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
Hollingsworth, Nisha
2020
Abstract
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 > 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 <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 (>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.Deep Blue DOI
Subjects
Polyelectrolytes Biodiesel Polymer physics Quartz Crystal Microbalance with Dissipation (QCM-D) Polymer membranes
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