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Storage Stability of 6FDA-DMB Polyamic Acid Solution Detected by Gel Permeation Chromatography Coupled with Multiple
Mei Hong1,2, Wei Liu1, Runxiang Gao1,2
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
Insights
Polyamic acid (PAA) solutions degrade over time, especially at higher temperatures and lower concentrations. This study reveals PAA hydrolysis accelerates degradation, unlike previously reported chain equilibration mechanisms.
Area of Science:
- Polymer Chemistry
- Materials Science
Background:
- Polyamic acid (PAA) is a crucial precursor for polyimide (PI) materials.
- PAA solution stability directly impacts the performance of final PI products.
- Understanding PAA degradation mechanisms is vital for material processing and longevity.
Purpose of the Study:
- To evaluate the stability of polyamic acid (PAA) solutions under various storage conditions.
- To elucidate the degradation mechanism of PAA by monitoring molecular parameters over time.
- To investigate the influence of temperature and concentration on PAA solution stability.
Main Methods:
- Synthesized PAA from 6FDA and DMB in DMAc.
- Stored PAA solutions at temperatures ranging from -18 to 25 °C and concentrations of 12 wt% and 0.15 wt%.
- Analyzed molecular parameters (Mw, Mn, Mw/Mn, Rg, [η]) using GPC-RI-MALLS-VIS.
Main Results:
- PAA solution stability decreased significantly with increasing temperature and decreasing concentration.
- Hydrolysis accelerated PAA degradation at higher temperatures, with Mw reduction up to 83.8% at 25 °C after 139 days.
- Diluted PAA solutions at 25 °C showed rapid degradation (Mw decrease of 52.8% in 10 h) due to higher water content and reduced chain entanglement.
- Observed simultaneous decrease in Mw and Mn, indicating degradation does not follow chain length equilibration.
Conclusions:
- PAA solution stability is highly sensitive to temperature and concentration.
- Hydrolysis is a primary degradation pathway for PAA, particularly in diluted solutions at elevated temperatures.
- The degradation mechanism differs from chain length equilibration, with implications for PAA processing and storage.
Abstract:
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.
