Quadruple hydrogen bonding containing supramolecular thermoplastic elastomers: Mechanical and morphological correlations

Abstract

We report the design of bioinspired, reversible supramolecular thermoplastic elastomers (TPEs) functionalized with ureido‐cytosine (UCyt) complementary quadruple hydrogen bonding (QHB) sites. The polymer contained a soft poly(n‐butyl acrylate) central block that imparted flexibility and two external, hard nucleobase‐containing blocks that contributed to structural integrity. In addition, the hard block with pendant QHB motifs served as efficient physical crosslinks to further enhance the thermomechanical performance, where the polymer service window extended up to 30 °C higher compared to the controls that bear dimeric hydrogen bonding units. The resulting UCyt copolymers also exhibited improved surface and bulk morphology, which self‐assembled into well‐ordered lamellar microstructures. Moreover, the polymer displayed an unexpected moisture‐resistant property with less than 1 wt % equilibrium moisture uptake even at 95% relatively humidity, which presumably correlated with its well‐ordered and densely‐packed morphology facilitated by strong hydrogen bonding. Variable temperature Fourier‐transform infrared spectroscopy experiments further confirmed the thermoreversibility of hydrogen bonding, indicating melt‐processablility and recyclability of the polymer. These physical properties verified quadruple bonding dominated behavior, and structure–property–morphology relationships suggest key design parameters for future TPEs. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 13–23

Quadruple Hydrogen Bond-Containing A-AB-A Triblock Copolymers: Probing the Influence of Hydrogen Bonding in the Central Block

“…On the contrary, the hard domains were composed of mostly UCyA that enabled the formation of densely packed crosslinks, thus dissociated at higher temperatures. Similarly, Long et al observed an increasing terminal flow temperature when hydrogen bonding unit increased in the copolymer [21]. Additionally, all UCyA copolymers flowed at~150 • C, suggesting minor effects of the central block on the terminal flow temperature.…”

“…An additional higher temperature transition step at~85 • C was associated with the external blocks. It is noteworthy that the T g of the CyA external block was slightly lower compared with the T g of a poly(CyA) homopolymer (~93 • C) [21], indicative of phase mixing that originated from interactions among the CyA units between the central and external blocks. Figure 3B depicts the thermal transition profiles of UCyA copolymers.…”

“…Lower thermal stability of the UCyA copolymers under N 2 limited the upper testing temperature to 150 • C (degradation at~160 • C). In fact, the slight increase of the heat flow near 140 • C was attributed to the onset of the thermal transition of the external blocks (Figure 3B) [21].…”

“…RAFT polymerization of nBA and CyA monomers yielded triblock copolymers with tunable chain lengths (Scheme 1 and Table 1). To generate polymers with a defined A-AB-A triblock architecture, a difunctional CTA, difunctional 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (di-DDMAT) was selected due to its excellent compatibility with acrylic-based monomers [18,21]. A series of macro-CTAs, i.e., copolymers of nBA and CyA, were prepared.…”

“…40 M −1 ) in chloroform (CHCl 3 ) [19,20]. To further probe the effects of hydrogen bonding strength on the bulk morphology and mechanical properties, Long et al reported UCy and Cy functionalized A-B-A triblock polymers and observed a 30 • C extended service window for the UCy-containing polymer compared to its Cy counterpart due to higher association strength of the UCy unit [21]. The increased upper service temperature afforded ureido cytosine acrylate (UCyA) copolymers as a novel high performance TPE.…”