Liu Xin,Wang Tiancheng,Li Jingru,Cheng Jue,Zhang Junying
Two series of 4, 4′-diphenylmethane diisocyanate (MDI) type polyurethanes based on poly(tetramethyleneoxide) glycol (PTMO) and polybutylene adipate glycol (PBA) as soft segments were synthesized by one-step bulk polymerization process. Various hard segments were obtained from hydroquinone ether derivatives with or without ether oxygen structure bonded with the aromatic units in main chain, including hydroquinone (HQE), hydroquinone bis(b-hydroxyethy) ether (HQEE), and 4-hydroxyethyl ethyl oxygen-1-hydroxy ethyl benzene ether (HQEE-L) as chain extenders. The effect of chain extender structure on aggregation structure, thermal and mechanical properties was studied. ATR-FTIR suggested that hydrogen bonding interactions were significantly changed in the presence of flexible ether oxygen structure in hard segment. HQEE-based polyurethanes and HQEE-L-based polyurethanes displayed microphase separation and microphase mixing behavior respectively based on DMA and DSC results. XRD curves confirmed that flexible ether oxygen chain bonded with the rigid aromatic units played a balancing role in the steric effect caused by aromatic chain extender. Thermal properties of polyurethanes were affected mainly by the hard segment packing while mechanical properties were influenced by both phase separation and phase mixing which possess strong hydrogen bonding interactions. The excellent elongation in PTMO/HQEE-L and the high ultimate tensile strength in PBA/HQEE-L can be attributed to the phase mixing with strong hydrogen bonding interactions presented in HQEE-L-based polyurethanes.
The development of an easy synthetic strategy combined with straightforward tailoring of physical properties and functionalities, such that optimal performance can be targeted for various applications, still remains challenging. Previously, we reported the construction of thermo- and water-responsive strong and tough supramolecular hydrogels based on the cooperatively enhancing effect between H-bonding and hydrophobic forces. Recently, the strategy was greatly simplified to a one-pot two-step approach. In this work, by simply changing the chain extenders used in the second step, different kinds of functional units are easily introduced into the hard segments in the main chain of the copolymers to endow the resultant strong and tough supramolecular hydrogels with various stimulus-responsive properties. A dynamic covalent bond (disulphide or imine bond, e.g. –S–S– or –CN–) in the main chain provides the resulting hydrogels with reduction or pH responsive degradation (gel-to-sol transition) on demand behaviors, respectively; the azobenzene unit endows the yielding hydrogels with UV-Vis controlled stiffness; the pyridine group containing supramolecular hydrogel shows metal ion responsive mechanical and fluorescent behavior. These results provide progress toward addressing the challenges of achieving structural and synthetic simplicity married with sophisticated functionalities.
Four chain extenders, 1,3-propanediol (PDO), 2-methyl-1,3-propanediol (MPO), 2,2-dimethyl-1,3-propanediol (NPG) and 2,2,4,4-tetramethyl-1,3-cyclobutanedio (CBDO) are chosen to synthesize model polyurethanes , respectively. Although with similar oxygen distance between the two hydroxyl groups , their volumes increase gradually according to computer simulation. As expected, the degree of microphase separation decreases with the increasing chain extender volume. Except for CBDO based polyurethane, the other three samples show systematic variations in mechanical properties . Various techniques including single-molecule force spectroscopy (SMFS), small-angle X-ray scattering (SAXS) and Fourier transform infrared spectroscopy (FTIR) are employed to investigate the structural changes after tensile break in different length scales. A picture of asynchronous fracture of microstructures during the tensile break of thermoplastic polyurethane elastomers can be obtained by the combination of different analytical methods. It is interesting to note that the macroscopic break may not affect the state of hydrogen bonding or the hard phase network. Sometimes, the hydrogen bonding state changes a lot while the phase network keeps almost the same, or vice versa.
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