POSS-functionalized polyphosphazene nanotube: preparation and effective reinforcement on UV-curable epoxy acrylate nanocomposite coatings

Abstract

This study presents an efficient method of preparing functionalized poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotube reinforced UV-curable materials. Octamercaptopropyl polyhedral oligomeric silsesquioxane (OMP-POSS) functionalized PZS (POPZS) nanotubes, prepared via a thiol–ene click approach, were covalently incorporated into epoxy acrylate (EA) by UV irradiation technology. The results of transmission electron microscopy and X-ray photoelectron spectroscopy indicated that OMP-POSS was successfully grafted onto the surface of PZS nanotubes. Dynamic mechanical analysis was employed to investigate the dynamic mechanical property of POPZS/EA nanocomposite coatings. The optimal reinforcing effect for EA matrix was observed at the 3.0 wt% loading of POPZS nanotubes. The storage modulus at 30 °C and glass transition temperature was dramatically improved by 88% and 16 °C, respectively, compared to those of pure EA. Moreover, the char yield at 800 °C of the nanocomposites was significantly increased, indicating the remarkably improved thermal stability. These extraordinary reinforcements of properties are attributed to effective reformative interfacial interaction between POPZS nanotubes and EA matrix by covalent linkage.

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1. Introduction

Phosphazene-containing polymers are versatile hybrid organic–inorganic materials with many outstanding properties such as fabulous thermal stability, biocompatibility and biodegradability. These polymers have been used widely as optical materials, biomaterials, membrane materials, electrical materials and hybrid materials.^1–4 Well-known with unique –P=N– structural units in macromolecular backbones and having multiple side groups, such as organic, organometallic or inorganic units, it is provided with tremendous flexibility to functionalize the materials through physical or chemical modifications.^5–9 In a prior study, phosphazene-containing nanotubes, formally called poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS), were synthesized through one-pot reaction using a simple process, controllable morphology, high yield and low cost.¹⁰ These synthesized nanomaterials possess high thermal stability, radiation resistance and fire retardancy.^11–13 Moreover, PZS nanotubes can be well dispersed in general solvents and polymer matrices, probably due to the possibility that organic components could provide some degree of affinity between organic molecules and polymers.^14–16 Furthermore, active groups such as hydroxyl groups can be obtained selectively on PZS nanotubes by altering the reaction condition and molar ratio of the monomer.^17,18 Together with the stunning controllability of the backbone, PZS nanotubes can be conveniently functionalized through introducing specific functional groups via covalent or noncovalent methods. Recently, novel epoxy-group modified phosphazene-containing nanotubes (EPPZTs) were cross-linked with epoxy resin to reinforce the resin matrix, endowing fabulous properties comparable to carbon nanotubes (CNTs).¹⁹ Compared to CNTs, PZS can be conveniently modified. Moreover, the low loading level, such as 0.1%, could significantly enhance mechanical properties and have a slight impact on thermal stability. Moreover, in our prior study,²⁰ the acryloyl-group functionalized PZS nanotubes turn out to have 45.0% and 36.3% maximum increases in the storage modulus and onset thermal degradation temperature, respectively, for the f-PZS/PUA nanocomposites over pure PUA. To date, there have been few literatures reporting the application of surface modified PZS in polymer nanocomposites.

Polyhedral oligomeric silsesquioxane (POSS) with a well-defined organic/inorganic hybrid structure has attracted increasing interest due to its unique cage-like molecular structure and physicochemical properties.^21–24 Typical POSS cages are represented by the formula (RSiO_1.5)_n, where the R generally refers to hydrogen or various functionalized groups.^25–27 Thus, every POSS molecule possessing eight organic groups can be endowed with high reactivity and compatibility in polymer matrices, compared to other inorganic nanoadditives such as graphene, clays or CNTs.^28,29 In the previous decades, numerous studies have reported the effects of application of POSS as nanoadditives on enhanced thermal properties, strength and modulus, impact resistance, increased oxidation and chemical resistance of polymer nanocomposites.^30,31 Among diverse POSS structures, the unique octamercaptopropyl POSS (OMP-POSS) with eight reactive thiol groups surrounding a cage-like core may be grafted on the surface of modified PZS nanotubes through a thiol–ene click reaction.³² Thus, it is expected to functionalize PZS nanotubes with OMP-POSS, resulting in well-dispersed polymer nanocomposites and improved interfacial interactions. However, study about the application of POSS functionalized PZS nanotubes in ultraviolet (UV)-curable materials has been scarcely reported.

To develop high-performance UV-curable coatings, we combined the reinforcement effect of the polyphosphazene nanotubes and OMP-POSS on the mechanical and thermal properties of the polymer matrix. In this work, 3-methacryloxypropyl trimethoxysilane was employed to modify PZS nanotubes, and then OMP-POSS was covalently grafted onto the PZS nanotubes via a thiol–ene click reaction. Subsequently, the POSS-modified PZS nanotubes (POPZS) were incorporated into epoxy acrylate (EA) by UV irradiation technology. The mechanical and thermal properties as well as transparency of POPZS/EA nanocomposites were investigated.

2. Experimental section

2.1 Materials

Hexachlorocyclotriphosphazene (HCCP) was purchased from Aldrich (U.S.) and purified through sublimation before use. Tetrahydrofuran (THF), methanol, ethanol, acetone and triethylamine (TEA) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China) and dried prior to use. 4,4′-Sulfonyldiphenol (BPS) was purchased from Shanghai Chemical Reagents Corp. (Shanghai, China). 3-Methacryl-oxypropyl trimethoxysilane (MPTES) and pentaerythritol tetrakis(2-mercaptopropionate) were provided by Nanjing Shuguang Chemical Group Co., Ltd. (Nanjing, China). EA resin was accessed from DSM-AGI Co., Ltd. (Taiwan, China). 2-Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), used as a photoinitiator, was from Shanghai Chemical Industry Co., Ltd. (Shanghai, China).

2.2 Synthesis of PZS nanotubes with active hydroxyl groups¹⁷

Typically, a given amount of BPS and TEA (2.08 g, 20.6 mmol) were dissolved in 125 mL THF in a three-necked flask equipped with a mechanical stirrer, dropping funnel and reflux condenser. Then the solution of HCCP (1.2 g, 3.45 mmol) in 125 mL of THF was added dropwise to the flask within 2 h under ultrasonication (53 kHz) at 40 °C. The temperature of this system was accurately controlled at 40 °C with ultrasonication for an additional 12 h. After completion of the reaction, the solvent was removed by distillation under reduced pressure, and the precipitate was washed with ethanol and deionized water three times. Finally, the resulting products were obtained under vacuum at 80 °C overnight. The yield was around 70%.

2.3 Synthesis of OMP-POSS functionalized PZS nanotubes (POPZS)

OMP-POSS was synthesized according to the method described in a prior study.³³ POPZS was prepared via thiol–ene click reaction, and the preparation route was illustrated in Scheme 1. As is well known, PZS contains active hydroxyl groups on its surface, which could provide the active sites to react with MPTES. First, PZS (2 g) was dispersed in deionized water (250 mL) by ultrasonication for 1 h. MPTES (20 mL) and HCl (4 mL) were subsequently added into the suspension mentioned above. The hydrolysis and condensation between MPTES and PZS were conducted simultaneously at 75 °C for 2 h to generate the modified PZS (KPZS). 2 g of KPZS obtained was dispersed in 150 mL of THF by sonication for 30 min. Subsequently, 4 g of OMP-POSS and 0.06 g of TEA were introduced into the suspension mentioned above, and the reaction was conducted at 70 °C for 24 h under nitrogen protection. The solid products (POPZS) were filtered and washed with THF and ethanol to remove the residual reagents, then dried under vacuum at 60 °C for 24 h.

Scheme 1 Schematic of the synthetic routes of the POPZS nanotubes and POPZS/EA nanocomposites.

2.4 Preparation of POPZS/EA nanocomposites

A typical procedure to prepare POPZS/EA nanocomposites with 0.1 wt% POPZS was described as follows: POPZS powder (10 mg) was dispersed in acetone (10 mL) in a 50 mL three-necked flask with the assistance of an ultrasonic bath (53 kHz) for 0.5 h at room temperature. EA (9.99 g) was subsequently incorporated into the abovementioned POPZS suspension, and the ultrasonic bath lasted for 2 h. The solvent was removed in a vacuum oven at 60 °C for 2 h. After cooling to room temperature, the photoinitiator Darocur 1173 (4 wt%) was added into the mixture by vigorous stirring. Then the blend was coated on a glass plate and dried in an oven at 60 °C for 2 h to remove the solvent completely. Finally, the film was irradiated for 30 seconds using an UV irradiation equipment (80 W m⁻¹ Lantian Co., China). The POPZS/EA nanocomposite coatings were named as POPZS/EA-x, where x was the weight percentage of POPZS nanotubes in the EA matrix.

2.5 Characterization

Fourier transform infrared (FTIR) spectra of PZS, KPZS and POPZS were obtained with a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). The samples were mixed with KBr powders and pressed into tablets before characterization.

X-ray photoelectron spectroscopy (XPS) test was performed with a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK). The excitation source was an Al K_α ray at 1486.6 eV.

X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Rigaku Co., Japan), using Cu K_α radiation (λ = 0.15418 nm), at a scanning rate of 4° min⁻¹.

Thermogravimetric analysis (TGA) was conducted on a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA) from 20 to 800 °C at a linear heating rate of 20 °C min⁻¹ under nitrogen atmosphere. The samples were run in triplicate: the temperature reproducibility of the instrument was 0.18 °C, whereas the mass reproducibility was 0.2%.

The morphology of PZS and POPZS nanotubes was examined by a JEM-2100F transmission electron microscope (Japan Electron Optics Laboratory Co., Ltd., Japan). PZS and POPZS nanotubes were dispersed in ethanol with ultrasonication for 0.5 h and then dripped onto copper grids for observation.

SEM was conducted by a high-resolution JEOL JSM-6700 field-emission scanning electron microscope (FE-SEM). The samples were dispersed in ethanol with ultrasonication for 0.5 h, and then dripped onto copper sheets for observation. The studied surfaces were sputter-coated with a thin layer of gold before observation.

Dynamic mechanical analysis (DMA) was performed on a PerkinElmer Pyris Diamond DMA apparatus from room temperature to 200 °C at a heating rate of 5 °C min⁻¹ in the tensile configuration. The frequency of dynamic oscillatory loading was 10 Hz. The storage modulus reproducibility of the instrument was 4–6%.

The transparency of five samples was studied using a DUV-3700 UV-Vis spectrometer (Shimadzu, Japan). The transmission mode was used and the wavelength ranges were set from 400 to 800 nm.

3. Results and discussion

3.1 Characterization of PZS and POPZS nanotubes

The FTIR spectra of PZS, KPZS and POPZS nanotubes are shown in Fig. 1a. From the infrared spectrum of bare PZS nanotubes, it can be observed that two sharp peaks at 1589 cm⁻¹ and 1490 cm⁻¹ are assigned to the stretching vibration of C=C groups in the sulfonyldiphenol units. The peaks at 1186 and 883 cm⁻¹ are associated with the absorption of P=N and P–N, respectively. Moreover, the characteristic absorption for O=S=O groups is observed at 1293 and 1153 cm⁻¹. The absorption peak at 941 cm⁻¹ corresponds to the P–O–Ar bands. The peaks at 3100 and 3073 cm⁻¹ are attributed to the stretching vibration of the hydroxyl in the phenolic groups. These results provide clear evidence for the occurrence of polycondensation reaction between HCCP and BPS. After the pretreatment of PZS nanotubes with MPTES, the new peaks at 1105 cm⁻¹ assigned to Si–O–C and 1635 cm⁻¹ corresponding to C=C in the MPTES, are observed in the IR spectrum of KPZS. In addition, two other emerging peaks at 1720 and 2954 cm⁻¹ are attributed to the C=O and methyl stretching, respectively. These results demonstrate that the MPTES has reacted with the active –OH in PZS nanotubes, and the nanotubes have been successfully modified. With further functionalization of PZS nanotubes with OMP-POSS, there are peaks at 2856 cm⁻¹ and 2927 cm⁻¹ involving the stretching of methylene groups. It appears the unique absorption peak of POSS is shown at 565 cm⁻¹,³³ whereas the new peak at 1014 cm⁻¹ is associated with the band of Si–O–Si asymmetric stretching, which means that OMP-POSS have successfully connected with the KPZS nanotubes.

Fig. 1 (a) FTIR spectra of the PZS, KPZS and POPZS nanotubes; (b) XPS S 2p spectra of PZS nanotubes and (c) POPZS nanotubes; (d) XPS survey scan spectra of PZS and POPZS nanotubes; (e) XRD patterns of PZS and POPZS nanotubes; and (f) TGA curves of PZS, KPZS and POPZS nanotubes under nitrogen atmosphere.

XPS offers plentiful information about the surface composition of PZS and POPZS nanotubes, which can further define their structures. Fig. 1b–d show the XPS scan spectra of PZS and POPZS nanotubes. Clearly, the surfaces of pure nanotubes were composed of C, O, P, N, and S elements; moreover, the N, P, and S atoms percentages are 5.28%, 6.01%, and 4.61%, respectively. The mole ratio of N : P : S is in accordance with the theoretical value for PZS with highly cross-linked chemical structure.³⁴ From the XPS survey scans of POPZS (Fig. 1d), it is observed that POPZS shows increased intensity in Si 2s and Si 2p peaks relative to pure PZS. A peak at 157.8 eV, corresponding to the O=S=O groups, is observed in the PZS S 2p spectrum (Fig. 1b). After the functionalization, the POPZS S 2p spectrum (Fig. 1c) exhibits a new peak at 163.0 eV, which is attributed to the S–C bond, confirming the attachment of OMP-POSS on the surface of PZS nanotubes. The XRD patterns for PZS and POPZS nanotubes further proved the abovementioned results. As shown in Fig. 1e, the very broad diffraction peak at 2θ values of around 15.0° corresponds to the reflection peak of pure PZS nanotubes, consistent with a previous study.¹² The only sharp peak at 13° is associated with the trimethylamine hydrochloride crystal, because the trimethylamine hydrochloride as a template is hard to wash out completely. However, after PZS nanotubes are functionalized with OMP-POSS through covalent reaction, the nanotubes are still amorphous and the broad diffraction peak shifts to 2θ = 20.0°, indicating successful modification.

The thermal stability of the PZS and POPZS nanotubes was investigated by TGA under nitrogen atmosphere, as shown in Fig. 1f. The onset degradation temperature (T_d) is defined as the temperature at 5 wt% mass loss. The T_d of bare PZS nanotubes is over 485 °C, and the char yield at 800 °C is approximately 56 wt%, revealing that the PZS nanotubes exhibit superior thermal stability. As for the KPZS nanotubes, a two-stage degradation process is observed. The first stage in the temperature range of 230–456 °C is attributed to the decomposition of labile MPTES compound at low temperature. The second stage is attributed to the decomposition of PZS substrate. However, the starting decomposition temperature of the second stage for KPZS nanotubes as substrate is approximately 460 °C, slightly earlier than that of bare PZS nanotubes, revealing that the surface functionalization results in slight instability of PZS nanotubes. POPZS presents a two-stage degradation behavior similar to KPZS. The first stage is attributed to the degradation of MPTES compounds and the grafted OMP-POSS on the surface of POPZS nanotubes. The char residue at 800 °C for KPZS and POPZS nanotubes under nitrogen is approximately 51 wt% and 54 wt%, respectively, slightly lower than that of pure PZS nanotubes (56 wt%). The results indicate that the grafted organic compound has little effect on the thermal stability of PZS nanotubes.

TEM and FE-SEM were employed to investigate the morphology and microstructure of PZS and POPZS. From the TEM images of PZS (Fig. 2a) and POPZS nanotubes (Fig. 2b), it can be observed that pure PZS and POPZS nanotubes present noodle-like hollow tubular structures. These nanotubes are several micrometers in length and approximately 40–50 nm in the outer diameters, most of the tube terminals closed, as was observed in the prior report.³³ The results of FE-SEM imaging of PZS (Fig. 2c) and POPZS nanotubes (Fig. 2d) reveal that the pure PZS and POPZS with noodle-like fiber shape entangle with each other. The pure PZS nanotubes are relatively smooth and clean without any clear extra phase covering. In contrast, the surfaces of POPZS are relatively rough and coated by a layer of POSS as extra phase (Fig. 2d), and the average diameter increases slightly compared with that of pure PZS. TEM and FE-SEM results demonstrate that MPTES and OMP-POSS are successfully grafted and uniformly distributed on the PZS nanotubes.

Fig. 2 TEM images of the PZS (a) and POPZS nanotubes (b); FE-SEM images of the PZS (c) and POPZS nanotubes (d).

3.2 Thermal properties of EA and POPZS/EA nanocomposites

The TGA curves of the EA and POPZS/EA nanocomposites under nitrogen atmosphere are presented in Fig. 3a and the corresponding data are summarized in Table 1. The initial degradation temperature (T_d) is defined as the temperature wherein the mass loss is 5 wt%. The addition of POPZS nanotubes has little effect on the T_d of EA nanocomposites. The thermal degradation process of POPZS/EA nanocomposites under nitrogen mainly shows a single mass-loss stage in the range of 300–550 °C, corresponding to the degradation of principal EA chains. The T_ds of cured EA films are integrally enhanced after adding the POPZS nanotubes in EA matrix. For example, POPZS/EA-1.0 exhibits the maximum increase of T_d (∼16 °C) compared to the pure EA. The pure EA leaves residual chars of 12 wt% at 800 °C. With the increase of POPZS nanotube loadings from 0.1 to 3.0 wt%, the char yields of EA nanocomposites at 800 °C are significantly improved, increasing from 14 wt% to 22 wt%, indicating increasing thermal stability. The thermal enhancement for EA nanocomposites is due to the fact that the randomly distributed PZS nanotubes connect with each other to form a network structure, and the network structure acts as a physical barrier to effectively cut off the heat and mass transfer between the inner materials and the surroundings, thus slowing the escape of the degradation products.

Fig. 3 (a) TGA curves of EA and POPZS/EA nanocomposites under nitrogen atmosphere; (b) storage modulus (E′) curves of the EA and POPZS/EA nanocomposites as a function of temperature; (c) tan δ curves of the EA and POPZS/EA nanocomposites as a function of temperature; and (d) UV-Vis spectra of the EA and POPZS/EA nanocomposites.

Table 1 TGA data and pencil hardness of the cured EA films

Sample	T0.05 (°C)	TMAX (°C)	Char (800 °C, wt%)	Pencil hardness
EA	349	422	12.0	4H
POPZS/EA-0.1	359	418	14.5	5H
POPZS/EA-0.5	353	418	15.4	5H
POPZS/EA-1.0	365	417	17.8	6H
POPZS/EA-3.0	356	407	21.7	6H

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3.3 Mechanical properties of EA and POPZS/EA nanocomposites

Dynamic mechanical analysis (DMA) has been widely employed for investigating the molecular motions and viscoelastic behavior of polymeric materials. The storage modulus (E′) was used to determine their relevant stiffness, as plotted in Fig. 3b, and loss modulus (E′′) was for damping characteristics for various applications. The ratio E′′/E′ is the loss tangent (tan δ), shown in Fig. 3c. The storage modulus of the cured nanocomposite coatings shows a gradual upward trend with increasing the loading of POPZS in EA matrices. The storage modulus at 30 °C for pure EA film is approximately 1618 MPa. As the content of POPZS nanotubes increases from 0.1 to 3.0 wt%, the storage modulus values at 30 °C of EA nanocomposites are dramatically improved by 2.5%, 23%, 41% and 88%, compared to that of pure EA film. These enhancements in the storage modulus are directly correlated to the high stiffness of POPZS nanotubes and the chemical bonding of the POPZS to the epoxy acrylate network.³⁵ Glass transition temperature (T_g) is determined from the temperature corresponding to the peak of tan δ curves (Fig. 3c). T_g shows an increasing trend similar to storage modulus with an increase of the POPZS content. At the 3 wt% loading of POPZS nanotubes, the T_g value of EA nanocomposite films increases to 138 °C, compared to the relatively low T_g of 122 °C for bare PZS nanotubes. The reason for this significant enhancement is attributed to strong interfacial interactions between POPZS and the EA matrix, and high stiffness of the POPZS nanotubes, reducing the flexibility of materials.

The pencil hardness of POPZS/EA nanocomposites is presented in Table 1. There is an increasing trend of pencil hardness with increasing the loading of POPZS nanotubes into the EA matrix. At 1.0 wt% POPZS loading, the pencil hardness of EA nanocomposites increases to 6H, higher than that of pure EA (4H), indicating the fabulous enhancement of scratch resistance for the EA composite. This result is due probably to the high stiffness of POPZS forming strong interfacial interactions with the EA matrix.

3.4 UV-Vis analysis of UV-cured POPZS/EA nanocomposite films

The effect of incorporated POPZS on the transparency of the resulting EA nanocomposite films was investigated by UV-Vis transmittance spectra (Fig. 3d). The pure EA film showed highly praised transparency with over 80% transmittance in the visible wavelength range of 400–800 nm, whereas the transmittance of the EA film gradually decreases with increasing the loading of POPZS. For instance, the transmittance at 700 nm is approximately 86.0% and 64.8% for the POPZS/EA-0.1 and POPZS/EA-1.0 samples, decreased about 3% and 14.2% compared to pure EA, respectively. Moreover, that of pure EA, the transmittance at 500 nm is 87.7%, 82.5% and 60.3% for the POPZS/EA-0.1 and POPZS/EA-1.0 samples, showing a downtrend of 5.9% and 31.2%. Although the POPZS powder shades and scatters the light and the transparency shows slightly compromised, especially in the high energy part, it still show priority in transparency, which broadens the application of the coatings in some fields.

3.5 Morphology of fractured surface of EA and POPZS/EA nanocomposites

Fig. 4 presents the SEM images of the cross-sections of pure EA (a) and POPZS/EA-3.0 (b). Compared to the smooth fractured surface of neat EA, POPZS/EA-3.0 nanocomposite exhibits a fairly rough fractured surface. In addition, what needs to be noticed is that no clear pulled-out POPZS nanotubes are observed, indicating the formation of stronger interfacial interactions between POPZS nanotubes and the EA matrix.

Fig. 4 (a) FE-SEM micrographs of the fractured sections of neat EA and (b) POPZS/EA-3.0 nanocomposite.

4. Conclusions

The novel OMP-POSS functionalized polyphosphazene nanotubes (POPZS) were fabricated to reinforce an EA matrix successfully. The addition of POPZS significantly enhanced thermal stability and the mechanical properties. The storage modulus of POPZS/EA-3.0 nanocomposite at 30 °C was increased by 88%, and the glass transition temperature of POPZS/EA-3.0 nanocomposite was increased by 16 °C, compared to those of pure EA. These significant enhancements are attributed to the high stiffness of POPZS and the formation of strong interfacial interaction. The EA nanocomposites with low loading of POPZS still retained high transparency. Therefore, the surface grafted functionalization of PZS nanotubes will provide a promising perspective to prepare UV-curable polymer nanocomposite coatings with outstanding comprehensive performance.

Acknowledgements

The study was financially supported by the National Basic Research Program of China (973 Program) (No. 2012CB719701), the National Natural Science Foundation of China (No. 21374111, No. 51203146), the Fundamental Research Funds for the Central Universities (No. WK2320000032) and the Opening Project of State Key Laboratory of Fire Science of USTC (No. HZ2013-KF05).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23309g

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