Minimization of Internal Molecular Free Volume: A Mechanism for the Simultaneous Enhancement of Polymer Stiffness, Strength, and Ductility

The prepn. and mech./structural characteristics were studied of a polyester contg. 21% triptycene monomer, vs. a ref. polyester homolog wherein benzene replaces the triptycene residue. Solvent-cast films and tension heat-treated (THT) films were studied by tensile deformation and wide-angle x-ray scattering. The addn. of triptycene units increases the Tg and, contrary to what is typically obsd., also increases the ductility of film samples. In comparison to the solvent-cast non-triptycene polyester films, the triptycene polyester films displayed a nearly 3-fold increase in Young’s modulus, an approx. 3-fold increase in strength, and a more than 20-fold increase in strain to failure. THT films of the triptycene polyester exhibited a modulus more than 7 times that of the non-triptycene as-cast polyester and strength greater than 14 times higher for roughly the same strain to failure. This unusually beneficial mech. behavior is primarily attributed to the ability of individual triptycene units to express what was termed as internal mol. free vol. (IMFV). The triptycene polymers adopt favorable conformations that minimize the IMFV, and the resultant assembly introduces two mechanisms for the enhancement of tensile mech. properties: mol. threading and mol. interlocking. [on SciFinder(R)]