In situ synthesis of MWCNT-graft-polyimides: thermal stability, mechanical property and thermal conductivity

Herein, MWCNT-graft-polyimides (MWCNT-g-PIs) were prepared by the in situ grafting method. Strengthening the interfacial interaction between MWCNTs and polyimide chains decreased their interfacial thermal resistance (RC). In contrast to the RC of 10% MWCNT/PIs, the RC of 10% MWCNT-g-PI decreased by 16.7%. Hence, MWCNT-g-PIs possessed higher thermal conductivity than MWCNT/polyimides (MWCNT/PIs). Meanwhile, the Tg values of all the samples (MWCNT/PIs and MWCNT-g-PIs) were greater than 399 °C (by DMA). Compared with MWCNT/PIs, 5% and 10% MWCNT-g-PIs showed enhancement in thermal stability in air. The storage modulus retentions were greater than 63% at 200 °C and 45% at 300 °C. Also, 5% and 10% MWCNT-g-PIs maintained the high tensile strength of pure PI, and the tensile modulus increased up to 2.59 GPa on increasing the loading amount of MWCNTs. This study sheds light on improving the thermal conductivity of polyimides effectively at relatively low loadings.


Introduction
In recent years, with the rapid development of highperformance microelectronic equipment and energy harvesting devices, the demand for heat sinks in industrial and electronic elds has dramatically increased. 1,2 However, the thermal conductivity of common polymers is quite low and ranges from 0.1 W m À1 K À1 to 0.3 W m À1 K À1 . Hence, their applications are severely limited in industrial and electronic elds due to heat accumulation. [3][4][5] It is important to increase the thermal conductivity of polymers to enhance the thermal diffusion and then reduce the heat accumulation. A simple and feasible method for enhancing the thermal conductivity of polymers involves introducing highly thermally conductive llers (carbon nanotubes, 4,6,7 graphites, [8][9][10] boron nitrides, 11,12 aluminum nitrides, 13,14 and aluminum oxides 15 ) into polymers.
Among all kinds of highly thermally conductive llers, carbon nanotubes (MWCNTs or SWCNTs) have been expected to be capable of improving the thermal conductivity of polymers effectively at relatively low loadings. [16][17][18][19] However, the poor thermal conductive performances of carbon nanotube composites are due to the high interfacial thermal resistance between carbon nanotubes and polymers.
Improving the ller/polymer interfaces can reduce "thermal resistance", and some methods have also been considered, such as non-covalent functionalization 7 and covalent functionalization. 6 Covalent functionalization involves graing some chemical functional groups (amines, silanes, polymers, etc.) onto carbon nanotubes.
In this paper, polyimide was selected as a polymer matrix owing to its outstanding thermal and mechanical properties and MWCNTs acted as thermally conductive llers. MWCNT-gra-polyimides (MWCNT-g-PIs) were obtained by the in situ graing method for reducing the interfacial thermal resistance between nanotubes and polyimide to enhance the thermal conductivity. The thermal stability, mechanical properties and thermal conductivity of MWCNT-g-PIs were studied. For comparison, MWCNT/polyimides (MWCNT/PIs) were prepared by a simple blending method.

Measurements
FTIR spectra were recorded on a Nicolet iS10 spectrometer at a resolution of 2 cm À1 in the range of 400-4000 cm À1 with reection mode. Dynamic Mechanical Analysis (DMA) was performed with a TA instrument (DMA Q800) at the heating rate of 5 C min À1 and a load frequency of 1 Hz in the lm tension geometry and T g was regarded as the peak temperature of tan d curves. Thermogravimetric analysis (TGA) was performed with the TA instrument 2050, with a thermal heating rate of 10 C min À1 in nitrogen or air atmosphere. The mechanical properties of the samples were studied at room temperature by a Shimadzu AG-I universal testing apparatus with a crosshead speed of 2 mm min À1 . Measurements were obtained at 25 C with lm specimens (about 50 mm thick, 6 mm wide and 40 mm long). The cross-section morphology of lms was observed by Scanning Electron Microscopy (SEM, NOVA NANOSEM 450, England). The lms were fractured in liquid nitrogen and coated with gold prior to test. Thermal conductivity measurements were performed at 25 C by thermal conductivity instrument of TC 3000 series based on ASTM D5930 Standard Test Method for Thermal Conductivity of Plastics by means of a Transient Line Source Technique. Thermal conductivity K (W m À1 K À1 ) was calculated by the following equation: Here, q represents the heat conducted per unit length of the wire, DT represents the temperature changes in the wire and t represents the measuring time. Samples with different MWCNT contents (0%, 5%, and 10%) in polyimide were synthesized via the blending method and designated as PI, 5% MWCNT/PI, and 10% MWCNT/PI, respectively. The preparation of 5% MWCNT/PI was used as a representative to illustrate the detailed synthetic procedure. First, 0.2202 g MWCNTs and 25 g DMAc were added into a three-neck ask and then, the mixture was subjected to ultrasonic dispersion at room temperature for 3 h. Subsequently, ODA (10 mmol, 2.002 g), PMDA (10 mmol, 2.181 g), and 14.6 g DMAc were added into the three-neck ask. The reaction mixture was slowly stirred for 24 h. Next, the mixture was casted on a glass plate, followed by a preheating program (60 C/10 h, 80 C/2 h, 100 C/2 h, 120 C/2 h) and an imidization procedure under vacuum (200 C/1 h, 250 C/1 h, and 300 C/1 h) to produce the 5% MWCNT/PI lm.
2.3.2 Preparing MWCNT-g-PIs by in situ graing method (Scheme 1). The different MWCNT contents (0%, 5%, and 10%) were graed on polyimide via the in situ synthesis method and the corresponding samples were named g-PI, 5% MWCNT-g-PI, and 10% MWCNT-g-PI. The preparation of 5% MWCNT-g-PI was used as a representative to illustrate the detailed synthetic procedure. First, 0.2204 g MWCNT-OH and 25 g DMAc were added into a three-neck ask and then, the mixture was subjected to ultrasonic dispersion at room temperature for 3 h. Subsequently, ODA (9.8 mmol, 1.962 g), PMDA (10 mmol, 2.181 g), and 14.7 g DMAc were added into the three-neck ask. The reaction mixture was slowly stirred for 2 h. At last, APTES (0.2 mmol, 0.0443 g) was introduced into the system, and the system underwent polymerization for 24 h. Then, the mixture was casted on a glass plate, followed by a preheating program (60 C/10 h, 80 C/2 h, 100 C/2 h, 120 C/2 h) and an imidization procedure under vacuum (200 C/1 h, 250 C/1 h, and 300 C/1 h) to produce the 5% MWCNT-g-PI lm. The chemical structures of MWCNT-g-PIs were characterized by FT-IR spectroscopy. Fig. 2 demonstrates the FT-IR spectra for MWCNT/PIs and MWCNT-g-PIs. All the samples exhibited characteristic imide absorptions at around 1776 cm À1 (asymmetrical C]O stretching), 1714 cm À1 (symmetrical C]O stretching), and 1366 cm À1 (C-N stretching). The spectra of MWCNT-g-PIs show the asymmetrical and symmetrical stretching vibrations of -CH 2 at 2921 cm À1 and 2846 cm À1 , respectively. These vibrations belonged to APTES and carbon nanotubes, and no existence of the characteristic absorption bands of the -NH 2 and -OH groups proved the successful Scheme 1 The preparation process of MWCNT-g-PIs.

Characterization of MWCNT/PIs and MWCNT-g-PIs
graing of polyimide chains on carbon nanotubes. The interaction between MWCNT and PI in MWCNT-g-PIs via coupling is illustrated in Fig. 3. Fig. 4 shows the wide-angle X-ray diffraction (XRD) curves of MWCNT/PIs and MWCNT-g-PIs. PI and g-PI only exhibited a diffuse peak at 2q ¼ 17.5 , whereas 5% and 10% MWCNT/PIs and MWCNT-g-PIs exhibited two diffuse peaks at 2q ¼ 17.3 and 24.9 , respectively. A small diffuse peak at 2q ¼ 24.9 was observed in the diffraction curves of 5% and 10% MWCNT/PIs and MWCNT-g-PIs, indicating that the carbon nanotubes were successfully incorporated into the polyimide matrix.  Table 1. The T 5% and T 10% values of PI were 559 C and 573 C under N 2 atmosphere, respectively. Compared with the values for PI, the T 5% and T 10% of g-PI decreased slightly under N 2 atmosphere; the values were 549 C and 569 C, respectively. However, the addition of carbon nanotubes improved T 5% and T 10% under N 2 atmosphere irrespective of whether by blending or graing. By the thermal degradation curves of MWCNTs and MWCNT-OH under N 2 atmosphere, we can infer that MWCNTs and MWCNT-OH have better thermal stability than PI, which results in the enhancement of T 5% and T 10% of the materials. The residual weight retentions at 800 C also improved under N 2 atmosphere; the values for 10% MWCNT/PI and 10% MWCNT-g-PI were 62.5% and 62.6%, respectively. In contrast to the values for PI, T 5% and T 10% had a marked decrease under air atmosphere for the materials prepared by the blending method. From the DTG curves of MWCNT/PIs in air, we can infer that the degradation of MWCNTs at a high-temperature stage is the main reason for the above-mentioned phenomenon. However, T 5% and T 10% had a marked increase under air atmosphere for the materials prepared by the graing method than the results obtained for the blending method. Aer graing, MWCNTs were tightly wrapped by polyimide chains owing to the covalent   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 13517-13524 | 13519

Thermal properties of MWCNT/PIs and MWCNT-g-PIs
Paper RSC Advances bond linkage between MWCNTs and polyimide chains, which strengthened the interfacial interaction and thus, the MWCNT degradation was delayed. The heat-resistance index (T HRI ) was calculated; 11,20 the results are listed in Table 1. In N 2 , the T HRI values of MWCNT/PIs and MWCNT-g-PIs increased aer the addition of MWCNTs. Under air atmosphere, the T HRI of MWCNT/PIs decreased aer the addition of MWCNTs, but T HRI of the MWCNT-g-PIs brought into correspondence with that of pure PI. In short, MWCNT/PIs and MWCNT-g-PIs exhibited good thermal stability in N 2 . However, MWCNT-g-PIs possessed better thermal stability than MWCNT/PIs in air. Nevertheless, the reduction in thermal stability for MWCNT/PIs in air was retained at an acceptable degree. The dynamic mechanical analyses of MWCNT/PIs and MWCNT-g-PIs are shown in Fig. 7 and 8. The storage modulus retentions of MWCNT/PIs and MWCNT-g-PIs at 200 C and 300 C were analysed and listed in Table 2. All the samples had good storage modulus retention at a high-temperature stage. The storage modulus retentions were greater than 63% at 200 C and 45% at 300 C. Meanwhile, the glass transition temperature (T g ) was analysed; it was determined by the peak temperature of the tan d curves and listed in Table 1. T g is possibly determined by two competitive factors: the free volume and the steric effect. 21,22 In the MWCNT-g-PI system, T g shows a decreasing trend with the increase in the loading amount of MWCNTs. The polyimide chains graed on the MWCNT surfaces disrupted the ordered chain structure of the polyimides and resulted in the increase in free volume. However, the T g values of all the samples were greater than 399 C.

Mechanical properties of MWCNT/PIs and MWCNT-g-PIs
For the nanocomposites, the mechanical property is affected by many factors, such as the polymer matrix, loading amount of inorganic nanollers, dispersion in the polymer matrix and interfacial interaction. 23 Based on the several aspects mentioned above, the mechanical properties of MWCNT/PIs and MWCNT-g-PIs were discussed. The tensile strength, tensile modulus and elongation at break results of MWCNT/PIs and MWCNT-g-PIs are summarized in Table 3. The tensile strength, tensile modulus and elongation at break of PI were 129 MPa, 2.39 GPa and 57.5%, respectively. PI showed good mechanical properties. Compared with the results for PI, the tensile strength and tensile modulus of g-PI had a slight increase because of the existence of crosslinking points by the self-polycondensation of the coupling agent at the ending of the  polyimide chains, which was also responsible for the decrease in the elongation at break of g-PI from 57.5% to 48.6%. Subsequently, the mechanical properties of MWCNT-g-PIs with different loading amounts were analysed. 5% and 10% MWCNT-g-PIs maintained the high tensile strength of PI. The tensile modulus increased up to 2.59 GPa on increasing the MWCNT loading. The elongation at break of MWCNT-g-PIs exhibited a reducing trend but was still more than 36%. The reduction in the elongation at break in our system was retained at an acceptable degree. The covalent bond linkage between MWCNTs and polyimide chains promoted the well-distributed dispersion of MWCNTs in polyimides and strengthened the interfacial interaction between MWCNTs and polyimide chains. Hence, MWCNT-g-PIs showed good mechanical properties. The mechanical properties of the sample prepared by the simple blending method (MWCNT/PIs) were also investigated. In this research, a short carbon nanotube (L/d ¼ 250) was selected, which could be easily dispersed in a polymer matrix and lead to the existence of p-p interactions between the carbon Table 1 Thermal properties of MWCNT/PIs and MWCNT-g-PIs

Sample codes
T g a ( C) a Measured by DMA at a heating rate of 5 C min À1 . b 5% weight loss temperature (T 5% ) and 10% weight loss (T 10% ) temperature measured by TGA. c Heat-resistance index (T HRI ) was calculated by the equation T HRI ¼ 0.49 Â [T 5% + 0.6 Â (T 30% À T 5% )]. d Residual weight retention at 800 C.    nanotubes and benzene rings in polyimide chains. Thus, MWCNT/PIs also exhibited good mechanical properties.

Morphology of MWCNT/PIs and MWCNT-g-PIs
Fig . 9 exhibits the SEM images of MWCNT/PIs and MWCNT-g-PIs. In Fig. 9, it can also be noticed that the MWCNTs disperse more homogeneously in MWCNT-g-PIs than in MWCNT/PIs due to covalent bond linkage, strengthening the interfacial interaction between MWCNTs and the polyimide matrix. A small portion of agglomerated MWCNTs can be seen in the 5% and 10% MWCNT/PI composites. This is one of the key factors that can affect the thermal conductivity of the resulting composites. Apparently, a higher ller content is required to form "thermal conductive pathways" when the llers agglomerate in the polymer matrix. A good dispersion of MWCNTs in polyimides may contribute to the improvement in thermal conductivity.

Thermal conductivity of MWCNT/PIs and MWCNT-g-PIs
The thermal conductivity properties of the MWCNT/PIs and MWCNT-g-PI composites are shown in Fig. 9. The increasing MWCNT loading enhanced the thermal conductivity of MWCNT/PIs and MWCNT-g-PIs because more and more MWCNTs participated in forming "thermal conductive pathways". However, the thermal conductivity of MWCNT-g-PIs increased faster than that of MWCNT/PIs at the same loading. The thermal conductivity of 10% MWCNT/PIs improved by 69.6% than that of pure PI. The thermal conductivity of 10% MWCNT-g-PIs increased by 87.0% than that of pure PI (Fig. 10). The well-distributed dispersion of MWCNTs in polyimides can be boosted to form "thermal conductive pathways" at the same loading, and strengthening the interfacial interaction between MWCNTs and polyimide chains by covalent bond linkage can decrease the interfacial thermal resistance (R C ) between nanotubes and the polymer matrix. The interfacial thermal resistance (R C ) was calculated by the Maxwell-Garnett-type effective medium approach (EMA) in our research as follows: 24