Poly (Lactic Acid)/Poly (Ethylene-co-butylene) Supramolecular Elastomers Bonded By Multiple Hydrogen Bonds

As a typical noncovalent interaction, the hydrogen bonding has the high selectivity and can direct the self-assembly of complementary molecular units. Therefore, the hydrogen bonding interactions have been widely used to prepare the supramolecular polymers.2-Ureido-4[1H]-pyrimidione (UPy) groups can form the quadruple hydrogen bonding interactions in dimerization. When the UPy groups are incorporated to the monomers and the terminals or side chains of oligomers, supramolecular polymers can be formed via the hydrogen bonding interactions between UPy dimers. Furthermore, the supramolecular elastomers with the ordered sequence structure can be prepared when the block copolymers with both soft and hard segments are terminally modified by UPy groups. The physical properties of such supramolecular elastomers can be finely tuned through changing the condensed matter structure of hard segments. Because of the strong hydrogen bonding interactions, the UPy-bonded supramolecular elastomers are generally less crystallizable or non-crystallizable. Therefore, the stereocomplex crystallization, which has the enhanced crystalline ability and faster crystallization than the common crystallization process, would be a feasible method to improve the crystallizability of supramolecular polymers. Due to the stimuli responsivity of hydrogen bonding interactions, the UPy-bonded supramolecular elastomers may have many functions such as the shape memory and self-healing properties.In this thesis, a series of poly(lactic acid)-poly(ethylene-co-butylene)-poly(lactic acid) (PLA-PEB-PLA) with different copolymer compositions and PLA stereostructures were first synthesized by the ring-opening polymerization of lactide using PEB as the macroinitiator. These copolymers include the poly(L-lactic acid)-poly(ethylene-co-butylene)-poly(L-lactic acid) (PLLA-PEB-PLLA) with crystallizable PLLA block and the poly(DL-lactic acid)-poly(ethylene-co-butylene)-poly(DL-lactic acid) (PDLLA-PEB-PDLLA) with the amorphous PDLLA block. The supramolecular elastomers were prepared via incorporating UPy groups to the terminals of PLA-PEB-PLA triblock copolymers, which were denoted as PLA-PEB-PLA-UPys. The prepared PLA-PEB-PLAs were viscous semifluids or brittle solids, while the PLA-PEB-PLA-UPys were transparent and elastic solids, which proved the formation of supramolecular polymers. As indicated by the differential scanning calorimeter (DSC) and wide-angle X-ray diffraction (WAXD) results, the quadruple hydrogen bonding of UPy dimers can drastically suppress the crystallizability of PLLA blocks. Because of the amorphous nature of PDLLA, PDLLA-PEB-PDLLAs and their UPy-bonded supramolecular polymers were all amorphous. The microphase-separated morphology of PLA-PEB-PLA oligomers and their supramolecular polymers was studied by small angle X-ray scattering (SAXS). As the volume fraction of PLA (fPLA) increased, the microphase-separated morphology changes from hexagonally packed cylindrical to lamellar structures and the average spacing of microphase-separated domains increases. Because of the chain extension of triblock oligomers by UPy dimerization, the UPy-bonded supramolecular polymers have improved mechanical properties. The storage moduli of supramolecular polymers and their precursors decrease abruptly in the temperature ranges of 50～20,20～60, and> 80℃, corresponding to the glass transitions of PEB, PLA, and softening of PLA. The prepared supramolecular polymers show shape memory behavior. PDLLA-PEB-PDLLA-UPys can resume to the permanent shape and the recovery ratios are nearly 100%. Because of the dynamic feature of self-complementary quadruple hydrogen bonding of UPy, the prepared supramolecular polymers have self-healing ability. After exposure under UV light, the supramolecular polymers with non-crystallizable PDLLA blocks can heal the cut. Because of the lower chain mobility of isotactic PLLA segments, the self-healing ability of PLLA-PEB-PLLA-UPy is suppressed.To enhance the crystallizability of supramolecular elastomers, the PLLA-PEB-PLLA-UPy and UPy end-functionalized poly(D-lactic acid)-poly(ethylene-co-butylene)-poly(D-lactic acid) (PDLA-PEB-PDLA-UPy) were further prepared. Upon blending the UPy end-functionalized PLLA-PEB-PLLA and PDLA-PEB-PDLA triblock oligomers, a series of PLLA-PEB-PLLA-UPy/PDLA-PEB-PDLA-UPy supramolecular elastomers were prepared. The crystallinity of stereocomplexed supramolecular elastomers is controlled by varying the mixing ratio of two amorphous supramolecular elastomers containing the enantiopure PLLA and PDLA blocks, respectively. DSC and WAXD results indicate that the stereocomplex crystallization, rather than homocrystallization, takes place in the PLLA-PEB-PLLA-UPy/PDLA-PEB-PDLA-UPy supramolecular elastomers. The crystallization of PLLA and PDLA blocks destroys the orderness of microphase-separated structure. The stereocomplexed supramolecular elastomers show large strength and modulus. The increase of crystallinity leads to the improved storage modulus and thermal resistance. The prepared stereocomplexed supramolecular elastomers have the excellent shape memory property, which can be tuned by varying the crystallinity or stereocomplex content. The deformation and recovery temperatures in shape memory experiments can be modulated from 70 to 100℃ with the increase of stereocomplex content.