Effects of amino-functionalized graphene oxide on the mechanical and

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Effects of amino-functionalized graphene oxide on the mechanical and thermal properties of polyoxymethylene Xiaoyu Meng, Mengliu Wang, Lide Yang, Hai-Mu Ye, Chuanbo Cong, Yuhua Dong, and Qiong Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02698 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Industrial & Engineering Chemistry Research

Effects of amino-functionalized graphene oxide on the mechanical and thermal properties of polyoxymethylene Xiaoyu Meng*†,‡, Mengliu Wang†, Lide Yang†, Haimu Ye†,‡, Chuanbo Cong†,‡, Yuhua Dong †, ‡

, Qiong Zhou†,‡

†

College of Science, China University of Petroleum (Beijing), Beijing, 102249, China.

‡

Beijing Key Laboratory of Failure, Corrosion and Protection of Oil/gas Facilities.

*

Corresponding author at: College of Science, China University of Petroleum (Beijing),

Beijing, 102249, China. Tel.: +86 10 89733973; fax: +86 10 89733973. E-mail address: [email protected] ABSTRACT: Polyoxymethylene (POM)/graphene oxide (GO) composites are fabricated using

a

melt

blending

method.

GO

is

modified,

respectively,

by

3-

aminopropyltriethoxysilane (KH550-GO) and polyethyleneimine (PEI-GO) to enable its uniform distribution in the POM matrix. The mechanical properties of the POM/GO composites are dramatically improved compared to those of pure POM even at a very low loadings of GO. The tensile strength, Young’s modulus, as well as storage and loss moduli increase by 17.1 %, 45.4 %, 97.3 % and 94.7 %, respectively. The thermal stability of the composites are also greatly promoted. Fourier transform infrared and X-ray photoelectron spectra reveal that the enhanced properties are mainly stemmed from the notable interfacial interactions between the ‒NH groups in GO sheets and POM chains. 1. INTRODUCTION Polyoxymethylene (POM) possesses an excellent combination of mechanical properties and a high application value in structural materials that require outstanding comprehensive performance.1-3

More efforts, however, still have to be carried out to satisfy increasingly

complex working conditions by various methods, such as forming POM/nanofiller composites.4-12 Graphene-based nanosheets are alternative nanofillers due to the intrinsic properties of graphene.13-20 The functional groups of graphene oxide (GO) can establish strong interactions with polymer matrices,21-23

and this interaction is crucial for effective

1 Environment ACS Paragon Plus

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enhancement. GO can also act as a diffusion barrier to improve the thermal stability of composites.24,25 Therefore, POM/GO composites are promising structural materials. Surface modification of nanofillers is a key method to enhance the properties of polymer composites.13,26 It usually modifies the compatibility and/or generate interactions between fillers and matrix. In case of GO sheets, which are easily well dispersed in aqueous media rather than organic media (e.g., polymers)14, the dispersion in POM are rather poor, and limited reinforcement were reported.27-32 Efforts have been therefore made regarding these modifications.33,34 For example, favorable surface functional groups have been introduced into GO sheets by various reactions.35-38 Pre-treated GO has demonstrated molecular-level dispersion and strong interfacial interactions.39-41 Studies have also shown that GO modified by amino groups exhibited dramatic strengthening effects.42-47 The incorporation of GO modified with low contents of organic amines improved the tensile strength of MC nylon/modified GO nanocomposites.27 The storage modulus of surface-imidized graphene nanocomposites increased by 25 % to 30 %,48 and the thermal stability improved compared to that of a polyimide/unmodified graphene nanocomposite.28 Modified GO significantly improved the mechanical properties of nanocomposites, with an addition of only 2 wt % modified GO32 Fewer reports have prepared POM/GO nanocomposites. Mohan et al.49 bridged graphene particles in a POM matrix by adding conducting polymers and found that the tensile strength of the resultant nanocomposite slightly decreased. The decreased tensile strength possibly stemmed from the weak interactions among the graphene derivative, conducting polymer, and thermoplastic polymer. In the present work, modified GO sheets were used as nanofillers by adopting surface amination to improve both the GO uniformity in the POM matrix and the interfacial interactions, and two kinds of amino molecules (small molecules and macromolecules) were used for comparison. 2. EXPERIMENTAL SECTION 2.1. Materials. POM 100p pellets (MI = 2.2 g/10 min) were purchased from DuPont (USA). Potassium persulfate was obtained from Tianjin Guang Fu Technology Development Co., Ltd. Phosphorus pentoxide was supplied by Tianjin Chemical Reagent Factory. Concentrated sulfuric acid (H2SO4, > 98 % purity), potassium permanganate, hydrochloric acid, and hydrogen peroxide (H2O2, > 30 % purity) were purchased from


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Beijing Chemical Factory. Natural graphite (NG, > 90 % purity) was purchased from Qingdao Jinrilai Graphite Co., Ltd. Polyethyleneimine (PEI) with a molecular weight of 1800 and N,N′-dicyclohexylcarbodiimide (DCC) with a molecular weight of 206 were purchased from Aladdin Reagents Co., Ltd. N,N′-dimethylformamide (DMF) was supplied by Beijing Chemical Company. 3-Aminopropyltriethoxysilane (KH550) was purchased from Beijing Shenda Fine Chemical Co., Ltd. 2.2. Preparation of GO and modified GO. GO was prepared from NG using modified Hummers’ method.50 Up to 0.25 g of GO was dispersed in DMF via ultrasonication for 60 min. PEI (0.5 g, MW = 1800) and DCC (0.25 g) were then incorporated into the dispersed solution and mechanically stirred at 120 °C for 24 h to fabricate a black flocculent product, which was then repetitively washed with tetrahydrofuran to remove the residual modifying agent. The PEI-GO product was dried at 80 °C under vacuum for 24 h. A certain amount of GO was collected and added to a 6 wt % KH550 solution (GO/KH550 quality ratio: 1/60). Afterwards, the premixed solution was ultrasonicated (100 W) for 1 h and then mechanically stirred at 80 °C for 24 h to fabricate a black flocculent product. The same washing and drying methods were then applied. Schematics for the surface modification of GO and melt-blending process with POM are shown in Fig. 1.42,43 2.3. Preparation of the composites. A given amount of GO or modified GO was dispersed in acetone, ultrasonicated (100 W) for 2 h to obtain a dispersed solution, and then mixed with POM while stirring the solution. After the solvent was completely volatilized, the polymer blends were melt compounded in a Brabender mixer at a rotating speed of 60 rpm for 10 min at 185 °C. The composites used in this study included three kinds of GO and modified GO (GO, KH550-GO, and PEI-GO) at different contents (0.1, 0.3, 0.5, 0.7, and 1.0 wt %). 2.4．． Characterization. The morphology of the GO layer was observed by atomic force microscopy (AFM) (NanomanVS). The morphologies of GO and the POM/GO composites were examined using transmission electron microscopy (TEM) with a JEM 2100 LaB6 microscope. Ultraviolet spectra (UV) were recorded on a UV-1700 instrument using a solution concentration of 0.5 mg/ml. Raman spectroscopy was conducted using an inViaReflex Raman system equipped with a 532 nm excitation laser. X-ray photoelectron



Figure 1. Schematics of the preparation of PEI-GO and KH550-GO and of the meltcompounding process with POM.

spectroscopy (XPS) of the nanocomposites samples was performed using a Thermo Fisher ESCALAB 250Xi system with a monochromatized AlKα X-ray source. Fourier transform infrared (FTIR) spectra of the samples were obtained on a Nicolet 670 instrument in absorbance mode using 32 scans within the scanning range of 4000 cm−1 to 400 cm−1. The samples were ground and pressed into KBr pellets. The nanocomposites with 1.0 wt % GO were used to perform XPS and FTIR measurements, and the samples were extracted with solvent to remove the free polymer chains. X-ray diffraction (XRD) was performed on a D8Focus machine using CuKα radiation (wavelength 0.154 nm) with a 2θ range from 5° to 45° at a scanning speed of 3°/min. The thermal stabilities of the samples were measured by thermogravimetric analysis (TGA, TG209) under argon gas from 20 °C to 800 °C at a heating rate of 10 °C/min. The mechanical properties were determined using an electronic


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universal tensile testing machine with a 50 mm/min speed at room temperature. The impact strength was measured with a GT-7045-MDH instrument. Dynamic mechanical analysis (DMA) was performed under ambient atmosphere using a Q800 DMA apparatus (TA Instruments) in tensile mode. The measurements were conducted at a constant frequency of 1 Hz, a strain amplitude of 0.2 mm from -100 °C to 100 °C, and a heating rate of 5 °C/min. The crystallization behavior was investigated by differential scanning calorimetry (DSC, DSC204F1, Netzsch) under argon flow. 3. RESULTS AND DISCUSSION 3.1. Morphologies of GO and the composites The morphology of GO is shown in Fig. 2. Fig. 2a shows a TEM image of the GO nanosheets, illustrating a flake-like single-layer structure. The selected area electron diffraction (SAED) pattern (inset in Fig. 2a) depicts an ordered graphitic structure.51 As shown in Fig. 2b, the size of the single-layer GO nanosheet is approximately 1 µm. The measured thickness of the GO nanosheet is approximately 1 nm (Fig. 2b), which is consistent with previous studies.41 Fig. 3a shows Raman spectra in 3500‒1000 cm−1 region of GO., and two strong characteristic bands were observed. One band at 1590 cm−1, denoted by G, corresponding to intrinsic Raman modes. The other band at 1349 cm−1, denoted by D, is induced by disorder. The intensity ratio of the D and G bands (ID/IG) is typically used to characterize the degree of disorder in graphite materials.52,53 As displayed in Fig. 3a, ID/IG markably increases, indicating the oxidation process has introduced disordered structures in the graphite lattice. The bands at 2701 and 3178 cm−1, corresponding to the overtones of

Figure 2. (a) TEM image and SAED pattern of the GO layers; (b) AFM image of singlelayer GO.



(a)

4.5 1349 1590

(b)

4.0

Raman Intensity (a.u.)

3178 GO NG

2.5 232

2.0 1.5

PEI-GO

Intensity (a.u.)

2701 2927

Abs (a.u.)

3.0 KH550-GO

(c)

PEI-GO KH550-GO GO

3.5

PEI-GO

296

1.0

KH550-GO 11.20° GO 26.54°

0.5 NG

0.0

1500

2000 2500 Wavenumber (cm-1)

3000

3500

200

300

400 500 600 Wavelength (nm) 110

(d)

2927

2849

1659

1565

PEI-GO

KH550-GO

1032 2927 2858

1639

1124 1574

3416 GO

700

800

10

20 30 2θ (Degree)

1622

1046

40

(e) NG

100 Weight loss (%)

1000

Absorbance

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

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90 80 70

PEI-GO

60

KH550-GO

50

NG

1573

GO

40

4000

3500

3000

2500 2000 1500 -1 Wavenumber(cm )

1000

500

100

200

300 400 500 600 Temperature(℃ )

700

800

Figure 3. (a) Raman spectra; (b) UV-vis spectra; (c) WAXD patterns; (d) FTIR spectra; (e) TGA curves for natural and modified GOs the D and the G bands, are typically referred as D* and G*, respectively. These bands are not present in defect-free graphite, and their relative intensity increases with the degree of disorder in graphite. The band at 2927 cm−1 is generated by a combination of D and G in the first-order spectrum, resulting from lattice disorder.54,55 The Raman results of modified GO suggest that the modification did not affect the graphitic structure. The intensities of the bands at 2701 and 3178 cm−1 increase in the KH550-GO spectra, which indicate more disordered structures. For PEI-GO, the ID/IG ratio further increases compared to that of GO, corresponding to increased disorder in the sheets. The surface of GO contains numerous hydrophilic groups, which facilitate good dispersion in aqueous solution. The absorption peak at 232 nm, ascribed to л→л* transitions of the aromatic graphene C=C bonds,

shifts to a shorter wavelength for

modified GO, (Fig. 3b). The peak at 296 nm , attributed to n→л* transitions of the carbonyl groups,56 indicating that the GO surface generates oxygen groups. After modification, the peak at 296 nm disappears resulting from the grafting reaction. Fig. 3c shows WAXD peak at 11.20°, corresponding to the (002) lattice, and the corresponding interlayer spacing of 0.7894 nm can be obtained using Bragg’s equation. The XRD curve of the pristine graphite contains a strong peak at 26.54°, corresponding to an interlayer spacing of 0.3355 nm, indicates that the interlayer spacing increases from 0.3355


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nm in the pristine graphite to 0.7894 nm in GO. This increased interlayer spacing is larger than that of NG, which is mainly ascribed to the functional groups in GO layers. After modification, the degree of disorder of the sheets evidently increases. However, the interlayer spacing of KH550-GO does not increase. It might originate from the partial reduction of GO by the amine, thus decreasing the number of oxidized groups. The interlayer spacing of PEI-GO increased can be due to the larger molecular weight of PEI. Fig. 3d illustrates the FTIR spectra of NG, unmodified GO, and modified GO. The C=C stretching band at 1573 cm−1corresponds to the graphene structure.57,58 In the FTIR spectrum of GO, the peaks at 3416, 1622, and 1046 cm−1 are attributed to -OH stretching vibrations, C=O stretching vibrations of the carboxylic groups (‒COOH), and C-O-C stretching vibrations, respectively, indicating these functional groups were successfully introduced. For PEI-GO, the band at approximately 1565 cm−1 is assigned to ‒NH stretching vibrations, indicating that the PEI molecules are grafted onto the GO surface. For KH550-GO, the characteristic peaks at 1574, 1032, 2927, and 2858 cm−1 are attributed to ‒NH, Si-O-Si, CH2, and ‒CH3 stretching vibrations, respectively, indicating that KH550 was grafted onto the GO surface. Thermal stabilities of GO and modified GO were analyzed by TGA. Fig. 3e shows the entire decomposition process from room temperature to 800 °C. NG does not decompose, whereas GO and modified GO exhibit different degrees of degradation , The weight losses at 100 °C corresponds to moisture volatilization. At approximately 200 °C, the apparent weight loss can be attributed to the decomposition of oxygen-containing groups. The initial decomposition of modified GO occurs at relatively high temperature, indicating its enhancement in thermal stability. This is likely originates from the partial reduction of modified GO by the amine.25,59,60 For the PEI-GO composites, an evident weight loss at 235 °C is due to the beginning of the grafted PEI detach from the GO surface. The curve gradually flattens at 480 °C, which provide by the GO skeleton after the complete detachment of the grafted groups. Similarly, the KH550-GO composites exhibit considerable weight loss at 480 °C can be caused by the grafted KH550 beginning to detach and then decompose at a higher temperature. Weight loss is provided by the GO skeleton at 600 °C.



The XPS spectra of various GOs are shown in Fig. 4a. The spectrum of NG shows only C 1s peaks at 285.0 eV, whereas the spectra of GO, KH550-GO, and PEI-GO exhibit an additional peak at 531.8 eV, corresponding to an O 1s signal. Moreover, the spectra of KH550-GO and PEI-GO exhibit a N 1s peak at 398.4 eV. As shown in Fig. 4b, GO exhibits a peak at 287.7 eV, corresponding to C=O bonding from the oxidation process. As shown in Fig. 4c, the O 1s peak intensity of modified GO evidently decreases mainly because of the partial loss of oxygen in the reaction process. Furthermore, the O 1s peak of modified GO shifts, which may be ascribed to the presence of hydrogen-bonding interactions in the modified GO sheets.61-65 The C 1s peaks of GO, PEI-GO, and KH550-GO are shown in Fig. 4d, 4e, and 4f, respectively. The C 1s peaks of GO (Fig. 4d) at 285.0, 285.4, 287.8, and 288.9 eV originate from C-C, C-OH, C=O, and COOR bonding, respectively, indicating that GO was successfully prepared.66 The C 1s spectra of KH550-GO and PEI-GO differ from that of GO. The C 1s peaks at approximately 285.8 and 288.0 eV originate from C-N and ‒NH-C=O bonding, respectively. As presented in Fig. 4f, the peak at 287.7 eV is assigned to C-N bonding. The ‒NH-C=O group arising from the reaction between PEI or KH550 and the carboxyl groups in GO67,68 demonstrates that PEI and KH550 are (b)

(a)

(c)

285.0(C-C)

O1s

KH550-GO O1s

N1s

C1s GO

287.8(C=O)

NG

100

300 400 Binding Energy (eV)

532.9

NG

Intensity (a.u.)

Intensity (a.u.)

PEI-GO

GO

Intensity (a.u.)

Si2p

532.3 KH550-GO

531.1

GO

600 282

500

284

PEI-GO

286 288 Binding energy (eV)

290

528

530

532 534 Binding energy (eV)

287.8(C=O)

280

282

284 286 288 290 Binding energy (eV)

292

294

Intensity (a.u.)

Intensity (a.u.)

288.9(C(O)/O)

285.8(C-N)

288.5(C-NHR)

280

538

285.4(C-O/C-NH2)

284.7(C-C) 285.0(C-C)

285.4(C-OH)

536

(f)

(e)

(d)

Intensity (a.u.)

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

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282

284 286 288 290 Binding energy (eV)

292

294

284.4(C-C)

280

287.8(C-NHR)

282

284

286 288 290 Binding energy (eV)

292

Figure 4. (a) Full survey XPS spectra, (b) C 1s spectra and (c) O 1s spectra of NG, GO, PEI-GO and KH550-GO; (d) C 1s spectra of GO, (e) KH550-GO and (f) PEI-GO.


294

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successfully grafted onto the GO surface via covalent bonds. The lower C(O)/O content in modified GO than that in GO is attributed to the reaction between PEI or KH550 and the carboxyl groups in GO. 3.2. Characterization of modified GO and the POM/GO composites TEM pictures of the modified GO morphology (Fig. 5) illustrate that the lamella structure of modified GO did not change. The large specific surface area can cause an unstable structure, as GO sheets would aggregate spontaneously to obtain reduced surface energy.

Figure 5. TEM images of (a) KH550-GO and (b) PEI-GO. With the addition of 0.5 wt % nanofiller, agglomerated GO sheets are observed (Fig. 6a) while the modified GO are uniformly dispersed in the POM matrix (Fig. 6b and 6d). As

Figure 6. TEM images of the POM/GO composites: (a) 0.5 wt % POM/GO; (b) 0.5 wt % POM/KH550-GO; (c) 1.0 wt % POM/KH550-GO; (d) 0.5 wt % POM/PEI-GO; (e) 1.0 wt % POM/PEI-GO.



shown in Fig. 6c and 6e, the modified GO tends to agglomerate as the GO content is increased to 1.0 wt %. The ‒NH groups on the modified GO surface can generate stronger interfacial interactions with the POM molecular chains. Both NG and modified GO over 1.0 wt% in composites displayed severe agglomerates (red circles in figure) can cause structural defects (Fig. 6a, 6c, and 6e) As shown in Fig. 7a, a relatively high N composition is achieved by PEI or KH550 grafting.69 The N 1s XPS peaks of KH550-GO at 399.5 and 402.0 eV are attributed to ‒NH‒ and ‒NH2 groups, respectively. The N 1s peak of PEI-GO at 399.7 is attributed to NH- groups. The observed binding energy shifts of the ‒NH‒ peaks for the POM/modified GO nanocomposites clearly indicate the presence of an effective interfacial interaction between POM and the -NH groups of modified GO.61-65 This interaction is beneficial for the good dispersion of GO in the matrix. The N 1s peaks of the POM/KH550-GO composites further increase, indicating stronger interfacial interactions. In Fig. 7b, the FTIR peak at 907 cm-1 is attributed to the C-O-C vibrations of the POM chains. The addition of the GO causes the peak to shift, and the addition of modified GO results in a greater shift, suggesting that stronger hydrogen-bonding interactions are formed between the modified GO and POM chains.70,71 (a)

(b)

399.7

N1s

POM/PEI-GO

Intensity (a.u.)

PEI-GO

Absorbance

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

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398.0 -NH-(399.5) 397.6

POM/KH550-GO

POM/GO

-NH2(402.0)

POM/KH550-GO

KH550-GO

POM

POM/PEI-GO

394

396

398 400 402 Binding energy (eV)

404

1000

975

950 925 900 Wavenumber(cm-1)

875

850

Figure 7. (a) N 1s XPS spectra; (b) FTIR spectra.

The incorporation of GO sheets in polymer matrices usually improves the thermal stability due to a physical barrier effect that retards diffusion of the degraded product. As shown in Fig. 8a, the thermal stabilities of the POM/GO nanocomposites decreases compared to that of the pristine POM, and it likely originates from polymer chain scission by the reactive hydroxyl radicals on the GO surface.72 The grafted groups on the modified


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GO can directly form hydrogen-bonding with the polymer matrix and become lipophilic, thus considerably promoting GO dispersion. These interactions are equivalent to the formation of a protective layer in the polymer. The protective layer of the modified GO prevents the POM degradation product from diffusion.24,25 Therefore, the addition of modified GO can improve the thermal stability of POM. The initial decomposition (a)

Weight loss (%)

100 80 60 40 20 0 250

0.3 wt % POM/GO 0.3 wt % POM/KH550-GO 0.3 wt % POM/PEI-GO POM

300

350 400 Temperature (oC)

450

(b)

Weight loss (%)

100 80 60 40

POM 0.3 wt % POM/KH550-GO 0.5 wt % POM/KH550-GO 0.7 wt % POM/KH550-GO 1.0 wt % POM/KH550-GO

20 0 250

300

350 Temperature (oC)

400

450

400

450

(c) 100

Weight loss (%)

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


80 60 40 20 0 250

POM 0.3 wt % POM/PEI-GO 0.5 wt % POM/PEI-GO 0.7 wt % POM/PEI-GO 1.0 wt % POM/PEI-GO

300

350 Temperature (oC)

Figure 8. (a) TGA plots of POM, 0.3 wt % POM/PEI-GO, and 0.3 wt % POM/KH550-GO; (b) TGA plots of POM, 0.3 wt % POM/KH550-GO, 0.5 wt % POM/KH550-GO, 0.7 wt % POM/KH550-GO, and 1.0 wt % POM/KH550-GO; (c) TGA plots of POM, 0.3 wt % POM/PEI-GO, 0.5 wt % POM/PEI-GO, 0.7 wt % POM/PEI-GO, and 1.0 wt % /POM/PEIGO.



temperatures of the composite materials with KH550-GO and PEI-GO increased by 18 °C and 12 °C, respectively, compared to that of pristine POM. In Fig. 8b and 8c, POM/KH550-GO and POM/PEI-GO present similar thermal stability characteristics. With an increase in the modified GO content, the thermal stability improves before declining with a further increase in the modified GO content. The GO sheets tend to agglomerate as the GO content increases, which hinders the physical barrier effect, resulting in a worsened thermal stability. 3.3. Mechanical properties of pure POM and the POM/GO composites Fig. 9 compares the mechanical properties of pure POM and the prepared POM/GO composites (including those with unmodified and modified GO). The typical stress-strain curves of POM and 0.3 wt % GO composites are shown in Fig. 10. For the POM/GO composite with unmodified GO, the tensile strength and elongation at break considerably decreased. However, the mechanical performances of the composites with the modified GOs were superior to those with unmodified GO. Compared to the tensile strength of pure POM, the tensile strength of the 0.5 wt % POM/KH550-GO composite improved from 57.9 MPa to 67.9 MPa, and the Young’s modulus of the 0.3 wt % POM/KH550-GO composite

(a)

POM/GO POM/KH550-GO POM/PEI-GO

68 64 60 56

35

Elongation at break (%)

Tensile Strength (MPa)

72

(b)


30 25 20 15

52 0.0

800

0.2

(c)

0.4 0.6 GO content (wt %)

0.8

0.0

1.0


40

0.2

0.4 0.6 0.8 GO content (wt %)

(d)

1.0


2

Impact strength (KJ/m )

750

Young's Modulus (MPa)

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

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700 650 600 550 500 450 0.0

0.2

0.4 0.6 0.8 GO content (wt %)

1.0

30 20 10 0 0.0

0.2

0.4 0.6 0.8 GO content (wt %)

1.0

Figure 9. Mechanical properties of pure POM and the nanocomposites: (a) tensile strength; (b) elongation at break; (c) Young’s modulus; (d) impact strength.


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improved from 485.9 MPa to 706.4 MPa. The tensile strength and Young’s modulus of the POM/PEI-GO composite improved from 57.9 and 485.9 MPa to 63.5 and 686.1 MPa, respectively, while the elongation at break only exhibited a slight decrease with an increasing amount of modified GO. Fig. 10 indicates that the pristine POM and composites show the similar deformation feature. A rounded yield point follows an initial elastic behavior at lower strains. The yield point appears at the similar strain. The mechanical properties of POM/GO composites are attributed to the uniform dispersion of the modified GO nanosheets in the POM matrix. However, once the GO content exceeds a certain limit, the sheets tends to spontaneously aggregate to reduce the surface energy, causing structural defects that degrade the composite mechanical properties. The increased composite modulus is mainly attributed to the large specific surface area of GO, which formed strong interactions with POM (verified by XPS). When the specific surface area of the particles increased, the interfacial area also increased, thereby force the polymer matrix unit (such as the chain segments) to rearrange. The decreased particle size led to a decrease in the GO packing fraction, as well as a consequent increased in the POM/GO composite modulus. The toughness can be determined by the area under the stress-strain curves73,74 or impact strength. In this work, the impact strength was analyzed (Fig. 9d). The impact strengths of the POM/GO nanocomposites decreased with an increasing GO loading. The impact strength of the 0.3 wt % modified GO nanocomposite was improved compared to that of pristine POM. In contrast to the 0.3 wt % modified GO nanocomposite, almost all the other nanocomposite impact strengths declined. 80 70 60

Stress (MPa)

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


50 40 30 POM 0.5 wt % POM/GO 0.5 wt % POM/KH550-GO 0.5 wt % POM/PEI-GO

20 10 0 0

5

10

15 20 Strain (%)

25

30

35

Figure 10. Typical stress-strain curves of POM and composites. 13 Environment ACS Paragon Plus


The DMA results are shown in Fig. 11. The storage modulus (E′) and loss modulus (E′′) increased after GO introduction. The increased E′ values of the composites indicate that the composite stiffness considerably improved, which is likely ascribed to both the enhanced crystallinity of POM and GO rigidity in the matrix after GO introduction.75,76 The E′′ values may mirror the consumed composite energies by the interfacial interaction and mutual friction between the matrix and filler4. The increased E′′ may be attributed to the stronger interaction between POM and modified GO. However, as the GO content further increases, the E′ and E′′ values of the composites decrease, as the excessive GO cannot be more extensively dispersed in the POM matrix.77,78 4.0x102

(a)

POM 0.1 wt % POM/KH550-GO 0.3 wt % POM/KH550-GO 0.5 wt % POM/KH550-GO 0.7 wt % POM/KH550-GO 1.0 wt % POM/KH550-GO

Storage Modulus (MPa)

4.0x103 3.5x103 3.0x10

3

2.5x10

3

2.0x103 1.5x103

(b)

POM 0.1 wt % POM/KH550-GO 0.3 wt % POM/KH550-GO 0.5 wt % POM/KH550-GO 0.7 wt % POM/KH550-GO 1.0 wt % POM/KH550-GO

3.5x102

Loss modulus (MPa)

4.5x103

3.0x102 2.5x102 2.0x102 1.5x102 1.0x102

1.0x103 5.0x101

5.0x102

0.0

0.0

-100

-50

0 50 100 Temperature (℃ )

-100

150

3

3.0x10

(c)

POM 0.1 wt % POM/PEI-GO 0.3 wt % POM/PEI-GO 0.5 wt % POM/PEI-GO 0.7 wt % POM/PEI-GO 1.0 wt % POM/PEI-GO

3.5x103 3.0x103 2.5x103 2.0x103 1.5x103 1.0x103

-50


150

2

(d)

POM 0.1 wt % POM/PEI-GO 0.3 wt % POM/PEI-GO 0.5 wt % POM/PEI-GO 0.7 wt % POM/PEI-GO 1.0 wt % POM/PEI-GO

2.5x102

Loss modulus (MPa)

4.0x10

Storage Modulus (MPa)

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

Page 14 of 26

2.0x102 1.5x102 1.0x102 5.0x101

5.0x102 0.0

0.0

-100

-50


150

-100

-50


150

Figure 11. DMA of POM and the POM/GO composites; (a) storage moduli of the POM/KH550-GO composites; (b) loss moduli of the POM/KH550-GO composites; (c) storage moduli of the POM/PEI-GO composites; (d) loss moduli of the POM/PEI-GO composites.

As shown in the WAXD patterns in Fig. 12, two apparent diffraction peaks, which correspond to the (100) and (105) crystal planes, are observed in all the samples. For the POM/GO composite, the (100) diffraction peak is observed at approximately 23°. With the


addition of GO in the composites, the (100) diffraction peak shifts toward lower angles, indicating an increase in the spacing of the interlayer. For the POM/modified GO composites, the (100) diffraction peak shifts toward higher angles, indicating that the interlayer spacing decreases. However, new diffraction peaks were not formed in the entire process, indicating that the improved mechanical properties of the POM/GO composite are not attributed to the formation of new crystalline phases. (a) 1.0 wt % POM/GO

Intensity (a.u.)

0.7 wt % POM/GO 0.5 wt % POM/GO 0.3 wt % POM/GO 0.1 wt % POM/GO POM

20

22

24 35 2θ θ (degree)

40

45

(b) 1.0 wt %POM/KH550-GO 0.7 wt %POM/KH550-GO

Intensity (a.u.)

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


0.5 wt %POM/KH550-GO 0.3 wt %POM/KH550-GO 0.1 wt %POM/KH550-GO POM

20

22

24 35 2θ (degree)

40

45

(c) 1.0 wt % POM/PEI-GO 0.7 wt % POM/PEI-GO

Intensity (a.u.)

Page 15 of 26

0.5 wt % POM/PEI-GO 0.3 wt % POM/PEI-GO 0.1 wt % POM/PEI-GO POM

20

22

24 35 2θ θ (degree)

40

45

Figure 12. WAXD patterns of pure POM and the POM/GO composites. (a) POM/GO; (b) POM/KH550-GO; (c) POM /PEI-GO. Crystallinity is an important factor that influences the physical properties, such as mechanical properties and melting point, of a crystalline polymer. The effect of GO on the crystallization behaviors of the POM/GO composites was analyzed via non-isothermal DSC experiments. As shown in Fig. 13, GO exhibits a heterogeneous nucleation effect during the composite crystallization because of the large specific surface area.79 Unmodified GO exhibited a considerably more efficient heterogeneous nucleation effect



than that of modified GO. However, hydrogen-bonding interactions were generated between the polymer matrix and surface groups of modified GO, and the ordered arrangement of the POM chains was hindered by GO. Both factors slightly increased the crystallization temperature of the composite material. As shown in the heating curves, the melting temperatures of the composites mainly increase because of the formation of hydrogen-bonding interactions between POM and modified GO. These hydrogen-bonding interactions reduce the flexibility of the molecular chains, resulting in a decreased melting entropy. (b)

(a)

1.0 wt % POM/GO

1.0 wt % POM/GO

0.7 wt % POM/GO

0.7 wt % POM/GO

0.5 wt % POM/GO

Endo

Endo

0.5 wt % POM/GO 0.3 wt % POM/GO

POM

POM

80

100

120 140 160 Temperature (℃ )

180

80

100

120

140

160

180

160

180

120 140 160 Temperature (℃ )

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Temperature (℃ )

(c)

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Endo


Endo

0.3 wt % POM/GO 0.1 wt % POM/GO

0.1 wt % POM/GO


0.3 wt % POM/KH550-GO 0.1 wt %POM/KH550-GO

0.1 wt %POM/KH550-GO POM

POM

80

100

120

140

160

180

80

100

Temperature (℃ )

120

(e)

(f)

1.0 wt % POM/PEI-GO

1.0 wt % POM/PEI-GO 0.7 wt % POM/PEI-GO

Endo



0.1 wt %POM/PEI-GO

0.1 wt % POM/PEI-GO

POM

POM

80

100

140

Temperature (℃ )

0.7 wt % POM/PEI-GO

Endo

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

Page 16 of 26

120 140 160 Temperature (℃ )

180

80

100

Figure 13. DSC plots of pure POM and the POM/GO composites: (a,c,e) crystallization curves; (b,d,f) melting curves.

4. CONCLUSIONS


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Single-layered GO sheets were exfoliated from graphite, modified and dispersed in a POM matrix to fabricate POM/GO composites. The GO modified with ‒NH groups possessed stronger interfacial interactions with the POM molecular chains and better dispersion in the POM matrix. Both thermal and mechanical performances of the composites with modified GO were superior to that of the composite with unmodified GO. The improved thermal stability was derived from the formation of a protective layer in the polymer. After surface modification, the strengthening effect was more pronounced, and the elongation at break was less reduced. In contrast to the 0.3 wt % modified GO composite, almost all the other nanocomposite impact strengths declined. The storage modulus and loss modulus increased after the introduction of GO, and a stronger interaction between POM and modified GO was also verified by the increased loss modulus. However, as the GO content exceeded a certain limit, the sheets spontaneously aggregated to reduce the surface energy, causing structural defects that degraded the composite mechanical properties. The WAXD results indicated that the improved mechanical properties of the POM/GO composite was not caused by the formation of any new crystalline phase. GO exhibited a heterogeneous nucleation effect, and the composite melting temperatures were improved mainly because of the formation of hydrogen-bonding interactions.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. ORCID Xiaoyu Meng: 0000-0002-5239-6866 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was financially supported by National Key Research and Development Plan (Grant No. 2016YFC0303700) and Science Foundation of China University of Petroleum, Beijing (Grant No. 2462015YO0602).



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Effects of amino-functionalized graphene oxide on the mechanical and

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