Functional Supramolecular Materials Based on Multiple Hydrogen-Bonding Motifs and the Associated Molecular Interaction Mechanism

• Author / Creator

  Chen, Jingsi

• Engineered multifunctional polymers with stimuli-responsive and self-healing properties have drawn increasing attention in recent years for the development of “smart” materials with a broad range of applications. Hydrogen bonding, as one of the most developed approaches in supramolecular chemistry, has been extensively exploited for generating self-assembled structures and functional materials. However, the integration of on-demand multifunctionalities in one platform still remains a challenge. Besides, the single-molecular level understanding of the interaction mechanism between hydrogen-bonding motifs, which plays a crucial role in developing desired hydrogen-bonded structures, is limited. In this thesis, a review of hydrogen-bonded supramolecular materials and their properties is presented first, followed by three original studies regarding the development of multifunctional supramolecular materials (i.e., micelles, conductive composites, conductive hydrogels) based on a hydrogen-bonding motif 2-ureido-4[1H]-pyrimidinone (UPy), and an original research elucidating the UPy dimerization mechanism is described as well.
  Polymeric micelles with core-shell structures have been recognized as attractive platforms for drug delivery. In the first project, a thermo-responsive core cross-linked polymeric micelle was developed based on a double hydrophilic block copolymer of poly(ethylene glycol)-b-poly(N-isopropylacrylamide) (PEG-b-PNIPAm), with UPy functionality incorporated throughout PNIPAm block to direct the core cross-linking via self-complementary multiple hydrogen bonds. The PNIPAm core of the micelles exhibited reversible thermo-sensitive swelling/shrinking behavior, where the stability and responsive properties of the obtained micelles with varying UPy side-group content were investigated by dynamic light scattering, atomic force microscopy and fluorescence spectroscopy. The temperature-controlled loading and release of pyrene molecules were demonstrated, indicating the potential of the synthesized micelles as drug carriers.
  Flexible conductive materials, which mimic human skin’s functions, has gained special interest in the development of next-generation electronics. In the second project, an electrically conductive composite combining stretchability, self-healing, adhesiveness and sensing capability was prepared via a facial solution casting method, by triggering the in situ polymerization of pyrrole in a supramolecular polymer matrix cross-linked by UPy groups. The elastomer composite with the incorporation of 7.5 wt% conductive polypyrrole displayed a mechanical strength of 0.72 MPa with an elongation over 300%, and a complete self-healing of both mechanical and electrical properties was achieved within 5 min. Besides, the elastic material exhibited strong adhesiveness to a broad range of inorganic and organic substrates, which was further fabricated as a strain sensor for the successful detection of large and subtle human motions including finger bending and pulse beating.
  In the third project, a novel type of multifunctional conductive polymer hydrogel was fabricated by incorporating UPy groups as cross-linking points into a conventional brittle polyaniline/poly(4-styrenesulfonate) (PANI/PSS) hydrogel, leading to a platform with the integration of high conductivity, excellent stretchability, injectability, and rapid self-healing capability. The formation of interpenetrating PANI/PSS network offered the hydrogel a superior conductivity of 13 S/m and a linear response to external strain, exhibiting accurate and reliable detection of various human activities, including large motions of human body (e.g., finger and wrist bending) and subtle movements of muscles (e.g., swallowing, speaking, pulse beating). Taking advantage of the reversibility of the non-covalent cross-links, the hydrogels could be facilely molded into different shapes and injected from a needle, along with a complete self-healing within 30 s upon damage.
  To further reveal the interaction mechanism of the hydrogen-bonding groups from a molecular perspective, in the fourth project, single-molecule force spectroscopy (SMFS) was employed to characterize the binding strength and dynamics of UPy-UPy dimers in aqueous environment, and the hydrophobic effect was investigated by tethering alkylene spacers of different lengths to the hydrogen-bonding moieties. The rupture force and unbinding energy of self-complementary UPy-UPy dimers were found to be remarkably enhanced with increasing spacer length, demonstrating cooperative effect between hydrogen bonding and hydrophobic interactions at the single-molecular level. In good agreement, molecular dynamics (MD) simulations on the interactions of UPy-UPy dimers also indicated higher structural stability with longer hydrophobic spacers.
  This thesis work has developed three novel multifunctional supramolecular materials based on a multiple hydrogen-bonding motif UPy, and has elucidated the related hydrogen-bonding interactions at the single-molecule level. This work expands the applicability of hydrogen-bonding motifs to different multifunctional materials ranging from drug carriers to flexible electronics, providing new insights and approaches to the development of advanced functional materials based on multiple hydrogen bonds for diverse biomedical and engineering applications.

• Subjects / Keywords

  • hydrogen bonding
  • 2-ureido-4[1H]-pyrimidinone
  • self-healing
  • single-molecule force spectroscopy

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-ge6z-ae62

• License

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