Synthesis of stretchable polyetherimides containing multiple alkyl side chains

Two series of diamine monomers, i.e., 4-[3,4,5-tris(n-alkan-1-yloxy)benzyl]-3,5-diaminobenzoate and 2,2′-bis{4-[3,4,5-tris(n-alkan-1-yloxy)benzoate]}-4,4′-biphenyldiamines containing multiple alkyl side chains, were synthesized in which the length of the alkyl side chains was varied from 5 to 18 ethylene units. Polyetherimides (PEIs) were prepared from these two diamines and bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydrides (BisADA) using a one-step method in 1-chloronaphthalene. All the polymers possessed good solubility in chlorinated solvents. The high-molecular weight PEIs could be solution cast into transparent, tough films. The polymers containing the side chains which were at least 16 methylene units could be crystallized. The melting points of the crystals increased as the length of the side chains increased. The glass transition temperatures (Tgs) of the PEIs decreased as the length of the side chains increased due to an internal plasticizing effect of the alkyl side chains. The PEIs containing multiple alkyl side chains showed excellent drawability with much higher elongations but lower tensile strengths and moduli compared with the commercial PEI, Ultem® 1000. The film's in-plane refractive index parallel to the drawing direction (n//) increased and its in-plane refractive index perpendicular to the drawing direction (n⊥) decreased during drawing mainly due to the orientation of main chains in the drawing direction. The film's in-plane birefringence increased with the drawing ratios.