The effect of UV radiation on polybenzoxazine/epoxy/OG-POSS nanocomposites

Abstract

Degradation against atomic oxygen (AO), ultra-violet (UV) and vacuum ultra-violet (VUV) radiation has to be controlled in order to maintain the longevity and performance of light weight and high strength nanocomposites used in space and nuclear applications. With this in mind, a new synthetic route has been used to develop a semi-organic precursor, [polyhedral oligomeric silsesquioxane (POSS)] reinforced polybenzoxazine/epoxy (PBZ/EP) nanocomposites. PBZ/EP/POSS nanocomposites have been developed by reinforcing varying weight percentages of (0, 1.0, 3.0, and 5.0 wt%) POSS into 1 : 1 (w/w) ratio of BZ and DGEBA epoxy (EP) matrix via thermal curing. The developed nanocomposites were tested under UV irradiation at the wavelength of 365 nm for a period of one week. The values of tensile strength and morphological behaviour before and after exposure to UV irradiation have been determined in order to assess their radiation resistant behaviour. It was ascertained that the data obtained from the value of tensile strength for 5 wt% POSS reinforced PBZ/EP has changed only to an insignificant extent when compared to that of before UV irradiation. From SEM and XPS analysis, it was observed that a passive inert silica protective layer has been formed after radiation, which protects the composite materials from further deterioration due to radiation. Data from thermal and dielectric studies indicate that the POSS incorporated system possesses better thermal and low dielectric properties than those of the neat PBZ/EP matrix.

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Introduction

Most satellites are being launched into low earth orbit (LEO) altitudes and positioned in the range between 200 km and 1000 km. LEO space environments present many obstacles for a successful spacecraft mission. The prevailing aggressive environment viz. atomic oxygen (AO), ultraviolet (UV) radiation, ionizing radiation, vacuum-ultraviolet (VUV), thermal cycles, micrometeoroids and orbital debris cause damage to satellite materials made from polymers.^1–7 Due to separate, combined or synergistic interactions with these space hazards, polymeric materials suffer a relatively rapid erosion, structural modification and surface roughening. This might lead to irreversible degradation of optical, thermal, electrical and mechanical properties.^8–12

A promising approach towards the production of LEO survivable polymers is to incorporate a hybrid polyhedral oligomeric silsesquioxanes (POSS) into the polymeric chains through covalent bonding. The introduction of POSS cage structures with various functionalities has recently made much attraction among material scientists. Polymer surfaces coated with silica have been shown to exhibit good radiation resistance; however, inherent or debris-induced defects in the coating will permit UV penetration and undercut the underlying polymer substrate. Hence, development of light weight polymeric materials which can sustain such erosion and still function under the harsh conditions of the LEO environment is essentially warranted. An attempt has been made in the present study to develop such materials for application in space environment for improved performance and longevity in the form of coatings and composites.^13–20

Polybenzoxazines are a class of thermosetting phenolic resins that have recently been developed to use in the electronics, aerospace, matrices for composite materials, high performance adhesives, nanoimprinting technologies and other industries as an attractive and alternative to other novolac-type phenolic resins. Although the high rigidity of a PBZ is a promising behaviour, reduction in rigidity may also be desired for certain applications.^21–26 Copolymerization or chemical blending is considered to be a potentially effective method to toughen polybenzoxazine.^27–29 In this regard, epoxy used as a potential modifier for PBZ matrix to increase the toughening behaviour of PBZ.

When compared with other conventional resinous systems, epoxy is expected possess high radiation resistance due to the presence of stable aromatic molecular structure which imparts inherent protection to the cured polymer matrices and composites.³⁰ When exposed to UV radiation, the excitation energy can be transferred either from one molecule to another or from aliphatic to aromatic part of the molecule as observed in organic compounds.³¹ Therefore, a protective behaviour of aromatic rings also contributes to the neighbouring aliphatic linkage of epoxy skeleton. This may be the reason for an increase of radiation resistance of epoxy when copolymerised with polybenzoxazine.

In the present work, POSS reinforced PBZ/EP copolymer based nanocomposites has been developed and their radiation resisting behaviour was studied against UV radiation.

Experimental

Materials

Bisphenol-A, aniline, paraformaldehyde, tetramethylammonium hydroxide (Me₄NOH) and methanol were received from SRL (India) Ltd. DGEBA epoxy (LY556) was received from Ciba-Geigy Ltd. Tetraethoxysilane (TEOS), chlorodimethylsilane was obtained from Sigma-Aldrich, India. Bisphenol-A/aniline based benzoxazine (BA-a) was synthesized as per reported procedure (yield 98%).²¹ Octakis(dimethylsiloxypropylglycidylether) silsisquioxane (OG-POSS) was synthesized as per the reported procedure (yield 85%).³²

Synthesis of PBZ/EP/POSS

20 g of BA-a and 20 g of DGEBA epoxy was taken and dissolved in 20 mL of THF. To the resulting product varying weight percentages of OG-POSS (0, 0.2 g (1%), 0.6 g (3%) and 1.0 g (5%)) was added and stirred for 12 h at 30 °C. After 12 h, the solution was cast on a smooth glass substrate and thermally treated at 50 °C for 1 day, and at 80, 100, 120, 140, 160, 180, 200 for 1 h each and 220 °C for 2 h. The thin and light brown coloured PBZ/EP/POSS films formed, were stripped from the glass substrates and then utilized for further characterization (Scheme 1).

Scheme 1 Schematic representation of PBZ/EP/POSS nanocomposites.

Characterisation

FT-IR spectrum was recorded on a Perkin Elmer 6X FT-IR spectrometer, India. About 100 mg of optical grade KBr was ground with sufficient quantity of solid sample to make 1 wt% mixture to prepare KBr pellets. After the sample was loaded, a minimum of 16 scans were collected for each sample at a resolution of ±4 cm⁻¹.

Wide-angle X-ray spectra were obtained using a Rich Seifert (Model 3000) diffractometer (with Cu Kα radiation (θ = 0.15418 nm)) for the ground powder of cured composites. The spectral window ranged from 2θ = 0° to 70°. A JEOL JEM-3010 analytical transmission electron microscope, operating at 300 kV with a measured point-to-point resolution of 0.23 nm, and the samples were prepared by dissolving the composites in DMF mounted on carbon-coated Cu TEM grids and dried for 24 h at room temperature (RT) to form a film and characterise the morphology of the nanocomposites. JEOL JSM Model 6360 scanning electron microscopy is used to find the surface morphology for before and after UV exposure of the nanocomposites. The surfaces of the specimens were coated with gold and were exposed to an accelerating voltage of 20 kV.

The radiation resistant behaviour was studied using HML compact-LC-MC-812, Heber multi lamp (12 nos) type photo reactor at room temperature. The power was 220/230 V, AC = 50 Hz and wavelength 365 nm.

Tensile strength was studied as per ASTM-D3039 using Instron testing machine (Model 6025, UK), at 10 mm per minute cross-head speed, using specimen with a width of 25 mm, length of 150 mm and thickness of 2 mm. A distance of 100 mm was kept in between the grips. Five specimens were tested for each sample.

The XPS was measured using an Omicron nanotechnology instrument at a pressure below 10⁻¹⁰ Torr. The sample was mounted on sample stubs. The wide scan, C 1s, O 1s and Si 2s, 2p core level spectra were recorded with a monochromatic Al-Kα radiation (photon energy = 1486.6 eV) at a pass energy of 15 V and electron take-off angle of 608°.

Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409 Thermogravimetric analyzer, USA. The samples (about 10 mg) were heated from ambient temperature to 800 °C under a continuous flow of nitrogen (20 mL min⁻¹), at 10 °C min⁻¹. The differential scanning calorimetric analysis (DSC) was performed on a Netzsch DSC-200, USA. Measurements were performed under a continuous flow of nitrogen (20 mL min⁻¹). All the samples (about 10 mg in weight) were heated from ambient temperature to 300 °C and the thermograms were recorded at a heating rate of 10 °C min⁻¹.

Dielectric constant was determined by impedance analyzer (Solartron impedance/gain phase analyzer 1260) using a platinum (Pt) electrode at 30 °C at a frequency range of 100 Hz to 1 MHz. This experiment was repeated four times under the same conditions.

Results and discussion

Radiation resistant behaviour of the POSS reinforced materials has improved because of the absorbed UV doze is distributed non-uniformly in the environment. The improvement of radiation resistant behaviour of the composite materials also influenced by the nature and percentage weight of POSS reinforcement used. The POSS reinforcement has significant effect due to the presence of cubic cage structures which impart stability to the PBZ/EP composite structure against radiation.

The, PBZ/EP and PBZ/EP/POSS nanocomposites (Scheme 1) were prepared by the reaction of BA-a, DGEBA epoxy and OG-POSS via thermal polymerisation and their molecular structures were confirmed by FT-IR spectra. Fig. 1 shows the FT-IR spectrum for OG-POSS. The appearance of peak at 925 cm⁻¹ represents the presence of glycidyl group and also the appearance of peak at 1106 cm⁻¹ confirms the presence of Si–O–Si linkage in the OG-POSS further the appearance of peak at 2934 and 2860 cm⁻¹ for symmetric and asymmetric aliphatic CH₂ groups. Fig. 2 represents the FT-IR spectra of neat, 1.0, 3.0 and 5.0 wt% OG-POSS reinforced PBZ/EP nanocomposites. From the figure the disappearance of peaks at 920–940 cm⁻¹ (for glycidyl groups present in DGEBA epoxy and OG-POSS), 1520 cm⁻¹ for trisubstituted benzene ring and 947 cm⁻¹ for oxazine ring, and the appearance of peak at 1505 cm⁻¹ represents the tetra-substituted benzene ring, 2964 cm⁻¹ and 2870 cm⁻¹ for symmetric and asymmetric aliphatic CH₂ groups, indicating that the DGEBA undergoes a complete curing reaction with BA-a to form an inter cross linked network structure. Further, the appearance of the peak at 1106 cm⁻¹ represents the Si–O–Si, which confirms the presence of covalently bonded POSS in the resulting nanocomposites.³² Fig. 3(a)–(d) represents the optical images of PBZ/EP, 1.0, 3.0 and 5.0 wt% PBZ/EP/POSS respectively. All the hybrid composite materials (PBZ/EP/POSS) showed a homogeneous and transparent morphology, this implies that the organic and inorganic constituents were completely dispersed at nano level and it was further confirmed from XRD and TEM analysis.

Fig. 1 FT-IR spectrum for OG-POSS.

Fig. 2 FT-IR spectra for neat PBZ/EP and PBZ/EP/POSS nanocomposites.

Fig. 3 Optical images of (a) PBZ/EP, (b) 1.0, (c) 3.0 and (d) 5.0 wt% of PBZ/EP/POSS.

XRD is used to characterize the organic–inorganic hybrid network structure of neat PBZ/EP, 1.0, 3.0 and 5.0 wt% OG-POSS reinforced PBZ/EP/POSS nanocomposites. The X-ray diffraction patterns for the neat PBZ/EP and PBZ/EP/POSS systems are shown in Fig. 4. From the figure, the XRD patterns of the PBZ/EP/POSS nanocomposites exhibit the broad amorphous peak at 2θ = 18.5°, and implies that the OG-POSS molecules are homogeneously dispersed in PBZ/EP networks through covalent bonding. The uniform and homogeneous dispersion was further confirmed by TEM analysis.

Fig. 4 XRD pattern for neat PBZ/EP and PBZ/EP/POSS nanocomposites.

Fig. 5 shows the TEM micrograph of 1.0, 3.0 and 5.0 wt% PBZ/EP/POSS nanocomposites. It can be seen that the POSS cores are well dispersed in the PBZ/EP matrix, but a number of dark spots appear at approximately 40 nm and even smaller. On the basis of their size, such dark spots are associated with POSS moieties. Thus, the associated POSS moieties present in the composite systems are evenly dispersed at a nanometer scale in the polybenzoxazine/epoxy network. No localized domains and no phase segregation were observed even at nanometer scale, implying that POSS cages were well dispersed in the matrix system and no POSS aggregation has occurred. The uniform and homogeneous morphology has a significant role in enhancing the ultimate properties of the resulting composite systems.

Fig. 5 TEM images of (a) 1.0, (b) 3.0 and (c) 5.0 wt% of PBZ/EP/POSS.

Effect of UV on tensile strength

The neat PBZ/EP, and 1.0, 3.0, and 5.0 wt% OG-POSS reinforced nanocomposites were subjected to ultra-violet radiation for a period of one week (168 h) at room temperature. The samples were placed in the multi lamp photo reactor and 365 nm UV lamp was used as a source for UV radiation. In order to evaluate the influencing effect of POSS on resistance to UV radiation, the values of tensile strength of 1.0, 3.0 and 5.0 wt% of POSS reinforced PBZ/EP nanocomposites were investigated (Table 1). The objective of assessing the values of tensile strength is that the physical performances such as mechanical property (i.e., tensile strength) can indicate the sustainability of the material against the radiation. The values of tensile strength of varying weight percentages of POSS reinforced composite system after treatment towards UV radiation was compared with before UV treatment and are presented in Table 1. The values of tensile strength of neat PBZ/EP and 5.0 wt% PBZ/EP/POSS are 74, 96 MPa and 56, 92 MPa respectively for before and after irradiation of UV. The values of tensile strength are increased with increase in percentage weight content of POSS into PBZ/EP matrix. From the table it is observed that after exposure of UV radiation the values of tensile strength of neat PBZ/EP system is decreased to about 24%, whereas in the case of POSS reinforced system the reduction in the value was only about 4%. This may be explained due to the presence of POSS molecules in the polymer systems which surface may be decompose initially under UV radiation exposure and subsequently forming an inert passive silica layer on the surface of the composite materials, which in turn protects the surface of hybrid composites from further attack of radiations. The inert silica passive layer formed on the surfaces of composites could act as a barrier to prevent the damage of surface and retain the properties of the hybrid composites.¹⁶

Table 1 Tensile strength properties of neat PBZ/EP and PBZ/EP/POSS nanocomposites

Sample	Tensile strength (MPa)
	Before UV radiation	After UV radiation
PBZ/EP	74 ± 3	56 ± 3
1.0 wt% PBZ/EP/POSS	83 ± 3	68 ± 3
3.0 wt% PBZ/EP/POSS	88 ± 3	74 ± 3
5.0 wt% PBZ/EP/POSS	96 ± 3	92 ± 3

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Effect of UV on SEM morphology

The surface morphology of neat PBZ/EP system and the PBZ/EP reinforced with glycidyl-functionalized POSS (OG-POSS) before and after exposure to UV radiation for a period of one week are compared in Fig. 6. From SEM images it was observed that the incorporation of POSS has improved the resistance to UV radiation. The improvement of UV radiation resistance may be explained due to the formation of the inert silica layer on the surface of the composite systems.

Fig. 6 SEM images for before UV radiation of (a) neat PBZ/EP, (b) 1.0 wt% and (c) 5.0 wt% of PBZ/EP/POSS and after UV radiation of (d) neat PBZ/EP, (e) 1.0 wt% and (f) 5.0 wt% of PBZ/EP/POSS.

In order to investigate the influencing mechanism of POSS on the improvement of radiation resistance, the SEM micrograph of neat PBZ/EP matrix and POSS reinforced composite systems were used. Fig. 6(a)–(f) represent the SEM images obtained before and after UV radiation PBZ/EP/POSS (0, 1.0 and 5.0 wt%) respectively. From the SEM, Fig. 6(a)–(c), it could be visualized that no inert layer was formed on the surfaces of the neat PBZ/EP and POSS incorporated PBZ/EP systems. After UV exposure there was no inert layer formed on the neat system (Fig. 6(d)) whereas in the case of POSS reinforced composite systems, an inert layers were formed on the surfaces of the composites and are shown in Fig. 6(e) and (f). From the SEM images it was observed that after UV radiation the POSS incorporated system has acquired a passive layer on the surface. The formation of inert silica layer was very stable and act as protecting layer against radiation. It could prevent and avoid the further erosion of surface of the system by high energy radiation particles, such as UV radiation and atomic oxygen. As a result, after irradiation the properties of composite materials are retained due to the incorporation of POSS into the matrix systems.

XPS analyses were carried out in order to obtain chemical information about the UV exposed PBZ/EP surfaces and are presented in Fig. 7. The surface composition of PBZ/EP/POSS hybrid films before and after UV exposure was investigated by XPS, the result is shown in Table 2. The O-to-Si atomic ratio is 2.015 for the sample before UV exposure; it is increased to 2.462 after the 168 h UV exposure. This behaviour indicates that complex chemical reactions occur during UV exposure. These changes in the O-to-Si atomic ratio resulting from exposure to the UV indicate the formation of SiO₂. The surface carbon concentration decreases from 56.948 at% for the pristine sample to 53.866 at% after 168 hours exposure. XPS spectra of Si 2p peaks obtained from the 5 wt% of POSS incorporated PBZ/EP surface before and after UV exposure are shown in Fig. 7(c). The Si 2p peak shifted form a binding energy of 102.4 eV, corresponding approximately to SiO_1.5, for the unexposed 5 wt% PBZ/EP/POSS hybrid film to a binding energy of nearly 101.7 eV, corresponding approximately to SiO₂,¹⁵ after this sample was exposed to 168 h of UV lamp. This binding energy shift indicates oxidation of the silicon into a passivation layer in the form of SiO₂.

Fig. 7 XPS for (a) 5.0 wt% of PBZ/EP/POSS before UV radiation (b) 5.0 wt% of PBZ/EP/POSS after UV radiation (c) Si 2p binding energy for 5.0 wt% of PBZ/EP/POSS before and after UV radiation.

Table 2 Elemental composition for 5.0 wt% PBZ/EP/POSS nanocomposites from XPS analysis

Atoms	Elemental composition (at%)
	Before UV radiation	After UV radiation
Si	15.774	20.306
Si/O	2.015	2.462
C	56.948	53.866
N	0.562	0.467
O	24.701	22.899

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Thermal properties

Fig. 8 depicts the TGA thermogram of PBZ/EP/POSS nanocomposites in a nitrogen atmosphere. By incorporating only a small amount of POSS cores, such as 1.0, 3.0 and 5.0 wt%, into the nanocomposite, there is no significant improvement was observed in the value of 5% and 10% mass loss temperature (T_d). Whereas the char yield (at 800 °C) of PBZ/EP/POSS nanocomposites is also obviously increased from 21 wt% in the neat PBZ/EP to 34.2 wt% in the case of 5 wt% of POSS reinforced nanocomposites. High char yield value is a direct indication of resistance to combustion. The improved char yield of PBZ/EP/POSS nanocomposites is due to the combined effects of high content of silica and aromatic moieties increased the cross-link density and retards the diffusion of gaseous fragments produced during the decomposition due to the closely packed POSS cores in the composite system.³²

Fig. 8 TGA thermogram for neat PBZ/EP and PBZ/EP/POSS nanocomposites.

The values of glass transition temperature (T_g) of neat PBZ/EP and OG-POSS incorporated PBZ/EP/POSS nanocomposites are presented in Fig. 9 and Table 3. The value of T_g of OG-POSS reinforced PBZ/EP nanocomposites are increased with increase in weight percentage of OG-POSS content. In the case of POSS reinforced polymer nanocomposites, an increase in T_g's interpreted on the basis of the nano reinforcement of POSS cages, which could restrict the motions of macromolecular chains. In this case, an enhancement of the value of T_g is not only ascribed to the nano reinforcement of POSS cages but also attributed to the additional crosslinking of the nano-composites resulting from the inter component reaction between OG-POSS and PBZ/EP.

Fig. 9 DSC thermogram for neat PBZ/EP and PBZ/EP/POSS nanocomposites.

Table 3 Thermal and dielectric properties for neat PBZ/EP and PBZ/EP/POSS nanocomposites

Sample	Tg (°C)	Td 5% (°C)	Td 10% (°C)	Char yield at 800 °C (%)	Dielectric constant (K)
PBZ/EP	171	264	327	21.0	4.72
1.0 wt% PBZ/EP/POSS	178	265	330	25.5	4.01
3.0 wt% PBZ/EP/POSS	183	267	331	29.8	3.28
5.0 wt% PBZ/EP/POSS	192	267	333	34.2	2.54

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Dielectric properties

The development of low k dielectric materials is one of the most desirable requirements for next-generation microelectronic devices. Development of new low-k materials can be obtained by introducing porosity or increasing of the free volume in the material system. Table 3 and Fig. 10 show the values of dielectric constant of neat PBZ/EP matrix and POSS reinforced PBZ/EP nanocomposites obtained from the impedance analysis. From the data it was observed that the higher percentage reinforcement of POSS (5.0 wt%) into PBZ/EP nanocomposites showed lower values of dielectric constant (k = 2.54) when compared to that of neat PBZ/EP (k = 4.72). POSS incorporated PBZ/EP system contributes to reduction in the value of dielectric constant with an increase in POSS, due to creation of porous voids by rigid and cage POSS structure. Further the POSS based nanocomposites can be used as better insulating material for wide range of industrial and engineering applications with improved performance and longevity.

Fig. 10 Dielectric constant for neat PBZ/EP and PBZ/EP/POSS nanocomposites.

Conclusion

Radiation resistant material based on polybenzoxazine as organic matrix and POSS as nano reinforcement has been developed with varying weight percentages of (0, 1.0, 3.0, and 5.0 wt %) POSS and PBZ/EP matrix and were characterized using different analytical techniques. The UV radiation resistant behaviour of neat PBZ/EP and POSS incorporated systems were investigated successfully, and correlated the data of tensile strength and SEM morphology obtained before and after exposure of UV radiation for a period of one week. From the values of tensile strength, it is noted that the 5.0 wt % of POSS incorporated PBZ/EP nanocomposites were retained tensile strength behaviour without appreciable change after exposure of UV radiation. The formation of inert silica passive layer was confirmed by SEM morphology. Data from thermal and dielectric studies infers that on increase in weight percentages of POSS into PBZ/EP system increases the thermal stability and decreases the dielectric constant of the resulting nanocomposites.

Acknowledgements

The authors thank BRNS, G. no: 2012/37C/9/BRNS, Mumbai, Govt. of India, for the financial support. The authors also thank Centre for Nanoscience and Technology, Anna University for providing Impedance analysis and PSG college of Technology, Coimbatore for TEM analysis facility.

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