrainsthe movement of the polymer chains bearing hydrogen bonds, which always results in a slow mechanical healing speed (typically over one day for complete healing). To address this problem, Wang et al²⁸ synthesized a self-healing nanocomposite by means of using modified graphene oxide (GO) that was treated with thionyl chloride as macroscopic crosslinking agents.When the weight percentage of modified GO changed from 1% to 4%, the tensile strength changed from 0.26 MPa to 0.8 MPa.As the above data, the relatively poor mechanical properties make nanocompositesunsuitable for practical applications in high strength domains. Similarly, Luan et al²⁹introducedGO modified by adding2,4-tolylene diisocyanate and hydroxyl terminated hyperbranched polyester into supramolecular polymer based on amino-terminated hyperbranched polymer which was synthesized from the hydroxyl-terminated hyperbranched polyester, succinic anhydride, diphenyl azidophosphate, and diethylenetriamine as the raw materials, and then obtained the self-healing elastomer via filling 1 wt% or 2 wt% modified GO, respectively. Nevertheless, when the content of modified GO was 2 wt%, the maximum mechanical strength was less than 0.6 MPa. In these two examples,though the healing speed of the supramolecular polymers are increasedand the glass transition temperaturedecreases, the small fraction of grafted groups on the surfaceof GO lowers the cross-linking degree of modified GO and polymers, and reduces the mechanical properties, thusconstrains the high-strength applications as in sports equipments, ballistic armors, and the aerospace industry. Hence, developing materials that possess parameters including adequate mechanical strength and quick self-healing speed, still faces daunting challenge.

GOhas excellent properties, such as high mechanical strength, high surface area and abundant functional groups,^30-32 for its highmechanical properties and multiple oxygen-containing groups, attractive to add GO into the self-healing materials. By this way, GO can better integrate with the organic substrate and produce functional materials with excellent mechanical properties.^33,34 In this paper, the modified Hummers method³⁵ is implemented to prepare GO, not only has more hydroxyl groups and carboxyl groups but contains a certain amount of epoxy groups. The incorporation of lowcontent ofreinforcement into the polymer system exerts a negligible effect on the density of hydrogen bonds or the movement of molecular chain containing the hydrogen bonds in that a spot of GO,even though, could significantly improvethe mechanical property of self-healing polymers.

To investigate the influence of GO, as a reinforcement, on the mechanical properties in  amino-terminated hyperbranched polymers, pure self-healing polymers(LP) andself-healing polymerscontaining different weight percentage of 3 wt%, 5 wt%, 7 wt% and 9 wt% of GO, termed as LP, LP-3, LP-5, LP-7 and LP-9, were synthesized.Through the mechanical property test, it is discovered that the mechanical property of the self-healing polymers gradually decreases with the increasing proportion of GO, and the mechanical strength is much higher than that of LPoverall. The optimum addition ratio of the reinforcementis 3 wt% and the tensile strength is up to 2.95 MPa.From the healing cycle test under various healing conditions, optimal healing condition of LP-3 can be obtained by contrast with the healing efficiency.After two broken pieces contact for 3hat 30^oC, the LP-3 sample heals to 50% of its original tensile strength, while the span of time will be shortened to 2 h when the temperature is 40^oC. Mechanical healing can be completed in 6h at 50 ^oC while 1h is enough at 60 ^oC, means shorter healing timematches with higher healing temperature and lower healing temperature acquires longer healing time. Even if 10 fracture-healing cycles under the appropriate healing condition, the mechanical strength of LP-3 can still be restored to 100%, which verifies the self-healing polymer composites (LP-GO) have a lower descent rate of healing efficiency compared with LP, which can be attributed to the loss of hydrogen bonds sites in the composites. The thermodynamic stability of LP-3, on account of the TGA&DTG analysis, is also greatly improved.

Experimental Section

Materials

Empol 1016 high pure Dimer Acid was purchased from BASF China (80% dibasic acids, 16% polybasic acids, 4% monobasic acids). The AR diethylenetriamine was available from Sigma Aldrich. Flake graphite (FG) was obtained from Qingdao Graphite Products Co., China. Chloroform,methyl alcohol, ethanol, and urea were supplied by Tianjin Tianli Fine chemical Co., Ltd., China. All the chemicals can be applied directly without further processing.

 

Preparationof materials

Preparation of GO

Thesynthesis principle of GO, abiding by the modified Hummer’s methods, is described in detail in the following reaction equations.The preparation process is divided into three stages: low temperature intercalation, medium temperature oxidation and high temperature hydrolysis stripping.In the intercalation stage, since concentrated sulfuric acid as intercalation agent cannot spontaneously react with FG, the chemical oxidation method should be applied - as the first react equation shows that KMnO₄ reacts with cold concentrated sulfuric acid at low temperature to generate the oxidative active component Mn₂O₇ - to slightly oxidize the edge and defects of FG and increase the interlayer spacing for intercalation of sulfuric acid and sulfate ion.In the oxidation stage, with the ascension of temperature, the strong oxidation effect of the oxidant gives rise to a large amount of oxidation of FG, which not only forms oxygen-containing functional groups on the surface, but continues increasing the interlayer spacing, as described in the second reaction equation.In the stripping phase, as the appropriate content of hydrogen peroxide is added to the reactants, the unreacted oxidant is reduced to colorless soluble MnSO₄, demonstrated in the third equation, and then GO was stripped by pickling, washing and ultrasound.Specifically, 1.0 g of FG, 10 mL and 90 mL of sulfuric acid and phosphate (1:9 by volume) were mixed with 6 g of potassium permanganate, and stirred in the 4^oC ice water bath for 1 h and at 50^oC for 24 h until a homogeneous dispersion was attained. The dispersion was put into ice water and blendeddropwisewith volume fraction 30% of hydrogen peroxide to turn itgolden. The resulted mixture was washed successivelywith volume fraction 5% of dilute hydrochloric acid solution and distilled water until the supernatant was neutral by centrifugation. The clean and sticky sendiment was sonicated for 1 h before the coarse product dried at 70 ^oC under vacuum (Fig. 1).

KMnO4+ H₂SO₄ →K₂SO₄+ Mn₂O₇ + H₂O

KMnO₄+H₂SO₄+C→MnSO₄+CO₂+SO₂+H₂O

KMnO₄+H₂SO₄+H₂O₂→MnSO₄+K₂SO₄+H₂O+O₄

Preparation of oligomers

A mixture of dimer acid (30 g)and 12.3 g of diethylenetriamine were stirred at 160^oC for 24 h under nitrogen atmosphere. Then the mixture was extracted three times by chloroform (50 mL)in the separation funnel. When the dispersion statically separated and the equilibrium was reached, the lower layer was released andthe upper layer was washed with 90 mL of the mixture methanol and water (1:2 by volume) to obtain the dendritic oligomer (Fig. 1).

Preparation of self-healing polymers

1.18 g of urea was slowly added to 5 g of oligomer and kept stirring up to 100^oC. When heating to 135^oC, increase temperature at the rate of 5^oC each hour till 160^oC. The obtained LP was then molded onto the samples for testing. For LP-3, extra 0.15g of GO wasconsidered to mix with 5 g oligomerapart from other reaction steps. Similarly, LP-5, LP-7 and LP-9 were obtained from mixing 5 g oligomer with 0.25 g, 0.35 g and 0.45 g of GO, respectively, in the same way. (Fig. 1).

Characterization

Infrared characterization of the molecular structure of natural flake graphite (FG) and GO samples were performed by means of theFourier transform infrared spectrometer (Perkin Elmer, Spectrum 400, USA)with a scan range of 400-4000 cm^-1. The contents of functional groups were conducted by XRD (Shimadzu, XRD-6100, Japan). Thermal stability was acquired from TGA&DTG (Perkin Elmer, Pyris1, USA). The mechanical tensile stress test was carried out with a WDW-2 type microcomputer controlled electronic universal testing machine (Jinan Yongke Testing Instrument Co., Ltd.). The thermal stability of LP and LP-GO was compared viausing a thermogravimetric analyzer (Perkin Elmer, Pyris 1, USA) as well. Their surface topography was performed by scanning electron microscopy (SEM) (Hitachi, SU8020).

Fig. 1. Schematic diagram of reaction mechanism and synthesis of LP-GO.

Results and discussion

Molecular structure and properties of GO

Surface groups of GO

Fig. 2ashows the infrared spectra of FG and GO. As can be seen from the curve ofFG, the characteristic absorption peaks of the functional groups of FG are relatively weak, and the infrared spectrum curve is rather smooth, compared to the curve of GO in whichthe chemical structure  can be definitely analyzed. In the vicinity of the high frequency region 3433 cm^-1, the stretching vibration assigned to -OH may come from water molecules adsorbed by GO because of the strong hygroscopicity. The absorption peaks near the 2928 cm^-1 and 2861 cm^-1 positions correspond to the antisymmetric and symmetric stretching vibrations of CH₂, respectively, and near the position of 1625 cm^-1 in the intermediate frequency region belongs to the C=O stretching vibration of carboxylic acid and carbonyl on the edge of GO. The C-O stretching vibration of the carboxyl group emerging at 1394 cm^-1and the C-O-C stretching vibration of GO surface was attributed near the position of 1117 cm^-1. In summary, the presence of these oxygen-containing groups indicates that FG has been oxidizedand these polar groups, especially the hydroxyl groups on the surface, make GO liable to form hydrogen bonds with water molecules, thereby explaining the  strong hydrophilicity ofGO

Thermal properties of GO

As shown in the Fig.2b, the thermal stability of FG is significantly higher than that of GO. FGdoes not exhibit a substantial loss of mass until above 600^oC, and before that,it shows good thermal stability, by contrast, the weight loss process of GO can be divided into three stages within the scope of the test temperature: 50~165^oC, 165~230^oC and above 230^oC. A thermogravimetric platform between 50 and 165^oCismainly due to the gasification of a little water adsorbed on the surface of GO. After 165 ^oCisthe major part of the weight loss contributed to the gasification of the bound water between the GO layers and the conversion of some reactive groups into carbon dioxideduring the heating process. Above 230 ^oC, the weight loss of the sample gradually drops to constant which boils down to the dissociation of the remaining oxygen molecules.

Molecular structure of GO

The XRD patterns of FG and GO can be found in Fig.2c. Among them, FG has a sharp, high-intensity diffraction peak at 2θ of 26.6°, which isthe characteristic diffraction peak ofFG and also demonstrates the spatial structure of FG sheets is extremely arranged neatly.

Fig. 2. a FTIR, b TGA&DTG, c XRD curve of GO and FG.

The other shows the XRD pattern of GO of which diffraction peak turns very small except a relatively strong diffraction peak at 2θ of 11.3° that indicates the structure of FG has been destroyed, the interlayer spacing is larger than that of FG and a new crystal structure has been in shape.

Microscopic topography and element content of GO

Fig.3a exhibits the high resolution SEM image of the stacked FG that agglomerates in a few places, and the multilayered structure can be observed clearly. In comparison, GO possesses a thin sheet structure, and the sheet has apparent wrinkles and edge curling as confirmed by the SEM result (Fig3b). The horizontal size of the GO nanosheets is between several hundred nanometers and several micrometers. While Fig3bcan not accurately characterize the thickness of the GO nanosheets, the thickness of the FG can be estimated from a few nanometers to several tens of nanometers from the width of the edge of the sheet and the width of the wrinkles. Hence, the samples prepared viathe modified Hummers method have topographical characteristics of GO. The bonding status of the C element existingin GOwere confirmed by XPS (Fig. 3c). According to the fitting analysis of the C element peak, the C element mainly consists of C-O bond (content 48.32%), C-C bond (content 24.95%), C=O bond (content 19.18%) and O-C=O bond (content7.55%). It was demonstrated that the polyaromatic ring system was modified by various oxygen-containing functional groups after the process of FG.

Fig. 3. a, b SEM images of FG and GO, c XPS spectra of GO.

Property analysis of self-healing polymers

Thermal properties of LP-GO

In order to maintain a reversible self-healing system, it is necessary to ensure that each component in the system is thermally stable during self-heal, namely no volatilization or degradation occurs. To measure and compare the relationship between LP and LP-GO when temperature changes in a nitrogen atmosphere, TGA&DTG analysis was taken into account. As the Fig.4a, mass loss of LP and LP-GO among 0~197 ^oC were all about 3.6%, nevertheless error in the scope of allowable can be thought as no quality loss between 0~197 ^oCdue to the instrument operation error and repeated use of the crucible, note that, which provides necessary conditions for self-heal to repeat many times. In addition, the rate of weight loss of LP-3 is slower relative to LP, indicating that the thermal stability of LP-3 is higher.

Mechanical property of LP-GO

To verify the good compatibility of GO with self-healing polymers and obtain suitable addition ratio of GO, the tensile test on self-healing polymers was carried out at 20 ^oCin a strain rate of 1 mm/min.First, the self-healing material was compacted with a pneumatic tablet press machine,and thencut into several groups of regular and flat splines with a blade. At the very least 6 samples were measured for each mass fraction of LP-GO. The tensile tester uniformly stretched the sample to obtain a smooth tensile profileand a series of data of tensile strength involving at break. Through performing a large number of parallel tests on each set of samples, the average tensile strength of each group was calculated, and the relationship between the addition ratio and the tensile strength was obtained. Detailed mechanical properties of LP and LP-GO composites are manifested in Fig. 4b.From the stress-strain curves of all materials, it is evident that LP-9 has largest strain at break up to 14% while LP is nearly 0.5%. To put it another way, the former is more elastic while the latter is brittle which can be transformed by applying different content of GO. The variation trend of mechanical properties are shown in Fig. 4c, that the tensile strength of LP-GO increases first and then decreases along with the increase of the addition ratio of GO. In specific, when the GO addition ratio is 3 wt%, LP-GO has maximum tensile strength at break up to 2.95 MPa, namely the optimum addition of GO is 3 wt%. It can be seen that the breaking stress (2.95 MPa) of LP-3 increases overfour times than LP (0.7 MPa).When the weight percentage of GO changes from 3 wt% to 9 wt%, the maximum strain decreases from 2.95 MPa to 1.02 MPa. On the one hand, the possible reason for the enhancement in mechanical strength of LP-GO before the GO addition ratio is less than 3wt%, is thatthe quantity of oxygen-containing functional groups gradually increases as the proportion of addition ascends, making GO more compatible with self-healing polymers. Besides, lower content allows GO to uniformly disperse in the composite material, which makes the stress distribution of the system uniform, thus the tensile propertiesand the elasticity of the LP-GO are enhanced. On the other hand, when the addition ratio exceeds 3 wt%, the break strain of LP-GO descends, which can be ascribed to the fact that GO nanosheets disturb the orientations of the polymer chains at high elongations, and consequently reduce the mobility of molecular chains.

Fig. 4. a TGA&DTG curves of LP and LP-3, b, c The tensile strain-stress curves and tensile stress of LP-GO with different GO contents.

 

Self-healing property of LP-GO

The healing properties were further tested by measuring the self-healing efficiency of LP-3 with the best mechanical properties.The two broken pieceswere first closely contacted with each other and healed at 30 ^oC for 1 h under vacuum. Subsequently, tensile strength after healing was measured and the healing efficiency was calculated by the ratio between tensile strength of the healing sample and the original sample. The span of healing time was prolonged to 2 h, 3 h and 6 h so as to explore the relationship between the healing time and healing efficiency, as demonstrated in Fig.5a, that the healing efficiency gradually increases as the growth of healing timeunder the constant temperature, and higher healing temperature comes with higher healing efficiency within the same healing time among 30 ^oC, 40 ^oC, 50 ^oC and 60 ^oC. Moreover, a conclusion is clearly shown in Fig. 5a that the healing samples can reach the original mechanical propertiesat 60 ^oC, 1 h and 50 ^oC, 6h, confirmed by the data listed in the Table 1 or the tensile strain-stress curves in Fig.5c below on the mechanical properties of the original LP-3 and self-healing LP-3. In view of shortening the process time, the condition of 60 ^oC for 1 h was adopted to perform the healing cycle test.

Table 1Summary of the mechanical properties of the original LP-3 and self-healing LP-3

Sample	Young’s Modulus (MPa)	Strain-at-break (%)	Stress-at-break (MPa)
Original LP-3	32.74	9.01	2.9503
Self-healing LP-3	33.48	8.81	2.9497

Furthermore, undergoing 10 fracture-healing cycle testsat 60 ^oC for 1 h, thoughLP-3 stillcompletely restore the originalmechanical properties each fracture-healing cycle later shown in Fig.5b, which proves the reversibility and stability of LP-GO healing performance.

Fig. 5.a Self-healing efficiency under different conditions of GO,b The self-healing efficiency of  LP-3 versus healing cycles, cThe tensile strain-stress curves of the original LP-3 and self-healing LP-3

FromFig.6d toFig.6f, illustrate the process, in part, of the fracture-healing cycle,and characterize the healing samples via SEM, of which images affirms that the physical interface at the fracture has been sufficiently healed such that the gap was indiscernible (Fig.6b) in contrast to the pristine surface in the identical position (Fig.6a). Obviously, the resulting healed sample can be subsequently subjected to 100% strain, which proves that GO as reinforcement has small effect on the self-healing performance of LP-GO.

Fig. 6.a, b SEM images of surface of original and self-healing LP-3, Inset b: SEM images of larger magnification,c the healing sample under 300 g of weightd, e, f Photographs portraying the self-heal of the nanocomposite with LP-3.

Conclusions

In conclusion, the self-healing polymer composite (LP-GO) was prepared by supramolecular polymer matrix with hydrogen bonds and appropriate proportion of GO, which not only has good self-healing properties (more hydrogen bond binding sites, as can be verified from the FTIR, TGA&DTG, XRD and SEM test analysis of GO) and thermal stability (TGA test of self-healing polymers), but effectively improves the defectsof poor mechanical properties in traditional self-healing polymers. In addition, the optimal addition ratio of GO is determined 3 wt% through the mechanical tensile stress test (more than this ratio is prone to agglomerate, less than is difficult to polymerize). The appropriate healing condition is at 60 ^oC for 1 h, and the excellent healing effect can be visually reflected from the SEM image. Eventually, the fracture-healing cycles is carried out under the appropriate healing condition to verify that the stability of the healing performance of LP-GOconforms to the conclusion that the intrinsic self-healing polymers can theoretically realize the infinite healing cycles.

There is no doubt that the preparation of LP-GO still has large space for further development, for instance, the preparation of high-quality single-layer GO and the compatibility of GO with polymers, etc., whereas in contrast to the self-healing systemreported previously, LP-GO system does not require healing agents, plasticizers or solvents and enhances the mechanical property. Most importantly, the material may have a promising application prospects depending on these properties.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Supported by Heilongjiang Natural Science Foundation (QC2017038), Fundamental Research Funds for the Central Universities (No. 2572018BC27), China Post-doctoral Science Foundation (No. 2016M601402 &2017M610212), Heilongjiang Postdoctoral Fund (No. LBH-Z16004, LBH-Z16089), and Heilongjiang Postdoctoral Special Fund (No. LBH-TZ13),University Research Training Program of Northeast Forestry University (KY2018016).

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