Small Angle X-Ray Scattering For Fractal Structure Exploration Of Of Sol-Gel Of Alumina,Silica And Polymer

The sol-gel process of alumina,silica and polymer plays a decisive role in the microstructure and macroscopical properties of the final material,and the evolution of fractal structure is one of the important parameters.In recent years,although researchers have made many researches on synthesis mechanism,the traditional characterization methods are still in the exploratory stage for the evolution of sol-gel particles such as growth,dissolution and rearrangement.SAXS method is a powerful tool for characterizing the sol-gel process.As long as the electron density difference exists in the system,the method will accurately reflect the structural information of the substance.Its advantages are as follows:1.The experimental results are reliable.Based on the statistical principle,the material structure is analyzed globally rather than locally.2.Non-destructive testing.In-situ analysis can be carried out without pretreatment and damage to the sample.3.Data analysis is reliable.It is supported by three mature theoretical systems of Guinier,Porod and Debye,which provide a basis for subsequent data analysis.4.Wide range of application.The acquisition of sample structure information only depends on the electron density difference of the system.SAXS method can analyze colloids,molecular sieves,polymers and biological macromolecules.Based on the principle of small angle electron cloud density fluctuation,and combining with other characterization method such as XRD,FTIR,SEM,DLS,Zeta potential and DSC,we can better analyze the growth mechanism of colloidal particles in sol-gel process.Based on the sol-gel system of aluminum hydroxide,the aluminosilicate of clinoptilolite（CP）and P（NIPAM-co-AA）system at different p H value and temperature,the fractal structure evolution and growth mechanism of different systems were investigated by SAXS method.The research contents and main results are as follows:Firstly,aluminum hydroxide sol was prepared by hydrolysis Al Cl₃·6H₂O with metal salt as precursor.Under the catalytic condition of acidic system,the structure evolution of aluminum hydroxide sols particles with aging time（0-135 min）was investigated by SAXS method.Combined with FTIR,SEM,DLS and Zeta potential characterization,the growth mechanism of aluminum hydroxide sols particles was proposed from the perspective of fractal.The results showed that with the extension of aging time,the mass fractal dimension D_m increased from 2.29 to 2.78 during 25min–85 min;the D_m value was in the range of 2.76-2.79 and basically did not change during 95 min-135 min.This indicated that when the aging time was 25 min-85 min,the primary particles of the system will rapidly reunite from the initial relative dispersion state to larger size clusters and finally form dense gels.The gel process had been completed,the size of the sol particles did not change during 95 min-135 min,and the adhesive block may break at 135 min.Based on Porod theory,the Porod curves were in positive deviation and no interface layer was formed during 25min-35 min.And the Porod curves showed a negative deviation during 45 min-135min,so there was a solvation layer in the phase.The maximum solvation layer thickness was 1.33 nm at 115 min,the minimum solvation layer thickness was 0.40nm at 65 min,but there was no linear relationship between the solvation layer thickness and the aging time.This indicated that the sol-gel process was nonlinear and unstable.Based on the pair distance distribution function（PDDF）and Monte Carlo model,the morphology of aluminum hydroxide sols particles was approximately flat disk.The maximum particle size of the system increased from 13.59 nm to 42.67 nm during 25 min-135 min,the radius of gyration R_g increased from 4.59 nm to 15.12nm during 25 min-135 min.And the particles size distribution of primary particles was 2-4 nm at 25 min,the particles size of the primary particles of the system was distributed between 25-32 nm at 135 min.It showed that with the extension of aging time,small particles gradually agglomerated into larger particles,which was a typical polydisperse system.In addition,the FTIR results showed that all the characteristic peaks of the aluminum hydroxide sol existed in the whole gel process,indicating that the hydrolysis and agglomeration process took place simultaneously.The SEM results showed that when the aging time was 85 min,the aluminum hydroxide sol had formed a dense rubber block macroscopically,and when the aging time was 135 min,the aluminum hydroxide rubber block broke.DLS results showed that when the aging time was 25 min,the PDI was 0.047,and the hydrated particles size distribution of secondary particles was between 107 nm and 712 nm,which showed that the dispersion of the system was good at this stage;When the aging time was 135 min,the PDI was 0.67 nm,and the hydrated particles size distribution was 91 nm-220 nm,1.7μm-5.6μm,it showed that the system presented a polydisperse state at this stage.The results of Zeta potential showed that the potential increased from+1.15m V to+3.71 m V when the aging time was 25 min-85 min and decreased gradually when the aging time was 105 min-135 min,it indicated that when the aging time was25 min-85 min,the instability of the system increased and the gel process took place.After that,the instability degree of the system decreased gradually,and the gel process was completed.Based on the above discussion,from the fractal point of view,the sol-gel growth mechanism of aluminum hydroxide sol was proposed as reaction controll agglomeration process.Secondly,based on the evolution of the fractal structure of the aluminum hydroxide sol under the catalytic condition of acidic system.Under the catalytic condition of alkaline system,the fractal structure evolution of the aluminosilicate formed clinoptilolite（CP）during sol-gel synthesis was studied.A possible growth mechanism was proposed.The fractal structure evolution of aluminosilicate sol during hydrothermal crystallization（0-72 h）was described by SAXS method combined with XRD,SEM and FTIR.The results showed that the fractal types were surface fractal（D_s）.When the crystallization time was 0-24 h,the D_s value increased from 2.26 to 2.44,which indicated that aluminosilicate species with high activity appeared in the system,there were primary structural units of[Si O₄]and[Al O₄]tetrahedron in the system,and the primary particles were further agglomerated by smaller size;When the crystallization time was 36 h,the D_s value decreased to 2.40,indicating that the synthesized CP changed from rough surface to smooth open structure,the primary structural units were assembled into secondary structural units with eight membered rings and ten membered rings.In addition,XRD results showed that HEU crystal structure composed of eight membered ring and ten membered ring appeared at this stage.And then,with the crystallization time from 48 h to 72 h,the D_svalue increased slowly and remained almost unchanged.All the strong characteristic peaks of clinoptilolite appeared when the crystallization reached 72 h,which indicated that the secondary structural units were further assembled into a three-dimensional structure of CP and the structure tended to be stable gradually.Finally,from the fractal point of view,the different p H value and temperature response properties of intelligent copolymer P（NIPAM-co-AA）were explored by using SAXS method combined with FTIR,SEM and DSC.The results showed that the p H value increased from 2.0 to 7.4 and the D_s value increased from 2.08 to 2.70 at25℃,indicating that the crosslinked network structure gradually became irregular and rougher from the initial regular state,when the temperature increased from 25℃to 37℃,the D_s value decreased from 2.72 to 2.41,which indicated that the crosslinked network structure of P（NIPAM-co-AA）changed from irregular and rougher to regular and dense.In other words,with the increased of p H value,D_s value gradually increased,because NIPAM chain existed in non-ionic form,the ionization degree of-COOH and-COO^-in AA chain gradually increased,and the hydrogen bond gradually formed in NIPAM chain and the isopropyl group in amide group in the system had hydrophobicity,larger electrostatic repulsion,and the swelling degree of copolymer increased.However,under stronger acidic conditions,the-CONH-and-COOH in NIPAM chains were protonated to-CONH₂⁺CH（CH₃）,but the-COOH in AA was free,the hydrogen bond between AA chains was formed,the electrostatic repulsion was small,and the copolymer shrinked.p H 7.4,the temperature increased from 25℃to 37℃,the D_s value decreased,and the copolymer shrinked compared with 25℃,it’s because the groups in the whole polymer chain randomly formed hydrogen bonds with the water molecules in the system,and the hydrophilic group-COOH in AA chain weaked the hydrophobic effects.Based on PDDF,the P（r）～r curves of copolymers showed a symmetrical"bell shape",it showed that the morphology of aggregates was ellipsoid.Based on Kratky plots curves,the aggregation degree of the copolymer was the largest at 25℃,p H 4.0 and p H 7.4.But the aggregation degree of the copolymer at 25℃,pH 6.0 was the lowest.