Macromolecules, Vol.52, No.17, 6585-6599, 2019
Thermoreversible Supramolecular Networks from Poly(trimethylene Carbonate) Synthesized by Condensation with Triuret and Tetrauret
In this work, poly(trimethylene carbonate) (PTMC)-based multiarm supramolecular polymers bearing self-complementary triuret and tetrauret hydrogen-bonding motifs (HBMs) at the chain ends were synthesized with various functionalities and HBM contents. These supramolecular structures were elaborated through condensation between diisocyanate 4,4'-methylenebis(cyclohexyl isocyanate) (H12MDI) and telechelic PTMC oligomers, urea/biuret, glycerol, and 1-pentanol in three steps. The equilibrium shifts of hydrogen bonding between HBMs were traced by H-1 NMR in dimethyl sulfoxide-d(6) and Fourier transform infrared (FT-IR) in bulk at different temperatures. Association constants (K-a) of triuret and tetrauret HBMs were measured to be 0.32 and 1.59 M-1, respectively, at low HBMs concentrations in CDCl3. HBMs content was found to have the greatest effect on T-g, whereas the functionality and HBM structure did not. Thermal stability of supramolecular polymers was enhanced by HBMs with tetrauret being more efficient than triuret. Network structures were formed due to the transient physical cross-links by hydrogen bonding between HBMs and evidenced by the high transient temperature from the elastic state to the liquid state (T-crossover) and good creep resistance. Increasing the functionality or HBMs content can increase the T-crossover,T- creep resistance, and Young's modulus in tensile tests. In contradiction to the measured K-a, triuret HBMs lead to the formation of denser physical network structures than their tetrauret counterparts, causing higher T-crossover and better dimensional stability consequently. Although the introduction of triuret or tetrauret HBMs decreases the biodegradability of supramolecular polymers, the influence of their structures is unapparent.