Terminal Functionalization with a Triptycene Motif That Dramatically Changes the Structural and Physical Properties of an Amorphous Polymer

A surprising terminal-group effect on the structural and physical properties of an amorphous polymer is reported. We recently demonstrated that triptycene derivatives with substituents at the 1,8,13-positions show specific self-assembly behavior, enabling the formation of a well-defined "2D + 1D" structure based on nested hexagonal packing of the triptycenes. Upon terminal functionalization with a 1,8-substituted triptycene (1,8-Trip), a liquid polymer, polydimethylsiloxane (PDMS, M_n = 18-24 kDa), turned into a highly viscous solid that exhibits birefringence at 25 °C. Small-angle and wide-angle X-ray scattering measurements revealed that the resulting telechelic PDMS assembles into a 2D + 1D structure, where layers of PDMS domains, formed between 2D assemblies of the triptycene termini, stack into a 1D multilayer structure with a layer spacing of 18-20 nm. Because of this structuring, the complex viscosity of the telechelic PDMS was dramatically enhanced, providing a value 4 orders of magnitude greater than that of the original PDMS. Remarkably, the structural and physical properties of PDMS were hardly changed upon terminal functionalization with another regioisomer of triptycene (1,4-Trip), which differs only in the substitution pattern.