Structural analysis of benzoxazine monomer [Bzo-BN]
The FT-IR spectra of the synthesized benzoxazine monomer [Bzo-BN] was shown in Figure 1. It can be seen from the figures that the benzoxazine ring is characterized by the absorption bands between 936 cm-1, due to the stretching vibrations of the oxazine ring. Moreover, the benzoxazine ring also gave its characteristic absorption band at 1219 cm-1, 1021 cm-1 due to the asymmetric and symmetric stretching vibrations of the C-O-C bond, stretching vibrations of the C-N-C bond respectively25. The nitrile (-CN) group show their characteristic absorption bands at 2246 cm-1. Figure 2, illustrates the 1H-NMR and 13C-NMR spectra of the benzoxazine monomer Bzo-BN. The oxazine ring protons (Ha, O-CH2-N and Hb, Ar-CH2-N) show two singlets at 5.4 and 4.6 ppm, respectively26-28. The aromatic ring protons are located between 6.5 - 8.0 ppm. The 13C-NMR spectrum shows the characteristic carbon resonances of Bzo-BN. The methylene carbons [O-CH2-N (C1) and Ar-CH2-N (C2)] of the oxazine ring resonate at 79 and 50 ppm, respectively. The carbon of the benzonitrile group (C3) resonates at 115 ppm. And the aromatic carbon attached to the benzonitrile group (C4) resonates at 94 ppm29. All other aromatic carbons resonate between 110 - 155 ppm, respectively.
Polymerization behavior of hybrids
DSC of the hybrids was performed to study the polymerization behavior of benzoxazine monomer in presence of TiO2 by monitoring the typical exothermic peak attributed to the ring opening polymerization of benzoxazine (Bzo-BN) and their hybrids (Bzo-BN/T0-T5). DSC showed that the neat benzoxazine monomer exhibits an exothermic curve with Tonset at 224 ℃, Tmax at 243 ℃ and Tfinal at 256 ℃. In comparison, for the hybrids, the exothermic curve shows that Tonset slightly shifted to lower temperature (218 ℃) and Tmax and Tfinal shifted to 251 and 262 ℃, respectively. This result indicates that the ring opening polymerization of benzoxazine monomer has been accelerated by the presence of nitrile group (act as a catalyst) and thus lowers its onset of curing temperature. The addition of TiO2 particles into the Bzo monomer shifts the maximum and final curing temperature to higher value, which could be due to the trapped TiO2 particles inside the monomer. Moreover, the ΔH value of Bzo-BN/TiO2 hybrids decreases with increasing TiO2 content. This is due to the fact that with increased TiO2 content, the Bzo content in the Bzo-BN/TiO2 hybrid decreased obviously, thus reducing their enthalpy values. Similar exothermic curing behavior was observed for BA-a/inorganic nano fibers (30 wt%), where, the maximum curing temperature of neat BA-a (240 ℃) was shifted to 250 ℃ with incorporation of the inorganic materials30,31.
Table 1 Data from DSC thermograms of Bzo-BN/TiO2 hybrids
S.No.
|
Sample
|
TiO2 ratio
|
T onset (oC)
|
T max(oC)
|
T final (oC)
|
1
|
Bzo-BN/TiO2
|
0
|
224
|
243
|
256
|
2
|
1
|
223
|
245
|
258
|
3
|
3
|
220
|
248
|
261
|
4
|
5
|
218
|
251
|
262
|
Morphology of PBz-BP/TiO2 composites
Figure 4 shows the SEM micrographs of PBz-BN/TiO2 composites. The smooth surface of pure polybenzoxazine is clearly visible from the figure. As the titanium content increases (from 1 to 5% by weight), the surface of the composites loses its smoothness and becomes rough. It also shows the formation of homogeneous hybrid material and fine dispersion of titanium particles. The small size of the titanium particles may be responsible for the transparency of the hybrid materials32. As the TiO2 content increases to 5% loading, few voids or gaps appear on the surface of the composites. AFM images (Fig. 5) of polybenzoxazine and their composites show that the size of the nodules formed by the Pbz-TiO2 particles is uniform and has a uniform distribution, as seen in the SEM images. The surface roughness of polybenzoxazine-silica hybrids was calculated from AFM measurements using the following equation33,
Rt = Rp + Rv
Where Rt is the total roughness of the measured sample, Rp is the maximum peak height of the profile, and Rv is the maximum valley depth of the profile. The total roughness was found to be 8, 83, 109, and 128 nm for Pbz-BP:T0, Pbz-BP:T1, Pbz-BP:T3, and Pbz-BP:T5, respectively. The roughness value increases with increasing titanium content from 1 to 5 wt%, which is consistent with the AFM images.
Surface Properties of the polybenzoxazine-titania hybrids
As is known, superhydrophobic surfaces can be prepared by combining low surface area and free energy materials with rough structures. In this case, Pbzo-BN with a network of hydrogen bonds and nitrile structures were used as a low surface area and free energy material34. The addition of TiO2 NPs was able to form rough structures. Figure 6 shows the WCAs of neat polybenzoxazine and polybenzoxazine-titania hybrid [Pbz-BP:T1, Pbz-BP:T3 and Pbz-BP:T5] coatings with different mass ratios of TiO2 NPs. The pure Pbz coating showed hydrophobicity with a WCA of 87°. With the increase of the additional amount of TiO2 NPs, the hydrophobicity of the Pbz/TiO2 also increased significantly with a linear increase in WCA from 87 to 146 °C. The hydrophobicity of polybenzoxazine was improved by incorporating TiO2 nanoparticles even at low concentrations (e.g. 5 wt% TiO2). The increased hydrophobicity of the [Pbz-BP:T5] hybrid is attributed to the air trapped in the voids of the rough surface and preventing water from entering the nanoparticles, leading to an increase in the contact angle with water35.
Dynamic mechanical Properties of titanium-modified polybenzoxazine hybrids
The dynamic mechanical properties of the hybrid materials in comparison to neat polybenzoxazine were examined. Fig. 7 shows the storage modulus (E′) and loss modulus (E″) as a function of temperature. The storage modulus of the neat Pbz drops slowly up to 125 °C, after which there is a sharp drop with a glass transition temperature (Tg) of 147 °C obtained from the maximum of loss modulus. The storage moduli of the composites in the glassy state increase with the inclusion of titanium particles, and rise more on further increasing the content of titanium in the hybrids36-38. This behavior indicates that the dispersed titanium nanoparticles are effective to reinforce polybenzoxazine matrix. The drastic improvement of the storage modulus of polybenzoxazine by hybridization with small amount of titanium nanoparticles (5%) can be attributed to the reinforcing role of the titanium nanoparticles and the increase of the polymerization degree of polybenzoxazine via their nitrile group on the ring opening polymerization similar to the role of silica, clay and other inorganic nanomaterials on polybenzoxazines nanocomposites as previously reported37,39. Also, the hybrids show higher Tg than the neat resin. The Tg of pure polybenzoxazine was found to be 147 °C; but for the composites, the Tg increased with increasing nano filler content (Tg of Pbz-BP/T5 is 164 °C). The hard and rigid inorganic regions create an obstacle to the movements of random chain segments in the matrix. This restricted mobility of the segmental molecular chains results with an increased Tg values. A similar increase in Tg values is observed in PBA-a composites with other inorganic nanofibers, ranging between 185 and 193 °C40-43.
Table 2 DMA data of Pbz-BN/TiO2 composites
S.No.
|
Sample
|
TiO2 ratio
|
DMA
|
Storage modulus
|
Tg (oC)
|
CLD
*105 mol/m3
|
1
|
PBz-BN/TiO2
|
0
|
2.81
|
147
|
3.7
|
2
|
1
|
3.15
|
151
|
4.1
|
3
|
3
|
3.21
|
157
|
4.2
|
4
|
5
|
3.26
|
164
|
4.3
|
Cross-link density of Pbz/TiO2 composites
The cross-link density (CLD), γc is the number of network chain molecules per unit volume of the cured polymer. The cross-linking density of highly cross-linked thermoset materials can be determined by modulus measurements using the modulus constitutive equation given below44,
γc = ɛ'/3RT
Where,
ɛ' = storage tensile modulus (from DMA)
T = temperature in K corresponding to the value of the storage modulus
R = gas constant
Table 2 shows the interconnect density of the Pbz/TiO2 composites. The crosslinking density of the pure polymer, i.e. Pbz-BN/T0, was found to be 3.7*10-5 mol m-3, while the crosslinking density of the composite, i.e. Pbz-BN/T5, was found to be 4.3*10-5 mol m-3. It can be seen that the crosslinking density obviously increases with the increase in the nanoparticle content in the composites. The main reason is that the nanoparticles behave as natural cross-linkers by forming intermolecular hydrogen bonds between the hydroxyl groups of the nanoparticles and the -OH, -CN, of the polybenzoxazine(s), thus limiting their molecular motion45,46.
Thermal stability of Pbz/TiO2 composites
The thermal stability of the hybrids was studied by thermogravimetric analysis to investigate the effect of dispersed titanium particles on the thermal stability of the polybenzoxazine matrix. Figure 8 shows the TGA profiles under nitrogen atmosphere of pure Pbz and the composites containing various percentages of titanium. For pure Pbz, T5 and T10 are 326 & 354 °C with a carbon yield of 42.6%. The incorporation of titanium particles in the Pbz matrix led to an increase in thermal stability. For example, the carbon yield increased from 42.6 to46.4, 51.2 and 53.7% with the addition of 0, 1, 3 and 5 wt% titania, suggesting that the dispersed titanium particles in the matrix act as a thermal insulator to protect the Pbz matrix. Also, the composites also contain nitrile group, by which the thermal stability showed a greater improvement. This could be attributed to the maximized adhesion between the organic (Pbz) and inorganic (titanium) sites, which further protect the organic matrix with thermal insulation47-49.
Table 3 TGA data of Pbz-BN/TiO2 composites
S.No.
|
Sample
|
TiO2 ratio
|
Ti (℃)
|
T5 (℃)
|
T10 (℃)
|
CY
|
LOI
|
1
|
PBz-BN/TiO2
|
0
|
286
|
326
|
354
|
42.6
|
34.5
|
2
|
1
|
296
|
351
|
374
|
46.4
|
36.1
|
3
|
3
|
308
|
364
|
386
|
51.2
|
38.0
|
4
|
5
|
313
|
378
|
403
|
53.7
|
39.0
|
Flame retardancy of Pbz/TiO2 composites
The flammability resistance of pure Pbz and Pbz-TiO2 composites is explained as a function of the limiting oxygen index (LOI) value. One way to calculate the LOI is by TGA analysis, knowing their corresponding carbon yield values. van Krevelen and Hofytzer used the equation50 given below to calculate the LOI,
LOI=17.5+0.4*CY
Where,
LOI is the oxygen limit index,
CY is the carbon yield (from TGA data).
The value of LOI for the pure Pbz-BN polymer was 34.5. And their composites showed LOI values of 36.1, 38.0 and 39.0 for Pbz-BN/T1, Pbz-BN/T3 and Pbz-BN/T5, respectively (Table 3). It could be observed that the LOI value of polybenzoxazine and their composites were higher than the threshold value (26). This clearly indicates that the incorporation of TiO2 nanoparticles in the Pbz system results in thermosets with good self-extinguishing and flame retardant properties.
Dielectric properties of Pbz/TiO2 composites
Dielectric materials can be used to store electrical energy in the form of charge separation when the electron distributions around their constituent atoms or molecules are polarized by an external electric field. The dielectric constant is directly related to the polarizability of the material and is therefore highly dependent on its chemical structure. Figure 9 shows the values of the dielectric constant and the dielectric loss of Pbz/TiO2 composites. The values of dielectric constant and dielectric loss were found to be in the range of 3.2 - 2.9 and 0.94 - 0.78, respectively for Pbz-BN/TiO2 composites, respectively (Table 4). Composites differ from micro composites in three aspects: they contain small amounts of fillers, the filler particles have sizes in the order of nanometers, and the interface between the filler and the polymer is large. Nanoparticles reduce the movement of the polymer chain by physical bonding51. The mobility of the charge carriers, also decreased with particle loading, suggesting that the nanoparticles disperse the carriers by reducing their mobility, leading to a decrease in permeability with increasing frequency52.
Table 4 Dielectric data of Pbz-BN/TiO2 composites
S.No.
|
Sample
|
TiO2 ratio
|
Dieletric
|
Constant
|
Loss
|
1
|
PBz-BN/TiO2
|
0
|
3.2
|
0.94
|
2
|
1
|
3.1
|
0.93
|
3
|
3
|
3.0
|
0.87
|
4
|
5
|
2.9
|
0.78
|