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

Abstract

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.

Association and relaxation of supra-macromolecular polymers

“…This entropic penalty is rarely offset by the enthalpic gain from endgroup association. 9 The stacked UPy aggregates reported for smaller functional molecules support the idea that some association to form branch points is likely but we note that large (e.g. >2 Upy dimers) aggregates are not required to contribute to strain hardening.…”

“…Polymers functionalised with reversibly associative substituents, driven by non-covalent interactions, have gained much interest in recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] in a variety of materials science applications. These include reversible blending of immiscible polymers, 16 self-healing, [17][18][19] improved processability 4,20,21 and smart adhesive surfaces.…”

“…However, Ishiwari et al have shown that some functional groups can cause significant structural organisation, even in relatively large polymers. 9 Our UPy end-functionalised polymers are expected to assemble end to end via hydrogen bonding of the UPy groups. The prospect of assembling polymer chains with a much higher effective Mw is desirable for multiple applications.…”

Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation

“…Thus, the T1 and T2 molecules on Au(111) likely assemble into a 2D nested hexagonal structure (Figure 2e), which is consistent with the packing of 1,8,13-trialkoxytriptycenes observed in X-ray diffraction experiments. 23−28 Consequently, also the centers of the phenyl groups align hexagonally with a separation of ca. 5 Å.…”

“…We have recently shown that 1,8,13-trisubstituted triptycenes exhibit superb self-assembling abilities to form well-defined, dense two-dimensional (2D) hexagonal structures through a nested packing of the aromatic blades. 23−28 This suggests that the trisubstituted triptycenes should offer a highly promising starting point for the development of ideal SAMs featuring well-controlled molecular density and orientation. Notably, due to the tridentate configuration, the present systems differ distinctly from previously reported monodentate triptycene-based monolayers with a single thiol or selenol group attached in the bridgehead position.…”