Graphene oxide Nano-composite

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Materials and Interfaces

Construction of Polyurethane-imide/Graphene oxide Nano-composite Foam with Gradient Structure and Its Thermal Mechanical Stability Chengjie Li, Bing Hui, and Lin Ye Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Industrial & Engineering Chemistry Research

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Construction of Polyurethane-imide/Graphene oxide Nano-composite Foam with Gradient Structure and Its Thermal Mechanical Stability Chengjie Li, Bing Hui, Lin Ye* State Key Laboratory of Polymer Materials Engineering Polymer Research Institute of Sichuan University, Chengdu 610065, China *: Corresponding author E-mail: [email protected] ABSTRACT: Graphene oxide (GO)@Fe3O4 nano-hybrid with ferromagnetism was prepared and polyurethane-imide copolymer (PUI)/GO@Fe3O4 nano-composite foams with gradient structure were first fabricated under magnetic field. Fe3O4 nanoparticles intercalated into GO layers with high grafting ratio, resulting in complete exfoliation of GO in matrix. For the composite foam, the average cell size, cell wall thickness and apparent density gradually decreased along magnetic field direction, forming gradient cell structure in foam. Both mechanical property and thermal stability of foam were remarkably improved by addition of GO@Fe3O4. Compared with A region, in B region with enriched distribution of GO@Fe3O4, the compressive strength and modulus increased by 12.5% and 7.0%, respectively. The thermal degradation temperature, storage modulus and Tg were obviously improved, indicating enhancement of thermal mechanical stability of PUI along magnetic field, revealing formation of gradient distribution of thermal mechanical property in foam, which showed potential application prospective in aerospace and defense area. KEYWORDS: Polyurethane foam, Graphene oxide (GO)@Fe3O4 hybrid, Gradient structure, Thermal mechanical stability 1. INTRODUCTION Polyurethane foam (PUF) has been widely applied in the fields of rail transport, automobile, construction, aviation, aerospace, and so forth due to its porous nature, low thermal conductivity and high mechanical properties.1-4 Unfortunately, the PUF is ACS Paragon Plus Environment

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prone to the cracking reaction at high temperature, and presents low heat resistance because of the thermal unstable groups on its molecules, such as urea, allophanate, carbamate and biuret. For example, its long-term service temperature is below 85 oC, while it can be used in the range of 80~100 oC only for a few days, and maintains its mechanical properties without obvious deterioration at 120 oC only for a few hours,5,6 and severe thermal degradation quickly occurs above 200°C. As a result, the processing or applications of PUF cannot satisfy the requirement in high temperature environments.7,8 Graphene with typical two-dimensional structure with one-atom-thick planar sheet of sp2 carbon atom arrayed in a hexagonal lattice, has caused concern in academic and industry due to its superior performances, such as excellent mechanical strength, superior electrical and high thermal conductivities,9-13 which promises its potential in widespread applications. In addition, it has a very large aspect ratio and exhibits extraordinary barrier property ascribed to its intrinsic lamellar structure, restricting the diffusion of oxygen into the polymer matrix and spread outward of the degradation products. Meanwhile, graphene possesses high electron affinity to capture free radicals generated by polymer degradation. Therefore, graphene has a distinctive stabilizing effect on polymers. Yan et al.15 prepared PU/graphene (GN) foams and found that only 0.3wt% loading of GN led to 36% improvement of the compressive modulus and 16 oC increase in the glass transition temperature of PUF. Piszczyk et al.16 studied the mechanical and thermal properties of PU/RGO (reduced GO) foams, which showed that 0.5wt% RGO addition resulted in 30% increase in mechanical strength and improvement of thermal stability, and the temperature value corresponding to 5% mass loss was higher by 16 °C than that of neat PU foam. Santiago-Calvo et al.17 prepared rigid polyurethane (RPU) foams using polyols by



adding graphene oxide (GO) via in situ polymerization. The foams with small amount of GO (0.017, 0.033 and 0.088 wt%) showed a reduction of cell size up to 33%, and the thermal conductivity decreased. Meanwhile the mechanical property showed no any improvement. However, very few reports can be available on PU/GO foam.18-20 Gradient foam, containing continuous variation of cells with significant difference in size, as compared to the foam with uniform cells, is a new kind of foam with properties and functions graded distribution along certain direction. Recently, the gradient foams have caught the attentions of many researchers since this asymmetric composition and structure in space showed good designability and great advantages in optimizing mechanical, thermal, sound absorption capacity, etc. of whole components, while the interfacial incompatibility problems can also be avoided.21-23 As a consequence, the gradient polymer foams are urgently needed for the specific purpose and applications.24 However, the gradient PUF is scarcely reported since most methods for producing polymer foams intrinsically induce uniform cells.18 Esmailzadeh et al.25 prepared graded PUF (GPUF) by applying a layer-by-layer casting method and the compressive strength of FGPUF increased dramatically. Li et al.26 prepared multilayered polyurethane/graphene foams (PUGF) by stacking single-layered PUGF and the gradient concentration of graphene was realized, which was proved to be a facile approach to enhance the microwave absorbing (MA) performance compared with PUF with uniform cell structure. No literature can be available on the thermal stability of the gradient PUF. Magnetic nanoparticles, such as Fe3O4, have attracted worldwide research attention for their potential applications in environmental, electronic and biological fields because of their excellent magnetic property and low toxicity.27-30 However, Fe3O4 nanoparticles tend to aggregate and are difficult to disperse uniformly in matrix


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when being used to prepare magnetic polymer materials. Graphene oxide (GO), as the most important and significant derivative of graphene, can load a large number of Fe3O4 nanoparticles, and inhibit the intrinsic aggregation of Fe3O4 particles in the meantime, while attributing the ferromagnetism to GO.31-33 In addition, GO sheets can be functionalized to obtain the tailored interface with PUF via formation of hydrogen bonding owing to the availability of abundant hydroxyl, carboxyl and epoxy groups on its surface.14,34 In this work, firstly, GO@Fe3O4 nano-hybrid with ferromagnetism was prepared by hydrothermal method. And then based on the heat-resistance polyurethane-imide copolymers (PUI), which was studied in our previous work,35 PUI/GO@Fe3O4 nano-composite foams with gradient structure were first fabricated via prepolymer foaming method under the applied magnetic field. The nano-composite foams with the controllable gradient distribution of the cell structure, the mechanical property and thermal mechanical stability were expected to be achieved. 2. EXPERIMENTAL SECTION 2.1 Materials Toluene-2,4-diisocyanate (TDI) was obtained from Wanhua Polyurethane Co. Ltd (Yantai, China). Polycarbonate diol (PCDL) ( Mn , 2000) was purchased from Asahi Kasei Co. Ltd (Nantong, China). Both pyromellitic dianhydride (PMDA) and 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol (TMP) with analytical grade were bought from Kermel Chemical Reagent Co. Ltd (Tianjin, China). Graphene oxide (GO) powder with micron grade was obtained from the Sixth Element Materials Technology Co. Ltd. (Changzhou, China). Dimethylsilicone fluid was commercially obtained from Longxu Co. Ltd (Shanghai, China). 2.2 Preparation of the gradient PUI/GO@Fe3O4 foams



The magnetic GO@Fe3O4 nano-hybrid was prepared through a facile one-pot method by precipitating irons salts onto GO sheets:26 0.1 g GO was dispersed into 100 ml deionized water under ultrasound for 30 min at room temperature, while the pH was adjusted to 11 with ammonium hydroxide (NH3·H2O). Then a certain amount of FeCl2·4H2O, FeCl3·6H2O and hydrazine hydrate were added into the GO dispersion under continuous stirring for 4 h. The resultant mixture solution was centrifugated and washed with deionized water for several times, finally yielding the ferromagnetic GO@ferroferric oxide (GO@Fe3O4) hybrid. The PUI/GO@Fe3O4 foams with gradient structure were prepared via prepolymer foaming method as follows: the stoichiometric amount of PCDL was dried under vacuum at 120 °C. Then the bulk temperature was lowered to 80 °C, an appropriate amount of TDI was added, and the prepolymer was obtained. Meanwhile, 1wt% content of GO@Fe3O4 nano-hybrid was dispersed in dimethyl formamide (DMF) by ultrasonication for 30 min. Afterwards the mixed GO@Fe3O4/DMF solution and the first part of PMDA were both added into the obtained prepolymer, and the reaction continued for 2 h. Finally, the curing agent (TMP), the residue PMDA as foaming agent and dimethylsilicone fluid as foaming stabilizer were added and stirred for 60 s. The resultant viscous mixture was poured into a foaming mold, cured and foamed in the oven under the external magnetic field, and thus the gradient PUI/GO@Fe3O4 nano-composite foams was obtained. 2.3 Measurements 2.3.1 FT-IR analysis The composition analysis of GO and GO@Fe3O4 samples was conducted with a Nicolet-560 Fourier-transform infrared spectrometer (FT-IR) (U.S.A), and the resolution of 4 cm-1 were applied.


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2.3.2 Raman analysis The structure characteristics of the GO and GO@Fe3O4 samples were analyzed with a RENISHAW Invia Raman microscope (U.K.) in the range of 300-3000 cm−1. Argon ion laser excitation source at 532 nm was applied. 2.3.3 XPS analysis The XPS measurement of the GO and GO@Fe3O4 was conducted with a XSAM 800 spectrometer (KRATOS Co., UK), using AlKa radiation (1486.6 eV) at a pressure of 2.0×10-7 Pa. 2.3.4 XRD analysis The XRD analysis of the GO and GO@Fe3O4 were conducted with Rigaku D/max III B X-ray diffraction equipment (XRD) (Japan) with the scanning range of 2θ=3-100°. Cu Kα radiation (λ=0.154 nm) was applied at voltage of 40 kV and current of 40 mA. The d-spacing of the GO layers was calculated with the following Bragg equation: 2d sin ߠ = ݊ߣ

(1)

where θ is the diffraction angle (o); n is the order of diffraction, and λ is the incident wavelength (nm). 2.3.5 TEM analysis The morphology of PUIF/GO@Fe3O4 foams was observed on JEOL JEM 100CX II TEM equipment (Japan). The acceleration voltage of 300 kV was applied. The samples were prepared by thin sections on a Leica ultramicrotome under cryogenic conditions. 2.3.6 SEM analysis The morphology of GO@Fe3O4 nano-hybrid and the fractured surface morphologies of PUI/GO@Fe3O4 foams were observed with a JEOL JSM-5900LV



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scanning electron microscope (SEM) (Japan) at the voltage of 15 kV. The water dispersion of the hybrid was deposited on the carbon-coated copper grids. The frozen dried foam samples were fractured in liquid nitrogen, and ion beam sputter-coated with gold. The energy-dispersive X-ray spectroscopy (EDS) measurement was conducted at the same time. 2.3.7 Apparent density According to standard ISO 845:2006, the apparent density of PUIF/GO@Fe3O4 foams with dimensions of 30×30×30 mm3 was measured and calculated with the following equation:

ρ=

m v

(2)

where ρ is the apparent density (kg·m−3) of the sample; m is the mass (kg) and v is the volume of the sample (m3). 2.3.8 Compressive properties The compressive test was carried out with a 5567 tensile testing machine from Instron Co. (U.S.A) by following ISO 844:244. The PUI/GO@Fe3O4 nano-composite foams were compressed to 10% strain. 2.3.9 TGA analysis The thermo-gravimetric analysis (TGA) of the PUI/GO@Fe3O4 foams was performed with a TA2950 thermobalance from TA Co. (U.S.A) under N2 atmosphere. The flow rate was 50 ml/min, and the heating rate was 10 °C/min. The granulated samples of about 5 mg were heated at room temperature to approximately 800 °C. 2.3.10 Dynamic mechanical analysis (DMA) The dynamic mechanical analysis (DMA) of PUI/GO@Fe3O4 nano-composite foams was carried out on TA Instrument Q800 DMA (U.S.A). A single cantilever mode was applied with heating rate of 3 °C/min and frequency of 1 Hz.


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3. RESULTS AND DISCUSSION 3.1 Intercalation structure of GO@Fe3O4 nano-hybrid GO@Fe3O4 nano-hybrid was prepared via hydrothermal method by precipitating irons salts onto GO sheets. The structural features of GO and GO@Fe3O4 nano-hybrid were analyzed by FTIR, as shown in Figure 1(a). It was found that GO exhibited typical absorption peaks at 3433 cm-1 (O-H stretching vibration), 2928 cm-1 and 2844 cm-1 (-CH2- asymmetric and symmetric stretching vibrations), 1734 cm-1 (C=O stretching vibration), 1625 cm-1 (O-H bending vibration, or C=C skeletal ring vibration),36,37 1399 cm-1 (C-OH stretching vibration), and 1063 cm-1 (C-O-C asymmetric stretching vibration). These vibrations revealed the existence of hydroxyl (O-H), carbonyl (C=O) and epoxy (C-O-C) groups on GO surface.38 For GO@Fe3O4 nano-hybrid, the characteristic peaks assigned to Fe-O bonds vibration in the crystalline lattice of Fe3O4 shifted to 890 and 583 cm-1 from 570 and 375 cm-1 respectively, as a result of the increasing surface bond force constant by interaction with GO.39-41 And further red-shift of O-H group from 3433 cm-1 to 3425 cm-1 suggested that Fe3O4 was absorbed and interacted with the hydroxyl groups (O-H) on GO sheets through strong hydrogen bonding interactions.42,43 Meanwhile, the peak at 1734 cm-1 corresponding to C=O groups of GO disappeared, indicating partial reduction of GO. Raman spectra of GO and GO@Fe3O4 nano-hybrid were shown in Figure 1(b). For GO sample, two peaks at 1355 cm-1 and 1590 cm-1corresponded to the D band and G band.44,45 For GO@Fe3O4 nano-hybrid, besides the above D/G bands, the characteristic peaks at 216 cm-1, 281 cm-1 and 664 cm-1 attributed to Fe3O4 particles were also observed. The intensity ratio of ID/IG, correlating to the disorder degree and the average size of the sp2 domains of GO,46 slightly increased for GO@Fe3O4 (1.321)



compared with GO (1.004), indicating that the disorder degree and defect density of GO increased resulting from the intercalation with Fe3O4. The element composition of GO and GO@Fe3O4 was analyzed through XPS spectra, as depicted in Figure 1(c). For GO sample, only two peaks at 286.2 eV (C1s) and 533.2 eV (O1s) were detected, and the C/O atomic ratio of GO was 3.98:1. For GO@Fe3O4 sample, besides the peaks assigned to C1s and O1s, the new peaks at 710-720 eV corresponding to Fe element was also observed, and the C/O/Fe atomic ratio of GO@Fe3O4 was 8.48:4.17:1, indicating the intercalation of Fe3O4 onto GO layers. The Fe core level spectrum of GO@Fe3O4 nano-hybrid with peak-fitting curves exhibited two typical peaks: Fe(2p)3/2 and Fe(2p)1/2 at 711.3 eV and 724.3 eV, respectively, further demonstrating the presence of Fe3O4 particles in GO layers,47,48 which was consistent well with FT-IR and Raman results. According to C/O/Fe atom ratio, the grafting ratio of Fe3O4 onto GO surface was calculated to be 52.56 wt%. Based on the above analysis, we can speculate that strong interaction formed between Fe3O4 and GO layers, which made Fe3O4 particles well intercalated into GO layers, as shown in Figure 1(d).


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Figure 1 FT-IR spectra (a), Raman analysis (b) and XPS analysis (c) of GO and GO@Fe3O4, and the intermolecular interaction between Fe3O4 particles and GO surface (d)

The XRD patterns of GO and GO@Fe3O4 nano-hybrid were illustrated in Figure 2(a). The sharp characteristic peak for (001) plane of GO could be found at 2θ=11.8°, with interlayer spacing of ~0.77 nm. For GO@Fe3O4 nano-hybrid, the diffraction peak for (001) plane of GO disappeared, and the new obvious diffraction peaks at 2θ=17.9°, 30.1°, 35.6°, 43.4°, 53.8°, 57.4°, 63.1° and 74.4° corresponding to Fe3O4 particles were observed,39,49 suggesting that GO was completely exfoliated resulting from the intercalation of Fe3O4 onto GO layers. Figure 2(b) showed that GO surfaces were decorated by a large number of spherical Fe3O4 nano-particles, and the Fe3O4 particles



were well-integrated with GO, indicating the strong interactions between Fe3O4 and GO. To intuitively observe the ferromagnetism of GO@Fe3O4 nano-hybrid, the external magnetic field was applied on its DMF dispersion, as depicted in Figure 2(c). It was found that GO@Fe3O4 nano-hybrid migrated and accumulated in strong magnetic field region.

Figure 2 XRD spectra (a) of GO and GO@Fe3O4, SEM images (b) of GO@Fe3O4 nano-hybrid at different magnifications and the photographs (c) of GO@Fe3O4 in DMF and its response to an external magnetic field

3.2 Preparation and cell microstructure of the gradient PUI/GO@Fe3O4 nano-composite foam The gradient PUI/GO@Fe3O4 nano-composite foam was synthesized via prepolymer foaming method under the applied magnetic field, as shown in Scheme 1. GO@Fe3O4 nano-hybrid with the content of 1wt% was added in the prepolymer


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formed by the reaction of PCDL with TDI, which generated the soft-segment of PUI. The hard-segment of PUI was generated by reaction of TDI with PMDA as the chain extender, and the product CO2 acted as the gas for bubble growth. The gradient distribution of GO@Fe3O4 in PUI matrix can be achieved along the magnetic field direction (from the A region to the B region).

Scheme 1 Schematic representation of synthesis of the gradient PUI/GO@Fe3O4 nano-composite foam

TEM was used to observe the dispersion of GO@Fe3O4 in the PUI/GO@Fe3O4 nano-composite foam, as shown in Figure 3. In the A region under the weak magnetic field, the GO@Fe3O4 nano-hybrid showed relatively sparse distribution, while enrichment distribution can be observed in the B region under strong magnetic field. In addition, it can be observed that GO@Fe3O4 was embedded and homogeneously



dispersed in their respective regions (marked by red arrows), illustratting the strong interfacial interaction and excellent compatibility between GO surface and PUI chains, thus resulting in the gradient distribution of GO@Fe3O4 in the PUI matrix.

Figure 3 TEM images of A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam

Figure 4 showed the SEM images of the fractured surface of PUI/GO@Fe3O4 nano-composite foam, and EDS element analysis of the A and B regions of the foam. The relevant data, such as average cell wall thickness and cell size, were illustrated in Figure 5(a). The composite foam showed a closed cellular structure with almost circular cell shape. The gradually changed cell morphology formed along the magnetic field direction, resulting in the gradient distribution of the cell structure for the whole foam. In the A region of the foam, the cell size showed a relatively wide distribution in the range of 100~450 µm, and the average cell size and the cell wall thickness were 194.4 µm and 47.3 µm, respectively. In the B region of the foam, the cell size showed a relatively uniform distribution in the range of 100~300 µm, and the average cell size and the cell wall thickness were 173.2 µm and 42.1 µm, respectively. Therefore, along the strong magnetic field direction, the cell morphologies with the thinner cell wall, the smaller cell size and the narrower cell size distribution formed. EDS result showed that A, B regions of the foam consisted of C, O and Fe elements, and the Fe element content in the B region was 2.96wt%, much higher than that in the


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A region (1.17wt%), suggesting that GO@Fe3O4 nano-hybrid densely arranged along the external magnetic field direction, resulting in the formation of the gradient cell structure for the whole foam. On the one hand, the well dispersed GO@Fe3O4 acted as nucleating agent to accelerate heterogeneous nucleation of bubble during foaming process.50-52 On the other hand, the strong interactions between GO@Fe3O4 layers and PUI molecular chains led to the formation of the GO@Fe3O4-centered secondary network structure, which restricted the cell to grow freely and reduced the average cell size.



Figure 4 SEM images (magnification: ×50), EDS spectra and the corresponding cell size distribution of A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam

The apparent density of the gradient PUI/GO@Fe3O4 nano-composite foam was illustrated in Figure 5(b). It was observed that compared with the neat PUI foam, the apparent density of the nano-composite foam increased, and in the A region of the


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foam, the apparent density (536.7 Kg/m3) was higher than that in the B region of the foam (515.6 Kg/m3), as a consequence of the increasing cell wall thickness.

Figure 5 The average cell wall thickness and cell size (a), the apparent density (b) of PUI foam and the gradient PUI/GO@Fe3O4 nano-composite foam

3.3 Compressive behavior of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 6(a) showed the compressive stress-strain curves of PUI foam and the gradient PUI/GO@Fe3O4 composite foam. It can be observed that all foams exhibited a multistage deformation response when applied with compressive loading, i.e., initial linear elasticity region (I), regulated by the overall elastic bending of cell walls; continuously increasing region (II), depending on collapse of weak cells structure; and yield plateau region (III), accompanied by the complete collapse and contact between opposing cell walls.53 The compressive strength and modulus of the foam were illustrated in Figure 6(b). It can be seen that, compared with the A region of PUI/GO@Fe3O4 nano-composite foam, a roughly 12.5% increase in compressive strength at 10% strain can be observed for the B region, from 10.35 to 11.64 MPa, and the compressive modulus at 10% strain increased by 7.0%, from 172 to 184 MPa. Moreover, the compressive strength and modulus of both A and B regions of PUI/GO@Fe3O4 nano-composite foam were much higher than that of PUI foam, displaying reinforcing effect of



GO@Fe3O4 on PUI foam.

Figure 6 The compressive stress-strain curve(a), compressive strength and modulus at 10% strain(b) and initial linear elasticity region (c) of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam

3.4 The thermal stability of the gradient PUI/GO@Fe3O4 nano-composite foam TGA curves under N2 atmosphere of PUI/GO@Fe3O4 nano-composite foams were shown in Figure 7 and the thermal degradation parameters were listed in Table 1. The onset degradation temperature (Tonset) and the end degradation temperature (Tend) represent the temperature at the intersection point of the tangent line at the fastest thermal weight loss rate with the extension line for the start and end of degradation, respectively. The peak degradation temperature (Tpeak) is the temperature at the point of the maximal thermal degradation rate. As shown in Figure 7, it was observed that the TGA curves showed a double inverse ‘S’ shapes, indicating three steps of degradation process for the gradient PUI/GO@Fe3O4 nano-composites foam. The first step of degradation occurred in the


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range of 320~400 °C, which was owing to the degradation of PCDL soft block, resulting in the formation of CO2, H2O and CO gas, etc. The second step of degradation occurred in the range of 440~510°C, which was attributed to the fracture of crosslinking point in hard segment. The third step of degradation appeared in the range of 570~665 °C, which may be attributed to the decomposition of imide group. Compared with the neat PUI foam, the gradient PUI/GO@Fe3O4 nano-composite foam showed higher thermal degradation temperatures and thermal stability, and the reason was probably due to the physical barrier effect of GO@Fe3O4 with a large specific surface area.54 Moreover, at the first degradation stage, Tonset, Tend and Tpeak for the B region of PUI/GO@Fe3O4 foam were 11 °C, 5 °C and 8 °C higher than that of the A region, respectively, while a slight increase at the second and third degradation stages can be observed. The above results indicated that the gradient nano-composite foam exhibited higher thermal stability in the enrichment region of GO@Fe3O4, and thus the gradient distribution of thermal stability formed under the applied magnetic field.



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Figure 7 TGA curves and the derivative thermograms of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Table 1 Thermal degradation characteristic temperatures of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam First stage

Second stage

Third stage

T1onset

T1peak

T1end

T2onset

T2peak

T2end

T3onset

T3peak

T3end

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

PUI

323

373

394

440

450

508

569

612

657

A

324

376

394

443

451

513

572

613

658

B

335

381

402

444

451

514

573

614

661

Sample

3.5 The thermal mechanical stability of the gradient PUI/GO@Fe3O4 nano-composite foam The dynamical mechanical property of the neat PUI foam and the gradient PUI/GO@Fe3O4 nano-composite foam was displayed in Figure 8, and the DMA parameters were listed in Table 2. It was found that below the glass transition


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temperature (Tg), the storage modulus (G’) for all samples decreased with increasing temperature, and tended to be the same when the temperature reached Tg and the foams entered high elastic state, due to relatively free motion of the chain segments. Compared with the neat PUI foam, the storage modulus of both A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam increased due to the reinforcing effect of GO@Fe3O4 on PUI. In addition, the storage modulus of the B region of PUI/GO@Fe3O4 nano-composite foam at room temperature was obviously higher than that of the A region, reaching as high as 560.83 MPa, which was attributed to the reinforcement of the enriched GO@Fe3O4 in the B region under the external magnetic field. As shown in Figure 8(b), all samples showed two characteristic loss peaks, assigning to Tg1 of the PCDL soft block and Tg2 of the hard block. Compared with the neat PUI foam, the Tg of both A and B regions of gradient PUI/GO@Fe3O4 nano-composite foam increased, due to the restriction of molecular chains and improvement of stiffness by addition of GO@Fe3O4. Moreover, the Tg of the B region of the composite foam was slightly higher than that of the A region, which was attributed to the more molecular chains entanglement due to the enrichment distribution of GO@Fe3O4 in the B region under the external magnetic field.

Figure 8 The dynamical mechanical property of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam



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Table 2 DMA parameters of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Room temperature Low temperature Sample Tg1/°C Tg2/°C modulus/MPa modulus/MPa PUI foam

670

419

22.8

227.5

A

754

473

25.6

228.4

B

823

561

27.3

229.1

3.6 The thermal conductivities of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 9 showed the thermal conductivities of the neat PUI foam and gradient PUI/GO@Fe3O4 nano-composite foam. Addition of GO@Fe3O4 resulted in an increase of the thermal conductivity compared with neat PUI foam. Meanwhile, the B region of PUI/GO@Fe3O4 nano-composite foam displayed a relatively higher thermal conductivity than that of the A region due to the enriched GO@Fe3O4 nano-hybrid, and thus the gradient distribution of thermal conductivity formed along the applied magnetic field direction.

Figure 9 The thermal conductivity of PUI foam, A and B regions of gradient PUI/GO@Fe3O4 nano-composite foam

4. CONCLUSIONS In this work, GO@Fe3O4 nano-hybrid with ferromagnetism was prepared by


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hydrothermal method, and polyurethane-imide (PUI)/GO@Fe3O4 nano-composite foam with gradient structure was first synthesized via prepolymer foaming method. An effective grafting of Fe3O4 onto GO surface was achieved by formation of hydrogen bonding between the hydroxyl groups on GO surface and Fe3O4 nanoparticles, which led to the complete exfoliation of GO in PUI matrix. For PUI/GO@Fe3O4 nano-composite foam, the gradually decline of the cell wall thickness, the average cell size and the apparent density could be observed along the external magnetic field direction, and thus cell structure with gradient distribution formed. In the B region with enriched distribution of GO@Fe3O4, it can be found that, compared with the A region, 12.5% and 7.0% increase in compressive strength and compressive modulus at 10% strain were observed, thus resulting in the formation of the gradient distribution of the mechanical property along the magnetic field direction. Meanwhile, the degradation process of the composite foam had three stages and the thermal degradation characteristic temperatures were improved by addition of GO@Fe3O4. For the B region of the composite foam, the thermal degradation characteristic temperature, the storage modulus and Tg increased, and a relatively high thermal conductivity was obtained compared with the A region, forming gradient distribution of the thermal mechanical stability and thermal conductivity along the magnetic field direction. Acknowledgements This work was supported by Joint Fund of National Natural Science Foundation of China and China Academy of Engineering Physics (NSAF) (No. U1530144). REFERENCES (1) Wang, C.; Wu, Y.; Li, Y.; Shao, Q.; Yan, X.; Han, C.; Wang Z.; Liu Z.; Guo, Z. Flame-Retardant Rigid Polyurethane Foam with a Phosphorus-nitrogen Single



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Caption Figure Captions Scheme 1 Schematic representation of synthesis of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 1 FT-IR spectra (a), Raman analysis (b) and XPS analysis (c) of GO and GO@Fe3O4, and the intermolecular interaction between Fe3O4 particles and GO surface (d) Figure 2 XRD spectra (a) of GO and GO@Fe3O4, SEM images (b) of GO@Fe3O4 nano-hybrid at different magnifications and the photographs (c) of GO@Fe3O4 in DMF and its response to an external magnetic field Figure 3 TEM images of A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 4 SEM images, EDS spectra and the corresponding cell size distribution of A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 5 The average cell wall thickness and cell size (a), the apparent density (b) of PUI foam and the gradient PUI/GO@Fe3O4 nano-composite foam Figure 6 The compressive stress-strain curve(a), compressive strength and modulus at 10% strain (b) and initial linear elasticity region (c) of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 7 TGA curves and the derivative thermograms of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 8 The dynamical mechanical property of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Figure 9 The thermal conductivity of PUI foam, A and B regions of gradient PUI/GO@Fe3O4 nano-composite foam



Table Captions Table 1 Thermal degradation characteristic temperatures of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam Table 2 DMA parameters of PUI foam, A and B regions of the gradient PUI/GO@Fe3O4 nano-composite foam


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Graphene oxide Nano-composite

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