Poly(vinyl alcohol) based hydrogen-bonded multilayers : from pH-controlled multi-stage dissolution to zwitter-wettable surfaces

[摘要] Understanding the mechanisms that govern the structure and function of synthetic polymer thin films is of fundamental and practical significance for developing a diverse range of functional surfaces including antifogging coatings, switchable surfaces and stimuli-responsive hydrogels. The first part of this thesis is focused on extending hydrogen-bonding driven polymer thin film assembly by developing a novel systematic framework in which poly(vinyl alcohol) (PVA) can be incorporated into LbL assembled multilayer thin films. Incorporation of PVA into multilayer thin films is advantageous due to its biocompatibility and ease of chemical functionalization and cross-linking. The optimal assembly conditions of PVA multilayer films were discovered through extensive investigation on the degree of PVA hydrolysis, molecular weight and the type of weak polyacids. Subtle variations due to the prevalence of PVA acetate moieties, characterized by the degree of hydrolysis, were shown to cause drastic differences both in self-assembly with its hydrogen-bonding partners as well as its overall pH-stability. The library of materials that can be hydrogen-bonded with PVA was further extended by assembling films with biologically relevant molecules such as tannic acid. This leads to enhanced pH-stability as a result of the high pKa value of tannic acid. Multiple stacks of hydrogen-bonded LbL structures with differing composition and properties were also assembled resulting in complex heterostructured architectures that sequentially dissolve with an increase in local pH conditions. The abundance of free hydroxyl and carboxylic acid groups in the PVA/PAA multilayer allows for enhanced pH stability up to physiological conditions using thermal and chemical methods which offer numerous opportunities for post-assembly functionalization. This was demonstrated by functionalizing PVA/PAA multilayers with poly(ethylene glycol methyl ether) (PEG) to generate a novel antifogging coating with switchable surface properties. To facilitate the characterization of the antifogging coatings a new protocol was developed that enables quantitative analysis of antifogging performance via real-time monitoring of transmission levels as well as image distortion. The antifogging PVA/PAA multilayers were shown to exhibit ;;zwitter-wettable;; behavior, whereby the multilayer film exhibited a facile, rapid absorption of molecular-level water into a film from the gas phase while simultaneously exhibiting very high contact angles for macroscopic liquid drops of water placed on the surface of the same film. An additional step of functionalizing this nano-blended PVA/PAA multilayer with PEG segments produced significantly enhanced antifog and even frost-resistant behavior which was due to the increase in the nonfreezing water capacity of the multilayer film. The PEG-functionalized PVA/PAA multilayers exhibited transient and reversible water contact angle behavior which was studied by both goniometry and dynamic tensiometry. The time-dependent wetting behavior of these coatings was attributed to the transient surface rearrangement of hydrophilic functional groups towards the surface in response to exposure to a liquid water environment. Using a simple first-order thermally-activated model, the kinetics of surface rearrangement was explored in detail. Finally, a model system was designed to study the zwitter-wettable phenomenon in more detail. The complex network of hydrophilic and hydrophobic moieties was decoupled into a heterostructured film consisting of a hydrophilic reservoir and a hydrophobic capping layer. Surface chemistry and roughness were previously believed to be the main factors controlling condensation of water on the film, however, the capacity of the film to transport water molecules was also found to be important for designing functional zwitter-wettable films.

[发布机构] Massachusetts Institute of Technology