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

Abstract

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 T g 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.

Gas permeation and physical aging properties of iptycene diamine-based microporous polyimides

“…Triptycene, as shown in Figure 1a, has three phenyl rings bound by a single hinge, which provides a high energy barrier to molecular twisting or deformation which keeps the angle between aromatic rings at 120 o . The rigid 3-dimensional, fully aromatic structure leads to poor polymer chain packing, which, in turn, results in high internal free volume (IFV) [26,27]. An extended iptycene building block, shown in Figure 1b, demonstrates even larger IFV than triptycene due to the addition of a benzene ring to the transverse aromatic arm in the 9,10-position of the triptycene unit.…”

High Performance Polyimide with High Internal Free Volume Elements

“…[16] Recently, some studies suggest that local cavities can be formed due to the inability for triptycene-based polymeric structures to pack efficiently. [17] Because of these features, triptycene-based polymers, or molecules have been developed for gas adsorption as well as membrane-based gas separation, providing large BET surface area within 600-1 800 m 2 Á g À1 as well as high microporosity.…”

Triptycene‐containing polyetherolefins via acyclic diene metathesis polymerization

“…This is in accordance with previous research, which has found that triptycene can increase the thermal stability of polymers. [18][19][20] Interestingly, the block copolymers do not show increasing thermal stability with higher triptycene content. Presumably, this is because the molecular weight of PT1 1 -b-PO 1 is lower than the weights of the PT1 1 -b-PO 3 copolymers.…”

“…Enhanced thermal stability is often also observed with triptycene incorporation, along with improvements in polymer mechanical properties including increased stiffness, toughness, ductility, and ultimate tensile strength. [19][20][21] In this study, we have systematically introduced triptycene into the backbone of ADMET-synthesized polyethers to investigate the thermal and mechanical effects of triptycene incorporation. Two different triptycene olefin monomers were synthesized to study the effect of incorporation into the polymer backbone in different geometries (Fig.…”