Multifunctional light-responsive graphene-based polyurethane composites with shape memory, self-healing, and flame retardancy properties

Journal Pre-proofs Multifunctional light-responsive graphene-based polyurethane composites with shape memory, self-healing, and flame retardancy properties Weining Du, Yong Jin, Shuangquan Lai, Liangjie Shi, Yichao Shen, Heng Yang PII: DOI: Reference:

S1359-835X(19)30435-X https://doi.org/10.1016/j.compositesa.2019.105686 JCOMA 105686

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

20 July 2019 31 October 2019 2 November 2019

Please cite this article as: Du, W., Jin, Y., Lai, S., Shi, L., Shen, Y., Yang, H., Multifunctional light-responsive graphene-based polyurethane composites with shape memory, self-healing, and flame retardancy properties, Composites: Part A (2019), doi: https://doi.org/10.1016/j.compositesa.2019.105686

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier Ltd.

Multifunctional light-responsive graphene-based polyurethane composites with shape memory, self-healing, and flame retardancy properties Weining Du a,b, Yong Jin a,b,*, Shuangquan Lai a,b, Liangjie Shi a,b, Yichao Shen a,b, Heng Yang a,b a Key

Laboratory of Leather Chemistry and Engineering (Sichuan University), Ministry of Education, Chengdu 610065, China

b

National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, China

* Corresponding author: [email protected] (Y. Jin); Tel: +86-13658027079

1

ABSTRACT: It is a challenge to manufacture light responsive polymer composite that possesses shape memory, self-healing, and flame retardancy capacities. Herein, multi-functionalized graphene oxide (mfGO) wrapped with nitrogen-, phosphorus-, and silicon- containing units was prepared via in-situ polymerization and subsequently was incorporated into a diselenide-containing polyurethane (dPTD) matrix to fabricate composite. The successful functionalization of mfGO was initially confirmed by a series of measurements. Taking advantage of the crystallization-induced and photo-thermal effects of mfGO as well as the dynamic exchange characteristic of diselenide bonds, the dPTD-mfGO2 composite containing 2 wt% of mfGO exhibited admirable shape memory and self-healing behaviors under visible-near infrared light within 3 min, and its shape memory characteristics and healing efficiencies were kept above 90% and 76% after three cycles, respectively. Further combustion experiments demonstrated that dPTD-mfGO2 composite showed superior LOI (24.9%) and UL-94 rating (V-2) without flaming drips, owing to the synergistic catalyzing carbonization and barrier effect of mfGO. Additionally, the dPTD-mfGO2 composite possessed an improved water contact angle of 109.5°. These findings suggest that the introduction of 2 wt% mfGO to the dPTD matrix can synergistically improve the toughness, shape memory, self-healing, flame retardancy, and water resistance as compared with the neat dPTD. This work provides a promising pathway to fabricate stimulus-responsive composite materials with versatile functions. KEYWORDS: Multi-functionalized graphene oxide; polyurethane composite; light responsive; self-healing; shape memory; flame retardancy

1. Introduction 2

Since the discovery of graphene in 2004 by Geim et al. [1], graphene has been considered as an efficient multi-functional additive for polymer composites due to its excellent properties such as high specific surface area, mechanical strength, and photo-thermal effect [2-4]. However, the pristine graphene tends to agglomerate and restack in the polymer matrix, which greatly limits the performances of the composites [5, 6]. In terms of cost efficiency and scalability, functionalized graphene (or graphene oxide) is an alternative to the pristine graphene, and capable of achieving a molecular-level dispersion of graphene in polymer matrix through intermolecular hydrogen bonding and covalent interactions [7]. Over the past decade, extensive studies have been devoted to fabricate graphene-based polymer (e.g., polyurethane, polyimide, polyethylene, and so on) composites with novel and improved properties [8-10]. Among these graphene-based composites, stimuli-responsive and multi-functional composites are highly desired now because of the various needs in the current applications. Compared with other existing external stimuli polymers (triggered by heat, electricity, magnetism, and chemicals), light responsive graphene-based polymer composites have the advantages of low noise, clean, wireless control, and local operation [11, 12]. Actually, graphene can absorb and convert near-infrared (NIR) light to thermal energy and act as photo-thermal heaters to increase the temperature of composites, thus capable of accelerating the intermolecular diffusion and re-entanglement of polymer chains [13, 14]. To date, light responsive graphene-based composites have been widely reported in the fields of shape memory [15-17] and self-healing [18-20]. For the shape memory materials, Yang et al. [15] prepared a novel graphene-vitrimer composite, and claimed the composite showed good shape memory behavior by NIR light trigger by photo-thermal conversion of graphene. Wang et al. [16] synthesized a type of graphene-elastin composite, and the composites exhibited rapid and tunable 3

motions under NIR light. With regard to the self-healing materials, Du et al. [18] reviewed the recent achievements of self-healing graphene-based composites. Typically, Huang et al. [19] proposed a graphene-thermoplastic polyurethane composite that possessed repeatable self-healing under NIR. However, there are few works in the literature focused on the light responsive graphene-based composites with both shape memory and self-healing properties [4, 14], and these composites still have poor target characteristics (i.e., healing rate and efficiency). More importantly, there are no reports on shape memory and self-healing polymer composites that feature flame retardancy performance currently. Polymer-based materials with efficient flame retardancy are capable of reducing the fire hazards and economic losses in practical application. Because the bare graphene or polymer matrix easily burn out upon exposure to a heat flow under air [21, 22], significant efforts have been applied to decorate graphene with flame retardant units to improve the flame retardancy of graphene-based polymer composites. Jin et al. [23] prepared functionalized GO with phosphorus-nitrogen dendrimer and further incorporated it into polyurethane, resulting in enhanced thermal stability and fire safety. Similar results were also reported by the relevant works in the literature [7, 21, 24]. Wang et al. [25] prepared octa-aminophenyl polyhedral oligomeric silsesquioxanes functionalized GO/epoxy composites, leading to an improved fire safety. In view of these facts, the graphene with nitrogen-, phosphorus-, and silicon-containing reactants have the capacity of minimizing the flammability of polymer matrices. However, until now there is no report on the simultaneous introduction of nitrogen-, phosphorus-, and silicon-containing reactants onto graphene which should dramatically improve the flame retardancy of the polymer composite. Along with this, the multi-functionalized graphene with multiple hydrogen-bonding sites can 4

greatly enhance the dispersibility and compatibility with polymer matrix, which in turn will improve the shape memory and self-healing properties of composite. So far, light responsive graphene-based composites still have the drawbacks of low healing rate and efficiency. Among the existing intrinsic self-healing chemistries, the dynamic diselenide bond with mild responsive characteristics and one-step manner of crack healing is relatively favorable to strengthen the self-healing property of polymer composite. The pioneer works on diselenide-containing polymers by Xu et al. showed an excellent metathesis reaction upon exposure to visible (VIS) light or heat [26, 27], and they further reported that the diselenide-containing polyurethane showed good healing efficiency under VIS light [28]. Combining with our previous studies [4, 29], we envisaged that the healing rate and efficiency for graphene-based composite containing diselenide bonds can be strengthened under simultaneously VIS-NIR light. To the best of our knowledge, there are no reports on graphene-based polyurethane composite that feature shape memory, self-healing, and flame retardancy properties simultaneously. In this work, we first synthesized multi-functionalized graphene oxide (mfGO) with nitrogen-phosphorus-silicon containing units, and then incorporated it into polyurethane matrices to fabricate composites. The prepared mfGO was initially characterized by the FT-IR, TG-DTG, XRD, Raman, XPS, and SEM-EDS measurements. Moreover, the dispersibility and compatibility of mfGO sheets with polyurethane matrix were evaluated by SEM. Incidentally, mechanical, crystallization, and photo-thermal behaviors of these polyurethane composites were studied. Their shape memory, self-healing, and flame retardancy were further systematically investigated and discussed.

2. Experimental section Detailed descriptions of Materials, Methods, and Characterizations sections are summarized in the 5

Supplementary material.

3. Results and discussion 3.1. Synthesis of multi-functionalized graphene oxide/polyurethane composites Fig. 1 illustrates schematically the synthetic routes of multi-functionalized graphene oxide/polyurethane composites, which consisting of two steps. In the first step, the mfGO sheet with polyethylenimine (PEI), 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), and isocyanatopropyltriethoxysilane (IPTS) units was synthesized by an in-situ method (Fig. 1a). In the second step, two typical polyurethane copolymers (dPTB and dPTD) containing different diol chain extenders such as 1,4-butanediol (BDO) and di(1-hydroxyethylene) diselenide (DiSe) were fabricated via a solution polymerization method (Fig. 1b). Details of the structural characterizations of DiSe monomer can be found in Fig. S1. For the neat dPTB and dPTD copolymers without fillers, their chemical structures and molecular weights were measured using 1H NMR and GPC, respectively, as shown in Fig. S2 and S3. Our previous studies reported that the introduction of 2 wt% functionalized graphene oxide to the polyurethane matrices significantly enhanced the relevant properties of polyurethane composites [4, 30]. Herein, a defined amount of mfGO (2 wt%) was physically incorporated with dPTB or dPTD copolymer during the polymerization process of the second step, and the interfacial interactions of mfGO sheets with the polyurethane matrices are schematically shown in Fig. 1c. The obtained dPTB and dPTD composites containing 2 wt% of mfGO were coded as dPTB-mfGO2 and dPTD-mfGO2, respectively. For comparison, the dPTD composite containing 2 wt% pristine GO was prepared by the same procedures and labeled as dPTD-GO2. The digital photographs of all prepared polyurethane films are shown in Fig. S4. 6

Detailed synthesis procedures can be found in the Methods section (Supplementary material).

Fig. 1. Schematic representation of synthesis routes of (a) mfGO, (b) dPTB or dPTD copolymers, and (c) dPTB-mfGO or dPTD-mfGO composites. 3.2. Evaluation of multi-functionalized graphene oxide The structural information and thermal stability of pristine GO, PEI-GO, DOPO-PEI-GO, and 7

mfGO were initially determined using FT-IR and TG-DTG, respectively. It can be seen from FTIR spectra (Fig. 2a) that the PEI, DOPO, and IPTS units were successfully covalently functionalized onto the surface of mfGO. As can be seen from the TG-DTG results (Fig. 2b, Fig. 2c, and Table S1), the mfGO exhibited a relative higher thermal stability over the majority of tested temperature range, which suggested indirectly the successful functionalization of mfGO. Detailed explanations for the FTIR and TG-DTG results are described in Supplementary material. Based on this, the chemical structures, compositions, and morphological features for the representative GO and mfGO were further investigated by XRD, Raman, XPS, and SEM-EDS techniques as following. XRD was employed to study the structural change of the GO and the final mfGO. As shown in Fig. 2d, GO exhibited a peak at 2θ=9.77° (representing the 002 reflection) [31], and the corresponding d-spacing value was 9.05 Å. In contrast, the 2θ peak of mfGO was shifted down to 7.42°, while the interlayer spacing was increased to 12.01 Å. The enlarged d-spacing was attributed to the intercalation of PEI, DOPO, and IPTS units between the GO sheets. In addition, mfGO showed a new obtuse peak at around 2θ=15-30°, mainly due to the existence of amorphous phosphorus- and silica- containing components [7, 32, 33]. Raman spectroscopy was carried out to estimate the interaction between graphene sheets. As shown in Fig. 2e, GO possessed two characteristic peaks at around 1359 cm−1 (D-band) and 1597 cm−1 (G-band) [34]. By contrast, a D-band at 1350 cm−1 and a G-band at 1591 cm−1 were observed for mfGO. Both the D-band and G-band of mfGO were shifted to lower wavenumbers in comparison with the pristine GO, which might be due to the strong interactions of the intercalated polymer chains with the mfGO sheet [35]. Moreover, the mfGO exhibited a higher ID/IG ratio than that of the pristine GO. This phenomenon could be explained by the restoration of sp2 carbon and reduction in 8

the average sizes of the sp2 domains after chemical functionalization of GO [36, 37]. XPS analysis was performed to characterize the chemical composition of GO and mfGO. It can be seen in Fig. 2f that both GO and mfGO possessed strong C 1s (284.6 eV) and O 1s (531.8 eV) peaks. By contrast to the pristine GO, mfGO showed four new peaks at 103.6, 133.7, 152.2, 191.5, and 399.8 eV, corresponding to the Si 2p, P 2p, Si 2s, P 2s, and N 1s peaks, respectively [7, 38, 39]. Moreover, the mfGO exhibited an increase in the intensity of C 1s peak and a reduction in the intensity of O 1s peak as compared to the GO (Table S2). These results further confirmed the existence of PEI, DOPO, and IPTS units on the mfGO surface. The microscopic morphologies of GO and mfGO were observed by SEM. As shown in Fig. 2g, the pristine GO exhibited a clean surface with slightly wrinkle, while the surface of mfGO was relatively coarse and possessed some irregular particles. These observations might be ascribed to the grafted phosphorus- and silica- containing moieties on the surface of mfGO sheets. In addition, SEM-EDS elemental mapping measurement was further conducted to detect the elemental composition and distribution of mfGO sheet, as shown in Fig. 2h. Combining with the results of elemental analysis (Table S2), three new elements (i.e., N, P, and Si) were detected in mfGO as compared to the pristine GO. Incidentally, the variation tendency of the elements was consistent with the results achieved by XPS measurement. It can be also seen in Fig. 2h that the N, P, and Si elements exhibited a good distribution on the mfGO sheets. Such morphological feature might not only contribute to the dispersibility and compatibility of mfGO in the polyurethane matrix, but also effectively improve the thermal stability of graphene-based polymer composites.

9

Fig. 2. (a) FTIR, (b) TG, and (c) DTG of pristine GO, PEI-GO, DOPO-PEI-GO, and mfGO. (d) XRD (inset, 2θ and d-spacing data), (e) Raman, (f) XPS (inset, enlarged region of 90-210 eV), (g) SEM, and (h) SEM-EDS mapping (C, red; O, green; N, blue; P, yellow; Si, purple) of GO and mfGO.

3.3. Physicochemical performances of polyurethane composites The dispersion level of the fillers in the polymer matrix is of great importance for the relevant performances of polymer composite. SEM analysis was initially conducted to evaluate the morphological features of the as-prepared polyurethane films (Fig. 3a). Clearly, the cross-section surfaces of neat dPTB and dPTD films were quite smooth. After introduction of 2 wt% mfGO into 10

polyurethane matrices, the dPTB-mfGO2 and dPTD-mfGO2 composites also possessed acceptable smooth surfaces without obvious agglomeration. However, the dPTD-GO2 composite film with the same loading of GO (2 wt%) exhibited relatively coarse surface and large aggregates. These results could be ascribed to the superior dispersion level of the mfGO sheets in the polyurethane matrices, and the polymer chains on the mfGO sheets could provide more active-sites to form hydrogen bonds (Fig. 1c), thus improving the compatibility of mfGO with the polyurethane matrices . Mechanical properties of the polyurethane materials play an important role in practical application. Fig. 3b presents the static mechanical data of as-prepared films, and the corresponding stress-strain curves are shown in Fig. S5. It could be found that dPTD film exhibited a relative lower tensile strength (σb) and elongation at break (εb) than that of dPTB film. This result might be attributed to the inherent lower bond energy of diselenide bond in the dPTD backbone chains. In comparison with the neat dPTB (σb=14.5 MPa, εb=1077%), both σb and εb for dPTB-mfGO2 composite were increased by 23% and 16%, respectively. Likewise, dPTD-mfGO2 composite showed 19% and 16% increase in σb and εb, respectively, as compared with the neat dPTD film (σb=13.1 MPa, εb=973%). This phenomenon could be explained by an effective load transfer from the polymer matrices to graphene sheets under external stress [40]. Unfortunately, the dPTD-GO2 with the same loading of GO filler showed significant reductions in σb and εb as compared to the neat dPTD film, mainly due to the poor dispersibility and large aggregates of pristine GO in the polyurethane matrix. This is consistent with the observations from SEM. DMA was further performed to study the dynamic mechanical properties of polyurethane films. Fig. 3c displays the loss factor (tan δ) and storage modulus (E′) versus temperature curves of as-prepared films. It could be seen that the tan δ peak temperatures for the neat dPTB and dPTD 11

films were 28.5 and 26.9 °C, respectively. By contrast, the tan δ peak temperatures of dPTB-mfGO2 (34.5 °C) and dPTD-mfGO2 (32.1 °C) films were significantly increased with the addition of mfGO. The results suggested the polyurethane chains near mfGO sheets were effectively restricted, leading to a higher tan δ peak temperature. However, the dPTD-GO2 (24.8 °C) showed a lower tan δ peak temperature as compared to the neat dPTD. This phenomenon could be ascribed to the aggregation of pristine GO and the weak interfacial interaction with polyurethane chains. It can be also found in Fig. 3c that dPTB-mfGO2 and dPTD-mfGO2 films possessed higher E′ values than neat dPTB and dPTD films through most of the test temperature range, respectively, while the dPTD-GO2 film showed a relative lower E′ as compared to the dPTD-mfGO2 film. These results were attributed to the excellent dispersibility and compatibility of mfGO (2 wt%) in polyurethane matrices and the strong interfacial interaction between them, and thus improving the rigidity and toughness of the polyurethane composites, similar to the results from static mechanical test. The thermal and crystallization performances were estimated by DSC and XRD. Fig. 3d presents the DSC curves of as-prepared films, and the relevant thermal data are summarized in Table S3. As shown in Fig. 3d, all prepared polyurethane films had two distinct transition temperatures including exothermic cold crystallization temperature (Tc) and endothermic melting temperature (Tm) that can dominate their shape memory performance as previously reported [41]. For all prepared polyurethane films, the Tc peaks at around ~15 °C are caused by the crystallization of PHA segment, while the Tm1 (~24 °C) and Tm2 (~38 °C) peaks are assigned to the melt of PTMG and PHA segment, respectively (Table S3) [5]. In comparison with the neat dPTB and dPTD films, the Tc values of polyurethane composite films were slightly decreased with the introduction of mfGO or GO sheets. Apart from the dPTD-GO2 film, the Tm values of the dPTB-mfGO2 and dPTD-mfGO2 films were 12

increased with the addition of mfGO. These phenomena suggested the nucleation effect of mfGO sheet on the crystallization of polyurethane segments [4, 42]. Additionally, there was only 0.4–1.5 °C variation of the two Tm peaks for all prepared polyurethane films except for dPTD-GO2, indicating the crystalline structures of polyurethane segments remain unchanged despite the presence of mfGO in the polyurethane matrices [43]. Combining with the melting enthalpy (ΔHm) results, the ΔHm1 (originated from PTMG moiety, <0.2 J/g) value was far lower than that of the ΔHm2 (originated from PHA moiety, 2.9-9.3 J/g), suggesting the formation of crystalline structure in the polyurethane films was mainly caused by the PHA segment. Therefore, the crystallinity of the polyurethane films can be calculated from the ΔHm data (see Supplementary material). It could be concluded that the dPTB-mfGO2 and dPTD-mfGO2 composites exhibited an increment in crystallinity, and enabled to facilitate the shape fixity capability during the shape memory process. XRD was further conducted to evaluate the crystalline structure and the crystallinity behavior of polyurethane films. Fig. 3e shows the XRD patterns of as-prepared films, and the relevant 2θ peak and d-spacing parameters are listed in Table S4. As can be seen in Fig. 3e, two main diffraction peaks at around 21.3 and 23.9 were observed for all prepared polyurethane films, corresponding to the 220 and 040 reflections of PHA component, respectively [44]. This result further confirmed the crystalline portion in polyurethane films was mainly caused by the PHA moiety, and the corresponding crystalline structure was slightly changed with the addition of mfGO sheets. In comparison with the neat dPTB and dPTD films, both the dPTB-mfGO2 and dPTD-mfGO2 composite films exhibited a diffraction peak at 2θ=~7.2°, which was assigned to the mfGO component. By contrast to the simple mfGO (d-spacing=12.01 Å), the d-spacing values of the mfGO in the dPTB-mfGO2 and dPTD-mfGO2 composites were increased to 12.35 and 12.26 Å, 13

respectively, which might be ascribed to the strong hydrogen-bonding interactions between mfGO sheets and polyurethane chains, leading to an enlarged d-spacing between graphene sheets. More interestingly, the peak intensities for the dPTB-mfGO2 and dPTD-mfGO2 composites were found to increase with the incorporation of mfGO. However, as if an equal loading of GO (2 wt%) was incorporated, the corresponding peaks for dPTD-GO2 was less pronounced. These phenomena indicated the introduction of mfGO and its good compatibility with the polyurethane matrices could effectively improve the crystallinity of polyurethane segments, similar to the results from DSC analysis.

14

Fig. 3. (a) Representative SEM micrographs (cross-section surfaces), (b) static mechanical data, (c) DMA (tan δ denotes loss factor; E' denotes storage modulus), (d) DSC, and (e) XRD patterns of dPTB, dPTB-mfGO2, dPTD, dPTD-mfGO2, and dPTD-GO2 films.

3.4. Shape memory behavior of polyurethane composites A conventional bending test was initially carried out to quantitatively evaluate the shape memory characteristics including shape fixity ratio (Rf), recovery ratio (Rr), and recovery time of as-prepared films under VIS-NIR light or heating, as shown in Table S5. In comparison with the neat dPTB and dPTD, the Rf values for dPTB-mfGO2 and dPTD-mfGO2 films increased to 96.7% and 95.2%, while their Rr increased to 95.5% and 93.1%, respectively. Both Rf and Rr values in this work were comparable to the Rf (86-99%) and Rr (78-100%) of the SMPs reported up to now [45]. The increment in Rf might be ascribed to the nucleation effect of mfGO2 sheet on polyurethane crystallization and the reestablishment of the hydrogen bond across the polymer chain interfacial surface during the fixing process [46], and the improvement in Rr could be attributed to the formed physically crosslinking network of mfGO2 in polyurethane. Unfortunately, the Rf and Rr values of dPTD-GO2 were even lower than those of the neat dPTD film, due to the poor dispersion level of pristine GO in the dPTD matrix. Upon illumination by VIS-NIR light, the recovery time of dPTB-mfGO2, dPTD-mfGO2, and dPTD-GO2 film was faster than the conventional heating method (Table S5). Moreover, the recovery processes for the neat dPTB and dPTD films were hardly completed under VIS-NIR light (Rr < 3%). These results could be attributed to the inherent photo-thermal effect of the inserted mfGO or GO (Fig. S6), and the NIR light played a dominant role. Repeatability of the shape memory process was further conducted to illustrate the durability of 15

as-prepared films, as shown in Fig. S7. It could be found that the Rf and Rr values for all prepared films were reduced to different extents with increasing cycle times, owing to the loosening of the polyurethane structure in repeating utilization. Intriguingly, the dPTB-mfGO2 and dPTD-mfGO2 composite films exhibited slightly reductions in shape memory characteristics, and the corresponding Rf and Rr values were kept above 90% after three cycles, suggesting an acceptable reliability. To intuitively illustrate the distinctive shape memory function as shape-customized materials, the representative dPTD-mfGO2 composite was transformed into different geometric shapes (Fig. 4a). Typically, the dPTD-mfGO2 composite with an original shape was first heated above the transition temperature (~45°C), and deformed it with stretching, circle, and spiral shapes, and subsequently cooled to fix the temporary shapes. As expected, all defined temporary shapes could almost recover their original shapes under VIS-NIR light. Along with this, the shape recovery process of dPTD-mfGO2 composite upon exposure to VIS-NIR light is presented in Fig. 4b. Under the illumination of VIS-NIR light, the temporary shape (spiral structure) of dPTD-mfGO2 composite underwent a series of unzipping, spiralling, and length stretching steps, and nearly recovery of its original shape within 10 s. In addition, the representative stretch-recovery process of dPTD-mfGO2 was further estimated by the DMA test (Fig. S8). It could be seen that the dPTD-mfGO2 film showed good shape memory behavior, and in particular the calculated Rf (96.1%) and Rr (91.8%) values from DMA curves were approximately consistent with the results from bending test (Table S5). Fig. 4c outlines schematically the possible shape memory mechanism of the graphene-based polyurethane composites via crystallization and photo-thermal behaviors. Specially, the crystalline regions in dPTD-mfGO2 composite will be loosened when heating above the transition temperature. Because the nucleation effect of mfGO filler on polyurethane crystallization, the temporary shape 16

could be effectively fixed during natural cooling. For the recovery process, the converted thermal energy from VIS-NIR light could be transferred through the polyurethane matrix to melt the crystals and release the temporarily stored strain energy.

Fig. 4. Shape memory behavior of the dPTD-mfGO2 composite film. (a) Photographs showing the representative dPTD-mfGO2 with original shape, temporary shapes (i.e., stretching, circle, and spiral shapes), and recovery shapes. (b) Photographs showing the shape memory process under visible-near infrared (VIS-NIR, 400-1100 nm) illumination. (c) Schematic showing the mechanism of the VIS-NIR light triggered shape memory behavior.

3.5. Self-healing behavior of polyurethane composites Tensile experiments were performed to quantitatively evaluate the self-healing behavior of as-prepared films. Fig. S9 presents the representative stress-strain curves of the original and the healed samples, and the associated healing efficiencies are shown in Fig. 5a. Clearly, the neat dPTB exhibited poor healing efficiencies (recovery of maximum tensile strength and elongation at break) 17

upon exposure to VIS-NIR light. In contrast, the healing efficiency for dPTD (31% and 41%) was higher than that of the dPTB (14% and 1%), might be due to the exchange reaction of the embedded diselenide bonds between the interface crack under VIS-NIR light (especially for the VIS light) [28]. Noticeably, the healing efficiencies for the dPTB-mfGO2 (39% and 70%) and dPTD-mfGO2 (80% and 96%) were significantly improved as compared to the neat dPTB and dPTD, respectively. These results were mainly ascribed to the photo-thermal effect of the incorporated mfGO (Fig. S6), and its capability of promoting the diffusion and rearrangement of the polymer chains, which at some extent, results in healing. However, the dPTD-GO2 exhibited a slight increase in healing efficiency as compared to the neat dPTD film, owing to the poor dispersibility of GO in the dPTD matrix. Among the prepared polyurethane films, dPTD-mfGO2 showed superior healing efficiencies of tensile strength and elongation at break. This work exhibited high level of healing efficiencies as compared to that of the light triggered graphene-based composites (~40%-99%) reported so far [2, 14, 19, 20]. These results were tentatively attributed to the synergistic photo-thermal effect of graphene and exchange reaction of diselenide bonds under VIS-NIR irradiation [4]. In addition, the healing efficiencies for the dPTB-mfGO2 were maintained at 76-90% after three cycle times (Fig. S10). For the visual illustration of the self-healing behavior, the representative dPTD-mfGO2 composite film was cut into two pieces and healed under VIS-NIR light for 3 min (Fig. 5b). It can be seen from Fig. 5b that the crack on the healed dPTD-mfGO2 was almost disappeared. Moreover, the dPTD-mfGO2 exhibited a relative superior healing capacity in comparison with the dPTB, dPTB-mfGO2, dPTD, and dPTD-GO2 films under VIS-NIR light (Fig. S11), similar to the results from the healing efficiency experiment. To better illustrate the self-healing behavior of 18

dPTD-mfGO2, the healed sample was bended and further loaded with a weight of 800 g. It could be found that the healed dPTD-mfGO2 did not fracture at the joint position under strong bending and stretching (Fig. 5c). Such admirable healing capacity might be attributed to the synergistic photo-thermal effect of graphene sheet and dynamic exchange characteristic of diselenide bonds. According to the previous works in the literature, the graphene and its derivatives possessed good photo-thermal effect under near-infrared light [4, 17, 47, 48], while the diselenide bonds exhibited dynamic exchange characteristic under visible light [26, 49]. To clarify the self-healing mechanism of dPTD-mfGO2 film in the present work, the photo-thermal effect and the dynamic exchange reaction were conducted under VIS-NIR light. Fig. 5d presents the changes in surface temperature over time for all prepared films under VIS-NIR light, and the corresponding thermographs are shown in Fig. S6. It could be seen that the neat dPTB and dPTD films exhibited non-obvious temperature changes upon exposure to VIS-NIR light even after a long time. Conversely, the surface temperatures for dPTB-mfGO2 and dPTD-mfGO2 containing 2 wt% of mfGO rapidly increased to ~55 °C in 16 s under irradiation, whereas the dPTD-GO2 showed less increase in temperature up to ~40 °C in 16 s. Such photo-thermal effects of the mfGO or GO come from the localized surface Plasmon resonance of graphene by NIR light, relying on the structural states of graphene (e.g., size, defects, and functionalization) [50]. In addition, 1H NMR and FT-IR measurements were employed to investigate the dynamic exchange property of diselenide bonds in small molecules system (Fig. 5e).

Equimolar

amounts

of

benzyl-urethane

end-capped

diselenide

(BI-DiSe-BI)

and

isopropyl-urethane end-capped diselenide (II-DiSe-II) were mixed in deuterated chloroform (CDCl3) and exposure under VIS-NIR light for 3 min at ambient conditions. The obtained results from 1H NMR and FT-IR confirmed the metathesis reaction between diselenide-containing small molecules 19

under VIS-NIR light, the corresponding explanations are summarized in Supplementary material. From above mentioned experimental results, the possible healing mechanism for dPTD-mfGO2 is schematically displayed in Fig. 8b. Specifically, the graphitic domains of graphene could adsorb energy from the VIS-NIR light and dissipate the energy to the polymer matrix to melt the crystals, and narrowing the crack surface by the assistance of shape memory effect, and, in turn promoting the interfacial chain diffusion and especially the dynamic diselenide exchange across the damaged surfaces, and thus greatly recovering the inherent performances.

Fig. 5. (a) Healing efficiency of dPTB, dPTB-mfGO2, dPTD, dPTD-mfGO2, and dPTD-GO2 films. (b) Digital photographs (I and II) and SEM micrographs (III and IV) of the representative dPTD-mfGO2 before and after healing under VIS-NIR light. (c) Digital photographs showing the self-healing behavior of the healed dPTD-mfGO2 with different shapes: (I) bending, (II) bearing a 20

weight of 800 g, and (III) after stretching. (d) Change in temperature over time for all prepared films upon expose to VIS-NIR light in air (inset, representative thermographs of the dPTD-mfGO2). (e) Exchange mechanism of the diselenide-containing small molecules under VIS-NIR light: (I) metathesis reaction between BI-DiSe-BI and II-DiSe-II (5 mmol each, in CDCl3), (II) 1H NMR and (III) FT-IR spectra obtained before and after the exchange reaction. (f) Schematic showing the possible self-healing mechanism under VIS-NIR light.

3.6. Thermal stability and flame retardancy of polyurethane composites TGA has been widely performed to investigate the thermal stability and degradation behaviors of polymer materials. Fig. 6a and 6b present the TG and DTG curves of as-prepared films, respectively, and the detailed data are summarized in Table S6. T5 and T80 are defined as the temperatures where weight losses are 5% and 80%, respectively. It could be found that all prepared films showed a two-stage thermal degradation within the temperature range of 50-600 °C. Strikingly, the dPTB-mfGO2 and dPTD-mfGO2 composite films containing 2 wt% of mfGO exhibited higher T5, T80, and Tmax (maximum decomposition temperature) values as compared to the neat dPTB and dPTD, respectively (Table S6), indicating the improvement of thermal stability over the whole temperature range. However, the T5, T80, and Tmax for dPTD-GO2 film were reduced to lower temperatures than those of the neat dPTD film, probably due to the unstable oxygen-containing groups of the pristine GO. It could be concluded that the dPTD-mfGO2 possessed superior thermal stability as compared to the dPTD-GO2 film, mainly assigned to the thermally stable phosphorusand silicon- containing units on the mfGO sheets [7, 25]. More interestingly, the char residues (at 600 °C) of polyurethane films were significantly increased with the incorporation of mfGO sheets. 21

Such increment in residual char could be attributed to the “tortuous path” effect of graphene sheets, and thus delaying the escape of volatile degradation substances and promoting the char yield [21, 51]. Actually, the efficiency of flame retardant fillers depends strongly on the structure and composition of the char residue when a condensed phase action is the main mechanism [25]. Consequently, exploring the properties of the resultant carbonaceous substances will provide an insight into understanding how the graphene filler acts in the condensed phase. Raman spectroscopy has been regarded as a powerful technique for characterizing carbonaceous materials[51]. Fig. S12 displays the Raman spectra of the char residues for as-prepared films heated in a muffle furnace at 600 °C for 10 min. And the integrated area ratio of ID/IG was employed to estimate the graphitization degree of the residual char [24]. That is, the lower of ID/IG value, the higher graphitization degree of carbonaceous materials. In comparison with neat dPTB (3.17) and dPTD (3.11) films, the ID/IG values for dPTB-mfGO2 and dPTD-mfGO2 composites significantly decreased to 2.83 and 2.78, respectively. By contrast, dPTD-GO2 film (2.99) showed a slightly reduction in ID/IG as compared to the neat dPTD film, due to the oxidation of the pristine GO at high temperature. The reduction in ID/IG value could be attributed to the increment of graphitized char, which could inhibit the thermal degradation of the polyurethane matrix [24, 52]. Cone calorimeter is an efficient tool for evaluating the combustion properties of polymer materials under real-world fire conditions. Herein, the flame retardancy of the representative dPTD, dPTD-mfGO2, and dPTD-GO2 films was measured by the cone calorimeter, and the relevant parameters including time to ignition (TTI), peak heat release rate (PHRR), time of peak heat release rate (TPHRR), total heat release (THR), and total smoke production (TSP) are listed in Table S7. It could be found that the TTI for dPTD-mfGO2 was delayed, whereas the TTI of dPTD-GO2 was 22

advanced, as compared to the neat dPTD. Moreover, the dPTD-mfGO2 and dPTD-GO2 composites showed 54% and 37% reductions in PHRR than that of the dPTD, respectively, a similar tendency was also observed for THR and TSP parameters. The reduction in PHRR and THR could be explained by the insulating barrier of graphene and the concomitantly formed carbonaceous block, thereby restraining the permeation of oxygen and the escape of volatile products during combustion [21]. Moreover, the effective smoke suppression for dPTD-mfGO2 composites could be ascribed to the synergistic catalyzing carbonization and barrier effect of mfGO2 [21, 24]. The residual chars for dPTD and its composites after the cone calorimeter test were further observed using a digital camera and SEM. Fig. 6d shows the digital photographs and SEM micrographs for the exterior residual chars. As can be observed from the macroscopic digital photographs, the neat dPTD was almost burned down in the central region after combustion, and many cracks are distributed on the surface of the residual char. However, the residual char for dPTD-GO2 was loosen and broken into small fragments, due to the severely combustion of GO at high temperature. By contrast, the incorporation of mfGO contributed to the formation of a relative continuous and compact char layer. As can be seen from the SEM micrographs, the residual char for neat dPTD showed a flat surface (Fig. 6d), and possessed loose and multi-porous features at high magnification (Fig. S13). This structure was caused by the emission of volatile gases during combustion. For the residual char of dPTD-GO2, the sample was burned into irregular fragments with a random distribution, similar to the observation from digital photographs. In comparison with dPTD and dPTD-GO2, the residual char for dPTD-mfGO2 was expanded, and its surface was much coarser. Additionally, the amounts of pores in the dPTD-mfGO2 char residue was significantly decreased as compared to the neat dPTD char residue. Concurrently, the dPTD-mfGO2 char residue 23

possessed a relative integral and compact carbonization block than that of the dPTD-GO2 char residue. These phenomena might be attributed to the coordinate catalytic carbonization of phosphorus- and silicon- containing mfGO. To confirm this conjecture, chemical composition of the residual chars was analyzed by SEM-EDS mapping technique. Table S8 summarizes the element percentages of the residual chars from SEM-EDS mapping analysis, and the SEM-EDS mapping micrographs of the representative dPTD-mfGO2 are shown in Fig. 6e. As can be seen, two new elements including P and Si were newly detected in dPTD-mfGO2 as compared to the residual chars of dPTD and dPTD-GO2. Moreover, the C, O, N, P and Si atoms exhibited a good distribution on the char surface of dPTD-mfGO2, indicated the P- and Si- containing components homogeneously distributed in the reinforced carbonization block. We further employed FT-IR spectroscopy to investigate the chemical structure of the residual chars, as shown in Fig. 6c. It could be found that the main peaks at around 3423, 2975-2875, 1629-1523 cm−1 were assigned to the H2O, C-H, and C=C components, respectively [7, 21]. In comparison with the residual chars of dPTD and dPTD-GO2, four new peak regions at around 1200 (P=O), 1161 (P-O), 1100-1000 (Si-O-Si/O-Si-C), and 755 (CAr-H) appeared in dPTD-mfGO2 char [7, 53]. These results indicated that the grafted P- and Si- containing units on the mfGO sheets might contribute to the growth of carbonaceous block with rigid silicon reinforced protective layer, and thus capable of achieving self-extinguish flames. The fire safety of dPTD and its composites was also estimated by limiting oxygen index (LOI) and UL-94 vertical burning experiments. As shown in Table S7 and Fig. S14, the neat dPTD showed a low LOI value of 19.9%, and burned with flaming drips. Addition of GO improved the LOI to some extent, but burned vigorously with a heavy flame, due to the drastically oxidation of pristine GO at 24

high temperature. For the dPTD-mfGO2 composite, the LOI was strikingly increased to 24.9%, and the flammability was obviously reduced with an UL-94 rating of V-2. Notably, the LOI value for dPTD-mfGO2 composite was comparable to the results reported in relevant published works [7, 24, 54]. The abovementioned results indicated that the introduction of mfGO (2 wt%) could effectively enhance the thermal stability and flame retardancy of polyurethane composite, and the probable mechanism is schematically shown in Fig. 7. Because of the “tortuous path” barrier effect of graphene sheet, the embedded mfGO in the dPTD matrix could hinder the release rate of the volatile products when the dPTD-mfGO2 composite is exposed to fire [21]. Incidentally, the degradation of mfGO could release few non-combustible gases to reduce the flammable volatiles and oxygen concentration during the combustion, and thus decreasing the flame propagation [7]. Moreover, the grafted phosphorus- and silicon- containing units were thermally oxidized into phosphorus and silicon layers covered on the mfGO surface, and further formed a synergistically strengthened carbonaceous protective layer [7, 25]. The formed carbonaceous barrier could decrease the oxygen and heat transportations and the release of volatile products, and enabling protect the underlying polymer composite [55]. Water contact angle measurement was carried out to evaluate the water resistance of composite films. As shown in Fig. S15, the water contact angles for the pure dPTB and dPTD films were 95.4° and 94.6°, respectively. After incorporating GO (2 wt%), the dPTD-GO2 exhibited a slightly decrease in water contact angle, owing to the hydrophilic feature of GO. By contrast, the dPTB-mfGO2 and dPTD-mfGO2 composite films containing 2 wt% of mfGO exhibited an increment in contact angles as compared to the pure dPTB and dPTD as well as dPTD-GO2 films, 25

mainly due to the existing of hydrophobic silica and urethane chains on the mfGO surface. Such improved water resistance probably contributes to the self-protective performance of composites in humid conditions.

Fig. 6. (a) TG and (b) DTG curves of dPTB, dPTB-mfGO2, dPTD, dPTD-mfGO2, and dPTD-GO2 films. (c) FTIR spectra for char residues of dPTD, dPTD-mfGO2, and dPTD-GO2 films after cone calorimeter test. (d) Digital photographs and SEM micrographs for exterior char residues of dPTD, dPTD-mfGO2, and dPTD-GO2 films after cone calorimeter test. (e) SEM-EDS mapping for exterior char residues of the representative dPTD-mfGO2 film after cone calorimeter test (C, red; O, green; N, blue; P, yellow; Si, purple).

26

Fig. 7. Schematic representation of the possible flame-retardant mechanism for dPTD-mfGO2 film (inset, digital photographs of the dPTD, dPTD-mfGO2, and dPTD-GO2 films before and after vertical burning test).

4. Conclusions In summary, we designed and synthesized a typical multi-functionalized graphene oxide/polyurethane composite (dPTD-mfGO) containing reversible diselenide bonds. The successful functionalization of mfGO was confirmed by FT-IR, TG-DTG, XRD, Raman, XPS, and SEM-EDS measurements. Moreover, the covalently grafted polymer chains on the mfGO sheets could improve the dispersion of mfGO in dPTD matrix and the compatibility between them. Considering the crystallization-induced and photo-thermal effects of mfGO and the dynamic exchange characteristic of diselenide bonds, the dPTD-mfGO2 composite (2 wt% mfGO) showed excellent shape memory and self-healing performances rapidly upon exposure to visible-near infrared (VIS-NIR) irradiation. In particular, the shape memory characteristics and healing efficiencies for dPTD-mfGO2 were maintained above 90% and 76% after three cycles, respectively. In addition, combustion experiments revealed that dPTD-mfGO2 composite possessed superior LOI (24.9%) and UL-94 rating (V-2) without flaming drips. Further contact angle measurement suggested the dPTD-mfGO2 composite 27

exhibited an enhanced water contact angle of 109.5°. These findings revealed that the introduction of 2 wt % mfGO to the dPTD matrix synergistically enhanced the toughness, shape memory, self-healing, flame retardancy, and water resistance in comparison with the neat dPTD film. We expect this work may shed light on the design of multifunctional stimulus-responsive composite materials.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21474065), the Sichuan Province Science and Technology Support Program (No. 2017GZ0422), the Fundamental Research Funds for the Central Universities, the National Innovation and Entrepreneurship Training Program for Undergraduate, and Sichuan University-Zschimmer & Schwarz CmbH & Co. KG Scholarships. The authors would thank Zhonghui Wang (College of Biomass Science and Engineering, Sichuan University) for her great help in FTIR analysis. The authors also acknowledge the valuable comments of potential reviewers.

References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666-669. [2] T.K. Jin, B.K. Kim, E.Y. Kim, H.K. Sun, M.J. Han, Synthesis and properties of near IR induced self-healable polyurethane/graphene nanocomposites, Eur. Polym. J. 49 (2013) 3889-3896. [3] S. Cheng, X. Chen, Y.G. Hsuan, C.Y. Li, Reduced graphene oxide-induced polyethylene crystallization in solution and nanocomposites, Macromolecules 45 (2011) 993-1000. [4] W. Du, Y. Jin, S. Lai, L. Shi, W. Fan, J. Pan, Near-infrared light triggered shape memory and self-healable polyurethane/functionalized graphene oxide composites containing diselenide bonds, Polymer 158 (2018) 120-129. [5] M. Bera, P.K. Maji, Effect of structural disparity of graphene-based materials on thermo-mechanical and surface properties of thermoplastic polyurethane nanocomposites, Polymer 119 (2017) 118-133. [6] L.C. Tang, Y.J. Wan, D. Yan, Y.B. Pei, L. Zhao, Y.B. Li, L.B. Wu, J.X. Jiang, G.Q. Lai, The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites, Carbon 60 (2013) 16-27. [7] P. Zhang, P. Xu, H. Fan, Z. Sun, J. Wen, Covalently functionalized graphene towards molecular-level dispersed waterborne polyurethane nanocomposite with balanced comprehensive performance, Appl. Surf. Sci. 471 (2019) 595-606. [8] M. Zhang, Y. Li, Z. Su, G. Wei, Recent advances in the synthesis and applications of graphene–polymer 28

nanocomposites, Polym. Chem. 6 (2015) 6107-6124. [9] I.A. Kinloch, J. Suhr, J. Lou, R.J. Young, P.M. Ajayan, Composites with carbon nanotubes and graphene: An outlook, Science 362 (2018) 547-553. [10] J. Wang, X. Jin, C. Li, W. Wang, H. Wu, S. Guo, Graphene and graphene derivatives toughening polymers: Toward high toughness and strength, Chem. Eng. J. 370 (2019) 831-854. [11] M.D. Hager, S. Bode, C. Weber, U.S. Schubert, Shape memory polymers: Past, present and future developments, Prog. Polym. Sci. 49-50 (2015) 3-33. [12] J. Leng, X. Lan, Y. Liu, S. Du, Shape-memory polymers and their composites: Stimulus methods and applications, Prog. Mater Sci. 56 (2011) 1077-1135. [13] D. Habault, H. Zhang, Y. Zhao, Light-triggered self-healing and shape-memory polymers, Chem. Soc. Rev. 42 (2013) 7244-7256. [14] M. Kashif, Y.W. Chang, Supramolecular hydrogen-bonded polyolefin elastomer/modified graphene nanocomposites with near infrared responsive shape memory and healing properties, Eur. Polym. J. 66 (2015) 273-281. [15] Z. Yang, Q. Wang, T. Wang, Dual-triggered and thermally reconfigurable shape memory graphene-vitrimer composites, ACS Appli. Mater. Inter. 8 (2016) 21691-21699. [16] E. Wang, M.S. Desai, S.-W. Lee, Light-controlled graphene-elastin composite hydrogel actuators, Nano Lett. 13 (2013) 2826-2830. [17] M. Ji, N. Jiang, J. Chang, J. Sun, Near-infrared light-driven, highly efficient bilayer actuators based on polydopamine-modified reduced graphene oxide, Adv. Funct. Mater. 24 (2014) 5412-5419. [18] Y. Du, D. Li, L. Liu, G. Gai, Recent achievements of self-healing graphene/polymer composites, Polymers 10 (2018) https://doi.org/10.3390/polym10020114. [19] L. Huang, N. Yi, Y. Wu, Y. Zhang, Q. Zhang, Y. Huang, Y. Ma, Y. Chen, Multichannel and repeatable self-healing of mechanical enhanced graphene-thermoplastic polyurethane composites, Adv. Mater. 25 (2013) 2224-2228. [20] S. Wu, J. Li, G. Zhang, Y. Yao, G. Li, R. Sun, C. Wong, Ultrafast self-healing nanocomposites via infrared laser and their application in flexible electronics, ACS Appli. Mater. Inter. 9 (2017) 3040-3049. [21] Y. Shi, B. Yu, Y. Zheng, J. Yang, Z. Duan, Y. Hu, Design of reduced graphene oxide decorated with DOPO-phosphanomidate for enhanced fire safety of epoxy resin, J. Colloid Interface Sci. 521 (2018) 160-171. [22] Y. Zhang, B. Wang, B. Yuan, Y. Yuan, K.M. Liew, L. Song, Y. Hu, Preparation of large-size reduced graphene oxide-wrapped ammonium polyphosphate and its enhancement of the mechanical and flame retardant properties of thermoplastic polyurethane, Ind. Eng. Chem. Res. 56 (2017) 7468-7477. [23] Y. Jin, G. Huang, D. Han, P. Song, W. Tang, J. Bao, R. Li, Y. Liu, Functionalizing graphene decorated with phosphorus-nitrogen containing dendrimer for high-performance polymer nanocomposites, Compos. Part A-Appl. S. 86 (2016) 9-18. [24] B. Yu, Y. Shi, B. Yuan, S. Qiu, W. Xing, W. Hu, L. Song, S. Lo, Y. Hu, Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites, J. Mater. Chem. A. 3 (2015) 8034-8044. [25] X. Wang, L. Song, H. Yang, W. Xing, B. Kandola, Y. Hu, Simultaneous reduction and surface functionalization of graphene oxide with POSS for reducing fire hazards in epoxy composites, J. Mater. Chem. 22 (2012) 22037-22043. [26] S. Ji, W. Cao, Y. Yu, H. Xu, Dynamic diselenide bonds: exchange reaction induced by visible light without catalysis, Angew. Chem. 53 (2014) 6781-6785. [27] S. Ji, F. Fan, C. Sun, Y. Yu, H. Xu, Visible light induced plasticity of shape memory polymers, ACS Appli. Mater. Inter. 9 (2017) 33169-33175. [28] S. Ji, W. Cao, Y. Yu, H. Xu, Visible-light-induced self-healing diselenide-containing polyurethane elastomer, Adv. Mater. 27 (2015) 7740-7745. [29] W. Du, Y. Jin, J. Pan, W. Fan, S. Lai, X. Sun, Thermal induced shape-memory and self-healing of segmented 29

polyurethane containing diselenide bonds, J. Appl. Polym. Sci. 135 (2018) https://doi.org/10.1002/app.46326. [30] W. Du, Z. Zhang, H. Su, H. Lin, Z. Li, Urethane-functionalized graphene oxide for improving compatibility and thermal conductivity of waterborne polyurethane composites, Ind. Eng. Chem. Res. 57 (2018) 7146-7155. [31] B. Yu, X. Wang, W. Xing, H. Yang, L. Song, Y. Hu, UV-curable functionalized graphene oxide/polyurethane acrylate nanocomposite coatings with enhanced thermal stability and mechanical properties, Ind. Eng. Chem. Res. 51 (2012) 14629-14636. [32] X. Lü, Z. Cui, W. Wei, J. Xie, J. Liang, J. Huang, J. Liu, Constructing polyurethane sponge modified with silica/graphene oxide nanohybrids as a ternary sorbent, Chem. Eng. J. 284 (2016) 478-486. [33] Y. Y. Xue, Y. Liu, F. Lu, J. Qu, H. Chen, L. Dai, Functionalization of graphene oxide with polyhedral oligomeric silsesquioxane (POSS) for multifunctional applications, J. Phys. Chem. Lett. 3 (2012) 1607-12. [34] P.-Q. Wang, T. Chen, B. Yu, P. Tao, Y. Bai, Tollen's-assisted preparation of Ag3PO4/GO photocatalyst with enhanced photocatalytic activity and stability, J. Taiwan. Institut. Chem. E. 62 (2016) 267-274. [35] B. Ramezanzadeh, E. Ghasemi, M. Mahdavian, E. Changizi, M.H.M. Moghadam, Characterization of covalently-grafted polyisocyanate chains onto graphene oxide for polyurethane composites with improved mechanical properties, Chem. Eng. J. 281 (2015) 869-883. [36] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.B.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558-1565. [37] P. Cui, J. Lee, E. Hwang, H. Lee, One-pot reduction of graphene oxide at subzero temperatures, Chem. Commun. 47 (2011) 12370-12372. [38] M.-Y. Shen, C.-F. Kuan, H.-C. Kuan, C.-H. Chen, J.-H. Wang, M.-C. Yip, C.-L. Chiang, Preparation, characterization, thermal, and flame-retardant properties of green silicon-containing epoxy/functionalized graphene nanosheets composites, J. Nanomater. 22 (2013) 1-10. [39] S. Ganguli, A.K. Roy, D.P. Anderson, Improved thermal conductivity for chemically functionalized exfoliated graphite/epoxy composites, Carbon 46 (2008) 806-817. [40] D. Chen, H. Zhu, T. Liu, In situ thermal preparation of polyimide nanocomposite films containing functionalized graphene sheets, ACS Appli. Mater. Inter. 2 (2010) 3702-3708. [41] M. Wei, M. Zhan, D. Yu, H. Xie, M. He, K. Yang, Y. Wang, Novel poly (tetramethylene ether) glycol and poly (ε-caprolactone) based dynamic network via quadruple hydrogen bonding with triple-shape effect and self-healing capacity, ACS Appli. Mater. Inter. 7 (2015) 2585-2596. [42] X. Wang, J. Zhao, M. Chen, L. Ma, X. Zhao, Z.M. Dang, Z. Wang, Improved self-healing of polyethylene/carbon black nanocomposites by their shape memory effect, J. Phys. Chem. B 117 (2013) 1467-1474. [43] J. Zhang, Z. Qiu, Morphology, Morphology, crystallization behavior, and dynamic mechanical properties of biodegradable poly (ε-caprolactone)/thermally reduced graphene nanocomposites, Ind. Eng. Chem. Res. 50 (2011) 13885-13891. [44] X. Li, Z. Hong, J. Sun, Y. Geng, Y. Huang, H. An, Z. Ma, B. Zhao, C. Shao, Y. Fang, Identifying the phase behavior of biodegradable poly (hexamethylene succinate-co-hexamethylene adipate) copolymers with FTIR, J. Phys. Chem. B. 113 (2009) 2695-2704. [45] Z. Ding, L. Yuan, G. Liang, A. Gu, Thermally resistant thermadapt shape memory crosslinked polymers based on silyl ether dynamic covalent linkages for self-folding and self-deployable smart 3D structures, J. Mater. Chem. A. 7 (2019) 9736-9747. [46] A. Raghu, G. Gadaginamath, H.M. Jeong, N. Mathew, S. Halligudi, T. Aminabhavi, Synthesis and characterization of novel Schiff base polyurethanes, J. Appl. Polym. Sci. 113 (2009) 2747-2754. [47] W. Xiong, Y. Chen, M. Hao, L. Zhang, T. Mei, J. Wang, J. Li, X. Wang, Facile synthesis of PEG based 30

shape-stabilized phase change materials and their photo-thermal energy conversion, Appl. Therm. Eng. 91 (2015) 630-637. [48] K. Shi, Z. Liu, Y.-Y. Wei, W. Wang, X.-J. Ju, R. Xie, L.-Y. Chu, Near-infrared light-responsive poly (N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels with ultrahigh tensibility, ACS Appli. Mater. Inter. 7 (2015) 27289-27298. [49] J. Xia, S. Ji, H. Xu, Diselenide covalent chemistry at the interface: stabilizing an asymmetric diselenide-containing polymer via micelle formation, Polym. Chem. 7 (2016) 6708-6713. [50] N.R. Han, J.W. Cho, Click coupled stitched graphene sheets and their polymer nanocomposites with enhanced photothermal and mechanical properties, Compos. Part A-Appl. S. 87 (2016) 78-85. [51] X. Wang, S. Zhou, L. Song, W. Xing, B. Yu, X. Feng, Y. Hu, Self-assembly of Ni–Fe layered double hydroxide/graphene hybrids for reducing fire hazard in epoxy composites, J. Mater. Chem. A. 1 (2013) 4383-4390. [52] B. Yu, W. Xing, W. Guo, S. Qiu, X. Wang, S. Lo, Y. Hu, Thermal exfoliation of hexagonal boron nitride for effective enhancements on thermal stability, flame retardancy and smoke suppression of epoxy resin nanocomposites via sol–gel process, J. Mater. Chem. A. 4 (2016) 7330-7340. [53] H. Ren, Y. Zhou, M. He, R. Xu, B. Ding, X. Zhong, Y. Tong, L. Fan, Z. Cai, Enhanced mechanical properties of silica nanoparticles-covered cross-linking graphene oxide filled thermoplastic polyurethane composite, New J. Chem. 42 (2018) 3069-3077. [54] X. Wang, W. Xing, X. Feng, B. Yu, L. Song, Y. Hu, Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism, Polym. Chem. 5 (2014) 1145-1154. [55] G. Yuan, B. Yang, Y. Chen, Y. Jia, Preparation of novel phosphorus-nitrogen-silicone grafted graphene oxide and its synergistic effect on intumescent flame-retardant polypropylene composites, RSC Advances 8 (2018) 36286-36297.

31

The authors declare no competing financial interest.

32