| Polyimide(PI)as a high-performance polymer,its film,fiber and other materials are widely used in aerospace and microelectronics fields.Polyimides are generally prepared by one-step or two-step method.Two-step method is the most commonly used.First,the solution of polyamic acid(PAA)is obtained by condensation of dianhydride and diamine in aprotic polar solvent at low temperature.Then polyimide was formed by chemical imidization of PAA with anhydride and tertiary amine or thermal imidization of PAA films or fibers.The properties of PAA solution and imidization process have significant influence on the properties of polyimide.However,polyamic acid as a precursor of polyimide,the determination of its molecular weight remains unsolved for a long time due to the complex interaction in PAA solution.Therefore,it is urgent to establish a simple and reliable method to determine the true molecular weight and its distribution and other molecular parameters of PAA.In addition,polyamic acid solution is unstable during storage,so it is necessary to study the degradation of polyamic acid solution under different storage conditions,so as to provide stable solution for regulation,processing,and further imidization of polyamic acid solution.In this dissertation,the solution properties of a fluorinated polyamic acid(6FDA-DMB)PAA was thoroughly studied.The optimized mobile phase was acquired after a series of gel permeation chromatography(GPC)experiments under different conditions to determine the true molecular weight and distribution of(6FDA-DMB)PAA.Then,the soluble polyimide was obtained by dehydration and cyclization under different thermal imidization conditions.The molecular parameters of(6FDA-DMB)PAA solution were studied at different storage temperatures and time.The purpose is to provide guidance for solution processing and solution property regulation of polyamic acid and polyimide.The main work of this dissertation is summarized below:1.The influence of the composition of the mobile phase including LiBr/DMF,LiBr/H3PO4/THF/DMF,LiBr/HAc/THF/DMF and LiBr/HAc/DMF on the elution behavior of(6FDA-DMB)PAA samples were studied by GPC coupled with multiple detectors.It was found that the electrostatic and hydrogen bond interaction were effectively inhibited by simultaneously adding LiBr and H3PO4 which prevent the adsorption of PAA on the GPC column.In order to solve the problem of mobile phase leakage caused by severe corrosion of H3PO4 on the GPC instrument,HAc was added to the mobile phase and could effectively replace H3PO4.Based on a series of experiments under different conditions,the optimized mobile phase composition suitable for characterization of the molecular weight of PAA was determined,namely 0.02MLiBr/0.2MHAc/DMF,which is mild and effective.And the mobile phase was proved to be valid for PAA with different chemical structures,which solved the difficult problem for determining the molecular weight of PAA.2.The molecular parameters of three polyamic acids including(6FDA-DMB)PAA,(6FDA-TFDB)PAA and(Ultem 1000)PAA were characterized in the optimized mobile phase 0.02MLiBr/0.20MHAc/DMF.The scale exponent a of the three polyamic acids in mobile phase is 0.74-0.78,v is 0.57-0.59 and ρ is 1.34-1.41,indicating that PAA molecular chains present random coil conformation and show good flexibility in the mobile phase.The lp values of the three PAA samples calculated by two different methods are 1.64-1.82nm and 2.64-3.66nm,respectively,indicating that the molecular chains show local rigidity in the mobile phase.The molecular weights of(6FDA-DMB)PAA,(6FDA-TFDB)PAA and(Ultem 1000)PAA samples measured by universal calibration and light scattering methods show discrepancy,which is related to the lower flexibility of PAA chain than PS.3.The effects of different thermal imidization conditions(i.e.,multi-step treatment and isothermal treatment)on the molecular weights of(6FDA-DMB)PAA were studied.FT-IR was used to study the degree of imidization of(6FDA-DMB)PAA films at different time and temperature of stepwise heating and isothermal treatment.The molecular weights of the samples under different imidization conditions were monitored by GPC.In the process of stepwise heating,no imidization reaction occurs at 80℃.When the temperature was 200℃ and the reaction time was 60 min,the reaction was slow and nearly 80%of the amic acid groups were changed into imide groups.Most imidization occurred at 250～300℃,and the degree of imidization was up to 90%.When the temperature is further raised to 300℃-350℃,the imidization is completed.In the process of multi-step treatment,the molecular weight decreased first and then increased with temperature.In the process of isothermal treatment,PAA samples were kept at 200℃,250℃ and 300℃for 60 min,respectively.When the temperature was raised from 200℃ to 300℃,the degree of imidization value was increased from76.61%,94.01%to 104.90%.A relatively high degree of imidization can be achieved by heating at a relatively high temperature for short time.For example,when(6FDA-DMB)PAA was treated at 350℃ for 10min,the imidization is 100%completed.In addition,during isothermal treatment,the relative molecular weight of(6FDA-DMB)PAA decreased first and then increased.The molecular weight of the final polyimide was smaller than the initial molecular weight of PAA.The Mw was decreased by 10.7%at 200℃ for 120min,39.0%at 250℃ for 60min,63.8%at 300℃ for 60minand 34.3%at 350℃for 60min.By comparing the two thermal imidization methods,pre-imidization at low temperature and then complete imidization at high temperature for a short time was recommended based on our results.Treatment at 150-250℃ should be avoid to obtain relatively stable(6FDA-DMB)PI samples with high molecular weights.4.The stability of concentrated and dilute solution of(6FDA-DMB)PAA at different storage temperature and time was studied by using GPC coulpled with multi-detectors.The 12%concentrated(6FDA-DMB)PAA solution was monitored at-18℃,-12℃,4℃ and 25℃ for 139 days,respectively.It was found that the molecular weights of PAA remained basically unchanged when stored at-18℃ for 139 days.When the storage temperature increases from-12℃ to 4℃ and 25℃,Mw decreases by 7.2%,34.7%and 83.8%,and Mn decreases by 4.7%,30.0%and 82.4%after storage for 139 days,respectively.With the increase of temperature,the average scission of each molecular chain increased from 0.05,0.43 to 4.67.When stored at the same temperature(25℃),the dilute solution of(6FDA-DMB)PAA(0.15%)is less stable than the concentrated solution.After being stored at 25℃ for 10h,the M and Mn of PAA dilute solution decreased by 52.8%and 48.7%,respectively,which was similar to the degradation degree of the concentrated solution stored at the same temperature for 15 days.PAA was almost linearly degraded in the dilute solution during storage at 25℃ for 10h.The simultaneous reduction of Mw and Mn indicates that PAA degradation does not follow the chain length equilibrium mechanism previously reported.The properties of polyamic acid solution and the changes of molecular weight during the process of thermal imidization were thoroughly investigated,which can provide guidance for the storage and imidization of polyamic acid solution. |
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