Investigating and Engineering Polymeric Interfaces: Elucidating the Swelling of Polyelectrolyte Brushes and Brush Bilayer Thin Films and the Development of Novel Electrospun Polymer Membranes for the Catalysis of Biodiesel
Creators: name: Hollingsworth, Nisha
Publisher: name: My University
Year: 2020
Subjects: subject: Polyelectrolytes
subject: Biodiesel
subject: Polymer physics
subject: Quartz Crystal Microbalance with Dissipation (QCM-D)
subject: Polymer membranes
subject: Chemical Engineering
subject: FOS: Chemical engineering
subject: Science

description: In this thesis, we examine a variety of engineered interfaces: polymer thin films—in the morphologies of brushes and LbL bilayers—and polymer membranes. First, this work investigates the swelling/de-swelling behavior of polyelectrolyte brushes of length 2 kDa–39 kDa of the weakly ionizable polyanion, poly(acrylic acid) (PAA) in the presence of KCl at concentrations from 0.1–1000 mM, at pH = 3, 7, and 9, and at grafting densities σ = 0.12–2.15 chains/nm2 using a Quartz Crystal Microbalance with Dissipation (QCM-D). After correcting our data for the effect of the solvent, we find that at pH 7 (above the pKa), at low and moderate grafting densities, our results in the high-salt “salted brush” (SB) regime (Cs &gt; 10 mM salt) agree with the predicted scaling H~Nσ^+1/3Cs^-1/3 , where H is the brush height [1,2]. In the low-salt "osmotic brush” (OB) regime (Cs &lt;10 mM salt), we find H~Nσ^+1/3Cs^+0.28-0.38, where the dependence on Cs agrees with scaling theory for a weakly charged brush, which predicts a +1/3 power [1], but the dependence on σ strongly disagrees with this theory, which predicts a -1/3 power. The predicted linearity in the degree of polymerization N is confirmed in both the OB and SB regimes [1,2]. Next, these PAA brushes are complexed with a polyelectrolyte of the opposite charge poly(ethylene imine) (PEI) to form LbL bilayers with a brush underlayer for investigation by QCM-D. We find that both brush and bilayer structures may or may not exhibit history-dependent swelling/de-swelling with respect to a pH cycle of pH 7-4-1-4-7 at fixed salt concentration, depending on their chain length and grafting density. We investigate chain lengths of 2 kDa–39 kDa, and use brush grafting densities of 0.023–0.87 chains/nm2. We find that at a low grafting density of 0.45 chains/nm2 and a high PAA molecular weight of 39 kDa, we see hysteresis in swelling over timescales of many minutes. For higher grafting densities (&gt;0.45 chains/nm2) and in short chains (2 kDa), we see little or no hysteresis, and at an intermediate chain length (14 kDa) and grafting density (0.06 chains/nm2), hysteresis is observed at short timescales but is greatly reduced at longer timescales. We also observe hysteresis in swelling with a KCl cycle of 100-20-100-1000-100 mM at fixed pH=7 for a long chain brush (39 kDa) at low grafting density (0.023 chains/nm2), but not for a short chain brush (2 kDa) at a high grafting density (0.86 chains/nm2). We also revisit the discrepancies between our previous findings and scaling predictions1 for brush height with respect to grafting density. We find that for a standalone PAA brush of sufficient chain length at a pH value at which it is partially charged (4.2), the brush height decreases with increased grafting density in the OB regime in agreement with theoretical predictions [1]. Finally, this work describes the development of electrospun polyacrylonitrile (PAN) membranes featuring novel chemistry to immobilize the heterogenous alkaline catalyst CaO onto the membrane fibers using dopamine-catalyst coordination. The resultant membranes were found to successfully achieve biodiesel when used in the transesterification of crude canola oil. References: [1]Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Theory of Ionizable Polymer Brushes. Macromolecules 1995, 28, 1491–1499. [2]Israels, R.; Leermakers, J. F. A. M.; Fleer, J.; Zhulina, E. B. Charged Polymeric Brushes: Structure and Scaling Relations. Macromolecules 1994, 27, 3249–3261.