Polyhexahydrotriazine (PHT) film layers are formed by a process comprising heating a first mixture comprising i) a solvent, ii) paraformaldehyde, and iii) a diamine monomer comprising two primary aromatic amine groups at a temperature of about 20° C. to less than 150° C. This heating step forms a stable polyhemiaminal (PHA) in solution, which can be cast on a surface of a substrate, thereby forming an initial film layer comprising the PHA. The initial film layer is heated at a temperature of 180° C. to about 280° C., thereby converting the PHA film layer to a PHT film layer. Young's moduli of about 8 GPA to about 14 GPA have been observed for the PHT film layers.