It is easy to see from the FTIR diagram of TiO2 nanoparticles and titanate nanotubes (TNTs) (Fig. 1) that the Ti-O infrared absorption curve in TNTs is different from that of raw anatase, and a wide and flat shoulder band in the anatase curve appears between 470 cm− 1~750 cm− 1, which indicates that there are two Ti-O-Ti vibration broadband and TNTs in 500 cm− 1 vibration band is not wide and flat but manifested as sharp and sudden appearance. This obvious change indicates that anatase causes certain changes in the vibration characteristics of Ti-O bonds after the formation of TNTs. The structure of TNTs is no longer anatase (octahedral coordination) structure. Since each monolayer of TNTs is equivalent to a "TiO2 molecular layer", the coordination of titanium atoms in TNTs does not reach the saturation state compared with the coordination structure of octahedron[11], resulting in certain changes in Ti-O bond length and vibration, so that the displacement of infrared absorption peaks changes or shifts accordingly.
Figure 1.FTIR spectra of TiO2 nanoparticles and TNTs
Figure 2 shows the SEM spectrum of titanium dioxide nanoparticles (anatase) and the TEM spectrum of titanate nanotubes, from Fig. 2(a) it can be seen that titanium dioxide nanoparticles have good crystalline performance but some agglomeration. From Fig. 2(b), it can be clearly seen that the hologeneous vacuum tubular structure of titanate nanotubes has openings at both ends, and a straight tube with an inner diameter of about 5 nm and an outer diameter of about 10 nm.
Figure 2. SEM image of TiO2 nanoparticles (a) and TEM image of TNTs (b)
Figure 3(a) shows the XRD comparison of titanium dioxide nanoparticles and titanate nanotubes, the results show that the titanate nanotubes are amorphous, and the raw material is anatase type 2θ = 25.4°, indicating the characteristic diffraction peak of TiO2. TNTs are amorphous because they are multilayer tubes, each single layer is "the thickness of a TiO2 molecular layer, and there are no lattices between layers that can stretch in three-dimensional space[12].
Figure 3(b) is the TG-DSC diagram of the prepared titanate nanotubes, from which it can be seen that TNTs have a large endothermic peak around 120°C, and a large weight loss occurs on the TG curve at the corresponding temperature, which is due to the thermal overflow of water adsorbed by TNTs. There is a large exothermic peak in the temperature range of 250 ~ 350°C, which should be the exothermic peak of TiO2 nanotubes from amorphous to anatase, and a large weightlessness occurs on the corresponding TG curve, because the strong binding force of Ti-OH leads to the strong ability of TNTs to bind water. In addition, the great weight loss of TNTs shows that TNTs also have a large specific surface area and strong adsorption performance, and TNTs have a capillary effect adsorption force is also very strong, which can adsorb a lot of water.
Figure 3 XRD curves of TiO2 nanoparticle and TNTs (a) and TG-DSC curves of TNTs(b)
Figure 4 is an FTIR spectrum of PI/TNTs composites and pure PI. It can be seen that compared to the infrared curve of pure PI, some peak positions in PI/TNTs composites have not changed, such as the C-NH telescopic vibration peak (1380 cm− 1). However, the C = O asymmetric telescopic vibration peak (1780 cm− 1) in pure polyimide was redshifted to 1650 cm− 1, which was caused by the large amount of -OH on the TNTs pipe wall combining with C = O on polyimide to form hydrogen bonds. In addition, the wide and strong absorption peak in the region of 450 cm− 1~850cm− 1 is consistent with the characteristic peak of TiO2, which proves the existence of the -Ti-O-Ti-O- chain. The absorption peak at 1076 cm− 1 on the infrared curve of the composite film is the telescopic vibration peak of the Ti-O-C bond[13], indicating that TNTs is crosslinked with the matrix.
Figure 4 FTIR spectar of PI/TNTs and pure PI
Figure 5 (a) and (b) are the comparison pictures of the brittle cross-sectional scanning electron microscope of pure polyimide and polyimide film doped with 5% titanate nanotube, respectively. From (a) in Fig. 5, it can be seen that the pure PI section is formed by the formation of polyimide by thermal iminylation of polyamino acid, resulting in neat and parallel linear "small ravines". Figure 5 (b) is a PI composite film doped with TNTs section electron microscopy, from the figure can be seen that the white tubular TiO2 is more uniformly dispersed in the matrix, there is no obvious agglomeration phenomenon, compared with Fig. 5 (a), it can be seen that the "small ravine" formed when the matrix polyimide crystallization does not appear, TNTs form a network structure of mutual chains in the matrix. From the preparation scheme 1, it can be explained that the outer wall of titanate nanotubes is connected with a large number of hydroxyl groups, resulting in TNTs can be well dissolved in the reaction solvent DMAc and maintain good dispersion in the matrix polyimide, and these large amounts of hydroxyl groups (-OH) form a large number of hydrogen bonds with carbonyl groups (C = O) on the polyimide distributed in the matrix, which is consistent with the results of infrared analysis. TNTs are straight tubes with a very small diameter, with good stiffness, and will not break even under high-frequency ultrasonic conditions, so that the steric hindrance effect of the polymer segment during the reaction process makes the inorganic phase TNTs unable to move freely in the matrix[14]. Through the infrared analysis of PI/TNTs composites and the observation of the cross-sectional morphology of composite films, PI/TNTs composites are expected to have good heat resistance and excellent mechanical properties of functional materials with potential application value.
Figure 5 SEM pictures of broken-sections of pure PI (a) and PI film (b) containing 5 wt% TNTs
Figure 6 is the PI(BTDA-ODA)/titanate nanotube XRD spectrum doped with different contents of TNTs. At 2θ angles of 26.5° and 18.6°, there is also a sharp peak and a broad peak, respectively. The orderliness of the molecular arrangement of the thin film is illustrated. The sharp peak of pure polyimide PI at 27.6° disappeared with the addition of TiO2 tubes, and the sharp peak intensity at 26.5° decreased with the increase of TiO2 nanotube doping content. This may be related to the fact that the formation of Ti-O-C bonds reduces the degree of order of the PI molecular chain. This is consistent with the results of FTIR analysis and cross-sectional topography analysis.
Figure 6 XRD spectra of PI/TNTs composite films
Figure 7 shows UV-Vis spectra of PI/TNTs. There were no characteristic absorption peaks in the visible range of 500 ~ 800 nm, indicating that the inorganic component TNTs were well dispersed in the composite film. It can be seen from the figure that with the increase of the doping amount of inorganic components, the absorption rate in the range of 500 ~ 800 nm gradually increases, indicating that the addition of TNTs nanotubes will affect the optical properties of the composite film, and the light transmittance decreases with the increase of inorganic components.
Figure 7 UV-Vis spectra of PI/TNTs composite films with unit micron thickness (1 µm)
Figure 8 is the TG curve of PI/TNTs composite film and pure PI, from which it can be seen that the thermal stability performance of polyimide doped titanate nanotubes is significantly enhanced, and the larger the amount of TNTs, the better the thermal stability performance of the composite film. Table 1 lists the three loss-in-weight temperatures and solids residues at 800°C. It can be seen that the weight loss temperature of 5%, 10% and 30% of the composite film increases with the increase of the TNT content of the reinforced phase in the composite film, for example, the 5% weight loss temperature of the composite film with a mass fraction of 3% TNTs reaches 535.4°C, which is 64.2°C higher than that of pure PI. Excluding the influence of moisture absorption of matrix PI, the improvement of the thermal stability performance of composite films can also be seen. In addition, the solid residue at 800°C increases with the increase of TNTs content.
Figure 8 TG curves of PI/TNT composite film and pure PI film
Table 1 Thermal parameters of PI/TNTs composite films
Sample name
|
5% weightlessness temperature
|
10% weightlessness temperature
|
30% weightlessness temperature
|
Residual amount at 800oC
|
Pure PI
|
471.2oC
|
539.2oC
|
609.1oC
|
55.6%
|
3 wt% TNTS/PI
|
535.4oC
|
561.0oC
|
635.5oC
|
61.7%
|
5 wt% TNTS/PI
|
553.0oC
|
573.4oC
|
653.4oC
|
63.2%
|
10 wt%TNTS/PI
|
566.2oC
|
587.3oC
|
680.7oC
|
66.8%
|
Figure 9 shows the DMA curve of a PI composite film with a TNTs content of 5wt%, and it can be seen from the figure that the dynamic mechanical curve of the composite film has a similar pattern to that of pure PI. Compared with the dynamic mechanical properties of pure PI films, PI/TNTs composite films have higher storage modulus and higher glass transition temperature, showing excellent dynamic mechanical properties. This is because a large number of hydroxyl groups on the surface of rigid and firm TNTs form a large number of hydrogen bonds with the groups in the matrix, resulting in TNTs restricting the movement of the chain break of PI molecules[15].
Figure 9 DMA curve of PI composite film containing 5 wt% TNTs
TNTs belong to the category of semiconductors, doped into the PI matrix will not cause the conductivity of the prepared composite materials to change too much, from the relationship between the resistivity change of PI composite film and the content of TNTs, it can be seen that the resistivity of the composite film is only reduced by three orders of magnitude when the content of TNTs is 20 wt%. Figure 10 is a comparative chart of resistivity of PI films with different TNTs contents, from which it can be seen that the resistivity of the enhanced relative composite film has little effect.
Figure 10 Comparison of resistivity (Ω·m) of PI/TNTs and pure PI films
Figure 11 shows the relationship between the dielectric constant and TNTs content of the prepared PI/TNTs composite film at three frequencies. It can be seen from the figure that the dielectric constant of PI composites increases with the increase of nano-TNTs content at three frequencies, and it also has the law of high dielectric constant at low frequencies[16]. The PI permittivity is 3.42 at 50kHz. The introduction of reinforced phase TNTs in PI composites has increased a certain amount of polar groups throughout the material, and it continues to increase with the increasing mass fraction of TNTs. Under the action of the electric field, these polar groups increase the polarization strength of the material, and the result is that the dielectric constant of the composite material increases[17]. It can be seen from the figure that the dielectric constant of the composite film increases with the increase of TNTs content, but the increase is very small.
Figure 11 Relationship between dielectric constants of PI composites and TNTs contents
From the ultraviolet visible pattern of PI/TNTs composite films, it can be seen that TNTs have strong absorption of ultraviolet light, and the UV light irradiation of PI/TNTs prepared by doping of TNTs prepared with anatase as raw material can have a certain impact on the dielectric properties of the composite films. Figure 12 shows the relationship between the dielectric constant of a PI/TNTs composite film with an inorganic content of 5 wt% over time under UV illumination of two wavelengths. Figure 12(a) shows the dielectric constant curve under ultraviolet irradiation with a wavelength of 365.0nm, from which it can be seen that ultraviolet light of this wavelength has the greatest effect on the increase of permittivity of PI/TNTs, and has little effect on pure PI. This is because the dielectric constant of pure PI is not sensitive to ultraviolet light, under ultraviolet light irradiation conditions, the valence band electrons of -O-Ti-O-Ti- in TNTs are excited to the conduction band, and a lot of electron hole pairs are produced on its surface, electrons reduce Ti4+ to Ti3+, then the surface bridge oxygen ions and holes react into a large number of oxygen vacancies, with the increase of irradiation time, vacancies are more and more and dense, these vacancies are located on the outer wall of TNTs, easy to accept charge, The titanate nanotubes in the matrix now become miniature cylindrical capacitors with uniform distribution[18]. Figure 12(b) shows the change curve of dielectric constant of PI/TNTs and pure PI under UV irradiation at a wavelength of 253.7 nm, and it can be seen that the dielectric constant change of the two PI materials under this wavelength of UV light irradiation is very small. This is because the band gap of anatase is 3.2 eV[19], and the absorption range is between 350 ~ 450nm, so the PI/TNTs composite material has strong absorption of 365.0nm ultraviolet light, but does not respond to ultraviolet light with a wavelength of 253.7nm. In short, PI/TNTs composite films have certain UV responsiveness[20].
Figure 12 Dielectric constants of PI/TNTs and pure PI irradiated with ultraviolte .(a,入=365.0nm; b,2 = 253.7nm)