TIAN Peijing, LI Man, YUAN Jian, ZHENG Weihong, WANG Jing

(1. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; 2. Shahe Research Institute of Glass Technology, Xingtai 054100, China; 3. State Key Laboratory of Silicate Materials for Architectures, Wuhan 430070, China)

Abstract: Aluminum sol, dry gel, and film were prepared by sol-gel and a dipping-withdrawing method. The effect of polyethylene glycol (PEG) on the behavior of aluminum sol, gel, and film was investigated by DLS, rotary viscometer, XRD, IR, EPMA, and light microscope, respectively. The solid gels drying at room temperature was identified as amorphous phase, and the addition of PEG nearly has no effect on the structure of aluminum dry gel. It is indicated that the sol viscosity decreases with the PEG content increasing, while the micelle size in sol decreases first then increases. The uniformity and attached particles of the film are improved to a certain extent after PEG addition. According to the analytic results, we demonstrate that the improvement is caused by low viscosity and small micelle size of sol resulting from the PEG addition.

Key words: PEG; alumina; sol-gel; film; dipping-withdrawing

1 Introduction

Aluminum oxide thin films are widely used as oxidation resistant[1], corona resistance[2], super-water-repellent[3]and luminescent[4]coating as it excellent physical and chemical properties. Among the ways to prepare alumina film, sol-gel method has been studied thoroughly by preceding scientists. The previous study about Al2O3prepared by sol gel method has confirmed that AlO(OH) (boehmite) and Al(OH)3(bayerite and gibbsite) are the hydrolysis products of aluminum salts and alkoxides. Andγ,δ,θ, α-alumina could be obtained after heat treatment at certain temperature[5]. It means that the aluminum hydroxide film could also be prepared by sol-gel method, for example, Zhang[6]synthesized superhydrophobic antireflective boehmite film by sol-gel and spin-coating method.

Regardless of the final film needed, satisfied film quality is necessary to meet performance in any application. Actually, uniformity, particles attached, cracking, and flaking are common defects for the film deposited from sol-gel method. To reduce these defects, some organic modifications, such as hexamethylsisilazane (HMDS)[7], ammonium polyacrylate (NHPA)[8], polyvinylpyrrolidone (PVP)[9]and polyethylene glycol (PEG)[10], were added to sol during the preparation process. Among these modifications, PEG has good compatibility in aqueous and organic solvent, and its main application varies with the molecular chain length. Besides preventing defects of film, PEG were widely used as pore forming agent[11,12], structure directing agents[13], surface active agent[14]or drug delivery[15]in sol-gel method. In this work, amorphous aluminum hydrous solid and film were prepared, and we explored the PEG effect on the viscosity and micelle size of sol, the structure of solid gel, as well as the quality of dipping-withdrawing film.

2 Experimental

2.1 Materials and procedure

Aluminum sols have been made from aluminum isopropoxide as precursor, water as solvent, and nitric acid as catalyst. Hydrolysis of aluminum isopropoxide (98%, SCR, China) was carried out with deionized water in a Pyrex flask, while mixing vigorously at 80 ℃ for 45 min under reflux condenser which was used to minimize evaporation. Then polyethylene glycol (PEG-1000, SCR, China) was added to the mixture at this temperature. The sol was peptized after adding nitric acid (65%, CR, China), while mixing at 90 ℃ for 24 h. The molar ratio of Al(i-Pro)3:H2O:HNO3: PEG=1:150:0.25:x was chosen to discuss the effect of PEG addition, and the samples with x=0, 0.025, 0.050, 0.075 were labeled as “A0”, “A1”, “A2”, and “A3”, respectively.

The coating was carried out on clean soda lime glass plates in a dipping-withdrawing manner (withdrawing speed: 3 mm/s) with aging time 7 days of sol. The coating films obtained were dried under air flow at room temperature, and the final film on the glass was transparent and colorless.

The solid gels were obtained from prepared sols by aging in the air and drying at room temperature for 3 months. As shown in Fig.1, the final gels are transparent and colorless solid.

Fig.1 Solid aluminum gels obtained from sols by aging in the air and drying at room temperature

2.2 Characteriztion

After 7 days aging at room temperature, sols were tested by following measurements. Size distribution of the micelle in sol was examined by dynamic light scattering (DLS) (ZEN3690，Malvern, UK). A 4 mW laser of wavelength 632.8 nm was used as the source, and three runs per measurement were acquired for each sample. The intensity average (Z-average) hydrodynamic diameter was calculated from measured diffusivities using the Stokes-Einstein equation. The viscosity of sol was tested by rotary viscometer (NDJ-5S, Mreay, China) using a 0# rotor at a rotation speed of 60 rpm.

The quality of coating film was observed by polarizing microscope (XPF-550C, Caikon, China) and X-ray electron probe microanalyzer (EPMA, JEOL JXA-8230, Japan). The powder of solid aluminum gels was used to complete the following two test. The crystalline phases are identified by X-ray diffraction analysis (XRD, UitimaⅣ, Rigaku, CuKα radiation, Japan). Copper Kα radiation, produced at 40 kV and 15 mA, scanned the range of diffraction angles (2θ) between 10° and 70° with a 2θ-step of 1 °/ min. Infrared spectra (IR) spectra were measured using a Thermo Nicolet Nexus spectrometer with KBr disks.

3 Results and discussion

3.1 Influence on aluminum sol

Fig.2 shows the viscosity change of sol with PEG addition. It could be noticed that the sol viscosity decreased from 1.68 to 1.46 mPa•s with the PEG content increased from 0 to 0.075 mol. For the structure of micelle in sol, when a charged particle is dispersed, an adsorbed double layer develops on its surface[16,17]. According to the theory, the structure of boehmite micelle could be described asWhere m means the number of AlOOH molecular in sol core, and is a large number about 103; n is the number of H+adsorpted on the sol core, and is much smaller than m; (n-x) and x are the NO3-number in the close layer and diffusion layer, respectively. Although the hydrogen bonding network for aluminum hydroxide (boehmite and gibbsite) are very different[18], the H+adsorpted on the sol core in micelle are provided by H2O. According to previous report, the oxygen atoms of the polyethylene oxide hydrophilic chain interact with the OH of the aluminum hydroxide by H-bond[19], while hydrogen bonding and coordination interaction exist between the surfactant PEG6000 and the boehmite particulates[20]. We infer that as a polar organic solvent, PEG 1000 with adsorption properties could replace the hydrogen bond between sol core and water, releasing more free water in sol. Increase of free water is the reason for decrease of sol viscosity after PEG addition.

Fig.2 The effect of PEG content on the viscosity of the sol

Unstable regime always appears at the early stage of aging for sol, such as silica sol[21,22]and titania sol[23], corresponding to unstable oscillations in the growing of the sol micelle, and is produced by the kinetic instability of the particles in sol. All the data of micelle size were tested after aging for 7 days, as these values remained relatively constant throughout the experimental period, which means the micelles in sol did not aggregate further to create larger clusters. The Z-average size is usually recognized as the accepted norm for presenting particle sizing results by DLS, so the Z-average size is recognized as micelle size in our work.

The micelle size of the sols are shown in Fig.3, it is observed that the addition of PEG could obviously affect the size of micelle in sol. The micelle size decreased from 91.06 to 60.95 nm with the PEG content increased from 0 to 0.050 mol, then increased to 77.13 nm as the PEG content reached 0.075 mol. It is believed that the bonding between the PEG and micelle reduced the free energy of the crystallites[20], the ether bonds (-O-) in the macromolecular chain of PEG bonds with -OH on the surface of aluminum hydroxide. When the particle surface covered with the macromolecular chain, two possibilities arise, (1) the steric hindrance would prevent ionic bond formation and restrain agglomeration between micelles, as a result, the micelle size reduced, as well as the dispersibility of the micelles was improved, or (2) the PEG chains connected with micelle were intertwined, inducing the micelle aggregation which was attributed to increased particle size. When the PEG content is below 0.05mol, the possibility of chain entanglement is low, and steric hindrance becomes stronger accompanied by the addition of PEG, so micelle size decreased as the increase of PEG content. With increase of PEG content, the chain entanglement becomes the dominant factor, resulting in the increase of micelle size. Therefore, the micelle size decreases firstly and then increases due to the competition between the two trends.

Fig.3 The effect of PEG content on the size of micelle in sol

3.2 Influence on aluminum gel

The XRD patterns of sample A0, A1, A2, and A3 are shown in Fig.4. In our previous work[24], boehmite was formed in the gel after drying at 90 °C in an oven under air flow during 48 h. But for the transparent solid gels drying at room temperature for 3 months, an obvious wide peak around 25° appeared in every sample, and no other sharp peak was observed, which showed that the main phase of these gels are all amorphous.

Fig.4 XRD patterns of the four aluminum gels after dring

Fig.5 shows the IR spectra of the four aluminum gels. The band at 3 432 and 1 636 cm-1are generally recognized as structural and molecular water stretching and bending vibrations, respectively. While the 3 432 cm-1can also be attributed to -OH stretching of hydrogen bonded aluminum oxy-hydroxide[25], confirming the dehydration of AlOOH. The peak at 2 427 and 1 763 cm-1is linked to C=O band as it had been proven that the gel is likely to absorb some CO2from air[26]. The peak at 1 384 cm-1are assigned to C-H bending vibrations of -CH3[27], and the band at 1 091 cm-1is mainly attributed to Al-O-C[28], and 955 cm-1are connected to C-C bending vibrations coming from PEG or other carbonaceous residues in solvent[29]. The peaks in the range 400-900 cm−1associated with the vibrations of Al-O bonds, which can be assigned to different coordination states of Al atoms: AlO6(794, 575, and 465 cm−1) and AlO4(825 cm−1)[30-32], indicating tetrahedral and octahedral coordination existence in amorphous gels. Comparing the IR spectra of four samples, no shift was found for their peaks position, indicating that all samples have the same functional group. But it could be noticed that the peaks at 1 152 and 1 213 cm-1appeared gradually with the PEG addition, and these two peaks could be recognized as C-O-C bonds[29]due to the added PEG. Therefore, the addition of PEG nearly have no effect on the structure of aluminum dry gel, and only amorphous aluminum hydrate could be obtained after drying at room temperature.

Fig.5 IR spectra of the four aluminum gels in the range of 4 000-400 cm-1 at room temperature

3.3 Influence on aluminum film

Fig.6 shows the micrographs of all films taken by optical microscopy and EPMA. It could be noticed that the film coated by A0 was non-uniform, and even some particles deposited on the surface of the substrate. The film uniformity was obviously improved in film coated by A1, A2, and A3, and the number and size of micro particles on the film reduced in A1 and A2. It is known that the sol viscosity decreased with the PEG content increased, while the micelle size decreased first then increased with the PEG content increased, corresponding to change trend of film uniformity and deposited particles respectively. The low viscosity is benefit to the sol flow accompanied with better uniformity, as viscosity is a liquid’s resistance against flow. And small micelle in sol means fewer clusters, reducing the deposited particles in film. It is easy to speculate that the improvement of film quality is caused by low viscosity and small micelle size in sol.

Fig.6 Optical microscope and EPMA images of films on glass

4 Conclusions

Aluminum sol, gel and film have been prepared via sol gel and dip-coating method. With molar ratio of Al(i-Pro)3:H2O:HNO3:PEG=1:150:0.25:x(x=0, 0.025, 0.050, 0.075), hydrolysis of aluminum isopropoxide was carried out. The sol viscosity decreased from 1.68 to 1.46 mPa•s with the PEG content increased, which is caused by the increase of free water accompanied with the addition of PEG. And the micelle size in sol decreased first from 91.06 to 60.95 nm, then increased to 77.13 nm with the PEG content increased, which is due to the competition between two trends: probability of chain entanglement and the steric hindrance. The film quality is related to the viscosity and micelle size in sol, and the addition of PEG could improve film quality to a certain extent. Transparent and colorless solid gels were obtained from sols after aging in the air and drying at room temperature for 3 months. All the gels are amorphous aluminum hydrate, and the addition of PEG nearly have no effect on its structure.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (WUT: 2020Ⅲ017GX) and the National Key Research and Development Program of China (No. 2016YFB0303700). The authors would like to thank Professor Lothar Wondraczek and Doctor Lenka Müller at University of Jena for their support of preliminary work.