Study On The Preparation And Structure-Properties Of Polyimides With Main Chain Containing Alicyclic Units

Aromatic polyimides continue to receive much attention because of their excellent thermal,mechanical,electrical and chemical properties.However,the aromatic polyimides are also responsible for the poor solubility and tractability,which cause difficulties in processing.The introduction of cycloaliphatic units into polyimide backbone would not only enhance processability but also maintain high thermal stability.Meanwhile,the cycloaliphatic units were also expected to bring many other properties,such as good solubility,optical transparency and dielectric properties as well as good thermal stability.So,these polyimidies have extensive application prospects in photosensitive materials,gas separation membranes,modern microelectronics industry,liquid crystal display,etc.The studies in this dissertation have exactly focused on the synthesis, characterization and structure-properties relationship of polyimides with main chain containing alicyclic units.These objectives were accomplished by designing and synthesizing several novel diamines containing alicyclic units via change the flexible linkage and substituent position.And then these novel diamines were employed in preparation of the modified polyimides via polycondensation with four dianhydrides. Meanwhile,the resulting polyimides were characterized in detail and the relationship between structures and properties were studied in profound terms.The knowledge get out from the research can make new contributions to the development of high performance and function polyimides.The final properties of polymers depend on the monomers to a great extent.So, the design,synthesis and development of novel polyimides mostly depend on the preparation of new monomers.In this dissertation,four novel diamine monomers: 1,4-bis（4-aminophenoxymethylene）cyclohexane（BAMC）,1,4-bis （3-aminophenoxymethylene）cyclohexane（mBAMC）,1,4-bis （4-aminobenzoyoloxymethyl）cyclohexane（BAZMC）and 1,4-bis （3-aminobenzoyoloxymethyl）cyclohexane（mBAZMC）were synthesized and characterized respectively.The characterization results revealed that these monomers were obtained with high purity and high yield,and can meet the needs of polymerization.The diamine monomers were synthesized by a two-step method. Firstly,dinitro componds were prepared from 1,4-cyclohexanedimethanol（CHDM） via etherification or esterification reaction.Secondly,objective diamines were obtained by catalytic reduction of dinitro componds.The etherification reaction was acomplished by Williamson method and Mitsunobu method respectively.A series of polyimides were prepared from the diamines and commercial aromatic dianhydrides,such as PMDA,ODPA,BTDA and BPDA via chemical imidization.The resulting polyimides were investigated by FTIR,NMR and EA, which revealed that the imide cyclization was accomplished completely.Judging from the inherent viscosities of precursor polyamic acids,the resulting polyimides were very high in molecular weight.Meanwhile,the reactivities of the monomers were analyzed according to their chemical structures and experimental phenomena,The results of reactivities of diamines were as follows:BAMC＞mBAMC＞BAZMC＞mBAZMC;the reactivities of dianhydrides have no large difference and the results were PMDA＞BTDA＞BPDA＞ODPA.All the modified polyimides were characterized and their properties were studied in detail.The results showed that some polyimides derived from diamines with m-ether and ester group and dianhydrides with ether and carbonyl group have excellent solubility in common organic solvents.It should be noted that the good solubility in even common low boiling point solvent is useful to produce polyimide films or coating at relatively low processing temperature,which is desirable for advanced applications.The crystallinity of the new polyimides was estimated by means of wide-angle X-ray diffraction analysis（WAXD）,from the results it can be concluded that the presence of alicyclic and aliphatic（-CH₂-O-,-CH₂-O-CO-）units decreased the intermolecular forces between the polymer chains and reduced the crystallinity of the polymers,but PI-5（mBAMC/PMDA）,PI-8（mBAMC/BPDA）and PI-9（BAZMC/ODPA）still showed a fair degree of crystallinity,and the melting temperature measured by DSC were at around 350℃,which reveled that these polyimides have the possibility of melt processing.Thermal properties of the polyimides were evaluated by means of DSC and TGA.The results showed that these polyimides have good thermal stability with high initial thermal decomposition temperature.DSC showed that most polyimides have two glass transition temperatures mainly because the introduction of modified group into polyimide backbone results in distinct soft and rind segments in macromolecular chains. Thermal degradation kinetics of the modified polyimides was studied.The apparent activation energy values obtained by Ozawa,Kissinger and KAS method were found to be lower than that of aromatic polyimides under nitrogen.This may be due to the aromatic polyimides are more stable and requires higher activation energy for its decomposition.Dynamic thermal decomposition of resulting polyimides was monitored by TG/IR technique,which illustrated that all the polyimides experienced a double-stage decomposition process in nitrogen atmosphere.The first stage can be interpreted to the degradation of flexible linkages.The second stage happened mainly due to the decomposition of heterocyclic structures.The IR spectra of evolved gases showed that hydrocarbons were the major evolved gases during the first stage degradation.The major degraded products were CO₂,CO and H₂O during the second decomposition.Next,four polyimide films were produced by polycondensation of BAMC and several dianhydrides via thermal imidization.Their properties were discussed and compared with polyimides powders produced by chemical imidization.The results indicated that step-heating method can overcome the problem of kinetic interrupt and the cyclizations of PAAs can accomplish completely at 250℃which reduced 50℃than traditional cyclization temperature.TGA results showed that the initial decomposition temperature of films were slightly high than polyimides obtained by chemical imidization because the macromolecular chain were crosslinked in the thermal treatment process.The results of investigation on thermal degradation kinetics indicated that the thermal decomposition mechanism of PI films and PI powders were difference.PI powders are susceptible to degradation at high temperatures.The result of dynamic thermal degradation revealed that the evolved gases of PI films were similar with that of PI powders.The major degraded products were hydrocarbons,CO₂,CO,H₂O and aromatic compounds.The results of mechanical property test indicated that the PI films have high static and dynamic modulus.The X-ray diffraction curves of these polyimides express a set of wider diffraction peaks,these should be evidence that indicate the polyimides holding amorphous morphologies. The UV-vis spectra of the modified polyimide films showed that the cutoff wavelength ranged from 400 to 420nm and the transmittances in the visible region was 80-90%.The transmittances of the films at 500nm were over 80%.The excellent optical transparency of the polyimides can probably be attributed to the decrease in ability for the formation of intermolecular charge-transfer-complexes（CTC）between the electron-donor segments（diamine）and electron-acceptor segments（dianhydrides） in the polymer backbones due to the presence of the alicyclic units,which are electron-withdrawing substituents,capable of reducing the CTC formation.In sum,experimental results indicate that the novel diamine monomers have good polymerizability and can be used to produce polyimides and other heterocyclic polymers.The novel polyimides obtained have excellent thermal stability,good mechanical properties and excellent optical transparency.The cycloaliphatic structure in the polymer backbones is not only responsible for enhancing the solubility but also is responsible for maintaining the thermal properties.So,these polyimides have a great potential application prospect in optical and microelectronic fields.