Helical polysilane wrapping onto carbon nanotube: preparation, characterization and infrared emissivity property study

Abstract

A helical polysilane/multi walled carbon nanotubes composite was fabricated by wrapping helical HPS copolymer around the surface of modified nanotubes through surface oxidation. HPS was pre-polymerized by Wurtz-type coupling reaction in chloroform (CHCl₃), demonstrating optical activity and adoption of a predominately signal-handed helical conformation. Various kinds of characterization including Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), thermogravimetric analysis (TGA), circular dichroism (CD) and Raman spectroscopy have been utilized to demonstrate HPS noncovalently wrapped around the nanotubes to protect their original graphite structure. The wrapped HPS exhibited enhanced thermal stability and increased optical activity after wrapping. The infrared emissivity property of the composites at 8–14 μm was investigated as well. These results indicate the HPS/f-MWNTs composites possess a much lower infrared emissivity value (ε = 0.576) than raw MWNTs, which resulted from synergistic effect of the particular helical conformation of HPS and improved interfacial interaction between the organic polymers and inorganic nanoparticles.

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1. Introduction

Multi walled carbon nanotube (MWNT) composites have been gradually attracting increasing attention due to their enhanced mechanical and structural properties.^1–3 MWNTs process advantages like light weight, high mechanical strength and electrical conductivity.^4,5 However, poor solubility and processability restrict their further fabrication of composite materials.^6–8 To overcome the disadvantages of raw MWNTs, various kinds of prerequisite treatments have been utilized.^9,10 Oxidation is a crucial requisite step owing to the incorporation of carboxyl group on the surface of MWNTs.^11,12 The presence of electron withdrawing groups facilitate the exfoliation of MWNT bundles, and thus improve their solubility in polar solvents.¹³ According to previous literature, the carboxyl functionalized MWNTs exhibit low infrared emissivity property at the value of 0.781.¹⁴ Moreover, the MWNTs grafted by helical polyacetylene demonstrate enhanced property in infrared radiation reduction which related to the synergistic effect of polymer and interfacial interaction of composite.

The solution blending offers a readily technique of polymer wrapping without affecting the electronic structure of the nanotubes.^15,16 The drive force of polymer wrapping could be attributed to weak noncovalent interactions including π–π, CH–π and van der Waals forces.^17–20 For instance, composite of polyester and MWNTs demonstrate excellent interfacial adhesion due to the intermolecular noncovalent interactions of electron withdrawing and donating groups. The incorporation of ester electron donating group promotes the noncovalent interaction force of polymer.^21–25 Additionally, the helical structure is also a significant driving force at the interface between chain-like polyester and MWNTs. In analogy to polyester, polysilane with hexyl functional groups possess a helical groove which was closely related to the wrapping behavior.^26,27 Thus, poly(di-n-hexylsilane) (PDHS) can be easily wrapped on the curved surface of MWNTs. Besides the superior wrapping behavior, helical polysilane also exhibits the extraordinary optical activity. HPS copolymerized by DCDHS and DCMMS are utilized to reduce infrared emission.²⁸ The incorporation of chiral methyl lactate provides both the single-handed helical conformation and methoxycarbonyl functional group, which ultimately contributed to low infrared emissivity. Helical polymer/inorganic composites are proved to be a common complexes pattern to reduce infrared emissivity.^29,30 Hence, the preparation of HPS/f-MWNTs nanocomposites show promising prospect in infrared stealthy application.

In this article, carboxyl-functionalized multiwall carbon nanotubes (f-MWNTs) were utilized to investigate the functionalization of MWNTs on the morphology and crystal structure of HPS in the nanocomposites. HPS/f-MWNTs nanocomposites were prepared via solution blending using THF as a mutual solvent. Helical polysilane (HPS) was copolymerized as the ratio of 9 : 11 which is capable to form a helical conformation. The HPS/f-MWNTs complexes were characterized by FT-IR, Raman, XPS, XRD, TEM, and SEM to record the wrapping process. In addition, the infrared emissivity property accompanied by CD and UV-vis were investigated to study the interfacial synergistic interaction of f-MWNTs in the polymer matrix on the reduction of infrared radiation.

2. Experimental section

2.1 Materials

All reagents and solvents were purchased from Aladdin Industrial Corporation or J&K and used without further purification unless otherwise stated. Solvents for the polymerizations (THF, xylene) were purified according to procedures of literature.²⁶ Allyl bromide was purchased from Adamas-beta. Karstedt's catalyst, platinum(0)-1,3-divinyltetramethyldisiloxane complex (19.0–21.5% as Pt), L(+)-lactic acid and dichlorodi-n-hexylsilane (DCDHS) were obtained from Tokyo Chemicals Industry. DCDHS was purified by fractionating distillation prior to use. Methyl 2-(allyloxy)propanoate was prepared according to the literature.³¹ Deuterated solvents were purchased from Beijing Seaskybio Technology Co. Ltd and stored over molecular sieves (4 Å). MWNTs (length 5–15 μm, diameter 40–60 nm, purity > 98%) were purchased from Shenzhen Nanotech Port Co., Ltd.

2.2 Instrument

FT-IR spectra were carried out on a Bruker ALPHA FT-IR spectrometer using KBr pellets. The spectra of ¹H NMR (300 MHz) and ¹³C NMR (75 MHz) were recorded using a Bruker AV 300. All spectra were referenced internally to residual proton signals of the deuterated solvent. UV-vis spectra were obtained on a Shimadzu UV-3600 spectrometer at room temperature. CD spectra were determined with a Applied photophysics Chirascan Ultrasensitive Spectroscopy using a 10 mm quartz cell at room temperature. UV absorptions were determined by a SHIMADZU UV3600 Series spectrophotometer. Calibration was carried out using poly(styrene) standards provided by Polymer Standards Service. TEM images were obtained on a JEM-2100 microscope operating at an accelerating voltage of 100 kV. The samples were prepared by dropping the complexes solution onto carbon covered copper grids with excess solvent evaporated. SEM images were performed on the LEO-1530VP microscope. XPS data of the samples were obtained on a Shimadzu (Amicus, Japan) instrument. Thermal analysis experiments were performed by a TGA apparatus operated in the conventional TGA mode (TA Q-600, TA Instruments) using heating rate of 10 K min⁻¹ under nitrogen atmosphere, and the sample weight was around 5 mg. GPC measurements were performed on a setup consisting of a Polymer Laboratories' PL-GPC220, and PLgel-5 μL-MIXED-C, 300 × 7.5 mm. Tetrahydrofuran was used as an eluent at 30 °C and at a flow rate of 1 mL min⁻¹. XRD measurements of the samples were recorded using a Rigaku D/MAX-R with a copper target at 40 kV and 30 mA. The powder samples were spread on a sample holder and the diffractograms were recorded in the range 5–70° at the speed of 4° min⁻¹. Sodium dispersions were conducted using a DF-101S magnetic stirrer purchased from YUHUA company. Infrared emissivity values of the samples were studied on a silicon substrate by using an IRE-2 Infrared Emissometer of Shanghai Institute of Technology and Physics, China. Microscopic Raman spectroscopy was carried out using a DXR Raman Microscope (Thermo Fisher Scientific Inc.) at room temperatures. A DXR 532 nm laser was used as the excitation source.

2.3 Synthesis of HPS copolymer

DCMMS monomer derived from the methyl 2-(allyloxy)propanoate was synthesized according to previous literature.²⁸ Copolymerization of DCMMS and DCDHS were carried out under the inert gas environment. In a glovebox, 1.035 g (45 mmol, 2.5 equiv.) sodium and 25 mL xylene were placed in a Schlenk flask, transferred to the vacuum line, and heated to 110 °C. The mixture was well dispersed by using a preheated homogenizer (DF-101S, YUHUA). After evaporation of xylene in high vacuum, 20 mL of THF and 20 mmol of dichlorodiorganosilane (DCMMS = 9 mmol, DCDHS = 11 mmol) were added via syringe. The mixture turned purple within 30 min and vigorous stirring for 24 h at room temperature. The reaction was then quenched with 100 mL of methanol. The precipitated polymer was filtered, washed with water and methanol, and dried in high vacuum. The cyclic fraction was separated via fractional precipitation from THF/2-propanol. M_n = 10 300, M_w/M_n = 1.2. Yields: 21–60%. ¹H NMR (300 MHz, CDCl₃): δ [ppm] = 4.02–3.89 (CH), 3.78–3.63 (CH₃–O), 3.57–3.40 (O–CH′H′′–CH), 3.36–3.23 (O–CH′H′′–CH), 1.81–1.55 (CH₂–CH₂–CH₂), 1.43–1.33 (CH–CH₃), 0.92–0.78 (Si–CH₃), 1.77–0.91 (C(CH₃)₂, Si–(CH₂)₅–CH₃, and CH₃–Si–CH₂). ¹³C NMR (75 MHz, CDCl₃): δ [ppm] = 104.68 (CH–COO–CH₃), 59.45 (CH₃–CH–O), 45.81 (CH₃–O), 45.93 (Si–CH₂–CH₂), 33.03 (Si–(CH₂)₂–CH₂–Pr), 31.53 (Si–(CH₂)₃–CH₂–Et), 22.58 (Si–CH₂–CH₂–Bu), 31.61 (CH₂–CH₂–CH₂), 29.63 (CH–CH₃), 16.62 (Si–(CH₂)₄–CH₂–Me), 16.18 (Si–CH₂–(CH₂)₂–O), 16.11 (Si–(CH₂)₅–CH₃), 14.02 (Si–CH₂–CH₂–Bu), −0.07 (Si–CH₃). FT-IR (cm⁻¹, KBr): 2957, 2920, 2856, 1758, 1740, 1570, 1464, 1405, 1378, 1340, 1258, 1186, 1075, 1015, 962, 887, 841, 793, 770, 722, 688.

2.4 Surface oxidization of MWNTs

The surface oxidization of MWNTs was performed with a steaming procedure originally described by Zhao et al.¹¹ In a typical experiment, MWNTs (0.5 g) were loaded on the porous SiO₂ griddle of a glass funnel and placed into a 100 mL Teflon-vessel, at the bottom of which concentrated HNO₃ (2.5 mL, 65 wt%) was added previously. Then the Teflon-vessel was sealed in the autoclave and reacted at 160 °C for 5 h. After the steaming treatment, the product was subsequently washed with distilled water and ethanol and dried at 80 °C for 24 h.

2.5 Preparation of HPS/f-MWNTs nanocomposites

The HPS/f-MWNTs nanocomposites were prepared through a solution mixing method with THF being the mutual solvent.²² First, appropriate amount of f-MWNTs were added into the THF at a concentration of 4 mg mL⁻¹. The mixture was sonicated with a BG-01 ultrasonic generator for 1 h to make a uniformly dispersed suspension. Then, HPS was dissolved in THF and stirred for 1 h to prepare the HPS solution. HPS solution was added to the f-MWNTs suspension and further sonicated for 6 h to obtain the HPS/f-MWNTs solution. After the completion of solution blending, the product was poured into a crystallizing dish to evaporate the solvent at room temperature for 24 h. In addition, the sample was further dried at 70 °C under vacuum for 3 days to remove the solvent completely. Through the aforementioned procedure, HPS was mixed with f-MWNTs at a mass percentage of 20 wt%. For the purpose of conciseness, the composites were abbreviated as HPS/f-MWNTs.

3. Results and discussion

3.1 Synthesis of methoxycarbonyl-based HPS copolymer

The methoxycarbonyl monomer DCMMS was designed and synthesized by linking chiral methyl 2-(allyloxy)propanoate moiety with dichloromethylsilane in order to introduce chiral functional group into the side-chain of the HPS copolymer. Chiral center in side-chain was connected with the HPS backbone exerted asymmetric interactions to form single-handed helical conformation. Methoxycarbonyl groups in MMS were incorporated as functional groups for further wrapping with f-MWNTs. The methoxycarbonyl-derived monomer was copolymerized with PDHS in distilled THF to prepare a HPS copolymer with moderate molecular weight (M_n ∼ 10 300) and suitable molecular weight polydispersity. HPS copolymer corresponding to their predicted structure was proved by various spectroscopic data. HPS is soluble in common polar solvents including chloroform and THF. The obvious Cotton effect of the HPS implies its predominately single-handed helical conformation.

3.2 Preparation of HPS/f-MWNTs nanocomposite

Scheme 1 despicts the preparation route of solution blending between HPS and f-MWNTs. To introduce f-MWNTs into a polymer matrix through noncovalent wrapping, MWNTs should be treated via chemical oxidation of exterior surfaces, which could render oxygen-containing groups such as carboxyl groups around the MWNTs side walls and ends. Carboxyl groups could provide more active end groups for further interacting with the methoxycarbonyl group in HPS. Finally, the presence of electrostatic force and van der Waals interaction lead to HPS/f-MWNTs nanocomposites, preventing the severe aggregation of f-MWNTs.

Scheme 1 Preparation of the HPS/f-MWNTs complexes.

The FT-IR spectra of products in different processing steps are shown in Fig. 1. For pristine MWNTs, the absorption peaks at 3445 and 1632 cm⁻¹ were assigned to the –OH functional group resulted from ambient atmospheric moisture. The weak absorption at 1405 cm⁻¹ assignable to C–H antisymmetric bend of the methyl groups. After HNO₃ steaming oxidation, an obvious peak emerged at 1715 cm⁻¹ was assigned to the C=O stretching vibrations of the carboxylic groups, demonstrating the completion of carbon nanotubes (CNTs) surface activation. For HPS/f-MWNTs composites, the emerged peaks around 2965, 2925 and 2859 cm⁻¹ were relevant to the stretching vibrations of methyl and methylene groups in HPS. The peaks at 1732 cm⁻¹ corresponding to C=O stretching of methoxycarbonyl group in the HPS, and the increased absorption at 1088 cm⁻¹ was assigned to the C–O–C stretching of ether between the methyl lactate and propyl group, demonstrating the HPS macromolecules have wrapped on the surface of MWNTs. In addition, the two overlapped peaks at 1186 cm⁻¹ which might represent the ρCH₃ rocking mode of HPS also confirmed the presence of HPS in composites.

Fig. 1 FT-IR spectra of raw MWNTs, f-MWNTs, and HPS/f-MWNTs.

Raman spectroscopy is important for the detection of carbon nanomaterials, especially for modified surface of carbon composites. Thus the Raman spectra of MWNTs, f-MWNTs and HPS/f-MWNTs are presented in Fig. 2. All of the spectra showed similar absorption peaks. The minor absorption around 245 cm⁻¹ was assignable to the radial breathing mode (RBM). The sharp absorption peaks around 1340, 1570 and 2680 cm⁻¹ were attributed to D-band, G-band and 2D-band respectively. The wide and low characteristic peak around 830 cm⁻¹ was assigned to the IR-active mode in graphite. The curve of HPS/f-MWNTs demonstrated smooth peaks range from 3060 to 3500 cm⁻¹ which were identified as the hexyl group vibration modes in HPS. Surface oxidation of f-MWNTs contribute to the improvement of D-band resulting from the increasing component of disorder structure.

Fig. 2 Raman spectra of raw MWNTs, f-MWNTs, and HPS/f-MWNTs.

XPS was performed to assess the surface modification of the MWNTs. In Fig. 3(A), raw MWNTs shows a maximum asymmetric peak at the value of 284.5 eV with a broadened tail at higher binding energy region was assigned to sp² hybridized carbons, and the similiar pattern of the asymmetric peak was utilized in other fittings of the sp² hybridized graphite-like carbons. The O_1s peak at the value of 533.2 eV indicates the presence of oxygen containing functional groups which were most likely introduced by surface oxidation. The C_1s envelope of the f-MWNTs in Fig. S2† XPS spectrum can be fitted by summing peaks from five types of Gaussian–Lorentzian shape peaks: sp³ hybridized diamond-like carbons (C–C at 285.2 eV), carbon oxygen bond (C–O at 286.2 eV), carbonyls (C=O at 286.8 eV), hydroxyls (C–OH at 287.5 eV), and carboxyls (O=C–OH at 288.9 eV), indicating the surface oxidization of nanotubes. Concerning the obtained HPS/f-MWNTs composites, the weak peaks related to the Si_2p and Si_2s in the HPS at 100.9 eV and 153.3 eV have been found, which resulted from the silicon backbone of HPS. In addition, the corresponding C_1s, O_1s and N_1s peaks in Fig. 3(B)–(D) were attributed to ether C–O–C (C_1s at 286.2 eV and O_1s at 532.2 eV) and esters –COO– (C_1s at 289.0 eV and O_1s at 532.6 eV) were also observed, suggesting that the HPS wrapped on MWNTs surface. To sum up, the FT-IR and XPS data demonstrated the HPS backbone have been wrapping onto the surface of MWNTs, schematic illustration has been shown in Scheme 1.

Fig. 3 XPS spectra of as-prepared samples: (A) wide scan survey of raw MWNTs, f-MWNTs, and HPS/f-MWNTs; (B) high resolution C_1s spectra of HPS/f-MWNTs; (C) O_1s spectra of HPS/f-MWNTs, and (D) Si_2p spectra of HPS/f-MWNTs.

The characteristic appearance can be seen in the Fig. S1,† HPS could readily soluble in THF with homogeneous mixture, THF solution of HPS turned to be orange-yellow in color. Although raw MWNTs were insoluble in THF, the f-MWNTs could easily dispersed in THF by surface oxidation. After wrapped by HPS, the HPS/f-MWNTs complexes could well dispersed in polar solvents like THF and CHCl₃. The solution of HPS/f-MWNTs complexes could be stable in couple of days without participating which resulted from the surface modification of MWNTs.

3.3 Morphological and structural analysis

In order to investigate the pattern of HPS and f-MWNTs interacting with each other, TEM and SEM measurements were applied. The morphology of f-MWNTs is measured by TEM and listed in Fig. 4. The f-MWNTs remained original diameter size (10–50 nm) which was similar to the pure and untreated MWNTs and crossing with each other. The surface oxidization or polymer wrapping resulted in the formation of composites without destructing internal tubular architectures of f-MWNTs. Meanwhile, the nanotubes had been covered by a distinct layer of polymers which were resulted from the HPS organic counterparts. The wrapping demonstrated excellent adherence as well as interactions between the HPS and the f-MWNTs. Moreover, a magnified HRTEM is inserted in Fig. 4(C), clear lattice fringe was observed which resulted from the graphite structure of individual nanotube. The lighter organic counterpart was coated onto the darker side wall of the nanotube, demonstrating the surface of MWNTs completely wrapped by the HPS.

Fig. 4 (A), (B) and (C) TEM images of typical morphologies of the HPS/f-MWNTs composites. Inset in typical morphologies (C): HRTEM of the sidewalls of HPS/f-MWNTs composites.

SEM images of the HPS/f-MWNTs composites are shown in Fig. 5(A). It can be clearly observed that the f-MWNTs had been thoroughly covered by the matrix polymer. Meanwhile, the f-MWNTs demonstrated interpenetrating network structure and well dispersed in the polymer. A single nanotube is presented in Fig. 5(B), which demonstrates helical conformation on the surface of tube. Obviously, the wrapped f-MWNTs shown roughened surface and partially embedded in the HPS matrix. These TEM and SEM images both confirmed the strong interfacial adhesion between the HPS and f-MWNTs.

Fig. 5 (A) and (B) typical SEM images of the HPS/f-MWNTs composites.

The crystalline structure of HPS, raw MWNTs, and HPS/f-MWNTs composites analyzed by XRD are shown in Fig. 6. HPS displayed a wide and smooth peak around 2θ ≈ 19° which was induced by the helical backbone and methoxycarbonyl functional group reflected its amorphous conformation. The raw MWNTs exhibited the typical diffraction pattern of the graphite structure at around 2θ ≈ 25.8°, 42.9°, 53.5° and 78.5°, corresponded to (002), (100), (004) and (006) diffraction planes, respectively. The diffraction pattern of the HPS/f-MWNTs composites displayed combined characteristic diffraction peaks of both the HPS and f-MWNTs, indicating the combination of the inorganic and organic components. The composites showed similar peak intensities of corresponding to the graphite structure of raw MWNTs without noticeable decreasing in the crystallinity. This phenomenon resulted from HPS surface wrapping maintained the conformation and orderliness of the nanotubes dispersed in polymer matrix. These data further demonstrated that the surface of nanotubes have been wrapped by HPS copolymer and the f-MWNTs maintained original the crystalline structure during the wrapping, coincidence with the TEM measurements.

Fig. 6 XRD patterns of pristine HPS, f-MWNTs and HPS/f-MWNTs composites.

3.4 UV-vis and CD spectra analyses

As described in previous work, the HPS copolymers bearing chiral methoxycarbonyl-based substituents exhibited intense Cotton effect in organic solvents due to their helical conformation. The optical activities of pristine HPS and HPS wrapped on f-MWNTs were confirmed by UV-vis and CD spectroscopies in THF. As presented in Fig. 7, the spectra of f-MWNTs demonstrated similar medium peak in UV-vis and CD spectra at a wavelength range from 200 to 280 nm. Besides the similar absorption, the CD curve of f-MWNTs exhibited an additional absorption range from 280 to 370 nm.

Fig. 7 (A) UV-vis and (B) CD spectra of pristine HPS, f-MWNTs and HPS/f-MWNTs composites in THF.

For pristine HPS, the wider UV absorption peak range from 200 to 350 nm corresponding to the combined characteristic band of the σ-conjugated silicon backbone of HPS and the methoxycarbonyl functional group. Meanwhile, the CD spectrum of HPS showed an intense obvious Cotton effect at ∼310 nm which was consistent with that of typical helical polysilane conformation, implying the predominantly single-handed helical structure of HPS. In addition, the weak absorption around 250 nm was attributed to the chiral center of methyl lactate.

The UV absorption spectrum of HPS/f-MWNTs composites demonstrated the strong sharp absorption centered around 210 nm and the shoulder peak range from 250 to 350 nm, which were related to the combined characteristic band of HPS and f-MWNTs, respectively. Compared to UV spectrum, HPS/f-MWNTs composites exhibited an obvious positive Cotton effect range from 280 to 400 nm in CD spectrum. Whereas, the CD signal of HPS/f-MWNTs composites range from 200 to 280 nm remained the similar intensity with HPS. The slight red-shift and strengthened intensity of the CD signal around 310 nm suggests that the wrapping HPS remained helical conformation and the improved orderliness. This phenomenon is resulted from the noncovalent interaction between the carboxyl of f-MWNTs and the methoxycarbonyl of HPS. Hence, the helical HPS has been effectively wrapped around the surface of f-MWNTs with enhanced optical activity. The UV and CD spectra features confirm that the wrapping of organic polymers on the surface of MWNTs has occurred as expected.

3.5 Thermal stability analysis

The TGA was employed for determining the amount of organics wrapped on f-MWNTs. The weight loss curves of raw MWNTs, HPS and HPS/f-MWNTs composites in nitrogen atmosphere are presented in Fig. 8. In the TGA curve of HPS/f-MWNTs, the weight loss at the range from 200 to 450 °C resulted from the thermal decomposition of the wrapped HPS. The 5% weight loss temperatures of the HPS and the composites were about 243 °C and 286 °C, reflecting improved thermal stability of the composites derived from the interaction between HPS and f-MWNTs. In addition, the amount of HPS wrapped onto the nanotubes was about 0.17 g g⁻¹ for composites.

Fig. 8 TGA curves of pristine HPS, f-MWNTs and HPS/f-MWNTs composites.

3.6 Infrared emissivity analysis

The property of infrared emissivity at wavelength of 8–14 μm had been investigated at room temperature and is presented in Table 1. Compared with raw MWNTs, the f-MWNTs demonstrated a lower infrared emissivity value from 0.802 to 0.781. This reduction resulted from the incorporation of oxygen containing functional groups like C=O and O–H. According to previous research, the oxygen containing groups like C=O could provide the lone pair electrons. The functional group C=O not only improved the unsaturated degree, but also formed the intermolecular hydrogen bond with O–H. Ultimately, the improved hydrogen deficiency facilitated the reduction of infrared emissivity. After wrapping by HPS, the HPS/f-MWNTs composites possessed enhanced property of low infrared emissivity at the value of 0.576 which coincided with the increased absorption of CD absorption. The helical backbone exhibited a sharp increased signal around 310 nm which reflected the improvement of backbone helicity. It has been reported that the reduction of infrared emissivity resulted from the helical conformation of polymer. The weak interaction between the pedant multihydroxy functional group and hexyl substituent led to the helical backbone of HPS. Moreover, the reduced infrared emissivity of the HPS/f-MWNTs were also resulted from intermolecular interactions of optical active wrapped HPS shell. Besides the helical conformation, medium electron withdrawing groups like methoxycarbonyl and ether also interacted with the electron donating carboxyl group of f-MWNTs. Thus the intra- and intermolecular interaction between HPS and f-MWNTs improved the degree of unsaturation and resulted in property of low infrared emissivity. To sum up, the functional group of HPS and f-MWNTs and their synergistic effect led to the elevated property of infrared emissivity.

Table 1 Infrared emissivity values of the samples

Samples	Infrared emissivity (εTIR at 8–14 μm)
MWNTs	0.802
f-MWNTs	0.781
HPS	0.598
HPS/f-MWNTs composites	0.576

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3.7 Cooperative interaction analysis

To further illustrate the wrapping behavior of the helical conformational HPS with f-MWNTs, computational studies with molecular mechanics calculations were conducted by using PCFF force fields (Materials Studio 7.0, Accelrys Inc., San Diego, CA). For the purpose of simplification, HPS oligomers with 60-Si repeating units and MWNTs with the chiral vector (7,5) (diameter: 8.17 Å, length: 88.95 Å) were employed as models of HPS and MWNTs, respectively. First of all, the HPS with most stable structures were presented, the dihedral angles were confined as particular degrees derived from the predicted UV maximum absorption of silicon backbone; thus, Φ_PDHS: 151° and Φ_HPS: 175° were set as the dihedral angles. Apparently, the conformation of polysilane could be classified into two categories determined by the wrapping ability. HPS with Φ of 175° possessed helical grooves induced by the chiral functional group bearing the helical silicon backbone [see, Fig. S3(A) in the ESI†]. The PDHS with Φ of 151° was incapable of warping onto the f-MWNTs due to their equally distributed alkyl side chains unable to form the grooves [see, Fig. S3(B) in the ESI†]. Hence, the relationship between the helical conformation of HPS and the surface of f-MWNTs could be easily demonstrated. Thus the f-MWNTs were covered by the helical grooves of the preoptimized HPS, then the potential energy minima of the HPS/f-MWNTs complexes was further optimized with no limitation of any configurations [see, Fig. 9(A) and (B)].

Fig. 9 Proposed structures of HPS/f-MWNTs complexes. (A) Pristine HPS, (B) HPS/f-MWNTs, (B) HPS/f-MWNTs, (C) projection view of HPS/f-MWNTs. To simplify the calculation, HPS with 60 monomers (with 60-Si repeating units) and MWNTs (7,5) were employed as a model of HPS and f-MWNTs, respectively. PCFF was used as a force field.

It is widely accepted that the noncovalent interaction played a significant role in promoting compatibility between HPS and CNTs, which including π–π, CH–π and van der Waals forces. The interaction between methoxycarbonyl of HPS and the carboxyl of f-MWNTs was assigned to the π–π interaction between the lone pair electron of oxygen atom. According to the UV absorption range from 200 to 250 nm, the HPS/f-MWNTs complexes exhibited a sharp absorption which assigned to the functional group of methoxycarbonyl and carboxyl group. The suddenly increasing of peak resulted from the π–π interaction. Besides the oxygen containing functional group, the alkyl side chain also contributed to the wrapping behavior of HPS onto f-MWNTs. Compared to the IR absorption in the region from 2800 to 3000 cm⁻¹, HPS/f-MWNTs complexes showed identical peak assignments of hexyl group with pristine HPS. This phenomenon reflects the negligible CH–π interaction between the single alkyl side chains of HPS and the carboxyl group of f-MWNTs. Although the CH–π interaction between the chiral functional group and the alkyl group is regarded as an essential drive force to form the helical conformation of HPS, the van der Waals forces are the major drive forces to push the polymer stick to the side wall of nanotubes. The morphology of composites demonstrated the excellent compatibility at the boundary of organic and inorganic part which supports the hypothesis presented above.

4. Conclusions

A novel type of optically active HPS/f-MWNTs composite has been prepared through noncovalent wrapping helical polysilane copolymers on the surface of oxidized MWNTs. The obtained composites not only possess the advantages of enhanced stability and processability, but also exhibit the improved property of low infrared emissivity. Optically active HPS wrapped on the surface of f-MWNTs maintains the helical structure and shows intensified orderliness than original helical polysilanes. The HPS/f-MWNTs composites demonstrate the superior property of the low infrared emissivity as low as 0.576 at the wavelength of 8 to 14 μm. Hence the solution blending of f-MWNTs and helical HPS offer a readily way of preparing the composites with low infrared emissivity. The interaction between f-MWNTs and helical HPS and their interaction mechanism have been discussed as well. These results may be profitably used as a guide towards preparing the type of helical polymers and conditions necessary to wrap MWNTs with a particular functionalized surface. The obtained newly class composites are significant to the infrared emission source in both military and civilian application.

Acknowledgements

The authors are supported by National Nature Science Foundation of China (51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province (BA2014100), the Fundamental Research Funds for the Central Universities (3207045301), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1417). Scientific Innovation Research Foundation of College Graduate in Jiangsu Province (KYLX_0161). Materials Studio 7.0 is provided by National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24052b

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