In situ sol–gel synthesis of silica reinforced polybenzoxazine hybrid materials with low surface free energy

Abstract

In the present work, silica reinforced polybenzoxazine (PBZ–SiO₂) hybrid materials possessing low surface free energy have been developed using a dimethylol-functional benzoxazine monomer (4HBA-BZ), tetraethoxysilane (TEOS) and 3-(isocyanatopropyl)triethoxysilane (ICPTS) through an in situ sol–gel process. Data from the contact angle measurement indicate that the hybrid materials are hydrophobic in nature and possess a low surface free energy. For example, a PBS₃ hybrid material (1 : 1 : 1 ratio of 4HBA-BZ : ICPTS : TEOS) exhibits a low surface free energy of 18.6 mJ m⁻² which is lower than that of poly(tetrafluoroethylene) (22.0 mJ m⁻²). Further data obtained from thermal studies indicate that the hybrid PBZ possesses higher values of the glass transition temperature (T_g), thermal stability and char yield than those of neat PBZ.

---

Introduction

Hydrophobicity is a highly desirable property for materials that are used in various industrial applications such as anti-wetting, anti-snow (or ice) adherence, anti-rusting, reduced frictional resistance etc.^1–3 Low surface energy polymeric materials have attracted great interest due to their diverse applications. Most of the low surface energy polymeric materials that have been developed are based on fluorine- or silicon-containing polymers. For example, poly(tetrafluoroethylene) (Teflon) and poly(dimethylsiloxane) are two well-known examples of polymeric materials possessing low surface free energy.^4–7 Teflon may be regarded as a typical standard lower surface energy material, displaying water repellency in combination with other desirable properties.⁸ The use of Teflon and fluorinated polymers is limited due to low oil repellency, poor processability and high cost.⁹ Much effort has been exerted in the search for low-cost polymeric materials exhibiting characteristics such as low surface free energy, good film-forming and better processability.^9–11

Furthermore, organo-siloxanes are also used to improve adhesion of a substrate and serve as an anchor using hydrogen and covalent bonds. At the same time, a siloxane may also greatly improve the hydrophobicity of a film surface.¹² The incorporation of an inorganic phase into an organic polymer matrix may be an effective approach to improve specific properties of organic polymers.¹³ Organic–inorganic hybrids that combine the advantages of both polymer and ceramic materials such as enhanced mechanical strength, thermal stability and improved processability have been synthesized through different routes. Among these, the in situ polymerization of metal alkoxide precursors, in the presence of polymer matrices via the sol–gel method is the most valuable and attractive.^14,15 The sol–gel route is a convenient method for the synthesis of hybrid materials consisting of organic polymers and inorganic compounds as it achieves a good dispersion of the inorganic compound and increases interfacial adhesion between the polymers.¹⁶ Intra- and intermolecular interactions are important for the surface properties of polymers. It is clear that enhanced intramolecular hydrogen bonding leads to a decrease in the surface free energy and increases the hydrophobic nature, whereas increasing the fraction of intermolecular hydrogen bonding leads to the opposite effect. These observations are in good agreement with the results of previous studies of the surface free energy effects.^10,17–19

Polybenzoxazines (PBZs) have been recently developed as a class of non-fluorine polymeric materials with strong intramolecular hydrogen bonds that result in extremely low surface free energy, even lower than that of Teflon. The low surface free energy of polybenzoxazine films was primarily caused by the transformation of intermolecular hydrogen bonds to intramolecular hydrogen bonds in the polybenzoxazine systems.^9,19–25

The distribution of hydrogen-bonded species in the polybenzoxazine is affected greatly by the amino groups in the Mannich bridges. For example, the network structure of the polybenzoxazine consists mainly of O–H⋯N intramolecular hydrogen bonds (proton-transfer equilibrium). Consequently, the surface free energy of the polybenzoxazine is low. This phenomenon can be explained by considering the fact that the strength of a hydrogen bond depends on the electronegativity of the group attached to the nitrogen atom. The polybenzoxazines have a low electron density because the electrons of the nitrogen atom are more delocalized. Therefore the polybenzoxazines display strong intramolecular hydrogen bonding, which results in relatively low surface free energies.^9,10,18

In the present work, PBZ–SiO₂ hybrid materials have been developed through an in situ sol–gel method and it was found that the PBZ–SiO₂ exhibited a lower surface free energy and a higher contact angle when compared to those of conventional fluoro-polymers, i.e. Teflon. Data from AFM and TEM analysis confirm the molecular level dispersion of silica in the polybenzoxazine matrix. In addition differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy have been used to study the chemical structure, thermal properties, and specific interactions that exist within the PBZ–SiO₂ hybrid materials. Data from thermal studies indicate an appreciable improvement in thermal behaviour.

Materials

4-Hydroxybenzyl alcohol (4HBA) (98%) was purchased from Spectrochem, India. 4,4′-Diaminodiphenylmethane (DDM) was obtained from Alfa Aesar. Xylene was purchased from RFCL limited, India. 3-(Isocyanatopropyl) triethoxysilane (ICPTS) was obtained from Fisher scientific, India. Tetraethoxysilane (TEOS) was obtained from Sigma-Aldrich chemical company, USA. Paraformaldehyde and N,N-dimethylformamide (DMF) were purchased from SRL, India.

Experimental

Synthesis of dimethylol-functional benzoxazine monomer (4HBA-BZ)

The benzoxazine monomer was synthesized through the Mannich reaction (Scheme 1) using 4-hydroxybenzyl alcohol (4HBA) (6.26 g, 0.05 mol), 4,4′-diaminodiphenylmethane (DDM) (5 g, 0.025mol) and paraformaldehyde (3.02 g, 0.10 mol) in xylene (100 mL) and the mixture was heated at 140 °C for 5 h. The reaction mixture was allowed to cool to room temperature. A mixture of hexane and methanol (1 : 1) was added to the reaction mixture and then stirred for 12 h. The yellow product obtained was then recrystallized from methanol, (Yield: 85%).

Scheme 1 Synthesis of 4HBA-BZ.

IR spectra (KBr, cm⁻¹): 1503 (tri-substituted benzene ring), 1278 (asymmetric stretching of C–O–C), 947 (oxazine ring). (Fig. 1)

Fig. 1 FT-IR spectra of (a) 4HBA-BZ and (b) PBZ.

¹H NMR spectra (DMSO-d₆, frequency: 500 MHz, ppm): 6.6–7.1 (14H, Ar protons), 5.3 (s, 4H, N–CH₂–O), 4.6 (s, 4H N–CH₂-Ar), 4.3 (s, 4H, –Ar–CH₂–O), 5.0 (2H, –OH) and 3.7 (Ar–CH₂–Ar). (Fig. S1 present in the ESI.†)

¹³C NMR spectra (DMSO-d₆, ppm): 152–118 (Ar carbon), 79.6 (N–CH₂–O), 61.4 (N–CH₂–Ar), and 45.3 (Ar–CH₂–OH). (Fig. S2 present in the ESI.†)

Preparation of the polybenzoxazine–silica hybrid (PBS)

To synthesize the PBZ–SiO₂ hybrid materials (Scheme 2), a series of procedures have been adopted. Initially the hybrid was prepared using 4HBA-BZ monomer, 3-(isocyanatopropyl)triethoxysilane (ICPTS) and TEOS in DMF as a solvent, in the presence of 0.1 N hydrochloric acid. The solvent was evaporated followed by curing, which resulted in the formation of a red brown transparent PBZ–SiO₂ hybrid film. The 4HBA-BZ monomer was dissolved in DMF and the solution was poured into a silane treated glass plate and cured at 100, 150, 200 °C for 60 min and 250 °C for 120 min to get a transparent polybenzoxazine (PBZ) film. Hybrid samples PBS_1–3 were prepared using 1 : 0 : 1, 1 : 1 : 0 and 1 : 1 : 1 (w/m/w) of 4HBA-BZ monomer, ICPTS and TEOS respectively. The TEOS was dissolved in DMF and an exact equivalent of the dilute acid was added with continuous stirring for 15 min. 4HBA-BZ and ICPTS were dissolved in DMF, stirred for 30 min and were mixed well to produce a clear solution which was then stirred for 60 min at room temperature. Finally, the solution was transferred into a silane treated glass plate and cured at 100 °C for 60 min and then post cured at 250 °C for 120 min. The PBZ–SiO₂ (PBS_1–3) hybrids were obtained as red brown transparent products. The curing cycle time of the hybrid materials is presented in Table 1.

Scheme 2 Schematic representation of PBZ–SiO₂ hybrid materials.

Table 1 Preparation of neat PBZ and PBZ–SiO₂ hybrid materials

Sample code	4HBA-BZ (g)	ICPTS (g)	TEOS (g)	Ratio (BZ : ICPTS : TEOS) (w/m/w)	DMF (mL)	0.1 N HClaq (mL)	Curing cycle temp (°C)/min
PBZ	0.1	—	—	1 : 0 : 0	5	—	100 + 150 + 200 °C/60 min, 250 °C/120 min
PBS1	0.1	—	0.1	1 : 0 : 1	5	0.035	100 + 150 + 200 °C/60 min, 250 °C/120 min
PBS2	0.1	0.1	—	1 : 1 : 0	5	0.022	100 + 150 + 200 °C/60 min, 250 °C/120 min
PBS3	0.1	0.1	0.1	1 : 1 : 1	5	0.057	100 + 150 + 200 °C/60 min, 250 °C/120 min

---

Characterization

FT-IR spectra were recorded on a Perkin Elmer 6X spectrometer. About 500 mg of optical-grade KBr was ground in a mortar with a pestle, and an amount of solid sample was ground with the KBr to make a 1 wt% mixture to make KBr pellets. After the sample was loaded, a minimum of 16 scans were collected for each sample at a resolution of ±4 cm⁻¹. ¹H NMR and ¹³C NMR spectra were recorded in DMSO-d₆ on a JEOL GSX 500 MHz spectrometer. ¹H NMR spectra were recorded at 500 MHz using an 8000 Hz spectral width, a relaxation delay of 3.5 s, a pulse width of 45, 32 K data points and DMSO-d₆ as the internal reference. ¹³C NMR spectra were obtained at 100.6 MHz using a 25 000 Hz spectral width, a relaxation delay of 1.68 s, a pulse width of 45, 32 K data points and DMSO-d₆ as the internal reference and TMS as the external reference.

Water contact angle measurements were obtained using a Rame-hart Inc. goniometer; 5 μL of deionized water and 5 μL of diiodomethane were used to measure the contact angle of the surface.

The surface morphology of the samples was examined with atomic force microscopy (AFM), using a AFM Park Instrument, XE-70. The mode used was non-contact with set points ranging from 5–15 nm. The images were acquired by non-contact mode using a Si–C cantilever. All the images were recorded under air atmosphere at room temperature. Scanning electron microscopy (SEM) was used to record the SEM images and the samples were prepared by coating gold on their surface and were recorded using a JEOL JSM-6360 instrument. Transmission electron microscopy (TEM), operating at 300 kV with a measured point-to-point resolution of 0.23 nm was used, using an analytical JEOL JEM-3010 instrument.

Thermogravimetric analysis (TGA) was performed using a Netzsch STA 409 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 under a continuous flow of nitrogen (20 mL min⁻¹) at a heating rate of 10 °C min⁻¹.

Results and discussion

The benzoxazine monomer was synthesized through the Mannich condensation reaction using 4-hydroxybenzyl alcohol (4HBA), 4,4′-diaminodiphenylmethane (DDM) and paraformaldehyde in the presence of xylene with the modified procedure reported by Ishida et al.²⁶ The structure of the monomer was confirmed by FT-IR (Fig. 1), ¹H NMR (Fig. S1†) and ¹³C NMR (Fig. S2†) spectra.

The FT-IR spectra of 4HBA-BZ and PBZ are shown in Fig. 1. From Fig. 1a, the 4HBA-BZ monomer was characterized by the peaks that appeared at 1278 cm⁻¹ for the aromatic ether (C–O–C) stretch, 947 cm⁻¹ for the oxazine ring and the peak that appeared at 1503 cm⁻¹ represents the tri-substituted benzene ring. After the thermal curing of the 4HBA-BZ monomer, the ring opening polymerization was further confirmed by FT-IR. From Fig. 1b, the peak at 1503 cm⁻¹ from the 4HBA-BZ monomer shifted to 1496 cm⁻¹ which demonstrates the conversion of the tri-substituted to tetra-substituted aromatic ring and the disappearance of bands at 947 cm⁻¹ further confirms the ring opening polymerization of benzoxazine and the formation of polybenzoxazine.²⁶

The PBZ–SiO₂ hybrid films were prepared through an in situ sol–gel method. The composition of the monomer with SiO₂ is presented in Table 1. Fig. 2 represents the FT-IR spectra of PBS_1–3. In the case of hybrid materials PBS_1-3, the absorption peak appeared at 1104 cm⁻¹ which confirms the presence of the –Si–O–Si– linkage in the PBZ–SiO₂ hybrid. This indicates that the formation of the Si–O–Si network through the polycondensation of the Si–OH groups within the polybenzoxazine matrix. The formation of the Si–O–Si network is also indicated by the C=O stretching of the non-hydrogen bonded carbonyl group of ICPTS at 1733 cm⁻¹ and the C–N stretching combined with N–H out-of-plane bending at 1540 cm⁻¹. Furthermore, a peak appeared at 1175 cm⁻¹ which confirms the presence of the Si–O–C linkage in the resulting system.

Fig. 2 FT-IR spectra of PBZ–SiO₂ hybrid materials.

The hydrophobicity of neat PBZ and PBZ–SiO₂ hybrid films was determined from the contact angle measurements using a goniometer (each sample was repeated 5 times). The surface contact angles were measured with 5 μl of water and diiodomethane as the probe liquids, the results are presented in Table 2. The values of the contact angle for neat PBZ, PBS_1–3 in water are 90.1, 98.5, 102.3 and 110.5°, respectively, and diiodomethane (DIM) are 63.5, 69.6, 71.1 and 78.3, respectively. The values of the contact angle for water and DIM against the surface of PBZ–SiO₂ are significantly increased when compared to that of neat PBZ (Table 2). Among the three systems, the PBS₁ system was prepared using 4HBA-BZ and TEOS through hydrogen bonding interactions to form the silica network, the PBS₂ system was prepared using 4HBA-BZ and ICPTS through chemical interaction to form a polysilsesquioxane network and the PBS₃ system was developed using 4HBA-BZ, ICPTS and TEOS to produce a perfect silica hybrid material. Among the systems mentioned above, the PBS₃ system possesses the lowest value of the surface free energy and the highest value of contact angle. The increase in hydrophobicity is mainly attributed to the difference in both the chemical composition of the hybrid composite material and its surface morphology. The increase in the value of contact angle of water indicates the increase in the hydrophobic nature of the hybrid material. This is due to the less polar nature of Si–O–Si linkages in the network system which reduces the value of surface free energy and increases the hydrophobic nature of the resulting hybrid systems.²⁷

Table 2 Contact angle and surface properties of neat PBZ and PBZ–SiO₂ materials

Sample code	Contact angle (θ)		Surface free energy
	Water	Diiodomethane	γ^d	γ^p	Γ
PBZ	90.1	63.5	26.6	3.0	29.5
PBS1	98.5	69.6	23.1	1.4	24.5
PBS2	102.3	71.1	22.3	0.9	23.1
PBS3	110.5	78.3	18.4	0.3	18.6

---

The surface free energy (γ_s) of the neat PBZ and the PBZ–SiO₂ hybrid was studied using contact angle measurements. The surface free energy of the organic–inorganic hybrid materials were calculated according to the geometric mean model.^27,28

cos θ = 2/γ_L[(γ^d_Lγ^d_s)^1/2 + (γ^p_Lγ^p_s)^1/2]⁻¹ (1)

γ_s = γ^d_s + γ^p_s (2)

where θ is the contact angle and γ_L is the liquid surface tension; and γ^d_s and γ^p_s are the dispersive and polar components of γ_L, respectively. As shown in Table 2, the values of surface free energy of the neat PBZ and the PBS_1–3 films are 29.5, 24.5, 23.1, and 18.6 mJ m⁻² respectively. Contrary to expectation, it was noticed that the value of the surface free energy of these non-fluorine and silicone-free polybenzoxazines are significantly lower than that of Teflon. It is believed that these PBZ–SiO₂ hybrids possess lower surface free energy than that of the neat PBZ system due to the presence of strong intramolecular hydrogen bonding. Ishida et al. have reported that the network structure of polybenzoxazine is supported by strong hydrogen bonding interactions between the polymer chains, which act as chemical cross-links.²⁹ Polybenzoxazines display strong intramolecular hydrogen bonding, which results in relatively lower surface free energy. It clearly indicates that enhanced intramolecular hydrogen bonding leads to a decrease in the surface free energy and an increase in the value of contact angle against water. These observations are in good agreement with the results of previous studies reported on surface-free-energy effects.^9,19 In conclusion, from analyses of expanded FTIR spectra (Fig. S3 present in ESI†) and surface free energy, it was noticed that the strong intramolecular hydrogen bonding between the hydroxyl groups of polybenzoxazines lowers their surface free energy. The lowest surface free energy that has been observed for the PBS₃ polybenzoxazine system was 18.6 mJ m⁻². This value is lower than that of Teflon (22.0 mJ m⁻²).^10,18 Both polybenzoxazines and fluoropolymers possess lower surface free energies, whereas the former are cost competitive and easier to process than those of the latter, and therefore polybenzoxazines are expected to replace fluoropolymers in biotechnological applications as well as low-surface-free-energy coatings.⁹

The surface morphology of the hybrid materials is one of the key factors for many industrial and engineering applications. The surface topology of the neat PBZ and the PBZ–SiO₂ hybrid nanomaterials was characterized using AFM techniques and the AFM image of the neat PBZ (Fig. 3a) shows a uniform and smooth surface morphology with no visible defects. The AFM images of the hybrids PBS_1–3 are presented in Fig. 3b–d respectively. The surface of hybrid thin film is quite smooth and has few visible defects, indicating that the synthesized hybrid materials have molecular level dispersion of organic and inorganic silica segments and that they are distributed uniformly at the nanoscale level in the polymer matrix. The surface of the hybrid shows a homogeneous surface indicating that the silica domains are located in the interior of the film or that the surface of the film is composed of silica to a significant part so that no localized areas of soft and hard material could be observed. From the study it is concluded that there is an effective interfacial interaction that exists between the silica and the polymer matrix. Furthermore, the silica phases are chemically bonded to the polymer matrix through covalent bonding interactions. The surface topography of the hybrid nanomaterials could be determined quantitatively in terms of surface parameters, such as the average roughness, R_a and the root mean square (RMS) roughness, R_q. The values of R_q are 0.227, 0.286, and 0.361 and those of R_a are 0.160, 0.227 and 0.287 for PBS_1–3 respectively. The R_q value was increased to 0.361 from 0.227 and that of R_a values are increased to 0.287 from 0.160 nm. The increases in values are due to the perfect silica network present in the PBS₃ system through chemical interaction as well as hydrogen bonding interaction between the organic and inorganic phases. The morphological behaviour and chemical composition of the PBZ–SiO₂ hybrid was further confirmed by SEM and EDAX (Fig. S4 and S5 present in ESI†) and TEM.

Fig. 3 AFM of (a) neat PBZ, (b) PBS₁, (c) PBS₂ and (d) PBS₃ hybrid materials.

HR-TEM has proven to be a powerful tool for studying the dispersion and dimension of nanosized materials embedded within a polymer matrix. Fig. 4 shows HR-TEM images of PBS_1–3. It is observed that nanosized SiO₂ chemically interact with the PBZ matrix and disperse uniformly throughout the hybrid network due to the attractive forces between the SiO₂ nanoparticles and PBZ matrix which results in an increase in the T_g value.³⁰

Fig. 4 TEM of (a) PBS₁, (b) PBS₂ and (c) PBS₃ hybrid materials.

From the TEM images the size of SiO₂ in the hybrid materials is found to be in the range between 5 nm and 15 nm (Fig. 4a–c). These results further strengthen the fact that the hybrid films showed good interfacial interaction between the SiO₂ and the PBZ matrix. The nanometer level dispersion of SiO₂ within the PBZ matrix also ascertains that there should be an influence on the thermal properties of the resulting hybrid materials.

The thermal stability of polymer composites is attributed to the restrictions in chain mobility resulting from the interaction between the inorganic reinforcement and the matrix. Furthermore, it is also observed that the value of T_g for the hybrid materials is consistently improved and higher than those of the neat materials. The values of T_g for the hybrid materials are influenced by the crosslink density of the matrix, dimension and interaction of the nano-reinforcement with the matrix. Table 3 and Fig. 5 present the DSC data of the hybrid materials containing different ratios of ICPTS, and TEOS in the benzoxazine matrix. The DSC data show that an increase in the percentage content of silica in the PBZ increases the T_g value from 254 to 263 °C, which is higher than that for the neat PBZ (249 °C) matrix. This may be due to the interaction of the nanosized silica with the polymer matrix which enhances the molecular complexities and further hinders the molecular motion of the polymer chains, thus contributing to an increased value of T_g. The restricted segmental mobility increases the T_g in addition to the overall improvement of the thermal stability of the materials due to the existence of the nanometer level dispersion of both organic and inorganic constituents.²⁵ The increase in the value of T_g (Table 3) also resulted from the occurrence of a strong interaction between the components involved in the interpenetrated network generated through the sol–gel process, which further reduces the molecular motion of the polymer chains, thus contributing to an enhanced T_g.¹⁵

Table 3 Thermal properties of the neat PBZ and the PBZ–SiO₂ hybrid materials

Sample code	Tg (°C)	Weight Loss			Char yield at 800 °C (%)
		10% (T^1d) %)	20% (T^2d) (%)	30% (T^3d) (%)
PBZ	249	353	390	414	21.9
PBS1	254	412	541	618	33.8
PBS2	258	385	518	616	36.2
PBS3	263	378	508	603	43.4

---

Fig. 5 DSC thermograms of neat PBZ and PBZ–SiO₂ hybrid materials.

The TGA thermograms of the hybrid matrix are shown in Fig. 6. The results indicate that the hybrid films (PBZ, PBS_1–3) reach 10% weight losses between 353 °C and 412 °C and are presented in Table 3. The reason for the increasing T_d may be due to the presence of the stable inorganic silica network in the hybrid materials. Obviously, the presence of the nanosized silica uniformly and homogenously distributed within the polybenzoxazine matrix has made the resulting hybrid materials become more stable against thermal decomposition. Thus, the presence of silica improved the values of char residue to a significant extent. At 800 °C, the char yield of the neat PBZ was 21.9 wt% whereas that of PBS_1–3 ranged between 33.8 and 43.4 wt% (Table 3). The silica nanoparticles distributed homogeneously in the PBZ matrix have made the materials more stable against thermal decomposition through interfacial interactions and the effect is more pronounced with increasing silica content. In addition, the presence of the partial ionic nature of the inorganic Si–O–Si skeleton contributed to the thermal stability. The silica provides an additional heat capacity which stabilises the materials against thermal decomposition. The loss of the organic materials from the segmental decomposition through gaseous fragments could be reduced by the well dispersed silica in the PBZ matrix. Furthermore, it was also observed that with the incorporation of silica into the PBZ matrix it has reduced the volatile decomposition. A further silica layer can form a protective layer on the surface of the material preventing further oxidation of the inner part of the matrix.

Fig. 6 TGA thermograms of the neat PBZ and the PBZ–SiO₂ hybrid materials.

Consequently, the thermal stability of the PBS hybrid materials are higher than that of the neat PBZ, which infers that the thermal stability of polybenzoxazine has appreciably improved due to the hybridization of the silica through chemical interaction.

Conclusion

The silica reinforced polybenzoxazine (PBZ–SiO₂) hybrid materials with low surface free energy have been successfully developed through an in situ sol–gel method. The developed PBZ–SiO₂ hybrid materials developed in this work exhibited higher values for the contact angle (hydrophobic in nature) and lowest surface free energy value of 18.6 mJ m⁻² was observed for the PBS₃ system. The value of the surface free energy is lower than that of conventional PTFE. The AFM and TEM images confirm the molecular level dispersion of the inorganic phase (SiO₂) into the organic phase (PBZ). The hybrid films exhibited an improved thermal stability, higher glass transition temperature (T_g) and higher char yield than those of the neat PBZ matrix. The hybrid materials with the higher value of T_g and contact angle, and low surface free energy make them a more suitable matrix material for use in electronic applications and water repellent coatings with enhanced performance and improved longevity.

Acknowledgements

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

References

1. E. Celia, T. Darmanin, E. T. Givenchy, S. Amigoni and F. Guittard, J. Colloid Interface Sci., 2013, 402, 1–18.
2. W. Ming, M. Tian, R. D. van de Grampel, F. Melis, X. Jia, J. Loos and R. vander Linde, Macromolecules, 2002, 35, 6920–6929.
3. L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang and D. Zhu, Adv. Mater., 2002, 1857–1860.
4. J. Liang, L. He, X. Zhao, X. Dong, H. Luo and W. Li, J. Mater. Chem., 2011, 21, 6934–6943.
5. E. Casazza, A. Mariani, L. Ricco and S. Russo, Polymer, 2002, 43, 1207–1214.
6. R. Bongiovanni, G. Malucelli, V. Lombardi, A. Priola, V. Siracusa, C. Tonelli and A. Di Meo, Polymer, 2001, 42, 2299–2305.
7. N. Garcıa, E. Benito, P. Tiemblo, M. M. B. Hasan, A. Synytska and M. Stamm, Soft Matter, 2010, 6, 4768–4776.
8. S. Wu, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982, p. 142.
9. C. F. Wang, Y. C. Su, S. W. Kuo, C. F. Huang, Y. C. Sheen and F. C. Chang, Angew. Chem., Int. Ed., 2006, 118, 2306.
10. S. W. Kuo, Y. C. Wu, C. F. Wang and K. U. Jeong, J. Phys. Chem. C, 2009, 113, 20666–20673.
11. D. L. Schmidt, C. E. Coburn, B. M. DeKoven, G. E. Potter, G. F. Meyers and D. A. Fischer, Nature, 1994, 368, 41.
12. J. Genzer and K. Efimenko, Science, 2000, 290, 2130–2133.
13. E. Celia, E. T. Givenchy, S. Amigoni and F. Guittard, Soft Matter, 2011, 7, 10057–10062.
14. A. Qu, X. Wen, P. Pi, J. Cheng and Z. Yang, Polym. Int., 2008, 57, 1287–1294.
15. S. Devaraju, M. R. Vengatesan, A. Ashok Kumar and M. Alagar, J. Sol-Gel Sci. Technol., 2011, 60, 33–40.
16. F. Pereira, K. Vallé, P. Belleville, A. Morin, S. Lambert and C. Sanchez, Chem. Mater., 2008, 20, 1710–1718.
17. T. Sun, G. Wang, L. Feng, B. Liu, Y. Ma, L. Jiang and D. Zhu, Angew. Chem., 2004, 116, 361–364.
18. H. C. Lin, C. F. Wang, S. W. Kuo, P. H. Tung, C. F. Huang, C. H. Lin and F. C. Chang, J. Phys. Chem. B, 2007, 111, 3404–3410.
19. L. Qu and Z. Xin, Langmuir, 2011, 27, 8365–8370.
20. C. H. Lin, S. L. Chang, T. Y. Shen, Y. S. Shih, H. T. Lin and C. F. Wang, Polym. Chem., 2012, 3, 935–945.
21. C. S. Liao, J. S. Wu, C. F. Wang and F. C. Chang, Macromol. Rapid Commun., 2008, 29, 52–56.
22. M. R. Vengatesan, S. Devaraju, K. Dinakaran and M. Alagar, J. Mater. Chem., 2012, 22, 7559–7566.
23. C. S. Liao, C. F. Wang, H. C. Lin, H. Y. Chou and F. C. Chang, J. Phys. Chem. C, 2008, 112, 16189–16191.
24. H. Dong, Z. Xin, Z. Lu and Y. Lv, Polymer, 2011, 52, 1092–1101.
25. T. Z. Kao, J. K. Chen, C. C. Cheng, C. I. Su and F. C. Chang, Polymer, 2013, 54, 258–268.
26. M. Baqara, T. Agag, H. Ishida and S. Qutubuddin, Polymer, 2011, 52, 307–317.
27. S. Devaraju, M. R. Vengatesan, M. Selvi, A. Ashok Kumar, I. Hamerton, J. S. Go and M. Alagar, RSC Adv., 2013, 3, 12915.
28. H. Ogihara, J. Xie, J. Okagaki and T. Saji, Langmuir, 2012, 28, 4605–4608.
29. H. Ishida and D. J. Allen, J. Polym. Sci., Part B: Polym. Phys., 1996, 34, 1019–1030.
30. T. Hanemann and D. V. Szabó, Materials, 2010, 3, 3468–3517.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra44511a

---