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

We have synthesized and analyzed the mechanical/structural characteristics of a polyester containing 21 wt % of a triptycene monomer and compared it to a reference polyester homologue wherein benzene replaces the triptycene residue. Solvent-cast films and tension heat-treated (THT) films were investigated by tensile deformation and wide-angle X-ray scattering. The addition of triptycene units increases the Tg and, contrary to what is typically observed, 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 approximately 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 mechanical behavior is primarily attributed to the ability of individual triptycene units to express what has been termed as “internal molecular free volume” (IMFV). We suggest that the triptycene polymers adopt favorable conformations that minimize the IMFV, and the resultant assembly introduces two mechanisms for the enhancement of tensile mechanical properties:  molecular threading and molecular interlocking.

Minimization جلد آزاد مولکولی مولکولی: یک مکانیزم برای افزایش همزمان سفتی و استحکام پلیمر.

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