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1. Layer-by-layer assembly of polymers and anisotropic nanomaterials using spray-based approach

   https://doi.org/10.1557/jmr.2020.44  

   De, Souvik; Patel, Anish; Lutkenhaus, Jodie L. (May 2020, Journal of Materials Research)

   Traditional dip-assisted layer-by-layer (LbL) assembly produces robust and conformal coatings, but it is time-consuming. Alternatively, spray-assisted layer-by-layer (SA-LbL) assembly has gained interest due to rapid processing resulting from the short adsorption time. However, it is challenging to assemble anisotropic nanomaterials using this spray-based approach. This is because the standard approach for fabricating “ all-polyelectrolyte ” LbL films does not necessarily give rise to satisfactory film growth when one of the adsorbing components is anisotropic. Here, polymers are combined with a model anisotropic nanomaterial via SA-LbL assembly. Specifically, graphene oxide (GO) is investigated, and the effect of anchor layer, colloidal stability, charge distribution along the carbon framework, and concentration of polymer on the growth and the film quality is examined to gain insight into how to achieve pinhole-free, smooth polymer/GO SA-LbL coatings. This approach might be applicable to other anisotropic nanomaterials such as clays or 2D nanomaterials for future development of uniform coatings by spraying. 

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2. Antifouling Coatings from Glassy Polyelectrolyte Complex Films

   https://doi.org/10.1021/acsami.3c11744  

   Akintola, John; Chen, Yuhui; Digby, Zachary A; Schlenoff, Joseph B (November 2023, ACS Applied Materials & Interfaces)

   Coatings that prevent or decrease fouling are sought for many applications, including those that inhibit the attachment of organisms in aquatic environments. To date, antifouling coatings have mostly followed design criteria assembled over decades: surfaces should be well/strongly hydrated, possess low net charge and maintain a hydrophilic character when exposed to the location of use. Thus, polymers based on ethylene glycol or zwitterionic repeat units have been shown to be highly effective. Unfortunately, hydrated materials can be quite soft, limiting their use in some environments. In a major paradigm shift, this work describes glassy antifouling films made from certain complexes of positive and negative polyelectrolytes. The dense network of electrostatic interactions yields tough materials below the glass transition temperature, Tg, in normal use, while the highly ionic character of these polyelectrolyte complexes ensures strong hydration. The close proximity of equal numbers of opposite charges within these complexes mimics zwitterionic structures. Films, assembled layer-by-layer from aqueous solutions, contained sulfonated poly(ether ether ketone), SPEEK, a rigid polyelectrolyte which binds strongly to a selection of quaternary ammonium polycations. Layer-by-layer buildup of SPEEK and polycations was linear, indicating strong complexes between polyelectrolytes. Calorimetry also showed complex formation was exothermic. Surfaces coated with these films in the 100 nm thickness range completely resisted adhesion of the common flagellate green algae, Chlamydomonas reinhardtii which were removed from surfaces at the minimum applied flow rate of 0.8 cm s-1. The total surface charge density of adsorbed cations, determined with a sensitive radioisotopic label, was very low, around 10% of a monolayer, which minimized adsorption driven by counterion release from the surface. The viscoelastic properties of the complexes, which were stable even in concentrated salt solutions, were explored using rheology of bulk samples. When fully hydrated, their Tgs were observed to be above 75 oC. 

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3. Selective hydrogen bonding controls temperature response of layer-by-layer upper critical solution temperature micellar assemblies

   https://doi.org/10.1039/d0sm01997f  

   Aliakseyeu, Aliaksei; Albright, Victoria; Yarbrough, Danielle; Hernandez, Samantha; Zhou, Qing; Ankner, John F.; Sukhishvili, Svetlana A. (March 2021, Soft Matter)

   null (Ed.)

   This work establishes a correlation between the selectivity of hydrogen-bonding interactions and the functionality of micelle-containing layer-by-layer (LbL) assemblies. Specifically, we explore LbL films formed by assembly of poly(methacrylic acid) (PMAA) and upper critical solution temperature block copolymer micelles (UCSTMs) composed of poly(acrylamide- co -acrylonitrile) P(AAm- co -AN) cores and polyvinylpyrrolidone (PVP) coronae. UCSTMs had a hydrated diameter of ∼380 nm with a transition temperature between 45 and 50 °C, regardless of solution pH. Importantly, micelles were able to hydrogen-bond with PMAA, with the critical interaction pH being temperature dependent. To better understand the thermodynamic nature of these interactions, in depth studies using isothermal titration calorimetry (ITC) were conducted. ITC reveals opposite signs of enthalpies for binding of PMAA with micellar coronae vs. with the cores. Moreover, ITC indicates that pH directs the interactions of PMAA with micelles, selectively enabling binding with the micellar corona at pH 4 or with both the corona and the core at pH 3. We then explore UCSTM/PMAA LbL assemblies and show that the two distinct modes of PMAA interaction with the micelles ( i.e. whether or not PMAA binds with the core) had significant effects on the film composition, structure, and functionality. Consistent with PMAA hydrogen bonding with the P(AAm- co -AN) micellar cores, a significantly higher fraction of PMAA was found within the films assembled at pH 3 compared to pH 4 by both spectroscopic ellipsometry and neutron reflectometry. Selective interaction of PMAA with PVP coronae of the assembled micelles, achieved by the emergence of partial ionization of PMAA at pH 4 was critical for preserving film functionality demonstrated as temperature-controlled swelling and release of a model small molecule, pyrene. The work done here can be applied to a multitude of assembled polymer systems in order to predict suppression/retention of their stimuli-responsive behavior. 

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4. Surface Functionalization Utilizing Mesoporous Silica Nanoparticles for Enhanced Evanescent-Field Mid-Infrared Waveguide Gas Sensing

   https://doi.org/10.3390/coatings11020118  

   Al Husseini, Diana; Karanth, Yashaswini; Zhou, Junchao; Willhelm, Daniel; Qian, Xiaofeng; Gutierrez-Osuna, Ricardo; Coté, Gerard; Lin, Pao; Sukhishvili, Svetlana (February 2021, Coatings)

   null (Ed.)

   This work focuses on the development of nanoparticle-based layer-by-layer (LbL) coatings for enhancing the detection sensitivity and selectivity of volatile organic compounds (VOCs) using on-chip mid-infrared (MIR) waveguides (WGs). First, we demonstrate construction of conformal coatings of polymer/mesoporous silica nanoparticles (MSNs) on the surface of Si-based WGs using the LbL technique and evaluate the coating deposition conditions, such as pH and substrate withdrawal speed, on the thickness and homogeneity of the assemblies. We then use the modified WGs to achieve enhanced sensitivity and selectivity of polar organic compounds, such as ethanol, versus non-polar ones, such as methane, in the MIR region. In addition, using density functional theory calculations, we show that such an improvement in sensing performance is achieved due to preferential adsorption of ethanol molecules within MSNs in the vicinity of the WG evanescent field. 

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5. Influence of Side Chain Hydrolysis on the Evolution of Nanoscale Roughness and Porosity in Amine-Reactive Polymer Multilayers

   https://doi.org/acs.chemmater.0c02095  

   Carter, Matthew C.; Wong, Matthew S.; Wang, Fengrui; Lynn, David M. (July 2020, Chemistry of materials)

   We report the influence of side chain hydrolysis on the evolution of nanoscale structure in thin films fabricated by the reactive layer-by-layer (LbL) assembly of branched poly(ethylenimine) (PEI) and poly(2-vinyl-4,4-dimethylazlactone) (PVDMA). LbL assembly of PEI and PVDMA generally leads to the linear growth of thin, smooth films. However, assembly using PVDMA containing controlled degrees of side chain hydrolysis leads to the growth of thicker films that exhibit substantial nanoscale roughness, porosity, and have resulting physicochemical behaviors (e.g., superhydrophobicity) that are similar to those of some thicker PEI/PVDMA coatings reported in past studies. Our results reveal that the degree of PVDMA partial hydrolysis (or carboxylic acid group content) influences the extent to which complex film features develop, suggesting that ion-pairing interactions between hydrolyzed side chains and amines in PEI promote the evolution of bulk and surface morphology. Additional experiments demonstrate that these features likely arise from polymer/polymer interactions at the surfaces of the films during assembly, and not from the formation and deposition of solution-phase polymer aggregates. When combined, our results suggest that nanoporous structures and rough features observed in past studies likely arise, at least in part, from some degree of adventitious side chain hydrolysis in the PVDMA used for film fabrication. Our results provide useful insight into molecular-level features that govern the growth and structures of these reactive materials, and provide a framework to promote nanoscale morphology reliably and reproducibly. The principles and tools reported here should prove useful for further tuning the porosities and tailoring the physicochemical behaviors of these reactive coatings in ways that are important in applied contexts. 

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