Storage Stability of 6FDA-DMB Polyamic Acid Solution Detected by Gel Permeation Chromatography Coupled with Multiple Detectors

Polyamic acid (PAA) is the precursor of polyimide (PI), and its solution’s properties have a direct influence on the final performances of PI resins, films, or fibers. The viscosity loss of a PAA solution over time is notorious. A stability evaluation and revelation of the degradation mechanism of PAA in a solution based on variations of molecular parameters other than viscosity with storage time is necessary. In this study, a PAA solution was prepared through the polycondensation of 4,4′-(hexafluoroisopropene) diphthalic anhydride (6FDA) and 4,4′-diamino-2,2′-dimethylbiphenyl (DMB) in DMAc. The stability of a PAA solution stored at different temperatures (−18, −12, 4, and 25 °C) and different concentrations (12 wt% and 0.15 wt%) was systematically investigated by measuring the molecular parameters, including Mw, Mn, Mw/Mn, Rg, and [η], using gel permeation chromatography coupled with multiple detectors (GPC-RI-MALLS-VIS) in a mobile phase 0.02 M LiBr/0.20 M HAc/DMF. The stability of PAA in a concentrated solution decreased, as shown by the reduction ratio of Mw from 0%, 7.2%, and 34.7% to 83.8% and that of Mn from 0%, 4.7%, and 30.0% to 82.4% with an increase of temperature from −18, −12, and 4 to 25 °C, respectively, after storage for 139 days. The hydrolysis of PAA in a concentrated solution was accelerated at high temperatures. Notably, at 25 °C, the diluted solution was much less stable than the concentrated one and exhibited an almost linear degradation rate within 10 h. The Mw and Mn decreased rapidly by 52.8% and 48.7%, respectively, within 10 h. Such faster degradation was caused by a greater water ratio and less entanglement of chains in the diluted solution. The degradation of (6FDA-DMB) PAA in this study did not follow the chain length equilibration mechanism reported in literature, given that both Mw and Mn declined simultaneously during storage.


Introduction
Polyimide (PI) is one of the high-performance polymers with merits, including hightemperature resistance, high mechanical strength, and excellent electrical properties, and is widely applied in aerospace, electronics, electrical, and other high-tech fields [1,2]. Usually, dianhydride and diamine are polycondensed in polar aprotic solvent such as N,Ndimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or N-methyl-2-pyrrolidone (NMP) to produce the precursor of PI: polyamic acid (PAA), which transforms to PI through thermal or chemical imidization.
Indeed, the properties of a PAA solution play a decisive role in the final performance of the PI products [3,4]. However, PAA solutions are relatively unstable upon storage and

Preparation of (6FDA-DMB) PAA Concentrated Solution
The synthesis of (6FDA-DMB) PAA was the same as our previous report [20]. Specifically, DMB was first dissolved in DMAc by stirring at room temperature under a nitrogen atmosphere. Next, stoichiometric amounts of 6FDA were added to react with DMB at 20 °C under a nitrogen atmosphere for 48 h to achieve a (6FDA-DMB) PAA solution with a concentration of 12 wt%. The solution was sealed and stored in the refrigerator at −18 °C. The formation of (6FDA-DMB) PAA is displayed in Scheme 1. Scheme 1. Formation of (6FDA-DMB) PAA through polycondensation.

Storage Conditions of (6FDA-DMB) PAA Concentrated and Diluted Solution
The concentrated solution of (6FDA-DMB) PAA was sealed in brown bottles and stored at different temperatures (−18 °C, −12 °C, 4 °C, and 25 °C) with a fluctuation range of ±1 °C for different days (0, 2, 10, 15, 30, and 139 days). Samples were taken out at various intervals for a GPC-RI-MALLS-VIS measurement.
The diluted solution of (6FDA-DMB) PAA was obtained by diluting the concentrated one to 0.15% and subsequently sealing it in brown bottles and storing it at 25 °C for different hours (0, 1, 3, 6, and 10 h). Samples were taken out periodically for a GPC-RI-MALLS-VIS measurement.

GPC-RI-MALLS-VIS Characterization
The GPC-RI-MALLS-VIS system includes a Waters 515 pump; Waters 717 plus autosampler; Waters 2414 differential refractive index (RI) detector; DAWN HELEOS II eighteen-angle laser light scattering (MALLS) detector (Wyatt Technologies, Santa Barbara, USA), including 22.5°, 28.0°, 32.0°, 38.0°, 44.0°, 50.0°, 57.0°, 64.0°, 72.0°, 81.0°, 90.0°, 99°, 108°, 117.0°, 126.0°, 134.0°, 141.0°, and 147.0°, where 99° is a dynamic light scattering (DLS) detector; ViscoStar viscometer (VIS, Wyatt Technologies); and two PLgel 10 μm MIXED-B LS columns (300 × 7.5 mm, Agilent Technologies, Beijing, China). The temperature of the columns and three detectors were 35 °C. The mobile phase was 0.02 M LiBr/0.20 M HAc/DMF. The flow rate was 1.0 mL/min, and the injection volume was 100 μL. Pure toluene of HPLC purity was used for calibration of the light-scattering photometer. The normalization coefficient of the MALLS detector was obtained using polystyrene narrow standards with a molecular weight of 19,000 g/mol. The calibration constant of the RI detector was determined also by this polystyrene narrow standard. The volume delay between the MALLS and RI detectors and the MALLS and VIS detectors were determined using the polystyrene narrow standard. The Berry method [21] with a linear fit was applied to the MALLS data processing.
All samples for GPC measurements were dissolved in the mobile phase, with a concentration of about 1.5 mg/mL, and filtered through 0.20 μm or 0.45 μm PTFE membranes.

Molecular Parameter Characterization of (6FDA-DMB) PAA
The optimal mobile phase for a GPC measurement of PAA is 0.02 M LiBr/0.20 M HAc/DMF, as determined by our group in a preliminary study [20]. The dn/dc of (6FDA-DMB) PAA in this mobile phase is 0.145 mL/g. The multi-detector elution curves of (6FDA-DMB) PAA are shown in Figure 1. As can be seen from Figure 1a, all the elution curves measured by the refractive index (RI) detector, light scattering (LS) at 90° detector, Scheme 1. Formation of (6FDA-DMB) PAA through polycondensation.

Storage Conditions of (6FDA-DMB) PAA Concentrated and Diluted Solution
The concentrated solution of (6FDA-DMB) PAA was sealed in brown bottles and stored at different temperatures (−18 • C, −12 • C, 4 • C, and 25 • C) with a fluctuation range of ±1 • C for different days (0, 2, 10, 15, 30, and 139 days). Samples were taken out at various intervals for a GPC-RI-MALLS-VIS measurement.
The diluted solution of (6FDA-DMB) PAA was obtained by diluting the concentrated one to 0.15% and subsequently sealing it in brown bottles and storing it at 25 • C for different hours (0, 1, 3, 6, and 10 h). Samples were taken out periodically for a GPC-RI-MALLS-VIS measurement.

GPC-RI-MALLS-VIS Characterization
The GPC-RI-MALLS-VIS system includes a Waters 515 pump; Waters 717 plus autosampler; Waters 2414 differential refractive index (RI) detector; DAWN HELEOS II eighteen-angle laser light scattering (MALLS) detector (Wyatt Technologies, Santa Barbara, USA), including 22. . The temperature of the columns and three detectors were 35 • C. The mobile phase was 0.02 M LiBr/0.20 M HAc/DMF. The flow rate was 1.0 mL/min, and the injection volume was 100 µL. Pure toluene of HPLC purity was used for calibration of the light-scattering photometer. The normalization coefficient of the MALLS detector was obtained using polystyrene narrow standards with a molecular weight of 19,000 g/mol. The calibration constant of the RI detector was determined also by this polystyrene narrow standard. The volume delay between the MALLS and RI detectors and the MALLS and VIS detectors were determined using the polystyrene narrow standard. The Berry method [21] with a linear fit was applied to the MALLS data processing.
All samples for GPC measurements were dissolved in the mobile phase, with a concentration of about 1.5 mg/mL, and filtered through 0.20 µm or 0.45 µm PTFE membranes.

Molecular Parameter Characterization of (6FDA-DMB) PAA
The optimal mobile phase for a GPC measurement of PAA is 0.02 M LiBr/0.20 M HAc/DMF, as determined by our group in a preliminary study [20]. The dn/dc of (6FDA-DMB) PAA in this mobile phase is 0.145 mL/g. The multi-detector elution curves of (6FDA-DMB) PAA are shown in Figure 1. As can be seen from Figure 1a, all the elution curves measured by the refractive index (RI) detector, light scattering (LS) at 90 • detector, and viscometer (VIS) were symmetrical and unimodal. In addition, the logarithm of molecular weight and the elution volume had a good linear relationship over a wide range of elution volume. Figure 1b shows the variation of scattered light intensity Kc/R(θ) with scattering angles sin 2 (θ/2). The data points from the lowest three detection angles and the highest detection angle deviated significantly from the overall linear relationship and were abandoned for calculating the molecular weights. These results indicate that the GPC separation is based on a size exclusion effect in the elution process of the (6FDA-DMB) PAA in columns. Figure 1c shows the molecular weight distribution curve for the original (6FDA-DMB) PAA. and viscometer (VIS) were symmetrical and unimodal. In addition, the logarithm of molecular weight and the elution volume had a good linear relationship over a wide range of elution volume. Figure 1b shows the variation of scattered light intensity Kc/R(θ) with scattering angles sin 2 (θ/2). The data points from the lowest three detection angles and the highest detection angle deviated significantly from the overall linear relationship and were abandoned for calculating the molecular weights. These results indicate that the GPC separation is based on a size exclusion effect in the elution process of the (6FDA-DMB) PAA in columns. Figure 1c shows the molecular weight distribution curve for the original (6FDA-DMB) PAA.  Table 1. The mass recovery of the PAA sample was 91.5%, implying there was almost no adsorption between PAA and the chromatographic columns. The Mw was 247.3 kg/mol, the Mn was 153.1 kg/mol, the Mw/Mn was 1.62, the zaverage radius of gyration Rg,z was 29.6 nm, and [η] was 161.4 mL/g.  Figure 2 shows the elution curves from detectors and the molecular weight distribution of (6FDA-DMB) PAA in the concentrated solution stored at −18 °C for different times. The detailed molecular parameters are listed in Table 2. The mass recoveries of the (6FDA-DMB) PAA samples stored at −18 °C for different days were all more than 90%. During the storage for 139 days, the Mw varied in the range of 241.8-253.1 kg/mol, and the Mn varied in the range of 147.7-154.0 kg/mol. The errors for both Mw and Mn were within 5%. All other molecular parameters, including molecular weight distribution Mw/Mn, the root-mean-square radius of gyration Rg, and intrinsic viscosity [η], changed little. These results show that the molecular parameters of (6FDA-DMB) PAA in a concentrated solution stored at −18 °C remain constant for 139 days.  Table 1. The mass recovery of the PAA sample was 91.5%, implying there was almost no adsorption between PAA and the chromatographic columns. The M w was 247.3 kg/mol, the M n was 153.1 kg/mol, the M w /M n was 1.62, the z-average radius of gyration R g,z was 29.6 nm, and [η] was 161.4 mL/g.

Molecular Parameter Changes of (6FDA-DMB) PAA in Concentrated Solution under Different Storage Conditions
GPC-RI-MALLS-VIS was employed to monitor the changes of molecular parameters such as M w , M n , M w /M n , R g , and [η], of the (6FDA-DMB) PAA in concentrated solution stored at −18 • C, −12 • C, 4 • C, and 25 • C for different times. Figure 2 shows the elution curves from detectors and the molecular weight distribution of (6FDA-DMB) PAA in the concentrated solution stored at −18 • C for different times. The detailed molecular parameters are listed in Table 2. The mass recoveries of the (6FDA-DMB) PAA samples stored at −18 • C for different days were all more than 90%. During the storage for 139 days, the M w varied in the range of 241.8-253.1 kg/mol, and the M n varied in the range of 147.7-154.0 kg/mol. The errors for both M w and M n were within 5%. All other molecular parameters, including molecular weight distribution M w /M n , the root-mean-square radius of gyration R g , and intrinsic viscosity [η], changed little. These results show that the molecular parameters of (6FDA-DMB) PAA in a concentrated solution stored at −18 • C remain constant for 139 days.   Figure 3 shows the elution curves obtained from RI and LS detectors and the molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at −12 °C for different amounts of days. The calculated molecular parameters are summarized in Table 3. All samples showed a mass recovery higher than 90%, implying that there was almost no adsorption of PAA to the GPC columns. At days 0, 2, 10, 15 Figure 3 shows the elution curves obtained from RI and LS detectors and the molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at −12 • C for different amounts of days. The calculated molecular parameters are summarized in Table 3. All samples showed a mass recovery higher than 90%, implying that there was almost no adsorption of PAA to the GPC columns. At days 0, 2, 10, 15, 30, and 139, the M w was 247. 3 Figure 4 shows the elution curves detected from RI and LS detectors and the corresponding molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at 4 °C for different times. It can be seen that the RI and LS elution curves for different storage times were not coincident and the initial elution volume delayed with the storage time, indicating that the molecular weight of (6FDA-DMB) PAA decreased with the storage time at 4 °C. The calculated molecular parameters are listed in Table 4. The mass recoveries of all samples were above 90%. The Mw decreased gradually from 247.3 kg/mol to 161.6 kg/mol and the Mn decreased from 153.1 kg/mol to 107.3 kg/mol when the storage time increased from 0 day to 139 days. A reduction in the polydispersity index Mw/Mn from 1.62 to 1.51 was observed after the solution was stored for 139 days. The Rg also showed a decrease from 29.6 nm to 23.4 nm, and the [η] decreased from 161.4 mL/g   Figure 4 shows the elution curves detected from RI and LS detectors and the corresponding molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at 4 • C for different times. It can be seen that the RI and LS elution curves for different storage times were not coincident and the initial elution volume delayed with the storage time, indicating that the molecular weight of (6FDA-DMB) PAA decreased with the storage time at 4 • C. The calculated molecular parameters are listed in Table 4. The mass recoveries of all samples were above 90%. The M w decreased gradually from 247.3 kg/mol to 161.6 kg/mol and the M n decreased from 153.1 kg/mol to 107.3 kg/mol when the storage time increased from 0 day to 139 days. A reduction in the polydispersity index M w /M n from 1.62 to 1.51 was observed after the solution was stored for 139 days. The R g also showed a decrease from 29.6 nm to 23.4 nm, and the [η] decreased from 161.4 mL/g to 127.0 mL/g after 139 days. Evidently, M w , M n , R g , and [η] show a gradual decline, and the molecular weight distribution becomes narrower with storage time.    Figure 5 displays the RI and LS signals as a function of elution volume and the molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at 25 °C for different times. Figure 5a,b show that the elution curves shifted to higher elution volume with the storage time, indicating that the molecular weights of (6FDA-DMB) PAA dramatically decreased with the storage time at 25 °C. The obtained molecular parameters are listed in Table 5. PAA samples had no adsorption on the column packings due to a mass recovery higher than 90%. After storage for 139 days, the Mw decreased from 247.3 kg/mol to 40.1 kg/mol, with a decrease ratio of up to 83.8%, and the Mn correspondingly decreased from 153.1 kg/mol to 27.0 kg/mol, with a reduction ratio of 82.4%. The   Figure 5 displays the RI and LS signals as a function of elution volume and the molecular weight distribution of (6FDA-DMB) PAA in a concentrated solution stored at 25 • C for different times. Figure 5a,b show that the elution curves shifted to higher elution volume with the storage time, indicating that the molecular weights of (6FDA-DMB) PAA dramatically decreased with the storage time at 25 • C. The obtained molecular parameters are listed in Table 5. PAA samples had no adsorption on the column packings due to a mass recovery higher than 90%. After storage for 139 days, the M w decreased from 247.3 kg/mol to 40.1 kg/mol, with a decrease ratio of up to 83.8%, and the M n correspondingly decreased from 153.1 kg/mol to 27.0 kg/mol, with a reduction ratio of 82.4%. The narrowing of the molecular weight distribution was indicated by the reduction of M w /M n from 1.62 to 1.49. The R g gradually decreased from 29.6 nm, 25.9 nm, 21.5 nm, 20.6 nm, and 18.8 nm to 11.6 nm, and the [η] gradually decreased from 161.4 mL/g, 137.5 mL/g, 117.2 mL/g, 105.4 mL/g, and 94.8 mL/g to 51.5 mL/g. The M w , M n , R g , and [η] absolutely decreased intensely with storage time at 25 • C, and the molecular weight distribution tended to be narrower. As shown in Figure 5d, the degradation of the polyamic acid was faster in the first 15 days, and the molecular weight was reduced to about half of the initial value; after that, the degradation gradually slowed down. Even though the solvent used in this study was HPLC grade, water was inevitably present. The o-carboxylic acid group acted as an intramolecular catalyst for the amide bond cleavage [22]. The resulting anhydrides reacted with trace water in the solvent to form diacids, which were inert toward the polyamic acid-forming reaction. As a result, the molecular parameters, such as M w , M n , R g , and [η], decreased with storage time.  Figure 5d, the degradation of the polyamic acid was faster in the first 15 days, and the molecular weight was reduced to about half of the initial value; after that, the degradation gradually slowed down. Even though the solvent used in this study was HPLC grade, water was inevitably present. The o-carboxylic acid group acted as an intramolecular catalyst for the amide bond cleavage [22]. The resulting anhydrides reacted with trace water in the solvent to form diacids, which were inert toward the polyamic acid-forming reaction. As a result, the molecular parameters, such as Mw, Mn, Rg, and [η], decreased with storage time.    The changes of M w , M n , and scissions per chain of (6FDA-DMB) PAA in a concentrated solution stored at −18 • C, −12 • C, 4 • C, and 25 • C for different times are summarized in Figure 6. The retention of M w and M n were obtained by calculating the percentage of molecular weights at any time to the original molecular weights. The scissions per chain were calculated using (M n,0 − M n,i )/M n,i , where M n,0 is initial M n and M n,i is M n at any time. The M w and M n of (6FDA-DMB) PAA remained basically unchanged at −18 • C with storage time, and no chain scission occurred. The M w and M n dropped slightly at −12 • C with storage time. After storage at −12 • C for 139 days, the reduction ratio of M w was 7.2%, while that of M n was 4.7%. The scissions per chain were about 0.05, which means that only one out of every 20 chains broke down. The M w and M n stored at 4 • C were reduced by 34.7% and 30.0%, respectively. Meanwhile, the R g and [η] also exhibited a decrease of 20.9% and 21.3%, respectively. Those reductions in molecular parameters suggested that PAA was degraded at 4 • C and one out of every 2.5 chains broke down, indicated by the scissions per chain value of 0.43. When the storage temperature increased to 25 • C, all the molecular parameters decreased much more rapidly compared with the other three temperatures. The M w and M n were found to decrease by 83.8% and 82.4%, respectively, after storage at 25 • C for 139 days. The breakage of amide bonds was about five scissions per chain. The R g and [η] also showed a pronounced loss of 60.8% and 68.1%, respectively. In addition, the decline rates of M w and M n were faster in the first 15 days by a drop of 53.2% and 51.4%, respectively. At this point, one break of amide bonds occurred on each chain, and then the chain scission slowed down. It is noteworthy that M w decreased faster than M n at different temperatures, which resulted in a narrower molecular weight distribution. At -12, 4 and 25 • C, the polydispersity index M w /M n decreased by 3.1%, 6.8%, and 8.0%, respectively, after storage for 139 days. Both percentages of decrease in M w and M n were the highest at 25 • C, followed by 4, -12, and -18 • C. This revealed that the storage stability of a PAA solution is enhanced at lower temperatures by inhibiting hydrolysis. Reductions in both M w and M n suggested that the degradation of PAA chains did not follow the reported chain length equilibration, where a reduction in M w and almost constant M n were observed [14]. The data at four temperatures (−18 • C, −12 • C, 4 • C, and 25 • C) and log (degradation rate) vs. 1/T curve were plotted as shown in Figure 7. Here, the degradation rate was defined as M w decreased in 139 days (kg/mol·day), which was calculated from |M w , 0 − M w , 139 |/139. The temperature threshold was about −6 • C, at which the dependence of degradation rate on temperature started to change.
in both Mw and Mn suggested that the degradation of PAA chains did not follow the reported chain length equilibration, where a reduction in Mw and almost constant Mn were observed [14]. The data at four temperatures (−18 °C, −12 °C, 4°C, and 25 °C) and log (degradation rate) vs. 1/T curve were plotted as shown in Figure 7. Here, the degradation rate was defined as Mw decreased in 139 days (kg/mol•day), which was calculated from |Mw,0 − Mw,139|/139. The temperature threshold was about −6°C, at which the dependence of degradation rate on temperature started to change.

Molecular Parameter Change of (6FDA-DMB) PAA in Diluted Solution Stored at 25 °C for Different Times
The 12% PAA solution was diluted to 0.15% solution, and the effect of concentration on PAA stability was investigated in detail. The GPC measurements of PAA in diluted solution were carried out after storage at 25 °C for 0, 1, 3, 6, and 10 h. Figure 8a,b show the elution curves from RI and LS detectors, and the relevant molecular parameters are presented in Table 6. The initial elution volume shifted to larger values with the elongation of storage time, indicating a decrease in molecular weights. The percentages of molecular weight retention are displayed in Figure 8c. The Mw and Mn dramatically decreased by 52.8% and 48.7%, respectively, after storage for 10 h. In addition, the [η] also significantly decreased by 54.4%. During the first 1.5 h of storage, Mw and Mn declined at similar rates, and the molecular weight distribution was almost unchanged. After 1.5 h, Mw declined faster than Mn, and the molecular weight distribution became narrower. The reduction ratio of Mw/Mn was 8.0% after being stored for 10 h. Figure 8d indicates that the PAA in the diluted solution was degraded basically linearly over time within 10 h of storage. The amide bonds broke down by about one scission per chain after storage for 10 h. This degradation degree was comparable with that of the concentrated solution stored at the same temperature for 15 days. Therefore, the degradation of PAA in the diluted solution was much more rapid than that in the concentrated one. One reason for this faster degradation The 12% PAA solution was diluted to 0.15% solution, and the effect of concentration on PAA stability was investigated in detail. The GPC measurements of PAA in diluted solution were carried out after storage at 25 • C for 0, 1, 3, 6, and 10 h. Figure 8a,b show the elution curves from RI and LS detectors, and the relevant molecular parameters are presented in Table 6. The initial elution volume shifted to larger values with the elongation of storage time, indicating a decrease in molecular weights. The percentages of molecular weight retention are displayed in Figure 8c. The M w and M n dramatically decreased by 52.8% and 48.7%, respectively, after storage for 10 h. In addition, the [η] also significantly decreased by 54.4%. During the first 1.5 h of storage, M w and M n declined at similar rates, and the molecular weight distribution was almost unchanged. After 1.5 h, M w declined faster than M n , and the molecular weight distribution became narrower. The reduction ratio of M w /M n was 8.0% after being stored for 10 h. Figure 8d indicates that the PAA in the diluted solution was degraded basically linearly over time within 10 h of storage. The amide bonds broke down by about one scission per chain after storage for 10 h. This degradation degree was comparable with that of the concentrated solution stored at the same temperature for 15 days. Therefore, the degradation of PAA in the diluted solution was much more rapid than that in the concentrated one. One reason for this faster degradation of PAA in the diluted solution was that the water ratio introduced by the solvent was much higher than that in concentrated solution. In addition, the molecular chains of (6FDA-DMB) PAA in the diluted solution were more likely to contact with water molecules than those in the concentrated solution, due to the difference in the entanglement of chains [23]. These factors accelerated the hydrolysis of amide bonds and the consequent molecular weight decline of PAA in the diluted solution. Similar to concentrated solutions, the degradation of PAA in the diluted solution did not conform to the chain length equilibration mechanism.

Conclusions
The solution stability of polyamic acid formed from the condensation reaction of 6FDA and DMB was studied by evaluating the variation of molecular parameters, especially Mw and Mn, during storage, using GPC-RI-MALLS-VIS. The effects of storage temperature, storage time, and solution concentration on the molecular parameters were systematically investigated.
The stability of PAA in concentrated solution (12%) presented a strong decline with increasing temperature. Specifically, the molecular weights basically remained unchanged at −18 °C for 139 days. When the storage temperature increased from −12 and 4 to 25 °C, the Mw decreased by 7.2%, 34.7%, and 83.8%, while the Mn decreased by 4.7%, 30.0%, and 82.4%, respectively, after storage for 139 days. The corresponding scissions per

Conclusions
The solution stability of polyamic acid formed from the condensation reaction of 6FDA and DMB was studied by evaluating the variation of molecular parameters, especially M w and M n , during storage, using GPC-RI-MALLS-VIS. The effects of storage temperature, storage time, and solution concentration on the molecular parameters were systematically investigated.
The stability of PAA in concentrated solution (12%) presented a strong decline with increasing temperature. Specifically, the molecular weights basically remained unchanged at −18 • C for 139 days. When the storage temperature increased from −12 and 4 to 25 • C, the M w decreased by 7.2%, 34.7%, and 83.8%, while the M n decreased by 4.7%, 30.0%, and 82.4%, respectively, after storage for 139 days. The corresponding scissions per chain increased from 0.05 and 0.43 to 4.67 with the increase of temperature. This dependence of the PAA degradation rate on temperature was caused by faster hydrolysis of amide bonds with trace water in the solvent at high temperatures.
The diluted solution (0.15%) exhibited worse stability than that of the concentrated one stored at the same temperature (25 • C), which was caused by faster degradation due to a higher water ratio and less entanglement of chains. The reduction ratio of M w and M n were 52.8% and 48.7%, respectively, after storage for 10 h, which was comparable with that of the concentrated solution stored at the same temperature for 15 days. The PAA in the diluted solution showed an almost linear degradation within 10 h of storage at 25 • C. Both reductions in M w and M n observed in this study suggest that the degradation of PAA does not follow the chain length equilibration mechanism.
In conclusion, storing PAA concentrated solutions in dry and sealed bottles below −18 • C is recommended to obtain a stable PAA solution for at least 4 months. Variations of molecular parameters during storage demonstrated in this study are helpful in tuning PAA solution properties and the consequent performances of PI films or fibers.  Institutional Review Board Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.