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  INTRODUCTION

  Supramolecular polymers^[1-4] represent dynamic systems that utilize noncovalent interactions to assemble easy-to-make molecules into architectures with the physical properties characteristic of traditional covalent polymers^[2]. Furthermore,supramolecular polymers typically exhibit emergent and stimuli-responsive properties (not commonly expressed by covalent polymers) that can be modulated through environmental variation (e.g.,temperature and light)^[5-8]. Such properties make supramolecular polymers an intriguing class of materials that have found applications as adhesives^[9],coatings^[10],scaffolds for tissue engineering^{[11, 12]},and healable materials^{[5, 13, 14]}.

  Especially,in the past decade,following the development of strong hydrogen-bonding dimers^[15-17],several research groups have applied these dimers for the formation of hydrogen-bonded supramolecular polymers^[18-22]. Because of the introduction of the strongly associating 2-ureido-4-pyrimidinone (UPy) motif as a quadruple hydrogen bonding moiety in telechelic supramolecular polymers^[23],numerous studies have been reported on its potential in supramolecular and polymeric materials^{[24, 25]}. As the self-complementary motif and high dimerization constant of 6 × 10⁷ (mol/L)-1 of UPy moieties in CHCl₃^[26],a high degree of supramolecular polymerization can be achieved^[27],which transforms the macroscopic properties of the low-molecular-weight supramolecular monomer into those of a high-molecular-weight polymer. UPy-functionalization of telechelic oligomers based on poly(ethylene butylene)^[28],polystyrene^[29],polyesters^[30],polycarbonates^[31],polysiloxanes^[32],and polyethers^[33] has been reported to enhance the mechanical properties when compared with their unfunctionalized counterparts. However,only the complex,lower efficient and high-cost chemical method based on the quadruple hydrogen bonding moiety modified polymers was addressed in the above reports. The simple,efficient and low-cost physical method has not yet been systematically proposed.

  Epoxy toughening has been an interesting and challenging topic for over four decades. Significant efforts have been paid on epoxies toughened with micrometer sized liquid rubbers^[34],core-shell rubber (CSR) particles^[35] and thermoplastic particles^[36]. The addition of rubbery toughening agents usually leads to an impressive toughening effect,but also tends to cause severe deterioration in the glass transition temperature (T_g),strength and other desirable properties,such as processability due to high viscosity. The introduction of thermoplastic particles will typically give a moderate toughening effect and cannot produce satisfactory results for low temperature or high rate test conditions. Consequently,there is still a significant need to develop alternative toughening approaches to greatly increase the fracture resistance of brittle epoxies without compromising other desirable physical and mechanical properties.

  In recent years,a new epoxy toughening approach using self-assembling block copolymers (BCPs) has drawn significant attention^{[37, 38]}. Incorporation of a small amount of dispersed,microphase separated BCP can produce improvements in fracture toughness without compromising the T_g and strength of the cured neat epoxy. In fact,BCPs that self-assemble into the well-defined micro/nanostructures due to phase separation are effective as nanodomain tougheners for epoxy resins.

  Supramolecular polymers can relax via the mechanism of dissociation and association of the hydrogen bonds,a process theoretically described by Cates et al^[27]. Indeed,Meijer has shown that dissociation and association of the hydrogen bonds contribute to the stress relaxation of hydrogen-bonded supramolecular polymers and demonstrated the use of a supramolecular polymer would allow a high degree of micro phase separation.

  Therefore,we anticipated that if replacing the covalent linkage with a dynamic quadruple H-bonding junction,the supramolecular polymer could act as block copolymer and be able to tough epoxy resin. Here we report on the improved toughness behavior of epoxy resin due to the micro phase separation of hydrogen-bonded supramolecular polymers in epoxy resin and propose a simple,efficient and low-cost physical method to expand the application of supramolecular polymers.

  EXPERIMENTAL

  Materials

  UPy containing supramolecular monomer D2000-UPy was synthesized in our lab. The epoxy resin matrix used in the current study was bisphenol A glycidyl ether epoxy resin (E-51),with an epoxy value of 0.51. A modified amine 593,which is an addition product by diethylenetriamine and butyl glycidyl ether,was used as curing agent. It was a transparent liquid with a long-chain structure; the amine value (mg·KOH/g) was 500-600,and the viscosity was 80-100 mPa·s (25 °C).

  Characterization Methods

  FTIR was recorded on a Nicolet MX-1IR spectrometer over the range of 500-4000 cm^-1 with a 2 cm^-1 resolution and 32 scans. Thermal properties of polymers were determined by differential scanning calorimetry (DSC) analysis,which was carried out with a TA Instrument DSCQ 200 under a steady flow of ultrahigh purity nitrogen. Samples were heated to 100 °C quickly,maintained for 3 min to erase the thermal history,and then cooled down at 5 K·min^-1 to -50 °C. Subsequently,the samples were heated again to 100 °C at the same rate. The data presented were determined from the cooling curve and the second heating curve. Thermomechanical properties of the samples were tested with a dynamic mechanical analyzer (DMA Q800,TA Instruments,USA) using a multi-frequency strain mode at a heating rate of 3 K·min^-1 from -70 °C to 80 °C at a frequency of 1 Hz. Scanning electron microscopy (SEM) (S4700,Hitachi Co.,Japan) was used to determine the morphologies of the blends at 1 kV. The samples were fractured under cryogenic conditions for 10 min and the surfaces were coated with a thin gold layer.

  Preparation of D2000-UPy Modified Epoxy Resins

  The ratio of E-51/D2000-UPy is in proportion,ranging from 100:0,100:1,100:2,100:5,and 100:10 by weight. The D2000-UPy was first dissolved in the E-51 and the mixtures were vigorously stirred at 80 °C until the mixtures became homogeneous. Then after the mixture cooled to room temperature,equi molar curing agent 593 with respect to the epoxide equivalence of E-51 was added under continuous stirring after the homogeneous mixtures were obtained again. The ternary mixture was poured into an tetrafluoroethylene mold after being degassed under vacuum for 30 min and cured at room temperature for 24 h to access a complete curing reaction.

  RESULTS AND DISCUSSION

  The supramolecular monomer used in this work comprised poly-(propylene glycol) bis(2-aminopropyl) ether chains (molecular weight 2000) and 2-ureido-4[1H]-pyrimidinone moieties (UPy),and was noted as D2000-UPy in the following discussion. D2000-UPy was prepared by the reaction between poly-(propylene glycol) bis(2-aminopropyl) ether and isocyanate-tailored UPy derivatives. Different amount of D2000-UPy was mixed and cured with bisphenol-A epoxies to prepare a series of supramolecular polymer incorporated epoxy resin samples.

  The impact strength values of neat epoxy and D2000-UPy supramolecular monomer incorporated epoxies are summarized in Fig. 1. A significant improvement in impact strength (by 300%) over neat epoxy is observed. The impact strength has been enhanced from 5 kJ/m² to about 21 kJ/m² upon incorporation of 10 wt% D2000-UPy supramolecular monomers. At the same time,the impact height of the samples were also improved,indicating the effective toughening of the incorporated D2000-Upy supramolecular monomers.

  

  Figure 1. Impact strength as a function of D2000-UPy supramolecular polymer content in the epoxy resin composites

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  To further investigate the impact fracture behavior of the D2000-UPy incorporated epoxies,the fracture surfaces of the samples observed by SEM are shown in Fig. 2. For neat epoxy,the fracture surface was very smooth,indicating a typical brittle fracture. When the D2000-UPy supramolecular monomers were added,additional feature like isolated linear cracks started to appear. With the increasing D2000-UPy contents,isolated,small parabolic and elliptical markings appeared and increased in number with increasing contents of D2000-UPy supramolecular monomers. These features were found apparently on the fracture surface. In fact,the presence of parabolic markings in epoxies systems has been reported before. A myriad of different types of flaws can initiate secondary cracks including inhomogeneities,microvoids,impurities from processing,and other secondary cracks. These defects are found at the foci of the parabolas,with the open end of the parabola pointing in the direction of primary crack propagation. Geometric features are readily visible due to a slight height variation between the primary and secondary crack planes,which can be either above or below the primary crack plane. Connecting these two crack planes requires the creation of new surface,which may aid in energy dissipation. Based on the previous reports,the main reason for this kind of energy dissipation is the micro phase separation. Therefore,SEM was used to observe the detailed morphology of cured epoxy thermosets containing D2000-UPy supramolecular monomers as presented in Fig. 3.

  

  Figure 2. SEM micrographs of impact fracture surfaces of E51 epoxies incorporated with different contents of D2000-UPy supramolecular monomers

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  Figure 3. SEM micrographs of cured epoxy thermosets containing different contents of D2000-UPy supramolecular monomers

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  It can be clearly observed D2000-UPy supramolecular monomers formed well-dispersed,spherical micro phase domains in the epoxy matrixes. As concentration of D2000-UPy was increased,the size of micro phase domains increased from about 300 nm at 1 wt% to about 600 nm at 10 wt% in diameter. The results were consistent with previous reports for micro phase separation of blended polymers and in turn demonstrated the improvement of impact behavior above.

  Hence,the parabolic marking formation in the impact fracture surfaces of D2000-UPy supramolecular polymer incorporated epoxies can be explained through the schematic illustration as shown in Scheme 1.

  

  Figure Scheme 1. A schematic illustration of parabolic marking formation

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  The stress field ahead of the primary crack causes secondary cracks forming from spherical micro phase domains on different planes. These secondary cracks propagate radially from their domains origins. Eventually,the primary and secondary cracks interact and create a new surface to join their respective planes,leaving behind a parabolic feature.

  So a question arises that what is the mechanism of the spherical domains formation or the micro phase separation in this supramolecular monomer incorporated epoxy composite system? Meijer et al. investigated the aggregation of UPy supramolecular thermoplastic elastomers^[39]. In the pure supramolecular polymers,the nano structure formation was a hierarchical process starting from the phase-separated melt with the dimerization of the UPy-units. The nano structure formation was the result of stack-to-stack of Upy dimerization through quadruply hydrogen bonds. If the UPy supramolecular monomers were introduced to the epoxy system,the nano structure may be changed due to the micro phase separation between epoxy matrixes and supramolecular polymers. In our study,the nano structure formed was spherical domain as shown in Fig. 3.

  In order to confirm the improving of impact resistance is the result of the incorporated D2000-UPy supramolecular polymeric nano structures due to the strong quadruply hydrogen bonds between UPy moieties,we tested the Jeffamine D2000 incorporated E51-593 epoxy system and found a different impact resistance behavior,as shown in Fig. 4. The results of these tests indicated that Jeffamine D2000 exhibited a toughen effect similar to UPy-tailored D2000,and the impact strength increased with increasing Jeffamine D2000 content. But it is important to note that the impact strength of epoxy thermosets containing 10% Jeffamine D2000 was 15.9 kJ/m²,while the epoxy thermosets containing 10% UPy-tailored D2000-UPy content is 20.7 kJ/m². That means UPy-tailored D2000-UPy showed a more obvious toughen effect compared to Jeffamine D2000,indicating the important role of quadruply hydrogen bonding between UPy moieties.

  

  Figure 4. Impact strength as a function of Jeffamine D2000 content in composites

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  FTIR studies with variable temperatures of the epoxy composites in the solid phase were performed to further investigate the micro phase separating processes. As shown in Fig. 5,neat epoxy shows no quadruply hydrogen bonding characteristic vibrations at 1650 cm^-1,which can be attributed to the strongly hydrogen-bonded urea carbonyls and no difference with the temperature variation. However,the vibration of urea carbonyls at 1650 cm^-1 was found in the starting spectrum of D2000-UPy incorporated epoxy composites at 20 °C and absorbance band was greatly weakened upon increasing temperatures. When the temperature was higher than 80 °C,the absorbance band became ignorable. This indicates the dissociation of quadruply hydrogen bonding between UPy moieties in the composite epoxy matrixes at a high temperature.

  

  Figure 5. Variable temperature FTIR curves of neat (a) and D2000-UPy supramolecular monomer incorporated epoxy resins (b)

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  Therefore,the formation of spherical domains in epoxy thermosets can be inferred. The lateral aggregation appears due to π-π stacking through the dimerized ureidopyrimidinone moieties,and then the aggregated domains exclude from the epoxy matrixes,while the D2000-UPy flexible chains miscible with the epoxy still maintain the stretched state. When the D2000-UPy supramolecular monomers incorporated thermosets suffered from the impact by external force,the crack began to propagate,the spherical domains fixed in epoxy thermosets through the stretched D2000-UPy flexible chains could produce more cracks and result in a larger energy dissipation. Hence the impact strength of the D2000-UPy supramolecular monomers incorporated epoxy thermosets was greatly improved,as shown in Scheme 2.

  

  Figure Scheme 2. Schematic formation of nanodomains of the D2000-UPy supramolecular polymers in epoxy thermosets matrixes

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  Further investigation of the mechanical properties of D2000-UPy supramolecular monomer incorporated epoxies were conducted. From the tensile tests in Fig. 6,it can be concluded that tensile strength of epoxy thermosets containing different contents of D2000-UPy supramolecular monomers showed a slight decrease compared with that of the neat epoxy resins,while the elongation was improved by the introduction of D2000-UPy supramolecular polymers.

  

  Figure 6. Tensile tests results of D2000-UPy supramolecular monomer incorporated epoxies: (a) stress-strain curves; (b) D2000-UPy content-dependant tensile strength (■) and elongation at break (●)

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  CONCLUSIONS

  In summary,we have shown that the use of supramolecular polymers allows micro phase separations in the epoxy matrixes. The spherical phase-separated microdomains are formed by performing a simple physical blending of supramolecular monomers and epoxies before curing. Increasing the amount of D2000-UPy supramolecular monomers has a strong influence on the size of spherical microdomains. The impact resistance is improved significantly through the addition of D2000-UPy supramolecular monomers,as the spherical supramolecular polymeric domains fixed in epoxy thermosets through the stretched D2000-UPy flexible chains could produce more cracks and result in energy dissipation. Moreover,the tensile properties comparable to those of the neat epoxy indicate this simple physical blending supramolecular monomers to epoxy can be a promising method to toughen epoxy resins. Besides,a simple,efficient and low-cost method is proposed to apply the unique supramolecular polymers in the industrial science.

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