| In recent years,with the development of electronic communication technology towards high frequency,high speed and high integration,higher requirements have been placed on the performance of copper clad laminates.The phenolic resin,epoxy resin and polyimide used in ordinary copper clad laminates can’t meet the needs of premium electronic products due to the shortcomings of high dielectric constant(Dk),large dielectric loss(Df)at high frequency,which will lead to significant transmission delay and transmission loss of signal.Therefore,searching for a dielectric material with low dielectric constant(low-Dk),low dielectric loss(low-Df),high mechanical strength,high thermal stability,low thermal expansion coefficient and low water absorption has become the focus of the high frequency communication industry.Thermotropic liquid crystal polymers(TLCPs)mainly represented by aromatic thermotropic liquid crystal polyesters are a class of polymers which can exhibit liquid crystal phase in melting state.TLCPs have been considered as ideal high performance 5G millimeter wave substrate and packing materials due to their outstanding properties.More specifically,the TLCPs exhibit stable dielectric constant and loss tangent in millimeter wave frequency range,low water absorption,low thermal expansion coefficient and good thermal stability.At present,the TLCP films and their raw resins used for copper clad laminates are mainly monopolized by American and Japanese enterprises.However,the domestication of TLCPs of film grade in our country moves slowly.TLCPs commonly have the characteristics of very low melt viscosity and easy orientation.Due to the lack of elasticity of the melt,the rupture of TLCP films is easy to occur during biaxial stretching and rotary blowing,making the processing of isotropic TLCP films very difficult.In this thesis,we focus on addressing mentioned two significant issues.The specific research contents are as follows:(1)The polymerization process and properties of HBA/HNA liquid crystal copolyester(trade name Vectra A950)consisted of 73 mol%HBA and 27 mol%HNA were investigated.There are many factors influence the molecular weight and properties of HBA/HNA liquid crystal copolyester,such as the purity of acetylated monomer,polycondensation temperature and time,vacuum degree,stirring speed,and additive.Copolyester with satisfactory color and properties was successfully synthesized under the following conditions:catalyst Mg(Ac)2(280 ppm);additive Irgafos 168(0.5 wt%)and Irganox 1010(0.5 wt%);polycondensation mixture was stirred for 1.5 h at 210～250℃,for 3 hat 250℃,for 1.5 h at 280℃,for 0.5 h at 290℃with reduced pressure,and then for 1 h at 310℃.The product has a melting point of 284.5℃,with excellent mechanical properties(tensile strength at 230 Mpa,charpy notched impact strength at 34.73 KJ/m2),low dielectric properties at high frequency(Dk=3.3@10GHz,Df=1.87×10-3@10GHz)and good thermal stability(Td5%=480℃at air condition).(2)We have carried the modification of TLCP to enhance the melt viscosity and melt strength by blending,in-situ melt polycondensation and reactive extrusion.The results show that the modification effects of reactive extrusion is the best,through which branched HBA/HNA liquid crystal copolyesters were gotten.The branching degree has slight effects on thermal properties,but noteworthy effects on rheological properties.Copolyester with higher branching degree exhibits higher storage and loss modulus,indicating that the melt strength and elasticity are significantly enhanced.The mechanical strength of branched copolyester is independent of branching degree,the tensile strength is about 245 MPa,and the charpy notched impact strength is about 30 KJ/m2.Copolyester with higher branching agent concentration exhibits higher Df(from 1.36×10-3 to 2.14×10-3 at 10 GHz).However,the Dk seems to be independent with branching degree(from 3.25 to 3.30 at 10 GHz).Base on the overall performance,the optimal content of ADR 4468 was found to be about 2～3 wt%. |
