Effect of Linker Chemistry, Linker Length and Temperature on The Thermal Conductivity of Vinylogous Urethane Dynamic Network

Covalent adaptable networks (CANs) have been heavily studied in recent years as promising candidates for recyclable polymer in applications like shape memory materials, shockwave energy-dissipative materials, and flexible electronics. However, the thermal management of polymeric materials in these devices is still an urgent issue, since polymers generally have low thermal conductivity (as 0.2 W/m K) which can lead to thermally-induced aging. Moreover, the effect of bond exchange dynamics, linker chemistry and temperature on the thermal conductivity of CANs is still unclear. In this work, a series of vinylogous urethane dynamic networks (VU-<i>x</i>E) were synthesized from tris(2-aminoethyl)amine and di-acetoacetates with ethylene linkers (E = ethylene) and varied linker length (<i>x</i> = 2-22 carbon atoms). Thermal conductivity was measured by frontside frequency-domain probe beam deflection (FD-PBD). In comparison to the ethylene network with benzene diboronic acid bonds, neither linker length dependence nor temperature dependence of thermal conductivity shows a monotonic trend as in the case of VU-<i>x</i>E network. The thermal conductivity of these VU networks at room temperature varies from 0.20 W/m K to &lt;0.1 W/m K as the linker length increases from <i>x</i> = 2 to <i>x</i> = 22. At high temperature approaching glass transition (T_g~ T_g + 20K), a drop of thermal conductivity is observed by FD-PBD and as the temperature grows higher (&gt; T_g + 30K), the thermal conductivity retains to that of low temperature (&lt; T_g). We hypothesized that this drop of thermal conductivity near T_g in FD-PBD is due to combined dynamics of monomer chain motion and bond exchange. The morphology of these VU-<i>x</i>E networks were also studied using WAXS for both fresh and annealed samples. Apart from amorphous halo peak at ~ <i>q</i>_I= ~14 nm^-1 and broad ethylene chain stacking peak at ~ <i>q</i>_II= ~4 nm^-1, no obvious crystalline peak is seen for all networks. Such a behavior is different from our previously studied ethylene network with boric acid dynamic bonds, where longer network (<i>x </i>&gt; 10) exhibits slow crystallization (with crystallinity&gt;78% in WAXS) at room temperature. We attributed this difference to the fact that VU bonds have a much slower bond exchange than boric acid. Furthermore, we investigated the effect of linker chemistry on the thermal conductivity, by changing the ethylene linker to ethylene oxide (VU-<i>x</i>EO, <i>x</i> = 6, 9, 12, 18, 36 atoms). Similar linker length dependence of thermal conductivity is observed for these ethylene oxide VU network. But no EO chain stacking peak <i>q_II</i> is seen in WAXS spectra as the linker length increases, suggesting that the presence of oxygen atoms in EO linker will disturb the chain stacking and make the network less capable of crystallization. This work provides new design rules for thermal conductivity of dynamic polymer networks and how to control the crystallization depending on linker length, chemistry, and dynamic bond type.