| Studies have shown that addition of cationic groups to a segmented polyurethane can dramatically change physical attributes such as solution behavior, mechanical strength, and viscoelastic properties. However, the specific interactions responsible for these changes are not well understood. A series of polyether polyurethane cationomers were synthesized using chain extenders with pendant trialkylammonium groups. The morphology and properties of the cationomers were studied as a function of ion-related variables using small-angle X-ray scattering (SAXS), tensile testing, dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC). Ion content affected phase separation and mechanical properties the most; the alkyl group length was of secondary importance; and the type of neutralizing anion had essentially no effect. Further, a lamellar microstructure typical of conventional polyurethanes was evident, with no apparent aggregation of ionic groups.; When the cationomers were annealed at room temperature for a period of approximately one month, DSC thermograms showed an endotherm centered near 70{dollar}spcirc{dollar}C which was not present in the unannealed polymer. Extended X-ray absorption fine structure temperature studies demonstrated that the endotherm corresponds to water leaving the coordination shell of the anion.; Fourier transform infrared temperature studies were completed to gain further insight into the nature of the specific interactions in these cationomers. Results showed that the interurethane N-H to C=O hydrogen bond found in typical polyurethanes was replaced by a stronger interaction between urethane N-H groups and the neutralizing anion. With no evidence of ionic aggregation, these interchain ties are apparently the primary interaction which "hold the cationomers together." A reversible shift in N-H hydrogen bonding, from the anion to urethane carbonyls, was observed on heating.; The flow transition in DMTA appears to be a direct consequence of this redistribution of hydrogen bonds. Since the number of N-H to anion bonds responsible for phase separation decreased at elevated temperatures, SAXS temperature studies and dynamic viscoelastic measurements were performed to determine if the cationomers passed through a microphase separation transition. Although considerable intersegmental mixing accompanied macroscopic flow, evidence of the lamellar microstructure persisted into the melt, and a homogenous phase was not observed. |
