Surface functionalization of MoS₂ with POSS for enhancing thermal, flame-retardant and mechanical properties in PVA composites

Abstract

Surface functionalization of molybdenum disulfide (MoS₂) was prepared by a simple reflux reaction between DITG-MoS₂ and octa-vinyl polyhedral oligomeric silsesquioxanes (OvlPOSS). The structure of OvlPOSS-MoS₂ was confirmed by XRD, FTIR and TEM. The SEM and TEM results of fracture surface exhibited that OvlPOSS-MoS₂ was dispersed well in the matrix due to the good interfacial interaction between the functionalized MoS₂ and PVA. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results indicated that the thermal decomposition temperature and the glass transition temperature (T_g) were improved. Compared with pure PVA, the maximum degradation temperature of the PVA/OvlPOSS-MoS₂ nanocomposites was increased by 23 °C, and the T_g of the PVA/OvlPOSS-MoS₂ was improved by 10.2 °C. Meanwhile, the peak of heat release rate (pHRR) and total heat release (THR) were decreased. The tensile stress was increased by 57% with addition of 2 wt% OvlPOSS-MoS₂. Moreover, the addition of OvlPOSS-MoS₂ significantly decreased the gaseous products, including hydrocarbons, carbonyl compounds and carbon monoxide, which was attributed to the synergistic effect of OvlPOSS and MoS₂: the adsorption and barrier effect of MoS₂ inhibited the heat and gas release and promoted the formation of graphitized carbons, while OvlPOSS improved the thermal oxidative resistance of the char layer.

---

1. Introduction

Recent progress in understanding 2D ordered crystals of graphene has drawn more and more scientists into the nanosheet field. Various 2D nanosheets such as transition metal dichalcogenides (TMDs, e.g. MoS₂ and WS₂), transition metal oxides (TMOs), and hexagonal boron nitride (h-BN) have been attracting tremendous attention due to the unusual properties associated with their ultrathin nanosheet structure.^1–6 Among these 2D layered materials, molybdenum disulfide (MoS₂) is one of the more commercially available members with a analogous structure to graphite, which is composed of three stacked atom layers (S–Mo–S) held together by van der Waals forces.⁷ Like some other members of the family of 2D layered materials, MoS₂ has many comprehensive applications due to their unmatched properties. For example, MoS₂ is frequently used as a solid lubricant in rigorous environments, lithium ion batteries, catalysts and sensors.^2,7–11 It is believed that a 2D ordered MoS₂ crystal structure with an exposed (002) crystal surface would be valuable to exploit many unique properties of a graphitic-like (002) plane, such as superb lubrication performance, mechanical strength, and others.

In the early ages of the polymer nanocomposites research, layered silicates, and layered double hydroxides (LDHs) are the most widely investigated.^12,13 However, very recently, the discovery of graphene with its combination of extraordinary physical properties and ability to be dispersed in various polymer matrices has created a new class of polymer nanocomposites.^14,15 It is well known that pristine MoS₂ is unsuitable for intercalation by large species, such as polymer chains, because MoS₂ as a bulk material has a pronounced tendency to agglomerate in polymer matrix which usually destroys the properties of the composites. Therefore, there has been great interest in the improvement of the dispersion of MoS₂ nanosheets in polymer matrix. Fortunately, the layered structure of MoS₂ enables easy intercalation of metal ions, such as Li⁺ and Mg²⁺.¹⁶ So it is convenient to prepare polymer nanocomposites by the intercalation of metal ions (Li⁺) and then exfoliated to single or few layers through the hydrolysis of the Li⁺. Like the graphene, single MoS₂ nanosheets were used to fabricate the polymer based nanocomposites, which can effectively enhance the mechanical, thermal properties and flame retardance.¹⁷ However, the additive used in the previous reports is bare MoS₂ so that high loading is needed to achieve enhanced properties. Up to now, the functionalization of MoS₂ nanosheets with coupling agents has been rarely reported. Intrigued by this, our current research is designed to fabricate functionalized MoS₂ nanosheets which can then improve the thermal stability, flame retardance and smoke suppression and mechanical properties of the polymer nanocomposites. Compared with previous report,¹⁷ only addition 2%wt OvlPOSS-MoS₂ into polyvinyl alcohol (PVA) can remarkably enhance the mechanical, thermal properties and flame retardance. Moreover, the addition of OvlPOSS-MoS₂ significantly decreased the gaseous products, including hydrocarbons, carbonyl compounds and carbon monoxide, which is attributed to the synergistic effect of OvlPOSS and MoS₂.

Herein, to improve the efficiency of MoS₂, we demonstrated a facile approach to functionalize MoS₂ with octa-vinyl polyhedral oligomeric silsesquioxanes (OvlPOSS). The synthetic route of the PVA/OvlPOSS-MoS₂ nanocomposites is demonstrated in Scheme 1. The thermal stability, fire resistance, smoke suppression and mechanical properties of the resulting materials were investigated. This work aims to study the influence of the surface functionalization of MoS₂ nanosheets on the thermal stability, fire retardation behavior, smoke suppression and mechanical properties of PVA composites. Meanwhile, mechanisms of the improved properties were discussed. Even more important is that the functionalization of MoS₂ nanosheets with OvlPOSS opens a new avenue to fabricate MoS₂-based nanocomposites effectively for more extensive applications.

Scheme 1 Illustration for the functionalization of OvlPOSS-MoS₂ nanosheets and PVA nanocomposites.

2. Experimental section

Raw materials

PVA (polymerization degree 1750 ± 50, CP), molybdenum disulfide (MoS₂, AP), dithioglycol (DITG, AP), tetrahydrofuran (THF, AP) and n-hexane (AP), were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The n-butyl lithium (2.2 M in hexane) was purchased from Alfa Aesar without further purification. Azodiisobutyronitrile (AIBN) was recrystallized from methanol. The distilled water was produced in our laboratory.

2.1. Preparation of Li_xMoS₂

Li_xMoS₂ was prepared by the solvothermal method according to the previous work.³⁰ In a typical procedure, MoS₂ (1.0 g) was dispersed in 36 mL hexane solution of n-butyl lithium (0.5 M). Then the mixture was transferred into a Teflon-lined stainless-steel autoclave with a volume of 50 mL. The autoclave was maintained at 100 °C for 4 h and finally cooled to room temperature naturally. After washed by 30 mL of anhydrous hexane for three times, the black precipitates were obtained and dried in vacuum at 60 °C for 3 h. The yield of products was about 80%.

2.2. Exfoliation of MoS₂

Single-molecule layers MoS₂ was achieved via the rapid hydrolysis and ultrasonication of Li_xMoS₂ in distilled water. In a typical experiment, Li_xMoS₂ (0.05 g) was hydrolysed in water (100 mL), and ultrasonicated at room temperature for 4 h to produce a colloidal suspension of single-molecule MoS₂ layers. The suspension was neutralized with 1 M nitric acid.²⁸

2.3. Functionalization of the MoS₂ nanosheets

The colloidal suspension of single-molecule MoS₂ layers collected from the experimental procedure in above section and excess amount of dithioglycol (DITG) were dissolved in tetrahydrofuran (THF) under stirring for 8 h. The surface-modified MoS₂ (DITG-MoS₂) were collected after being centrifugally separated at 4800 rpm for 10 minutes, washed with THF for several times and then dried in vacuum at 60 °C for 24 hours. The yield of products was about 75%. DITG-MoS₂ was added into a solution of OvlPOSS in THF. A stream of nitrogen was bubbled through the mixture for 30 min. Some of azodiisobutyronitrile (AIBN) was added under nitrogen. Then, the mixture was heated at 80 °C for 6 h. After cooling to room temperature, the black precipitates were obtained by vacuum filtration and dried in vacuum at 60 °C for 3 h.

2.4. Preparation of PVA/OvlPOSS-MoS₂ nanocomposites

PVA/OvlPOSS-MoS₂ nanocomposites were prepared by the ultrasonic solvent blending and casting technique. To prepare PVA nanocomposites with 2 wt% OvlPOSS-MoS₂, 3.92 g PVA was dissolved in deionized water at 95 °C in a flask with magnetic stirring to prepare PVA aqueous solution. Then, 0.08 g OvlPOSS-MoS₂ was introduced into the PVA solution which was then stirred at 95 °C for 72 h. At last, the blending was cast onto glass plates and dried at 40 °C in an oven for 24 h to form flat membranes which were peeled off and further heated at 80 °C for 48 h to remove residual water. The obtained membranes were cut into pieces for tests. For comparison, pristine PVA, PVA/MoS₂ composites with equivalent filler content were also prepared under similar processing conditions.

2.5. Characterization

Several analytical techniques were used to characterize the synthesized products. The powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded with a Japan MXPAHF X-ray diffractometer equipped with graphite monochromatized Cu Kα irradiation (λ = 0.154056 nm), employing a scanning rate of 0.02° s⁻¹ in the 2θ range of 3–60°. Infrared (IR) spectrum analyses were operated on samples pelletized with KBr powder in the range of 4000–400 cm⁻¹ using an infrared Fourier transform spectrophotometer (Nicolet, ZOSX). Microstructures of the products were observed by JEOL JSM-2010 field-emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) images were obtained on a Hitachi model H-800 transmission electron microscope with an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., New Castle, DE) under an air flow of 25 mL min⁻¹. The temperature was increased from 30 to 700 °C at a linear heating rate of 20 °C min⁻¹. The thermogravimetric analysis/infrared spectrometry (TG-IR) was performed using the TGA Q5000 IR thermogravimetric analyzer which was coupled with the Nicolet 6700 FT-IR spectrophotometer via the transfer line. The transfer line was heated to 250 °C. Each sample specimen (about 5 mg) was carried out at a heating rate of 20 °C min⁻¹ from room temperature to 700 °C under a nitrogen flow at a flow rate of 35 mL min⁻¹. The fire resistance of samples was evaluated with an MCC-2 microscale combustion calorimeter (Govmark Organization, Inc., Farmingdale, NY). The test sample was put in a ceramic crucible in air and heated at 80–600 °C at a linear heating rate of 60 °C min⁻¹. The heat release data were calculated with the operating software of the MCC-2 instrument. Cone calorimeter test was carried out on the cone calorimeter, following the procedures in ISO5660. Square specimens (100 × 100 × 3 mm³) were irradiated at a heat flux of 35 kW m², corresponding to a mild fire scenario. Differential scanning calorimetry (DSC) was performed using a Q2000 DSC instrument (TA Instruments Inc.). Samples (2–4 mg) were heated from 0 to 150 °C at a linear heating rate of 10 °C min⁻¹; the temperature was kept at 150 °C for 10 min and then decreased from 150 to 0 °C at a linear rate of 10 °C min⁻¹. This heating–cooling cycle was repeated, and the data obtained from the second heating section were plotted. The tensile strength and elongation at break were measured according to the Chinese standard method (GB 13022-91) with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., Changchun City, China) at a crosshead speed of 50 mm min⁻¹. Nanocomposite membranes were cut into special pieces, and 10 parallel runs were performed for each sample to obtain averages. Dynamic mechanical analysis (DMA) was performed using a DMAQ800 apparatus (TA Instruments Inc.) at a fixed frequency of 10 Hz in the temperature range from 30 to 100 °C.

3. Result and discussion

3.1. Structural properties

To confirm the successful functionalization of MoS₂ nanosheets with POSS, XRD and FTIR analyses were conducted on the pristine and modified MoS₂ nanosheets. The representative XRD patterns of MoS₂ and OvlPOSS-MoS₂ samples are depicted in Fig. 1A. An intense reflection at 2θ = 14.48° (corresponding interlayer distance was 0.61 nm) is observed for unmodified MoS₂, which is attributed to the (002) plane of MoS₂. In comparison with bulk MoS₂, it can be noted that the (002) peak of OvlPOSS modified MoS₂ shifts to 2θ = 11.40°, which corresponds to d-spacing of 0.78 nm. It has been generally accepted that the expansion of the interlayer space is due to successful functionalization of MoS₂ nanosheets with the OvlPOSS molecules. FTIR analysis spectra of bulk MoS₂ and OvlPOSS modified MoS₂ (Fig. 1B) provides another evidence for the organic modification. After the functionalization of MoS₂ by OvlPOSS, some new absorption bands in the spectra of OvlPOSS-MoS₂ appear. Compared to MoS₂, the strong absorption band at around 1105 cm⁻¹ is attributed to the Si–O–Si group in POSS.¹⁸ These results in conjunction with the XRD results could be considered as indication of successfully functionalization.

Fig. 1 XRD patterns (A) and FTIR spectra (B) of MoS₂ and OvlPOSS-MoS₂ nanosheets.

The PVA-based nanocomposites were prepared by the ultrasonic solvent blending and casting technique with addition of PVA and MoS₂ or OvlPOSS-MoS₂. The morphology and structure of polymer/layered inorganic nanocomposites are generally characterized by XRD and TEM analyses. Fig. 2 shows the XRD patterns of the virgin PVA, PVA/MoS₂, and PVA/OvlPOSS-MoS₂ composites. The neat PVA exhibits a broad crystalline peak at 2θ = 19.5°, corresponding to the (101̄) and (101) crystalline reflections of PVA. For the PVA/OvlPOSS-MoS₂ system, it displays similar characteristics diffraction peaks to PVA matrix. The diffraction peaks observed in the XRD patterns of the MoS₂ disappear from the patterns of PVA/OvlPOSS-MoS₂ composites. The composites exhibit none of the MoS₂ diffraction peaks, implying the possible exfoliation of the MoS₂ layered structure. In contrast, the strongest diffraction peak (002) of the MoS₂ is still observed in the XRD patterns of PVA/MoS₂ composite, indicating the presence of stacked structure of MoS₂.

Fig. 2 XRD patterns of pure PVA, PVA/MoS₂ and PVA/OvlPOSS-MoS₂.

Fourier transform infrared is an effective instrument for assessing the interaction between inorganic particles and polymer matrix,¹⁹ as a supplementary technique to XRD and TEM. The condensed phase products of pure PVA and its composite (PVA/OvlPOSS-MoS₂) were characterized by FTIR (Fig. 3). From the analyses of FTIR spectra of the PVA/OvlPOSS-MoS₂, the characteristic peaks of both the OvlPOSS-MoS₂ and PVA matrix are identified in the spectra of the composite (PVA/OvlPOSS-MoS₂). The FTIR assignments indicate that OvlPOSS-MoS₂ has been incorporated into the PVA matrix successfully.

Fig. 3 FTIR spectra of PVA and PVA/OvlPOSS-MoS₂.

Although the XRD and the FTIR spectra demonstrated MoS₂, OvlPOSS-MoS₂ and the interaction of OvlPOSS-MoS₂ in the PVA nanocomposite, TEM and SEM analyses could provide direct visual information that OvlPOSS-MoS₂ in PVA matrix. As can be seen from the Fig. 4, the MoS₂ sheets exhibit a typically flat nanoplatelet with several hundred nanometers (Fig. 4A), but some granules emerge on the flat nanosheet for OvlPOSS-MoS₂ derived from the grafting OvlPOSS. Moreover, it can be clearly seen that the fractured surface of neat PVA is quite smooth, while that of the nanocomposite sample is very rough. It is also worthy to note that a uniform dispersion of the MoS₂ sheets within the polymer matrix was achieved, with no visible aggregation of MoS₂. This phenomenon is in good agreement with the previous literatures.^20,21 Meanwhile, from the ultrathin section of PVA/OvlPOSS-MoS₂ composite, thin MoS₂ sheets with a high aspect ratio are randomly oriented over the entire imaging area (Fig. 4E). Therefore, combined with the XRD, FTIR results, it can be concluded that a homogeneous dispersion state is formed in the case of the PVA nanocomposite.

Fig. 4 TEM images of (A) MoS₂, (B) OvlPOSS-MoS₂, SEM images of the fractured sections of (C) neat PVA and (D) PVA/OvlPOSS-MoS₂ composite and TEM images of ultrathin section of PVA/OvlPOSS-MoS₂ composite (E).

3.2. Thermal behavior of the nanocomposites

As we know, the addition of layered materials usually improves the thermal stability and fire resistance of polymer matrix, because 2D layered materials can decrease the heat conduction. The thermal stability of PVA and its composites were evaluated by TGA under air atmosphere, as shown in Fig. 5. As can be observed, the thermal degradation process of pure PVA mainly contains three stages based on the TGA profile, which mainly correspond to the vaporization of water molecules, the decomposition of the macromolecular chains, and the oxidation of char residue, respectively. Increased thermal stability is typical in the polymer/layered compound nanocomposites, usually attributed to the heat and mass barrier effects of the layered nano-compounds, which slow the diffusion of heat and pyrolysis products.²² In comparison with neat PVA, the maximum decomposition temperatures (T_max) of PVA/MoS₂ nanocomposites were increased due to the physical barrier effect of MoS₂. When MoS₂ was modified by OvlPOSS to convert into OvlPOSS-MoS₂, PVA/OvlPOSS-MoS₂ depicts the best thermal stability, a 23 °C increment compares to that of pure PVA. In the case of previous report,¹⁷ the introduction of 5%wt neat MoS₂ can only increase 2 °C in comparison with that of pure PVA. When the neat MoS₂ amount was increased to 10%wt, the maximum decomposition temperature (T_max) of PVA/MoS₂ nanocomposite was decreased.

Fig. 5 TGA thermograms of pure PVA, PVA/MoS₂ and PVA/OvlPOSS-MoS₂ nanocomposites in air atmosphere.

The glass transition temperature (T_g) is an important thermal parameter which can be used to evaluate the segmental mobility of the polymer chains. To understand how the PVA/OvlPOSS-MoS₂ could affect on the glass transition behavior of the matrix polymer in the nanocomposites, differential scanning calorimetry (DSC) analysis was carried out. T_g values of the samples above were tested and calculated by DSC. The DSC curves of the prepared samples are presented in Fig. 6. After OvlPOSS-MoS₂ is incorporated, and T_g is found to increase, improving by 10.2 °C compared with that of pure PVA. The increase of the T_g may be attributed to the great restriction of polymer's chain motions which prove the network and good interfacial interactions between the OvlPOSS-MoS₂ nanosheets and polymer chains. By comparing the T_g from previous work,¹⁷ the maximum improvement of T_g (3.6 °C) is lower than our data.

Fig. 6 DSC curves of PVA nanocomposites marked with glass transition temperature.

3.3. Fire hazards evaluated using a MCC and cone calorimeter

Microscale combustion calorimetry (MCC) is a thermal combustion analysis instrument that directly measures the heat of combustion of the gases evolved during controlled heating of milligram-sized samples.²³ Several parameters can be obtained from MCC, such as heat release rate (HRR), heat release capacity (HRC), total heat release (THR), etc., which are very important to reflect the combustion properties of materials, allowing a reasonable estimation of fire hazard using small quantities of samples. Fig. 7 shows typical MCC results of neat PVA and its composites. By compared with pure PVA, the PVA nanocomposites achieved significant improvements in flame resistance. As depicts in Fig. 7, the pHRR and THR of all the samples are lower than pure PVA. Both pHRR and THR show the lowest value when OvlPOSS-MoS₂ was added. Compared with the pure PVA, the pHRR and THR decreased apparently, falling by 41% and 16%, respectively. However, when the concentration of MoS₂ reaches 5 wt% in the previous literature,¹⁷ the pHRR only decreased by 33%.

Fig. 7 HRR and THR curves of pure PVA, PVA/MoS₂ and PVA/OvlPOSS-MoS₂ nanocomposites from MCC test.

Fig. 8 HRR and THR versus time curves of PVA and its composites obtained from cone calorimeter test.

Cone calorimeter is a widely used device for measuring the flammability of various materials in real-world fire conditions.²⁴ The HRR curves for PVA and its composites are shown in Fig. 8. It can be observed that neat PVA burns extremely rapidly after ignition and the pHRR value reaches to 375 kW m². It is well-known that layered materials are usually used to impart the flame retardant properties to polymers due to its unique 2D nanosheet structure.^25,26 As expected, incorporating MoS₂ nanosheets into PVA makes the pHRR decrease to 297 kW m². For PVA/OvlPOSS-MoS₂, the pHRR is also reduced to 268 kW m², corresponding to a 29% decrease compared to that of pure PVA. Also, the THR values of PVA/OvlPOSS-MoS₂ are significantly reduced by 31%, compared to that of pristine PVA.

3.4. Flame retardant mechanism

Flame retardant additives in polymers may play a key role either in the condensed phase or in the gas phase or also in both phases at the same time. On the basis of the previous reports, various nanoparticles, such as organo-modified clays, layered silicate, layered double hydroxides (LDH), carbon nanotubes, graphene or polyhedral silsesquioxanes (POSS), act as a ‘char reinforcer’ or ‘char expander’ in the condensed phase.^18,27–31 When a condensed phase impact is the main mechanism of the flame retardant additives, the efficiency depends strongly on the structure and composition of the char during burning. Therefore, investigating the properties and the structure of the resultant carbonaceous layers will provide an insight into understanding how the flame retardant additives act in the condensed phase.

From the results of TGA, DSC, MCC and cone tests, it is facilely capable of finding the analogous changing trend: the addition of OvlPOSS-MoS₂ into PVA matrix can effectively improve the thermal properties and the glass transition temperature (T_g), diminished the pHRR and the THR value. In order to elucidate the significant advantages of PVA/OvlPOSS-MoS₂ composites, the residues of the composites after thermal degradation in air atmosphere were investigated, and the corresponding SEM images of the char residue are displayed in Fig. 9. As indicated in Fig. 9A, residual char of pure PVA presents a rough and looser char layer with a mass of holes disperse on the surface. Moreover, it displays porous and incompact surface at high-magnification SEM image (Fig. 9B). However, when 2 wt% of OvlPOSS-MoS₂ is added into PVA polymer, the holes become fewer (Fig. 9C) and the porous and incompact surface changes into more compact one. The dense char layer can lower efficiency of heat and volatiles transference since the strong hindering effect, so it can provide better flame shield for the underlying material during combustion. Flame retardant additives in polymers may have an action either in the condensed phase or in the gas phase or also in both phases at the same time.

Fig. 9 SEM images of surface morphology of the residue of PVA composites after calcination in air: (A and B) PVA; (C and D) PVA/OvlPOSS-MoS₂.

Raman spectroscopy offers a powerful tool for characterizing carbonaceous materials. Fig. 10 shows the Raman spectra of the residual char of PVA and PVA/OvlPOSS-MoS₂ obtained from the calcination at 600 °C for 20 min in muffle furnace. As can be observed, the Raman spectra of the two samples exhibit similar shape, with two peaks at 1604 cm⁻¹ and 1362 cm⁻¹. The characteristic peak at 1604 cm⁻¹ is called G band, corresponding to the first-order scattering of the E_2g mode, while the other is called D band, arising from the activation in the first order scattering process of sp³ carbons.³² As shown in Fig. 10, the peak intensity ratio I_(D)/I_(G) between D band and G band of PVA/OvlPOSS-MoS₂ is much lower than that of PVA, suggesting the improvement of the graphitized carbons in the residual char and thermally stable char structure of the PVA/OvlPOSS-MoS₂.³³ The high content of graphitized carbons in the residual char is known to be compact and efficient in terms of thermal insulation, which provides a protective shield that leads to a decrease in heat and mass transfers between the flame and the material.

Fig. 10 Raman curve of the residue of PVA and PVA/OvlPOSS-MoS₂ nanocomposite (PVA3) after calcination in air.

The TG-FTIR technique is a useful tool in dynamical analysis as it monitors continuously both the time-dependant evolution of the gases and the weight of the residue. It has been widely used in polymer thermal degradation, which can make a great contribution to the understanding of thermal degradation mechanism. To investigate the influence of OvlPOSS-MoS₂ on the evolved gaseous volatiles during pyrolysis, the volatile components of PVA and PVA/OvlPOSS-MoS₂ were investigated by TG-FTIR technique. FTIR spectra obtained at the maximum evolution rate during the thermal decomposition of PVA and its OvlPOSS-MoS₂ composite are presented in Fig. 11. Some small molecular gaseous decomposition products evolved from PVA and PVA/OvlPOSS-MoS₂ composite are identified unambiguously by characteristic strong FTIR signals, such as –C–H groups for allyl alcohol, acetone, ethers and various hydrocarbons (3100–2800 cm⁻¹), CO₂ (2360 cm⁻¹), CO (2180 cm⁻¹) carbonyl-containing compounds (1720 cm⁻¹), and aromatic compounds (1605, 1510 and 1460 cm⁻¹).^34,35 PVA/OvlPOSS-MoS₂ exhibits that the typical thermal degradation process of the composites is similar to pure PVA. However, the intensity of gas emission of PVA/OvlPOSS-MoS₂ composites is much lower than that of the virgin PVA. In order to provide a clear comparison, the intensity of typical gaseous organic volatiles of pure PVA and the PVA/OvlPOSS-MoS₂ composites are represented in Fig. 12. It can be obviously observed that the addition of OvlPOSS-MoS₂ decreased the evolution of all gaseous volatiles significantly. The decrease of these combustible gases could be attributed to the enhancement of char formation and the physical barrier effect of the nanosheets. The reduced amount of the organic volatiles further leads to the inhibition of smoke, because the organic volatiles may crack into smaller hydrocarbon molecules and smoke particles. The gaseous hydrocarbons are condensed and the smoke particles are aggregated to form smoke.³⁶ Combined with the results of thermal stability, MCC, cone results, SEM images and Raman spectra of the residual char in this study, it is reasonably believed that the thermal stability and fire resistance properties of PVA nanocomposites are strongly affected by physical barrier effect and charring effect. Based on analysis above, an illustration of proposed model for the evolution of the charred layer for flame retarded PVA samples is presented in Scheme 2.

Fig. 11 IR spectra of gasified pyrolysis products for pure PVA and PVA/OvlPOSS-MoS₂ at the maximum evolution rate.

Fig. 12 Intensity of characteristic peaks for pyrolysis products of pure PVA and PVA/OvlPOSS-MoS₂ composites.

Scheme 2 Model of the char forming of PVA/OvlPOSS-MoS₂ composites.

3.5. Mechanical properties

It is interesting to note that the structures and thermal properties of PVA/OvlPOSS-MoS₂ nanocomposites were not the only aspect of the composites influenced. According to the literature, the mechanical performance of the nanocomposites could be enhanced by the rigid LDH layered nanocrystals in PVA matrix. Therefore, it is speculated that the addition of the MoS₂ nanosheets will improve the mechanical performance of the PVA nanocomposites by the strong interfacial interaction between OvlPOSS-MoS₂ and PVA matrix. The typical stress–strain behaviors for the films of PVA, PVA/MoS₂, and PVA/OvlPOSS-MoS₂ nanocomposites are presented in Fig. 13. The tensile stress of PVA/OvlPOSS-MoS₂ nanocomposite increase by 57%, compared with that of the pure PVA film. Zhou et al., reported that the largest increase in tensile strength was 24% in PVA nanocomposites,¹⁷ which is much lower than our data. The increase to some extent of the stress of the PVA/OvlPOSS-MoS₂ nanocomposite films can be ascribed to the homogeneous dispersion of OvlPOSS-MoS₂ in the polymer matrix and strong interfacial interactions between both components. Similar results have been observed for polymer/carbon nanotubes and polymer/graphene nanocomposites.^37,38 The elongation at break of the nanocomposite films gradually decreases with the addition of nanoparticles, which is due to the lamellar barrier effect of the nano-flakes restricting the segmental motion of the polymer chains in the nanocomposites. Furthermore, the storage modulus is a measure of the stiffness for polymer, thus DMA was carried out to evaluate the mechanical properties. The storage modulus and loss tangent curves are shown in Fig. 14. All the composites display higher storage module than pure PVA, and sample PVA/OvlPOSS-MoS₂ have the largest increase in comparison with neat PVA. The glass transition temperature (T_g) is determined from the peak temperature of tan δ curves. As expected, a great increase in T_g values for PVA/OvlPOSS-MoS₂ is achieved. The pure PVA matrix displays a T_g of 57.7 °C, while for the composite film with 2 wt% OvlPOSS-MoS₂, the T_g is significantly increased to 78.9 °C with an increment of 21.2 °C. The enhanced T_g indicates a strong confinement effect of OvlPOSS-MoS₂ to the PVA chains. The results further proved the strong interfacial adhesion between the PVA/OvlPOSS-MoS₂ and the PVA matrix.

Fig. 13 Typical stress–strain behaviors for the films of pure PVA, PVA/MoS₂ and PVA/OvlPOSS-MoS₂ composites.

Fig. 14 DMA curves of pure PVA, PVA/MoS₂ and PVA/OvlPOSS-MoS₂ composites.

4. Conclusion

In this article, to obtain high-performance properties of PVA, OvlPOSS-MoS₂ was incorporated into PVA using ultrasonic solvent blending, and casting. The OvlPOSS-MoS₂ were homogeneously dispersed in the PVA matrix and resulted in various property enhancements. The thermal stability and fire resistance were significantly improved. The maximum degradation temperature of the PVA/OvlPOSS-MoS₂ nanocomposites was increased by 23 °C, the T_g of the PVA/OvlPOSS-MoS₂ was improved 10.2 °C. Meanwhile, the peak of heat release rate (pHRR) and total heat release (THR) decreased. The improvement of fire retardant properties was mainly attributed to the fact that addition of OvlPOSS-MoS₂ leads to the formation of the compact and insulating char layer to protect the inner polymer matrix from further burning. Moreover, the TG-IR results indicated that the total pyrolysis products of PVA/OvlPOSS-MoS₂ were significantly reduced, and specially the hydrocarbons, aromatic compounds and carbon monoxide were significantly reduced, compared to those of neat PVA. The notable reduction of the fire hazards is mainly due to the physical barrier effect and charring effect that could slow down the release of combustible gas, especially hydrocarbons and aromatic compounds. The tensile stress was increased by 57% upon addition of 2 wt% OvlPOSS-MoS₂ due to strong interfacial interactions between both components. As a conclusion, our strategy for preparing high-performance PVA/OvlPOSS-MoS₂ nanocomposites is facile and efficient, indicating promising use in academic research and practical applications.

Acknowledgements

The work was financially supported by the National Key Technology R&D Program (2013BAJ01B05), the National Basic Research Program of China (973 Program) (2012CB719701), and Specialized Research Fund for the Doctoral Program of Higher Education (20103402110006).

References

1. J. Feng, X. Sun, C. Z. Wu, L. L. Peng, C. W. Lin, S. L. Hu, J. L. Yang and Y. Xie, J. Am. Chem. Soc., 2011, 133, 17832–17838.
2. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147–150.
3. R. F. Service, Science, 2009, 324, 875–877.
4. G. Wang, X. G. Zhu, Y. Y. Sun, Y. Y. Li, T. Zhang, J. Wen, X. Chen, K. He, L. L. Wang, X. C. Ma, J. F. Jia, S. B. B. Zhang and Q. K. Xue, Adv. Mater., 2011, 23, 2929–2932.
5. C. Y. Zhi, Y. Bando, C. C. Tang, H. Kuwahara and D. Golberg, Adv. Mater., 2009, 21, 2889–2893.
6. Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537–1541.
7. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274–10277.
8. J. Haider and M. S. J. Hashmi, AIP Conf. Proc., 2010, 1315, 1365–1370.
9. K. Bindumadhavan, S. K. Srivastava and S. Mahanty, Chem. Commun., 2013, 49, 1823–1825.
10. Y. M. Shi, Y. Wang, J. I. Wong, A. Y. S. Tan, C. L. Hsu, L. J. Li, Y. C. Lu and H. Y. Yang, Sci. Rep., 2013, 3, 2169,  DOI:10.1038/srep02169.
11. H. Li, Z. Y. Yin, Q. Y. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang and H. Zhang, Small, 2012, 8, 63–67.
12. Z. Matusinovic and C. A. Wilkie, J. Mater. Chem., 2012, 22, 18701–18704.
13. G. Scocchi, P. Posocco, J. W. Handgraaf, J. G. E. M. Fraaije, M. Fermeglia and S. Pricl, Chem.–Eur. J., 2009, 15, 7586–7592.
14. T. Kuilla, S. Bhadra, D. H. Yao, N. H. Kim, S. Bose and J. H. Lee, Prog. Polym. Sci., 2010, 35, 1350–1375.
15. R. Verdejo, M. M. Bernal, L. J. Romasanta and M. A. Lopez-Manchado, J. Mater. Chem., 2011, 21, 3301–3310.
16. K. Chang, W. X. Chen, L. Ma, H. Li, H. Li, F. H. Huang, Z. D. Xu, Q. B. Zhang and J. Y. Lee, J. Mater. Chem., 2011, 21, 6251–6257.
17. K. Q. Zhou, S. H. Jiang, C. L. Bao, L. Song, B. B. Wang, G. Tang, Y. Hu and Z. Gui, RSC Adv., 2012, 2, 11695–11703.
18. X. Wang, L. Song, H. Y. Yang, W. Y. Xing, B. Kandola and Y. Hua, J. Mater. Chem., 2012, 22, 22037–22043.
19. X. M. Yang, L. A. Li, S. M. Shang and X. M. Tao, Polymer, 2010, 51, 3431–3435.
20. X. Zhao, Q. H. Zhang, D. J. Chen and P. Lu, Macromolecules, 2011, 44, 2392.
21. V. Alzari, D. Nuvoli, R. Sanna, S. Scognamillo, M. Piccinini, J. M. Kenny, G. Malucelli and A. Mariani, J. Mater. Chem., 2011, 21, 16544–16549.
22. Y. W. Cao, J. C. Feng and P. Y. Wu, Carbon, 2010, 48, 3834–3839.
23. X. Wang, L. Song, H. Y. Yang, W. Y. Xing, H. D. Lu and Y. Hu, J. Mater. Chem., 2012, 22, 3426–3431.
24. K. C. Cheng, C. B. Yu, W. J. Guo, S. F. Wang, T. H. Chuang and Y. H. Lin, Carbohydr. Polym., 2012, 87, 1119–1123.
25. C. L. Bao, Y. Q. Guo, L. Song, Y. C. Kan, X. D. Qian and Y. Hu, J. Mater. Chem., 2011, 21, 13290–13298.
26. C. L. Bao, L. Song, W. Y. Xing, B. H. Yuan, C. A. Wilkie, J. L. Huang, Y. Q. Guo and Y. Hu, J. Mater. Chem., 2012, 22, 6088–6096.
27. S. Pack, T. Kashiwagi, C. H. Cao, C. S. Korach, M. Lewin and M. H. Rafailovich, Macromolecules, 2010, 43, 5338–5351.
28. A. B. Morgan, R. H. Harris, T. Kashiwagi, L. J. Chyall and J. W. Gilman, Fire Mater., 2002, 26, 247–253.
29. T. Kashiwagi, F. M. Du, J. F. Douglas, K. I. Winey, R. H. Harris and J. R. Shields, Nat. Mater., 2005, 4, 928–933.
30. B. X. Du and Z. P. Fang, Nanotechnology, 2010, 21, 315603.
31. J. W. Gilman, W. H. Awad, R. D. Davis, J. Shields, R. H. Harris, C. Davis, A. B. Morgan, T. E. Sutto, J. Callahan, P. C. Trulove and H. C. DeLong, Chem. Mater., 2002, 14, 3776–3785.
32. M. Fang, K. G. Wang, H. B. Lu, Y. L. Yang and S. Nutt, J. Mater. Chem., 2009, 19, 7098–7105.
33. P. Lespade, R. Aljishi and M. S. Dresselhaus, Carbon, 1982, 20, 427–431.
34. H. M. Zhu, J. H. Yan, X. G. Jiang, Y. E. Lai and K. F. Cen, J. Hazard. Mater., 2008, 153, 670–676.
35. K. Wu, L. Song, Y. Hu, H. D. Lu, B. K. Kandola and E. Kandare, Prog. Org. Coat., 2009, 65, 490–497.
36. Y. Y. Dong, Z. Gui, Y. Hu, Y. Wu and S. H. Jiang, J. Hazard. Mater., 2012, 209, 34–39.
37. R. K. Layek, S. Samanta and A. K. Nandi, Carbon, 2012, 50, 815–827.
38. X. F. Zhang, T. Liu, T. V. Sreekumar, S. Kumar, V. C. Moore, R. H. Hauge and R. E. Smalley, Nano Lett., 2003, 3, 1285–1288.

---