Uncharged Lewis bases yield polydimethylsiloxane ionomers with amidinium alkyldithiocarbamate side chains

Abstract

Ionomers exhibiting good thermal stability have been prepared by mixing three uncharged components—a poly(dimethyl)siloxane (PDMS) with randomly substituted amidine side chains, an alkylamine, and carbon disulfide. In essence, the three uncharged components produce charged materials, polyelectrolytes with amidinium dithiocarbamate ion pairs. The preparation, structures, and properties of the ionomers at the microscopic and macroscopic levels are reported as a function of PDMS chain length, degree of amidine substitution, and length of the alky group of the amine component. Whereas ionomers with hexyldithiocarbamate anions are viscous liquids at room temperature, those with octadecyldithiocarbamate groups are semi‐crystalline solids, exhibiting multiple thermal transitions between 0 °C and the temperature at which they become isotropic phases. The structural natures of those transitions have been characterized from analyses of data from differential scanning calorimetry, X‐ray diffraction, and rheology measurements. Among the interesting properties exhibited by the ionomers are strong adhesion to different surfaces, and high ultimate shear stress considering their relatively low amidinium contents.

Effects of cyclic and acyclic amidine side-chains on the properties of polysiloxane ionomers constructed in situ from three uncharged components

“…This trend is nearly the same as that for polysiloxane-based ionomers with 6-7% amidinium hexyldithiocarbamate side chains (1a-6). 9 The trend may be present at lower frequencies, as well, but the magnitudes of the moduli at o10 rad s À1 are below the detection limit of our rheometer.…”

“…Although there is a difference in their molecular weights, they are within the same order of magnitude, and a comparison of the properties of 1-6 and 2-6 (as well as 1-18 and 2-18) can be used to measure the effect of the bulkier cations on polymer dynamics; rheology data for polysiloxane-based ionomers with amidine side chains with 6-7% or 20-25% by mole fraction and their corresponding ionomers can be found in the literature. 9 A correlation between G 00 E o and G 0 E o 1.7 was found for 2-6 at high angular frequencies (i.e., 410 rad s À1 ). This trend is nearly the same as that for polysiloxane-based ionomers with 6-7% amidinium hexyldithiocarbamate side chains (1a-6).…”

“…Amidinium salts can be generated easily through the reaction of an amidine with an alkylamine and either CO 2 (forming an amidinium carbamate) or CS 2 (forming an amidinium dithiocarbamate); 30,31 the same procedure generates the analogous imidazolinium alkylcarbamates and alkyldithiocarbamates. As noted in our previous work, 9 amidinium alkyldithiocarbamate ionomers are thermally stable at room temperature and allow the bulk properties of these ionomers to be studied. The same is not true for amidinium alkylcarbamate-based systems which revert to a mixture of amidine, amine, and CO 2 when mild heat is applied.…”

“…Syntheses of the analogous amidine and amidinium alkyldithiocarbamate ionomers have been reported in a recent publication. 9 Many details of the synthetic steps and characterization protocols are presented in ESI. † Structural analyses of the neat materials include differential scanning calorimetry (DSC) measurements, variable temperature powder X-ray diffraction (PXRD), and polarizing optimal microscopy (POM).…”

“…Previously, we investigated the in situ construction of neat ionomeric materials constituted from two uncharged components-polysiloxanes with amino side chains and carboxylic acids or simple triatomic molecules-and from three uncharged components-polysiloxanes with amidine side chains, an alkylamine, and one of several small gaseous molecules, principally CO 2 or CS 2 . 5,8,9 Whereas the ionomers employing CO 2 are stable under one atmosphere of that gas, they revert slowly to their non-ionomeric polysiloxane and alkylamine mixtures in air at room temperature. However, the ionomers employing CS 2 are stable at room and at elevated temperatures in the air for reasons dealing with the different thermodynamic stabilities of CO 2 and CS 2 ; vide infra.…”