Thermal, mechanical and conductive properties of imidazolium-containing thiol-ene poly(ionic liquid) networks

Abstract

A series of imidazolium-based bis(trifluoromethylsulfonyl)imide [NTf 2 ] poly(ionic liquid)s (PILs) were prepared by conducting the solventless thiol-ene 'click' photopolymerization of bisallylimidazolium [NTf 2 ] and pentaerythritol tetrakis(3-mercaptopropionate) (PTMP). The thiol:ene molar ratio was varied in order to examine changes in the thermal, mechanical and conductive properties of the resulting polymer networks. The 1.0:2.0 thiol-ene PIL network exhibited the highest glass transition temperature (T g of-5.0 °C) and storage modulus (E′ of 9.24 MPa at 100 °C) values. Temperature-dependent ionic conductivities were found to rely on both the T g /crosslink density as well as the IL content, with the highest ionic conductivity observed for the 1.0:3.0 thiol-ene PIL network (1.42 x 10-5 S/cm at 25 °C). Application of Vogel-Fulcher-Tammann and Williams-Landel-Ferry theories revealed the impact of both T g /crosslink density and free ion concentration on temperature-dependent conductivity data, and indicated lower free volumes and fragilities for the thiol-ene networks relative to other non-network PILs.

Correlating structure with ionic conductivity in bis(phosphonium)‐containing [NTf2] thiol–ene networks

“…The second step ( ca 410 °C) correlates relatively well with the decomposition of the phosphonium IL. Thus, for this specific thiol–ene network architecture, the choice of cation (phosphonium versus imidazolium) appears to have little effect on the minimum decomposition temperature threshold, as previously reported imidazolium‐containing thiol–ene PIL networks exhibited similar T d5% values (340–360 °C) but with only a single‐step decomposition …”

“…A slight decrease in E ′ occurred when the thiol‐to‐ene molar ratio was off‐stoichiometry (PILs 3f and 3g ). Apparent crosslink density ( ρ x ) was calculated following rubber elasticity theory: ρ x = E ′/3 RT , where E ′ is the storage modulus (Pa) at a temperature above T g , R is the gas constant (8.314 J mol −1 K −1 ) and T is the absolute temperature (K) …”

“…The ionic conductivities of these systems followed Vogel–Fulcher–Tammann (VFT) dependence for all of the PILs according to the following equation: where σ o is the high‐temperature ionic conductivity limit, T o is the Vogel temperature, T is the experimental temperature and D is the strength parameter (related inversely to the fragility of polymer dynamics) . Such fitting is attributed to the relationship that exists between polymer chain relaxation dynamics and the long‐range diffusion of ionic species.…”

“…Thiol–ene polymer networks were photopolymerized following a previously published procedure from our laboratory . The thiol‐to‐ene functional group ratio was stoichiometric for the photopolymerizations of all monomers except 2a where two off‐stoichiometric ratio PIL networks (1.0:0.75 and 1:1.25 thiol‐to‐ene functional group ratios) were also prepared.…”

“…Our efforts in the field of PILs have primarily focused on the synthesis of covalently crosslinked networks using thiol–ene ‘click’ photochemistry. Our initial structure–property studies involved the photopolymerization of bisallylimidazolium [NTf 2 ] with pentaerythritol tetrakis(3‐mercaptopropionate) (PTMP) at various thiol‐to‐ene molar ratios . PIL networks were found to display a wide range of T g values (−5 to −32 °C) and mechanical properties ( E ′ at 100 °C of 1.5–9.5 MPa) with good ionic conductivities (10 −5 to 10 −7 to at 30 °C, 30% RH).…”