Mechanical properties and thermal analysis of graphene nanoplatelets reinforced polyimine composites

Si Zhang; Zhengjin Xiong; Jian Zhang; Xueting Zhang; Yuhang Chen; Yun Chen

doi:10.1515/epoly-2022-0060

Article Open Access

Mechanical properties and thermal analysis of graphene nanoplatelets reinforced polyimine composites

Si Zhang , Zhengjin Xiong , Jian Zhang , Xueting Zhang , Yuhang Chen and Yun Chen

Published/Copyright: August 22, 2022

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From the journal e-Polymers Volume 22 Issue 1

Abstract

The polymer with imine bonds (C═N) synthesized by condensation of aldehydes and amines was called polyimine. Graphene nanoplatelets (GNPs) were blended into polyimine by imine dynamic chemistry, and GNPs/polyimine (GNPs-P) composites were fabricated by heat-pressing. A series of thermal and mechanical properties have been tested for the matrix and GNPs-P composites. Thermogravimetric analyzer showed that the GNPs were able to improve the thermal stability of the GNPs-P composites. From the test of mechanical properties, GNPs-P composite with 0.5 wt% GNPs was superior to the matrix in bending and tensile properties. The bending and tensile strengths were 92.65 and 73.05 MPa, with an improvement of 18% and 5%. GNPs-P composites with 1 wt% GNPs showed the most significant advancement in impact properties, reaching an impact strength of 11.745 kJ·m⁻² with a gain of 21.6%. Cross-sectional observations using scanning electron microscope proved that the GNPs-P composites have brittle fractures. A small number of GNPs could synergize with the matrix by bridging the cracks, creating a crack diffusion resistance and a load transfer reinforcement effect, which improved the mechanical properties of the GNPs-P composites.

Keywords: polyimine; graphene nanoplatelets; composites; thermal properties; mechanical properties

1 Introduction

Polyimine, prepared from aldehyde and amine, is an emerging thermoset material, which can undergo imine metathesis. Thus, unlike typical thermoset materials, polyimine materials have many special properties such as plasticity, high ductility, recyclability, and self-healing due to the metathesis of imines (1,2). It is worth noting that polyimine is easy to synthesize and can be shaped by heat-pressing under relatively low pressure and temperature. This feature provides unlimited possibilities for today’s industrial applications. However, polyimine materials can only be used as special functional materials because their mechanical properties are weaker than those of commonly used plastics such as polypropylene (PP) and polyethylene (3). Therefore, polyimine composites are developed to improve their mechanical properties so that they can have greater application prospects. Attempts have been made to add inorganic (4,5,6) or organic reinforcing phases (7) to the matrix, leading to significant improvements in various properties of the polyimine composites such as mechanical properties, thermodynamic properties, and chemical stability.

Graphene is a naturally occurring carbonaceous material with a single layer of two-dimensional honeycomb-like crystalline structure, which has excellent optical, electrical, and mechanical properties. It is also the thinnest and strongest material in the world. Its high specific surface area of 2,630 m²·g⁻¹ (8) and high strength are used as an ideal reinforcing phase for polymers (9,10,11,12) to improve various aspects of polymer properties. Ayesha adds graphene to shape memory polymer. Presence of graphene has caused fast switching of temporary shape to the original shape in polymer/graphene nanocomposites (13). Yang et al. used oscillatory pressure-assisted sintering to add graphene nanoplatelets to SiC matrix composites, it was found that the relative density, flexural strength, hardness, and fracture toughness of SiC–GNPs composites were effectively improved (14). Zhao added GNPs into PP uniformly and found that the addition of GNPs could improve the comprehensive properties of nanocomposite, including antistatic properties, mechanical properties, and thermal stability (15).

In this paper, terephthalaldehyde (TA), diethylenetriamine (DETA), and triethylenetetramine (TETA) with a molar ratio of 3:0.9:1.4 were used to obtain polyimine (16). GNPs-P composites were successfully prepared by heat-pressing the synthesized polyimine with GNPs under certain conditions. Various properties of the matrix and GNPs-P composites were characterized and tested by fourier transform infrared spectrometer (FTIR), diffraction of X-rays (XRD), thermogravimetric analyzer (TGA), derivative thermogravimetry (DTG), scanning electron microscope (SEM), tensile tests, bending tests, impact tests, and hardness tests.

2 Experimental methods

2.1 Materials

TA, DETA, and TETA were purchased from Aladdin industries (China), with the purity of 98%, 99%, and 99%; GNPs were from Beijing Deke Daojin Science and Technology Co., Ltd. (China), with carbon purity of >99.5%. All chemicals were used at the time of receiving without further purification.

2.2 Polyimine preparation

The aldehyde and amine can produce polyimine by the imine complex decomposition reaction. Dynamic covalent chemistry was used to form the imine bonds of polyimine. The dynamic reaction of imines involved two distinct processes: imine condensation/hydrolysis and imine exchange. Therefore, the polymer materials with imine bonds could be formed, broken, and exchanged under certain stimuli. This article mainly formed imine through the condensation of the aldehyde group in TA and amine group in DETA and TETA (16,17). First, 6.34 mL of TA was measured in a 200 mL beaker, and 50 mL of ethyl acetate was added and stirred magnetically at 500 rpm for 30 min to ensure complete dissolution of the aldehyde. Then, 0.97 mL of DETA and 2.11 mL of TETA were measured and dispersed by sonication for 10 min to make a homogeneous mixture. The mixture of DETA and TETA was slowly added to the ethyl acetate solution of TA and stirred for about 5 min, gradually precipitating a yellow solid until the rotor did not stir reaction was completed. The yellow solid obtained from the reaction was dried in an oven at 60°C overnight to ensure that the solid was dry. The dried product was crushed into powder by a pulverizer and passed through a 200-mesh sieve to obtain polyimine powder.

2.3 GNPs-P composites preparation

GNPs-P composites have been prepared by dynamic covalent chemistry of imines (18,19,20). A series of GNPs-P composites were prepared by incorporating varied contents of GNPs into polyimine. The resultant GNPs-P composites were named as GNPs-P-0.25, GNPs-P-0.5, GNPs-P-0.75, GNPs-P-1, GNPs-P-1.5, and GNPs-P-2, corresponding to the nanocomposites with 0.25, 0.5, 0.75, 1, 1.5, and 2 wt% of GNPs, respectively. The specific preparation process was as follows: a quantitative amount of polyimine powder and GNPs were mixed through an YXQM planetary ball mill at a speed of 100 rpm for 60 min to ensure a homogeneous mixture of both. The resulting blends were poured into a fixed mold, and the mixture powder can be heat-pressed at 9 MPa, 80°C, to obtain a rectangular specimen of 35 mm × 5 mm × 4 mm by imine dynamic covalent chemistry. The thickness of the sample can be changed by varying the mass of the mixture. The 35 mm × 5 mm × 2 mm rectangular specimen was placed in a dumbbell mold and rubbed with 2,000 mesh sandpaper to obtain a 35 mm × 5 mm × 2 mm dumbbell specimen.

2.4 Testing and characterization

FTIR testing was carried out on an FTIR spectrometer manufactured by Thermo Fisher (USA); polyimine and GNPs-P composites powder were prepared using KBr pressed sheets.

XRD analysis was carried out on an XRD-6000 manufactured in Japan. The test conditions were as follows: voltage, 40 kV; current, 30 mA; anode Cu; wavelength, λ = 0.154 nm; scanning angle, 5–80° at room temperature; and scanning speed, 4°·min⁻¹ (21). Both polyimine and GNPs-P composites samples were in powder form.

TGA analysis was carried out on a Q600 TGA from TA, USA, weighing 5 mg of the solid sample at a time, with a temperature range of 23–800°C and a ramp rate of 10°C·min⁻¹. The whole test was carried out under N₂ protection.

SEM testing was carried out on a JEM-2100F (Japan Electron Co., Ltd.) SEM with impact sections and gold-sprayed surfaces before testing.

The tensile performance test was carried out on a WDW-005-type microcomputer-controlled electronic universal testing machine (Jinan Hengsi Shanda Instruments Co., Ltd.) at room temperature. The sample was a 35 mm × 5 mm × 4 mm rectangular sample, and the tensile speed was 1 mm·min⁻¹; each group of experiments was carried out four times in parallel and took the average value.

Bending performance test in WDW-005 type of microcomputer-controlled electronic universal testing machine (Jinan Hengsi Shanda Instruments Co., Ltd.) for three-point bending test was carried out at room temperature; the sample was a 35 mm × 5 mm × 4 mm rectangular one, and the speed was 1 mm·min⁻¹; each group of experiments was performed as four parallel tests, and the average value was taken.

The impact performance test was carried out on an XJJ-50S-type liquid crystal display simple beam impact tester (Jinan Hengsi Shanda Instruments Co. Ltd.) with pendulum energy of 7.5 J. The specimens were not chipped.

Hardness testing was carried out on an HBRVS-187.5 digital display Brauvel hardness tester in HRR mode with a 12.70 mm diameter ball indenter and 35 mm × 5 mm × 4 mm rectangular specimens.

3 Results and discussion

3.1 Polyimine and GNPs-P composites structures

3.1.1 FTIR analysis

The FTIR was employed to verify that GNPs were successfully incorporated into the matrix. In Figure 1, we can find that the spectrum of GNPs has two characteristic peaks, one is at near 1,640 cm⁻¹ caused by C═C double bond stretching vibration, and the other is at 3,460 cm⁻¹ from –OH group. The characteristic peak of imine (15) caused by C═N double bond is at 1,630 cm⁻¹ for the matrix. Because the two peaks from C═C double bond of GNPs and C═N double bond of polyimine are relatively close, the existence of GNPs is not obvious in the infrared spectrum of GNPs-p composites when the amount of GNPs is low. When the addition amount of GNPs is the maximum value (2%), two characteristic peaks near 1,640 cm⁻¹ were found for the spectrum of GNPs-P-2 (marked with a circle in Figure 1). Therefore, GNPs have been added to the matrix. In order to confirm the incorporation of graphene into polyimine, we also jointly verified the successful synthesis of GNPs-P composites using XRD tests.

Figure 1

FTIR spectra of GNPs, polyimine, and different GNPs-P composites.

3.1.2 XRD analysis

Figure 2 shows the XRD spectra of the GNPs, the GNPs-P composites, and polyimine. As shown in Figure 2, the spectrum of GNPs exhibited a strong peak at about 26.5°, and the spectrum of polyimine had a broad peak at around 20°. It can also be found that each GNPs-P composite has imine peaks at 2θ = 20°, and the peak of GNPs is also found at 2θ ≈ 26.5°. Therefore, it can be shown that GNPs have been incorporated into the matrix.

$Figure 2 The X-ray diffraction patterns of polyimine, GNPs, and different GNPs-P composites.$

Figure 2

The X-ray diffraction patterns of polyimine, GNPs, and different GNPs-P composites.

3.1.3 TGA analysis

In order to explore the effect of the addition of GNPs on the thermal stability of the matrix, TGA was used to analyze the relationship between the mass loss of the material and the temperature (22,23). It can be seen from Figure 3a that the five groups of samples begin to decompose at 85°C, and the weightless water evaporates before 85%. There are two main reaction stages in the whole process. One stage is at 260°C due to the opening of functional groups, and the loss of hydroxyl groups leads to weight loss. When the temperature reaches 450°C, the reaction rate reaches the maximum, and the carbonization weight loss of the sample tends to be gentle. By comparing the matrix and GNPs composites, it is found that GNPs can improve the degradation rate of the composites. For example, in Figure 3b, the matrix and GNPs composite have the fastest reaction rate at 450°C. Through comparison, it can be found that with the gradual addition of GNPs, the reaction temperature also increases. These results suggest that GNPs, as a layered material (24), can increase the volume by their high physical barrier properties and high thermal conductivity. Also, the interfacial interaction between the matrix and GNPs improves the resistance of oxygen molecular attack at high temperatures, thereby improving the thermal stability of the composites.

Figure 3

(a) TGA curves and (b) DTG curves of polyimine and different GNPs-P composites.

3.2 GNPs-P composites properties

3.2.1 Mechanical properties of GNPs-P composites

Figure 4 shows the mechanical properties of the polyimine and the GNPs-P composites with different mass fractions. As shown in Figure 4a, the tensile strength tends to increase first and then decrease as the GNPs content gradually increased, reaching a maximum of 73.05 MPa at 0.5 wt% GNPs content. GNPs-P composites show a 5% increase in tensile strength compared to the matrix. However, the tensile strength of the GNPs-P composites decreased rapidly with the continued addition of filler. When the GNPs content reached 1.5 wt%, the tensile strength of GNPs-P composites was only 30.75 MPa.

Figure 4

Different mechanical property curves of polyimine and different GNPs-P composites: (a) tensile strength, (b) bending strength, (c) impact strength, and (d) hardness strength.

The bending property curve and tensile property of GNPs-P composites were consistent (Figure 4b). When the content of GNPs was 0.5 wt%, the bending property of the composites reached the maximum, and the bending strength reached 92.65 MPa, which was 18% higher than that of the matrix. However, when the GNPs addition was more than 0.5 wt%, the addition of filler caused a significant decrease in the flexural properties of the GNPs-P composites, with a reduction of around 31.6% compared to the matrix at an addition level of 1.5 wt%. The mechanism affecting tensile strength was the same as that of bending strength. Small amounts of GNPs provide an excellent synergistic effect on the toughness of the matrix, dispersing internal local stresses and thus increasing the tensile and flexural strength of the GNPs-P composites. When the addition of GNPs exceeded 0.5 wt%, the aggregation of GNPs destroyed the tensile and bending resistance of the matrix itself. While forming stress concentration points inside (25,26), the bending performance decreased significantly.

In Figure 4c, the impact strength of the GNPs-P composites decreased slightly with the addition of GNPs, then increased and finally decreased sharply. When the GNPs content reached 1 wt%, the impact strength of the GNPs-P composites reached 11.745 kJ·m⁻², an increase of 21.6% compared to the matrix. However, at 1.5 wt% GNPs, the impact strength was only 7.59 kJ·m⁻², a decrease of 21.4% compared to the matrix. GNPs is a nanosheet material; its large specific surface area and nanothickness effectively prevent silvering, thus improving the toughness of the GNPs-P composites. Also, at additions above 1 wt%, the agglomerated GNPs generally affected the impact properties of the matrix. Bending and tensile properties show the same phenomenon. A comprehensive analysis of the mechanical properties of the GNPs-P composites showed that small amounts of GNPs can have some strengthening effect on the matrix.

Figure 4d shows the hardness of the polyimine and GNPs-P composites for each mass fraction. Due to the large hardness of the polyimine, the GNPs-P composites were able to maintain high values, all-around 126 HRR.

3.2.2 Micromorphology

Figure 5 shows the SEM of GNPs and a tensile section for each material. The “river-like” appearance can be seen in each stretch section, which indicated that the fracture type of polyimine and GNPs-P composites were brittle (27,28). The section of GNPs-P composites was rougher than polyimine (Figure 5a), because GNPs were scattered in the matrix. When the content of GNPs was 0.5 wt% (Figure 5b), some GNPs are scattered in the matrix. On the cross section, some turning cracks appear near the GNPs, the mechanical properties of the composites can be improved through the crack guiding mechanism. Therefore, GNPs forced the crack to turn to dissipate the loss of efficiency when the crack moved forward and increased the energy consumption by increasing the total fracture area, so as to improve the tensile strength of the material (Figure 4a). However, when the addition of the reinforcing phase exceeded 0.5 wt%, the agglomeration (Figure 5c–e) caused by uneven dispersion will affect the overall mechanical strength. The agglomerated GNPs will cause large cracks, which will not only fail to enhance the effect, but also affect the strength of the matrix (Figure 4a).

$Figure 5 SEM images of tensile fracture surfaces of (a) polyimine and GNPs-P composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, (e) 2 wt%, and (f) GNPs.$

Figure 5

SEM images of tensile fracture surfaces of (a) polyimine and GNPs-P composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, (e) 2 wt%, and (f) GNPs.

Interestingly, the phenomenon shown in the bending section (Figure 6) was consistent with that in Figure 5, dimples and GNPs appeared in the bending section (Figure 6b). The GNPs are dispersed in the matrix when the added amount of GNPs is 0.5 wt%. The scattered GNPs can bridge the cracks and help the substrate to bear the load in the cracks, which indirectly improves the flexural strength of the GNPs-P composite (Figure 4b). The aggregates and rough surfaces in Figure 6c–e also lead to the reduction in bending strength, indicating that 0.5 wt% GNPs can effectively improve the properties of the matrix, and more than 0.5% will affect the material properties.

$Figure 6 SEM images of bending fracture surfaces of (a) polyimine and GNPs-P composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, and (e) 2 wt%.$

Figure 6

SEM images of bending fracture surfaces of (a) polyimine and GNPs-P composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, and (e) 2 wt%.

In the impact section (Figure 7), 1 wt% of GNPs showed a good lifting effect, which may be due to the rapid and large energy and short time in the impact process. 1 wt% of GNPs can better produce synergy with the matrix (29,30,31), disperse stress, and absorb impact energy, to indicate the strength. If it exceeds 1 wt%, agglomeration appeared, which reduces the total contact area between GNPs and matrix, to reduce the impact strength.

$Figure 7 SEM images of impact fracture surfaces of (a) polyimine and GNPs composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, and (e) 2 wt%.$

Figure 7

SEM images of impact fracture surfaces of (a) polyimine and GNPs composites with different contents of GNPs: (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, and (e) 2 wt%.

Due to different test methods, the force and action mode of materials are different; therefore, it is not surprising that distinct testing conditions may require different amounts of GNPs particles to exhibit the best performance.

4 Conclusions

The GNPs-P composites were prepared by heat-pressing under mild and convenient conditions without catalyst.
The mechanical properties of the GNPs-P composites were tested to show that the tensile strength increased by 5% to 73.05 MPa, the bending strength increased by 18% to 73.05 MPa, and the flexural strength reached 92.65 MPa when the GNPs was added at 0.5 wt%. The impact strength increased by 21.6% to 11.745 kJ·m⁻².
TGA results showed that the GNPs-P composites can improve thermal stability compared to the polyimine.
A small number of GNPs can create a synergistic effect, creating a load transfer reinforcement effect to enhance the mechanical strength. Beyond 1 wt%, agglomerated GNPs can damage the matrix properties and affect the overall mechanical properties.

Acknowledgment

The authors thank the research funding below.

Funding information: This work is funded by the Natural National Natural Science Foundation of China (Youth Science Foundation, Grant No. 51705214), and general project of National Natural Science Foundation of China (Grant No. 51875003).
Author contributions: Si Zhang: designed and performed the experiments; prepared the manuscript with contributions from all co-authors; Zhengjin Xiong: experimental design, data sorting, participation in manuscript modification; Jian Zhang: designed the experiments; Xueting Zhang: designed the experiments; Yuhang Chen: designed the experiments; Yun Chen: designed the experiments and analyzed experimental data.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2022-01-19

Revised: 2022-06-01

Accepted: 2022-06-20

Published Online: 2022-08-22

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-0060

Keywords for this article

polyimine; graphene nanoplatelets; composites; thermal properties; mechanical properties

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