The system was then parsed by CHARMM and both ends of each PET chain were methylated. An initial box of 90 Å× 90 Å × 90 Å was built around the crystal film to generate the in vacuo model. TIP3P water molecules were inserted into the box using CHARMM to generate the solvated system. The energy of both systems was minimized via 500 steps of steepest descent minimization followed by 500 steps of adopted-basis Newton-Raphson minimization. Equilibration was carried in OpenMM for 1 ns while a position restraint was applied on all heavy atoms with a force of 500 kJ mol -1 nm 2 .