Study On In-situ-formed Sheet-like Silica Compound During PET Synthesis

Organic-inorganic hybrid materials have been paid great attention due to their unique opportunity to combine the remarkable features of organic materials with those of inorganic materials. In-situ sol-gel process provides a versatile way to prepare functionalized nanohybrids because it can achieve the molecular lever blending between organic-inorganic materials. The key factor is how to combine the sol-gel process with polymer synthesis, process and application to develop high performance nanohybrids.Polyester materials such as PET and PBT have been widely used in plastic,coating and fiber. So it is very useful to study the morphology control and formation mechanism of in-situ-fabricated silica compound during the polyester synthesis.In this dissertation, we try to learn how to in-situ fabricate sheet-like silica compound and how to control its morphology during PET synthesis.The procedure of PET synthesis includes two quite different steps: esterification under high pressure and polycondensation under high vacuum. The sheet-like silica compound can be in-situ formed at the beginning of esterification. So our study focus on the esterification step and systemetically examine the various factors, such as raw materials molar ratio, introduction mode of the precursor and their dosage, temperature, stirring rate and esterification degree, which influence the morphology and size of in-situ formed silica compound. Two esterification mode, direct esterification and transesterification, and two introduction mode, in-situ sol-gel process from precursor and sol have been studied.The bis?2-hydroxyethyl? terephthalate ?BHET?/silica compound nanocomposites were prepared by in-situ sol-gel process from precursor tetraethyl orthosilicate?TEOS? during direct esterification. It is found that the ethanol is distilled out at the beginning of the direct esterification process, the in-situ formed silica compound could be sheet-like or/and round-like. It tends to be round-like with reaction temperature increasing.The BHET/silica compound composites were prepared by in-situ sol-gel process from precursor TEOS during transesterification. It is found that the sheet-like silica compound can also be in-situ formed but must exist at catalyst during transesterification. It suggests that the sheet-like silica compound can be in-situ formed under no existence of H₂O, which is from the direct esterification of PTA and EG. The sheet-like silica compound tends to become thinner from 500 nm to 300 ran as the dosage of precursor increase from 0.5% to 3.0%, and become 200nm thickness as the molar ratio of DMT and EG reduces to 1:24. It is shown by WAXD patterns that the in-situ formed silica compound has unique crystalline structure with new diffraction peaks at 2-theta 6.9°.The BHET base composites with sheet-like silica compound were also synthesized by in-situ sol-gel process from silica sol in EG during direct esterification . The silica sol was prepared at the molar ratio TEOS:H₂O:CH₃COOH=1:2:6.5 and 70?. It is shown that the key factors, which influence the size of in-situ formed sheet-like silica compound, are also the molar ratio of PTA and EG, and the dosage of precursor. The in-situ formed silica compound also had unique crystalline structure with new diffraction peaks at 2-theta 6.9°as shown by WAXD patterns. It is also found that the sheet-like silica compound has already been formed when the degree of esterification attain 30%.Poly?ethylene terephthalate? ?PET?/silica compound nanocomposites have been prepared by in-situ sol-gel process from precursor during transesterification ?method A? and silica sol in EG during direct esterification ?method B?. Only granule-like with the diameter at about 50nm is observed as shown by SEM images and no new diffraction peaks occur from both of the resulted PET/silica compound composites and WAXD patterns. The intrinsic viscosity of PET/silica compound composites tends to decrease as silica compound content increases.