Sol-gel transition of polyamic acid solution during chemical imidization and its effect on the drawing behavior of polyimide films

Herein, sol-gel transition of 3,3′,4,4′-biphenyldianhydride (BPDA)/p-phenylene diamine (PDA) polyamic acid (PAA) solution were investigated by adding excessive dehydration reagent, and it was found that increasing the amount of dehydration reagent and pyridine content, as well as the temperature could accelerate the sol-gel transition. The maximum strain of PAA film at the gel stage of sol-gel transition increases from 1.5 to 5.8 compared to the pure PAA film, and the better drawing property enables it has the potential to prepare PI films with more excellent mechanical properties and lower coefficient of thermal expansion (CTE). When the solvent content of PAA gel film increases from 20% to 85%, the tensile strength and modulus of the corresponding PI film with a draft ratio of 1.5 increase from 488 MPa to 12.1 GPa to 591 MPa and 17.3 GPa, respectively. Therefore, the PI film drawn from the gel film with higher solvent content has better mechanical properties. Besides, the corresponding amorphous and orientation composite models with different solvent content were constructed and analyzed based on molecular dynamic (MD) simulation. The FFV of amorphous composite model decreases with the decrease of solvent content, indicating that the model with higher solvent content has less intermolecular entanglement and interaction during the drawing process. The mean square end-to-end distance and XRD patterns demonstrate that the molecular chain in the orientation model drawn from the amorphous model with higher solvent content have more regular and extended arrangement along the drawing direction. Accounting for the reason that PI film drawn from the gel film with higher solvent content exhibits more excellent mechanical properties.