Preparation, Characterization And Mechanism Of Metakaolinite-polymer Composites

Composites of organic polymers with layered clay minerals have been received significant interest in the past decades. Because the composite has excellent properties, price moderate and easy fabrication, it has been paid close attention all the while. The preparation of the composite is to use some layered silicate and organic polymer to synthesize through intercalative polymerization or polymer intercalation. Recently, the study on polymer-clay composite is mainly focused on polymer-montmorillonite composite, polymer-kaolinite composite, polymer-vermiculite composite and so on. Among these composites, the composite of polymer-montmorillonite is the most in-depth and comprehensive. But the studies on other composites are still at basic stage.Metakaolinite is derived from dehydroxylation of kaolinite at 450-980℃. It is based on the amorphous structure, but some of its structure is still ordered and retained original layered structure. The alumina polyhedron of metakaolinite becomes transformation of 6-coordination to 4-coordination, resulting in the distortion of the alumina polyhedron and decline of its ordered arrangement. Because the alumina polyhedron sheets of metakaolinite include 4-,5-, and 6-coordinated aluminum ions, its properties are very similar toγ-Al2O3, which is called "active aluminum oxide" and has porosity, high specific area and good absorbability and forms rather strong coordinate bonds with many organic compounds. At present, it is only reported that metakaolinite is used in the preparation of molecular sieve and cement. In this paper, metakaolinite was modified with various kinds of intercalative agents. Then, metakaolinite-polymer composites were synthesized by different methods in the presence of the modified metakaolinite. Besides, the traditional concept about the polymer just being organic polymer is changed. The organic polymer-inorganic polymer composite with excellent properties was synthesized in the presence of modified metakaolinite and polymers on the condition of acidity. The main research content in our study is that the preparation condition, mechanism and structure of composites were researched by various measurements in order to provide theoretical basis and widen the practice range of layered clay. Our main studies include the following aspects:1. The preparation of modified metakaolinite complex is a key step to the preparation of polymer-metakaolinite composite. Therefore, the complexes were prepared by using various kinds of agents to modify metakaolinite and investigated by X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), Scanning electron microscopy (SEM), Differential scanning calorimetry and Thermogravimetric analysis (DSC-TG) and Transmission electron microscopy (TEM) in order to discuss the preparation conditions (grinding time, intercalative agents content and heating) and the delaminating mechanism. The results showed that potassium acetate and urea were inserted into the interlayer of metakaolinite, resulting in the intercalation or probable exfoliation of metakaolinite layers. That was, the metakaolinite/potassium acetate complex and metakaolinite/urea complex were successfully synthesized. However, other intercalative agents were not successfully inserted. Moreover, the preparation condition of intercalative complexes, such as the content of intercalative agents, heat treatment and grinding time, was discussed. The results displayed that heating promoted the insert of intercalative agents into the interlayer of metakaolinite, leading to the delaminating or exfoliation of metakaolinite. Besides, the increase of grinding time and the content of intercalative agents was also favorable for the delaminating. In particular, when the grinding time was more than one hour and the content of intercalative agents was more than that of metakaolinite, the result of delaminating would be better. In addition, we presumed the mechanism of modified metakaolinite complexes. For metakaolinite/potassium acetate complex, potassium acetate was firstly coordinated with water molecules, and then moved into the interlayer of metakaolinite by mechanical grinding, resulting in metakaolinite layers being expanded. As heat treatment, water volatilization also leaded to the expansion of metakaolinite layers. In intercalated complex, acetate ions were coordinated with the gibbsite-type layers through water molecules bridging. Besides, the hydrated K+ cation stayed near the silica-type layers with negative charge in order to keep charge balance. For metakaolinite/urea complex, urea was inserted into the interlayer of metakaolinite by mechanical grinding. The decomposition of urea between the laminar silicates occurred because urea could be broken down into CO2 and NH3 at 100℃. The escape of gas finally leaded to the expansion of metakaolinite layers.2. Metakaolinite/polyacrylamide nanocomposites were successfully prepared by in-situ intercalative polymerization using acrylic amide, sodium hydrogen sulfite and ammonium peroxydisulfate in the presence of different contents of metakaolinite clay modified with potassium acetate. The preparation and mechanism of composite were discussed by XRD, FTIR, Raman, SEM, DSC-TG and Atomic force microscopy (AFM). The results indicated that metakaolinite/polyacrylamide composite was successfully synthesized. In the composite, metakaolinite was dispersed in the polymer matrix at the nanoscale (the largest lamellar spacing of metakaolinite being 1.45 nm). The FTIR and Raman spectroscopy displayed that the acrylamide chains were connected with metakaolinite by the hydrogen-bonding during the polymerization. Besides, the structure of metakaolinite layer became change. The Si-O polyhedron and Al-0 polyhedron were distorted and resulted in the decline of symmetry. This result was agreement with the group-theory analysis. The thermal analysis showed that when the content of modified metakaolinite was less than 10%, the composites had higher decomposition temperature and glass transition temperature in comparison with the pure PAM. Otherwise, the decomposition temperature and glass transition temperature descended. The thermodynamics analysis showed that the grinding, the polymerization of acrylamide and the interaction between metakaolinite and polyacrylamide were feasible to the intercalation during the preparation of metakaolinte/polyacrylamide composite. The main disadvantage was that the acrylamide molecule had to insert into the limited space, leading to the increase entropy. But the entropy was counteracted when the metakaolinite layers were expanded and exfoliated. As a result, various factors were favorable for the preparation of metakaolinite/polyacrylamide composite on the basis of thermodynamics analysis. Therefore, we discussed on the mechanism of the composite. At first, metakaolinite and potassium acetate were mixed and ground. The inserting of potassium acetate into the interlayer of metakaolinite resulted in the expansion of layers and twist of partial layers. Then, the modified metakaolinite and acrylamide were mixed. Acrylamide was intercalated into the interlayer of metakaolinite by grinding. The polymerization of acrylamide occurred in the presence of the catalyst and activator. As a result, the metakaolinite layers were further expanded or exfoliated. The nanocomposite of metakaolinite/polyacrylamide, which included the dispersion of metakaolinite in the polymer matrix at the nanoscale, was formed.3. Metakaolinite/polyethylene glycol composites were successfully prepared by solution intercalation. The preparation and mechanism of composite were discussed by XRD, FTIR, SEM, DSC-TG and AFM. The results showed metakaolinite was disordered dispersed in the polymer matrix. And the most of metakaolinite particles was about 100-200 nm and few of particles was at the micrometer scale. In the series of composite, the growth of some crystal face of polyethylene was different with the content of metakaolinite though polyethylene in all composite had same crystal type. The thermal analysis showed that the decomposition temperature of composite increased but the loss weight of composite decreased due to the presence of modified metakaolinite. It was because that the dispersion of metakaolinite leaded to increase the symmetry and regularity of composite. In addition, the heat resistance of metakaolinite could improve the the decomposition temperature of polyethylene glycol. According to the experimental results and thermodynamics analysis, we still supposed the mechanism of metakaolinite/polyethylene glycol composite:After modified with potassium acetate, metakaolinite were mixed with polyethylene glycol by grinding. The polyethylene glycol chains were intercalated into the interlayer of metakaolinite, resulting in further expansion or exfoliation of layers. Finally, the metakaolinite/polyethylene glycol composites were formed.4. Two geopolymer-polymer composites were synthesized in the presence of aqueous phosphoric acid as an activator and different content of modified metakaolinite, polyvinyl alcohol and polyethylene glycol at room temperature. The composites were investigated by XRD, FTIR, Raman, SEM and DSC-TG. And the compressive strength of composites was also tested. The results indicated that the polymerization of metakaolinite happened in the aqueous phosphoric acid, leading to the formation of geopolymer (inorganic polymer) with the structure of three-dimensional cross-linked polysialate chains (([-Si-O-Al-O-P-O-]n). Meanwhile, the organic polymers were uniformly mixed with geopolymer in the preparation process. The thermal analysis showed that composites displayed good thermal stability because of organic polymer being protected by geopolymer. In addition, the synthesized composites showed high compressive strength and hardness. In geopolymer-polyvinyl alcohol composites, the composite GPVC-5 showed the highest compressive strength (61.5MPa) and composite GPVC-10 displayed the maximum relaxation time compared with other composites. The main reason was that polyvinyl alcohol chains were favorable flexibility. As a result, the addition of the polyvinyl alcohol would improve the tenacity of inorganic-organic polymer composites. In geopolymer-polyethylene glycol composites, composite exhibited the highest hardness (Vickers hardness:5543 HV0.5kg). It was because that polyethylene glycol recrystallized and formed good crystal during the preparation of composites. As a result, the composites showed rigidity as a whole. The mechanism of composites was discussed according to analysis results. Phosphoric acid was inserted into the interlayer of metakaolinite by grinding. And it was connected with Al-O sheets by the coordination bond due to the high activity of Al-0 sheets. The phosphate ester polymers were formed by the dehydroxylation between phosphoric acid. Besides, polyvinyl alcohol could participate in the polymerization reaction due to the interaction between polyvinyl alcohol and phosphoric acid in the preparation process. Finally, the geopolymer-organic polymer composites, which consisting of the interconnection of organic polymer chains and inorganic polymer chains in three dimensions, were formed. For geopolymer-polyethylene glycol composite with the addition of polyethylene glycol, the hydroxyl group of polyethylene glycol was connected with phosphate ester polymers through the hydrogen bond. And the hydroxyl group was also connected with silicate sheets by the charge effect. Finally, the composite with the structure of interconnection between polyethylene glycol and geopolymer was formed.