Thermally Induced Liquid Crystal Polymer Composition And Synthesis Technology

Thermotropic liquid crystalline polymers (TLCPs) have received a great deal of attention from both academia and industry in the last decades, due to their high performance and wide applications. At present, only a few companies in USA and Japan grasp the key technology to manufacture such materials. Many efforts have been made on the R&D of TLCPs for many years in our country, and its commercialization has been started in recent years, but a number of technical problems have been encountered in practice during the commertializing process. Therefore, it is of great importance to conduct the relevant applied and fundamental studies for TLCP commertializing. In this thesis, a new composition of TLCP was investigated and a monomer was found to vary effectively melting temperature (T_m) of TLCP and widen the processing temperature window. Solid-state polymerization of TLCP was also studied and the influence of various factors was disclosed on the solid state polymerization. These researches would be a valuable fundament for the commercialization of TLCPs in our country.1. 4,4'-diphenyloxide dicarboxylic acid (DODA), as a comonomer, was melt-polymerized for the first time with 4-acetoxybenzoic acid (ABA), hydroquinone diacetate (HQA), 2,6-naphthalene dicarboxylic acid (NDA) and terephthalic acid (TA) to make TLCPs with a good processing properties. With introduction of the ether linkage into the polymer chain, the melting point of liquid crystalline copolyester was lowered and the processing temperature window was expanded, so that the copolyester could be easily to process with conventional moulding technology. Meanwhile, the reaction rate and melt viscosity could be well controlled in the late stage of polymerization due to the presence of DODA unit in chains. The statistical analysis on the compositions and melting temperatures of liquid crystalline copolyesters synthesized in the range of monomer mole ratio, ABA 50%-65%, HQA 17.5%-25%, NDA 10.5%-17.5% and TA 5.25%-10%, shows that the amount of each monomer has different effects on the melting point of copolyester. The TLCPs with different melting points can be synthesized by adjust the mole ratio of monomers, sometimes, the TLCPs with high melting points are required in the case of high heat distortion temperatures.,2. The crystallization and melting behaviors of wholly aromatic liquid crystalline copolyester K(ABA / HQA / NDA / TA = 57 / 21.5 / 12.9 / 8.6) and copolyester L(ABA/ HQA/NDA/ DODA/TA= 57 / 21.5 /10.4/2.5 / 8.6) were investigated by using differential scanning calorimetry (DSC). Both of copolyesters exhibit double melting peaks after isothermal crystallization at various temperatures for different time. The high temperature peak corresponds to the melting of the crystals formed in the "fast" crystallization process, and it does not change with isothermal crystallization conditions. While the low temperature melting peak relates to the melting of the crystals formed in the "slow" crystallization process, which is controlled by diffusion, and the low temperature melting peak gradually shifts toward higher temperature and becomes larger with increasing annealing time. A small amount of ether linkage greatly changes the crystallization and melting behaviors of the copolyester. Since the incorporation of ether linkage makes polymer chain more irregular and flexible, the crystallization of chains would be more difficult and thus the high temperature melting peak would be small. However, the increase of chain mobility enhances annealing effect and structural rearrangement of copolyester during isothermal crystallization. The enthalpy (â–³H) and entropy (â–³S) during melting transition as well as equilibrium melting point (T_m^o) of the copolyesters were obtained by extrapolation, and small â–³H, small â–³S and high T_m^o indicate typical characteristics of rigid main-chain liquid crystals and all of them are related to the irregularity, high rigidity of the chain and disordered conformation in crystals.3. A fundamental study on solid-state polymerization (SSP) of TLCP was performed for the first time in this thesis. The banded texture and mechanical properties of the TLCPs (ABA/HQA/NDA/TA=50/25/15/10) showed a substantial variation after SSP. The influences of various operating parameters, such as reaction time, reaction temperature, pellet size of the samples and nitrogen gas flow velocity, on SSP were investigated, and the SSP reaction mechanism and rate were discussed. The molar mass of SSP products was characterized by measurement of melting temperature (T_m) and inherent viscosity (I. V) It was found that the T_m increases with solid-state polymerization time in a certain range of molar mass and then trends a level at higher melting temperature, while I.V. increases with solid-state polymerization time in a broad molar mass range. Based on the increase in I. V. and T_m of the copolyesters, evidently, it can be concluded the molar mass of the copolyester substantially increases during SSP process.4. A diffusion- and reaction-controlled kinetic model for the SSP of TLCP was developed and the SSP was simulated for the first time on the basis of the model proposed. Specially, the model developed for SSP of TLCP in this study includes the main reversible chemical reaction, acidolysis, for chain extension of TLCP and a one-dimensional unsteady-state diffusion process of the by-product acetic acid. Computer simulation indicated that the variation picture of acetic acid concentration, carboxyl end-group concentration, molar mass of copolyesters with solid-state polymerization time from the core to the surface of a pellet, and that the influence of reaction temperature nitrogen gas flow rate and pellet size on the variation picture. The simulation results are essentially coincided with experimental observations, and it provides a better understanding of the SSP process of TLCP and a theoretical basis for polymerization process and equipment design.5. The SSP mechanism of TLCPs is strongly dependent on the operation conditions, such as reaction time, reaction temperature, pellet size of the samples and nitrogen gas flow velocity. At the lower temperature, the SSP reaction rate is mainly controlled by the chemical reaction. As the temperature increases, the SSP reaction rate turns to be determined by the by-product diffusion: interior diffusion for large-sized sample, surface diffusion for lower N₂ velocity. The SSP reaction mechanism is changed to chemical reaction control again with increasing reaction time due to the decrease in reactive end-group concentration.