Polyelectrolyte Multilayers with Tunable Properties - Synthesis, Layer Assembly, and Post-modification

Tao Jiang*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

Layer-by-layer (LbL) assembly is a highly versatile approach to the fabrication of thin films forLayer-by-layer (LbL) assembly is a highly versatile approach to the fabrication of thin films for surface modification. The polyelectrolyte multilayers (PEMs) prepared accordingly has been extensively studied, demonstrating tremendous potential in a wide range of applications. The functionality of a PEM film is dependent on the physicochemical properties of the film, which are determined by its chemical composition and the internal structure. This PhD thesis aims to raise strategic approaches to systematically tune the physicochemical properties of a PEM film by utilization of tailor-designed synthetic polyelectrolytes and post-assembly chemical modification. This PhD work in total
involves four studies, two published journal articles, one manuscript, and one unpublished study. In the first study (Paper 1), I proposed a novel approach to systematically tune the growth mechanism and water content of a PEM film by incorporation of PEGMEMA-based copolymers with variable PEG side chain lengths. To do so, I copolymerized poly (ethylene glycol) methyl ether methacrylate (PEGMEMA) with a charge-bearing co-monomer, 2-aminoethyl methacrylate (AMA). PEGMEMA-based polycations were obtained and successfully co-deposited with alginate by LbL assembly. Upon increasing the PEG side chain length, the PEM film demonstrated a shift in growth mechanism, as well as an increased water content. In addition, the film was stabilized against disintegration upon pH variation by partial cross-linking of the amino groups with the treatment of glutaraldehyde. Finally, the cross-linked PEM film was found to show a pHresponsive behavior, exhibiting cationic, zwitterionic, and anionic charging states as a function of the pH value, associated with a film contraction and swelling behavior, as well as water content changes. In the second study (Paper 2), I raised an approach of fabricating a single-component, cross-linked, and surface-grafted polyelectrolyte thin film with tunable layer thickness. In this regard, I expanded the concept of copolymerization in Paper 1 and prepared a PEGMEMA-based polyanion, which was co-deposited with the PEGMEMA-based polycation in an LbL assembly process. After cross-linking with EDC/NHS, the excess amino groups in the film were quenched to provide a II single-component anionic PEGMEMA-based polyelectrolyte layer. The obtained polyelectrolyte layer showed an enhanced BSA repellence compared to the bare substrate and the zwitterionic film before amine quenching. While the first two studies emphasized the bulk properties and the internal, the third study (Paper 3) focused on tuning the interfacial property of the PEM film. I successfully prepared a PEM film comprising PAMA and polymethacrylic acid (PMAA) with reactive amino groups situated selectively in the outer layer. Therefore, a selective tuning of the chemical composition of the outer layer is achieved by a conjugation reaction with functionalized carboxylic acids catalyzed by EDC/NHS. As an illustrative example, I successfully grafted undecanoic acid and m-PEG3-COOH, onto the outer layer of the film, obtaining two polyelectrolyte films showing different surface hydrophilicity but a similar pH-responsiveness. Finally, the fourth study was to further investigate the surface interaction of the two PEM films prepared by colloidal probe atomic force microscopy (CP-AFM) using a hydrophobic colloidal probe. The adhesion force and energy were examined with three varying parameters, namely the surface hydrophilicity, pH value, and the effect of a multivalent cation (Ca2+). The pH-responsive behavior of the films was confirmed in the force measurement, following the same trend as observed in paper 3 with QCM-D. The film modified with undecanoic acid showed a higher adhesion force, which follows our hypothesis that the strong hydrophobic-hydrophobic interaction
facilitates the adhesion. However, it was expected that the film modified with undecanoic acid show high adhesion energy at pH 9 in the presence of Ca2+. This trend was not observed, which was attributed to the relatively low grafting density of the hydrophobic alkyl chains.surface modification. The polyelectrolyte multilayers (PEMs) prepared accordingly has been extensively studied, demonstrating tremendous potential in a wide range of applications. The functionality of a PEM film is dependent on the physicochemical properties of the film, which are determined by its chemical composition and the internal structure. This PhD thesis aims to raise strategic approaches to systematically tune the physicochemical properties of a PEM film by utilization of tailor designed synthetic polyelectrolytes and post-assembly chemical modification. This PhD work in total involves four studies, two published journal articles, one manuscript, and one unpublished study. In the first study (Paper 1), I proposed a novel approach to systematically tune the growth mechanism and water content of a PEM film by incorporation of PEGMEMA-based copolymers with variable PEG side chain lengths. To do so, I copolymerized poly (ethylene glycol) methyl ether methacrylate (PEGMEMA) with a charge-bearing co-monomer, 2-aminoethyl methacrylate (AMA). PEGMEMA-based polycations were obtained and successfully co deposited with alginate by LbL assembly. Upon increasing the PEG side chain length, the PEM film demonstrated a shift in growth mechanism, as well as an increased water content. In addition, the film was stabilized against disintegration upon pH variation by partial cross-linking of the amino groups with the treatment of glutaraldehyde. Finally, the cross-linked PEM film was found to show a pHresponsive behavior, exhibiting cationic, zwitterionic, and anionic charging states as a function of the pH value, associated with a film contraction and swelling behavior, as well as water content changes. In the second study (Paper 2), I raised an approach of fabricating a single-component, cross-linked, and surface-grafted polyelectrolyte thin film with tunable layer thickness. In this regard, I expanded the concept of copolymerization in Paper 1 and prepared a PEGMEMA-based polyanion, which was co-deposited with the PEGMEMA-based polycation in an LbL assembly process. After cross-linking with EDC/NHS, the excess amino groups in the film were quenched to provide a II single-component anionic PEGMEMA-based polyelectrolyte layer. The obtained polyelectrolyte layer showed an enhanced BSA repellence compared to the bare substrate and the zwitterionic film before amine quenching. While the first two studies emphasized the bulk properties and the internal, the third study (Paper 3) focused on tuning the interfacial property of the PEM film. I successfully prepared a PEM film comprising PAMA and polymethacrylic acid (PMAA) with reactive amino groups situated selectively in the outer layer. Therefore, a selective tuning of the chemical composition of the outer layer is achieved by a conjugation reaction with functionalized carboxylic acids catalyzed by EDC/NHS. As an illustrative example, I successfully grafted undecanoic acid and m-PEG3-COOH, onto the outer layer of the film, obtaining two polyelectrolyte films showing different surface hydrophilicity but a similar pH-responsiveness. Finally, the fourth study was to further investigate the surface interaction of the two PEM films prepared by colloidal probe atomic force microscopy (CP-AFM) using a hydrophobic colloidal probe. The adhesion force and energy were examined with three varying parameters, namely the surface hydrophilicity, pH value, and the effect of a multivalent cation (Ca2+). The pH-responsive behavior of the films was confirmed in the force measurement, following the same trend as observed in paper 3 with QCM-D. The film modified with undecanoic acid showed a higher adhesion force, which follows our hypothesis that the strong hydrophobic-hydrophobic interaction facilitates the adhesion. However, it was expected that the film modified with undecanoic acid show high adhesion energy at pH 9 in the presence of Ca2+. This trend was not observed, which was attributed to the relatively low grafting density of the hydrophobic alkyl chains.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages71
Publication statusPublished - 2020

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