Synthesis And Application Of Azobenzene Polyimide As Photoalignment Layer In Liquid Crystal Devices

Polyimides?PIs?are well-known materials for their excellent properties including thermal stability,mechanical,film-forming,chemical stability,and dielectric.They have been widely used in many high technology fields,such as aerospace,microelectronics,separation membrane,and liquid crystal?LC?alignment etc.It is the way to design PI-based photo-alignment film to introduce photosensitive groups into the PI backbone through the main chain or side chain.Among photosensitive compounds,azobenzene?azo?and its derivatives,are believed to be the most typical examples.Azo can isomerize reversibly between the trans and cis forms upon light irradiation or thermal treatment,accompanied by physical and chemical changes in the molecular shape and dipole moment.Azo-containing PIs?Azo-PIs?not only have the unique photo-responsive properties of the azo chromophores,but also carry on the outstanding thermal stability and orientation properties of PI materials.The side-chain azo-PIs have attracted much more attention because of the simple synthesis,the flexibility in molecular design,and lower energy barrier of photoisomerism.Thus,design and synthesis of novel side-chain azo-PIs and exploring the effects of molecular structures on their orientation properties,are of great importance to their potential application in LC devices?LCDs?.In this thesis,we reported the design and synthesis of a series of side-chain azo-PIs,as well as the test on their application as photo-alignment films,as shown in the following:?1?A series of photoresponding azo-PIs were prepared by a conventional two-step approach,i.e.,co-polycondensation of 4,4?-diaminodiphenyl ether?ODA?,4,4?-oxydiphthalic anhydride?ODPA?,and azo-containing diamines?Sn?bearing different alkyl spacer lengths at low temperature.All of the azo-PI films showed outstanding thermal stability with the glass transition temperature?T_g?in the range of 174²45°C.Their weight loss temperature of 5%(T_d5)and 10%(T_d10)were in the range of 370⁴65°C and 407⁵22°C in nitrogen,respectively.In addition,the char yields of all azo-PIs at 700°C in nitrogen still remained 41%or more of their original weights.The azo-PI films fabricated through a casting method showed good dimensional stability and mechanical performance with the tensile strengths of 64¹27 MPa,elongation at breaks of 3.1⁹.6%,respectively.The azo-PI films exhibited extremely high birefringence by using 405 nm continuous laser as the pump light.The birefringence values of PI1-S6 and PI1-S9 were found to be higher than those of the other films,with high?n values of 0.0134 for PI1-S6 and 0.0122 for PI1-S9 at room temperature.The birefringence decay to 54.8%of the saturation value at 150°C was comparable to that at room temperature.Finally,polarizing optical microscope?POM?was used to evaluate the alignment behaviors of the LCs by placing LC cells between its two crossed polarizers.All of the azo-PI films exhibited good alignment performance towards LC molecules in the LC cell.The pretilt angle of the azo-PIs was 1.1⁴.7°,and azimuthal anchoring energy was 6.63⁸.04×10^-6 J/m².?2?In order to improve the efficiency of LC alignment of PI,the mesogens units were introduced into the molecular structures.In detail,a series of co-PIs were prepared based on co-polycondensation of a pyrimidyl diamine monomer with a cyanobiphenyl mesogenic unit?M6?,S6,ODA,and ODPA.All of the co-PI films showed outstanding thermal stability with T_g over 200°C,T_d5 and T_d10 were in the range of 410⁴39°C and 430⁵19°C in nitrogen,respectively.In addition,the char yields of all co-PIs at700°C in nitrogen still maintained at 47.5%or more of their original weights.Co-PI films showed good dimensional stability and mechanical performance with the tensile strengths of 93.7¹10.5 MPa,elongation at breaks of 6.1⁷.6%.The birefringence value of PI-5 was found to be highest among the co-PI films,which is 0.0117 at room temperature.The birefringence decreased to 95.73%after relaxation in darkness for 80s at room temperature.The orientation stability and azimuthal anchoring energy of co-PIs were effectively improved by the introduction of cyanobiphenyl mesogenic units.?3?A novel nonplanar diamine containing azo moieties?DAzoTM?was synthesized via nucleophilic substitutuion reactions using 1-bromo-6-?4-methoxyazobenzene-4'-oxy?hexane,p-hydroxybenzaldehyde,and aniline as starting materials.Then it was polymerized with ODA,and ODPA through the two-step co-polycondensation procedure to yield azo-PIs.The thermal imidization kinetics parameters including activation energy and frequency factor of the conversion of PAA to PI were measured and calculated according to Ozawa and Kissinger methods.The results revealed that the introduction of azo units through a long spacer could effectively lower the activation energy barrier of imidization by reducing intermolecular interactions.All the azo-PI films still showed good thermal stability,with high T_d5values of above 400°C and T_g above 210°C.In addition,azo-PI films showed good mechanical performance and stability dielectric properties with the tensile strengths of105.2¹10.5 MPa,elongation at breaks of 8.5⁹.3%.The azo-PIs also had low water absorption of 0.76¹.21%.Furthermore,systematic study of the orientation properties of azo-PIs with different alignment conditions.The azo-PI films exhibited perfect photoalignment properties towards LC molecules among the direction vertical to UV polarization,and an appropriate pretilt angel can be obtained via adjusting the ratio of azo content.?4?Soluble PIs are important for their real application.Here,DAzoTM and ODA were polymerized with three dianhydrides?ODPA;4,4?-hexafluoroisopropylidene diphthalic anhydride,6FDA;1,2,4,5-cyclohexanetetracarboxylic dianhydride,CHDA?through the two-step copolycondensation at low temperature to produre PI?PI-ODPA,PI-6FDA?,and the one-step high-temperature copolycondensation at high temperature to produre semi-aliphatic PI?PI-CHDA?.All of the azo-PI films showed outstanding thermal stability with T_g over 241°C,T_d5 and T_d10 were in the range of 421⁴84°C and 461⁵45°C in nitrogen,respectively.The PI-CHDA was found to be easily dissolved in organic solvents,such as N-methyl-2-pyrrolidone?NMP?,N,N-dimethylformamide?DMF?,N,N-dimethylacetamide?DMAc?,dimethyl sulfoxide?DMSO?,m-Cresol,chlorlform?CHCl₃?,and dichloromethane?CH₂Cl₂?at room temperature.All of the azo-PI films exhibited good alignment performance towards LC molecules in the LC cell,in which the PI-CHDA films showed low threshold voltage and high voltage holding ratio?97.4%?.?5?M6 and ODA were polymerized with ODPA through the two-step low-temperature copolycondenation to produre PIx-M6 with high transparency and orientation properties.All of the PI films showed outstanding thermal stability with T_g in the range of 194²45°C,T_d5 and T_d10 were in the range of 420⁵12°C and 436⁵51°C in nitrogen,respectively.In addition,the char yields of all PIx-M6 system at700°C in nitrogen still maintained 47%or more of their original weights.PI films showed good dimensional stability and mechanical performance with the tensile strengths of 90¹27 MPa,elongation at breaks of 4.3⁹.3%.These PI films exhibited highly optical transparency with transmittance of up to 86%in the wavelength range450⁷00 nm.Evaluation of the adhesion force between PI films and LCs with a super-sensitive microelectro-mechanical balance system were performed.From these results,we have verified that appending mesogenic groups on the side chains of PI strengthens intermolecular interactions between the LC and PI surfaces,and conquently enhances the surface anchoring energy.Compared with the unmodified PI,the anchoring energy of the PIx-M6 system was improved more than 2.2-fold.