Browsing by Author "Johnson, EC"

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    Competitive specific ion effects: a neutron reflectometry study of thermoresponsive
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Robertson, H; Johnson, EC; Gresham, IJ; Prescott, SW; Nelson, A; Wanless, EJ; Webber, GB
    Specific ion effects are phenomena that depend on the identity of ions present in a system, and not merely their valence or concentration. For example, the Hofmeister series orders ions on their ability to either stabilise (salting-in ions) or destabilise (salting-out ions) proteins and is vital in biochemistry.[1] In recent years, polymer brushes, which consist of end-tethered polymer chains to a substrate, have been used as exemplar systems to investigate specific ion effects. [2] The effective solvent quality influences the conformation of these brushes (collapsed or expanded), which is directly linked to application properties (e.g. switchable adhesion and self-cleaning). We have performed a significant body of work on brush conformation in single salt electrolytes.[2-4] However, our understanding of the influence that mixed electrolytes have on the behaviour of polymer brushes is currently limited, which is necessary for real-world applications. Here, we present the behaviour of poly(ethylene glycol) methyl ether methacrylate (POEGMA) brushes in a variety of both pure and mixed electrolytes, as studied with neutron reflectometry. Reflectometry allows for the extraction of volume fraction profiles, which can provide detailed information regarding the influence of ion specificity on polymer brush conformation. Consistent results were also obtained from other techniques, such as ellipsometry, which is used to track overall changes in brush thickness. In the presence of electrolytes composed of ions from the same end of the Hofmeister series (salting-in and salting-in or salting-out and salting-out), a non-monotonic concentration-dependent influence of the two ions was observed. The specific ion effects imparted by two salting-in ions were dependent on the influence of the ions with the polymer chains. In contrast, the impact of two salting-out ions was dependent on the available solvent molecules. In the presence of electrolytes composed of ions from opposite ends of the Hofmeister series (salting-in and salting-out), ion behaviour was observed to be temperature-dependent.[3] Much can be gained by improving our knowledge of ion specificity and understanding the subtle structural changes of a brush are essential in order to unravel the dominant drivers behind specific ion effects.
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    Competitive specific ion effects: a neutron reflectometry study of thermoresponsive polymer brushes in mixed electrolytes
    (Elsevier, 2020-10-26) Robertson, H; Johnson, EC; Gresham, IJ; Prescott, SW; Nelson, A; Wanless, EJ; Webber, GB
    Grafted poly(ethylene glycol) methyl ether methacrylate (POEGMA) copolymer brushes change conformation in response to temperature ('thermoresponse'). In the presence of different ions the thermoresponse of these coatings is dramatically altered. These effects are complex and poorly understood with no all-inclusive predictive theory of specific ion effects. As natural environments are composed of mixed electrolytes, it is imperative we understand the interplay of different ions for future applications. We hypothesise anion mixtures from the same end of the Hofmeister series (same-type anions) will exhibit non-additive and competitive behaviour. © 2023 Elsevier B.V.
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    Examining the structural and mechanical implications of surfactants on neutral polymer brushes through neutron reflectometry
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Gresham, IJ; Willott, JD; de Vos, WM; Johnson, EC; Humphreys, BA; Murdoch, TJ; Wanless, EJ; Webber, GB; Nelson, A; Prescott, SW
    Layers of densely-tethered polymers (polymer brushes) are of interest due to their potential applications as nano-actuators, biocompatible coatings, and switchable lubricating or antifouling surfaces. These applicable properties are dependant on the structure of the polymer interface, so it is important that the structural effects of common compounds and relevant environmental variables be understood. Neutron Reflectometry (NR) is the only technique capable of providing detailed structural resolution of solvated multi-component polymer brush systems due to its penetrating power and the possibility of isotopic substitution. Here we present a Neutron Reflectometry study on the effects of surface-active molecules (surfactants) on two neutral polymer brushes, poly(ethylene oxide) (PEO) and Poly(N-isopropylacrylamide) (PNIPAM), focusing on small, single tail surfactants. PEO is a widely used biocompatible polymer with a range of medical and commercial applications, whilst PNIPAM is a well known thermoresponsive polymer, undergoing a swollen to collapsed transition over its critical solution temperature (CST) of 32˚C. We show that these two polymers exhibit similar yet distinct interactions with surfactants, with the observed differences having implications for the mechanism of brush-surfactant interaction. The presence of surfactants was found to raise the CST of PNIPAM; we show that this effect is dependent strongly on surfactant identity and concentration. As part of this work we have developed new modelling techniques for the analysis of NR data from polymer brush interfaces. These advancements will be explained in the context of the data at hand, and their applicability to other soft diffuse interfaces will be briefly discussed. © The Authors.
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    Imaging the invisible: resolving polymer brush structure through a freeform bayesian analysis of neutron reflectometry data
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Gresham, IJ; Murdoch, TJ; Johnson, EC; Grant, W; Wanless, E; Prescott, SW; Nelson, A
    Surfaces covered with densely tethered polymer chains possess desirable properties and are ubiquitous in natural and human-made systems. These properties stem from the diffuse structure of these polymer brush interfaces; consequently, resolving their structure is key to better understanding and designing polymer brush systems. We have been using the PLATYPUS neutron reflectometer at the ACNS to achieve this structural resolution over the past six years, contributing to our understanding of brush structure, as well as fundamental polymer physics. However, the analysis of collected reflectometry data is not without significant challenges; Inflexible models preclude viable structures and the uncertainty around accepted profiles (known as spread) is challenging to quantify. Furthermore, there is no guarantee of profile uniqueness in reflectivity analysis - multiple structures may match the data equally well. Quantifying profile uniqueness and determining the structures that agree with collected data (known as multimodality) has not been previously attempted on brush systems. Historically, data analyses have used least-squares approaches, which do not satisfactorily determine profile spread and bypass the possibility of c. Here we will briefly document our journey in modelling neutron reflectometry data collected from polymer brush systems, culminating in the presentation of our developed methodology. In this methodology, we model our brush with a freeform profile that minimises assumptions regarding polymer conformation while only producing physically reasonable structures. This model is built within refnx‘s Bayesian statistical framework, which enables the characterisation of structural uncertainty and multimodality through Markov Chain Monte Carlo sampling. We demonstrate the rigour of our approach via a round-trip analysis of a simulated system before applying it to real data, examining the well-characterised collapse of a thermoresponsive brush. The method we describe is directly applicable to reflectometry experiments on soft and diffuse systems, but may also be generalised to other instruments where the “inverse problem” hampers data analysis.
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    Maximum flux: using time-resolved neutron reflectometry to improve our understanding of surface-initiated polymerisation
    (Australian Nuclear Science and Technology Organisation, 2021-11-24) Gresham, IJ; Prescott, SW; Nelson, A; Robertson, H; Johnson, EC; Webber, GB; Wanless, EJ
    Polymer brushes are dense arrays of surface-tethered polymers that possess desirable qualities, such as lubricity and fouling resistance, provided that their structure and chemistry are correctly tuned [1]. Surface-initiated polymerisation (SIP) is the primary method for synthesising these brushes with the physicochemical properties required to imbue surfaces with the aforementioned qualities. However, previous work [2,3] indicates that polymers synthesised by SIP deviate from polymers produced via solution polymerisation, likely due to the proximity of initiators in the tethered case. This deviation is not well understood, which impedes the structural characterisation of the resulting brushes. As structure dictates behaviour [1], understanding the nature of the brushes produced by SIP facilitates the rational design of functional brush coatings. Here we present a study of brushes synthesised via SIP of the well-characterised polymer poly(N-isopropyl acrylamide) (PNIPAM) using time-resolved neutron reflectometry (NR). First, we demonstrate that we can control the polymer initiator density and examine the relationship between molecular weight and grafting density. We then observe a series of SIP reactions from surfaces with different initiator densities in situ using time-resolved NR. To our knowledge, this is the first time that the structure of a growing polymer brush has been directly observed. The results confirm that a high initiator density leads to poor control early in the reaction, and explain several phenomena observed by previous NR experiments [4,5]. This experiment paves the way for further kinetic experiments on Platypus and will be of interest to anyone interested in the dynamic assembly of interfaces over timescales of 10 minutes to several hours. © 2021 The Authors