Preparation And Properties Of Polyimides Containing Cycloaliphatic Structure

Aromatic polyimide has excellent comprehensive properties,such as good heat resistance,high mechanical strength,good dielectric properties,and outstanding radiation resistance.However,most of the conventional polyimides have yellow or brownish red color,poor solubility and high processing temperature,which limit their applications in some fields.The development of some special fields requires polyimide materials with special properties,such as high transparency,good solubility and low dielectric constant,which has been one of the research hotspots.This paper briefly introduced the development history,classification,performance,application and synthesis methods of polyimides,and summarizes structure modification methods and their structure-properties relationships of polyimides.On this basis,from a perspective to design and synthesize alicyclic based monomers,a series of functional moieties including hydrogenated bisphenol A,isosorbide,isomannitol,etc.were introduced into the backbone of polyimides.The chemical structures of monomers and polyimides were confirmed by FT-IR,¹H NMR,¹³C NMR,COSY,HSQC,HRLC-MS.The properties of polyimide were systematically tested by DSC,DMA,TGA,TMA,UV-Vis,WAXD,dielectric constant instrument,universal testing machine,etc.,and the results were discussed in detail.The main research contents and conclusions were as follows:（1）In this work,HBPA based dinitro isomer mixture were synthesized via nucleophilic substitution,and dinitro 1a’and 1a were successfully separated by vacuum distillation under the monitor of DSC and ¹H NMR.Corresponding diamines were respectively polycondensed with five commercial dianhydrides via thermal imidization to obtain ten kinds of polyimide films.The glass transition temperature（T_g）was208²92°C（DMA）,the 5%thermal weight loss in the nitrogen atmosphere is 419⁴34°C,and the carbon residue rate at 14%³4%,indicating their good thermal stability.Polyimides containing equatorial C-O owned higher decomposition temperature than polyimides containing axial C-O.¹H NMR illustrated that no conformation changes took place during the process of polyimides.The tensile strength,tensile modulus and elongation at break were 72¹17 MPa,1.1³.2 GPa and 8.3¹2.6%,respectively.Most of the obtained polyimide films possessed excellent solubility in common organic solvents including DMAc,DMF,NMP and low-boiling solvents like dichloromethane and chloroform.Moreover,these polyimides were transparent,when the dianhydride was ODPA,BPADA or 6FDA,no apparent difference was found either in transmittance or solubility between two series of polyimides,this could be the reason that twisted and flexible ether linkages,as well as bulky substituent led to“already weakened”inter-and intramolecular CT interaction and cohesive force.However,when came to rigid and stiff dianhydride,e.g.BPDA,PI-3’took an obvious advantage over PI-3 on aspects of transmittance and solubility,which was possibly owing to larger molecular chain d-space imparted by axial C-O.（2）（Trans,trans）-HBPA was successfully recrystallized from HBPA.The purity,melting point and chemical structure of（trans,trans）-HBPA were confirmed by GC-Mass,DSC and ¹H NMR,respectively.The dinitro,diamine and dianhydride monomers（HBPADA）were synthesized via nucleophilic substitution,reduction,hydrolysis,and cyclization.PI-（1-5）were prepared from HBPADA and five commercial diamines,and PI-6 from HBPADA and diamine bearing（trans,trans）-HBPA unit.¹H NMR spectra of the polyimides indicated that isomers of HBPA moieties had no conformation changes during the preparation of polymers.The T_g of the polyimides was 214²66°C（DSC）,the 5%thermal weight loss in the nitrogen atmosphere was 386⁵23°C,and the carbon residue rate was at 1.8%⁵2%,thermal expansion coefficient（CTE）locate was at56⁶2 ppm×K^-1,indicating their good thermal stability.The tensile strength,tensile modulus and elongation at break were 85¹22 MPa,0.8¹.4 GPa and 8¹1%,respectively,which were parallel to aromatic PI-7.When the content of the alicyclic residues was 52%,the tensile strength of the polyimide decreased.These polyimides exhibited good solubility in DMAc,DMF,NMP,etc.,some of them were soluble in low boiling solvents like chloroform and dichloromethane.Also,the polyimide films possessed high transmittance,low water uptakes and dielectric constant.Suitable alicyclic content in the backbone of polyimide can improve the transmittance and solubility,but when the content was too high,the thermal and mechanical properties of the polyimide were sacrificed.（3）Two dinitro monomers were designed and synthesized via solvent free melt-heating method,after reduction,two diamines bearing isosorbide moiety were polycondenced with five kinds of common dianhydrides via a two-step thermal imidization.The T_g of polyimides was 207²80°C（DSC）,the 5%thermal weight loss in the nitrogen atmosphere was 427⁵36°C,and the carbon residue rate was 31%⁵4%,thermal expansion coefficient was 46 ppm×K^-162 ppm×K^-1,indicating their good thermal stability.The tensile strength,tensile modulus and elongation at break of these polyimides were 92¹31 MPa,2.1³.2 GPa and 7²4%,respectively.The comparsion of isosorbide based PI-（1-7）and PI-8 illustrated that isosorbide did not compromise the mechanical property of polyimides.These polyimide films can be dissolved in DMAc,DMF,NMP,as well as in low boiling point solvents.The introduction of the fluorinated group and isosorbide into the molecular skeleton of the polyimide improved the transmittance of the polyimide（72-87%at 450 nm）,and trifluoromethyl was more effective than isosorbide in increasing solubility and transmittance of polyimide films.Also,these polyimides exhibited lower water absorption and dielectric constant,which was related to the higher fluorine content and the introduction of isosorbide in the polyimide backbone.（4）IMDA bearing 1,4;3,6-dianhydrocyclohexitol was prepared via nucleophilic substitution,hydrolysis,and cyclization.IMDA was polycondenced with four commercialized diamines and three alicyclic diamines to produce PI-（1-7）.The T_g of these polyimides was 214³00°C（DSC）,the 5%thermal weight loss in the nitrogen atmosphere was 415⁴53°C,and the carbon residue rate was 33%⁴7%,CTE located at 25 ppm×K^-155 ppm×K^-1,indicating their good thermal stability.The tensile strength,tensile modulus and elongation at break were 79¹53 MPa,1.9⁴.2 GPa and 6¹5%,respectively.The mechanical of polyimide films still maintained high level when the biobased content was up to 30%,implying these alicyclic residues possessed rigid and stiff structure.These polyimides were colorless and soluble in commen solvents.It was concluded that 1,4;3,6-dianhydrohexitol fragment in dianhydride was more determinant in solubility and transmittance of polyimides than that in diamine.Meanwhile,low water uptakes and dielectric constant had close relationships with hydrophobic effects of alicyclic moieties and fluorine atoms.