Abstract
Supramolecular block copolymers (PS-DAT-sb-PI-Thy) were synthesized via noncovalent hydrogen bonding between well-defined thymine end-functionalized polyisoprene (PI-Thy) and diaminotriazine (DAT) end-functionalized polystyrene (PS-DAT). Three covalently linked block copolymers were also synthesized for comparison with the noncovalent supramolecular block copolymers. The complementary DAT/Thy interaction resulted in the microphase separation of the supramolecular block copolymer system. Detailed characterization of all functionalized homopolymers and block copolymers was carried out via proton nuclear magnetic resonance (1H NMR) spectroscopy, gel permeation chromatography, matrix-assisted laser desorption/ionization-time of flight mass spectrometry, and differential scanning calorimetry. The self-assembly process of supramolecular block copolymers was evidenced by transmission electron microscopy. Small-angle X-ray scattering was also performed to study the microphase separation of supramolecular and covalently linked block copolymers. Comparison of microphase separation images of supramolecular block copolymers and the corresponding covalently linked analogues reveals differences in d-spacing and microdomain shape.
Highlights
Block copolymers have undergone an extraordinary evolution from simple surfactant materials to an expansive class of macromolecules
Reversibility, directionality, and self-organization nature make hydrogen bonding one of the most common motifs to design and construct supramolecular polymers with a complex macromolecular architecture. 2,6Diaminotriazine (DAT) and thymine (Thy) are two of the most common triple hydrogen-bonded systems for the construction of supramolecular structures.[2,3,5,11ā14] Meijer and his group invented the quadruple hydrogen-bonded system to construct complex macromolecular architectures.[15ā18] Ikkala et al used phenolic āOH and pyridine nitrogen to form hierarchical architectures of small surfactant molecules with block copolymers.[19ā22] In the case of the DAT/Thy system, strong heteroassociation can be formed with the help of three complementary hydrogen bonds between DAT and Thy groups, whereas homoassociation through two hydrogen bonds results in a weak interaction.[13]
The moderate association constant for the DAT/Thy system is strong enough to hold the microstructure in a supramolecular block copolymer
Summary
Block copolymers have undergone an extraordinary evolution from simple surfactant materials to an expansive class of macromolecules. The decrease in Tg of polystyrene from 92 Ā°C (PS15KNH2) to 74 Ā°C in PS15K-DAT-sb-PI15K-Thy can be explained by the disruption of polystyrene chain packing in the presence of polyisoprene chains which is similar to the covalently linked block copolymer PS15K-b-PI15K-OH, whereas Tg corresponding to polystyrene (84 Ā°C) did not show any significant change in the mixture of two homopolymers without the hydrogen bonding end group (PS15K-NH2-Mix-PI15K-OH). The formed structure depends on the interplay among all miscibility parameters and attractive hydrogen bond interactions to form either the ordered microphase-separated structure similar to the covalently linked block copolymers[31] (e.g., lamellae, cylinders) or a more disordered, self-aggregated homopolymer-swollen structure because of the formation of homoassociations in addition to the heteroassociations. Such a large lamellar periodicity due to domain swelling from the respective associated homopolymers would lead to a Bragg peak at a āqā value that is very close to the main beam and likely would not be resolved in SAXS
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