Probing Single-Molecule Polymer-Solid Interactions in Water by Single Molecule Force Spectroscopy and Its Preliminary Applications

  • Author / Creator
    Yuechao, Tang
  • The interactions between polymers and solid-water interfaces involve various interesting physical processes, which are fundamental questions for both scientific research and a variety of practical applications, such as underwater adhesives, anti-fouling and stabilization of interested materials/particles. This study focuses on investigating the fundamental single-molecule interactions between polymers and solid surfaces in an aqueous environment using Single Molecule Force Spectroscopy (SMFS). With the help of stimuli-responsive oligo ethylene glycol copolymer, the influence of polymer hydrophobicity on single-molecule adhesion force was studied using one polymer-substrate pair. By changing the NaCl concentration, the polymer underwent a transition from a hydrophilic to a hydrophobic state due to the suppressed hydration of side chains. It was found that the single-molecule adhesion force on a MoS2 basal surface in the presence of 1 mM NaCl was around 53 pN. The force almost remained constant in low NaCl concentration up to 0.5 M, followed by a significant increase with increasing NaCl concentration before approaching a plateau at NaCl concentration as high as 1 M, which indicated that the single-molecule adhesion force was sensitive to polymer hydrophobicity. To further understand the probed dependence of single-molecule force on polymer hydrophobicity, the anisotropic property of MoS2 was utilized to separate the contributions of electric double layer interaction (EDL), van der Waals interaction (vdW) and hydrophobic attraction from total single-molecule interactions. The observed pH-independent force excludes the contribution of EDL to the total single-molecule interaction. Compared with the hydrophobic basal surface of MoS2, the single-molecule adhesion force between the hydrophilic edge surface of MoS2 and the studied copolymer was found to be smaller than the lower detection limit of SMFS (9 pN). Thus, the vdW interaction was demonstrated to play a minor role in single-molecule interaction between the studied polymer and MoS2 surface. The overall investigations ruled out the dominant role of EDL and vdW interactions on single-molecule interactions and illustrated that the hydrophobic attraction might be the main driving force of single-molecule interaction between hydrophobic polymers and surfaces. To capture the underlying physics, a simple thermodynamic model was derived in the third part of this study. Based on the law of energy conservation, the energy inputted into the system by peeling the polymer chain from a solid-water interface under external force was related to the change of entropic free energy and adhesion free energy of the polymer chain. As the driving force holding the polymer on the MoS2-water interface was demonstrated to be mainly hydrophobic attraction, in common with other investigations, the adhesion free energy was quantified using hydrophobic hydration free energy. The single-molecule forces of polymers on MoS2 and highly oriented pyrolytic graphite (HOPG) surfaces were used to test the validity of the derived model. The good agreement between the experimental data and theoretical model further confirmed the dominant role of hydrophobic attraction in single-molecular interactions between polymers and hydrophobic surfaces. In the fourth part of this thesis, the in-depth understanding of SMFS investigation between different polymers and MoS2 surfaces was used to navigate the structure design of polymers for MoS2 exfoliation. The performance of MoS2 for various applications can be significantly improved when exfoliated into a single-layer. The key for the efficient aqueous exfoliation relies on the proper selection of polymer that is able to functionalize and stabilize the exfoliated MoS2 nanosheets in water. Therefore, the understanding and quantification of polymer-solid interaction in water is highly desired for the selection of suitable organic functionalization agents. Among the polymers studied, cationic poly (vinylbenzyl trimethyl ammonium chloride) (PVBTA) was indicated to be promising for efficient exfoliation of MoS2 due to its good water compatibility and strong interaction with both the basal and edge surfaces of MoS2. Compared with previously reported values, scaled production of single-layer MoS2 nanosheet exfoliated by the selected polymer was achieved to be one magnitude higher concentration using magnitude less treatment time. These results demonstrated that the insights from SMFS investigation is highly effective to optimize the functionalization and stabilization of interested materials in practical applications. To be noted, this study provided an efficient approach to discover appropriate surface active polymer for effective functionalization of MoS2 and could be applied in a variety of practical applications demanding optimization of materials, including underwater adhesion, dispersion or functionalization.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.