INVESTIGATION OF REOKINETIC IN SOL-GEL PROCESS POLYMER-SILICA MATERIALS

ABSTRACT

Last time the creation of nanosize hybride sorbents is of great interest because it allow to combine varions properties in one materials. Sol-gel process includes hydrolysis reaction filling by alcoxy- or hydroxysylanes condensation. At first at alcoxy groups hydrolysis silanoli groups Si-OH are formed; and at condensation through hydroxyl and alcoxyl groups the chains Si-O-Si are formed. In the course of further hydrolysis and condensation siloxane network of hydrogel is formed by olygomers cross-linking. The process goes through embryo formation (nucleation) and particles growth following by aglomeration.

The main goal of this investigation is the investigation of rheokinetic regularities of three-dimensional polymeric nanostructures on the base of tetraetoxysilane under varying of catalyst content in reaction system with the use of viscosimetry method, which allows observing the process of structure formation up to fluidity loss by the system under investigation.

АННОТАЦИЯ

Последний раз создание наноразмерных гибридных сорбентов представляет большой интерес, поскольку позволяет объединить свойства варионов в одном материале. Золь-гелевый способ включает заполнение реакции гидролиза конденсацией алкокси- или гидроксисиланов. Вначале при гидролизе алкоксигрупп образуются силанольные группы Si-OH; и при конденсации через гидроксильные и алкоксильные группы образуются цепи Si-O-Si. В процессе дальнейшего гидролиза и конденсации формируют силоксановую сеть гидрогеля путем сшивания олигомеров. Процесс проходит через образование эмбрионов (зародышеобразование) и рост частиц с последующей агломерацией.

Основной целью данного исследования является исследование реокинетических закономерностей трехмерных полимерных наноструктур на основе тетраэтоксисилана при изменении содержания катализатора в реакционной системе с использованием метода вискозиметрии, что позволяет наблюдать процесс формирования структуры вплоть до потери текучести исследуемой системой.

Keywords: Nanosize hybride sorbents, sol-gel, TEOS, Polyetoxysiloxane, oligomer, polietilenglicol, viscosimetry method, isothermical conditions, “Rheotest-2”

Ключевые слова: Наноразмерные гибридные сорбенты, золь-гель, ТЭОС, полиэтоксисилоксан, олигомер, политиленгликол, метод вискозиметрии, изотермические условия, «Rheotest-2»

Experimental

Polyetoxysiloxane oligomer was investigated, obtained on the base of TEOS. Reaction system compositions are:

Polyetoxysiloxane (PES) oligomer formation reaction was carried out at 25⁰C 24 h. The oligomer obtained was colorless liquid with viscosity 0,14 Pa.

Hardening reaction – nanoparticles formation was carried out in isothermical conditions at 25⁰C under catalyst content varying.

Modification of nanostructures on PES base in the hardening process was carried out by insertion of various quantities of polietilenglicol in hardening systems.

Viscosity changes in the process of hardening was estimated on rotation viscosimeter “Rheotest-2” (Germany) with working unit of coaxial cylinders in the conditions of high shear velocities (D=656sec^-1).

Results and Discussion

The mechanism of oligomer obtaining and its further hardening occurs by sol-gel process scheme, including hydrolysis reaction followed by alcoxy or hydroxysilanes condensation [1-3]. At the beginning under alcoxy group hydrolysis, silanol groups ≡Si-OH are formed and under condensation on hydroxyl and alcoxyl groups ≡Si-O-Si≡ chains are formed. Under the further hydrolysis and condensation siloxane network of hydrogel is formed by oligomers cross-linking. The process goes through nuclei formation and particles growth followed their agglomeration.

Thus at well controlled hydrolysis conditions (solvent, catalyst, temperature) and tetraalcoxysilane condensation in the presence of transformed polymer it is possible to get materials where organic polymers are included homogeneously in silica three-dimensional network, forming two-phase system. In display of physical and chemical properties by these systems both organic and inorganic phases play the important role. Synergism between two phases can be explained exceptionally by domens size influence and interface properties.

The common scheme of three-dimensional polymeric micro- spherical silica gel consists of three stages. On the first stage hydrolytic polycondensation of initial alcoxysilanes takes place with the formation of PES oligomer. On the second stage hydrosol is formed by addition of components to regulate of porosity in alcohol solutions. After hydrosol drying at 120⁰ C hydrogel is formed followed with xerogel- silica gel formation.

It was of significance to investigate the kinetics of sol-gel process of porous microspherical polymer-silica sorbent obtaining to investigate the effect of some factors, influencing on sol-gel process and properties of reaction end product.

On the structure on xerogel forming, hardening reaction kinetics and mechanism structural features of reacting components and their ratio with hardening in reaction system influence. In the hardening process the complex of chemical and physical-mechanical properties changes. Schematically PES oligomer hardening process can be divided into 2 stages.

On the first stage the growth of linear macromolecules and their branding take place, but common network enveloping all material volume is absent. On the second stage such network is formed. Eventually, in the systems with the increasing of topological complexity of molecules in the course of reaction, when the transition takes place in the scope of liquid phase from liquid aggregate state in hard (high-elastic or glass-like) the velocity of network polymer formation process will be determined at the every stage of functional groups interaction and various relaxation processes. Formation of concrete topological forms developing at polyfunctional molecules (oligomers) condensation depends on the nature of substances reacting and ratio of different elementary reaction velocities and determines the concrete value of the optical transformation degree, which is responsible for fluidity loss-the moment, which is named by various terms- “structurizing”, gel-formation.

On the Tabl.1 the experimental data of dynamical viscosity for hardening PES with various catalyst content (3,6,9,12,15% in relation to oligomer) depending on time. As the initial reagents 10% PES oligomer in ethanol and ammonia hydroxide as a catalyst were used.

It is seen from rheokinetical curves that it is possible to escape 3 steps of PES oligomer gel formation the initial induction period when the system remains the viscous Newtonian liquid with practically constant viscosity the stage of intensive gel-formation with rapid increasing of all parameters measured and completing period when the limiting values of dynamical viscosity is reached and the gel formed is stabilized. The time constants, characterizing these steps are determined: t_o - the end of induction period  t_1/2 - half period of gel formation process to the time of full process proceeding. As it is seen from the results cited in the table 1 this clearly give the main regularities of gel formation process namely the existence of induction period, auto catalytically character of viscosity increase and obtaining of limiting gel state.

Table 1.

The dependence of PES oligomer hardening time on catalyst (I) and polietilenglicol on catalyst content gap

From the data cited it is seen clearly the catalyst influence on PES oligomer gel formation kinetics. The decrease of catalyst leads to significant increase of time constants. It is especially distinctly expressed at 3% catalyst addition.

Experimentally by viscosimetry method the determination of full gel formation process due to hardening and loss of system flow. The T_k value is determined by calculation through extrapolation. Mathematical treatment of experimental results obtained allowed deriving the equation describing the change of dynamical viscosity on time as follow: h=h₀exp(k_ht) [4].

On the Tabl.1 the influence of polietilenglicol solution on the process of PES oligomer hardening is shown. As it is seen from Table 1 the increase of 5% polietilenglicol concentration in system in hardened system leads to slowing down of gel formation reaching point. It can be explained by the chemical interaction of polietilenglicolwith silanol groups of PES oligomer product hydrolysis that possibly fades the part of active centers and leads to less cross linked structure of polymer.

So, in can be noted that polietilenglicol solution influences actively on the process of PES oligomer hardening and slowing down the the hardening process, forming more spatial polymer network.

Conclusion

The rheokinetical regularities of polyetoxysilane oligomers hardening under varying of catalyst content in system and modifying reagent were investigated. It was shown, that introduction of polietilenglicol in systems and increase of its content leads to slowing down of gel formation process and formation of less cross-linked structures.

Some technological parameters of obtaining and regulation of polymer polietilenglicol nanostructural particles were estimated.

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