Threading and Interlocking: A Mechanism for the Simultaneous Enhancement of Polymer Stiffness, Strength, and Ductility
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Threading and Interlocking: A Mechanism for the Simultaneous Enhancement of Polymer Stiffness, Strength, and Ductility Lokman Torun1, Alex J. Paraskos2, Nicholas T. Tsui3, Timothy M. Swager2, and Edwin L. Thomas3 1 Materials Institute, TUBITAK Marmara Research Center, P. K. 21, Gebze, Kocaeli, 41470, Turkey 2 Department of Chemistry, MIT, 77 Mass Ave, Cambridge, MA, 02139 3 Department of Materials Science and Engineering, MIT, 77 Mass Ave., Cambridge, MA, 02139 ABSTRACT We have synthesized polyester systems containing pendant iptycene units and compared their mechanical/structural properties to a homologous reference polymer wherein benzene replaces iptycene units. Iptycenes have unique structural properties called internal molecular free volume (IMFV). The incorporation of iptycene into polyester backbones results in a polymer chain contour resembling “molecular barbed wire.” The contribution of iptycene to the mechanical properties of polyesters is significant and robust across concentration and processing conditions. 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. We proposed that the presence of triptycene introduces two mechanisms for the enhancement of tensile mechanical properties: molecular threading and molecular interlocking. INTRODUCTION Enhancement of mechanical performance of fibers has been a focus from both scientific and technological point of views. The present understanding for constructing high performance fibers with superior mechanical properties is by creating high axial chain alignment, strong sideway hydrogen bond interactions between adjacent chains and effective π−π contacts between polymer sheets. With homopolymeric systems, improvements to stiffness and ductility are mutually exclusive. For example, increasing crystallinity, increasing chain alignment or increasing Tg raise stiffness. Strategies for enhancing ductility include the reverse of the techniques for increasing stiffness. Only seldomly, improvements to the mechanical properties circumvent the stiffness/ductility tradeoff. No general methods are known for achieving simultaneous enhancements exclusively with a homopolymer systems. [1]-[13] Entanglements significantly reduce mechanical performance of fibers and additional means are necessary to increase interchain interactions to increase energy absorbing capacity of polymeric materials. In this regard, we envisioned that manipulating certain unique design principle and incorporating subunits with internal molecular free volume (IMFV) into the polymer backbone could result in gearing and interlocking of the polymer chains and in doing so increasing the barrier to movement of adjacent polymer chains past one another [14]. To test this approach, we synthesized polyester systems containing iptycene subunits incorporated into the backbone and compared their mechanical properties of solvent cast films to that of a model polyester system
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